Wastewater Alkalinity Enhancement

This Protocol provides the requirements and procedures for the calculation of net carbon dioxide equivalent (CO₂e) removal 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 Greenhouse Gas (GHG) Statement can be accurately applied and in which the carbon dioxide (CO₂) captured is durably stored for over 1000 years.

Conventional wastewater treatment plants (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 primary treatment, in which large particles are removed via screens and gravity settling; secondary treatment, in which pollutants and unreacted biomass (sludge) are removed and effluent is clarified; and tertiary treatment, in which effluent is disinfected for discharge. During secondary treatment, solid and dissolved organic carbon is transformed into CO₂ through biological processes, which results in emission of biogenic CO₂^1,2. Additionally, the secondary treatment process leads to emission of both methane (CH₄) and nitrous oxide (N₂O)², which are 29.8 and 273 times more potent greenhouse gasses (GHGs), respectively, than CO₂ on 100-year timescales³. However, biogenic emissions from wastewater treatment are not typically counted in standard GHG accounting frameworks^{4, 5}.

In Wastewater Alkalinity Enhancement, rock or mineral feedstock 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, it has high potential for scaling.

This Protocol specifically applies to municipal and industrial WWTPs that reduce biological oxygen demand (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 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 (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 Project refers to the system boundaries of the retrofitted process, as described in Section 7.3. Carbon Credits are generated from this Protocol relative to Business As Usual (BAU) operations of the existing WWTP. For example, calculation of $CO_2e_{GHG Emissions}$ considers only emissions that are above BAU (see Section 8.1).

This Protocol is developed to adhere to the requirements of ISO 14064-2: 2019 -- Greenhouse Gases -- Part 2: Specification with guidance at the project level for quantification, monitoring, and reporting of greenhouse gas emission reductions or removal enhancements. The Protocol ensures:

• consistent, accurate procedures are used to measure and monitor all aspects of the WAE process required to enable accurate accounting of net CO₂e removals
• consistent system boundaries and calculations are utilized to quantify net CO₂e removal for WAE projects
• requirements are met to ensure the CO₂e removals are additional
• evidence is provided and verified by independent third parties to support all net CO₂e removal claims

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:

• Isometric Standard
• ISO (International Organization for Standardization) 14064-2: 2019 – Greenhouse Gases – Part 2: Specification with guidance at the project level for quantification, monitoring and reporting of greenhouse gas emission reductions or removal enhancements

Additional reference standards that inform the requirements and overall practices incorporated in this protocol include:

• ISO 14064-3: 2019 - Greenhouse Gases - Part 3: Specification with Guidance for the verification and validation of greenhouse gas statements
• ISO 14040: 2006 - Environmental Management - Lifecycle Assessment - Principles & Framework
• ISO 14044: 2006 - Environmental Management - Lifecycle Assessment - Requirements & Guidelines

This protocol was developed based on the current state of the art and current publicly available science regarding Wastewater Alkalinity Enhancement. As Wastewater Alkalinity Enhancement is a novel CDR approach, with limited published literature, the 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 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 for Wastewater Alkalinity Enhancement are safe and have a demonstrable net-negative climate impact. The scope of this protocol will be expanded in future iterations to allow for more discharge locations and different types of wastewater treatment facilities. To be eligible for crediting under this Protocol, projects and associated operations must meet all of the following project conditions.

To ensure safety of human and environmental health, eligible projects must:

• Operate within the existing permits, or an approved modified permit, of a wastewater treatment plant.
• Characterize feedstock prior to usage according to the Rock and Mineral Feedstock Characterization Module
• Submit proof of an official partnership with partner facilities to ensure that the project is consensual. This can be evidenced by documents such as a Memorandum of Understanding, written consent, or other agreement with involved parties, such as the WWTP, wastewater utility, contracted operators, or contracted feedstock supplier.

To ensure net-negative climate impacts, eligible projects must:

• Do one of the following:
  • Add alkaline feedstock to a wastewater treatment plant (WWTP) that otherwise would not do alkalinity enhancement such as liming, as evidenced from historical records of plant operations
  • Increase the alkaline feedstock dosing rate above the BAU operations in a WWTP that does currently add alkalinity (e.g. in the form of lime, NaOH, MgOH₂, etc.) for process control. See Section 8.3 for details on calculating the counterfactual in this case.
• Be considered additional, in accordance with the requirements of Section 5.4.
• Provide a net-negative CO₂e impact (net CO₂e removal) as calculated in compliance with Section 8, on a cradle to grave GHG assessment basis.
• Provide long duration storage (>1000 yr) of CO₂ in seawater and/or carbonate minerals.

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 currently includes storage of removed CO₂ as Dissolved Inorganic Carbon in the Ocean. Thus, eligible Projects must operate in WWTPs that discharge directly into the ocean from coastal outfalls, or to rivers where the discharge will be transported to the ocean. Most rivers will reach their basin outlet within 45 days⁸. Long term storage in inland waters (e.g. lakes) is being explored for future iterations.
• This protocol quantification framework is developed for WWTP treating biogenic waste with activated sludge reactors and variations on activated sludge processes, which is the most common form of biological treatment. Alternative types of biological treatment may require modifications to the quantification approaches described in Section 8, and are allowed on a case-by-case basis in consultation with Isometric.
• Alkalinity addition occurs in the biological treatment portion of the facility.
• Projects must operate as retrofits to existing WWTP facilities.

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 must document project characteristics in a Project Design Document (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:

• detailed feedstock characterization following the Rock and Mineral Feedstock Characterization Module,
• description of discharge site and WWTP business as usual operations, following Section 10
• description of the mitigation plan according to the environmental and social risk assessment in adherence with Appendix 1, including an accompanying robust monitoring plan to ensure efficacy,
• description of the quantification strategy for net CO₂e removal following Section 8,
• description of all measurement and methods used to quantify processes relevant to the calculation of net CO₂e removal, cross-referenced with relevant standards where applicable,
• documentation of official permitting and Memorandum of Understanding with partner WWTP

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 (VVB) must consider the following requisite components:

1. Validate that the feedstock adheres to the requirements listed in the Mineral and Rock Feedstock Characterization Module.
2. Verify that the quantification approach adheres to requirements of Section 8, including demonstration of required records.
3. Verify that the Environmental & Social Safeguards outlined in Section 6 are met.
4. Verify that the project is compliant with requirements outlined in the Isometric Standard.

The threshold for Materiality, 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 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:

• control issues that erode the verifier’s confidence in the reported data;
• poorly managed documented information;
• difficulty in locating requested information;
• noncompliance with regulations indirectly related to GHG emissions, removals or storage

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:

• VVBs must be able to demonstrate accreditation from:
  • an International Accreditation Forum member against ISO 14065 or other relevant ISO standard, including but not limited to ISO 14034, ISO 17020, ISO 17029; or
  • a relevant governmental or intergovernmental regulatory body.
• Alternatively, on a case-by-case basis, if VVBs are able to demonstrate to Isometric that they satisfy all required Verification needs and competencies required for the relevant Protocol and follow the guidelines of ISO 19011 or other relevant standards, they may be approved.

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 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 scenarios and baselines 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:

• regulatory requirements or other legal obligations for project implementation change or new requirements are implemented
• project financials indicate carbon finance is no longer required, potentially due to, for example
  • increased tipping fees for waste feedstocks
  • sale of co-products that make the business viable without carbon finance
  • reduced rates for capital access
  • subsidies for WWTP emissions reduction

Any review and change in the determination of additionality shall not affect the availability of carbon finance and Credits for the current or past Crediting Periods, 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 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:

• required measurements used for net CO₂e removal calculation
• emission factors utilized, as published in public and other databases
• values of measured parameters from process instrumentation, such as electricity usage from utility power meters
• laboratory analyses, including analysis of seawater chemistry and alkaline feedstocks

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 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:

• Project Design Document
• GHG Statement
• Measurements taken
• Model specifications and output
• Emission factors used
• Scientific literature used

The Project Proponent can request certain information to be restricted (only available to authorized buyers, the Registry 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.

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 socio-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:

• Project Proponents must identify jurisdictional authorities receiving water bodies of the project site, and as outlined in Section 4 (Applicability).
• Projects must operate within the existing permits, or an approved modified permit, of a wastewater treatment plant.
• Project Proponents must hold an official partnership, such as through an MOU, with the wastewater facility the project intends to integrate with.

Compliance:

• Project Proponents must comply with all national and local laws, regulations and policies.
• Where relevant, projects must comply with international conventions and standards governing human rights and uses of the environment, when conducted within or where it is foreseeable for it to impact Party jurisdictions.

Environmental and social risk assessment in adherence with Section 3.7 of the Isometric Standard must be completed 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:

• Potential risks associated with feedstocks include land use impacts from sourcing, production, preparation, storage and distribution, such as land degradation, land occupation, dust pollution, deforestation and localized watershed contamination.

Co-products and waste:

• Generation of waste products or additional waste must be accompanied by a plan which ensures safe handling, containment and disposal.

Pollution prevention:

• Additional pollutants, such as nutrients or toxic elements, from dissolution of feedstock(s), which may result in bioaccumulation in biota or water quality impacts for downstream water users. Rock and Mineral Feedstock characterization is mandatory as a first line of defense for safeguarding against the release of harmful pollutants.

Ecological impacts:

• Shock to the ecosystem due to rapid or sudden changes in carbonate system parameters
• Changes in carbonate chemistry, such as pH, could directly help or harm aquatic life depending on the magnitude and direction of the pH shift
• Cascading impacts of altered carbonate chemistry, nutrient fields or particle deposition on biodiversity and ecosystem functions at and downstream of the receiving water body

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:

• have a plan for safeguarding working conditions, especially for safe handling and sampling of wastewater, activated sludge and fine particulate feedstocks
• adhere to all health and safety standards set by the partner facility
• consider the risks to water justice for users of the receiving water body and downstream water bodies
• consider the risks to environmental justice due to historical inequities in siting of wastewater treatment facilities

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:

• instrument malfunctions lead to data-gaps in required monitoring
• effluent exceeds thresholds outlined in the PDD
• regulatory non-compliance, e.g. danger to ecosystem health detected (such as by the local community or government agency)
• compromised health and/or safety of workers and/or local stakeholders

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 $CO_2e_{Removal,RP}$.

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 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 (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.

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.

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 activites that are shared with BAU WWTP activities. Only activities above BAU are accounted for within the system boundary. 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

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 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 the reduction of CH₄ and N₂O emissions. Crediting the avoidance of non-CO₂ GHG 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

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

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 additonal 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 separate 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 scenario considers the biogenic CO₂ generated during biological treatment that would have outgassed to the atmosphere within the confines of the WWTP, e.g. the outgassing that is expected to occur over the residence time of the Control Volume. 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. Details on how to calculate CO₂e_{Counterfactual, RP} for different scenarios is described in Section 8.3.

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:

$CO_2e_{Removal, RP} = CO_2e_{Stored, RP} - CO_2e_{Counterfactual, RP} - CO_2e_{Emissions, RP}$

Equation 1

Where

• $CO_2e_{Removal, RP}$ represents the total net CO₂e removal for reporting period, RP, in tonnes of CO₂e.
• $CO_2e_{Stored, RP}$ represents the total CO₂ removed from the conversion of biogenic CO₂ in wastewater to stable dissolved inorganic carbon due to the Project activity for reporting period, RP, in tonnes CO₂e.
• $CO_2e_{Counterfactual, RP}$ represents the total counterfactual CO₂ removed from the atmosphere for the reporting period, RP, in tonnes of CO₂e. This term is calculated using BAU operations. See Section 8.3.
• $CO_2e_{Emissions, RP}$ represents the total GHG emissions associated with the Project including leakage, over a reporting period, RP, in tonnes of CO₂e.

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. It is important to note that 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 a metal carbonate (MCO₃) as an alkaline feedstock follows the chemical reaction:

$MCO_{3 (s)} + CO_{2 (aq)} + H_2O_{(l)} \rightarrow M^{2+}_{(aq)} + 2HCO_{3 (aq)}^{-}$

Equation 2

For a metal carbonate, CO₂ conversion has a 1:1 molar ratio to MCO₃ consumption and CO₂ consumption. Thus, CO₂ conversion to bicarbonate (HCO₃^- ) can be measured by direct measurement of solid feedstock, (ie. MCO_{3 (s)} ), or dissolved weathering products (ie. M²⁺_(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:

$CO_2e_{Stored, RP} = m_{Dissolved, RP} * \frac{M_{CO_2}}{M_{FS}} - CO_2e_{Losses, RP}$

Equation 3

Where

• $m_{Dissolved, RP}$ is the total mass of feedstock dissolved within a reporting period, RP, in tonnes;
• $M_{CO_2}$ is the molar mass of CO₂ in tonnes/mol;
• $M_{FS}$ is the molar mass of the chosen feedstock in tonnes/mol;
• $CO_2e_{Losses, RP}$ are losses of CO₂ due to riverine and oceanic processes, in tonnes CO₂e. See Section 8.2.3 for calculation of this term.

The mass of feedstock dissolved within a reporting period, $m_{Dissolved, RP}$, can be determined by:

$m_{Dissolved,RP} = \sum_{i}^{T} ( \dot m_{Dissolved,i} * \Delta t_i)$

Equation 4

Where

• $m_{Dissolved, RP}$ is the total mass of feedstock dissolved within a reporting period, RP, in tonnes;
• $\dot m_{Dissolved,i}$ is the average feedstock dissolution rate over time interval, i, in tonnes/day;
• $\Delta t_i$ is the duration of time period, i, over which measurements are averaged, in days;
• $T$ is the total number of time intervals in the reporting period RP to be summed over.

$\dot m_{Dissolved}$ 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.

Figure 2: Example process diagram, control volume and monitoring locations for Wastewater Alkalinity Enhancement. The control volume, 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.

$\dot m_{Dissolved}$ can be calculated based on the following options:

• Option 1: Mass balance on solid feedstock, where the difference between solid feedstock into and out of the control volume is assumed to be the result of feedstock dissolution.
• Option 2: Mass balance on conservative dissolved weathering products, such as Ca²⁺ or Mg²⁺, where the difference in weathering product concentration between the influent and effluent is assumed to the result of feedstock dissolution.

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.

The mass flow rate of solid feedstock is defined as:

$\dot m_{FS} = Q * FS * 10^{-9}$

Equation 5

Where

• $\dot m_{FS}$ is mass flow rate of feedstock in tonnes/day;
• $Q$ is flow rate in L/day;
• $FS$ is mass concentration of feedstock in mg/L;
• A factor of 10^-9 is used to convert units from mg to tonnes.

Alternative formulations of flow rate can be used where applicable. All calculations must be submitted for verification.

The mass balance of the solid feedstock within the control volume V (Figure 2) can be defined as:

$\frac{dFS}{dt} = \dot m_{FS, Dosing} - \dot m_{FS, Effluent} - \dot m_{FS, WAS} - \dot m_{Dissolved}$

Equation 6

Where:

• $\frac{dFS}{dt}$ is the change in feedstock mass in the control volume over time t;
• $\dot m_{FS, Dosing}$ is the mass flow rate of feedstock added in tonnes/day;
• $\dot m_{FS, Effluent}$ is the mass flow rate of feedstock in the effluent stream in tonnes/day;
• $\dot m_{FS, WAS}$ is the mass flow rate of feedstock in the waste activated sludge (WAS) stream in tonnes/day;
• $\dot m_{Dissolved}$ is the mass of feedstock dissolved in tonnes/day.

By assuming steady state, $\frac{dFS}{dt} = 0$, no feedstock present in the influent, and the control volume shown in Figure 2, $\dot m_{Dissolved}$ can be calculated as:

$\dot m_{Dissolved} = \dot m_{FS, Dosing} - \dot m_{FS, Effluent} - \dot m_{FS, WAS}$

Equation 7

Where:

• $\dot m_{FS, Dosing}$ is the mass flow rate of feedstock from dosing in tonnes/day;
• $\dot m_{FS, Effluent}$ is the mass flow rate of the feedstock in the effluent stream in tonnes/day:
  • $\dot m_{FS, Effluent} = Q_{Effluent} * FS_{Effluent} * 10^{-9}$ (Equation 8)
    • $Q_{Effluent}$ is the flow rate of plant effluent in L/day;
    • $FS_{Effluent}$ is the concentration of feedstock suspended in plant effluent in mg/L;
    • A factor of 10^-9 is used to convert units from mg to tonnes
• $\dot m_{FS, WAS}$ is the mass flow rate of feedstock in the waste activated sludge (WAS) stream in tonnes/day:
  • $\dot m_{FS, WAS} = Q_{WAS} * FS_{WAS} * 10^{-9}$ (Equation 9)
    • $Q_{WAS}$ is the flow rate of waste activated sludge in L/day;
    • $FS_{WAS}$ is the concentration of feedstock settled in waste activated sludge in mg/L.
    • A factor of 10^-9 is used to convert units from mg to tonnes.

Requirements for measurements of concentration of the feedstock in effluent and waste activated sludge will vary depending on the feedstock used:

• When carbonate feedstocks, such as calcite (CaCO₃) are used, $FS_{Effluent}$ and $FS_{WAS}$ can be assumed to be equivalent to the total inorganic carbon (TIC) in samples taken from these streams. This approach assumes that any inorganic carbon present in plant effluent and waste activated sludge comes from dosing of carbonate feedstocks, which is a conservative assumption. Methods for TIC analysis must be described in detail in the PDD and must include any standards or SOPs used. Acceptable analyses may include, but are not limited to, use of a TIC analyzer.

• Where non-carbonate feedstocks are used, $FS_{Effluent}$ and $FS_{WAS}$ must be determined from direct measurements of relevant dissolved weathering products (e.g. Ca²⁺, Mg²⁺) in each stream, relative to the baseline. Acceptable methods include, but are not limited to, sample digestion, or analysis on ICP-MS or ICP-OES. Note that for $FS_{Effluent}$, samples must be acidified prior to filtration. Methods used for cation analysis must be described in detail in the PDD and must include any standards or SOPs used.

The molar flow rate of a dissolved weathering product, C^X, is defined as:

$n_{C^x}= Q * [C^x]$

Equation 10

Where

• $n_{C^x}$ is the molar flow rate of dissolved weathering product C^x, in mol/day;
• $Q$ is flow rate in L/day;
• $[C^x]$ is the molar concentration of dissolved weathering product C^X, in mol/L

The mass balance of the dissolved weathering product, C^X, within the control volume V (Figure 2) can be defined as:

$\frac{d[C^x]}{dt} = (1/V)* [(n_{C^x, Influent} - n_{C^x, WAS} -n_{C^x, Effluent}) + (\dot m_{Dissolved}/M_{FS})+n_{C^x, NonCarbAcid}]$

Equation 11

Where:

• $\frac{d[C^x]}{dt}$ is the change in molar concentration of dissolved weathering product, C^X, in the control volume over time, in mol/(L day);
• $V$ is the volume of the control volume, in L;
• $n_{C^x, Influent}$ is the molar flow rate of dissolved weathering product C^x in the influent stream in mol/day;
• $n_{C^x, WAS}$ is the molar flow rate of dissolved weathering product C^x in the WAS stream in mol/day;
• $n_{C^x, Effluent}$ is the molar flow rate of dissolved weathering product C^x in the effluent stream in mol/day.
• $\dot m_{Dissolved}$ is the mass of feedstock dissolved in tonnes/day.
• $M_{FS}$ is the molar mass of the feedstock, in tonnes/mol;
• $n_{C^x, NonCarbAcid}$ is the molar flow rate of dissolved weathering product C^x resulting from non-carbonic acid dissolution of feedstock within the control volume, in mol/day.

Assuming steady state, $\frac{d[C^x]}{dt} = 0$, the rate of feedstock dissolved can be calculated in the aqueous phase using the following equation:

$\dot m_{Dissolved} = ( M_{FS})*(n_{C^x, Effluent} + n_{C^x, WAS} -n_{C^x, Influent}-n_{C^x, NonCarbAcid})$

Equation 12

Where

• $n_{C^x, Effluent}$ is the molar flow rate of dissolved weathering product C^x in the effluent stream in mol/day:
  • $n_{C^x, Effluent}= Q_{Effluent} * [C^x]_{Effluent}$ (Equation 13)
    • $Q_{Effluent}$ is the flow rate of plant effluent in L/day;
    • $[C^x]_{Effluent}$ is the molar concentration of dissolved weathering product C^x in the effluent stream mol/L;
• $n_{C^x, WAS}$ is the molar flow rate of dissolved weathering product C^x in the effluent stream in mol/day:
  • $n_{C^x, WAS}= Q_{WAS} * [C^x]_{WAS}$ (Equation 14)
    • $Q_{WAS}$ is the flow rate of plant effluent in L/day;
    • $[C^x]_{WAS}$ is the molar concentration of dissolved weathering product C^x in the effluent stream mol/L;
• $n_{C^x, Influent}$ is the molar flow rate of dissolved weathering product C^x in the effluent stream in mol/day:
  • $n_{C^x, Influent}= Q_{Influent} * [C^x]_{Influent}$ (Equation 15)
    • $Q_{Influent}$ is the flow rate of plant effluent in L/day;
    • $[C^x]_{Influent}$ is the molar concentration of dissolved weathering product C^x in the influent stream mol/L;
• $M_{FS}$ is the molar mass of the feedstock, in tonnes/mol.
• $n_{C^x, NonCarbAcid}$ is the molar flow rate of dissolved weathering product C^x resulting from non-carbonic acid dissolution of feedstock within the control volume:
  • $n_{C^x, NonCarbAcid} = \sum_{i} ( Q_i*[C^x]_i)$ (Equation 16), where:
    • $Q_i$ is the flow rate of stream i;
    • $[C^x]_i = \sum_{y} (a_y*n_y$) (Equation 17)
      • $a_y$ is the concentration of anion y;
      • $n_y$ is the charge of anion y

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 $\frac{dFS}{dt} = 0$, 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 $CO_2e_{Stored}$, 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 WAS buildup 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 $CO_2e_{Stored}$.

There are two options available for quantifying $m_{Dissolved}$ 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 $m_{Dissolved}$. Project Proponents must check that the results of both options for calculating $m_{Dissolved}$ agree within propagated error of the relevant measurements once per Reporting Period. If this is not the case, an audit will 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.

In the absence of project activities, wastewater treatment plants have direct CO₂ emissions from evasion of biogenic CO₂^14,15. 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. The quantity of emissions that are released within the residence time of the control volume is referred to as counterfactual biogenic CO₂ emissions.

Wastewater Alkalinity Enhancement reduces or eliminates direct outgassing of biogenic CO₂ emissions from the secondary treatment process. CO₂e_Stored cannot, by definition, exceed the counterfactual biogenic CO₂ emissions. Thus, Project Proponents are required to provide a conservative estimate of the counterfactual biogenic CO₂ emissions as the upper limit for CO₂e_Stored within the residence time of the control volume, summed across the full timescale of the reporting period. For example, a default emissions factor of 560g CO₂/kg COD removed during treatment can be used ¹⁵.

There are several methods that can be used to provide an estimate of counterfactual biogenic CO₂ emissions:

• Measurement of dissolved CO₂ concentrations prior to aeration and in the outflow from secondary treatment. The difference in these values can be taken as the CO₂ loss during the treatment process. This method requires extensive baselining of CO₂ concentrations across the full range of typical environmental conditions that may impact the efficiency of microbial communities within the secondary treatment process. At a minimum, Project Proponents must measure baseline CO₂ concentrations in the relevant streams during standard operational conditions 3 times per quarter for 1 year prior to the onset of Project activities, due to the dependence of CO₂ solubility and microbial activity on temperature. Where available, data from partner facilities may be substituted for direct measurements. Where direct measurements are required, Project Proponents must describe the methods used for measurement of CO₂ concentrations. This Protocol recommends the equilibration method, in which a volume of fluid is equilibrated with the headspace of a vessel (typically a syringe) and the headspace gas is collected and injected into a vacuum-sealed vial (Vacutainer) for analysis on a gas chromatograph. Care should be taken to ensure that gas samples are properly stored. Other methods may be suitable and must be described in the PDD. Due to the high pCO₂ of wastewater with a high organic load, it is not recommended to measure the dissolved inorganic carbon (DIC) of the fluid, as outgassing is likely to occur during sample collection.

• 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:

  • Concentration of CO₂
  • pH
  • Flow rate of the fluid
  • Flow rate of the gas
  • Temperature
  • Gas pressure within the system

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 CO₂e_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. If it cannot be justified that these losses are negligible, it is expected that these losses are quantified and subtracted in the calculation of $CO_2e_{Stored, RP}$.

Processes that can lead to losses include:

• Re-equilibration of the DIC system
• Carbonate precipitation
• Reduction in natural alkalinity fluxes
• Changes in biotic calcification (for ocean discharge only)

The sum of these processes are considered $CO_2e_{Losses, RP}$.

$CO_2e_{losses,RP} = \Sigma^{N}_{i} CO_2e_{losses,i,RP}$

Equation 18

Where:

• $CO_2e_{losses, i, RP}$ represents the CO₂ loss associated with each loss term, i, over the Reporting Period, RP, in tonnes CO₂.
• $N$ is the total number of loss terms. The loss terms which must be considered for ocean and river discharge are described below.

Each individual loss term is quantified as a total number in tonnes, which may be calculated as a percentage of $CO_2e_{Stored,RP}$. For example,

$CO_2e_{losses,i,RP} = (1-\eta_i) * CO_2e_{stored,RP}$

Equation 19

Where:

• $\eta_i$ is the fraction of $CO_2e_{stored, RP}$ that is retained after a given loss process, i.

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 subtracted from gross carbon removal.

Figure 3. Example schematic of where downstream losses may occur, and the spatial zones which must be considered for coastal and river discharges.

Losses which must be addressed in each spatial zone are indicated in Table 4.

Table 4. Losses and their corresponding spatial zones. All losses in column 1 "Initial 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.

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 them. Due to the difficulty and uncertainty in quantifying the impact of these processes at this time, acceptable treatment of loss terms in this Protocol include:

• avoiding the likelihood of these losses by identifying avoidance strategies around conditions which lead to non-negligible loss terms, with corresponding monitoring to demonstrate adherence to those guardrails
• estimating a conservative upper limit of loss based on scientific literature, first principles calculations, and/or experimentation
• process-based modeling studies
• direct measurements
• alternative approaches that are sufficiently justified

Data, measurements and evidence used in the quantification of losses must be publicly disclosed. Example recommendations for each loss term are discussed below. Much of the existing research in these loss terms 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 loss terms in the relevant carbonate chemistry parameter space for their specific process.

The near-field zone comprises the spatial extent of initial mixing, 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:

$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^- \leftrightarrow 2H^+ + CO_3^{2-}$

Equation 20

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 estimate the difference between the measured DIC at the WWTP effluent and the theoretical equilibrium DIC of the effluent and ambient water mixture calculated at ambient environmental conditions (temperature, salinity, pH, and pCO₂). For example, 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:

$Ca^{2+} + 2HCO_3^{-} \rightarrow CaCO_3 + CO_2 + H_2O$

Equation 21

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 hours^{18, 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 state^{25, 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 ocean^{29, 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:

• Domain:
  • Full river network through which bicarbonate and carbonate ions will be transported
• Inputs:
  • River characteristics:
    • Baseline calcite saturation index (SI_c)
    • pH
    • pCO₂
    • Alkalinity
  • Effluent characteristics:
    • Discharge rate
    • Alkalinity fluxes
    • Cation fluxes
• Outputs:
  • SI_c along river*
  • pH along river
  • Efficiency or discount factor on river transport loss for re-equilibration of DIC
  • Efficiency or discount factor on river transport loss for carbonate precipitation

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 (SI_c) is a useful parameter to determine the likelihood of carbonate precipitation. Calcite saturation index (SI_C) is calculated as:

$SI_c = \log_{10} \frac{\alpha Ca^{2+} * \alpha CO_3^{2-}}{K_{sp(calcite)}}$

Equation 22

With

$K_{sp(calcite)} = \alpha_{sat} Ca^{2+} * \alpha_{sat} CO_3^{2-}$

Equation 23

Where:

• $\alpha$ is the measured solution activities of those ions
• $\alpha_{sat}$ is the ion activities at saturation

When SI_c > 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 SI_c > 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 circulation^38,39,40. Typically, the ocean has a higher pH than rivers and the increased presence of CO₃^2- in oceans can reduce the total storage of terrestrially exported DIC. 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:

1. Using the Renforth and Henderson (2017) uptake efficiency equation
2. Using a 3D Earth Systems Model or ocean physical-biogeochemical model to explicitly simulate ocean circulation and air-sea CO₂ fluxes⁴⁰
3. Alternate approaches may be considered on a case by case basis, provided it is sufficiently described and justified in the PDD.

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:

$$\frac{\Delta C_T}{\Delta A_T} = \eta = (S \cdot 10^{-3.009} + 10^{-1.519}) \text{ln}(pCO_2)-(S \cdot 10^{-2.100}) - \\ (T \cdot pCO_2)(S \cdot 10^{-7.501} - 10^{-5.598}) - (T \cdot 10^{-2.337}) + 10^{-0.102}$$

Equation 24

Where

• $\eta$ is the uptake efficiency of CO₂ upon mixing with ocean waters, dimensionless
• $\Delta C_T$ is the change in Total DIC in the ocean as a result of WAE, in mol
• $\Delta A_T$ is the change in Total Alkalinity in the ocean as a result of WAE, in mol
• $pCO_2$ is the local ocean partial pressure of CO₂ at the discharge site, in μatm
• $T$ is the local surface ocean temperature, in °C
• $S$ is the local ocean salinity, in %

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 22). The efficiency can be used in Equation 17 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:

• the model used and domain
• inputs and forcing data, including atmospheric forcing and initial conditions
• carbonate system representation and parameterization of air-sea CO₂ fluxes
• representation of WAE project, e.g. as represented through an additional flux of DIC and alkalinity into the ocean at a particular river mouth
• baseline simulations

Furthermore, the model must be well-validated and skillful for the purpose that it is used for. Proof of model valdiation can be achieved through either:

1. A track record of use in science, industry, or government applications, which is demonstrated through multiple peer-reviewed papers, or proof of usage in a number of previous applications.
2. Newly developed models without a track record of usage must be validated against reputable data sources, which include quality-controlled in situ measurements and public datasets adhering to FAIR (Findable, Accessible, Interoperable and Reusable) principles⁴². Sufficient model validation data must be provided with the PDD.

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.

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

For projects operating in WWTPs that do not add any alkalinity for their process control, the counterfactual in CO₂e_{Counterfactual, RP} is considered to be 0, since CO₂e_Stored,RP is calculated relative to a BAU where no alkaline feedstock is added to the WWTP and the amount of limestone present in the WWTP after primary treatment is negligible.

For projects in which the partner facility does add alkalinity, then the baseline alkalinity dosing rate must be calculated.

Type: Emissions

$CO_2e_{Emissions, RP}$ is the total GHG emissions associated with a Reporting Period, ${RP}$. This can be calculated as:

$CO_2e_{Emissions,\ RP} = CO_2e_{Establishment,\ RP} + CO_2e_{Operations,\ RP} + CO_2e_{End-of-Life,\ RP} + CO_2e_{Leakage,\ RP}$

Equation 25

Where

• $CO_2e_{Emissions, RP}$ represents the total GHG emissions for a Reporting Period, in tonnes of CO₂e.
• $CO_2e_{Establishment, RP}$ represents the GHG emissions associated with project establishment, represented for the Reporting Period, in tonnes of CO₂e, see Section 8.4.1.
• $CO_2e_{Operations, RP}$ represents the total GHG emissions associated with operational processes for a Reporting Period, in tonnes of CO₂e, see Section 8.4.2.
• $CO_2e_{End-of-Life, RP}$ represents GHG emissions that occur after the Reporting Period and are allocated to a Reporting Period, in tonnes of CO₂e, see Section 8.4.3.
• $CO_2e_{Leakage, RP}$ represents GHG emissions associated with the project’s impact on activities that fall outside of the system boundary of a project, over a given Reporting Period, in tonnes of CO₂e, see Section 8.4.4.

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 activites beyond the WWTP BAU operations are included in the calculation of $CO_2e_{Emissions, RP}$. This is set out in Table 1 and Figure 1.

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:

• as a one time deduction from the first removal
• allocated to removals as annual emissions over the anticipated project lifetime
• allocated per output of product (i.e., per ton CO₂ removed) based on estimated total production over project lifetime

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 process will be triggered in accordance with the Isometric Standard, to account for any remaining emissions.

GHG emissions associated with $CO_2e_{Operations, RP}$ 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 $CO_2e_{Operations, RP}$.

$CO_2e_{Operations, RP}$ 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.

$CO_2e_{End-of-Life, RP}$ 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 $CO_2e_{End-of-Life, RP}$ 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 $CO_2e_{Establishment, RP}$.

Given the uncertain nature of $CO_2e_{End-of-Life, RP}$ emissions, assumptions must be revisited at each Crediting Period and any necessary adjustments made. Furthermore, if there are unexpected $CO_2e_{End-of-Life, RP}$ 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.

$CO_2e_{Leakage, RP}$ 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:

• Feedstock replacement
• Consumables replacement

$CO_2e_{Leakage, RP}$ 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 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 ⁴³.

Examples of fuel consumption may include, but are not limited to handling equipment, such as fork trucks or loaders.

The Energy Use Accounting module provides guidance 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, 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.

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:

• transportation of equipment, feedstock and consumables to the facility
• transportation of sludge to storage site
• transportation and shipping related to collection and analysis of samples

The Transportation Emissions Accounting Module provides guidance 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.

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:

• feedstock, consumables and associated equipment produced, constructed and utilized explicitly for the CDR project
• process equipment (i.e., pumps, mixers, blowers, flow control, measurement instruments)
• equipment related to waste handling and disposal

The Embodied Emissions Accounting Module sets out the calculation approach to be followed including allocation of embodied emissions, life cycle stages to be considered, data sources and emission factors.

The primary storage reservoir of the CO₂ removed through Wastewater Alkalinity Enhancement is Dissolved Inorganic Carbon (DIC) in the ocean. The durability and reversal risks of this storage reservoir are discussed in the 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 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 categorized as having a Very Low Risk Level of Reversal according to the Isometric Standard Risk Assessment Questionnaire. The Buffer Pool corresponding to this lowest risk score is 2% and is intended as an additional precaution against unknowns.

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:

• Project Proponents must provide a site description of the discharge location, including the dominant circulation and modes of variability, as well as the frequency of various environmental conditions. This must include, at minimum, information on seasonal patterns, predominant currents, periods of maximum stratification and periods when receiving water quality conditions may be more critical.
• Project Proponents must develop a conceptual model of dispersion of discharge, and produce a mixing zone model, which will likely already be a part of the existing WWTP discharge permit. The mixing zone model will estimate initial dispersal of the discharge, and will dictate the recommended reach of the monitoring efforts. Sensitivity analysis is required to demonstrate adherence to water quality limits at the boundary of the mixing zone under varying environmental conditions.
• Project Proponents must characterize the baseline of relevant parameters at the points of measurement (Figure 4), including carbonate chemistry parameters (e.g. pH, total alkalinity, DIC and pCO₂) and trace metal concentrations of trace metals that will or may be released into receiving waters.
• Project Proponents must have sufficient plans for restoring water quality conditions prior to discharge in the event that negative environmental impacts result from project activities.
• Rocks and minerals utilized as feedstock must be sufficiently characterized following Isometric’s Rock and Mineral Feedstock Characterization Module.

In addition, the following pre-deployment parameters should be reported in the PDD.

• [C^x] within the WWTP reservoirs should be measured and reported by the Project Proponent
• Information on how dosing rate and maximum CDR potential were calculated, including any partner data that was used in those calculations
• Energy usage of the WWTP (e.g. from pumping, aeration, mixing, maintenance and/or cleaning)
• The amount of WWTP solid/WAS generation during business as usual WWTP operation and the fate of the WAS (e.g. incineration, anaerobic digestion, aerobic digestion)
• Historical emissions from the WWTP plant, as well as additional data on how future projected BAU emissions can be modeled
• Control volume, in L, within the boundaries identified by the Project Proponent
• Retention and settling times of solids/WAS within the WWTP.

This Protocol requires and recommends monitoring to occur at the following locations, which are illustrated in Figure 4. 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).

1. Influent (A): The influent stream of wastewater entering the control volume. Typically for control volumes that encompass the secondary treatment step, the influent stream measurements will be taken after primary treatment and before entering secondary treatment.
2. Feedstock dosing (B): Measurements of alkaline feedstock dosing into the control volume.
3. Effluent (C): The effluent stream exiting the control volume.
4. Waste activated sludge (D): Measurements of the waste activated sludge that is settled out of the water at the end of the secondary treatment step, and prior to transport and disposal of the WAS (e.g. incineration or landfill) or onsite solids treatment (e.g. anaerobic or aerobic digestion).
5. Edge of mixing zone (E): This refers to measurements made in the receiving waters, in the vicinity of the WWTP discharge. The region near the discharge may be referred to as a mixing zone, or mixing length, and is where signals of the WWTP discharge will be highest before being diluted farther away from the discharge location.

Monitoring locations and a description of the control volume must be included in the monitoring plan submitted in the PDD.

Figure 4 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 ($Q_{Influent}$, L/day) and the concentration of the dissolved weathering product ($[C^x]_{Influent}$; mol/L), both of which are used for the aqueous phase mass balance for quantifying the amount of feedstock dissolved in Equation 12. This is either used as the primary approach for quantifying $\dot m_{Dissolved}$, or must be used as a validation check (see Section 8.2). In addition, measurements of anions are required for determining the amount of non carbonic acid weathering (see Section 8.2.1.2, Equations 16 and 17).

The addition of feedstock into the control volume must be quantified in terms of the feedstock mass flow rate ($\dot m_{FS, Dosing}$; tonnes/day) for the solid phase mass balance (See Equation 7). This is used either as the primary approach for quantifying $\dot m_{Dissolved}$, or is required as a validation check (see Section 8.2). For projects where slurry dosing is used, the flow rate $Q_{dosing}$ (L/day) must also be reported here.

Measurements of the flow rate ($Q_{Effluent}$; L/day), dissolved weathering product concentration ( $[C^x]_{Effluent}$ ; mol/L), and concentration of suspended feedstock ($FS_{Effluent}$; tonnes/day) are required in the effluent stream. These measurements are used in both the aqueous and solid phase mass balance calculations (see Equations 7 and 12). In addition, measurements of anions are required for determining the amount of non carbonic acid weathering (see Section 8.2.1.2, Equations 16 and 17).

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 initial 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 8.2.2.1 for initial mixing losses.

Measurements of the waste activated sludge are used in both the aqueous and solid phase mass balances (see Section 8.2), as well as for confirming the WWTP is operating in steady state (see Section 8.2.2.1). Required measurements of the WAS include the dissolved weathering product concentration ($[C^x]_{WAS}$; mol/L), flow rate ($Q_{WAS}$; L/day), and concentration of feedstock settled in the WAS ($FS_{WAS}$; 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 required for determining the amount of non carbonic acid weathering (see Section 8.2.1.2, Equations 16 and 17).

To confirm steady state operations, Project Proponents must additionally report the total volume of activated sludge within the control volume (L) , the volume of waste activated sludge removed from the control volume (L/day) and the volume of waste activated sludge recycled through the control volume (L/day).

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 is 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, the true spatial extent of initial 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 high⁴⁴. 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:

1. Safety thresholds are imposed on the effluent characteristics and controlled prior to discharge.
2. Action thresholds are measured at the edge of the mixing zone. Parameters measured are indicators of water quality and/or unintended environmental changes. Action thresholds can be absolute or relative, and can be determined by regulatory limits, relative to natural variability based on historical data or relative to a control site. For example, the action threshold on pH could be 6.5 < pH < 9 or ±0.2 relative to natural variability, whichever is more conservative. Action thresholds may vary seasonally, and should be consistent with monitoring requirements under official permitting.

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.

The following authors from Isometric contributed to writing this Protocol:

• Jing He, Ph.D.
• Ella Holme, Ph.D.
• Jennifer Yin, Ph.D.
• Sophie Gill, Ph.D.
• Emma Marsland

Isometric would like to thank the following reviewers of this Protocol:

• Jeff Peeters
• Kevin Sutherland, Ph.D.
• Adam Ward, Ph.D.