| Emissions Source | Unit | kg CO₂–e/unit | CO₂/unit (kg CO₂–e) | CH₄/unit (kg CO₂–e) | N₂O/unit (kg CO₂–e) |
|---|---|---|---|---|---|
| Water Supply | |||||
| Water supply emission factors (m3) | m3 | 0.0377557 | 0.0365414 | 0.00115538 | 0.0000589379 |
| Water supply emission factors (per capita) | per capita | 4.25025 | 4.11355 | 0.130064 | 0.00663478 |
9 Water supply and wastewater treatment emission factors
Emissions result from energy use in water supply and wastewater treatment plants. Some treatment plants also generate emissions from the treatment of organic matter. We calculated the emission factors using data from Water New Zealand and New Zealand’s Greenhouse Gas Inventory 1990–2024.
9.1 Overview of emission factor changes from 2025 to 2026
| Section | Total EFs | EFs added | EFs removed | EFs changed | Explanation for change |
|---|---|---|---|---|---|
| Wastewater Treatment | 8 | 0 | 0 | 8 | Domestic wastewater parameters changed, including septic tank volume, correction factor, MCF and methane inputs. |
| Water Supply | 2 | 0 | 0 | 2 | WaterNZ population and organic-product inputs changed. |
For detailed information on the emission factor changes, download the Emission factor changes CSV file from Appendix G.
9.2 Water supply
Table 9.2 provides water supply emission factors. We calculated the factors using Water New Zealand data.
9.2.1 GHG inventory development
Users should collect data on cubic metres (m3) of water used, if available. In the absence of this information, the per capita emission factor can be applied.
Applying the equation E = Q x F this means:
- E = emissions from the emissions source in kg CO2-e within the reporting period
- Q = quantity of water used (m3) or persons using water supply (per capita)
- F = appropriate emission factors from Table 9.2
9.2.1.1 WATER SUPPLY: EXAMPLE CALCULATION
An entity’s assets have water meters. Throughout the reporting year they use 1,000 m3 of water.
| Gas | Calculation | Emissions (kg CO₂-e) |
|---|---|---|
| CO₂ emissions | 1,000 x 0.0365414 kg CO₂-e per m3 | 36.5 kg CO₂-e |
| CH₄ emissions | 1,000 x 0.00115538 kg CO₂-e per m3 | 1.16 kg CO₂-e |
| N₂O emissions | 1,000 x 0.0000589379 kg CO₂-e per m3 | 0.0589 kg CO₂-e |
| Total CO₂-e emissions | 1,000 x 0.0377557 kg CO₂-e per m3 | 37.8 kg CO₂-e |
Note: Numbers may not add due to rounding.
9.2.2 Emission factor derivation methodology
We adopted the Water New Zealand 2020/21 National Performance Review1 methodology to calculate the water supply emission factors. The Water New Zealand review gathered data from participating water industry bodies, which represent approximately 75 per cent of New Zealand’s population. 27 participants in the survey provided reliable information on the energy use of their water systems, which was used to calculate national averages. In the 2020/21 period, the operation of water supply pumps used 757 TJ of energy to supply 471 million m3 of water, and treatment plants used an estimated 1130 TJ of energy in the treatment of about 409 million m3 of water. This equates to a median energy intensity of 1.6 megajoules (MJ) of energy per cubic metre of water supplied and 2.8 MJ of energy per cubic metre of water treated.
We used a weighted average of participant energy use and water supply data to calculate the emission factors.
We calculated the emission factors for each gas by summing the weighted averages from each participant’s data. The basic equation for each gas is as follows:
\[ \begin{aligned} \dfrac{ \text{energy use} }{ \text{water supply} } &\times \text{electricity emission factor} \times \text{unit conversion factor} \end{aligned} \]
This equation gives the emissions per m3 of water supplied, where the following values were used to calculate the emission factors:
- energy use = the gigajoule (GJ) of energy used by the water system that year
- water supply = m3 of water supplied that year
- electricity emission factor = the relevant gas emission conversion factor (ie, CO2, N2O, CH4)
- unit conversion factor = 277.778 (converting GJ to kWh).
If entities do not know the volume of water used, they can estimate it based on a calculated per capita (per person) emission factor. To develop a per capita emission factor, we used an average of 116 m3 of water per person per year, which is calculated from the following equations and information:
The first equation:
\[ \begin{aligned} \text{average volume of water supplied per person} &= \dfrac{ \text{water supplied} }{ \text{population served by WWTP} } \end{aligned} \]
The second equation:
\[ \begin{aligned} \text{emission factors for water supplied per capita} &= {} \\ &\text{average volume of water supplied per person} \\ &\times \text{emission factors for water supplied in } \mathrm{m}^3 \end{aligned} \]
Where the following data were used to calculate the emission factors:
9.2.3 Assumptions, limitations and uncertainties
The data adopted from Water New Zealand do not account for emissions outside those associated with the national electricity grid, and therefore, may underestimate the total GHG emissions depending on the water supplier’s facilities and processes.
The assumptions used for water supply per person are inherently uncertain and entities should only use them in the absence of water volume data. They do not account for factors such as: seasonal use of water and water-intensive activities (such as gardening, lifestyle choices and geography). Therefore, per person water supply reflects only an average of the water supply per person. Furthermore, the figure is based on a national average of water usage throughout the year and may overestimate emissions from office use per capita. This is because employees do not spend all their time in the office, and it is likely that most of their water usage will be outside working hours.
9.3 Wastewater treatment
We recommend that users refer directly to the Water New Zealand’s guidelines4 for emission factors for specific types of wastewater treatment plants. Weighted average emission factors for wastewater treatment remain in the measuring emissions guide for general use.
We converted energy use (kWh) to GHG emissions and added these to the treatment process emissions to give the total emissions from wastewater treatment in New Zealand.
We provide wastewater treatment emission factors in Table 9.3 and Table 9.4. Some industries produce wastewater that is particularly high in biological oxygen demand (BOD). For this reason, we developed industrial wastewater emission factors for the meat, poultry, pulp and paper, wine and dairy sectors. Manufacturing entities in these sectors should use specific industrial wastewater factors. All other entities should use the domestic wastewater factors. Where the domestic wastewater treatment type is unknown, we suggest using the average for wastewater treatment plants (see Table 9.3).
| Emissions Source | Unit | kg CO₂–e/unit | CO₂/unit (kg CO₂–e) | CH₄/unit (kg CO₂–e) | N₂O/unit (kg CO₂–e) | Uncertainties |
|---|---|---|---|---|---|---|
| Domestic Wastewater | ||||||
| Average for wastewater treatment plants (m3) | m3 of water supplied | 0.494999 | 0.0628711 | 0.203746 | 0.228381 | The uncertainty for domestic and industrial wastewater methane and nitrous oxide emission factors are ±40% and ±90% respectively |
| Average for wastewater treatment plants (per capita) | per capita | 45.6807 | 5.80203 | 18.8026 | 21.0761 | The uncertainty for domestic and industrial wastewater methane and nitrous oxide emission factors are ±40% and ±90% respectively |
| Septic tanks | per capita | 175.2 | 149.9 | 25.3 | The uncertainty for domestic and industrial wastewater methane and nitrous oxide emission factors are ±40% and ±90% respectively | |
| Emissions Source | Unit | kg CO₂–e/unit | CH₄/unit (kg CO₂–e) | N₂O/unit (kg CO₂–e) | Uncertainties |
|---|---|---|---|---|---|
| Industrial Wastewater | |||||
| Dairy processing | m3 of milk | 0.102242 | 0.102242 | The uncertainty for domestic and industrial wastewater methane and nitrous oxide emission factors are ±40% and ±90% respectively | |
| Wine | tonne of crushed grapes | 5.79403 | 5.79403 | The uncertainty for domestic and industrial wastewater methane and nitrous oxide emission factors are ±40% and ±90% respectively | |
| Meat (excl poultry) | tonne of kills | 52.5761 | 50.05 | 2.52606 | The uncertainty for domestic and industrial wastewater methane and nitrous oxide emission factors are ±40% and ±90% respectively |
| Poultry | tonne of kills | 51.7323 | 48.125 | 3.60731 | The uncertainty for domestic and industrial wastewater methane and nitrous oxide emission factors are ±40% and ±90% respectively |
| Pulp & paper | tonne of product | 11.7936 | 11.7936 | The uncertainty for domestic and industrial wastewater methane and nitrous oxide emission factors are ±40% and ±90% respectively | |
9.3.1 GHG inventory development
Domestic water users should collect data on m3 of water sent to treatment. If metered water data is not available, the per capita emission factor can be applied instead. Industrial entities can calculate the emissions using appropriate activity data and the correlating emission factors.
Applying the equation E = Q x F this means:
- E = emissions from the emissions source in kg CO2-e within the reporting period
- Q = quantity of water treated (m3) or persons using water facilities (per capita)
- F = appropriate emission factors from Table 9.3 and Table 9.4.
9.3.1.1 WASTEWATER: EXAMPLE CALCULATION
During the reporting period an entity uses 100 m3 of water in its offices. They assume that all water is also sent to be treated. This entity also owns a winery that crushed 10 tonnes of grapes during the reporting period.
The office wastewater is domestic, therefore:
| Gas | Calculation | Emissions (kg CO₂-e) |
|---|---|---|
| CO₂ emissions | 100 x 0.0628711 kg CO₂-e per m3 of water supplied | 6.29 kg CO₂-e |
| CH₄ emissions | 100 x 0.203746 kg CO₂-e per m3 of water supplied | 20.4 kg CO₂-e |
| N₂O emissions | 100 x 0.228381 kg CO₂-e per m3 of water supplied | 22.8 kg CO₂-e |
| Total CO₂-e emissions | 100 x 0.494999 kg CO₂-e per m3 of water supplied | 49.5 kg CO₂-e |
The winery wastewater is industrial wastewater (wine), therefore:
| Gas | Calculation | Emissions (kg CO₂-e) |
|---|---|---|
| CO₂ emissions | 10 x NA | NA |
| CH₄ emissions | 10 x 5.79403 kg CO₂-e per tonne of crushed grapes | 57.9 kg CO₂-e |
| N₂O emissions | 10 x NA | NA |
| Total CO₂-e emissions | 10 x 5.79403 kg CO₂-e per tonne of crushed grapes | 57.9 kg CO₂-e |
The total wastewater emissions are:
49.5 kg CO₂-e + 57.9 kg CO₂-e = 107 kg CO₂-e
Note: Numbers may not add due to rounding.
9.3.2 Emission factor derivation methodology
9.3.2.1 Domestic wastewater treatment
We derived the domestic wastewater treatment plant emission factors from the total energy use emissions in the wastewater treatment plants, and the gases emitted during the treatment process.
The emission factors for septic tanks are sourced directly from Water New Zealand (2021).
Since direct carbon dioxide emissions from wastewater treatment are biogenic, the methodologies described here for all treatment types other than septic tanks are only for methane and nitrous oxide. We calculated the emission factors using equations, methodologies and parameters outlined in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, as well as the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories.
To calculate methane emissions, first calculate the total organic product in domestic wastewater (TOW):
\[ \begin{aligned} \text{total organic product in domestic wastewater} &= {} \\ &\sum_i P_i \times \operatorname{BOD} \times I \end{aligned} \]
Where the following data were used to calculate the emission factors:
- P = the population for wastewater treatment plant 𝑖
- 𝑖 = type of treatment plant
- BOD = 26 (kg/capita/year) country-specific, per-capita Biological Oxygen Demand
- I = the correction factor for additional industrial and commercial BOD (default 1.25 or 1 for septic tanks but varies for several sites).
Then calculate methane emissions per capita:
\[ \begin{aligned} \text{methane emissions (kg }\mathrm{CH_4}\text{ per capita)} &= {} \\ &\frac{ \mathrm{MCF} \times \mathrm{B_0} \times \mathrm{TOW} \times \mathrm{GWP} }{ \text{population served} } \end{aligned} \]
Where the following data were used to calculate the emission factors:
- MCF = 0.02528, the weighted-average methane correction factor (MCF) for wastewater treatment plants in 2021
- B0 = 0.625, converts the BOD to maximum potential methane emissions
- TOW = the total organic product in wastewater from the equation above
- GWP = 28 (IPCC AR5), converts methane into CO2-e
- population served = the population served by all wastewater treatment plants.
To calculate methane emissions per water volume, divide methane emissions per capita by the average water volume (m3) treated per capita (101 m3).
To calculate nitrous oxide emissions from wastewater treatment plants we used two equations. The first equation calculates the amount of nitrogen per person:
\[ \begin{aligned} \text{per capita nitrogen in effluent} \;(\text{kg N per year}) &= {} \\ &\text{protein} \times F_{\mathrm{NPR}} \times F_{\mathrm{NON\text{-}CON}} \times F_{\mathrm{IND\text{-}COM}} \end{aligned} \]
Where the following data were used:
- protein = annual per capita protein consumption (36.135 kg per year from Beca, 2007)
- FNPR = fraction of nitrogen in protein (0.16, IPCC 2006)
- FNON-CON = factor for non-consumed protein added to the wastewater (1.4, IPCC 2006)
- FIND-COM = factor for industrial and commercial co-discharged protein into the sewer system (1.25, IPCC 2006).
Table 9.5 details the values used in the equation above.
| Calculation component | Number | Additional information | Source |
|---|---|---|---|
| Population | 1 | This is a per-person calculation | |
| Per capita protein consumption | 36.135 | kg/year | Beca 2007, 99g/day |
| Fraction of N in protein | 0.16 | IPCC default | |
| Fraction of non-consumption protein | 1.4 | IPCC default | |
| Fraction of industrial and commercial co-discharged protein | 1.25 | IPCC default | |
| N removed with sludge | 0 | Default is zero | IPCC default |
Then the second equation calculates nitrous oxide emissions based on the result from the first equation:
\[ \begin{aligned} \mathrm{N_2O}\ \text{emissions (kg CO}_2\text{e per capita)} &= {} \\ &\text{per capita nitrogen in effluent} \\ &\times \mathrm{EF}_{\text{effluent}} \times \frac{44}{28} \times \mathrm{GWP} \end{aligned} \]
Where the following data were used:
- per capita nitrogen in effluent = from equation above
- effluent = emission factor of 5.0e-3 kg N2O-N/kg N (IPCC 2006)
- 44/28 ratio of N2O to N2
- GWP = 265 (IPCC AR5).
Divide these emissions per capita by the average volume of water treated (96 m3) per capita to give the emissions per m3.
9.3.2.2 Industrial wastewater treatment
As with domestic wastewater, we derived the emission factors for industrial wastewater treatment from the total energy use emissions in the wastewater treatment plants and the gases emitted during the treatment process.
For the purpose of this guide, it is assumed there are no direct carbon dioxide emissions from the treatment of wastewater, as all carbon dioxide emissions are biogenic. Therefore, we have calculated only methane and nitrous oxide emissions.
The equation to calculate methane emissions is:
\[ \begin{aligned} \text{methane emission factor}\;(\text{kg}\,\mathrm{CO_2e}\,\text{t}^{-1}) &= {} \\ &\mathrm{mbCOD} \times \mathrm{EF} \times \mathrm{GWP} \end{aligned} \]
Where:
- mbCOD = the unit biodegradable chemical oxygen demand load in kg per tonne of material processed
- EF = CH4 emission factor (kg CH4/kg CODb)
- GWP = global warming potential.
The following tables detail the information used in the calculations to provide the industrial wastewater treatment emission factors.
| Factor | Pulp and paper | Meat (excl poultry) | Poultry | Wine | Source |
|---|---|---|---|---|---|
| Biodegradable chemical oxygen demand load (kg CODb/tonne) | 36 | 50 | 50 | 12.42 | Cardno 2015 |
| CH4 emission factor (kg CH4/kg CODb) | 0.0117 | 0.03575 | 0.034375 | 0.016661 | Cardno 2015 |
| GWP | 28 | 28 | 28 | 28 | IPCC AR5 |
Sources: Cardno 20155
It is assumed that the methods used to treat wastewater from dairy processing do not result in methane emissions.
The equation used to calculate nitrous oxide emissions is:
\[ \begin{aligned} \text{nitrous oxide emission factor} \;(\text{kg}\,\mathrm{CO_2e}\,\text{t}^{-1}) &= {} \\ &\mathrm{mbCOD} \times \mathrm{N{:}COD} \times \mathrm{EF} \times \frac{44}{28} \times \mathrm{GWP} \end{aligned} \]
Where:
- mbCOD = unit biodegradable COD load (kg CODb/t)
- N:COD = total nitrogen to biodegradable COD ratio
- EF = Nitrous oxide emission factor (kg N2O/kg CODb)
- 44/28 = ratio of N2O to N2
- GWP = global warming potential.
Table 9.7 details the information used in the calculations to provide the industrial wastewater treatment emission factors. Note that for dairy processing, users should first convert the quantity of milk to tonnes using a density factor of 1.031 tonnes per m3.
| Factor | Dairy processing | Meat (excl poultry) | Poultry | Source |
|---|---|---|---|---|
| Biodegradable chemical oxygen demand load (kg CODb/tonne) | 2 | 50 | 50 | Cardno 2015 |
| Total N: biodegradable COD ratio | 0.044 | 0.09 | 0.09 | Cardno 2015 |
| Nitrous oxide emission factor (kg N2O/kg CODb) | 0.00279 | 0.001348 | 0.001925 | Cardno 2015 |
| GWP | 265 | 265 | 265 | IPCC AR5 |
Based on the Cardno report6 we assume that there are no nitrous oxide emissions from the methods used to process wastewater from the wine and pulp and paper industries.
9.3.3 Assumptions, limitations and uncertainties
We calculated these emission factors on the available data using industry-wide sources and international default factors where appropriate. As the wastewater emissions include electricity emissions, the same electricity emissions uncertainties carry through. Table 9.8 details the uncertainties with this source category.
| Emission Source | Uncertainty in activity data | Uncertainty in emission factors |
|---|---|---|
| Domestic and industrial CH4 | 10% | 40% |
| Domestic and industrial N2O | 10% | 90% |
Water New Zealand 2020/21 National Performance Review: www.waternz.org.nz/Attachment?Action=Download&Attachment_id=5573.↩︎
Water New Zealand report: www.waternz.org.nz/Attachment?Action=Download&Attachment_id=3142.↩︎
Ministry for the Environment’s wastewater treatment plants database.↩︎
Water New Zealand, Carbon Accounting Guidelines for Wastewater Treatment: CH4 and N2O↩︎
Cardno (2015) Greenhouse Gas Emissions from Industrial Wastewater Treatment – Inventory Basis Review.↩︎
Cardno (2015) Greenhouse Gas Emissions from Industrial Wastewater Treatment – Inventory Basis Review.↩︎