Greenhouse Gas Accounting and Reduction Pathways for Wastewater Treatment Plants: Current Method, Parameter Values and Policy Levers

1. Stated up front: no mandatory dedicated standard exists

This is the point most in need of correction. To date China has no mandatory national standard for greenhouse gas accounting specific to municipal wastewater treatment plants. The only documented method dedicated to this sector is the Technical Guideline for Accounting of Greenhouse Gases under Co-control of Pollutant Removal at Municipal Wastewater Treatment Plants (trial), issued by the Ministry of Ecology and Environment in April 2018. Its foreword states that it is the first release and was developed under the Department of Science, Technology and Standards, which makes it a trial technical guideline rather than a mandatory standard. A 2021 reply by the Ministry of Ecology and Environment to a proposal from a national legislator also states as a next step that it will "study and establish a standard system for accounting greenhouse gases under co-control of pollutant removal at wastewater treatment plants", confirming that no such system existed at that time.

A second point concerns framing. The guideline accounts for the net greenhouse gas reduction under co-control of pollutant removal, not a corporate greenhouse gas emission inventory in the usual sense. Clause 6.3.2 defines the net figure with methane recovery as a negative term and emissions from COD removal, sludge treatment, total nitrogen removal and electricity consumption as positive terms; a negative result indicates net reduction and a positive result net increase. This is not the same as the "what to account and how to account" framing of the GB/T 32150 series, and the two cannot be added together or substituted for one another.

2. Accounting boundary and greenhouse gas types

Chapter 4 defines the boundary:

• Boundary unit: following territorial management, the boundary is the corporate legal entity or the independently accounted local municipal wastewater treatment plant, classified under industry code 4620 of the National Economy Industry Classification (GB/T 4754-2017);
• Treatment technologies: physical methods (gravity separation, centrifugal separation, screening, heat exchange), chemical methods (chemical coagulation, coagulation sedimentation, coagulation dissolved air flotation, neutralization, chemical precipitation, redox) and biological methods (aerobic, anaerobic, combined aerobic-anaerobic);
• Pollutant types: methane (CH4), chemical oxygen demand (COD), total nitrogen (TN) and sludge;
• Greenhouse gas types: methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2);
• Activity data: two cases - pollutant removal quantity used as the activity level for greenhouse gas reduction (with CH4 being both a pollutant and a greenhouse gas), and the quantity of sludge treated in on-site sludge facilities used as the activity level.

Appendix A gives four mechanism statements useful for deciding which stages must be included: anaerobic degradation of organic matter produces CH4; anaerobic digestion of sludge produces large amounts of CH4; N2O is generally regarded as a product of incomplete nitrification or incomplete denitrification, linked to loss or absence of N2O reductase activity in denitrifying bacteria; and CO2 arises as an indirect emission from electricity consumed by the treatment process.

3. Five equations and all parameter values

Chapter 6 provides the following methods and parameters, which can be transferred directly into an accounting sheet:

3.1 Methane recovery (equation 1)

Annual recovered mass = annual recovered volume x 0.717 kg/m3, where 0.717 is the density of CH4 at standard conditions (one standard atmosphere and 0 degrees Celsius). Activity data come from metering instruments.

3.2 Reduction from recovered methane (equation 5)

Reduction = annual recovered CH4 mass x global warming potential of CH4, taken as 21. The guideline explicitly sets the GWP of CH4 at 21.

3.3 Emissions from COD removal (equations 6 and 7)

• Parameters involved: annual COD removal, annual sludge dry matter generation, organic content of sludge dry matter, annual CH4 recovery, CH4 emission factor, and GWP of 21;
• Emission factor = MCF x B0, where MCF (methane correction factor) is 1 for fully anaerobic treatment and 0 for fully aerobic treatment, and B0 (maximum methane producing potential) is 0.25 t CH4 per t COD.

3.4 Emissions from sludge treatment (equation 8)

• DOCf, the fraction of degradable organic carbon in sludge dry matter, is taken as 50%;
• F, the fraction of degradable organic carbon that converts to CH4, is taken as 50%;
• MCF is 1 for fully anaerobic and 0 for fully aerobic conditions;
• The CH4/C molecular weight ratio is taken as 16/12.

3.5 N2O emissions from total nitrogen removal (equation 9)

• The amount of nitrogen converted to nitrous oxide per unit of nitrogen removed: 0 in the aerobic zone and 0.005 t N2O-N per t N in the anoxic zone;
• The N2O/N2 molecular weight ratio is 44/28;
• The global warming potential of N2O is taken as 310.

3.6 CO2 emissions from electricity use (equation 10)

Emissions = annual electricity consumption (MWh/a) x grid CO2 emission factor (t CO2/MWh) x global warming potential of CO2, equal to 1. Table 1 of the guideline gives recommended regional grid factors: North China 0.8843, Northeast China 0.7769, East China 0.7035, Central China 0.5257, Northwest China 0.6671 and South China 0.5271 t CO2/MWh. The table notes that the data come from the 2012 values in the study of average CO2 emission factors of China's regional power grids for 2011 and 2012, and recommends using the latest data published by the competent government authority.

3.7 Estimating sludge generation (equation 4)

Sludge dry matter generation = daily treatment volume x generation coefficient x annual operating days, where EFs, the sludge dry matter generated per 10,000 m3 of wastewater treated per day, has a recommended value of 1.0 to 2.0 t per (10,000 m3 per day), and the guideline permits substituting a measured value.

4. The GWP basis must be declared

The values CH4 = 21 and N2O = 310 used in the guideline are the 100-year global warming potentials from an early IPCC assessment report, and differ from the values updated in later IPCC assessment reports. Two engineering consequences follow. First, carbon figures from different reports cannot be compared directly unless they use the same set of GWP values. Second, when reporting to a competent authority or a carbon market, the GWP values stipulated by that reporting system take precedence and the values of 21 and 310 from the guideline should not be assumed by default. The guideline itself recommends using the latest data for the grid emission factor; the same logic applies to GWP.

5. Quality control: three rates at 90% and cross-checking within 5%

Clause 7.2 gives hard indicators that can be used directly for internal audit:

• Validity: data completeness rate = (records with complete key variables / total records in the database) x 100%;
• Accuracy: data correctness rate = (records free of illegal values, logical errors and missing data / total records) x 100%;
• Authenticity: data traceability rate = (sampled data consistent with verification / total sampled data) x 100%;
• All three indicators must reach 90% or above;
• Within the same reporting period, the difference between data from different sources must be kept within 5% (for example, material consumption verified against purchase invoices and requisition records, electricity consumption verified against meter readings).

Clause 7.1 additionally requires preparing an accounting plan and a monitoring plan with defined monitoring methods, training accounting personnel, verifying data both longitudinally (comparison across years) and laterally (comparison across sources and methods), and promptly testing, controlling, maintaining or replacing measuring instruments that no longer meet requirements.

6. Uncertainty: one formula and one worked example

Appendix C specifies that where an estimate is the product of n estimates, the total uncertainty is Uc = square root of (Us1 squared + Us2 squared + ... + Usn squared). The worked example in Appendix C: a plant with uncertainties of plus or minus 5% for CH4 recovery, 10% for COD removal, 15% for sludge organic content and 3% for the emission factor yields Uc = square root of (5% squared + 10% squared + 15% squared + 3% squared) = 18.9%.

Clause 6.4 lists three sources of uncertainty: lack of completeness (emission mechanisms not fully understood and monitoring results unavailable), data gaps (use of substitute or empirical data) and measurement error (instruments, calibration or imprecise standards). Each data item used should be assessed and annotated against these three sources.

7. Eight parts of the report

Chapter 8 requires the accounting report to contain eight parts: basic information on the enterprise, accounting boundary, data sources, accounting results, uncertainty analysis, quality control and quality evaluation, a statement of authenticity, and annexes, with the detailed format given in Appendix D.

8. Reduction pathways: four that can be found in policy documents

The Methane Emission Control Action Plan, issued on 2023-11-07 by the Ministry of Ecology and Environment and ten other departments, provides pathways and targets directly relevant to wastewater treatment:

• Biogas from sludge anaerobic digestion with enhanced recovery and use: projects with suitable conditions are encouraged to generate biogas by sludge anaerobic digestion and to strengthen recovery and use, with urban sludge reaching a harmless disposal rate of over 90% by 2025;
• Efficient methanization of high-strength organic industrial wastewater: promotion of high-efficiency methane production technologies for high-strength organic industrial wastewater, together with matching high-efficiency treatment technologies;
• Negotiated influent concentrations to reduce methane generation: industries discharging wastewater with high organic content and good biodegradability are encouraged to negotiate influent pollutant concentrations with municipal wastewater treatment plants in accordance with regulations, thereby reducing methane generation;
• Stronger supervision and data quality: full implementation of standards including those for pollutant discharge from municipal wastewater treatment plants, strengthened supervision of methane emission data quality, exploration of satellite remote sensing for detecting abnormal methane emissions, and revision of technical specifications for methane monitoring, accounting, reporting and verification with timely updating of default emission factors.

Read together with the equations in section 3, four levers emerge clearly: recovering methane (a negative term), reducing the MCF of anaerobic stages by converting them to aerobic operation or covering and collecting gas, cutting fugitive emissions from sludge anaerobic digestion, and reducing electricity consumption (indirect CO2). The electricity item can be quantified directly with the regional factors in Table 1 of the guideline, making it the easiest to quantify and to verify.

9. What has changed on the general standard side

The generic basis at enterprise level is GB/T 32150, General guideline of the greenhouse gas emissions accounting and reporting for industrial enterprises. Two facts matter: GB/T 32150-2015 (issued 2015-11-19, effective 2016-06-01) was abolished on 2026-07-01 and fully replaced by GB/T 32150-2025, which was issued on 2025-12-31 and became effective on 2026-07-01. Its competent authority changed from the former National Development and Reform Commission to the Ministry of Ecology and Environment, while the managing body remains the National Technical Committee on Carbon Emission Management Standardization (SAC/TC 548), and the ICS classification changed from 13.020 to 13.020.10. A wastewater treatment plant performing enterprise-level accounting under the general standard must therefore move to the 2025 edition and update its rules for boundary definition and emission factor selection accordingly.

10. Implementation recommendations

1. Clarify the purpose: a co-control net reduction figure (the guideline approach) or an enterprise-level emission inventory (the GB/T 32150-2025 approach). The two must not be combined;
2. Fix the boundary according to Chapter 4 and map CH4, N2O and CO2 against COD, total nitrogen and sludge item by item;
3. Prefer measured values for parameters and fall back on the recommended defaults only when measurements are unavailable (B0 0.25, DOCf 50%, F 50%, EFs 1.0 to 2.0, anoxic zone 0.005);
4. Take grid emission factors and GWP values from the reporting system or the latest provisions of the competent authority, and state the basis in the report;
5. Run internal quality control against the three 90% rates and the 5% cross-check rule, and report uncertainty with the Appendix C formula;
6. Rank reduction options as follows: biogas recovery from sludge anaerobic digestion, gas collection covers on anaerobic stages, electricity reduction, and negotiated influent concentrations.

Note: process parameters are taken from the trial technical guideline issued by the Ministry of Ecology and Environment in April 2018, which is guidance rather than a mandatory standard; the GB/T 32150-2025 facts come from the standard information record and should be confirmed against the official text.