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Wastewater Alkalinity Enhancement
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This Protocol (A document that describes how to quantitatively assess the net amount of CO₂ removed by a process. To Isometric, a Protocol is specific to a Project Proponent's process and comprised of Modules representing the Carbon Fluxes involved in the CDR process. A Protocol measures the full carbon impact of a process against the Baseline of it not occurring.) provides the requirements and procedures for the calculation of net carbon dioxide equivalent (CO2e) removal (The term used to represent the CO₂ taken out of the atmosphere as a result of a CDR process.) from the atmosphere via Wastewater Alkalinity Enhancement (WAE). This Protocol is developed for application in Wastewater Alkalinity Enhancement processes in which a cradle-to-grave (Considering impacts at each stage of a product's life cycle, from the time natural resources are extracted from the ground and processed through each subsequent stage of manufacturing, transportation, product use, and ultimately, disposal.) Greenhouse Gas (GHG) Statement can be accurately applied and in which the carbon dioxide (CO2) captured is durably stored for over 1,000 years.
Conventional wastewater treatment plants (A facility that receives and treats municipal and/or industrial wastewater.) (WWTP) use a combination of physical, chemical and biological processes to remove soluble and insoluble contaminants prior to effluent discharge, which is essential to safeguard both human and environmental health. Commonly, WWTPs consist of preliminary and primary treatment, in which large particles are removed via screens and gravity settling, and secondary treatment, in which pollutants and unreacted biomass (sludge) are removed and effluent is clarified and disinfected for discharge. Finally, some WWTPs with more stringent discharge limits (e.g. those that discharge into recreational or drinking water bodies) receive a tertiary treatment, which may consist of targeted approaches for treating specific constituents before discharge.
Secondary treatment is typically a large source of biogenic CO2 emissions in WWTPs as that is the step where a majority of solid and dissolved organic carbon in the wastewater is transformed into CO2 through biological processes1,2. The secondary treatment process also leads to emissions of both methane (CH4) and nitrous oxide (N2O)2, which are 29.8 and 273 times more potent GHGs, respectively, than CO2 on 100-year timescales3. This Protocol focuses on the removal of biogenic CO2 emissions and not other GHGs. It is important to note that biogenic CO2 emissions from wastewater treatment are not typically counted in standard GHG accounting frameworks4, 5, thus the removal of this biogenic CO2 through WAE qualifies as carbon dioxide removal (Activities that remove carbon dioxide (CO₂) from the atmosphere and store it in products or geological, terrestrial, and oceanic Reservoirs. CDR includes the enhancement of biological or geochemical sinks and direct air capture (DAC) and storage, but excludes natural CO₂ uptake not directly caused by human intervention.) (CDR), rather than emissions avoidance.
In Wastewater Alkalinity Enhancement, rock or mineral feedstock (Raw material which is used for CO₂ Removal or GHG Reduction.) is added into the secondary treatment process. This feedstock reacts with carbonic acid generated through the degradation of organic matter to produce stable bicarbonate (HCO3-) ions, analogous to natural rock weathering processes that regulate atmospheric CO2 concentrations on geologic timescales6. When the effluent is discharged into the ocean or into a river that drains into the ocean, the constituent bicarbonate is stably stored on millennial timescales. Because Wastewater Alkalinity Enhancement leverages existing wastewater infrastructure and permits, it has high potential for scaling.
This Protocol specifically applies to municipal and industrial WWTPs that reduce biochemical oxygen demand (The amount of dissolved oxygen consumed by microorganisms while breaking down organic matter in water over a specific time period.) (BOD) using biological treatment. In contrast, chemical treatment for wastewater treatment from industrial processes such as mining or oil and gas extraction is ineligible under this Protocol. The goals of biological treatment are to a) convert solid and dissolved organic matter into biomass (sludge) and CO2; b) remove excess nutrients, such as phosphorus and nitrogen, from the wastewater stream; and c) clarify effluent prior to disinfection and discharge. Several methods for biological treatment exist in practice, but the most common example is the activated sludge process, in which a bacterial biomass suspension (or activated sludge) is used for pollutant removal7. Depending on WWTP design and specific requirements for treatment, activated sludge can remove nitrogen and phosphorus in addition to organic materials. In the activated sludge process, the sludge is settled in clarifier tanks prior to disinfection. A portion of this sludge is recycled through the system and a portion, termed waste activated sludge (A type of biological wastewater treatment process that uses microorganisms to break down organic waste.) (WAS) is removed. WAS is referenced throughout this Protocol with respect to solid-phase sampling (see Section 11.4).
For the purposes of this Protocol, a Wastewater Alkalinity Enhancement process must be retrofitted into existing WWTP operations. A retrofit is defined as the introduction of new materials, products or technologies to an existing process or facility. See Section 4 for more details on eligible projects. A Project (An activity or process or group of activities or processes that alter the condition of a Baseline and leads to Removals or Reductions.) refers to the system boundaries of the retrofitted process, as described in Section 7.3.
This Protocol is developed to adhere to the requirements of ISO (A worldwide federation (NGO) of national standards bodies from more than 160 countries, one from each member country.)14064-2: 2019 -- Greenhouse Gases -- Part 2: Specification with guidance at the project level for quantification, monitoring, and reporting of greenhouse gas emission reductions (Lowering future GHG releases from a specific entity.) or removal enhancements. The Protocol ensures:
Note that throughout this Protocol, use of the word "must" indicates a requirement, whereas "should" indicates a recommendation.
Specific standards and protocols which are utilized as the foundation of this Protocol and for which this Protocol is intended to be fully compliant with are the following:
Additional reference standards that inform the requirements and overall practices incorporated in this Protocol include:
This Protocol was developed based on the current state of the art and publicly available science regarding Wastewater Alkalinity Enhancement. As Wastewater Alkalinity Enhancement is a novel CDR approach, with limited published literature, this Protocol incorporates requirements that may be highly stringent to minimize risk and for the purposes of environmental and social safeguarding.
This Protocol will be altered in future versions as the science underlying this pathway (A collection of Removal or Reduction processes that have mechanisms in common.) evolves, and the overall body of knowledge and data across all processes is increased, for example regarding feedstock supply, conversion, ecosystem impacts and durable storage. Future updates will also increase the scope of eligible projects for this Protocol, see Appendix 2 for planned areas of improvement for future versions.
This Protocol will be reviewed at a minimum every 2 years and/or when there is an update to scientific published literature which would affect net CO2e removal quantification or the monitoring and modeling guidelines outlined in this Protocol.
The aim of this Protocol is to ensure that projects seeking carbon removal Credits (A publicly visible uniquely identifiable Credit Certificate Issued by a Registry that gives the owner of the Credit the right to account for one net metric tonne of Verified CO₂e Removal or Reduction. In the case of this Standard, the net tonne of CO₂e Removal or Reduction comes from a Project Validated against a Certified Protocol.) for Wastewater Alkalinity Enhancement are safe and have a demonstrable net-negative climate impact. The scope of this Protocol may be expanded in future versions to allow for more discharge locations and provide additional guidance for different types of wastewater treatment facilities. To be eligible for crediting under this Protocol, projects and associated operations must meet all of the following conditions.
To ensure human and environmental health, eligible projects must:
To ensure additional net-negative climate impacts, eligible projects must:
The following applicability requirements are to limit the scope of eligible projects that the present version of this Protocol is developed for. However, these may be expanded in future versions.
This Protocol only quantifies CDR within the project facility (i.e. within a closed system). Additional uptake of CO2 that occurs in the open ocean after discharge is not eligible for crediting under this Protocol, but may be eligible for further crediting under the Ocean Alkalinity Enhancement from Coastal Outfalls Protocol.
The following topics are covered briefly in this Protocol due to their inclusion in the Isometric Standard, which governs all Isometric Protocols. See in-text references to the Isometric Standard for further guidance.
For each specific project to be evaluated under this Protocol, Project Proponents (The organization that develops and/or has overall legal ownership or control of a Removal or Reduction Project.) must document project characteristics in a Project Design Document (The document that clearly outlines how a Project will generate rigorously quantifiable Additional high-quality Removals or Reductions.) (PDD) as outlined in Section 3.2 of the Isometric Standard. The PDD will form the basis for project verification and evaluation in accordance with this Protocol, and must include consideration of processes unique to Wastewater Alkalinity Enhancement projects, such as:
Projects must be validated and net CO2e removals verified by an independent third party, consistent with the requirements described in this Protocol as well as in Section 4 of the Isometric Standard.
The Validation and Verification Body (Third-party auditing organizations that are experts in their sector and used to determine if a project conforms to the rules, regulations, and standards set out by a governing body. A VVB must be approved by Isometric prior to conducting validation and verification.) (VVB) must consider the following requisite components:
The threshold for Materiality (An acceptable difference between reported Removals/emissions or Reductions/emissions and what an auditor determines is the actual Removal/emissions or Reduction/emissions.), considering the totality of all omissions, errors and mis-statements, is 5%, in accordance with Section 4.3 of the Isometric Standard.
Verifiers should also verify the documentation of uncertainty (A lack of knowledge of the exact amount of CO₂ removed by a particular process, Uncertainty may be quantified using probability distributions, confidence intervals, or variance estimates.) of the GHG statement as required by Section 2.5.7 of the Isometric Standard. Qualitative Materiality issues may also be identified and documented, such as:
Project validation and verification must incorporate site visits to project facilities in accordance with the requirements of ISO 14064-3, 6.1.4.2, including, at a minimum, site visits during validation and initial verification. Validators should, whenever possible, observe project operations to ensure full documentation of process inputs and outputs through visual observation (see Section 4 of the Isometric Standard).
A site visit must occur at least once per project validation at each location.
Verifiers and validators must comply with the requirements defined in Section 4 of the Isometric Standard. In addition, VVB teams shall maintain and demonstrate expertise associated with the specific technologies of interest.
All VVBs are approved by Isometric independently and impartially based on alignment with Conflict of Interest policies, rotation of VVB policies, oversight on quality and the following requirements:
CDR via Wastewater Alkalinity Enhancement can often be a result of a multi-step process (such as quarrying, alkaline feedstock processing, transportation, on-site operations and monitoring), with activities in each step managed and operated by a different operator, company or owner. When there are multiple parties involved in the process, and to avoid double counting (Improperly allocating the same Removal or Reduction from a Project Proponent more than once to multiple Buyers.) of net CO₂e removal, a single Project Proponent must be specified contractually as the sole owner of Credits. Contracts must comply with all requirements defined in Section 3.1 of the Isometric Standard.
The Project Proponent must be able to demonstrate additionality through compliance with Section 2.5.3 of the Isometric Standard. The counterfactual (An assessment of what would have happened in the absence of a particular intervention – i.e., assuming the Baseline scenario.) scenarios and baselines (A set of data describing pre-intervention or control conditions to be used as a reference scenario for comparison.) utilized to assess additionality must be project-specific, and are described in Section 8 of this Protocol.
Additionality determinations must be reviewed and completed every two years, at a minimum, or whenever project operating conditions change significantly, such as the following:
Any review and change in the determination of additionality will not affect the availability of Carbon Finance and Credits for the current or past Crediting Periods (The period of time over which a Project Design Document is valid, and over which Removals or Reductions may be Verified, resulting in Issued Credits.), but, if the review indicates the Project has become non-additional, this shall make the Project ineligible for future Credits.
The uncertainty in the overall estimate of net CO₂e removal as a result of the Project must be accounted for. The total net CO₂e removal for a specific Reporting Period must be conservatively (Purposefully erring on the side of caution under conditions of Uncertainty by choosing input parameter values that will result in a lower net CO₂ Removal or GHG Reduction than if using the median input values. This is done to increase the likelihood that a given Removal or Reduction calculation is an underestimation rather than an overestimation.) determined, and projects must conduct an uncertainty analysis for the net CO₂e removal calculation in compliance with requirements outlined in Section 2.5.7 of the Isometric Standard.
Projects must report a list of all key variables used in the net CO₂e removal calculation and their uncertainties, as well as a description of the uncertainty analysis approach, including:
The uncertainty information should at least include the minimum and maximum values of each variable that goes into the net CO₂e removal calculation (see Section 8 for more details). More detailed uncertainty information should be provided if available, as outlined in Section 2.5.7 of the Isometric Standard.
In addition, a sensitivity analysis (An analysis of how much different components in a Model contribute to the overall Uncertainty.) that demonstrates the impact of each input parameter’s uncertainty on the final CO2e removal uncertainty must be provided. Details of the sensitivity analysis method must be provided such that a third party can reproduce the results. Input variables may be omitted from an uncertainty analysis if they contribute to a < 1% change in the net CO₂e removal. For all other parameters, information about uncertainty must be specified.
In accordance with the Isometric Standard, all evidence and data related to the underlying quantification of net CO₂e removal and environmental monitoring will be available to the public through Isometric’s Science Platform. That includes:
The Project Proponent can request certain information to be restricted (only available to authorized Buyers (An entity that purchases Removals or Reductions, often with the purpose of Retiring Credits to make a Removal or Reduction claim.), the Registry (A database that holds information on Verified Removals and Reductions based on Protocols. Registries Issue Credits, and track their ownership and Retirement.) and VVB) where it is subject to confidentiality. However, that does not apply to any numerical data produced or used as part of the quantification of net CO2e removal, or to any monitoring data that is required for regulatory compliance.
Following the Isometric Standard, Credits issued under this Protocol are contingent on the implementation, transparent reporting, and independent verification of comprehensive safeguards. These safeguards encompass a wide range of considerations, including environmental protection, social equity, community engagement and respect for cultural values. The process mandates that safeguard plans be incorporated into all major project phases, with detailed reports made accessible to stakeholders. Adherence to and verification of environmental and social economic safeguards is a condition for all Crediting Projects.
Since Projects under this Protocol are retrofitted into existing WWTP operations, the scope of environmental and social safeguarding is applicable for the additional impacts relative to BAU.
Projects must adhere to the following governance and legal requirements:
Official permitting:
Compliance:
An environmental and social risk assessment in adherence with Section 3.7 of the Isometric Standard must be completed by a subject matter expert to identify potential risks, followed by the development of tailored mitigation plans. These plans must encompass specific actions to avoid, minimize or rectify identified impacts. Effective implementation of these measures must also be accompanied by a robust monitoring plan to detect negative impacts and stop projects when necessary (see Section 11). Since Wastewater Alkalinity Enhancement is integrated with existing wastewater treatment plants, in many cases, the existing monitoring requirements for BAU wastewater treatment operations may be sufficient.
Environmental and social risk identification, assessment, avoidance and mitigation planning will be unique to each Project’s technological, environmental and social contexts. The severity of these risks vary based on site specifics and the wastewater effluent characteristics.
The Project Proponent must conduct an environmental risk assessment which adheres to Section 3.7.1 of the Isometric Standard.
When assessing aquatic environmental risks, it is important to holistically consider the risk compared to the baseline scenario of business as usual wastewater treatment. For context, existing pollutant and nutrient loads from wastewater discharge have a documented impact of leading to acidification, hypoxia, and toxic algae blooms. Alkalinity enhancement has previously been demonstrated as an intervention to mitigate these environmental impacts. Generally, it is expected that Wastewater Alkalinity Enhancement will reduce the risks already contained within business as usual wastewater discharge.
Particular environmental risks associated with Wastewater Alkalinity Enhancement which must be assessed, avoided and/or mitigated are:
Feedstock sourcing:
Co-products and waste:
Pollution prevention:
Ecological impacts:
The Project Proponent must conduct a social risk assessment which adheres to Section 3.7.2 of the Isometric Standard on Social Impacts.
In particular, Wastewater Alkalinity Enhancement projects must:
Per Section 3.5 of the Isometric Standard, Project Proponents must demonstrate active stakeholder engagement throughout project planning and operation, ensuring that all risk mitigation strategies contribute to sustainable project outcomes. Local stakeholders situated in the vicinity of the project site may contribute an in-depth understanding of the local system and provide invaluable insights and recommendations on the potential risks, necessary safeguards and specific monitoring needs. Relevant local stakeholders may include municipal utilities operators, local members of academia, Indigenous groups, environmental groups, and citizen associations. The Stakeholder Input Process must adhere to requirements outlined in Section 3.5 of the Isometric Standard, and evidence of these meetings must be submitted in the PDD.
Project Proponents must include in the PDD a plan for information sharing, emergency response and conditions for stopping or pausing alkaline feedstock dosing. Plans for pausing or stopping dosing must be in place in instances where:
The Reporting Period represents an interval of time over which removals are calculated and reported for verification. The total net CO₂e removal is calculated using a series of measurements for a specified Reporting Period, and is written hereafter as [math: CO_2e_{Removal,RP}]. See Section 8.1 for details on how [math: CO_2e_{Removal,RP}] is calculated. Only CO2 is included for the quantification of gross removal, while all GHG emissions are included when calculating the net removal (see Section 7.2 and Table 1).
GHG emission calculations must include all emissions related to the project activities that occur within the Reporting Period. This includes: a) any emissions associated with project establishment allocated to the Reporting Period, b) any emissions that occur within the Reporting Period, c) any anticipated emissions that would occur after the Reporting Period that have been allocated to the Reporting Period and d) leakage (The increase in GHG emissions outside the geographic or temporal boundary of a project that results from that project's activities.) emissions that occur outside of the system boundary as a result of induced market changes that are associated with the Reporting Period.
The scope of this Protocol includes GHG sources, sinks and reservoirs (A location where carbon is stored. This can be via physical barriers (such as geological formations) or through partitioning based on chemical or biological processes (such as mineralization or photosynthesis).) (SSRs) associated with a Wastewater Alkalinity Enhancement (WAE) Project. A cradle-to-grave GHG Statement must be prepared encompassing the GHG emissions relating to the activities outlined within the system boundary. The system boundary must include all SSRs controlled by and related to the Project, including but not limited to the SSRs in Figure 1 and Table 1.
The system boundary must include all GHG SSRs from activities related to the Credits delivered within the Reporting Period that are associated with the establishment of the Project, operations and end-of-life activities that occur after the Reporting Period.
As noted in Section 7.3, the baseline scenario assumes that the activities associated with the WAE component of the wider WWTP facility do not take place and no additional infrastructure associated with the WAE project is built. Therefore, some elements of the process included within the system boundary will only apply to the additional emissions attributed to activities that are above business as usual (BAU) emissions for the WWTP. Any emissions from sub-processes or process changes that would not have taken place without the involvement of the CDR process must be fully considered in the system boundary. This allows for accurate consideration of additional, incremental emissions induced by the CDR process. For example, if a WWTP was using NaOH as a pH amendment as part of their BAU operations, and a WAE project replaces NaOH with a lower emission feedstock, then this constitutes a process change and the resulting new feedstock manufacturing and transport emissions must be fully accounted for in the system boundary. If a WWTP is using the same feedstock as the BAU scenario and increasing dosing, then only the feedstock emissions associated with the increased feedstock need to be accounted for.
If any GHG SSRs within Table 1 are deemed not appropriate to include in the system boundary, they may be excluded if robust justification and appropriate evidence is provided. Exclusions that are based on no activity above BAU WWTP activities must be evidenced.
[Image: **Figure 1** System Boundaries]
Figure 1 System boundaries for a Wastewater Alkalinity Enhancement (WAE) project integrated into an existing Wastewater Treatment Plant (WWTP). Any components in color with a dotted boundary are those where emissions are associated with WAE activities that are shared with BAU WWTP activities. Only activities above BAU are accounted for within the system boundary. The accounting of feedstock manufacturing and transport emissions depend on the specific WWTP BAU operations, see Table 1 for more details. Components in grayscale with a dotted boundary are those where activities are unlikely to be accounted for within the system boundary but which are an integral part of the process, and so are included for completeness.
Table 1. Scope of activities and GHG SSRs to be included by the removal project
| Activity | GHG source, sink or reservoir | GHG | Scope | Timescale |
|---|---|---|---|---|
| Project Establishment | Initial surveys and feasibility studies | All GHGs | Any embodied, energy and transport emissions associated with surveys or feasibility studies required for establishment of the project site for WAE CDR activities. | Before dosing starts - must be accounted for in the first Reporting Period or amortized in line with allocation rules (See Section 8.4.1). |
| Equipment & materials manufacture | All GHGs | Embodied emissions (Life cycle GHG emissions associated with production of materials, transportation, and construction or other processes for goods or buildings.) associated with equipment and materials manufacture for project establishment (lifecycle Modules A1-39). | ||
| Equipment & materials transport to site | All GHGs | Transport emissions associated with transporting equipment and materials associated with WAE CDR processes to the project site(s) (lifecycle Module A49). Must include any additional material or infrastructure requirements at the WWTP as a result of WAE, for example any additional load bearing structures. | ||
| Construction and installation | All GHGs | Emissions associated with construction and installation of the project site(s) (lifecycle Module A59) for WAE CDR activities and any additional infrastructure requirements to WWTP as a result of WAE activities. | ||
| Misc. | All GHGs | Any SSRs not captured by categories above. Any SSRs not captured by categories above. Only those that occur over the level of BAU WWTP activities should be accounted for. | ||
| Operations | Energy use | All GHGs | Electricity and fuel consumption associated operational processes, including marginal pumping, pre-treatment, waste-water treatment, solids separation and disposal, and discharge of treated to the environment. | Over each Reporting Period - must be accounted for in the relevant Reporting Period (See Section 8.4.2) |
| Feedstock manufacturing and transport | All GHGs | Feedstock raw material extraction and manufacturing including rock quarrying, crushing, grinding and drying. Feedstock transport from source manufacturer to project site. | ||
| Consumables (other than feedstock) | All GHGs | Embodied emissions associated with consumables required for operation of the project site (excluding feedstock). | ||
Waste Activated Sludge (WAS) transport | All GHGs | Transport of waste activated sludge (WAS) to disposal locations. | ||
| Incineration of Waste Activated Sludge (WAS) | CO2 | CO2 released from the incineration of waste activated sludge (WAS), if applicable. | ||
| Landfill disposal of Waste Activated Sludge (WAS) | All GHGs | Emissions associated with the landfilling of waste activated sludge (WAS), if applicable. | ||
| Sampling required for MRV | All GHGs | Monitoring, including consumables used for measurement and transportation or shipping of samples for laboratory analysis and sample processing for MRV. | ||
| Staff travel | All GHGs | Flight, car, train or other travel required for the project operations, including contractors and suppliers required on site. | ||
| Surveys | All GHGs | Embodied, energy and transport emissions associated with undertaking required surveys e.g. environmental impact surveys. | ||
| CO₂ Stored | CO₂ | The gross amount of CO₂ removed and durably stored from the Wastewater Alkalinity Enhancement process as ocean dissolved inorganic carbon (DIC). | ||
| Maintenance of project site | All GHGs | To include maintenance (lifecycle Modules B29), repair (B3), replacement (B4) and refurbishment (B5) activities associated with equipment, buildings and infrastructure. | ||
| Misc. | All GHGs | Any SSRs not captured by categories above. For example, accidental or unintended release of treated waters may result in secondary precipitation and ocean outgassing, or unneutralized streams may result in ocean outgassing. If these events occur, their impacts must be quantified. Only those that occur over the level of business as usual WWTP activities should be accounted for. | ||
| End-of-Life | End-of-life of emissions | All GHGs | To include anticipated end-of-life emissions (lifecycle Modules C1-49). | After Reporting Period - must be accounted for in the first Reporting Period or amortized in line with allocation rules (See Section 8.4.1) |
| Ongoing sampling and monitoring required for MRV | All GHGs | Embodied, energy and transport emissions related to ongoing monitoring of carbonate mineral storage site, measurement equipment, and energy use associated with monitoring equipment, sampling systems, laboratory analysis, operation and management of monitoring facility buildings and handling equipment. | ||
| Ongoing surveys | All GHGs | Embodied, energy and transport emissions associated with undertaking long-term required surveys e.g. ecological surveys. | ||
| Misc. | All GHGs | Any emissions source, sink or reservoir not captured by categories above. |
The Project Proponent must consider all GHGs associated with SSRs, in alignment with the United States Environmental Protection Agency’s definition of GHGs, which includes: carbon dioxide (CO₂), methane (CH4), nitrous oxide (N₂O) and fluorinated gasses such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3). For CO₂ capture and CO₂ leakage, only CO₂ is expected to be included as part of the quantification. For all other activities, all GHGs must be considered. For example, CO2, CH4 and N2O are all associated with diesel consumption.
All GHGs must be quantified and converted to CO₂e. GHGs must be converted to CO₂e in the GHG Statement using the 100-yr GWP (A measure of how much energy the emissions of 1 tonne of a GHG will absorb over a given period of time, relative to the emissions of 1 ton of CO₂.) for the GHG of interest, based on the most recent volume of the IPCC Assessment Report (currently the Sixth Assessment Report)10.
Miscellaneous GHG emissions are those that cannot be categorized by the GHG SSR categories provided in Table 1. The Project Proponent is responsible for identifying all sources of emissions directly or indirectly related to project activities and must report any outside of the SSR categories identified as miscellaneous emissions.
Emissions associated with a project's impact on activities that fall outside of the system boundary of a project must also be considered. This is covered under Leakage in Section 8.4.4.
Ancillary Activities
Ancillary activities, such as supplementary research and development activities and corporate administrative activities, that are associated with a project but are not directly or indirectly related to the issuance of Credits can be excluded from the system boundary.
Secondary Impacts on GHG Emissions
Wastewater Alkalinity Enhancement has additional impacts on GHG emissions beyond the scope of this Protocol, for example by reducing CH4 and N2O emissions, or by increasing energy efficiency at WWTPs. Crediting the avoidance of emissions is beyond the scope of this Protocol, and thus is not considered.
Considerations for Waste Input Emissions
Embodied emissions associated with system inputs considered as waste products can be excluded from the system boundary provided the appropriate criteria are met. For energy inputs, for example the use of waste heat, refer to the Energy Accounting Module. For other waste inputs, the following criteria shall be considered.
If EC1 in Table 2 is satisfied then this is sufficient to exclude embodied emissions from the system boundary. Market leakage emissions associated with waste inputs may also be excluded from the system boundary as compliance with EC1 would result in no change to the waste producer behavior (no market leakage) and indicates there are no alternative users of the waste product (no replacement emissions).
Table 2. Waste input emissions exclusion criteria, EC1
| Criteria | Description | Documentation required |
|---|---|---|
| EC1 | No payment was made for the material, or only a “tipping fee” is paid. | Feedstock purchase or removal records between Project Proponent and feedstock supplier demonstrating price paid, amount, Buyer, seller and date. Additionally, a signed affidavit from the Project Proponent stating that no in-kind compensation was made to the feedstock supplier must be provided. |
If EC2 and EC3 in Table 3 are both satisfied then this is sufficient to exclude embodied emissions from the system boundary. Market leakage emissions associated with waste inputs may also be excluded from the system boundary as compliance with EC2 and EC3 would result in no significant change to the waste producer behavior (no market leakage) and there are no alternative use cases for the waste product (no replacement emissions).
Table 3. Waste input emissions exclusion criteria, EC2 and EC3
Criteria | Description | Documentation required |
|---|---|---|
EC2 | The amount of the waste product used by the CDR project was not already being utilized as a valuable product by another party for non-CDR uses. Therefore, the producer of the waste product has no alternative use case for the waste product. | Feedstock purchase or removal records between Project Proponent and feedstock supplier demonstrating price paid, amount, Buyer, seller and date. Additionally, an affidavit from the waste supplier identifying that there are no alternative use cases for the waste product. |
EC3 | Payments for the waste product do not constitute a significant share of upstream operations revenue for the waste producer. | Feedstock purchase or removal records between Project Proponent and feedstock supplier demonstrating price paid, amount, Buyer, seller and date. Additionally, an affidavit from the waste supplier that documents that payments from the Project do not constitute a significant share of upstream operations revenue. |
Considerations for Project Activities Integrated into Separate Practices
WAE projects eligible under this Protocol are integrated into existing WWTP (see Section 4), and thus will be reliant on processes occurring separately from the CDR activity. This may include operations of bioreactors, existing flowmeters and other monitoring equipment and making use of discharge outfalls. Activities that were already occurring and would continue to occur without the WAE project may be omitted from the system boundary, provided that evidence that the activity was already occurring and would have continued to occur in the absence of the WAE project can be provided. These principles are reflected in Table 1 and Figure 1.
Any impacts on the WWTP as a result of WAE that would require changes to the existing separate facility, such as additional infrastructure requirements associated with heavier loads for incorporating WAE processes, shall be considered within the system boundary. Incremental increases in activity from BAU, such as incremental additional energy requirements, must be accounted for in the system boundary and evidenced in full.
The separate facility must not include any CO2 removals associated with the WAE CDR activity in their company GHG accounting to avoid double counting of removals.
Under this Protocol, a Project must be a retrofit to an existing WWTP facility (see Section 4). The baseline scenario for a WAE project assumes the WAE activities associated with CDR do not take place and the WWTP operates in a BAU manner.
The counterfactual removal scenario for each Project will depend on if the BAU operations of the WWTP include some level of alkalinity addition or not. If the BAU operations use a high carbon intensity feedstock (e.g. NaOH) such that the feedstock manufacturing emissions exceed the potential CO2 drawdown in the WWTP, then the carbon removal potential of the BAU operation may be considered 0 as it is a net-emitting process. Projects operating under this scenario must be cleared ahead of time in consultation with Isometric. See Section 8.3 for details on calculating the counterfactual, [math: CO_2e _{Counterfactual,\ RP}], for different scenarios.
In some instances, it may be necessary to consider weathering of alkaline feedstock that would have occurred without the Wastewater Alkalinity Enhancement project. For example, if the feedstock used is a waste product that was not mined or quarried specifically for project activities and was stored in open-air conditions, some degree of surficial weathering may be expected over timescales relevant to a project lifetime. Project Proponents using these feedstocks must account for the counterfactual weathering of the feedstock.
Net CO₂e removal from Wastewater Alkalinity Enhancement for each Reporting Period, RP, must be calculated conservatively so as to give high confidence that, at minimum, the estimated net CO₂e was removed.
The net CO₂e removal equation is:
[math: CO_2e_CO*2e*{Removal,\ RP} = CO_2e_CO*2e*{Stored,\ RP} - CO_2e_CO*2e*{Counterfactual,\ RP} - CO_2e_CO*2e*{Emissions,\ RP}]
Equation 1
Where
During biological treatment, organic carbon is oxidized to form CO2, which may then be released to the atmosphere. Outgassing of CO2 largely occurs during secondary treatment, where mixing, aeration, carbonic acid generation and contact with the atmosphere are enhanced.
Alkalinity addition, such as through addition of carbonates or hydroxides, convert CO2 to carbonate and bicarbonate ions, and prevent the release of CO2 to the atmosphere. As a reminder, Wastewater Alkalinity Enhancement projects prevent the release of biogenic CO2 to the atmosphere, which is not counted in standard GHG accounting frameworks5. Thus, Wastewater Alkalinity Enhancement qualifies as carbon dioxide removal, rather than emissions avoidance.
For example, the dissolution of calcium carbonate (CaCO3) as an alkaline feedstock follows the chemical reaction:
[math: CaCO_CaCO*{3 (s)} + CO_CO*{2 (aq)} + H_2O_H*2O*{(l)} \rightarrow Ca^{2+}_{(aq)} + 2HCO_{3 (aq)}^{-}]
Equation 2
In this example, CO2 conversion has a 1:1 molar ratio to CaCO3 consumption and CO2 consumption. Thus, CO2 conversion to bicarbonate (HCO3-) can be measured by direct measurement of solid feedstock, (ie. CaCO3(s)), or dissolved weathering products (ie. Ca2+(aq)).
The total CO2 that is converted to bicarbonate ions in the wastewater stream is determined using direct measurements within the WWTP and subtracting any potential losses upon effluent discharge.
The total amount of carbon stored can be determined as follows:
[math: CO_2e_{Stored,\ RP} = m_{Dissolved,\ RP} *\frac{M_{CO_2}}{M_{FS}}* ((1-P)*X_{carbonic}* \eta_{Losses,\ RP} - P*X_{non-carbonic})]
Equation 3
Where
The mass of feedstock dissolved within a Reporting Period, [math: m_{Dissolved,\ RP}], can be determined by:
[math: m_{Dissolved,\ RP} = \sum_{i}^{T} ( \dot m_{Dissolved,\ i} * \Delta t_i) ]
Equation 4
Where
Note that this quantification approach assumes the use of a pure mineral feedstock. In cases where a feedstock of mixed mineralogy is used, Project Proponents must make conservative assumptions in quantifying CDR. For example, a Project Proponent may choose to allocate all carbon removal to the mineral phase with lowest conversion potential (e.g., if a feedstock contains MgO and MgCO3, these phases store 2 and 1 moles of CO2 per mole of feedstock, respectively), or a Project Proponent may choose to assume that the phase with the lowest conversion potential dissolves first (e.g., if a feedstock is 90% MgO and 10% MgCO3 by mass, the first 10% of dissolved weathering products by mass are assumed to come from MgCO3). All assumptions and calculations must be documented in the PDD.
[math: \dot m_{Dissolved}] is determined using a mass balance on a control volume (see Figure 2 for an example). Project Proponents must define the boundaries of their control volume in the PDD. For example, for an activated sludge system, the control volume should encompass the total volume of the secondary treatment process, including the bioreactor and secondary clarifier (Figure 2). All inputs and outputs of the control volume must be clearly defined and identified in the PDD. Modifications to mass balance equations may be required based on the control volume boundaries, and must be described in the PDD. All calculations must be submitted for verification.
[Image: **Figure 2** Control Volume]
Figure 2: Example process diagram, control volume and monitoring locations for Wastewater Alkalinity Enhancement. The control volume, [math: V], shown in pink, may encompass any volume inclusive of the biological treatment and solids separation. The control volume and exact input and output locations may differ depending on the WWTP.
[math: \dot m_{Dissolved}] can be calculated based on the following options:
The choice of these metrics used for quantification will depend on site-specific WWTP design. The equations used for both options are detailed below. Project Proponents must specify which measurement phase will be used for primary quantification in the PDD.
Note that Figure 2 represents one possible WWTP configuration, and as such the equations given below represent the minimum terms that must be considered for quantification. Additional considerations may be required based on WWTP design. All site-specific considerations must be documented in the PDD. Examples of site-specific variations include (but are not limited to):
The mass flow rate of solid feedstock is defined as:
[math: \dot m_{FS} = Q * FS * 10^{-9}]
Equation 5
Where
Alternative formulations of flow rate can be used where applicable, this must be described in the PDD.
The mass balance of the solid feedstock within the control volume, [math: V], (Figure 2) can be defined as:
[math: \frac{dFS}{dt} = \dot m_m*{FS,\ Dosing} - \dot m_m*{FS,\ Effluent} - \dot m_m*{FS,\ WAS} - \dot m_m*{Dissolved}]
Equation 6
Where:
By assuming steady state, [math: \frac{dFS}{dt} = 0], no feedstock present in the influent, and the control volume shown in Figure 2, [math: \dot m_{Dissolved}] can be calculated as:
[math: \dot m_{Dissolved} = \dot m_{FS,\ Dosing} - \dot m_{FS,\ Effluent} - \dot m_{FS,\ WAS}]
Equation 7
Where:
Requirements for measurements of concentration of the feedstock in effluent and waste activated sludge will vary depending on the feedstock used:
A portion of the feedstock may be dissolved by non-carbonic acids, in which case there is no conversion of CO2 to stable bicarbonate within the WWTP and CDR cannot be credited for any non-carbonic acid weathering that occurs. Furthermore, for carbonate feedstocks, dissolution by non-carbonic acids leads to emission of CO2. The proportion of feedstock dissolved by non-carbonic acid, [math: P], must be determined:
[math: P = \frac{\dot m_{NCA}}{\dot m_{Dissolved}}]
Equation 10
Where
The recommended approach to determine [math: \dot m_{NCA}] is measuring anion concentrations in the aqueous phase:
[math: \dot m_{NCA} = \frac{\sum_{i} (Q_i*[A^x]_i)}{1 * 10^6}]
Equation 11
Where:
Note that, where anion concentrations are not available, alternative estimations of [math: \dot m_{NCA}] or [math: P] may be used. All data sources and calculations must be provided in the PDD and approved by Isometric.
The molar flow rate of a dissolved weathering product, [math: C^x], is defined as:
[math: n_{C^x}= Q * [C^x]]
Equation 13
Where
The mass balance of the dissolved weathering product, [math: C^x], within the control volume V (Figure 2) can be defined as:
[math: \frac{d[C^x]}{dt} = (1/V)* [(n_{C^x,\ Influent} - n_{C^x,\ WAS} -n_{C^x,\ Effluent}) + (\dot m_{Dissolved}/M_{FS})]]
Equation 14
Where:
Assuming steady state, [math: \frac{d[C^x]}{dt} = 0], the rate of feedstock dissolved can be calculated in the aqueous phase using the following equation:
[math: \dot m_{Dissolved} = ( M_{FS})*(n_{C^x,\ Effluent} + n_{C^x,\ WAS} -n_{C^x,\ Influent})]
Equation 15
Where
The proportion of feedstock dissolved by non-carbonic acid, [math: P], must be determined.
[math: P = \frac{n_{C^x,\ NCA}*M_{FS}}{\dot m_{Dissolved}}]
Equation 19
Where
The recommended approach to determined [math: n_{C^x,\ NCA}] is measuring anion concentrations in the aqueous phase.
[math: n_{C^x,\ NCA} = \sum_{i} ( Q_i*[C^x]_i)]Equation 20
Where:
Note that, where anion concentrations are not available, alternative estimations of [math: n_{C^x,\ NCA}] or [math: P] may be used. All data sources and calculations must be provided in the PDD and approved by Isometric.
Project Proponents must conduct the following validation checks each Reporting Period. Discrepancies beyond the analytical precision of the methods used must be reported to the VVB and Isometric, and addressed with a system audit.
This framework assumes that [math: \frac{dFS}{dt} = 0], meaning that no feedstock accumulates in the control volume; this is the functional definition of steady state for the purpose of this Protocol. Verifying this assumption is important for determining [math: CO_2e_{Stored}], as accumulated feedstock is not accounted for in Equation 7 and may thus lead to overcrediting. Project Proponents are required to confirm this assumption through measurements of any feedstock accumulation in the system, either through sludge sampling devices or during routine cleaning and maintenance of the bioreactors. The frequency of these measurements will depend on site-specific operations and must be justified in the PDD. In instances where feedstock is accumulating, the amount of accumulated feedstock must be conservatively estimated and appropriately deducted from [math: CO_2e_{Stored}].
There are two options available for quantifying [math: m_{Dissolved}] which are described in Section 8.2.1. Both Option 1 (solid phase measurements) and Option 2 (aqueous phase measurements) are expected to result in the same estimate for [math: m_{Dissolved}]. Project Proponents must check that the results of both options for calculating [math: m_{Dissolved}] agree within propagated error of the relevant measurements once per Reporting Period. If this is not the case, an audit must be performed to determine the cause of the discrepancy.
Several studies have documented the sorption of metals and organic compounds to waste activated sludge 11,12,13. This may be a possible cause of a discrepancy in mass balance calculations. If mass balance calculations cannot be resolved, Project Proponents should investigate sorption onto WAS by extracting sorbed weathering products from dried solid samples that have been washed with deionized water to remove any non-sorbed weathering products. Project Proponents are required to detail the extraction procedure in the PDD.
It is anticipated that new technologies and methods for quantifying and validating wastewater alkalinity enhancement will emerge in the coming years. These new technologies will be regularly assessed by the Isometric science team for inclusion in this Protocol. New technologies and methods that have some demonstrated efficacy when compared to conventional solid-phase and aqueous-phase mass balance approaches may be used as a validation approach. New technologies and methods that have demonstrated efficacy and have gathered wider scientific consensus may be used for either quantification or validation.
In the absence of WAE project activities, wastewater treatment plants have direct CO2 emissions from evasion of biogenic CO214,15. Secondary biological treatment contributes the majority of direct CO2 emissions of wastewater treatment due to the generation of carbonic acid and the aeration process enhancing equilibration with the atmosphere.
Wastewater Alkalinity Enhancement reduces or eliminates direct outgassing of biogenic CO2 emissions from the secondary treatment process. This Protocol only credits the avoided biogenic CO2 emissions that occur within the confines of the WWTP, and not any reduced outgassing that might occur upon discharge into receiving waters. As a validation check, Project Proponents are required to calculate the amount of biogenic CO2 outgassing within the residence time of the control volume, summed across the full timescale of the Reporting Period. This sets an the upper limit for [math: CO_2e_{Stored}].
There are several methods that can be used to provide an estimate of total biogenic CO2 emissions:
Modeling of outgassing in secondary treatment based on known fluid parameters. Model construction will be dependent on the project site and should follow best practices for the relevant treatment system 16. At a minimum, models should consider the following parameters within the biological treatment tanks:
Alternatively, an upper limit may be established from the total biogenic CO2 production, which may be calculated from measurements of biological oxygen demand (BOD) in the influent stream. This provides a more crude estimate as it assumes that all of the biogenic CO2 produced is outgassed within the WWTP.
There may be some instances where addition of alkalinity may result in increased efficiency of organic matter degradation within the control volume. This would increase the CO2 available to react with feedstock relative to baseline conditions. This may increase the upper limit of [math: CO_2e _{Stored}], if sufficient evidence is provided (e.g. through modeling changes in the biological treatment process from feedstock dosing, based on expected dissolution rates and system conditions).
The effluent exiting the WWTP is enriched in TA and DIC compared to the receiving waters. After discharge into the receiving waters, losses may reduce the efficiency of CO2 stored as a result of Wastewater Alkalinity Enhancement.
Processes that can lead to losses include:
Losses may occur in a number of locations, depending on the Project. In general, losses must be considered in the near field of the discharge site, as that is where the risk of losses are highest. For river discharges, losses may also occur along the transport pathway from the effluent discharge to the final storage reservoir, the ocean. As such, losses along river transport and upon entering the ocean must also be considered. Any losses that cannot be justified as negligible must be quantified and deducted from gross carbon removal.
[Image: **Figure 3** Losses]
Figure 3. Example schematic of where downstream losses may occur, and the spatial zones which must be considered for coastal and river discharges.
The overall loss discount for a Wastewater Alkalinity Enhancement project is the product of the loss factors associated with each loss process:
[math: \eta_{Losses,\ RP} = \prod_{i=1}^{N} \eta_i]
Equation 22
Where:
The loss processes which must be considered for ocean and river discharge are indicated in Table 4.
Table 4. Losses and their corresponding spatial zones. All losses in column 1 "Mixing zone of discharge site" are to be considered for both coastal and riverine discharge, unless otherwise indicated. Transport losses in columns 2 and 3 apply only to riverine discharge. The risk of these losses must be described in the PDD, and justified if they are negligible or need to be quantified.
| Loss process | 1) Mixing zone of discharge site | Transport (riverine discharge only) | |
|---|---|---|---|
| 2) Along river to estuary | 3) At ocean | ||
| Re-equilibration of DIC | x | x | x |
| Carbonate precipitation | x | x | |
| Natural alkalinity reduction | x | ||
| Biotic calcification | x (coastal discharge only) |
In the PDD, Project Proponents must describe the risk of these losses, as well as either (1) a justification of why the losses are negligible or (2) a strategy for quantifying a corresponding loss factor. Due to the difficulty and uncertainty in quantifying the impact of these processes at this time, acceptable treatment of losses in this Protocol include:
Data, measurements and evidence used in the quantification of loss factors must be publicly disclosed. Example recommendations for each loss process are discussed below. Much of the existing research in these losses have been motivated by research in Enhanced Weathering, River Liming, and Ocean Alkalinity Enhancement, which may not exactly simulate the carbonate chemistry state of the effluent from the Wastewater Alkalinity Enhancement process described in this Protocol. Project Proponents are recommended to conduct research on these potential losses in the relevant carbonate chemistry parameter space for their specific process.
The near-field zone comprises the spatial extent of initial mixing upon discharge, sometimes known as the mixing zone. Generally, the risk of losses are modulated by saturation states and environmental conditions including TSS. Since the effluent is controlled and solids are separated within the treatment plant, the influence of high effluent saturation states and TSS is expected to be limited. Additionally, existing thresholds for wastewater treatment plant permits provide an additional guardrail against these potential downstream losses. The risk of these losses declines further from the discharge site, since the perturbation from the wastewater effluent will become increasingly dilute.
Re-equilibration of DIC
Concept
Equilibrium speciation of DIC is primarily dependent on pH, and to a lesser extent temperature, salinity and pressure:
[math: CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^- \leftrightarrow 2H^+ + CO_3^{2-} ]
Equation 23
The release of CO2 due to re-equilibration of DIC may occur due to mixing of fluids. This may occur upon initial mixing of effluent with the receiving water body, or at river confluences and when rivers discharge to the ocean.
Quantification
The recommended quantification approach for outgassing upon initial mixing is to use a two endmember mixing model to predict the DIC and TA of the mixture of effluent and receiving waters. The difference between the observed DIC and predicted DIC of the mixture can be used to estimate the loss by CO2 outgassing. This can be done using CO2SYS or PHREEQC. Alternate approaches, such as using a physical-biogeochemical model for the discharge site, can also be used.
Carbonate Precipitation
Concept
Secondary precipitation of calcium carbonate could cause CO₂ outgassing by the following reaction:
[math: Ca^{2+} + 2HCO_3^{-} \rightarrow CaCO_3 + CO_2 + H_2O]
Equation 24
Calcium carbonate precipitation may result in a reduction in carbon loss by 50% for non-carbonate feedstocks or 100% for carbonate feedstocks.
Discussion
In rivers and coastal areas, higher suspended particulates may increase nucleation. Early research suggests there is a relationship between increased alkalinity loss with higher TSS in the receiving water body17. Thus, the risk of secondary precipitation is most pronounced in the mixing zone of the outfall, where the carbonate chemistry and TSS perturbation are largest.
Limiting pH and the saturation state has been shown to be effective at avoiding this result, and laboratory research to characterize the critical thresholds that trigger precipitation under close-to-natural conditions are ongoing 18, 19, 20, 21, 22. Furthermore, precipitation dynamics occur on a timescale between minutes to hours18, 20, which suggests that dilution could be an effective risk mitigation strategy23.
Quantification
An example avoidance strategy is setting a threshold on TA and saturation state, with consideration of environmental conditions including carbonate chemistry variables, TSS and dilution at the site22. Continuous monitoring of carbonate chemistry variables and TSS is recommended to ensure that conditions for secondary precipitation are avoided. In some cases, secondary precipitation can be identified by an observed increase in turbidity. Monitoring of turbidity is recommended, however it may be difficult to isolate a signal from secondary precipitation over natural fluctuations.
Natural Alkalinity Flux Reduction
Concept
Increased alkalinity in wastewater effluent can potentially reduce the natural alkalinity flux from marine or river sediments24. This risk may be exacerbated by projects with settling particles that result in local alkalinity enrichment in marine or river sediments, and the potential impacts on the net removal calculation is uncertain at this time. More research in this area is needed and the Protocol will be updated with future advancements.
Quantification
A recommended avoidance strategy for Wastewater Alkalinity Enhancement projects is to limit accumulation of alkalinity on the sea bed through careful design of discharge rates, TSS thresholds, and solids separation prior to discharge, which will likely already be done as part of the WWTP. If this risk cannot be avoided, then additional monitoring or modeling is needed to assess the likely impact. Potential approaches may include numerical modeling of particle transport in receiving waters, sediment sampling, measuring benthic alkalinity fluxes.
Changes in Biotic Calcification
Concept
For coastal discharges, increases in marine biotic calcification can cause CO₂ outgassing. The carbonate chemistry conditions promoted by coastal discharge of alkalinity-enhanced wastewater could promote calcification due to the lowered H+ or elevated saturation state25,26,27, 28. Alkaline feedstock dissolution may release trace metals which has the potential to fertilize blooms of calcifiers28.
Discussion
Early stage research manipulating Total Alkalinity with the aim of simulating OAE has found no significant increase in biologically produced calcium carbonate at elevated alkalinity in the ocean29,30. However, the Black Sea, a naturally elevated alkaline environment, harbors extensive blooms of the coccolithophores 31,32, a major group of calcifying plankton. This is thought to be due to the favorable carbonate chemistry promoted by the elevated alkalinity regime28.
This is still an area where more research is needed, particularly through mesocosm and field trials, albeit there is a rich body of literature on lab and mesocosm scale species-specific responses to changing seawater carbonate chemistry 33. The risk of outgassing due to biotic calcification may be project and location specific. Recently published meta-analyses synthesizing data from ocean acidification studies for OAE supports this claim that species and functional group specificity is likely 34. Coastal areas with significant benthic calcification of CaCO3 sediments may be especially susceptible to this feedback.
Quantification
A recommended avoidance strategy is setting thresholds on pH and TA based on what has been shown in previous studies to have no significant increase in biologically produced CaCO335, or as determined for the specific deployment site), and monitoring for changes in ocean biota.
For riverine discharge, the WWTP effluent generated alkalinity is transported through rivers and eventually, to the oceans. The transport of carbonate and bicarbonate ions along the river may result in carbon losses through the re-equilibration of DIC or calcium carbonate precipitation. Risks of these losses are more pronounced at major confluences along the river network. Project Proponents must quantify a loss discount for both re-equilibration and precipitation along rivers and upon entering the ocean.
Quantification
To account for potential losses along river transport, Project Proponents must estimate potential losses along the river network through measurement or models. Recent publications have outlined modeling approaches that combine baseline river geochemical data, equilibrium modeling of water chemistry and scenarios of terrestrially exported DIC, which may serve as useful references 36,37.
The minimum requirements for river transport loss models are:
Project Proponents are required to submit a detailed description of their modeling approach, including the model used, the river/watershed data used in model construction and the source of that data. Alternate approaches may be considered on a case by case basis.
*Note on calcite saturation index in rivers
Calcite saturation index (SIC) is a useful parameter to determine the likelihood of carbonate precipitation. Calcite saturation index (SIC) is calculated as:
[math: SI_c = \log_{10} \frac{\alpha Ca^{2+} * \alpha CO_3^{2-}}{K_{sp(calcite)}}]
Equation 25
With
[math: K_{sp(calcite)} = \alpha_{sat} Ca^{2+} * \alpha_{sat} CO_3^{2-}]
Equation 26
Where:
When SIC > 0, a river is considered supersaturated. Although supersaturation with respect to calcium carbonate does not necessarily result in calcium carbonate precipitation, typically, calcium carbonate precipitation is predicted at a SIC > 1 37.
Discussion
Once alkalinity reaches the ocean, changes in pH, temperature or salinity can shift the carbonate system and result in a re-equilibration of DIC. Estimates from peer-reviewed studies suggest that marine losses of terrestrially exported DIC could amount to 10-30% loss of carbon, depending on temperature, salinity, pCO₂, ocean circulation38,39,40. Typically, the ocean has a higher pH than rivers which shifts the carbonate equilibrium towards a higher proportion of CO32- in oceans. As a result, the amount of DIC stored per unit of alkalinity decreases between inland waters and the ocean. In addition, changes in salinity and saturation state upon reaching the ocean can lead to calcium carbonate precipitation41.
Quantification
Outgassing upon entering the ocean must be quantified using regional data specific to the area where the river reaches the ocean. This loss can be estimated with one of the following approaches:
Option 1 assumes thermodynamic equilibrium with the atmosphere and provides an upper limit on the expected ocean losses. Option 2 considers the 3D ocean circulation and does not assume equilibrium with the atmosphere, which may lead to fewer losses in certain regions if the exported DIC is subducted out of atmospheric contact for long periods of time.
Option 1
The Renforth and Henderson (2017) uptake efficiency equation is:
[math: \frac{\Delta C_T}{\Delta A_T} = \eta = (S \cdot 10^{-3.009} + 10^{-1.519}) \text{ln}(pCO_2)-(S \cdot 10^{-2.100}) - \\ (T \cdot pCO_2)(S \cdot 10^{-7.501} - 10^{-5.598}) - (T \cdot 10^{-2.337}) + 10^{-0.102}]
Equation 27
Where
Project Proponents should use the oceanographic conditions from publicly available, locationally-specific time-series data, such as the NOAA climate indices list, OceanSODA-ETHZ, or equivalent, to the uptake efficiency (Equation 28). The efficiency can be used in Equation 23 to determine the size of the loss term. Equilibrium conditions can be assumed.
Option 2
An alternative approach to quantifying ocean losses is to use a 3D Earth Systems Model or ocean physical-biogeochemical model to explicitly simulate ocean transport and air-sea CO2 fluxes. For example, Kanzaki et al., 2023 used an Earth system model to estimate the ocean leakage of CO2 from terrestrial enhanced weathering projects40. Similar model-based approaches may be used, however the calculation of ocean losses must be regionally specific to the project-site. A globally averaged loss factor may not be used at this time since it may not be conservative, given the large variability that exists in different regions of the ocean.
Projects Proponents must submit a detailed description of their modeling approach in the PDD, including the following:
Furthermore, the model must be well-validated and skillful for the purpose that it is used for. Proof of model validation can be achieved through either:
Discharge of Undissolved Alkalinity
Any undissolved feedstock which is released in the effluent as TSS may dissolve in the open environment depending on local saturation states and enhance alkalinity in the receiving water body. This will result in increased pH, total alkalinity (TA), and potentially facilitate additional carbon uptake via air-sea gas exchange if the alkaline-enriched waters remain in contact in the atmosphere.
Additional solids separation prior to discharge may be required to prevent release of undissolved feedstock to the open environment. Projects which aim to discharge undissolved feedstock into the ocean and quantify open ocean CDR must refer to the Ocean Alkalinity Enhancement from Coastal Outfalls Protocol. Projects that wish to seek Credits under both the WAE and OAE Protocols must consult with Isometric ahead of time to establish clear boundaries between the Projects and ensure no potential double-counting.
Biological Fertilization
The release of elements (such as iron, silica, nitrogen and phosphorus) and DIC-enriched waters could lead to increases in primary production or changes in phytoplankton community structure, which may have a broader impact on biological carbon export.
There is significant nutrient export from wastewater treatment plants under BAU operations, and the additional fertilization from alkaline feedstocks are relatively small in comparison to the existing nutrient export from WWTPs. Furthermore, it is challenging to attribute additional primary production above the baseline scenario to the CDR intervention. For these reasons, the potential impacts of increased primary production on carbon removal are not accounted for in this Protocol. Project Proponents must select alkaline feedstocks which minimize the risk of fertilization and ensure effluent concentrations of key nutrients are not increased beyond the standard range for the wastewater treatment plant.
Type: Counterfactual
[math: CO_2e_{Counterfactual,\ RP}] represents the amount of CO2 removed from the conversion of biogenic CO2 in wastewater to bicarbonate in the absence of the WAE project, under the WWTP's BAU operations. This term represents the counterfactual scenario of the [math: CO_2e_{Stored,\ RP}] term, and does not include GHG emissions in the counterfactual scenario. The quantification of GHG emissions relative to BAU operations is described in Section 8.4.
The calculation of [math: CO_2e_{Counterfactual,\ RP}] depends on the BAU scenario for the WWTP (Figure 4).
For projects operating in WWTPs that do not add any alkalinity for their process control, the counterfactual in [math: CO_2e_{Counterfactual,\ RP}] is considered to be 0, since [math: CO_2e_{Stored,\ RP}] is calculated relative to a BAU where no alkaline feedstock is added to the WWTP to react with the biogenic CO2 generated during wastewater treatment. This assumes there is no limestone or feedstock present in the WWTP after primary treatment.
For projects in which the partner WWTP does add alkalinity, [math: CO_2e_{Counterfactual,\ RP}] is calculated as follows:
See Appendix 3 for further explaination on this approach.
[Image: **Figure 4** Counterfactual]
Figure 4 Decision tree for determining CO2eCounterfactual, RP for a WAE project.
Type: Emissions
[math: CO_2e_{Emissions,\ RP}] is the total GHG emissions associated with a Reporting Period, [math: {RP}]. This can be calculated as:
[math: CO_2e_CO*2e*{Emissions,\ RP} = CO_2e_CO*2e*{Establishment,\ RP} + CO_2e_CO*2e*{Operations,\ RP} + CO_2e_CO*2e*{End-of-Life,\ RP} + CO_2e_CO*2e*{Leakage,\ RP}]
Equation 28
Where
The following sections set out specific quantification requirements for each variable. Projects applicable under this Protocol are integrated with existing WWTP, so only the additional activities beyond the WWTP BAU operations are included in the calculation of [math: CO_2e_{Emissions,\ RP}]. This is set out in Table 1 and Figure 1. Emissions relative to BAU must be positive, i.e. avoided emissions relative to BAU are not eligible for crediting.
GHG emissions associated with project establishment should include all historic emissions incurred as a result of project establishment, including but not limited to the SSRs set out in Table 1.
Project establishment emissions occur from the point of project inception up until the first Reporting Period. Establishment emissions may be accounted for in the following ways, with the allocation method selected and justified by the Project Proponent:
The anticipated lifetime of the Project should be based on reasonable justification and should be included in the Project Design Document (PDD) to be assessed as part of project validation.
Allocation of project establishment emissions to removals must be reviewed at each Crediting Period renewal and any adjustments made. If the Project Proponent is not able to comply with the allocation schedule described in the PDD, e.g. due to changes in delivered volume or anticipated project lifetime, the Project Proponent should notify Isometric as early as possible in order to adjust the allocation schedule for future removals. If that is not possible, the Reversal (The escape of CO₂ to the atmosphere after it has been stored, and after a Credit has been Issued. A Reversal is classified as avoidable if a Project Proponent has influence or control over it and it likely could have been averted through application of reasonable risk mitigation measures. Any other Reversals will be classified as unavoidable.) process will be triggered in accordance with the Isometric Standard, to account for any remaining emissions.
GHG emissions associated with [math: CO_2e_{Operations,\ RP}] should include all emissions associated with operational activities, including but not limited to the SSRs set out in Table 1. In particular for WAE projects, the state of Waste activated sludge must be described in the PDD. If the WAS is incinerated, the CO2 emissions from incineration must be included in the calculation of [math: CO_2e_{Operations,\ RP}].
[math: CO_2e_{Operations,\ RP}] emissions must be attributed to the Reporting Period in which they occur. Allocation may be permitted in certain instances, on a case by case basis in agreement with Isometric.
[math: CO_2e_{End-of-Life,\ RP}] includes all emissions associated with activities that are anticipated to occur after the Reporting Period, but are directly or indirectly related to the Reporting Period. For example, this could include ongoing sampling activities for MRV for the specific deployment (directly related), or end-of-life emissions for the project facility (indirectly related to all deployments).
GHG emissions associated with [math: CO_2e_{End-of-Life,\ RP}] may occur from the end of the Reporting Period onwards, and typically through to completion of project site deconstruction and any other end-of-life activities.
GHG emissions associated with activities that are directly related to each deployment must be quantified as part of that Reporting Period. GHG emissions associated with activities that are indirectly related to all deployments may be allocated in the same ways as set out in [math: CO_2e_{Establishment,\ RP}].
Given the uncertain nature of [math: CO_2e_{End-of-Life,\ RP}] emissions, assumptions must be revisited at each Crediting Period and any necessary adjustments made. Furthermore, if there are unexpected [math: CO_2e_{End-of-Life,\ RP}] emissions associated with a Reporting Period, or the Project as a whole, that occur after the Project has ended, then the Reversal process will be triggered to compensate for any emissions not accounted for.
[math: CO_2e_{Leakage,\ RP}] includes emissions associated with a project's impact on activities that fall outside of the system boundary of a project. It includes increases in GHG emissions as a result of the Project displacing emissions or causing a knock on effect that increases emissions elsewhere. As an example, creating a market for feedstocks may generate new revenue in the source sector that alters producer behavior in ways that result in additional GHG emissions.
It is the Project Proponent's responsibility to identify potential sources of leakage emissions, however, for an Wastewater Alkalinity Enhancement project the following must be considered as a minimum:
[math: CO_2e_{Leakage,\ RP}] emissions must be attributed to the Reporting Period in which they occur. Allocation may be permitted in certain instances, on a case by case basis in agreement with Isometric.
This section sets out specific requirements relating to quantification of energy use as part of the GHG assessment. Emissions associated with energy usage result from the consumption of electricity or fuel.
Examples of electricity usage may include, but are not limited to the operation of process equipment (i.e., pumps, mixers, blowers, flow control, measurement instruments). WAE may increase or decrease energy consumption for secondary treatment. For example, reduced oxygen transfer efficiency requiring increased blower energy or increased mixing energy required to keep feedstock in suspension. If this is the case for a WWTP under this Protocol, a means to monitor energy consumption before/after implementation should be outlined in the PDD. In cases where it is not possible to measure this directly, energy could be estimated based on rotating equipment duty points (e.g.; flow, pressure) and a pump curve.44
Examples of fuel consumption may include, but are not limited to handling equipment, such as fork trucks or loaders.
The Energy Use Accounting Module v1.2 provides requirements on how energy-related emissions must be calculated in a CDR project so that they can be subtracted in the net CO₂e removal calculation. It sets out the calculation approach to be followed for intensive facilities and non-intensive facilities and acceptable emissions factors.
In alignment with the Energy Use Accounting Module v1.2, the consequential impact of electricity usage on the system it is procured from must be included in calculations for intensive projects, as determined in the Energy Use Accounting Module v1.2.
Refer to Energy Use Accounting Module for the calculation requirements.
This section sets out specific requirements relating to quantification of transportation emissions as part of the GHG Statement.
Emissions associated with transportation include transportation of products and equipment as part of a Reporting Period’s activities. Examples may include, but are not limited to:
The Transportation Emissions Accounting Module v1.1 provides requirements on how transportation-related emissions must be calculated in a CDR project so that they can be subtracted in the net CO₂e removal calculation. It sets out the calculation approach to be followed and acceptable emissions factors.
Refer to Transportation Emissions Accounting Module for the calculation requirements.
This section sets out specific requirements relating to quantification of embodied emissions as part of the GHG Statement. Embodied emissions are those related to energy use or other emissions during the manufacture of equipment and materials used in a process.
Examples of project-specific materials and equipment that must be considered as part of the embodied emission calculation include but are not limited to:
The Embodied Emissions Accounting Module v1.0 sets out the calculation approach to be followed including allocation of embodied emissions, life cycle stages to be considered, data sources and emission factors.
Refer to Embodied Emissions Accounting Module for the calculation guidelines.
The primary storage reservoir of the CO2 removed through Wastewater Alkalinity Enhancement is Dissolved Inorganic Carbon (DIC) in the ocean. The durability (The amount of time carbon removed from the atmosphere by an intervention – for example, a CDR project – is expected to reside in a given Reservoir, taking into account both physical risks and socioeconomic constructs (such as contracts) to protect the Reservoir in question.) and reversal risks of this storage reservoir are discussed in the Ocean DIC Storage Module.
Ocean DIC Storage Module
Future versions of this Protocol will include expanded storage options, such as storage of DIC in lakes.
As outlined in Section 2.5.9 of the Isometric Standard, the Buffer Pool (A common and recognized insurance mechanism among Registries allowing Credits to be set aside (in this case by Isometric) to compensate for Reversals which may occur in the future.) is a mechanism used to insure against Reversal risks that may be observable and attributable to a particular project through monitoring.
Based on present understanding, reversals in the global ocean Dissolved Inorganic Carbon (DIC) reservoir will not be directly observable with measurements and attributable to a particular project. Reversals of DIC in the ocean is a system-wide storage uncertainty that should be addressed through further scientific research. Projects applicable to this Protocol are generally categorized as having a Very Low Risk Level of Reversal according to the Isometric Standard Risk Assessment Questionnaire. TheRisk Bufferof Poolreversal corresponding to this lowest risk scoreinformation is 2%given andin isAppendix intended4: asRisk anof additionalReversal precautionQuestionnaire, againstwith unknownsfurther information provided within the relevant storage module storage module.
For more details on Reversals, refer to Sections 2.5.9 and 5.6 of the Isometric Standard.
All pre-deployment requirements must be described in the PDD, as outlined in Section 5.1. The requirements are as follows:
This Protocol requires and recommends monitoring to occur at the following locations, which are illustrated in Figure 5. The monitoring locations are defined relative to a control volume, which in most cases encompasses the secondary treatment portion of the WWTP and includes the bioreactor and settling tank (see Section 8.2.1 for more details).
Note that exact monitoring locations may differ for various WWTPs depending on plant-specific processes. For example, plants that do Garrett wasting (i.e. remove WAS from the aeration tank) may have a separate WAS stream to monitor. Plant-specific monitoring locations and a description of the control volume must be included in the monitoring plan submitted in the PDD.
[Image: **Figure 5** Monitoring locations]
Figure 5 Monitoring locations relative to a control volume representing the secondary treatment step in a WWTP. Monitoring locations referred to throughout this Protocol are shown in green. Note that the control volume provided is an example, and the exact control volume may be drawn differently for other WWTPs. Exact locations of inputs and outflows from the control volume may look different for projects.
The following sections summarize the measurements that must be reported in each of the monitoring locations. The purpose of monitoring is for quantification of the net CDR. Some of these measurements may already be collected as part of the WWTP BAU operations, and some may be collected by the Project Proponent. The breakdown of data collection between the Project Proponent and partner WWTP should be specified in the PDD.
Required measurements in the influent stream entering the control volume include: the flow rate ([math: Q_{Influent}], L/day) and the concentration of the dissolved weathering product ([math: [C^x]_{Influent}]; mol/L), both of which are used for the aqueous phase mass balance for quantifying the amount of feedstock dissolved in Equation 15. This is either used as the primary approach for quantifying [math: \dot m_{Dissolved}], or must be used as a validation check (see Section 8.2). In addition, measurements of anions are recommended for determining the amount of non-carbonic acid weathering (see Section 8.2.1.2, Equations 10-12 and/or 19-21). If anion measurements are not available, an alternative approach for determining non-carbonic acid weathering is required.
The addition of feedstock into the control volume must be quantified in terms of the feedstock mass flow rate ([math: \dot m_{FS,\ Dosing}]; tonnes/day) for the solid phase mass balance (See Equation 7). This is used either as the primary approach for quantifying [math: \dot m_{Dissolved}], or is required as a validation check (see Section 8.2). For projects where slurry dosing is used, the flow rate [math: Q_{dosing}] (L/day) must also be reported here.
Measurements of the flow rate ([math: Q_{Effluent}]; L/day), dissolved weathering product concentration ( [math: [C^x]_{Effluent}] ; mol/L), and concentration of suspended feedstock ([math: FS_{Effluent}]; tonnes/day) are required in the effluent stream. Depending on the type of feedstock (e.g. carbonate vs non-carbonates), different approaches are used for determining the feedstock concentration in the effluent, such as TIC for carbonate feedstocks (see Section 8.2 for more details). These measurements are used in both the aqueous and solid phase mass balance calculations (see Equations 7 and 15). In addition, measurements of anions are recommended for determining the amount of non-carbonic acid weathering (see Section 8.2.1.2, Equations 10-12 and/or 19-21). If anion measurements are not available, an alternative approach for determining non-carbonic acid weathering is required.
Depending on how the Control Volume is drawn, the effluent may refer to the wastewater stream exiting the secondary treatment and entering tertiary treatment, or it may refer to the final effluent exiting the WWTP into receiving waters. Additional measurements at the final effluent are needed for monitoring of environmental impacts and the calculation of losses upon mixing with receiving waters. These include measurements of at least two carbonate system parameters (pH, DIC, TA, pCO2) and TSS. See Section 11.3.2 for environmental monitoring, and Section 8.2.3.1 for losses upon mixing with receiving waters.
Measurements of the waste activated sludge are used in both the aqueous and solid phase mass balances (see Section 8.2). Required measurements of the WAS include the dissolved weathering product concentration ([math: [C^x]_{WAS}]; mol/L), flow rate ([math: Q_{WAS}]; L/day), and concentration of feedstock settled in the WAS ([math: FS_{WAS}]; mg/L). Depending on the type of feedstock (e.g. carbonate vs non-carbonates), different approaches are used for determining the feedstock concentration in the WAS, such as TIC for carbonate feedstocks (see Section 8.2 for more details). Measurements of anions are recommended for determining the amount of non-carbonic acid weathering (see Section 8.2.1.2, Equations 10-12 and 19-21). If anion measurements are not available, an alternative approach for determining non-carbonic acid weathering is required.
Monitoring for feedstock accumulation in the control volume is required to confirm the steady state assumption that no feedstock accumulates in the control volume. See Section 8.2.2.1 for more details.
Confirmation of steady state operations may be demonstrated with sludge sampling devices in the WAS removal, WAS recycling streams and effluent streams or periodically during routine cleaning and maintenance of bioreactors.
The aims of monitoring in receiving waters are to demonstrate permit compliance, monitor environmental conditions, conduct ongoing monitoring for quantification of downstream losses, and establish processes for adaptive management to ensure that Project activities are stopped if negative impacts are identified.
All measurements in receiving waters are recommended. It may be the case that no additional measurements beyond the WWTP BAU are needed if it can be demonstrated that the WAE project either (1) does not change effluent characteristics compared to BAU, or (2) effluent changes are improved or reduce environmental risks compared to BAU. For example, if BOD in the final effluent is decreased as a result of the WAE project compared to the BAU of the WWTP, then it may not be necessary to do additional monitoring of dissolved oxygen in receiving waters as an indicator of ecosystem health.
The receiving waters monitoring plan should span the general locations listed below. This Protocol does not prescribe exact monitoring locations, as this will be site specific. Project Proponents are responsible for determining appropriate monitoring locations. It is recommended that Project Proponents use models to plan and optimize sampling design. A diagram of the monitoring locations, including the location in the water column, must be included in the monitoring plan submitted in the PDD.
The area where initial dilution takes place is often called the mixing zone (or "a reasonable mixing zone" or "zone of initial dilution"), typically on the scale of ~100m, depending on the scale of discharge. A mixing zone is a regulatory concept, which describes a spatial area surrounding the discharge infrastructure where water quality criteria can be exceeded. Permits which allow for a mixing zone require water quality criteria to be met at the edge of a mixing zone. A mixing zone is a simplified representation of initial mixing of discharge with receiving waters, the true spatial extent of this mixing changes dynamically with environmental conditions.
Detection of a measurable signal may be difficult beyond the mixing zone, especially for small-scale deployments. Beyond the mixing zone, receiving waters are expected to be well-mixed, which can provide more representative measurements of the impact of the discharge. Thus, environmental monitoring should be focused on the edge of the mixing zone.
It is recommended that Project Proponents monitor temperature, salinity, at least two carbonate chemistry parameters (pH, TA, DIC, pCO₂), dissolved oxygen (DO), turbidity and total suspended solids (TSS) at the edge of the mixing zone. Temperature and salinity are necessary for determining physical conditions, and should be co-located with other measured quantities. Two carbonate chemistry parameters (pH, TA, DIC, pCO₂) can be used to characterize the carbonate system. It is a known limitation of using pH and pCO₂ measurements to constrain the full carbonate system that uncertainty can be high45. It is therefore recommended a third carbonate system measurement is collected and measured to constrain the full carbonate system (including the state of carbonate disequilibrium) and recalibrate autonomous sensors. DO, turbidity and TSS are water quality indicators. Action thresholds are placed on pH, TSS and DO (see Section 11.3.3). Monitoring Chl-a and dissolved inorganic nutrients are also recommended.
Tailored biological and ecological monitoring for specific projects and sites must be determined by conducting an environmental risk assessment and mitigation strategy (see Section 6).
Project Proponents are recommended to conduct periodic ecological surveys to monitor phytoplankton community abundance and composition, benthic community abundance and diversity and presence and potential interactions with species of special concern (IUCN listed, commercially exploited or keystone species). Functional diversity should be considered in addition to taxonomic diversity. Survey sites should include the immediate vicinity of the discharge pipe and specific benthic habitats within the affected area of the project activities to be identified and determined on a site-by-site basis.
Action thresholds are not imposed on ecological indicators due to the difficulty of establishing ecological baselines and attributing ecological changes, particularly compared to BAU operations of WWTP. Data collection remains imperative for establishing a foundation for future analysis and enabling the assessment of cumulative impacts of Wastewater Alkalinity Enhancement, including assessing co-benefits and restoration impacts. Ecological data must be made publicly available, and the location of where data is stored publicly must be included in the PDD.
Project Proponents must conduct an environmental risk assessment and develop a mitigation strategy (see Section 6.3). Thresholds on environmental monitoring parameters are used to determine safe limits for discharge, mitigate environmental risks and identify negative environmental impact.
Two types of thresholds are used:
Effective implementation of these measures must also be accompanied by a robust monitoring plan to detect negative impacts and stop projects when necessary.
Violations are determined based on the threshold type, and specifics of the site, permit and intended activities. For example, exceedance of safety thresholds may be determined by daily mean or max values and exceedance of action thresholds in the mixing zone may be determined by weekly and/or monthly mean or max values. All violations need to be reported, and gaps in ocean monitoring data must be justified.
Credits cannot be issued for time periods without sufficient measurements to demonstrate compliance with action and safety thresholds. The handling of data gaps must be reported in the PDD. Removal activity from discharges that occur during time periods of safety threshold violations will not be eligible for crediting. Action threshold violations must trigger adaptive management plans. Failure to adequately address environmental risks or remediate any harm will lead to a project being subject to Credit cessation.
Table 5 below summarizes the required and recommended monitoring parameters. Required measurements indicate the minimum common set of measurements needed for all projects under this Protocol. Additional required measurements may be necessary based on specifics of the Project, site and environmental risk mitigation plan.
Table 5. Summary of required and recommended measurements for monitoring. Required measurements indicate the minimum common set of measurements needed for all projects under this Protocol. Additional required measurements for ocean monitoring may be necessary based on specifics of the Project, site, and environmental risk mitigation plan. The exact monitoring plan must be described in the PDD.
| Location | Parameter | Requirement Scope | Reason for Measurement | Methods | Frequency |
|---|---|---|---|---|---|
| Influent (A) | Q, Flow rate (L/day) | Required for all projects | Required for determining volume of water processed, used in carbon removal calculation | Flow meter | At least once per residence time at the onset of project activities until steady state (defined as dFS/dt=0 or d[Cx]/dt=0) is reached. Ongoing throughout deployment. |
[Cx], Concentration of dissolved weathering products (mol/L) | Required for Option 2 | Required for quantifying amount of carbon removed | Discrete bottle samples (e.g., ICP-MS, ICP-OES) or using an ion-selective electrode). | ||
| Anions | Recommended for all projects. | Recommended to determine non-carbonic acid weathering. If anion measurements are not available, an alternative approach for determining non-carbonic acid weather is required (see Section 8.2.1.1 and 8.2.1.2) | Ion chromatography; measured ions must include, at a minimum, Cl-, NO3-, PO43- and SO42- | ||
| Feedstock dosing (B) | Q, flow rate (L/day) | Required for all projects where slurry dosing is used | Required for determining volume of water processed, used in carbon removal calculation | Flow meter | At least once per residence time at the onset of project activities until steady state (defined as dFS/dt=0 or d[Cx]/dt=0) is reached. Ongoing throughout deployment. |
mFS, Dosing, mass flow rate of feedstock dosing (tonnes/day) | Required for all projects | Required for determining the amount of carbon removed. | Weigh scales of alkaline feedstock, or calculated from mass concentration of feedstock and dosing flow rate | ||
| Effluent (C) | QEffluent, Flow rate (L/day) | Required for all projects | Required for determining volume of water processed, used in carbon removal calculation | Flow meter; where not directly measured, Project Proponents may assume flow rate is constant throughout the system | At least once per residence time at the onset of project activities until steady state (defined as dFS/dt=0 or d[Cx]/dt=0) is reached. Ongoing throughout deployment. |
FSEffluent, concentration of suspended feedstock in effluent (mg/L) | Required for Option 1 | Required to determine total amount of undissolved feedstock in effluent | May be derived from TSS and analytical measurement (e.g., TIC) of feedstock relative to other solid materials present. | ||
[Cx], concentration of dissolved weathering products (e.g. Ca2+, Mg2+) (mol/L) | Required for Option 2 | Required for quantifying amount of carbon removed | Discrete bottle samples (e.g., ICP-MS, ICP-OES) or using an ion-selective electrode). | ||
| Anions | Recommended for all projects | Recommended to determine non-carbonic acid weathering. If anion measurements are not available, an alternative approach for determining non-carbonic acid weather is required (see Section 8.2.1.1 and 8.2.1.2) | Ion chromatography; measured ions must include, at a minimum, Cl-, NO3-, PO43- and SO42- | ||
| Total Suspended Solids (TSS) | Required for all projects at final effluent prior to discharge | Required for monitoring water quality, assessing and mitigating risk of downstream losses, and may be used for determination of FSEffluent. | Discrete bottle samples and filtering directly onto pre-weighed filter and subsequent measurement of mass change through drying (for example, in an oven), or TSS analyzers from existing WWTP infrastructure. | ||
At least two carbonate system parameters: pH, DIC, TA, pCO₂ | Required for all projects at final effluent prior to discharge | Required for constraining carbonate system and determining outgassing upon mixing with receiving waters | Sensor (for pH or pCO₂) or discrete bottle samples | ||
| Waste Activated Sludge (WAS) (D) | QWAS, Flow rate (L/day) | Required for all projects | Required for determining volume of water processed, used in carbon removal calculation | Flow meter | At least once per residence time at the onset of project activities until steady state (defined as dFS/dt=0 or d[Cx]/dt=0) is reached. Ongoing throughout deployment |
FSWAS, concentration of settled feedstock in WAS (mg/L) | Required for Option 1 | Required for quantification of carbon removed | Methods may vary based on feedstock | ||
[Cx], concentration of dissolved weathering products, e.g. Ca2+, Mg2+ (mol/L) | Required for Option 2 | Required for quantifying amount of carbon removed | Discrete bottle samples (e.g., ICP-MS, ICP-OES) or using an ion-selective electrode). Samples must be acidified prior to preparation for measurement. | ||
| Total Inorganic Carbon (TIC) | May be used to determine FSWAS for projects utilizing carbonate feedstocks | Where carbon feedstocks are used, FSeffluent and FSWAS can be assumed to be equivalent to TIC in samples taken from these streams | Discrete bottle samples taken from streams. Acceptable analyses include TIC analyzer or LOI analyses. | ||
| Anions | Recommended for all projects | Recommended to determine non-carbonic acid weathering. If anion measurements are not available, an alternative approach for determining non-carbonic acid weather is required (see Section 8.2.1.1 and 8.2.1.2) | Ion chromatography; measured ions must include, at a minimum, Cl-, NO3-, PO43- and SO42- | ||
Within the control volume, i.e. biological treatment tanks | Feedstock accumulation | Required for all projects | Required for assessing steady state assumption of no feedstock accumulation in control volume | Sludge sampling devices, or during routine cleaning and maintenance of bioreactors | Dependent on site-specific operations and must be justified. |
| Receiving waters, i.e. edge of mixing zone | Temperature | Recommended for all projects | Recommended for determining physical conditions and making calculations to fully constrain the seawater carbonate system | Sensor | (a) at high frequency when ramp-up is occurring and shortly after any dosing rate changes are made, (b) with low frequency when the dosing rate is occurring at a steady state (defined as dFS/dt=0 or d[Cx]/dt=0) |
| Salinity | Recommended for all projects | Recommended for determining physical conditions and making calculations to fully constrain the seawater carbonate system | Sensor | ||
Any two carbonate system paramters: pH, DIC, TA, pCO2 | Recommended for all projects | Recommended to constrain the seawater carbonate system, monitor water quality and local carbonate saturation state | Sensor (for pH or pCO₂) or discrete bottle samples | ||
| Third carbonate system parameter | Recommended for all projects | Recommended to assess the local state of carbonate disequilibrium | Sensor (for pH or pCO₂) or discrete bottle samples | ||
| Dissolved Oxygen (DO) | Recommended for all projects | Recommended as indicator of general ecosystem health | Sensor | ||
| Total Suspended Solids (TSS) | Recommended for all projects | Recommended for monitoring water quality and for mitigation of losses | Discrete bottle samples and filtering directly onto pre-weighed filter and subsequent measurement of mass change through drying (for example, in an oven) or derived from turbidity measurements with routine validation from bottle samples | ||
| Turbidity | Recommended for all projects | Recommended as part of ecosystem monitoring because elevated turbidity can impact photosynthesis and benthic ecosystem health | Sensor or discrete water sample | ||
| Chlorophyll-a | Recommended for all projects | Recommended as an indicator of general ecosystem health (phytoplankton primary productivity) | Sensor or discrete water sample | ||
| Dissolved inorganic nutrients | Recommended for all projects | Recommended as an indicator of general ecosystem health (available nutrients for primary productivity) and nutrient limitation impacts from secondary precipitation. | In situ sensor or discrete water sample via mooring or routine boat sampling | ||
| Deployment area | Plankton community | Recommended for all projects, may be required as part of project-specific environmental risk mitigation strategy | Recommended periodically for monitoring of ecosystem health, documentation of shifting baseline ecology and contributions to primary research | Plankton tow or other methods | At agreed upon intervals, before, during and after deployment |
| Benthic community | Recommended for all projects, may be required as part of project-specific environmental risk mitigation strategy | Recommended periodically for monitoring of ecosystem health, documentation of shifting baseline ecology and contributions to primary research Recommended for water and tissue samples in keystone biological species to track potential trace and heavy metal accumulation | Benthic survey or other methods |
The following authors from Isometric contributed to writing this Protocol:
Isometric would like to thank the following reviewers of this Protocol:
Deployment area
The area over which ecological and biological indicators should be monitored for changes due to chronic exposure to modified carbonate chemistry, determined on a project-specific basis in the PDD. This is likely to be larger than the mixing zone.
DIC
Dissolved inorganic carbon
EPA
Environmental Protection Agency
GHG
Greenhouse gas
Mixing Zone
A regulatory concept describing the spatial area surrounding the discharge infrastructure where water quality criteria can be exceeded.
MRV
Monitoring, Reporting and Verification
PDD
Project design document
Reporting Period
The Reporting Period (RP) represents an interval of time over which removals are calculated and reported for verification. See Section 7.1 for full details.
SOP
Standard Operating Procedure
This Appendix is general for marine CDR approaches: Ocean Alkalinity Enhancement (OAE), Direct Ocean Capture (DOC), Wastewater Alkalinity Enhancement
Field measurements play an important role in:
Broadly, field measurements may include moored in-situ sensors, ship-based or autonomous transecting and profiling, discrete bottle sampling, field surveys, aerial imaging and remote sensing. There is significant variability in the specific instruments and field methods that may be used to conduct collect data.
Project Proponents must provide thorough documentation on planned and completed field campaigns, including details on the campaign design (spatial and temporal coverage, sampling density, data collection techniques, instrument resolution), metadata, data processing, analytical and statistical techniques, instrument calibration and data quality control. In the sections that follow, we highlight some recommended references and best practices to ensure high quality data generation from field measurements.
Collecting discrete bottle samples alongside continuous autonomous sensor measurements is recommended. Both measurement types offer advantages, for example autonomous sensors can offer wider temporal coverage, while bottle samples are crucial for ground truthing absolute values. Additionally, bottle samples can be used to calibrate drift of long term autonomous sensor deployments.
To facilitate accessibility, transparency and interoperability of research relevant data, data collected from field campaigns must adhere to FAIR data principles 42. Project Proponents should follow the data standards and controlled vocabularies described in Guide to Best Practices in Ocean Alkalinity Research: Chapter 13–Data reporting and sharing for ocean alkalinity enhancement research 46. Metadata should be complete according to the NOAA ocean acidification metadata template 47.
Alkalinity is used in multiple fields that may have different terminology and definitions. For example, total or titration alkalinity is widely used in oceanography, whereas charge balance alkalinity is commonly used in freshwater systems and by geologists48. This Protocol uses the oceanographic community terminology when referring to measurements taken in the ocean, where total alkalinity (TA) for seawater is defined following Dickson 1981 based on acid titration to the CO2 equivalence point49. However, alternative definitions of alkalinity that may be more appropriate for specific projects (e.g. for non-seawater systems, such as effluent from a wastewater stream) should be specified in the PDD.
There are also different scales used for pH measurements which may differ by over 0.150. For example, the National Bureau of Standards (NBS) scale is suitable for freshwater systems and waters with low ionic strength, while for seawater, the total hydrogen ion scale is more commonly used51. This Protocol recommends the total hydrogen ion scale for seawater pH measurements, following the SOPs below in the Guide to Best Practices for Ocean CO2 Measurements. Care should be taken to account for differences in pH scales, such as when comparing pH measurements against regulatory thresholds.
For discrete bottle samples, we refer Project Proponents to the Best Practice Data Standards for Discrete Chemical Oceanographic Observations 52. Additional considerations for discrete bottle sampling of carbonate chemistry are discussed in Guide to Best Practices in Ocean Alkalinity Research: Chapter 2 (Seawater carbonate chemistry considerations for ocean alkalinity enhancement research: theory, measurements and calculations) 53.
Recommended analytical methods for bottle samples include:
| Parameter | Recommended Method |
|---|---|
| Carbonate Chemistry parameters (pH, DIC, TA) | Guide to Best Practices for Ocean CO2 Measurements |
| Nutrients | Methods of Seawater Analysis |
| Salinity | TEOS-10 |
| Dissolved Oxygen | Methods of Seawater Analysis |
| Chl-a | Welschmeyer 1994 |
| TSS | Methods of Seawater Analysis |
| Trace metals | GEOTRACES cookbook |
The following methods are approved by the Clean Water Act, EPA:
For autonomous instruments, Project Proponents are recommended to follow manufacturer provided user manuals for instrument preparation, maintenance, calibration and quality control. Environmental and deployment factors which may affect the quality and interpretation of data, (e.g. instrument orientation, interference, presence of bubbles, sediment, and biology) must be described.
Adoption of innovative sensor technologies is encouraged. For novel sensors, additional information that would typically be available from a manufacturer would also have to be provided. These include detection range, resolution, accuracy, performance under different environmental conditions (i.e., temperature ranges) and response time. The expected measurement conditions must be within the sensor’s range.
Project Proponents must perform instrument calibration before deployment, and recalibrate instruments at an interval recommended by the manufacturer. Calibration curves or equations for the instrument or sensor must be documented. Post-deployment calibration is recommended to account for instrument drift over the deployment duration.
Project Proponents must develop a maintenance plan for moored instruments deployed longer than 30 days. This should include plans to clean or replace sensors that may be impacted by biofoul, such as optical sensors, conductivity cells or flow through sensors.
Data processing techniques must be documented, including quality control, filtering, despiking and statistical analysis. The recommended quality control process differs between instruments. Data should be reported using quality control flags consistent with International Oceanographic Data and Information Exchange (IODE)54.
The quality control manuals for autonomous instruments prepared by the Integrated Ocean Observing System are a recommended resource.
Quality control checks could include:
Data Analysis Methods for Physical Oceanography is a recommended resource for statistical analysis methods 55.
Tracer studies track effluent using a tracer that is either naturally present or added. They can be useful for mCDR projects to:
Ideal tracers are not harmful to the environment, have near zero background concentration, are conservative or decay very slowly in relation to the duration of field work, mix freely and can be measured in the field at low concentrations. As such, examples of tracers that may be used in DOC are fluorescent dyes (i.e. rhodamine dye) and gas tracers (SF6, CH3, SF5, 3He).
Fluorescent dye tracers, such as rhodamine dye, are preferable because they can be detected from aerial imaging or high frequency in situ sensors. Rhodamine dye degrades in days to weeks, and can be helpful for characterizing near-field mixing. Gas tracers have a lifespan on the order of years, and can be useful for large scale experiments, however, they have high Global Warming Potential (A measure of how much energy the emissions of 1 tonne of a GHG will absorb over a given period of time, relative to the emissions of 1 ton of CO₂.).
Tracers can be injected into the effluent to increase the trackability of the effluent. Alternatively, the tracer dye can be released directly into the water column as a slug release, to help constrain localized dispersion characteristics for coastal models.
If a tracer study is conducted to characterize pCO2-plume transport, the sampling plan must include vertical profiling along transects. A sufficient number of studies are required to produce a sensitivity analysis. Sampling plan, data analysis and interpretation will be evaluated on a case by case basis.
The US EPA Operating Procedure56 and USGS Procedure for Dye Tracer Measurements57 are recommended resources.
The following checklist can be referenced for tracer release studies using rhodamine dye to validate an ocean model or characterize plume transport.
Study planning:
Materials preparation:
Release:
Sampling (ship-based or underway system):
Data quality and interpretation:
Below are some hypothetical examples that are for illustrative purposes only, to demonstrate some of the site and project considerations when determining the appropriate monitoring duration and frequency for a particular parameter and use case.
Example 1
An OAE project is monitoring for changes in ocean pH near the discharge site to make sure the change in pH does not exceed a threshold of +/-0.2 of the natural variability of pH. The dosing period is 1 week, and the residence time of the region where pH sensors are deployed is 1 day. At this site, it is known from previous studies and published literature that the dominant temporal modes of variability for pH are diurnal and seasonal, due to biological activity. Monitoring duration: Because the activity and dosing period are much shorter than the seasonal variability in this case, it is not necessary for the baseline characterization to span a full year to resolve the seasonal cycle. Instead, the baseline monitoring for a few days before deployment is sufficient. There is ongoing monitoring during the 7 day dosing period, and the post-dosing monitoring for pH should additionally span at least the residence time. To be conservative and to collect additional safety data though, the Project decides to extend post-dosing monitoring for 1 week. Monitoring frequency: To resolve the diurnal cycle variability, sampling should be conducted at a higher frequency than daily, e.g. sampling at minimum every 6 hours would result in at least 4 measurements per day and would be sufficient here. Luckily, the Project uses continuous pH sensors, which allows them to sample much more frequently to fully resolve the diurnal cycle.
Example 2
An environmental risk of a mineral OAE project that the local community is concerned about is bioaccumulation of metals in a nearby oyster farm. To address this, the Project is monitoring the concentration of trace metals in oyster tissues before and after the project activity. The Project is dosing feedstock particles into the ocean for 1 week, but some of the feedstock will accumulate on the seafloor and it can take up to 3 months for the particles to fully dissolve. The farm grows oysters in the ocean for 1 year before harvesting. Monitoring frequency: The accumulation of any metals in oysters will take some time, especially because it will take a few months after project dosing completes for the feedstock particles to dissolve. Thus it was decided to sample the oysters once a month for the first 3 months following the Project since that is the period during which particle dissolution is expected to occur, and then once a season after that. Monitoring duration: In this case, the community would like to know the impacts on the oysters that are being harvested for food. So the post-deployment monitoring lasts for 1 year post-dosing (taking seasonal samples) to span the oysters growth cycle and to ensure that metal concentrations do not exceed the local food safety regulatory limits. A control sample was collected from a number of oysters pre-deployment.
This Protocol only quantifies the carbon removal within-the-fence of the wastewater treatment plant. The release of CO2 from carbonic acid in the wastewater stream is assumed to be imminent and avoided through the addition of alkalinity. CO2-reactive alkalinity that is released in the effluent may facilitate carbon uptake in the open environment, which must be quantified through the River Alkalinity Enhancement Protocol or Ocean Alkalinity Enhancement from Coastal Outfalls Protocol.
In future versions of this Protocol, we plan to explore the following additional applicability criteria:
Future versions of this Protocol will also include:
This document is a companion to the Isometric Wastewater Alkalinity Enhancement Protocol V1.0, providing supporting information regarding the rationale and factors considered when determining the requirements of the Protocol, specifically relating to the treatment of CDR vs emissions avoidance in WWTPs. This document should be read in conjunction with the Protocol and is provided as guidance. Should there be any discrepancy or inconsistency between this companion document and the Protocol itself, the requirements of the Protocol will prevail.
Many CDR projects are more emissions intensive than their counterfactual scenarios because they involve building new facilities (e.g. Direct Air Capture, Electrolytic Seawater Mineralization, projects involving pyrolysis). However, some CDR activities are integrated into existing practices and infrastructure, such as Enhanced Weathering or Wastewater Alkalinity Enhancement. In these instances, the counterfactual business-as-usual (BAU) of existing practices might be more emissions intensive than the CDR project. For example, a WWTP may use NaOH, a carbon intensive pH amendment, in their normal operations. A WAE project operating in that plant would have to switch the NaOH out for a lower carbon intensity feedstock in order to do CDR. This scenario raises the question of how to ensure we are only crediting for CDR and not emissions reductions. The Isometric Standard dictates that credits should only be issued for carbon removal, not emissions avoidance or reductions, and it is very important to clearly distinguish between the two.
The net CDR that is eligible for crediting is given by:
net CDR = drawdown - counterfactual BAU drawdown - project emissions
The purpose of the counterfactual BAU term is to solely account for any CDR that would have happened in the absence of the project activity. This avoids crediting emissions reductions because the counterfactual emissions are excluded from the equation.
It is important to define what project activities are considered CDR vs. emissions reductions. The difference is that in a net-zero world, CDR activities are those that can remove residual emissions and lead to net negative emissions, whereas emissions reduction activities will never get us below net-zero.
For WAE, we have the following possible BAU scenarios:
A WWTP does use a prior pH amendment
a. pH amendment emissions - pH amendment drawdown potential > 0
b. pH amendment emissions - pH amendment drawdown potential < 0
In Scenario 1, there is no CDR occurring, so counterfactual BAU drawdown = 0.
In Scenario 2a, the WWTP is using a pH amendment whose production emissions exceed its drawdown potential when dissolved in wastewater. This activity will never be net-negative no matter the dosing amounts. In this case, the “drawdown” from the pH amendment dissolving in wastewater is doing the job of emissions reductions instead of CDR, as it is compensating for the emissions of producing the pH amendment. There is no CDR occurring, so counterfactual BAU drawdown = 0.
In Scenario 2b, the WWTP is using a low carbon intensity pH amendment such that this activity is net-negative and can be defined as CDR, as opposed to emissions reductions. In this case, the full drawdown from the pH amendment dissolving in wastewater does need to be included in the counterfactual BAU drawdown term.
This risk assessment identifies the pathway specific risk factors relevant to a carbon removal project. The relevant risk factors identified as part of a risk assessment are included in the monitoring plan requirements for the project, with details included in the Project Design Document. Project specific risk factors inform the required duration of monitoring along with the monitoring requirements set out in the Protocol and the requirements set out in the Monitoring Section of the Isometric Standard.
The risk score, as determined by the Risk of Reversal Questionnaire, will determine a project’s buffer pool contribution. Projects must re-assess their reversal risk at the renewal of each crediting period, or if monitoring identifies a reversal-related risk, or if an actual reversal event takes place. In any event, projects should reassess their reversal risk at a minimum every 5 years.
The Risk of Reversal Questionnaire questions that pertain to this protocol, drawn from the programme-level Risk of Reversal Questionnaire defined in Appendix B: Risk Reversal Questionnaire of the Isometric Standard, include the following:
| # in Isometric Standard Questionnaire | Question | If answered “Yes” | If answered “No” |
|---|---|---|---|
| 1 | Is a reversal directly observable with a physical or chemical measurement as opposed to a modeled result? | Proceed to questions 2-9 | Proceed to questions 8-9 |
| 5 | Does this approach have a material risk of reversal due to natural disasters including, but not limited to, floods, storms, earthquakes, fires, etc.? | Add 1 to Risk Score | |
| 6 | Does this approach have a material risk of reversal due to human-induced events from outside actors, such as change in farming practices, change in ownership and management of project sites, or similar? | Add up to 2 to Risk Score | |
| 8 | Is there 10+ years of monitoring and/or lab data demonstrating low project risk? | Minus up to 2 to Risk Score | |
| 9 | Does this pathway have a documented history of reversals? | Add 2 to Risk Score | |
| 10 | Is there one or more project-specific factors that merit a high risk level? | Add up to 2 to Risk Score |
Risk Score Categories
Project specific risk factors will depend on the form of carbon being stored (i.e., organic vs. inorganic), the method of storage (e.g., mineralization, encapsulation), the location of carbon storage (e.g., subsurface, ocean), and the proximity of that carbon to potential agents of reversal.
For projects with carbon storage as inorganic carbon, the presence of the following risk factors must be reflected in the risk score corresponding to question 10:
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