Chapter 6: Wastewater Treatment and Discharge

CHAPTER 6

WASTEWATER TREATMENT AND
DISCHARGE

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Volume 5: Waste

Authors
Deborah Bartram (USA), Michael D. Short (Australia), Yoshitaka Ebie (Japan), Juraj Farkaš (Slovakia), Céline
Gueguen (France), Gregory M. Peters (Sweden), Nuria Mariana Zanzottera (Argentina), M. Karthik (India)

Contributing Authors
Shuhei Masuda (Japan)

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Chapter 6: Wastewater Treatment and Discharge

Contents
6

Wastewater Treatment and Discharge .......................................................................................................6.7
6.1

Introduction .........................................................................................................................................6.7

6.1.1 Centralised treatment systems ......................................................................................................6.10
6.1.2 Decentralised treatment systems of domestic wastewater (onsite sanitation) ...............................6.11
6.1.3 Emissions from receiving waters ..................................................................................................6.12
6.1.4 Changes compared to 1996 Guidelines and Good Practice Guidance .........................................6.12
6.1.5 Changes compared to 2006 IPCC Guidelines ...............................................................................6.12
6.2

Methane emissions from wastewater .................................................................................................6.13
6.2.1 Methodological issues ..................................................................................................................6.13
6.2.2 Domestic wastewater ....................................................................................................................6.14
6.2.2.1 Choice of method ..............................................................................................................6.14
6.2.2.2 Choice of emission factors ................................................................................................6.18
6.2.2.3 Choice of activity data .......................................................................................................6.21
6.2.2.4 Time series consistency .....................................................................................................6.29
6.2.2.5 Uncertainties......................................................................................................................6.29
6.2.2.6 QA/QC, Completeness, Reporting and Documentation ....................................................6.30
6.2.3 Industrial wastewater ....................................................................................................................6.30
6.2.3.1 Choice of method ..............................................................................................................6.30
6.2.3.2 Choice of emission factors ................................................................................................6.33
6.2.3.3 Choice of activity data .......................................................................................................6.35
6.2.3.4 Time series consistency .....................................................................................................6.35
6.2.3.5 Uncertainties......................................................................................................................6.35
6.2.3.6 QA/QC, Completeness, Reporting and Documentation ....................................................6.35

6.3

Nitrous oxide emissions from domestic wastewater ..........................................................................6.35
6.3.1 Methodological issues ..................................................................................................................6.35
6.3.1.1 Choice of method ..............................................................................................................6.36
6.3.1.2 Choice of emission factors ................................................................................................6.38
6.3.1.3 Choice of activity data .......................................................................................................6.39
6.3.2 Time series consistency ................................................................................................................6.43
6.3.3 Uncertainties .................................................................................................................................6.43
6.3.4 QA/QC, Completeness, Reporting and Documentation ...............................................................6.44

6.4

Nitrous oxide emissions from industrial wastewater .........................................................................6.44
6.4.1 Methodological issues ..................................................................................................................6.44
6.4.1.1 Choice of method ..............................................................................................................6.44
6.4.1.2 Choice of emission factors ................................................................................................6.47
6.4.1.3 Choice of activity data .......................................................................................................6.47
6.4.2 Time series consistency ................................................................................................................6.48
6.4.3 Uncertainties .................................................................................................................................6.49
6.4.4 QA/QC, Completeness, Reporting and Documentation ...............................................................6.49

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Appendix 6A.1 Non-biogenic (fossil) CO2 emissions from wastewater treatment and discharge: Basis for
Future Methodological Development......................................................................................6.51
Annex 6A.1

Summary data for pit latrine use, no sanitation facility, and groundwater use by country......6.54

Annex 6A.2

Derivation of the maximum CH4 producing potential (Bo) for domestic wastewater .............6.57

Annex 6A.3

Estimation of default methane conversion factors for CH4 in centralised wastewater treatment
plants treating domestic wastewater ........................................................................................6.58

Annex 6A.4

Calculation of MCF for methane emissions from sewage discharges .....................................6.59

Annex 6A.5

Estimation of default emission factors for N2O in domestic wastewater treatment plants ......6.61

Annex 6A.6

Estimation of default emission factors for N2O in effluent .....................................................6.63

Annex 6A.7

List of countries by region included in Table 6.10a ................................................................6.64

References................................................................................................................................................6.65

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Chapter 6: Wastewater Treatment and Discharge

Equations
Equation 6.1 (Updated) CH4 emissions from domestic wastewater for each treatment/discharge pathway orsystem, j ......................................................................................................................6.17
Equation 6.1a (New)

Total CH4 emissions from domestic wastewater treatment and discharge ..................6.17

Equation 6.2

CH4 emission factor for each domestic wastewater treatment/discharge pathway orsystem .........................................................................................................................6.18

Equation 6.3 (Updated) Total organically degradable material in domestic wastewater ...................................6.21
Equation 6.3a (New)

Total organics in domestic wastewater by treatment/discharge pathway or system ...6.22

Equation 6.3b (New)Organic component removed as sludge from aerobic treatment plants .......................6.27

Equation 6.3c (New)Organic component removed as sludge from septic systems ......................................6.28

Equation 6.3d (New)

Total organics in treated domestic wastewater effluent .............................................6.28

Equation 6.4

Total CH4 emissions from industrial wastewater ........................................................6.33

Equation 6.5

CH4 emission factor for industrial wastewater ............................................................6.33

Equation 6.9 (Updated) N2O emissions from domestic wastewater treatment plants........................................6.37
Equation 6.7 (Updated) N2O emissions from domestic wastewater effluent.....................................................6.38
Equation 6.10 (New)

Total nitrogen in domestic wastewater by treatment pathway ....................................6.40

Equation 6.10a (New)

Estimation of protein consumed ..................................................................................6.41

Equation 6.8 (Updated) Total nitrogen in domestic wastewater effluent ..........................................................6.42
Equation 6.11 (New)N2O emissions from industrial wastewater treatment plants .......................................6.44

Equation 6.12 (New)N2O emissions from industrial wastewater effluent ....................................................6.47

Equation 6.13 (New)

Total nitrogen in industrial wastewater .......................................................................6.47

Equation 6.14 (New)

Total nitrogen in industrial wastewater effluent ..........................................................6.48

FiguresFigure 6.1 (Updated)

Wastewater treatment systems and discharge pathways ...............................................6.8

Figure 6.1a (New)

Percentage of low-income country populations using pit latrines as a primary sanitation
facility .........................................................................................................................6.12

Figure 6.2 (Updated)

Decision tree for CH4 emissions from domestic wastewater.......................................6.15

Figure 6.3 (Updated)

Decision tree for CH4 emissions from industrial wastewater treatment ......................6.32

Figure 6.4 (New)

Decision tree for N2O emissions from domestic wastewater ......................................6.37

Figure 6.5 (New)

Nitrogen in domestic wastewater treatment ................................................................6.40

Figure 6.6 (New)

Decision tree for N2O emissions from industrial wastewater ......................................6.46

Figure 6A.1 (New)

Correlation between influent total nitrogen (TN) loading and N2O emissions ...........6.61

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TablesTable 6.1 (Updated)

CH4 and N2O emission potentials for wastewater and sludge treatment and discharge
systems ..........................................................................................................................6.9

Table 6.2

Default maximum CH4 producing capacity (Bo) for domestic wastewater .................6.18

Table 6.3 (Updated)

Default MCF values and resultant EFs for domestic wastewater by type of treatment
system and discharge pathway, j .................................................................................6.20

Table 6.4

Estimated BOD5 values in domestic wastewater for selected regions andcountries ......................................................................................................................6.22

Table 6.5

Suggested values for urbanisation (U) and degree of utilisation of treatment, discharge
pathway or method (Ti,j) for each income group for selected countries ......................6.24

Table 6.6

Example of the application of default values for degrees of treatment utilisation (T)by income groups ........................................................................................................6.26

Table 6.6a (New)Removal of organic component from wastewater as sludge (Krem) according totreatment type ..............................................................................................................6.27

Table 6.6b (New)

Wastewater treatment organics removal fractions (TOWREM) according to treatment
type ..............................................................................................................................6.28

Table 6.7 (Updated)

Default uncertainty ranges for domestic wastewater ...................................................6.29

Table 6.8 (Updated)

Default MCF values and resultant EFs for industrial wastewater ...............................6.34

Table 6.8a (New)

Default EF values for domestic and industrial wastewater .........................................6.39

Table 6.10a (New)

Default factors for domestic wastewater .....................................................................6.41

Table 6.10b (New)

Estimate on use of food waste disposal in sewer ........................................................6.42

Table 6.10c (New)

Wastewater treatment nitrogen removal fractions (NREM) according to treatment
type ..............................................................................................................................6.43

Table 6.11 (Updated)

N2O methodology default data ....................................................................................6.43

Table 6.12 (New)

Examples of industrial wastewater data ......................................................................6.48

Table 6.13 (New)

Default uncertainty ranges for industrial wastewater ..................................................6.49

Table 6Ap.1 (New)

Summary of literature investigating fossil organic carbon in wastewater ...................6.52

Table 6A.1 (New)

Summary data for pit latrine use, no sanitation facility, and groundwater use
by country ...................................................................................................................6.54

Table 6A.2 (New)MCFs based on measured CH4 in full-scale domestic wastewater treatment plants ...6.58

Table 6A.3 (New)

Summary of literature investigating methane emissions from wastewater
discharge .....................................................................................................................6.60

Table 6A.4 (New)

Default N2O emission factors for domestic wastewater treatment plants ...................6.61

Table 6A.5 (New)N2O emission factors in full-scale domestic wastewater treatment plants ..................6.62

Table 6A.6 (New)

List of countries by region included in Table 6.10a ....................................................6.64

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Chapter 6: Wastewater Treatment and Discharge

6 WASTEWATER TREATMENT AND
DISCHARGE
Users are expected to go to Mapping Tables in Annex 1, before reading this chapter. This is required to correctly
understand both the refinements made and how the elements in this chapter relate to the corresponding chapter
in the 2006 IPCC Guidelines.

6.1

INTRODUCTION

Wastewater can be a source of methane (CH4) when treated or disposed anaerobically or when dissolved CH4enters aerated treatment systems. It can also be a source of nitrous oxide (N2O) emissions. Carbon dioxide (CO2)emissions from wastewater are not considered in the IPCC Guidelines because these are generally derived frommodern (biogenic) organic matter in human excreta or food waste and should not be included in national total
emissions. Appendix 6Ap.1 presents a discussion of non-biogenic (fossil) CO2 emissions from wastewater
treatment and discharge, where fossil organic carbon is present in wastewater or treatment sludge.
Wastewater originates from a variety of domestic, commercial and industrial sources and may be treated on site
(uncollected), sewered to a centralised plant (collected) or disposed untreated nearby or via an outfall. Domestic
wastewater is defined as wastewater from household water use, while industrial wastewater is from industrial
practices only. 1 Treatment and discharge systems can sharply differ between countries and for rural and urban
areas. Also, treatment and discharge systems can differ for rural and urban users, and for urban high income andurban low-income users.
Sewer systems may consist of networks of open channels or closed underground pipes. Occasional stagnant
conditions and heat provide favourable anaerobic condition for methane generation in closed and open sewers. In
urban areas in developing countries and some developed countries, sewer systems may consist of networks ofopen canals, gutters, and ditches, which are referred to as open sewers. These systems are subject to heating fromthe sun and the sewers may be stagnant allowing for anaerobic conditions to emit CH4 (Doorn et al. 1997). In
most developed countries and in high-income urban areas in other countries, sewers are usually closed andunderground. Wastewater in closed underground sewers likely generate CH4, but there are insufficient data
available to quantify the emissions from these collection systems. However, research shows that significant
amounts of CH4 can be formed within closed sewer collection systems and enters centralised wastewater
treatment plants (WWTPs) as dissolved CH4 in the wastewater, where it is then emitted during treatment.
The degree of wastewater treatment varies in most developing countries. In some cases, industrial wastewater isdischarged directly into bodies of water, while major industrial facilities may have comprehensive in-plant
treatment. Domestic wastewater is treated in centralised plants, pit latrines, septic systems or disposed of in
unmanaged lagoons or waterways, via open or closed sewers. In some coastal cities domestic wastewater isdischarged directly into the ocean. Pit latrines are lined or unlined holes of up to several metres deep, which may befitted with a toilet for convenience. The pits are used alternatively, and the contents used as manure after few
months’ usage.
The most common wastewater treatment methods in developed countries are centralised aerobic wastewater
treatment plants and lagoons for both domestic and industrial wastewater. To avoid high discharge fees or tomeet regulatory standards, many large industrial facilities pre-treat their wastewater before releasing it into thesewage system. Domestic wastewater may also be treated in on-site septic systems. These are advanced systems
that may treat wastewater from one or several households. They consist of an anaerobic underground tank and adrainage field for the treatment of effluent from the tank. Some developed countries continue to dispose of
untreated domestic wastewater via an outfall or pipeline into a water body, such as the ocean.
Centralised wastewater treatment methods can be classified as primary, secondary, and tertiary treatment. In
primary treatment, physical barriers remove larger solids from the wastewater. Remaining particulates are thenallowed to settle. Secondary treatment consists of a combination of biological processes that promotebiodegradation of wastewater constituents by microorganisms. Secondary treatment processes include aerobic
stabilisation ponds, trickling filters, and activated sludge processes, as well as anaerobic reactors and lagoons.
Tertiary treatment processes are used to further purify the wastewater of pathogens, contaminants, and remaining
1 Because the methodology is on a per person basis, emissions from commercial wastewater are estimated as part of domestic

wastewater. To avoid confusion, the term municipal wastewater is not used in this text. Municipal wastewater is a mix ofhousehold, commercial and non-hazardous industrial wastewater, treated at wastewater treatment plants.

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nutrients such as nitrogen and phosphorus compounds. This is achieved using one or a combination of processes
that can include maturation/polishing ponds, biological processes, advanced filtration, carbon adsorption, ion
exchange, and disinfection.
Sludge is produced in all of the primary, secondary and tertiary stages of treatment. Sludge that is produced inprimary treatment consists of solids that are removed from the wastewater.
Sludge produced in secondary and tertiary treatment results from biological growth in the biomass, as well as thecollection of small particles. This sludge must be treated further before it can be safely disposed of. Methods ofsludge treatment include aerobic and anaerobic stabilisation (digestion), conditioning, centrifugation, composting,and drying. Some sludge is incinerated before land disposal. Emissions from anaerobic sludge digestion, where
the digester’s primary use is for treatment of wastewater treatment solids, should be reported under Wastewater
Treatment. Land disposal, composting, and incineration of sludge is considered in Volume 5, Section 2.3.2 in
Chapter 2, Waste Generation, Composition, and Management Data, Section 3.2 in Chapter 3, Solid Waste
Disposal, Section 4.1 in Chapter 4, Biological Treatment and Disposal, and Chapter 5, Incineration and OpenBurning of Waste, respectively. N2O emissions from sludge and wastewater spread on agricultural land areconsidered in Section 11.2, N2O emissions from managed soils, in Chapter 11, N2O Emissions from Managed
Soils, and CO2 Emissions from Lime and Urea Application, in Volume 4 of the Agriculture, Forestry, and Other
Land Use (AFOLU) Sector.
Figure 6.1 from the 2006 IPCC Guidelines has been updated and simplified to show the different pathways for
wastewater treatment and discharge, for wastewater that is not collected and for wastewater that is collected. The
figure specifies whether the discharge or end use of the wastewater is reported in this chapter. Figure 6.1(Updated) also shows sludge treatment pathways in grey and clarifies whether the emissions are reported in thischapter. Table 6.1 has been updated to reflect the main wastewater treatment and discharge systems in developed
and developing countries and their potential to emit CH4 and N2O.
Figure 6.1 (Updated) Wastewater treatment systems and discharge pathways
Uncollected wastewater
Untreated
Decentralised treatment
Latrines
Septic tanks
Constructed wetlands

Freshwater, estuarine or marine environments(Emissions reported in Volume 5, Chapter 6)
Agricultural soils(Emissions reported in Volume 4, Chapter 11)
Land disposal of wastewater(Emissions reported in Volume 5, Chapter 3)

Onsite industrial treatment plant
Collected wastewater
Untreated

Wastewater

Centralised treatment
Aerobic
Anaerobic
Convention activated sludge
Biological nutrient removal
Constructed wetlands

Sludge treatment offsite(Emissions reported in Volume 5, Chapter 2)
Incineration(Emissions reported inVolume 5, Chapter 5)

Composting(Emissions reported inVolume 5, Chapter 4)

Landfilling(Emissions reported inVolume 5, Chapter 3)

Land application(Emissions reported inVolume 4, Chapter 11)

Sludge treatment at wastewater treatment plants(Emissions reported in Volume 5, Chapter 6)

Sludge6.8

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Chapter 6: Wastewater Treatment and Discharge

TABLE 6.1 (UPDATED)
CH4 AND N2O EMISSION POTENTIALS FOR WASTEWATER AND SLUDGE TREATMENT AND DISCHARGE SYSTEMS

Untreated or Treated Systems

While modulated by oxygen status, CH4 is generated
in a range of freshwater and estuarine environments.
Among them, stagnant or oxygen deficientenvironments are probable sources of N2O.

Non-aquatic environment (soils)

Emissions are considered in Volume 4 when applied
to agricultural land.

Sewers (closed and underground)

Likely source of CH4/N2O. However, insufficient data
exist to quantify emission factors that address thevariation in sewer type and operational conditions.

Sewers (open)

Stagnant, overloaded open collection sewers or
ditches/canals are likely significant sources of CH4.

Onsite sludge treatment1

Anaerobic treatment

Collected
Treated

CH4 and N2O emission potentials

Freshwater, estuarine, or marinedischarge

Aerobic treatment

Untreated

Discharge from Collected or
Uncollected Systems

Types of treatment and disposal

Centralised aerobic
wastewater treatment plants

May produce limited CH4 from anaerobic pockets.
May also liberate CH4 generated in upstream sewer
networks during turbulent and/or aerobic treatment
processes.
Poorly designed or managed aerobic treatment
systems produce higher CH4 due to reduced removalof organics in sludge during primary treatment.
Plants with nutrient removal processes are sources of
CH4 and N2O.

Aerobic shallow ponds

Unlikely source of CH4/N2O.
Poorly designed or managed aerobic systems produce
CH4.

Anaerobic lagoons

May be a significant source of CH4.
Insignificant source of N2O.

Facultative lagoons2

Source of CH4 from anaerobic layer.

Constructed wetlands

May be source of CH4 and N2O. See 2013 Supplement
to the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories: Wetlands (IPCC 2014).

Anaerobic reactors

May be a significant source of CH4 if emitted CH4 is
not recovered or flared.

Sludge anaerobic treatment
in centralised aerobic
wastewater treatment plant

Sludge may be a significant source of CH4 if emitted
CH4 is not recovered or flared. In addition, sludge
digestion and handling may be a source of fugitive
CH4 from biogas recovery operations. See Chapter 4
for more details.

Composting

Emissions are considered in Volume 5, Chapter 4.

Incineration and open
burning

Emissions are considered in Volume 5, Chapter 5.

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TABLE 6.1 (UPDATED) (CONTINUED)
CH4 AND N2O EMISSION POTENTIALS FOR WASTEWATER AND SLUDGE TREATMENT AND DISCHARGE SYSTEMS

Uncollected

Types of treatment and disposal

CH4 and N2O emission potentials

Septic tanks (without dispersion field)

Source of CH4.
Frequent solids removal reduces CH4 production.

Septic system (including a septic tank and asoil dispersal system)

Source of CH4 (tank) and N2O (soil dispersal system).
Frequent solids removal reduces CH4 production.

Open pits/Latrines

Pits/latrines are likely to produce CH4 whentemperature and retention time are favourable.

1
For onsite sludge treatment, see Chapters 4 and 5 for emissions methodology, but note that emissions for onsite systems should bereported under the Wastewater Treatment and Discharge category.
2
Facultative organisms can function in the presence or absence of molecular oxygen. In a facultative lagoon, the layer of water near thesurface contains dissolved oxygen due to atmospheric reaeration and algal respiration, a condition that supports aerobic and facultative
organisms. The bottom layer of the lagoon includes sludge deposits and supports anaerobic organisms. The intermediate anoxic layer–
the facultative zone–ranges from aerobic near the top to anaerobic at the bottom (US EPA 2002b).

6.1.1

Centralised treatment systems

Centralised wastewater treatment systems may include a variety of treatment processes spanning the hierarchy oftreatment levels. Primary treatment involves mechanical processes such as screening, grit removal andsedimentation. Secondary treatment involves biological processes in which microorganisms convert inorganicand organic nutrients into settleable solids which can be removed by sedimentation and degradation metabolites
which are transferred to the atmosphere. Secondary biological treatment processes may be augmented withchemical disinfection. Commonly used secondary treatment processes include activated sludge, trickling filters
and lagoons. In some countries simple disinfection process concludes tertiary treatment; however, elsewheremore advanced tertiary treatment involves the use of enhanced biological nutrient removal processes(nitrification–denitrification), and other advanced physical and chemical processes.
Wastewater treatment processes can range from simple technologies such as lagooning and wetlands, to more
technologically advanced treatment technology designed for stringent nutrient removal. Wetlands can beconstructed or semi-natural systems and may be used as the primary method of wastewater treatment, or as apolishing treatment step following settling and biological treatment. Constructed wetlands develop naturalprocesses that involve vegetation, soil, and associated microbial assemblages to trap and treat incoming
contaminants through a combination of biodegradation, volatilisation, sorption and sedimentation process (IPCC2014).
Sewer collection systems provide an environment conducive to the formation of CH4, which can be substantial
depending on the configuration and operation of the collection system (Guisasola et al. 2008). Recent research
has shown that at least a portion of CH4 formed with the collection system enters the centralised system where it
contributes to CH4 emissions from the treatment system (Foley et al. 2015). Although there are insufficient data
to quantify emissions directly from the sewer collection system, the emission factors presented for centralised
treatment plants account for dissolved methane entering the treatment systems.
Soluble organic matter is generally removed using biological processes in which microorganisms consume theorganic matter for maintenance and growth. The resulting biomass (sludge) is removed from the effluent prior todischarge to receiving environments. Microorganisms can biodegrade soluble organic material in wastewater
under aerobic and/or anaerobic conditions, where the latter condition produces CH4. During collection andtreatment, wastewater may be accidentally or deliberately managed under anaerobic conditions. In addition, thesludge may be further biodegraded under aerobic or anaerobic conditions, such as with anaerobic digestion.
The generation of N2O may also result from the treatment of domestic wastewater during both nitrification anddenitrification of the nitrogen (N) present, usually in the form of urea, proteins, and ammonia. Ammonia N isconverted to nitrate (NO3−) through the aerobic process of nitrification. Denitrification occurs underanoxic/anaerobic conditions, whereby aerobic or facultative organisms reduce oxidized forms of nitrogen (e.g.,nitrite, nitrate) in the absence of free oxygen to produce nitrogen gas (N2). N2O is an intermediate product ofboth nitrification and denitrification processes. No matter where N2O is formed it is usually stripped to the air in
aerated parts of the treatment process.
A common example of a centralised treatment system configuration is an activated sludge wastewater treatment
system. Following grit removal and primary treatment for solids removal, wastewater is sent to an activatedsludge reactor for carbonaceous biochemical oxygen demand removal, followed by secondary clarification where
solids are allowed to settle from the wastewater. Clarified effluent may be disinfected prior to discharge.

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Chapter 6: Wastewater Treatment and Discharge

Secondary clarifier sludge is pumped out from the bottom of the clarifier. Of this sludge, a portion is sent back to
the activated sludge treatment process (return activated sludge) and the remainder (waste activated sludge) is
combined with primary treatment sludge before being sent to sludge handling (such as gravity thickening). The
thickened sludge may also be processed onsite in an anaerobic digester followed by further dewatering, such as
by centrifuge. Recovered wastewater from thickening and dewatering operations, such as filtrate from the gravity
thickener or centrate from the centrifuge) are returned to the influent stream at the headworks to the wastewater
treatment system.
CH4 generated in the sewer system enters the treatment plant as dissolved methane and is emitted during
turbulent/aerated treatment steps (Daelman et al. 2012). Although the CH4 emissions from centralised aerobic
plants may be small, they are not zero and may be substantial for some plants receiving sewage from expansive
sewer networks (Willis 2017). In addition, anaerobic conditions can form within the treatment system increasing
the potential for CH4 generation from an otherwise aerobic system. These refinements introduce a new MCF
associated with these systems, as well as a revised N2O emission factor for centralised wastewater treatment
systems.
Occasionally wastewater treatment plants could be hydraulically or organically overloaded, resulting in degraded
plant performance. In these cases, organic matter and nutrients that would normally be removed by the treatment
system instead pass through untreated and are discharged to the aquatic environment. The 2006 IPCC Guidelines
previously provided different emission factors for “well managed” centralised aerobic treatment systems versus“not well managed” centralised aerobic treatment systems. However, these refinements remove those factors andinstead provide one factor for these types of systems. In the case of overloaded systems, the amount of organics
removed by the system in sludge (S) should reflect the performance achieved by the system.

6.1.2

Decentralised treatment systems of domestic
wastewater (onsite sanitation)

Depending on national circumstances, domestic wastewater not connected to a centralised wastewater treatment
plant may be directly discharged in natural aquatic environments (rivers, lakes, oceans, etc.) or treated using
onsite sanitation. The most common onsite treatment systems are holding tanks, septic systems and latrines.
A septic system is usually composed of a septic tank, generally buried in the ground, and a soil dispersal system.
Solids and dense materials contained in the incoming wastewater (influent) settle in the septic tanks as sludge.
Floatable material (scum) is also retained in the tank. The sludge settled on the bottom of the tank undergoes
anaerobic digestion. Partially treated water is discharged in the dispersal system. The liquid fraction remains in
the tank for only a short period, with the hydraulic retention time (HRT) varying from 24 to 72 hours dependingon tank volume and hydraulic load. The solid fraction accumulates and remains in the tank for several years,
during which time it degrades anaerobically. The solids retention time (SRT) depends on the sludge withdrawal
frequency. The gas produced from anaerobic sludge digestion (mainly CH4 and largely biogenic CO2) rise to theliquid surface and are usually released through vents. Gases produced in the effluent dispersal system (mainly
N2O and biogenic CO2) are released through the soil.
A latrine usually consists of a slab over a pit which may be two metres or more in depth. A wide range ofconfiguration options exists for latrines (simple pit latrines, ventilated latrines, composting latrines, etc.) havingin common that little (e.g., pour flush latrines) or no water is used to flush excreta into the pit. Pit latrines are
utilised by more than 1.5 billion people throughout the world, especially in low-income countries (see new
Figure 6.1a and new Annex 6A.1). Pit latrine gaseous emissions depend in part on local groundwater level.
Anaerobic conditions favourable to CH4 emissions occur when the water table is high and the organic waste in
the pit is submerged.
In the absence of latrines, people resort to open defecation. Open defecation is not considered as a source of CH4,
as anaerobic conditions are considered unlikely.
In some high-income countries, onsite aerated wastewater treatment systems are used and enable a moreadvanced level of treatment than septic tanks in reducing the load of organics and nutrients in domestic effluent.
The process usually involves a first step of sedimentation and anaerobic digestion, a second step of aerobic
treatment and last step of clarification and disinfection. The treated effluent is discharged into the environment
via surface irrigation or infiltration through an absorption trench.

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Figure 6.1a (New) Percentage of low-income country populations using pit latrines as a
primary sanitation facility (Graham & Polizzotto 2013)

6.1.3

Emissions from receiving waters

Dissolved CH4 and N2O that is generated in sewers, or present in untreated or treated discharges, has thepotential to be released (Short et al. 2014; Short et al. 2017). A strong correlation between the condition of theaquatic environment and the generation of CH4 and N2O has been observed (e.g., Smith et al. 2017). Therefore,
where wastewater is then discharged to aquatic environments with nutrient-impacted/eutrophic conditions (i.e.,water bodies which are rich in nutrients and very productive in terms of aquatic animal and plant life), theadditional organic matter in the discharged wastewater is expected to increase emissions. Many waterways arenaturally eutrophic, while others have been altered by human impacts and are subject to eutrophication as a result.
Surface marine waters are typically supersaturated (Ward et al. 1987; Conrad & Seiler 1988) and freshwaters
highly supersaturated (Stanley et al. 2016) with CH4 irrespective of their trophic state, so this refinement does
not distinguish between eutrophic and oligotrophic receiving waters, but it does distinguish between default
waters (rivers and streams) and lakes and reservoirs where CH4 emissions are higher.

6.1.4

Changes compared to 1996 Guidelines and Good
Practice Guidance

No refinement.

6.1.5

Changes compared to 2006 IPCC Guidelines

The 2006 IPCC Guidelines included combined equations to estimate CH4 emissions from wastewater and fromsludge removed from the wastewater. However, in some cases, this combined equation caused confusion among
inventory compilers when calculating CH4 emissions from aerobic systems with anaerobic sludge digestion. In
these cases, some compilers estimated zero CH4 emissions from well operated wastewater treatment systems,
and then subtracted emissions associated with sludge digestion operations without first estimating the CH4
emissions from sludge treatment, resulting in negative emissions. In this refinement, we discuss the proper way
to use the equation in such situations, and we present an update to provide guidance on the calculation of theorganic component removed in sludge. Countries must estimate the amount (mass) of sludge they generate fromwastewater treatment and default data are provided for a number of countries. With this additional guidance, theuse of a default value of zero for sludge removal from aerobic treatment systems and septic systems is no longer
applicable.
In addition, certain emission factors for CH4 emissions from domestic and industrial wastewater treatment havebeen updated to reflect additional measurement data on emissions from septic systems and centralised WWTPs.
Furthermore, the CH4 emission factors for wastewater discharged to aquatic environments have been updated
and a new emission factor for discharge to reservoirs, lakes, and estuaries is introduced. The calculation of CH4
emissions from effluent discharged to aquatic systems has been updated to include the discharge of treatedeffluent and to reflect the removal of organics that occurs during treatment.

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Chapter 6: Wastewater Treatment and Discharge

The 2019 Refinement also includes new guidance on how to estimate N2O emissions from domestic andindustrial wastewater and presents updated guidance to estimate N2O emissions from centralised WWTPs.
Furthermore, the N2O emission factors for wastewater discharged to aquatic environments have been updated
and the calculation of N2O emissions from effluent discharged to aquatic systems has been updated to reflect theremoval of nitrogen that occurs during treatment.

6.2

METHANE EMISSIONS FROM WASTEWATER

6.2.1

Methodological issues

Emissions are a function of the amount of organic waste generated and an emission factor that characterises theextent to which this waste generates CH4.
Three tier methods for CH4 from this category are summarised below:
The Tier 1 method applies default values for the emission factor and activity parameters. This method is
considered good practice for countries with limited data.
The Tier 2 method follows the same method as Tier 1 but allows for incorporation of a country specific emission
factor and country specific activity data. For example, a specific emission factor for a prominent treatment
system based on field measurements could be incorporated under this method. The amount of sludge removed
for incineration, landfills, composting, and agricultural land should be taken into consideration. In addition,
countries that are able to categorize wastewater discharge by the type of waterbody should use Tier 2 emission
factors for estimating emissions from discharge.
For a country with good data and advanced methodologies, a country specific method could be applied as a Tier
3 method. For example, a more advanced country-specific method could be based on plant-specific data fromlarge wastewater treatment facilities, using country-specific measurements of organics discharged to aquatic
environments, or may draw on country-specific water quality data for aquatic environments receiving wastewater
inputs.
Wastewater treatment facilities that receive wastewater from collection systems, particularly pressurized sewers
and gravity-fed sewers that are closed, can liberate CH4 in aerobic systems from dissolved CH4 that enters thetreatment system (Daelman et al. 2012; Short et al. 2017).
Wastewater treatment facilities can include anaerobic process steps. CH4 generated at such facilities can berecovered and combusted in a flare or energy device. The amount of CH4 that is flared or recovered for energy
use should be subtracted from total emissions through the use of a separate CH4 recovery parameter. The amountof CH4 which is recovered or flared is expressed as R in updated Equation 6.1. R refers to CH4 recovered fromwastewater treatment, such as methane captured on filters exhausted from covered (anaerobic) treatment ponds,
as well as CH4 recovered from anaerobic sludge digestion. The approach to estimation of emissions fromwastewater treatment in this chapter covers all emission sources and sinks (recovery) at a wastewater treatment
plant.
Note that only a few countries may have sludge removal data and CH4 recovery data. The information on sludge
generation has improved and statistical data on sludge can be found in databases of Eurostat and Organization for
Economic Co-operation and Development (OECD). The lack of data for CH4 recovery was taken into account inthis refinement by recommending the use of the methodology in Section 4.1, Chapter 4 of Volume 5, 2006 IPCC
Guidelines which estimates emissions on the basis of total sludge anaerobically digested; thus, the information
on R is not required in this calculation. Default sludge removal factors are provided in this refinement. The
default for CH4 recovery is zero. If a country selects to report CH4 recovery, it is good practice to distinguishbetween flaring and CH4 recovery for energy generation, which should be reported in the Energy Sector taking
into account the avoidance of double counting emissions from flaring and energy used.
Emissions from flaring are not significant, as the majority of CO2 emissions are of biogenic origin, and the CH4
and N2O emissions are very small so good practice in the Waste Sector does not require their estimation.
However, if it is wished to do so these emissions should be reported under the Waste Sector. A discussion ofemissions from flares and more detailed information are given in Volume 2, Energy, Chapter 4.2. Emission fromflaring is not treated at Tier 1.

2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

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Volume 5: Waste

6.2.2

Domestic wastewater

6.2.2.1

C HOICE OF METHOD

An updated decision tree for domestic wastewater is included as updated Figure 6.2 and should be used todetermine the tier approach that is applicable to the country.
This section is an update to the 2006 IPCC Guidelines. In general, the overall steps for good practice ininventory preparation for CH4 from domestic wastewater have been updated as follows:

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2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Discharge

Figure 6.2 (Updated) Decision tree for CH 4 emissions from domestic wastewaterStart

Are wastewater
treatment pathways
characterised?

Are activity data
available to categorise
discharge by type ofwaterbody?

No

Collect data on
the share ofwastewater
treatment ineach pathway

Yes

Yes

Is a countryspecific method
available?

No

No

Tier 1 for discharge:
Estimate emissions from discharge
to all aquatic environments using
default Tier 1 emission factor andmethodology.

Tier 2 for discharge:
Estimate emissions from discharge
to aquatic environments using
default Tier 2 emission factors andmethodology.

Yes

Tier 3 for discharge:
Estimate emissions from discharge
to aquatic environments using
country-specific emissions data
and methodology.

Are facility-specificCH4 emissions data
available?

Yes

No

Is a countryspecific method
available?

Yes

Tier 3 for treatment:
Estimate emissions fromtreatment using country-specificmethod based on measured
emissions data from facilities.

No
Are countryspecific emission factors
available for the key
pathways?

Yes

Tier 2 for treatment:
Estimate emissions using
country-specific emission
factors (Bo, MCF, etc.) and/or
activity data and default
methodology.

No

Is this a key
category1?

Yes

No

Develop countryspecific emissions
factors and/orcollect activity data

Tier 1 for treatment:
Estimate emissions fromtreatment using default
emission factors andmethodology.

1

See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section
4.1.2 on limited resources), for discussion of key categories and use of decision trees.

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Volume 5: Waste

Step 1:

Use updated Equation 6.3 to estimate organically degradable material TOW in wastewater prior totreatment. Use new Equation 6.3a to estimate total organics in domestic wastewater for eachwastewater treatment/discharge pathway or system, j (TOWj). Use new Equation 6.3d to estimate
total organics in treated wastewater effluent discharged (TOWEFFtreat).

Step 2:

Use new Equations 6.3b and 6.3c to estimate the amount of organic component removed in sludge, S,from aerobic treatment plants and septic systems.

Step 3:

Select the pathway and systems (see updated Figure 6.1) according to country activity data. Use
Equation 6.2 or the updated Table 6.3 to obtain the emission factor for each domestic wastewater
treatment/discharge pathway or system.

Step 4:

Use updated Equation 6.1 to estimate emissions and adjust for possible sludge removal and/or CH4
recovery of treatment/discharge pathway or system, j, in inventory year. Use new Equation 6.1a tosum the emissions across all treatment/discharge pathways or systems.

Step 5:Use Equation 4.1 and emission factors in Table 4.1 to estimate methane emissions from anaerobic
digestion of sludge.

To determine the use of each type of treatment or discharge system, it is good practice to refer to national
statistics (e.g., from regulatory authorities). If these data are not available, wastewater associations orinternational organisations such as the World Health Organization (WHO) may have data on the system usage.
Otherwise, consultation with sanitation experts can help, and expert judgment can also be applied (see Chapter 2,
Approaches to Data Collection, in Volume 1). Urbanisation statistics may provide a useful tool, e.g., city sizes
and income distribution.
These updates are presented because, in some cases, the original Equation 6.1 caused confusion among inventory
compilers when calculating CH4 emissions across multiple wastewater treatment/discharge pathways or systems
across multiple income groups. These updates allow for a more stepwise process in estimating emissions
throughout the country.
It is good practice for countries to treat the wastewater treatment system and onsite sludge treatment system as
separate pathways. As an example, for an activated sludge treatment process, calculate the emissions directly
associated with the aerobic treatment system as one pathway, and calculate the emissions and report any CH4
recovery directly associated with the anaerobic sludge digestion system as a separate pathway using theemissions methodology provided in Table 4.1, Chapter 4 of Volume 5. Net emissions from both systems should
be summed together and reported under wastewater treatment and discharge. In no circumstances should acountry report negative emissions. As discussed in Chapter 4, the reporting of anaerobic digestion of sludge isunder biological treatment only if transferred from the wastewater treatment plant to anaerobic facilities codigesting sludge with MSW or other waste.
It is important that CH4 emissions from sludge that is managed using landfills, incineration, composting, biogas
production, or used in agriculture are not included in the wastewater treatment and discharge category. The data
should be consistent across the sectors, and categories, amount disposed at SWDS, applied to agricultural land,
incinerated or used elsewhere should be equal to the amount organic component removed as sludge in updated
Equation 6.1. Wastewater and sludge that is applied on agricultural land should be considered in Volume 4 for
AFOLU Sector, Section 11.2, N2O emissions from managed soils, in Chapter 11, N2O Emissions from Managed
Soils, and CO2 Emissions from Lime and Urea Application.
Wastewater treatment system/pathway usage often differs for rural and urban residents. Also, in developing
countries, there are likely to be differences between urban high-income and urban low-income residents. Hence,emissions are calculated by each income group fraction. It is good practice to treat the three categories: rural
population, urban high-income population, and urban low-income population separately. It is suggested to use aspreadsheet, as shown in Table 6.5.
Equation 6.1 should be used to estimate CH4 emissions from every treatment system and discharge pathway j(hereafter referred to as treatment/discharge pathway or system), presented in Table 6.3 (Updated) that areappropriate for the country, including the discharge of treated or untreated wastewater.
CH4 emissions from the following discharge pathways must be considered in the inventory for treated anduntreated wastewaters (if occurring in the country):•

Discharge to reservoirs, lakes, and estuaries;•

Discharge to aquatic environments other than to reservoirs, lakes, and estuaries;•

Discharge to sewers (with a distinction between stagnant and flowing (open or closed) sewers).

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Chapter 6: Wastewater Treatment and Discharge

For the discharge of treated wastewater, the TOW should reflect the organics in the wastewater as discharged(see Equation 6.3d (New). As noted in the table, discharges to soil should be reported in Volume 4.
CH4 emissions from the following wastewater treatment systems must be considered in the inventory (ifoccurring in the country):•

Centralised, aerobic treatment plant;•

Anaerobic reactor (e.g., upflow anaerobic sludge blanket digestion (UASB));•

Anaerobic lagoons (with a distinction between shallow and facultative lagoons and deep lagoons);•

Constructed wetlands;•

Septic systems (with a distinction between stand-alone septic tanks and septic tanks with land dispersal
field);•

Latrines (depending of the climate).

Emissions from anaerobic digestion of wastewater treatment sludge should be estimated using Equation 4.1 andincluded in the sum of emissions using Equation 6.1A (New). The emission factor used in Equation 4.1 accounts
for CH4 recovery, therefore the amount of methane recovered from anaerobic digestion should not be subtracted.

EQUATION 6.1 (UPDATED)CH4 EMISSIONS FROM DOMESTIC WASTEWATER FOR EACH TREATMENT/DISCHARGE PATHWAY
OR SYSTEM, J
(TOW j − S j ) • EFj − R j 
CH 4 Emissions=j

Where:
CH4 Emissionsj = CH4 emissions from treatment/discharge pathway or system, j, in inventory year, kg
CH4/yrTOWj= organics in wastewater of treatment/discharge pathway or system, j, in inventory year,
kg BOD/yr. See Equation 6.3a.

Sj= organic component removed from wastewater (in the form of sludge) fromtreatment/discharge pathway or system, j, in inventory year, kg BOD/yr. See Equations
6.3b and 6.3c. For wastewater discharged to aquatic environments, there is no sludge
removal (Sj = 0) and no CH4 recovery (Rj = 0). For wastewater treatment systems, pleasesee Section 6.2.2.3 for additional guidance on how to estimate S, organic component
removed as sludge, if country-specific data are not available.

j= each treatment/discharge pathway or systemEFj= emission factor for treatment/discharge pathway or system, j, kg CH4/kg BOD. See
Equation 6.2 or updated Table 6.3.

Rj

= amount of CH4 recovered or flared from treatment/discharge pathway or system, j, ininventory year, kg CH4/yr. Default value is zero.
EQUATION 6.1A (NEW)TOTAL CH4 EMISSIONS FROM DOMESTIC WASTEWATER TREATMENT AND DISCHARGE=
CH 4 Emissions∑ [CH Emissions ] • [10 ]
j

4

j

−6

Where:
CH4 Emissions= CH4 emissions in inventory year, Gg CH4/yr

CH4 Emissionsj = CH4 emissions from treatment/discharge pathway or system, j, in inventory year, kg
CH4/yrj= each treatment/discharge pathway or system

10-6= conversion of kg to Gg

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Volume 5: Waste

6.2.2.2

C HOICE OF EMISSION FACTORSThis section represents an update to Section 6.2.2.2 of the 2006 IPCC Guidelines.
The emission factor for a wastewater treatment and discharge pathway and system is a function of the maximum
CH4 producing potential (Bo) and the methane correction factor (MCF) for the wastewater treatment anddischarge system, as shown in Equation 6.2. The Bo is the maximum amount of CH4 that can be produced from agiven quantity of organics (as expressed in Biochemical Oxygen Demand (BOD) or Chemical Oxygen Demand(COD) in the wastewater and represents the complete conversion of organic C to biogas. The MCF indicates theextent to which the CH4 producing capacity (Bo) is realised in each type of treatment and discharge pathway andsystem. Thus, it is an indication of the degree to which the system is anaerobic.

EQUATION 6.2
CH4 EMISSION FACTOR FOR
EACH DOMESTIC WASTEWATER TREATMENT/DISCHARGE PATHWAY OR SYSTEMEF=Bo • MCFjj
Where:EFj= emission factor, kg CH4/kg BODj= each treatment/discharge pathway or systemBo= maximum CH4 producing capacity, kg CH4/kg BOD

MCFj= methane correction factor (fraction). See updated Table 6.3.

Table 6.2 includes default maximum CH4 producing capacity (Bo) for domestic wastewater. It is good practice touse country-specific data if available for emission factors, which are made up of Bo and MCF values. If a country
chooses to introduce country-specific data for Bo based on independent wastewater analyses, they must also
update the MCF because the MCFs presented in Table 6.3 were developed using the default Bo values. For
domestic wastewater, a COD-based value of Bo can be converted into a BOD-based value by multiplying with afactor of 2.4. New Annex 6A.2 provides further explanation of the basis for these default Bo values to allow
countries to consider if these values are appropriate for the specific characteristics of their waste streams.
TABLE 6.2
DEFAULT MAXIMUM CH4 PRODUCING CAPACITY (BO) FOR DOMESTIC WASTEWATER
0.6 kg CH4/kg BOD
0.25 kg CH4/kg COD
Based on expert judgment by lead authors and on Doorn et al. (1997)

Because the Bo and MCF values must be used together, updated Table 6.3 now also includes the resultant default
CH4 emission factors for each wastewater treatment and discharge pathway. In addition, the MCFs in Table 6.3of the 2006 IPCC Guidelines have been updated to reflect revisions to the following specific wastewater
treatment and discharge pathways and systems (see also new Annex 6A.3).

DISCHARGE FROM TREATED OR UNTREATED SYSTEMS
Updated Table 6.3 presents updated default MCFs associated with the discharge of wastewater to a water bodyand it is good practice to apply the Tier 1 MCF to discharges of both treated and untreated wastewater. The BOD
of treated wastewater is typically 5–25 mg/L (Hammer & Hammer Jr. 2012; Tchobanoglous et al. 2014), which
provides a substrate for the formation of CH4 in a receiving water body. Furthermore, much of the dissolved CH4that is generated in sewers is released either in the treatment plant headworks or to the receiving water body inthe case of untreated or primary treated discharges (Short et al. 2014; Short et al. 2017). Recent evidence points
to the operation of both microbial and non-microbial methanogenic pathways in nature (e.g., Jugold et al. 2012)and strong relationships between the nutrient status of a receiving water body and the rate of generation of CH4
have been observed (e.g., Smith et al. 2017). Despite this relationship, most rivers, estuaries and coastal waters
are considerably supersaturated with CH4, irrespective of their nutrient status (Patra et al. 1998; Grunwald et al.
2009; Ward et al. 2017), while open oceans are slightly supersaturated (Tilbrook & Karl 1995; Oudot et al. 2002;
Castro-Morales et al. 2014). Supersaturated conditions occur when the rate of methanogenesis exceeds the rate at
which the CH4 is oxidised and/or transferred to the atmosphere and is important as it governs the driving force

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2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Discharge(mass transfer coefficient) and likely water-to-air CH4 emission from a receiving environment. So, while theconditions of the receiving water body play a modulating role in relation to the rate of methanogenesis, theaddition of organic matter from sewer discharges is generally expected to increase CH4 emissions in freshwaterand coastal environments. Recent reviews of measurements of CH4 generation indicate that a significant
proportion of the CH4 emitted from freshwater systems has its origins in carbon deposited on sediments (Deemer
et al. 2016) which is one reason why methanogenesis is more intense in lakes and reservoirs than fast-flowing
rivers.
Using the same stoichiometric relationship as was used to calculate the default Bo value (see new Annex 6A.2), a
ratio of 0.938 kg C per kg COD is obtained. Deemer et al. (2016) performed an extensive review ofmeasurements of CH4 and CO2 originating in aquatic systems, and independent measurements of both CH4 andCO2 emission fluxes around the world. Using these data in combination with information on the partitioning of
global carbon flows in freshwater systems Tranvik et al. (2009) and a default Bo of 0.25 kg CH4/kg COD, a Tier2 MCF of 0.035 is calculated for rivers. Where it is possible to separately identify discharges to slow-flowing
systems such as lakes and reservoirs, a Tier 2 MCF of 0.19 is presented (see updated Table 6.3 and new Annex
6A.4).
If countries are not able to collect activity data regarding the distribution of discharge to reservoirs, lakes, andestuaries as compared to discharges to other aquatic environments, they should use the default Tier 1 factors. The
default Tier 1 MCF for CH4 generation for discharges to all aquatic environments is presented as the mean of the
two Tier 2 MCFs, or 0.11.
Concerning decentralised treatment systems for domestic wastewater, CH4 and N2O emissions from effluent
infiltration into soil must be considered.

CENTRALISED, AEROBIC TREATMENT PLANTS
The MCF for centralised aerobic treatment plants has been updated in Table 6.3 to reflect the potential for
generation of CH4 from these systems. In addition, there are no longer separate MCFs for “well managed” and“not well managed” systems and it is good practice to estimate CH4 from all centralised, aerobic treatment plants.
If country-specific data are available to differentiate whether systems are overloaded or not well managed, thesesituations should be reflected in the calculation of TOW (for inflow overload) or Smass (for systems that are notwell managed and therefore not achieving the expected removal of sludge).

IMPACT OF TEMPERATURE ON DECENTRALISED TREATMENT SYSTEMS
OF DOMESTIC WASTEWATER
Temperature affects wastewater treatment processes, in particular decentralised systems where no external
supplemental heat is provided (uncontrolled temperature) and anaerobic digestion for which the optimal
temperature is 30–38°C. At lower temperatures, the rate of anaerobic digestion decreases and CH4 production
becomes unlikely below 12°C. Inside septic tanks, the temperature is uncontrolled and is related to atmospherictemperature as well as volumes of household hot and cold water used and discharged. There may also be a
gradient of temperature inside the septic tank, with warmer conditions at the bottom (sludge layer) and colder at
the top (Leverenz et al. 2010). Therefore, in countries having seasonal temperature variability, when thetemperature in septic tanks cools, the rate of digestion slows, the SRT increases, sludge accumulates, and CH4
emissions decrease. When the liquid temperature warms, the rate of digestion increases, sludge accumulated
during the cold season decomposes, gas solubility in the liquid decreases and CH4 emissions increase. Thissituation can produce a ‘spring boil’ phenomenon, wherein warmer weather conditions give rise to increased
anaerobic microbial activity, increased gas production, and decreased solids removal efficiency due to theresuspension of settled and incoming solids. Accordingly, there is a seasonal variability of CH4 emissions(Leverenz et al. 2010); however, at this time, insufficient data exist to establish a temperature-dependent
emission factor associated with these systems. Countries that experience significant seasonal temperature
variations may wish to consider the development of a country-specific emission factor.

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Volume 5: Waste

TABLE 6.3 (UPDATED)DEFAULT MCF VALUES AND RESULTANT EFS FOR DOMESTIC WASTEWATER BY TYPE OF TREATMENT SYSTEM AND
DISCHARGE PATHWAY, J1
Type of treatment
and discharge
pathway or system

Comments

MCF 1(Range)EF2(kg CH4/kg
BOD)

EF2
(kg CH4/kg
COD)

Discharge from treated or untreated system
Discharge toaquatic
environments (Tier1)

Most aquatic environments including rivers
are supersaturated in CH4. Nutrient
oversupply will increase CH4 emissions.
Environments where carbon accumulates in
sediments have higher potential formethane generation.

0.11(0.004 – 0.27)

0.068

0.028

Discharge toaquatic
environments
other thanreservoirs, lakes,
and estuaries (Tier2)

Most aquatic environments including rivers
are supersaturated in CH4. Nutrient
oversupply will increase CH4 emissions.

0.0353(0.004 – 0.06)

0.021

0.009

Discharge toreservoirs, lakes,
and estuaries (Tier2)

Environments where carbon accumulates in
sediments have higher potential formethane generation.

0.193(0.08 – 0.27)

0.114

0.048

Discharge to soil

Sludge and/or wastewater discharge to soil
may be a source of CH4 for fertilisation

Stagnant sewer

Open and warm

Flowing sewer(open or closed)

Fast moving, clean. (Insignificant amountsof CH4 from pump stations, etc.)

Emissions reported in Volume 4
0.5(0.4 – 0.8)

0.3

0.125

0

0

0

0.034
(0.003 – 0.09)

0.018

0.0075

Wastewater treatment system

Centralised, aerobic
treatment plantSome CH4 can be emitted from settlingbasins and other anaerobic pockets. May
also emit CH4 generated in upstream sewer
networks during turbulent and/or aerobic
treatment processes. For treatment plantsthat are receiving wastewater beyond thedesign capacity, inventory compilers
should judge the amount of organic
material removed in sludge accordingly.

Anaerobic reactor(e.g., upflow
anaerobic sludge
blanket digestion(UASB))

CH4 recovery is not considered here.

0.8(0.8 – 1.0)

0.48

0.2

Anaerobic shallow
lagoon andfacultative lagoons

Depth less than 2 metres, use expert
judgment.

0.2(0 – 0.3)

0.12

0.05

Anaerobic deep
lagoonDepth more than 2 metres

0.8(0.8 – 1.0)

0.48

0.2

Constructedwetlands

See 2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories:
Wetlands (IPCC 2014)

Septic tank

Septic tanks emit CH4

0.55(0.4 – 0.72)

0.3

0.125

Septic tank + land
dispersal field

Septic tanks emit CH4; negligible emissions
come from land dispersal field

0.55(0.4 – 0.72)

0.3

0.125

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Chapter 6: Wastewater Treatment and Discharge

TABLE 6.3 (UPDATED) (CONTINUED)DEFAULT MCF VALUES AND RESULTANT EFS FOR DOMESTIC WASTEWATER BY TYPE OF TREATMENT SYSTEM AND
DISCHARGE PATHWAY, J1
Comments

MCF 1(Range)EF2(kg CH4/kg
BOD)

EF2
(kg CH4/kg
COD)

LatrineDry climate, ground water table lower thanlatrine, small family (3–5 persons)0.1(0.05 – 0.15)

0.06

0.025

LatrineDry climate, ground water table lower thanlatrine, communal (many users)

0.5(0.4 – 0.6)

0.3

0.125

LatrineWet climate/flush water use, ground watertable higher than latrine

0.7(0.7 – 1.0)

0.42

0.175

Type of treatment
and discharge
pathway or system

Sludge treatment system
Anaerobic digester
for sludge

See Chapter 4 for emissions methodology

See Chapter 4, Table 4.1

Composting

Emissions reported in Volume 5, Chapter 4

See Chapter 4, Table 4.1

Incineration andopen burning

Emissions reported in Volume 5, Chapter 5

See Chapter 5

Sources:1

Based on expert judgment by Lead Authors of this section.

2

Emission factors calculated using default Bo and default MCF.

3

See Annex 6A.4.

See Annex 6A.3 (Czepiel et al. 1993; Kozak et al. 2009; Bellucci et al. 2010; Wang et al. 2011; Daelman et al. 2013; Kyung et al.
2015; Delre et al. 2017).
4

5

Leverenz et al. 2010; Diaz-Valbuena et al. 2011; Truhlar et al. 2016.

6.2.2.3

C HOICE OF ACTIVITY DATA

The activity data for this source category is the total amount of organically degradable material in the wastewater(TOW). This parameter is a function of human population and BOD generation per person. It is expressed interms of biochemical oxygen demand (kg BOD/year). The equation for TOW is:
EQUATION 6.3 (UPDATED)TOTAL ORGANICALLY DEGRADABLE MATERIAL IN DOMESTIC WASTEWATER

TOW =P • BOD • 0.001 • 365

Where:
TOW= total organics in wastewater in inventory year, kg BOD/yr

P= country population in inventory year, (person)

BOD= country-specific per capita BOD5 in inventory year, g/person/day. See Table 6.4.

0.001

= conversion from grams BOD to kg BOD

Total population statistics should be readily available from national statistics agencies or international agencies(e.g., United Nations Statistics, see https://population.un.org/wpp/). Table 6.4 includes BOD default values forselected countries. It is good practice to select a BOD default value from a nearby comparable country whencountry-specific data are not available. The degree of urbanisation for a country can be retrieved from various
sources, (e.g., Global Environment Outlook, United Nations Environment Programme and World Development
Indicators, World Health Organization). The urban high-income and urban-low income fractions can be
determined by expert judgment when statistical or other comparable information is not available.

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TABLE 6.4
ESTIMATED BOD5 VALUES IN DOMESTIC WASTEWATER FOR SELECTED REGIONS AND COUNTRIES
BOD5(g/person/day)

Range

Reference

Africa37

35 – 45

1

Egypt

34

27 – 411

Asia, Middle East, Latin America

40

35 – 45

1

India

34

27 – 411West Bank and Gaza Strip (Palestine)

50

32 – 68

1

Japan

42

40 – 45

1

Brazil50

45 – 55

2

Canada, Europe, Russia, Oceania

60

50 – 701

Denmark62

55 – 681

Germany

62

55 – 68

1

Greece57

55 – 60

1

Italy

60

49 – 60

3

Sweden75

68 – 82

1

Turkey38

27 – 50

1

United States

85

50 – 120

4Country/Region

Note: These values are based on an assessment of the literature. Please use national values, if available.
Reference:
1.

Doorn and Liles (1999).

2.

Feachem et al. (1983).

3.

Masotti (1996).

4.

Metcalf and Eddy (2003).

This section is updated to include a new equation for the calculation of total organics in wastewater (TOWj) bytreatment/discharge pathway or system (see new Equation 6.3a).
EQUATION 6.3A (NEW)TOTAL ORGANICS IN DOMESTIC WASTEWATER BY TREATMENT/DISCHARGE PATHWAY OR
SYSTEMTOW=
j∑ [TOW • U • T • I ]
i

i

ij

j

Where:
TOWj= total organics in wastewater in inventory year, kg BOD/yr, for income group i andtreatment/discharge pathway or system, j.

TOW= total organics in wastewater in inventory year, kg BOD/yr. See updated Equation 6.3 for
TOW in wastewater prior to treatment or wastewater that is discharged without treatment andnew Equation 6.3d for TOW in treated wastewater effluent.

Ui= fraction of population in income group i in inventory year. See Table 6.5.

Tij

= degree of utilisation of treatment/discharge pathway or system, j, for each income group
fraction

Ij= correction factor for additional industrial BOD discharged into treatment/discharge pathway
or system j (for collected the default is 1.25, for uncollected the default is 1.00)

The factor I values in new Equation 6.3a are from the 2006 IPCC Guidelines. It expresses the BOD fromindustries and establishments (e.g., restaurants, butchers or grocery stores) that is co-discharged with

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Chapter 6: Wastewater Treatment and Discharge

domestic wastewater. In some countries, information from industrial discharge permits may be available
to improve I. Otherwise, expert judgment is recommended.
Table 6.5 includes default values of Ui and Ti,j for selected countries.

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Volume 5: Waste

TABLE 6.5
SUGGESTED VALUES FOR URBANISATION (U) AND DEGREE OF UTILISATION OF TREATMENT, DISCHARGE PATHWAY OR METHOD (TI,J) FOR EACH INCOME GROUP FOR SELECTED COUNTRIESUrbanisation(U) 1
Fraction of Population
Country
AfricaNigeria
Egypt
Kenya
South Africa
Asia
China
India
Indonesia
Pakistan
Bangladesh
Japan
EuropeRussiaGermany5
United Kingdom
FranceItaly
North America
United States
CanadaLatin America
and Caribbean
BrazilMexicoOceania
Australia and
New Zealand

6.24

Rural urban-high2 urban-low2

Septic
Tank

Degree of utilisation of treatment or discharge pathway or method for each income group (Ti,j )3
U=ruralU= urban high income
U=urban low income
Septic
SepticLatrine Other Sewer4 None
Latrine Other Sewer4 None
Latrine Other Sewer4 None
Tank
Tank

0.52
0.57
0.62
0.39

0.10
0.09
0.08
0.12

0.38
0.34
0.30
0.49

0.02
0.02
0.02
0.10

0.28
0.28
0.28
0.28

0.04
0.04
0.04
0.04

0.10
0.10
0.10
0.10

0.56
0.56
0.56
0.48

0.32
0.15
0.32
0.15

0.31
0.05
0.31
0.15

0.00
0.10
0.00
0.00

0.37
0.70
0.37
0.70

0.00
0.00
0.00
0.00

0.17
0.17
0.17
0.17

0.24
0.24
0.24
0.24

0.05
0.05
0.05
0.05

0.34
0.34
0.34
0.34

0.20
0.20
0.20
0.20

0.59
0.71
0.54
0.65
0.72
0.20

0.12
0.06
0.12
0.07
0.06
0.80

0.29
0.23
0.34
0.28
0.22
0.00

0.00
0.00
0.00
0.00
0.00
0.20

0.47
0.47
0.47
0.47
0.47
0.00

0.50
0.10
0.00
0.00
0.00
0.50

0.00
0.10
0.10
0.10
0.10
0.30

0.3
0.33
0.43
0.43
0.43
0.00

0.18
0.18
0.18
0.18
0.18
0.00

0.08
0.08
0.08
0.08
0.08
0.00

0.07
0.07
0.00
0.00
0.00
0.10

0.67
0.67
0.74
0.74
0.74
0.90

0.00
0.00
0.00
0.00
0.00
0.00

0.14
0.14
0.14
0.14
0.14
0.10

0.10
0.10
0.10
0.10
0.10
0

0.03
0.03
0.03
0.03
0.03
0

0.68
0.53
0.53
0.53
0.53
0.90

0.05
0.20
0.20
0.20
0.20
0

0.27
0.06
0.10
0.24
0.32

0.73
0.94
0.90
0.76
0.68

0.00
0.00
0.00
0.00
0.00

0.30
0.20
0.11
0.37
0.42

0.10
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00

0.60
0.80
0.89
0.63
0.58

0.00
0.00
0.00
0.00
0.00

0.10
0.05
0.00
0.00
0.04

0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00

0.90
0.95
1.00
1.00
0.96

0.00
0.00
0.00
0.00
0.00

NA
NA
NA
NA
NA

NA
NA
NA
NA
NA

NA
NA
NA
NA
NA

NA
NA
NA
NA
NA

NA
NA
NA
NA
NA

0.22
0.20

0.78
0.80

0.00
0.00

0.90
0.90

0.02
0.02

0.00
0.00

0.08
0.08

0.00
0.00

0.05
0.05

0.00
0.00

0.00
0.00

0.95
0.95

0.00
0.00

NA
NA

NA
NA

NA
NA

NA
NA

NA
NA

0.16
0.25

0.25
0.19

0.59
0.56

0.00
0.00

0.45
0.45

0.00
0.00

0.10
0.10

0.45
0.45

0.00
0.00

0.20
0.20

0.00
0.00

0.80
0.80

0.00
0.00

0.00
0.00

0.40
0.40

0.00
0.00

0.40
0.40

0.20
0.20

0.08

0.92

0.00

0.90

0.02

0.00

0.08

0.00

0.05

0.00

0.00

0.95

0.00

NA

NA

NA

NA

NA

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Chapter 6: Wastewater Treatment and Discharge

TABLE 6.5 (CONTINUED)SUGGESTED VALUES FOR URBANISATION (U) AND DEGREE OF UTILISATION OF TREATMENT, DISCHARGE PATHWAY OR METHOD (TI,J) FOR EACH INCOME GROUP FOR SELECTED COUNTRIES
Notes:
1.
Urbanization projections for 2005 (United Nations, 2002).
2.
Suggested urban-high income and urban low income division. Countries are encouraged to use their own data or best judgment.
3.
Ti.j values based on expert judgment, (Doorn and Liles, 1999).
4.
Sewers may be open or closed, which will govern the choice of MCF, see Table 3.3
5.
Destatis, 2001.
Note: These values are from the literature or based on expert judgment. Please use national values, if available.

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Volume 5: Waste

ExampleTable 6.6 includes an example. Categories with negligible contributions are not shown. Note that the table caneasily be expanded with a column for MCF for each category. The degree of urbanization for this country is 65
percent.
TABLE 6.6
EXAMPLE OF THE APPLICATION OF DEFAULT VALUESFOR DEGREES OF TREATMENT UTILISATION (T) BY INCOME GROUPS
Treatment or discharge system or pathway
Urban high-income

Urban low-income
Rural

T (%)

Notes

To sea10

No CH4

To aerobic plant1

20

Add industrial component

To septic systems

10

Uncollected

To sea10

Collected

To pit latrines

15

Uncollected

To rivers, lakes, sea

15

To pit latrines

15

To septic tanks

5

Total

100%

UncollectedMust add up to 100%

The degree of treatment utilisation (T) does not take into account emissions associated with the discharge oftreated effluent.
1

Reference: Doorn and Liles (1999)

This section is updated to also include new equations for the calculation of organic components removed as
sludge from aerobic treatment plants and from septic systems (see Equations 6.3b and 6.3c, respectively). Notethat the estimate of TOWij in Equation 6.3a is specific to the wastewater treatment system or pathway. Inventory
compilers should consider that sludge recovered from septic tanks may be transferred to centralised WWTPs. In
these cases, it is good practice to include this additional organic load when estimating TOW in influent to thecentralised WWTP.
The organic component removed from wastewater as sludge, S, in Equations 6.1 and 6.4, is not explained indetail in the 2006 IPCC Guidelines. This section provides an update through the introduction of new Equations
6.3b and 6.3c, which provide default calculations of S for aerobic treatment plants and septic systems,
respectively. The default value of S for all other systems is zero. For aerobic treatment systems, some inventory
compilers incorrectly defined the variable “S” in Equation 6.1 as the mass of sludge removed rather than theorganic component removed from wastewater as sludge. It is important to note that the organic component
removed from wastewater as sludge is not equivalent to the mass (tonnes) of sludge produced from wastewater
treatment. Instead, the organic component removed as sludge is a function of sludge produced from wastewater
treatment (Smass) and a sludge factor (Krem) which indicates how much organic matter is removed from thetreatment process in sludge per kilogram of sludge produced.

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Chapter 6: Wastewater Treatment and Discharge

EQUATION 6.3B (NEW)ORGANIC COMPONENT REMOVED AS SLUDGE FROM AEROBIC TREATMENT PLANTS

Saerobic = ( Smass • K rem • 1000 )Where:
Saerobic= organic component removed from wastewater (in the form of sludge) in aerobic treatment
plants, kg BOD/yr

Smass= amount of raw sludge removed from wastewater treatment as dry mass, tonnes/year

Krem= sludge factor, kg BOD/kg sludge. See Table 6.6a.

1000= conversion factor for tonnes to kilograms

New Table 6.6a provides factors associated with the amount of organics in wastewater removed in sludge.
Aerobic WWTPs with primary treatment only remove about 0.5 kg of influent BOD per kg of primary sludge bysedimentation. The aerobic stage of treatment removes 1.5 kg BOD per kg of secondary sludge. The sludge
produced in aerobic WWTPs with primary sedimentation contains approximately 70 percent of primary sludge
and 30percent of secondary sludge by weight. Aerobic WWTPs without primary treatment remove about 1.16 kg
BOD per kg of sludge, depending on process type. Countries where data are available should estimate a countryspecific Krem as a ratio between BOD removed and sludge generated for each process listed in Table 6.6a.
Country-specific Krem should be estimated as a weighted average by individual processes.
TABLE 6.6A (NEW)REMOVAL OF ORGANIC COMPONENT FROM WASTEWATER AS SLUDGE (KREM) ACCORDING TO TREATMENT
TYPE1,2

Default
Treatment TypeRange

(kg BOD/kg dry masssludge)Mechanical treatment plants (primary sedimentation sludge)

0.5

0.4 – 0.6

Aerobic treatment plants with primary treatment (mixed primary and secondary
sludge, untreated or treated aerobically)

0.8

0.65 – 0.95

Aerobic treatment plants with primary treatment and anaerobic sludge digestion(mixed primary and secondary sludge, treated anaerobically)

1.0

0.8 – 1.2

Aerobic wastewater treatment plants without separate primary treatment

1.16

1.0 – 1.5

Sources:1
BOD, or biochemical oxygen demand, is a proxy indicator of the quality (or organic content) of the waste, but it is not a
direct mass measurement of the organic content. Just as the mass of oxygen in carbon dioxide is greater than the mass ofcarbon, it is feasible for the BOD of a carbon-containing molecule to exceed 1 when the molecule is metabolised to carbon
dioxide.
2
Based on expert judgment by Lead Authors of this section using the following references: Pescod (1992); Davies (2005);
Foladori et al. (2010); WEF (2010); Wisconsin Department of Natural Resources (2010); Serón et al. (2011).

Concerning septic systems, emissions depend on the fraction of the population managing their septic tank inaccordance with the sludge removal instructions of their septic tank/system (F in new Equation 6.3c). The default
value for F is 0.5 and corresponds to the situation where 50percent of the population managing their septic
system are complying with the sludge removal instructions. This default value is for countries where there is noeffective regulation or administrative requirements for sludge removal in septic systems. In countries with such
regulations or requirements, some evidence of maintenance controls should be provided (for instance theexistence of a local public service responsible for onsite sanitation). It is good practice to assess the F value
using available data on sludge removal practices among the population using septic systems.

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Volume 5: Waste

EQUATION 6.3C (NEW)ORGANIC COMPONENT REMOVED AS SLUDGE FROM SEPTIC SYSTEMS=S septic TOWseptic • F • 0.5
Where:
Sseptic= organic component removed from wastewater (in the form of sludge) in septic systems, kg
BOD/yr

TOWseptic = total organics in wastewater in septic systems inventory year, kg BOD/yr
F= fraction of the population managing their septic tank in compliance with the sludge removal
instruction of their septic system

0.5= fraction of organics in wastewater removed in sludge when septic tank is managed inaccordance with sludge removal instructions

For discharges of treated wastewater, inventory compilers should estimate the amount of TOW in effluent(TOWEFFLUENT) as shown in new Equation 6.3d.
EQUATION 6.3D (NEW)TOTAL ORGANICS IN TREATED DOMESTIC WASTEWATER EFFLUENTTOW=EFFtreat∑ TOW • T • (1 − TOWj

REM , jj)Where:TOWEFFtreat= total organics in the treated wastewater effluent discharged to aquatic environments ininventory year, kg BOD/yr

TOW= total organically degradable material in domestic wastewater in inventory year, kg
BOD/yr. See Equation 6.3 (Updated).

TJ= degree of utilisation of treatment system j in inventory year ( ∑ Tij ). See Table 6.5.
ij= each wastewater treatment type used in inventory year

TOWREM,j= fraction of total wastewater organics removed during wastewater treatment per
treatment type j. See new Table 6.6b. Pathways for organics removal include loss tosludge and biological decomposition.

TABLE 6.6B (NEW)WASTEWATER TREATMENT ORGANICS REMOVAL FRACTIONS (TOWREM) ACCORDING TO
TREATMENT TYPE

Treatment TypeDefault

Range

0

0 – 0.1

Primary (mechanical treatment plants)

0.40

0.25 – 0.50

Primary + Secondary (biological treatment plants)

0.85

0.80 – 0.90

Primary + Secondary + Tertiary (advanced
biological treatment plants)

0.90

0.80 – 0.95

Septic tank/septic system

0.625

0.50 – 0.60

Latrines – Dry climate, groundwater table lower
than latrine, small family (3–5 persons)0.1

0.05 – 0.15

Latrines – Dry climate, groundwater table lower
than latrine, communal (many users)

0.5

0.4 – 0.6

Latrines – Wet climate/flush water use,
groundwater table higher than latrine

0.7

0.7 – 1.0

Untreated systems

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Chapter 6: Wastewater Treatment and Discharge

TABLE 6.6B (NEW) (CONTINUED)WASTEWATER TREATMENT ORGANICS REMOVAL FRACTIONS (TOWREM) ACCORDING TO
TREATMENT TYPE

Sources:1
Based on expert judgment by Lead Authors of this section using the following references: Pescod (1992);
WEF (2009); Schaider et al. (2017) .

6.2.2.4

T IME SERIES CONSISTENCYNo refinement.

6.2.2.5

U NCERTAINTIES

Chapter 3, Uncertainties, in Volume 1 provides advice on quantifying uncertainties in practice. It includes
guidance on eliciting and using expert judgments which in combination with empirical data can provide overall
uncertainty estimates. This section provides an update to Table 6.7 to provide default uncertainty ranges for new
or updated emission factor and activity data of domestic wastewater. The following parameters are believed to bevery uncertain:•

The degrees to which wastewater in developing countries is treated in latrines, septic tanks, or removed bysewer, for urban high, urban low income groups and rural population (Ti,j).

•

The fraction of sewers that are ‘open’, as well as the degree to which open sewers in developing countries
are anaerobic and will emit CH4. This will depend on retention time and temperature, and on other factors
including the presence of a facultative layer and possibly components that are toxic to anaerobic bacteria(e.g., certain industrial wastewater discharges).

•

The amount of industrial TOW that is discharged into open or closed domestic sewers for each country isvery difficult to quantify.
TABLE 6.7 (UPDATED)DEFAULT UNCERTAINTY RANGES FOR DOMESTIC WASTEWATER
Parameter

Uncertainty Range

Emission Factor
Maximum CH4 producing capacity (Bo)± 30%

Fraction treated anaerobically (MCF)

The MCF is technology dependent. See Table 6.3. Thus, theuncertainty range is also technology dependent. The uncertainty range
should be determined by expert judgment, bearing in mind that MCF
is a fraction and must be between 0 and 1. Suggested ranges areprovided below.
Untreated systems, ± 50%
Lagoons ± 30%
Centralised plant, digester, reactor, ± 10%

Activity Data
Human population (P)± 5%Biochemical oxygen demand (BOD)± 30%

Fraction of population income group (U)Good data on urbanisation are available, however, the distinction
between urban high income and urban low income may have to be
based on expert judgment. ± 15%

Degree of utilisation of treatment/
discharge pathway or system for each
income group (Ti,j)Can be as low as ± 3% for countries that have good records and only
one or two systems. Can be ± 50% for an individual method/pathway.
Verify that total Ti,j = 100%

Correction factor for additional industrial
BOD discharged into sewers (I)For uncollected, the uncertainty is zero %. For collected theuncertainty is ± 20%

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Volume 5: Waste

TABLE 6.7 (UPDATED) (CONTINUED)DEFAULT UNCERTAINTY RANGES FOR DOMESTIC WASTEWATER
Parameter

Uncertainty Range

Amount of sludge removed fromwastewater treatment (Smass)± 30%

Sludge factor (Krem)± 25%

Fraction of the population managing their
septic system complying with the sludge
removal instruction (F)Can be as low as ± 3% for countries that have good records on
implementation. Can be ± 50% if based on expert judgment.

Amount of CH4 recovered or flared (R)For systems with measured data, the uncertainty is equal to theuncertainty of the measurement system.

Source:Based on expert judgment by Lead Authors of this section.

6.2.2.6

QA/QC, C OMPLETENESS , R EPORTING AND
D OCUMENTATIONNo refinement.

6.2.3

Industrial wastewaterIndustrial wastewater may be treated on site or released into domestic sewer systems. If it is released into thedomestic sewer system, the emissions are to be included with the domestic wastewater emissions. This section
deals with estimating CH4 emissions from on-site industrial wastewater treatment. Only industrial wastewaterwith significant carbon loading that is treated under intended or unintended anaerobic conditions will produce
CH4. Organics in industrial wastewater are often expressed in terms of COD, which is used here.

6.2.3.1

C HOICE OF METHOD

Assessment of CH4 production potential from industrial wastewater streams is based on the concentration of
degradable organic matter in the wastewater, the volume of wastewater, and the propensity of the industrial
sector to treat their wastewater in anaerobic systems. Using these criteria, major industrial wastewater sources
with high CH4 gas production potential can be identified as follows:•

pulp and paper manufacture;•meat and poultry processing (slaughterhouses);•alcohol, beer, starch production;•organic chemicals production;•other food and drink processing (dairy products, vegetable oil, fruits and vegetables, canneries, juice making,etc.).

Both the pulp and paper industry and the meat and poultry processing industries produce large volumes ofwastewater that contain high levels of degradable organics. The meat and poultry processing facilities typically
employ anaerobic lagoons to treat their wastewater, while the paper and pulp industry also use lagoons andanaerobic reactors. The non-animal food and beverage industries produce considerable amounts of wastewaterwith significant organic carbon levels and are also known to use anaerobic processes such as lagoons andanaerobic reactors. Anaerobic reactors treating industrial effluents with biogas facilities are usually linked withrecovery of the generated CH4 for energy. Emissions from the combustion process for energy should be reported
in the Energy Sector.
The method for estimating emissions from industrial wastewater is similar to the one used for domestic
wastewater. An updated decision tree for industrial wastewater is included as updated Figure 6.3 and should be
used to determine the tier approach that is applicable to the country.
The development of emission factors and activity data is more complex because there are many types ofwastewater, and many different industries to track. The most accurate estimates of emissions for this source
category would be based on measured data from point sources. Due to the high costs of measurements and the

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2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Dischargepotentially large number of point sources, collecting comprehensive measurement data is very difficult. It issuggested that inventory compilers use a top-down approach that includes the following general steps:
Step 1:Use Equation 6.6 to estimate total organically degradable carbon in wastewater (TOW) for industrial
sector i

Step 2:

Select the pathway and systems (updated Figure 6.1) according to country activity data. Use Equation
6.5 to obtain emission factor. For each industrial sector estimate the emission factor using maximum
methane producing capacity and the average industry-specific MCF.

Step 3:Use Equation 6.4 to estimate emissions, adjust for possible sludge removal and or CH4 recovery andsum the results.

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Volume 5: Waste

Figure 6.3 (Updated) Decision tree for CH 4 emissions from industrial wastewater treatment
Start

Identify major industrial
sectors with large potentials
for CH4 emissions (seeSection 6.2.3.1).

For these
industrial sectors, is a country-specificmethod from individual facilities orcompanies available?

Tier 3 for treatment anddischarge:
Estimate emissions using
bottom-up data.

Yes

No

For these
industrial sectors, are
COD and wastewater outflow
data available?

Yes

Are
country-specific emission
factors for selected industrial
sectors available?

Estimate emission factors
using a review ofNo
industry wastewater
treatment practices.

No

Is
Industrial wastewater a
key category1?

Yes

Collect COD and
outflow for eachindustrial sector.

Yes

Tier 2 for treatment:
Estimate CH4 emissions using countryspecific emission factors for treatment
and estimate emissions from discharge
to aquatic environments using default
Tier 2 emission factors.

Yes

Tier 2 for discharge:
Estimate emissions from discharge toaquatic environments using default Tier
2 emission factors and methodology

No

Estimate outflow using
industrial production
data.

Tier 1 for treatment:
Estimate all emissions
using default emission
factors (Bo, MCF, etc.)
and methodology.

Are activity
data available to categorise
discharges by type ofwaterbody?

No

Tier 1 for discharge:
Estimate emissions from discharge toaquatic environments using default Tier 1
emission factor and methodology

1

See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section 4.1.2 on limited resources),for discussion of key categories and use of decision trees.

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Chapter 6: Wastewater Treatment and Discharge

The general equation to estimate CH4 emissions from industrial wastewater is as follows:
EQUATION 6.4
TOTAL CH4 EMISSIONS FROM INDUSTRIAL WASTEWATER=
CH 4 Emissions∑ ( TOW − S ) • EF − R  •10
i

i

i

i

i

−6

Where:
CH4 Emissions= CH4 emissions in inventory year, Gg CH4/yr

TOWi= total organically degradable material in wastewater from industry i in inventory year,
kg COD/yri= industrial sector

Si= organic component removed from wastewater (in the form of sludge) in inventory
year, kg COD/yrEFi= emission factor for industry i, kg CH4/kg COD for treatment/discharge pathway orsystem(s) used in inventory year
If more than one treatment practice is used in an industry this factor would need to bea weighted average.

Ri= amount of CH4 recovered or flared in inventory year, kg CH4/yr

10-6= conversion of kg to Gg

The amount of CH4 which is recovered or flared is expressed as R in Equation 6.4.

6.2.3.2

C HOICE OF EMISSION FACTORSThis section represents an update to Section 6.2.3.2 of the 2006 IPCC Guidelines.
As stated in the 2006 IPCC Guidelines, there are significant differences in the CH4 emitting potential of different
types of industrial wastewater dependent on the type and form of constituents present in the wastewater. To theextent possible, data should be collected to determine the maximum CH4 producing capacity (Bo) in eachindustry.
As mentioned before, the MCF indicates the extent to which the CH4 producing potential (Bo) is realised in eachtype of treatment method. Thus, it is an indication of the degree to which the system is anaerobic. See Equation
6.5.
EQUATION 6.5
CH4 EMISSION FACTOR FOR INDUSTRIAL WASTEWATEREF=Bo • MCFjj
Where:EFj= emission factor for each treatment/discharge pathway or system, kg CH4/kg COD,
(See Table 6.8.)

j= each treatment/discharge pathway or systemBo= maximum CH4 producing capacity, kg CH4/kg COD

MCFj= methane correction factor (fraction). See Table 6.8.

Good practice is to use country- and industry-sector specific data that may be available from government
authorities, industrial organisations, or industrial experts. If country-specific data are available to determine the
Bo for a particular industry, industry-specific MCFs must be developed for the calculation of CH4 emissions.
However, if country-specific data are not available, it is good practice to use the IPCC default factors listed inTable 6.2.
In determining the MCF, which is the fraction of waste treated anaerobically, expert judgment is recommended.
A peer-reviewed survey of industry wastewater treatment practices is one useful technique for estimating these
data. Surveys should be conducted frequently enough to account for major trends in industry practices (i.e., every
3-5 years). Chapter 2, Approaches to Data Collection, in Volume 1, describes how to elicit expert judgment for2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

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Volume 5: Waste

uncertainty ranges. Similar expert elicitation protocols can be used to obtain the necessary information for othertypes of data if published data and statistics are not available. Table 6.8 includes default MCF values, which are
based on expert judgment.
In addition, the MCFs in Table 6.8 have been updated to reflect revisions to certain wastewater treatment anddischarge pathways and systems, as described in Section 6.2.2.2.
TABLE 6.8 (UPDATED)DEFAULT MCF VALUES AND RESULTANT EFS FOR INDUSTRIAL WASTEWATER
Type oftreatment anddischarge
pathway orsystemMCF 1(Range)Comments

EF2(kg CH4/kg
BOD)

EF2
(kg CH4/kg
COD)

Discharge from treated or untreated system
Discharge toaquatic
environments(Tier 1)

Most aquatic environments including rivers
are supersaturated in CH4. Nutrient
oversupply will increase CH4 emissions.
Environments where carbon accumulates in
sediments have higher potential for methane
generation.

0.11(0.004 – 0.27)

0.068

0.028

Discharge toaquatic
environments
other thanreservoirs,
lakes, andestuaries (Tier2)

Most aquatic environments including rivers
are supersaturated in CH4. Nutrient
oversupply will increase CH4 emissions.

0.0353(0.004 – 0.06)

0.021

0.009

Discharge toreservoirs,
lakes, andestuaries (Tier2)

Environments where carbon accumulates in
sediments have higher potential for methane
generation.

0.193(0.08 – 0.27)

0.114

0.048

Discharge tosoil

Sludge and/or wastewater discharge to soil
may be a source of CH4 for fertilisation

Emissions reported in Volume 4

Wastewater treatment system

Centralised,aerobic
treatment plantSome CH4 can be emitted from settlingbasins and other anaerobic pockets. For
treatment plants that are receiving
wastewater beyond the design capacity,
inventory compilers should judge the amountof organic material removed in sludge
accordingly.

0(0 – 0.1)

00Anaerobic
reactor (e.g.,
upflow
anaerobic
sludge blanket
digestion(UASB))

CH4 recovery is not considered here.

0.8(0.8 – 1.0)

0.48

0.2

Anaerobic
shallow lagoon
and facultative
lagoons

Depth less than 2 metres, use expert
judgment.

0.2(0 – 0.3)

0.12

0.05

Anaerobic
deep lagoon

Depth more than 2 metres.

0.8(0.8 – 1.0)

0.48

0.2

Constructedwetlands

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See 2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories:
Wetlands (IPCC 2014)2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Discharge

TABLE 6.8 (UPDATED) (CONTINUED)DEFAULT MCF VALUES AND RESULTANT EFS FOR INDUSTRIAL WASTEWATER
Type oftreatment anddischarge
pathway orsystemMCF 1(Range)Comments

EF2(kg CH4/kg
BOD)

EF2
(kg CH4/kg
COD)

Sludge treatment system
Anaerobic
digester forsludgeSee Chapter 4 for emissions methodology

See Chapter 4, Table 4.1

Composting

Emissions reported in Volume 5, Chapter 4

See Chapter 4, Table 4.1

Incinerationand openburning

Emissions reported in Volume 5, Chapter 5

See Chapter 5

Sources:1

Based on expert judgment by Lead Authors of this section.

2

Emission factors calculated using default Bo and default MCF.

3

See Annex 6A.4.

6.2.3.3

C HOICE OF ACTIVITY DATA

No refinement.

6.2.3.4

T IME SERIES CONSISTENCYNo refinement.

6.2.3.5

U NCERTAINTIESNo refinement.

6.2.3.6

QA/QC, C OMPLETENESS , R EPORTING AND
D OCUMENTATIONNo refinement.

6.3

NITROUS OXIDE EMISSIONS FROMDOMESTIC WASTEWATER

This section refines Section 6.3 of the 2006 IPCC Guidelines by providing update and new guidance for
estimating domestic N2O emissions from WWTPs and provides new guidance on the estimation of N2Oemissions from wastewater after disposal of untreated wastewater or treated wastewater effluent into aquatic
environments by accounting for losses of nitrogen prior to disposal.

6.3.1

Methodological issues

Nitrous oxide emissions can occur as emissions from WWTPs or as emissions from receiving aquatic
environments following the disposal of untreated or treated wastewater effluent. This section describes how toestimate the N2O produced during wastewater treatment, sludge treatment that occurs within the wastewater
treatment system, and disposal of the wastewater. More recent research and field surveys have revealed thatemissions in sewer networks and from nitrification or nitrification-denitrification processes at WWTPs,
previously judged to be a minor source, may in fact result in more substantial emissions. N2O is generated as aby-product of nitrification, or as an intermediate product of denitrification. There are many factors affecting N2O

2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

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Volume 5: Waste

emissions from wastewater treatment systems such as the temperature and dissolved oxygen concentration of thewastewater, and the specific operational conditions.
Emissions also result from untreated wastewater or wastewater treatment effluent that is discharged into aquatic
environments. It is important to note that emissions are dependent on the nutrient-impacted status andoxygenation level of the aquatic environment receiving the wastewater discharge. The current methodology inthe 2006 IPCC Guidelines appropriately captures discharge to relatively clean and/or well-oxygenatedenvironments. However, in the case of discharge to nutrient-impacted waters such as eutrophic lakes, estuaries
and rivers, or locations where stagnant conditions occur, emissions can be significantly higher.
Three tiers of methods for estimating N2O emissions from this category are summarised below:
The Tier 1 method applies default values for the emission factor and activity parameters. This method is
considered good practice for countries with limited data.
The Tier 2 method follows the same method as Tier 1 but allows for incorporation of a country-specific emission
factor and country-specific activity data. For example, a specific emission factor for a prominent treatment
system based on field measurements could be incorporated under this method. Use of country-specific emission
factor in the same region or climate area is also good practice. The amount of sludge removed for incineration,
landfills, and agricultural land should be taken into consideration.
For a country with good data and advanced methodologies, a country-specific method could be applied as a Tier
3 method. For example, a more advanced country-specific method could be based on plant-specific emissions
data from large wastewater treatment facilities or using country-specific measurements of nitrogen discharged toaquatic environments of varying nutrient-impacted status. Direct measurement methods would provide a moreaccurate measurement of N2O production from each treatment plant.

6.3.1.1

C HOICE OF METHOD

A decision tree for domestic wastewater is included in new Figure 6.4.
The steps for good practice in inventory preparation for N2O from domestic wastewater are as follows:
Step 1:

Use new Equation 6.10 to estimate total nitrogen in wastewater and updated Equation 6.8 to estimate
total nitrogen in wastewater effluent.

Step 2:

Select the pathway and systems (see updated Figure 6.1) according to country activity data. Select theemission factor for each domestic wastewater treatment/discharge pathway or system.

Step 3:

Use updated Equation 6.9 to estimate emissions from wastewater treatment and sum the results foreach treatment pathway/system.

Step 4:

Use updated Equation 6.7 to estimate emissions from effluent, accounting for losses of nitrogen that
occur within the wastewater treatment process including sludge removal and sum the results for eachpathway/system. Emissions should also be calculated for nitrogen discharged as untreated wastewater.

As described earlier, the wastewater characterisation will determine the fraction of wastewater treated or
disposed of by a particular system. To determine the use of each type of treatment or discharge system, it is goodpractice to refer to national statistics (e.g., from regulatory authorities). If these data are not available,
wastewater associations or international organisations such as the WHO may have data on the system usage.
Otherwise, consultation with sanitation experts can help, and expert judgment can also be applied (see Chapter 2,
Approaches to Data Collection, in Volume 1). Urbanisation statistics may provide a useful tool, e.g., city sizes
and income distribution.
If activity data are available to categorize discharges to nutrient-impacted environments, these refinements
introduce a new Tier 3 emission factor for those discharges.

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2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Discharge

Figure 6.4 (New) Decision tree for N 2 O emissions from domestic wastewaterStart

Are wastewater
treatment pathways
characterised?

No

Collect data on
the share ofwastewater
treatment ineach pathway

Are activity data
available to categorise
discharge by type ofwaterbody?

No
Tier 1 for discharge:
Estimate emissions from discharge
to reservoirs, lakes, and estuaries
using default emissions factors
and methodology.

Yes

Are facility-specific
N2O emissions data
available?

Yes

Is a countryspecific method
available?

Yes

Yes

Is a countryspecific method
available?

No

Yes

Tier 3 for discharge:
Estimate emissions fromdischarge to reservoirs, lakes, andestuaries using country-specificemissions data and methodology.

Tier 3 for treatment:
Estimate emissions fromtreatment using country-specificmethod based on measured
emissions data from facilities.

NoNo

Are countryspecific emission factors
and/or activity data
available?

Yes

Tier 2 for treatment:
Estimate emissions fromtreatment using countryspecific emission factors and/or
activity data and default
methodology.

Yes

Develop countryspecific emissions
factors and/orcollect activity dataNo

Is this a key
category1?

No

Tier 1 for treatment:
Estimate emissions fromtreatment using default
emission factors andmethodology.

1

See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section
4.1.2 on limited resources), for discussion of key categories and use of decision trees.

EQUATION 6.9 (UPDATED)N2O EMISSIONS FROM DOMESTIC WASTEWATER TREATMENT PLANTS44
N 2O Plants= ∑ ( U i • Tij • EF j ) • TN DOM •DOM
28
 i, j

Where:

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Volume 5: Waste

N2O Plants DOM = N2O emissions from domestic wastewater treatment plants in inventory year, kg N2O/yr
TNDOM= total nitrogen in domestic wastewater in inventory year, kg N/yr. See new Equation 6.10.

Ui= fraction of population in income group i in inventory year. See Table 6.5.

Tij

= degree of utilisation of treatment/discharge pathway or system j, for each income group
fraction i in inventory year. See Table 6.5.

i= income group: rural, urban high income and urban low incomej= each treatment/discharge pathway or systemEFj= emission factor for treatment/discharge pathway or system j, kg N2O-N/kg N

The factor 44/28 is for the conversion of kg N2O-N into kg N2O.
It is also required to estimate direct N2O emissions arising from the discharge of wastewater (treated or untreated)
into aquatic receiving environments. The methodology for emissions from effluent is similar to that of N2Oemissions explained in Volume 4, Section 11.2.2 in Chapter 11 N2O Emissions from Managed Soils, and CO2
Emissions from Lime and Urea Application. The simplified general equation is as follows:
EQUATION 6.7 (UPDATED)N2O EMISSIONS FROM DOMESTIC WASTEWATER EFFLUENT

N 2 OEFFLUENT , DOM= N EFFLUENT , DOM • EFEFFLUENT •

44
28

Where:
N2OEFFLUENT,DOM= N2O emissions from domestic wastewater effluent in inventory year, kg N2O/yr

NEFFLUENT,DOM= nitrogen in the effluent discharged to aquatic environments, kg N/yr. See updated
Equation 6.8.

EFEFFLUENT= emission factor for N2O emissions from wastewater discharged to aquatic systems,
kg N2O-N/kg N

The factor 44/28 is the conversion of kg N2O-N into kg N2O.

6.3.1.2

C HOICE OF EMISSION FACTORSNew Table 6.8a includes default Tier 1 EF values for use in estimation of N2O emissions from waters receiving
treated or untreated effluent (see Annex 6A.5). These refinements to the emission factors build on the current
default value for EFEFFLUENT provided in Table 6.11 of the 2006 IPCC Guidelines. This existing emission factor
is adequate for all discharges when using the Tier 1 methodology. However, it is expected to generate anunderestimate of N2O emissions in the case of discharge to nutrient-impacted waters such as eutrophic lakes,
estuaries and rivers, or locations where stagnant conditions occur. Research published between 1978 and 2017provide data indicating that higher N2O emissions occur when wastewater is discharged to nutrient-impacted(eutrophic) or hypoxic aquatic receiving environments. Accordingly, Table 6.8a provides a new EFEFFLUENT of
0.019 g N2O-N/g N (95percent confidence limits 0.0041–0.091) for use with a Tier 3 method (see Annex 6A.6
for a discussion of information used to develop this new emission factor). The compiler can make use of manydifferent indications of whether this higher EF value should be employed. Harmful algal blooms and hypoxia are
the two most common symptoms of waterway nutrient over-enrichment (eutrophication). One possibility is toconsider the dissolved oxygen status of the receiving waters. Waterway hypoxia is becoming increasingly
common globally and definitions of hypoxia can vary depending on temperature, salinity, and the particular biota
of interest. Dissolved oxygen concentrations of between 0.1–3.0 mg/L (or <30percent of the oxygen saturation
concentration) are typically classified as hypoxic (Vaquer-Sunyer & Duarte 2008; Naqvi et al. 2010; Rabalais etal. 2010) and are likely to result in increased N2O yields from microbial metabolism of discharged wastewater
nitrogen.
Oxygen status can be a highly dynamic indicator in practice, and compilers may have better access to
information regarding the nutrient (trophic) status of receiving waters. Various thresholds for the boundary
between mesotrophic and eutrophic conditions have been proposed, such as Secchi depth (suggestions range
from 2 to 3.6 m), total phosphorus (20–75 μg/L), total nitrogen (0.3–5 mg/L), benthic chlorophyll (70 mg/m2)
and suspended chlorophyll concentration (5–30 μg/L) (Dodds et al. 1998; Burns et al. 2009; Mateo-Sagasta &Burke 2010). Compilers may also consider whether there have been reports of algal blooms, sulphurous odours,

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2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Discharge

fish kills or other indications of eutrophic and/or hypoxic conditions in the discharge environment. Global
datasets exist which may assist compilers in assessing the risk of nutrient impacts in different regions (Diaz et al.
2011; Xie & Ringler 2017). It is important to note that the context for application of this new Tier 3 EFEFFLUENT
factor is not necessarily the same as for nitrogen runoff from agriculture, due to the combination of reducing
conditions and high organic loading in many wastewaters which are unlikely to exist in agricultural runoff
systems.
TABLE 6.8A (NEW)DEFAULT EF VALUES FOR DOMESTIC AND INDUSTRIAL WASTEWATER
Type of treatment anddischarge pathway or system

EF1 (kg N2ON/kg N)

Comments

Range

Discharge from treated or untreated system, EFEFFLUENT
Freshwater, estuarine, andmarine discharge (Tier 1)

Based on limited field data and on specificassumptions regarding the occurrence ofnitrification and denitrification in rivers
and in estuaries

0.00520.0005 – 0.075

Nutrient-impacted and/or
hypoxic freshwater, estuarine,and marine environments (Tier3, if needed)

Higher emissions are associated withnutrient-impacted/hypoxic water such as
eutrophic lakes, estuaries and rivers, or
locations where stagnant conditions occur.
See section 6.3.1.2 for more information.

0.01920.0041 – 0.091

Discharge to soil

Emissions reported in Volume 40.01610.00016 – 0.045

Wastewater treatment system, EFplants
Centralised, aerobic treatment
plant

N2O is variable and can be significant

Anaerobic reactor

N2O is not significant

0

0 – 0.001

Anaerobic lagoons

N2O is not significant

0

0 – 0.001

Constructed wetlands

See 2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas
Inventories: Wetlands (IPCC 2014)

Septic tank

N2O is not significant

Septic tank + land dispersal
field

N2O is emitted by the soil dispersal system

LatrineN2O is not significant

0

0 – 0.001

0.00450 – 0.0050

0 – 0.001
0

Sludge treatment system
Anaerobic digester for sludge

N2O is not significant

0Composting

See Chapter 4 for emissions methodology

See Chapter 4, Table 4.1

Incineration and open burning

See Chapter 5 for emissions methodology

See Chapter 5

Sources: Based on scientific literature and expert judgment by Lead Authors of this section.
1

See Annex 6A.5.

2

See Annex 6A.6.

6.3.1.3

C HOICE OF ACTIVITY DATA

The methodology to calculate the amount of N content in wastewater effluent (NEFFLUENT,DOM) presented in
Equation 6.8 of the 2006 IPCC Guidelines currently does not account for the removal of ammonia-N via
nitrification-denitrification processes in the sewer and the WWTP, or the removal of N in sludge, which can
result in a significant overestimate of N discharged to aquatic receiving environments. Typical total N
concentration in raw urban sewage is about 40 mg/L (range 20–70 mg/L) (Tchobanoglous et al. 2014), whereas
effluent treated in conventional activated sludge facilities (with nitrification) has a total of about 25 mg/L. Plants
with biological nutrient removal (with denitrification) regularly achieve an effluent total N of 5 mg/L or less. The
difference is due both to N removed in sludge (see Table 2.4A (New), Chapter 2 of Volume 5 regarding the N
content of sewage sludge) versus N loss to the atmosphere (see new Figure 6.5).

2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

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Volume 5: Waste

This section updates the methodology presented in the 2006 IPCC Guidelines to estimate the amount of N
content in wastewater entering treatment (TNDOM), the loss or removal of N through the treatment process (either
through biological conversion or removal with sludge) (NREM), and the N content in wastewater discharged toaquatic systems (NEFFLUENT,DOM).
Figure 6.5 (New) Nitrogen in domestic wastewater treatment
Industrial andcommercial protein
disposed to sewer(FIND-COM)

N2, N2O losses toatmosphere duringtransport andtreatment

Nitrogen inhouseholdproducts (NHH)
Nitrogen inprotein available(ProteinSUPPLY ●FNPR)

Nitrogen inprotein consumed(Protein ● FNPR)

Food waste
disposed to sewer(FNON-CON)

Wastewater
treatment

Nitrogen intreatedwastewater

N2O
emissionsafterdischarge

Nitrogenremoved insludgeNITROGEN IN DOMESTIC WASTEWATER (TN D O M )The activity data that are needed for estimating TNDOM in domestic wastewater include the population associatedwith the wastewater, the average annual per capita protein consumed (kg/person/yr), and factors to account for N
in non-consumed protein entering the wastewater and other N from household, industrial, and commercial
sources co-discharged into the sewer system. The total N in wastewater for each treatment pathway is estimatedas follows:
EQUATION 6.10 (NEW)TOTAL NITROGEN IN DOMESTIC WASTEWATER BY TREATMENT PATHWAY

TN DOM _ j = ( Ptreatment _ j • Protein • FNPR • N HH • FNON −CON • FIND −COM )Where:TNDOM_j= total annual amount of nitrogen in domestic wastewater for treatment pathway j, kg N/yr

Ptreatment_j= human population who are served by the treatment pathway j, person/yrProtein= annual per capita protein consumption, kg protein/person/yr

FNPR= fraction of nitrogen in protein, default = 0.16 kg N/kg protein

FNON-CON= factor for nitrogen in non-consumed protein disposed in sewer system, kg N/kg N. See
new Table 6.10a.

FIND-COM= factor for industrial and commercial co-discharged protein into the sewer system, kg
N/kg N

NHH= additional nitrogen from household products added to the wastewater, default is 1.1(some country data are in new Table 6.10a).

If national statistics on protein consumed or protein supply are not available, Food Balance Sheets of FAOSTAT
can be used as activity data on per capita “protein supply quantity.” This information represents the total amountof protein available to the population but must be adjusted to reflect the fraction of protein consumed (FPC),
according to the new Equation 6.10a.

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2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Discharge

EQUATION 6.10A (NEW)ESTIMATION OF PROTEIN CONSUMED

Protein = ProteinSUPPLY • FPC
Where:
ProteinSUPPLY = annual per capita protein supply, kg protein/person/yr
FPC= Fraction of protein consumed. Default regional values are listed in the new Table 6.10a

Food that is not consumed may be disposed to the sewer (e.g., as result of the use of food waste disposals insome countries) and a factor for non-consumed protein (FNON-CON) should be used to reflect this additional N
entering wastewater (see new Table 6.10a). If food waste is disposed with solid waste, it is assumed that no
additional N is entering wastewater and FNON-CON = 1.
Bath and laundry water may contain household chemicals (detergents, shampoos, softeners, dishwashing agents,
WC fresheners, cosmetics, etc.) and a factor for N from household products (NHH) should be used. The default
factor for NHH is 1.1 (Henze et al. 2008; Tjandraatmadja et al. 2008), but additional regional factors are provided
in Table 6.10a.
TABLE 6.10A (NEW)DEFAULT FACTORS FOR DOMESTIC WASTEWATER
Proteinconsumed2 as
fraction of protein
supplyFNON-CON2 in casefood waste isdisposed in sewer

Additional N
fromhouseholds’
chemicals

Europe

0.85

1.09

1.08

North America and Oceania

0.80

1.13

1.17 (USA)1.07 (Australia)

Industrialised Asia

0.86

1.08

No data

Sub-Saharan Africa

0.98

1.01

No data

North Africa, West and central Asia

0.90

1.06

No dataSouth and Southeast Asia

0.96

1.02

1.13 (India)

Latin America

0.92

1.04

No data

Region1

1

See Annex 6A.7 for a list of countries by region

2

Based on FAO (2011)2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

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Volume 5: Waste

TABLE 6.10B (NEW)ESTIMATE ON USE OF FOOD WASTE DISPOSAL IN SEWER1
Share of households disposing food waste in
sewer

Country
USA

50%

Canada10%

Australia

12%

New Zealand

30%

UK

5%

Ireland1 – 2%

Italy

Supported

Czech Republic<1% / not allowed

Denmark

Rare

Belgium

Restricted / not allowed

The Netherlands

Not allowed

Germany

Not allowed

1 Based on EPA (2008) and expert judgment by Lead Authors

Wastewater from industrial or commercial sources that is discharged into the sewer may contain protein (e.g.,
from grocery stores and butchers). The default for this fraction is 1.25 for centralised treatment and 1 fordecentralised treatment systems (septic system, latrines, discharge).

NITROGEN IN WASTEWATER EFFLUENT (N E F F L U E N T , D O M )The total nitrogen in wastewater effluent is estimated as follows:
EQUATION 6.8 (UPDATED)TOTAL NITROGEN IN DOMESTIC WASTEWATER EFFLUENTN EFFLUENT=, DOM∑ (TNj

DOM• T j ) • (1 − N REM , j ) 

Where:NEFFLUENT,DOM= total nitrogen in the wastewater effluent discharged to aquatic environments ininventory year, kg N/yr

TNDOM

= total nitrogen in domestic wastewater in inventory year, kg N/yr. See new Equation
6.10.

Tj

= degree of utilisation of treatment system j in inventory year ( ∑ Tij ). See Table 6.5
i

6.42

j

= each wastewater treatment type used in inventory year

NREM

= fraction of total wastewater nitrogen removed during wastewater treatment per
treatment type j. See new Table 6.10c. Pathways for N removal include transfer tosludge and nitrification–denitrification with concomitant N loss to the atmosphere.

2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Discharge

TABLE 6.10C (NEW)WASTEWATER TREATMENT NITROGEN REMOVAL FRACTIONS (NREM)ACCORDING TO TREATMENT TYPE

Treatment TypeDefault

Range

No treatment

01–3

01–3

Primary (mechanical)

0.101–3

0.05 – 0.201–3

Secondary (biological)

0.401–3

0.35 – 0.551–3

Tertiary (advanced biological)

0.801–4

0.45 – 0.851–4

Septic tank

0.151–3

0.10 – 0.251–3

Septic tank + land dispersal field

0.685

0.62 – 0.735

Latrine0.126

0.07 – 0.216

Sources:

6.3.2

1

Kristensen et al. (2004)

2

Van Drecht et al. (2009)3

Based on expert judgment by Lead Authors of this section.

4

Ekama and Wentzel (2008)

5

Andreoli et al. (1979)

6

EMEP/EAA (2016)

Time series consistency

No refinement.

6.3.3

Uncertainties

Large uncertainties are associated with the IPCC default factors for N2O. Updated Table 6.11 below includes
uncertainty ranges based on expert judgment.
TABLE 6.11 (UPDATED)N2O METHODOLOGY DEFAULT DATA
Definition

Default ValueRange

N2O emission factor

See Table 6.8a

See Table 6.8a

P

Number of people in country

Country-specific± 10 %

Protein

Annual per capita protein consumption
Fraction of nitrogen in protein(kg N/kg protein)
Degree of utilisation of large WWT plants

Country-specific± 10 %

0.16

0.15 – 0.17

Country-specific± 20 %

Factor to adjust for non-consumed protein, basedon available protein

1.0 for countries with noin-sink disposals,
1.1 for countries with insink disposals

0.9 – 1.2

Factor to adjust for non-consumed protein, basedon consumed protein

1.1 for countries with noin-sink disposals,
1.25 for countries with insink disposals

1.0 – 1.4

Factor to allow for co-discharge of industrial
nitrogen into sewers. For countries with significant
fish processing plants, this factor may be higher.
Expert judgment is recommended.

1.25

1.0 – 1.5

Emission Factor
EF
Activity Data

FNPR
TplantFNON-CON

FIND-COM

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Volume 5: Waste

TABLE 6.11 (UPDATED) (CONTINUED)
N2O METHODOLOGY DEFAULT DATA

NREM

6.3.4

Definition

Default Value

Range

Factor to account for losses of nitrogen prior todischarge

See Table 6.10c

See Table 6.10c

QA/QC, Completeness, Reporting andDocumentation

No refinement.

6.4

NITROUS OXIDE EMISSIONS FROM
INDUSTRIAL WASTEWATER

6.4.1

Methodological issues

This section refines the 2006 IPCC Guidelines by adding new guidance for estimating N2O emissions fromindustrial WWTPs and refines the estimation of N2O emissions from wastewater after disposal of untreatedwastewater or wastewater treatment effluent into aquatic environments by accounting for losses of nitrogen prior
to disposal.

6.4.1.1

C HOICE OF METHOD

A decision tree for industrial wastewater is included in new Figure 6.6.
The steps for good practice in inventory preparation for N2O from industrial wastewater are as follows:
Step 1:

Use new Equation 6.13 to estimate total nitrogen in wastewater.

Step 2:

Select the pathway and systems (see updated Figure 6.1) according to country activity data. Select theemission factor for each industrial wastewater treatment/discharge pathway or system.

Step 3:

Use new Equation 6.11 to estimate emissions from wastewater treatment and sum the results for eachpathway/system.

Step 4:

Use new Equation 6.12 to estimate emissions from effluent, accounting for losses of nitrogen that
occur within the wastewater treatment process, including sludge removal, and sum the results for eachpathway/system.

As described earlier, the wastewater characterisation will determine the fraction of wastewater treated or
disposed of by a particular system. To determine the use of each type of treatment or discharge system, it is goodpractice to refer to national statistics (e.g., from regulatory authorities). If these data are not available, industry
associations may have data on the system usage. Otherwise, consultation with industry experts can help, andexpert judgment can also be applied (see Chapter 2 Approaches to Data Collection, Volume 1).
EQUATION 6.11 (NEW)N2O EMISSIONS FROM INDUSTRIAL WASTEWATER TREATMENT PLANTS 44=  ∑ (Ti. j • EFj • TN INDi )  •N 2 O PlantsIND
 i
 28

Where:
N2O PlantsIND = N2O emissions from industrial wastewater treatment plants in inventory year, kg N2O/yr

6.44

TNINDi= total nitrogen in wastewater from industry i in inventory year, kg N/yr. See new
Equation 6.13.

Ti,j= degree of utilisation of treatment/discharge pathway or system j, for each industry i ininventory yeari

= industry

2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Dischargej= each treatment/discharge pathway or systemEFj= emission factor for treatment/discharge pathway or system j, kg N2O-N/kg N. See Table
6.8a (New).

The factor 44/28 is for the conversion of kg N2O-N into kg N2O.

2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

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Volume 5: Waste

Figure 6.6 (New) Decision tree for N 2 O emissions from industrial wastewater
Start

Identify major industrial
sectors with large potentials
for N2O emissions(see Table 6.12).

For these
industrial sectors,
is a country-specific methodfrom individual facilities
or companies
available?

Are activity data
available to categorise
discharge by type ofwaterbody?
Yes

Tier 3 for treatment:
Estimate emissions
using bottom-up data.

Yes

For these
industrial sectors, are
N and wastewater outflow
data available?

No

Are country-specificemission factors for selectedindustrial sectors available?

Yes

Yes

No

Tier 1 for discharge:
Estimate emissions fromdischarge to reservoirs,
lakes, and estuaries using
default emissions factors andmethodology.

No

Is a countryspecific method
available?

Tier 3 for discharge:
Estimate emissions fromdischarge to reservoirs, lakes,
and estuaries using countryspecific emissions data andmethodology.

No

Estimate emission factors
using a review of industry
wastewater treatment
practices.

No

Is industrial
wastewater a key
category1?

Yes

Collect N
and outflow
for eachindustrial
sector.

Yes

Tier 2 for treatment:
Estimate N2O emissions
using country-specificemission factors.

No
Estimate outflow
using industrial
production data.

Tier 1 for treatment:
Estimate emissions using
default data.

1

See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section 4.1.2 on limited resources), for
discussion of key categories and use of decision trees.

It is also required to estimate N2O emissions from wastewater treatment effluent that is discharged into aquatic
environments. The methodology for emissions from effluent is like that of N2O emissions explained in Volume 4,
Section 11.2.2, Chapter 11, N2O Emissions from Managed Soils, and CO2 Emissions from Lime and Urea
Application. The simplified general equation is as follows:

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Chapter 6: Wastewater Treatment and Discharge

EQUATION 6.12 (NEW)N2O EMISSIONS FROM INDUSTRIAL WASTEWATER EFFLUENT

N 2 OEffluentIND= N EFFLUENT , IND • EFEFFLUENT •

44
28

Where:
N2OEffluentIND = N2O emissions from industrial wastewater effluent in inventory year, kg N2O/yr
N EFFLUENT, IND= nitrogen in the industrial wastewater effluent discharged to aquatic environments, kg
N/yr. See new Equation 6.14.

EFEFFLUENT= emission factor for N2O emissions from wastewater discharged to aquatic systems, kg
N2O-N/kg N

The factor 44/28 is for the conversion of kg N2O-N into kg N2O.

6.4.1.2

C HOICE OF EMISSION FACTORSNew Table 6.8a includes default EF values for N2O emissions.

6.4.1.3

C HOICE OF ACTIVITY DATATOTAL NITROGEN IN INDUSTRIAL WASTEWATER (TN I N D )The activity data for this source category is the amount of total nitrogen (TN) in the industrial wastewater
entering treatment (TNIND). This parameter is a function of industrial output (product) P (tonnes/yr), wastewater
generation W (m3/ton of product), and total N concentration in the untreated wastewater (kg TN/m3). See new
Equation 6.13. The following steps are required for determination of TNIND:(i)

Identify the industrial sectors that generate wastewater with large quantities of N, by evaluating
total industrial product, N in the wastewater, and wastewater produced.

(ii)

Identify industrial sectors that use treatment systems with N2O emissions factors (see new Table
6.8a). Experience has shown that usually three or four industrial sectors are key.

For each selected sector estimate total N in the industrial wastewater (TNIND).
EQUATION 6.13 (NEW)TOTAL NITROGEN IN INDUSTRIAL WASTEWATER

TNINDi = Pi • Wi • TN i
Where:TNINDi= total nitrogen in wastewater entering treatment for industry i, kg TN/yri= industrial sectorPi= total industrial product for industrial sector i, t/yr

Wi

= wastewater generated for industrial sector i, m3/t productTNi= total nitrogen in untreated wastewater for industrial sector i, kg TN/m3

Industrial production data and wastewater generation rates may be obtained from national statistics, regulatory
agencies, wastewater treatment associations or industry associations. In some cases, quantification of the N
loading in the wastewater may require expert judgment. In some countries, N content and total water usage persector data may be available directly from a regulatory agency. New Table 6.12 provides examples that could be
used as default values for industries that may be considered key sources of N2O. These values should be usedwith caution, because they are industry-, process-, and country-specific.

2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

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Volume 5: Waste

TABLE 6.12 (NEW)EXAMPLES OF INDUSTRIAL WASTEWATER DATA
Industry Type

Wastewater Generation W

Range for W

Total Nitrogen (TN)TN Range(m3/tonne)(m3/tonne)(kg/m3)(kg/m3)

Alcohol refining

242

16 – 322

2.42

0.94 – 3.862

Beer & malt

6.32

5.0 – 9.02

0.05530.025 – 0.083

Fish processing52

2 – 820.602

0.21 – 0.982

Iron and steel
manufacturing51

0.004 – 10.44

0.251

0.0004 – 0.5244

Meat & poultry132

8 – 182

0.192

0.17 – 0.202

Nitrogen fertiliser

2.892

0.46 – 8.32

0.52

0.1 – 0.82

Plastics & resins

0.65

0.3 – 1.25

0.256

No range provided

Starch production92

4 – 182

0.92

0.8 – 1.102

1

Based on expert judgment by Lead Authors of this section.

2

IPCC (2014)

3

Simate et al. (2011)4

US EPA (2002a)

5

Doorn et al. (1997)6

Li et al. (2016)

NITROGEN REMOVED FROM WASTEWATER (N R E M )Nitrogen removal by different treatment facilities can range from 10–85 percent. Default values for the fractionof nitrogen removed by type of wastewater treatment system are presented in Table 6.10c.

NITROGEN IN WASTEWATER EFFLUENT (N E F F L U E N T , I N D )The total nitrogen in the industrial wastewater effluent is estimated as follows:
EQUATION 6.14 (NEW)TOTAL NITROGEN IN INDUSTRIAL WASTEWATER EFFLUENT=N EFFLUENT, IND∑ TNj

INDi• T j • (1 − N REM , j ) 

Where:NEFFLUENT, IND = total annual amount of nitrogen in the industrial wastewater effluent, kg N/yr
TNINDi= total nitrogen in wastewater entering treatment for industry i, kg TN/yr

Tj= degree of utilisation of treatment system j in inventory year ( ∑ Tij ). See Table 6.5.
ij= each wastewater treatment type used in inventory year

NREM,j= fraction of total wastewater nitrogen removed during wastewater treatment per treatment
type j. See new Table 6.10c.

6.4.2

Time series consistency

Once an industrial sector is included in the inventory calculation, it should be included for each subsequent year.
If the inventory compiler adds a new industrial sector to the calculation, then he or she should re-calculate theentire time series so that the method is consistent from year to year. General guidance on recalculation of
estimates through time series is provided in Volume 1, Chapter 5, Time Series Consistency.
If a country decides to incorporate plant emissions into the estimate, this change must be made for the entire timeseries. Potential sludge removal should be treated consistently across years in the time series.

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Chapter 6: Wastewater Treatment and Discharge

6.4.3

Uncertainties

Uncertainty estimates for EF, P, W and TN are provided in new Table 6.13. The estimates are based on expert
judgment.
TABLE 6.13 (NEW)DEFAULT UNCERTAINTY RANGES FOR INDUSTRIAL WASTEWATER
Parameter

Uncertainty Range

Emission Factor
N2O emission factor± 30%

Activity Data
Industrial production (P)± 25%
Use expert judgment regarding the quality of data source to assign moreaccurate uncertainty range.

Wastewater/unit production (W)These data can be very uncertain as the same sector might use different
waste handling procedures at different plants and in different countries.
The product of the parameters (W•TN) is expected to have lessuncertainty. An uncertainty value can be attributed directly to TN
concentration. –50 %, +100% is suggested (i.e., a factor of 2).

TN/unit wastewater (TN)

Source: Expert Judgment by Lead Authors of this section.

6.4.4

QA/QC, Completeness, Reporting andDocumentationQUALITY ASSURANCE / QUALITY CONTROL (QA/QC)
It is good practice to conduct quality control checks and quality assurance procedures as outlined in Chapter 6,
QA/QC and Verification, of Volume 1. Below, some fundamental QA/QC procedures include:•

For industrial wastewater, inventory compilers may review the secondary data sets (e.g., from national
statistics, regulatory agencies, wastewater treatment associations or industry associations) that are used toestimate and rank industrial TN waste output. Some countries may have regulatory control over industrial
discharges, in which cases significant QA/QC protocols may already be in place for the development of thewastewater characteristics on an industry basis.

•

For industrial wastewater, inventory compilers should cross-check values for EFs against those from othernational inventories with similar wastewater characteristics.

•

If sludge removal is reported in the wastewater inventory, check for consistency with the estimates for
sludge applied to agriculture soils, sludge incinerated, and sludge deposited in solid waste disposal.

•

For countries that use country-specific parameters or higher tier methods, inventory compilers should crosscheck the national estimates with emissions using the IPCC default method and parameters.

COMPLETENESS
Completeness for estimating emissions from industrial wastewater depends on an accurate characterisation ofindustrial sectors that produce nitrogen-laden wastewater. In most countries, approximately 3–4 industrial sectors
will account for the majority of the nitrogenous wastewater volume, so the inventory compilers should ensurethat these sectors are covered. Periodically, the inventory compilers should re-survey industrial sources,
particularly if some industries are growing rapidly.
This category should only cover industrial wastewater treated onsite. Emissions from industrial wastewater
released into domestic sewer systems should be addressed and included with domestic wastewater.
Some sludge from industrial wastewater treatment may be incinerated or deposited in landfills or on agriculturallands. This constitutes an amount of N that should be subtracted from effluent N. It is good practice to beconsistent across sectors: the amount of N that is removed as sludge should be equal to the amount of sludge
disposed at landfills, applied to agricultural soils, incinerated, or treated elsewhere.

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Volume 5: Waste

REPORTING AND DOCUMENTATION
It is good practice to document and report a summary of the methods used, activity data and emission factors.
When country-specific methods and/or emission factors are used, the reasoning for the choices as well as
references to how the country-specific data (measurements, literature, expert judgment, etc.) have been derived(measurements, literature, expert judgment, etc.) should be documented and included in the reporting.
If sludge is incinerated, landfilled, or spread on agricultural lands, the quantities of sludge and associated emissions
should be reported in the waste incineration, solid waste disposal systems, or agricultural categories, respectively.
More information on reporting and documentation can be found in Volume 1, Chapter 6, Section 6.11
Documentation, archiving and reporting.

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Chapter 6: Wastewater Treatment and Discharge

Appendix 6A.1 Non-biogenic (fossil) CO2 emissions fromwastewater treatment and discharge: Basis for Future
Methodological Development
The 2006 IPCC Guidelines assume that organic carbon present in wastewater derives from modern (biogenic)
organic matter in human excreta or food waste (Doorn et al. 2006). Consequently, CO2 emissions fromwastewater treatment according to those Guidelines are also considered wholly biogenic and are discounted frominternational greenhouse gas accounting inventories, since they do not represent a transfer of carbon from thelithosphere to the atmosphere. In comparison, fossil organic carbon (that with turnover timescales exceeding 106
years) is considered to have a role in climate change and is accounted for in international greenhouse gas
emissions inventories. In the 2006 IPCC Guidelines, it is assumed that emissions from biogas flaring are notsignificant because the CO2 emissions are of biogenic origin and the CH4 and N2O emissions are very small sogood practice in the Waste Sector does not require their estimation. If countries wish to report such emissions,they should be reported under the Waste Sector. A discussion of emissions from flares and more detailed
information are given in Volume 2, Energy, Chapter 4.2. However, the presence of fossil organic carbon in
sewage also implies the emission of additional fossil CO2 from wastewater treatment facilities, sludge
management, and environmental recipients of treated or untreated wastewater.
Data emerging since the 2006 IPCC Guidelines indicate that wastewater contains an appreciable amount of nonbiogenic (fossil) organic carbon, with this fossil carbon thought to be derived from the use of petroleum-based
products (domestically and commercially). These products include: cosmetics; pharmaceuticals; surfactants;
detergents and food additives (Law et al. 2013). Additionally, direct dosing of synthetic, fossil-derived organic
substrates (e.g., methanol) can occur at wastewater treatment plants to enhance denitrification performance(Schneider et al. 2015). Despite early indications of the potential for significant fossil organic carbon fractions insewage sludge (Turekian & Benoit 1981), fossil carbon in wastewater was first detailed by Griffith et al. (2009)
following the sampling of treated effluent from 12 predominantly domestic wastewater treatment plants within
the Hudson and Connecticut River watersheds, USA. Since then, several studies have surveyed fossil organiccarbon in wastewater from Japan (Nara et al. 2010; Toyoda et al. 2011), Australia (Law et al. 2013), Denmark(Yoshida et al. 2014) and North America (Schneider et al. 2015).
Research published since the 2006 IPCC Guidelines (Griffith et al. 2009; Law et al. 2013; Yoshida et al. 2014;
Schneider et al. 2015; Tseng et al. 2016) gives an emerging consensus figure for the fossil wastewater organiccarbon fraction at some 4–14percent; although more recent work suggests this can be as high as 28percent(Tseng et al., 2016) or 51percent (Nara et al. 2010; recalculated by Tseng et al. 2016) in some influents.
Schneider et al. (2015) put the fossil content of activated sludge process off-gases at 10–15percent. Law et al.
(2013) put the subsequent fossil carbon fraction in anaerobic digester biogas CO2 at 2.1±0.2percent due togreater recalcitrance of fossil carbon during anaerobic digestion, and total additional scope 1 wastewater
treatment plant emission load from previously unaccounted fossil carbon somewhere between 2–12percent.
Tseng et al. (2016) put this figure at some 13 to 24percent higher without and with energy recovery respectively.
Based on the above data, countries are encouraged to evaluate if such emissions should be reported, particularly
those countries that have higher levels of fossil carbon in wastewater. In addition, future improvements to the
IPCC Guidelines should include a method for estimating these non-biogenic emissions associated with
wastewater treatment operations and wastewater discharges.

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Volume 5: Waste

TABLE 6AP.1 (NEW)SUMMARY OF LITERATURE INVESTIGATING FOSSIL ORGANIC CARBON IN WASTEWATER
Fossil C fraction in various
wastewater streams

Wastewater treatment system

Detection/measurement approach

Effluent from 12 WWTPs(using conventional activatedsludge) sampled, USA

Effluent grab samples; 14C analysed
via isotope ratio mass spectrometry

25% of treated wastewater dissolved
organic carbon (DOC) and 14% oftreated wastewater particulate organiccarbon (POC)

Griffith et al. (2009)

WWTPs handle predominantly domestic
wastewater. Secondary treated effluent
sampled only (i.e. not raw influent), so
results may overestimate true fossil carbon
fraction in raw sewage, since heterotrophic
bacteria are known to preferentially utilize
young (14C-enriched) carbon for assimilation(Raymond & Bauer 2001). Average reported
effluent DOC concentration was 8.7 mg/L.

Four large activated sludge
WWTPs in Brisbane, Australia(two receiving domestic
sewage and two receiving 15%
industrial load)Range of sampling locations(influent, primary- and secondarytreated effluent, waste activatedsludge (WAS), digested biosolids)

Fossil fraction of total organic carbon(TOC) in domestic WWTP influent 4–
7% and 8–14% for WWTPs receiving
15% industrial load (5–14% overall
range); ≈29–50% of this influent
fossil C is transformed to CO2 duringsecondary activated sludge treatment(1.4–6.3% of influent TOC). Higher
fossil C fraction in WAS from
WWTPs receiving industrial load (8–14%) versus domestic WWTPs (6–7%). 2.1(±0.2)% of biogas CO2 isfossil in origin.

Law et al. (2013)

Higher fossil C fractions in WWTPs
receiving industrial wastewater load.
Equivalent concentrations of fossil organiccarbon in raw wastewater were between 6–35
mg/L. Majority of fossil organic carbon inraw wastewater is present in particulate form(>80%; 5 to 29 mg/L), whereas dissolved
fossil carbon levels are relatively small (1–6
mg/L).

Influent to Avedøre WWTP,
Denmark

Single 24-hour flow proportionate
composite influent sample collectedin February 2013. Radiocarbon
isotope ratio method (ASTMD6866-12).

14(±3)% in influent wastewater

Yoshida et al. (2014)

Avedøre WWTP receives 15% industrial
load from adjacent pharmaceuticals industry.
Figure of 14% is corrected for cellulosic
biogenic carbon from toilet paper which may
contain elevated levels of 14C due tohistorical atomic bomb detonations andunderestimate fossil C fraction.

Modified Ludzack–Ettinger(MLE) activated sludge
process with biologicalnitrification–denitrification,USA

14C content of emitted CO
2

11.4–15.1% (mean 12.83%) based on
measured CO2 emissions fromsecondary treatment reactor

Schneider et al. (2015)

Three municipal WWTPs andwaste stabilisation ponds, two
industrial WWTPs

Raw and partially treatedwastewater, gas and sludge samples
taken during 2010–2013

2–28% in the primary influent

Tseng et al. (2016)

6.52

measured twice a day for five daysin early spring using floating
chambers

Reference

Comments

Article provides a tabulation of results from 6
separate research papers.

2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Chapter 6: Wastewater Treatment and Discharge

TABLE 6AP.1 (NEW) (CONTINUED)
SUMMARY OF LITERATURE INVESTIGATING FOSSIL ORGANIC CARBON IN WASTEWATER
Fossil C fraction in various
wastewater streams

Wastewater treatment system

Detection/measurement approach

Assumed conventional
activated sludge-based WWTP
with primary gravity
sedimentation, Japan

Fresh domestic sewage, primarytreated and secondary-treatedwastewater sampled∆14C values (‰) of sewage DOC was
more negative (14C ages in the orderof ≈1000–5000 years) than nearby
lake and river water; no fossil C
fraction given.

Nara et al. (2010)

No information on relative fossil C fraction
given.

A2O WWTP, Japan

13 wastewater samples collectedalong the treatment train during
March, 2008

δ13C signature of −50.7‰ (depletedwith respect to background air)Toyoda et al. (2011)

WWTP received municipal wastewater2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

Reference

Comments

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Volume 5: Waste

Annex 6A.1 Summary data for pit latrine use, no sanitation
facility, and groundwater use by country
TABLE 6A.1 (NEW)SUMMARY DATA FOR PIT LATRINE USE, NO SANITATION FACILITY, AND GROUNDWATER USE BY COUNTRY
Country

Data
Source1

ReportYear

Pit Latrine Use
for Sanitation(%)

No Sanitation
Facility (%)

Groundwater
Use forDrinking (%)

Afghanistan

MICS

2010–2011

42.5

17.7

62.9

Albania

DHS

2008–2009

24.0

0.0

20.8

Algeria

MICS

2006

2.2

4.9

13.4

Angola

MICS

2001

31.0

38.7

37.5

Armenia

DHS

2010

30.4

0.0

2.5

Azerbaijan

DHS

2006

58.8

0.3

25.7

Bangladesh

DHS

2007

60.1

7.5

90.8

Belarus

MICS

2005

27.5

0.0

12.5

Belize

MICS

2006

40.7

2.1

7.5

Benin

DHS

2006

11.7

69.5

38.6

Bhutan

MICS

2010

52.7

3.4

1.5

Bolivia

DHS

2008

25.7

28.4

7.9

Bosnia andHerzegovina

MICS

2006

2.5

0.1

15.7

Botswana

MICS

2000

57.2

16.2

2.9

Brazil

DHS

1996

42.2

Burkina Faso

MICS

2006

1.0

62.6

68.9

Burundi

MICS

2005

92.7

3.0

69.0

Cambodia

DHS

2010

5.7

55.3

50.0

Cameroon

MICS

2006

82.6

7.5

49.9

Central African
Republic

MICS

2006

75.4

22.3

69.1

Chad

DHS

2004

24.3

74.1

77.2

China

CHS04

2004

49.9

2.3

20.1

Colombia

DHS

2010

0.7

4.8

3.2

Comoros

MICS

2000

95.0

0.7

7.9

Congo

DHS

2005

84.4

10.2

30.6

Côte d'Ivoire

MICS

2006

42.7

34.0

51.8

Cuba

MICS

2010–2011

25.7

1.0

18.2

Democratic People's
Republic of Korea

MICS

2009

37.4

0.0

10.8

Democratic Republic of
the Congo

MICS

2010

80.0

14.5

59.1

Djibouti

MICS

2006

73.4

3.9

2.7

Dominican Republic

DHS

2007

47.6

36.0

9.7

Egypt

DHS

2008

0.4

4.0

Eritrea

DHS

2002

74.3

45.2

6.54

9.0

21.3

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Chapter 6: Wastewater Treatment and Discharge

TABLE 6A.1 (NEW) (CONTINUED)
SUMMARY DATA FOR PIT LATRINE USE, NO SANITATION FACILITY, AND GROUNDWATER USE BY COUNTRY
Country

Data
Source1

ReportYear

Pit Latrine Use
for Sanitation(%)

No Sanitation
Facility (%)

Groundwater
Use forDrinking (%)

Ethiopia

DHS

2011

56.0

38.2

50.6

Gabon

DHS

2000

92.8

3.0

40.0

GambiaMICS

2005–2006

80.6

4.4

41.0

Georgia

MICS

2005

57.8

0.0

25.7

Ghana

MICS

2010–2011

56.6

0.5

0.1

GuatemalaDHS

1998–1999

40.9

13.2

14.9

Guinea

DHS

2005

67.2

30.3

60.7

Guinea-Bissau

MICS

2006

4.0

31.1

79.1

Guyana

DHS

2009

43.3

1.0

3.5

Haiti

DHS

2005–2006

32.0

0.0

45.0

Honduras

DHS

2005–2006

39.8

16.7

15.8

India

DHS

2005–2006

12.9

56.2

58.6

Indonesia

DHS

2007

3.8

8.1

52.3

Iraq

MICS

2006

28.9

2.5

3.4

Jamaica

MICS

2005

77.2

0.6

3.3

Jordan

DHS

2009

43.5

0.0

0.0

Kazakhstan

MICS

2006

62.3

0.0

22.5

Kenya

DHS

2008–2009

67.3

14.5

41.4

Kyrgyzstan

MICS

2005–2006

82.0

0.1

10.1

Lao People's
Democratic Republic

MICS

2006

31.7

50.1

48.7

Lesotho

DHS

2009

66.8

35.6

36.2

Liberia

DHS

2007

20.1

54.7

76.2

Madagascar

DHS

2008–2009

35.1

43.7

53.3

Malawi

DHS

2010

84.9

9.9

75.3

Maldives

DHS

2009

27.8

1.0

1.3

Mali

DHS

2006

60.2

19.6

69.9

Mauritania

MICS

2007

35.1

45.5

37.7

Mongolia

MICS

2005

67.1

13.4

60.7

Montenegro

MICS

2005–2006

7.7

0.3

9.2

MoroccoDHS

2003–2004

1.7

15.9

16.3

Mozambique

MICS

2008

52.7

41.8

55.9

MyanmarMICS

2009–2010

74.9

7.0

73.6

Namibia

DHS

2006–2007

11.7

53.4

16.6

Nepal

DHS

2011

21.0

38.4

46.9

Nicaragua

DHS

2001

59.1

13.9

25.3

Niger

DHS

2006

21.5

78.0

74.3

Nigeria

MICS

2007

58.9

27.7

47.6

Pakistan

DHS

2006–2007

13.7

28.4

55.6

2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories

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Volume 5: Waste

TABLE 6A.1 (NEW) (CONTINUED)
SUMMARY DATA FOR PIT LATRINE USE, NO SANITATION FACILITY, AND GROUNDWATER USE BY COUNTRY
Country

Data
Source1

ReportYear

Pit Latrine Use
for Sanitation(%)

No Sanitation
Facility (%)

Groundwater
Use forDrinking (%)

Peru

DHS

2011

1.9

12.0

6.6

Philippines

DHS

2008

11.7

9.6

39.0

Republic of Moldova

DHS

2005

62.5

0.0

56.0

Rwanda

DHS

2010

96.6

1.1

59.0

Samoa

DHS

2009

10.0

0.1

3.8

Sao Tome and Principe

DHS

2008–2009

23.1

57.7

5.9

Senegal

DHS

2010–2011

57.1

16.5

27.6

Serbia

MICS

2010

4.5

0.0

8.5

Sierra Leone

MICS

2010

63.4

28.9

54.1

Somalia

MICS

2006

37.2

53.8

25.6

South Africa

DHS

2003

36.7

8.1

3.8

Sudan

MICS

2000

55.2

32.4

40.7

Suriname

MICS

2006

19.6

6.3

3.1

Swaziland

MICS

2010

69.7

15.4

19.3

Syrian Arab Republic

MICS

2006

18.2

1.0

7.7

Tajikistan

MICS

2005

85.5

0.4

14.2

TFYR Macedonia

MICS

2005

6.9

3.1

7.0

ThailandMICS

2005–2006

1.4

0.8

12.3

Timor-Leste

DHS

2009–2010

28.0

35.8

48.7

Togo

MICS

2010

31.5

55.7

55.2

Trinidad and Tobago

MICS

2006

15.0

0.1

1.2

Turkey

DHS

2003

22.8

0.5

7.3

Turkmenistan

DHS

2000

71.3

0.6

22.9

Uganda

DHS

2006

66.4

11.8

73.5

Ukraine

DHS

2007

47.2

0.0

28.0

United Republic of
Tanzania

DHS

2010

78.8

15.9

48.3

Uzbekistan

MICS

2006

87.4

0.0

20.1

Vanuatu

MICS

2007

77.3

3.2

22.7

Venezuela

MICS

2000

6.7

4.4

2.1

Viet Nam

MICS

2010–2011

18.2

6.4

43.7

Yemen

MICS

2006

42.1

21.4

35.9

Zambia

DHS

2007

57.1

23.5

47.1

Zimbabwe

DHS

2010–2011

42.6

28.3

64.0

Sources:1
MICS: Multiple Indicator Cluster Surveys, UNICEF (http://www.childinfo.org/mics.html); DHS: Demographic and Health Surveys,
USAID (http://www.measuredhs.com); CHS04: Economic, Population, Nutrition, and Health Survey, data accessed from WHO/UNICEF
Joint Monitoring Programme reports (http://wssinfo.org)

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Annex 6A.2 Derivation of the maximum CH4 producing potential(Bo) for domestic wastewater
The maximum CH4 producing potential (Bo) for domestic wastewater is calculated theoretically by the anaerobic
decomposition of glucose considering the total degradation of organic matter:

C6 H12 O6 → 3CO2 + 3CH 4
One mole of glucose weighs 180 g and produces 3 moles of methane which weighs 3 × 16 = 48 g. Therefore, themethane production rate per gram of glucose is 48 g /180 g = 0.27 g methane / g glucose.
The complete oxidation of one mole of glucose (180 g) requires six moles of oxygen (6 × 32 g = 192 g)

C6 H12 O6 + 6O2 → 6CO2 + 6 H 2 O
Then, the oxygen demand rate per gram of glucose is 192 g / 180 g = 1.067 g oxygen / g glucose
Finally, the maximum CH4 producing potential (Bo) for domestic wastewater based of COD content is 48 g CH4/ 192 g oxygen = 0.25 kg CH4 / kg COD.
The COD/BOD ratio of a specific wastewater indicates the amount of organic matter difficult to degrade. For
domestic wastewater, a typical COD/BOD ratio is 2.4 based on empirical tests. However, the ratio between thecomponents in any given domestic wastewater stream may vary due to contributions from other sources,
particularly wastewater contributions from commercial or industrial sources. The expected range of COD/BOD
ratios is between 1.5 and 3.5 (Henze et al. 2008).
Inventory compilers should compare country-specific data on COD/BOD ratio in domestic wastewater to IPCC
default values of 2.4. If inventory compilers use country-specific COD/BOD ratio, they should provide
documented justification why their country-specific values are more appropriate for their national circumstances.
It should be noted that the emission factors are made up of a) the maximum CH4 producing capacity (Bo) and b)
MCF which indicates the extent to which the Bo is released in each wastewater treatment/discharge pathways or
systems. If a country chooses to introduce country-specific data for Bo based on measured composition ofwastewater, they must also update the MCF because the MCFs were developed using the default Bo values.

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Annex 6A.3 Estimation of default methane conversion factors forCH4 in centralised wastewater treatment plants treating domestic
wastewater
We reviewed scientific literature to find measured CH4 emissions from full-scale domestic wastewater treatment
plants (excluding anaerobic sludge digestion). Although much literature exists, relatively few studies relate to
full-scale treatment plants and provide key information such as influent organics in wastewater or organics
removed in sludge. We reviewed and accepted data from 14 wastewater treatment plants. MCFs were calculated
from data presented in studies, including measured CH4 emissions from wastewater treatment plant (notincluding sludge digesters), influent organics in wastewater, estimates of organics removed in sludge, and the
IPCC default Bo value. Some data were not accepted for use because the treatment system co-treated asignificant percentage of non-domestic wastewater, such as food waste. The references used, along with the typeof treatment system studied and the resulting MCF, are listed in Table 6A.2. Although CH4 emissions vary by
the type of wastewater treatment system used, more on-site exhaustive monitoring data are required to develop
different MCFs for different treatment processes. Overall, an MCF of 0.03 was calculated for aerobic wastewater
treatment systems with a standard deviation of 0.024 and a range of 0.003–0.09.
TABLE 6A.2 (NEW)MCFS BASED ON MEASURED CH4 IN FULL-SCALE DOMESTIC WASTEWATER TREATMENT PLANTS

Type of treatment process(Name of plant)

Reference

MCF1

Activated sludge (Stickney)

Bellucci et al. (2010)

0.017

Activated sludge (Northside)

Bellucci et al. (2010)

0.004

Activated sludge (Egan)

Bellucci et al. (2010)

0.014

Activated sludge

Czepiel et al. (1993)

0.013

Biological nutrient removal (Kralingseveer)

Daelman et al. (2013)

0.03

Biological nutrient removal (Kortenoord)

Daelman et al. (2013)

0.02

Biological nutrient removal (Papendrecht)

Daelman et al. (2013)

0.04

Sequencing batch reactor (Holbæk)Delre et al. (2017)

0.038

Activated sludge (Källby)

Delre et al. (2017)

0.048

Biological nutrient removal (Lundtofte)

Delre et al. (2017)

0.014

Biological nutrient removal (Lynetten)

Delre et al. (2017)

0.015

Activated sludge

Kozak et al. (2009)0.09

5-stage Bardenpho

Kyung et al. (2015)

0.07

Anaerobic/anoxic/oxic (A2O) process

Wang et al. (2011)

0.003

MCF shown was calculated from data presented in studies, including measured CH4 emissions from wastewater treatment plant (notincluding sludge digesters), influent organics in wastewater, estimates of organics removed in sludge, and the IPCC default Bo value.
1

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Chapter 6: Wastewater Treatment and Discharge

Annex 6A.4 Calculation of MCF for methane emissions fromsewage discharges
Data were obtained from a recent global review article by Deemer et al. (2016) for CO2 and CH4 emissions fromdifferent environments. At equilibrium with current atmospheric conditions, CO2 is relatively soluble (over 500μg/L) compared with CH4 (about 40 ng/L) at 25 degrees Celsius. Therefore, a reliance on studies that only
measure diffusive flux will underestimate the relative emission of CH4. Deemer et al. (2016) raised previous
estimates of CH4 emissions by excluding data from sources that did not include assessment of ebullition.
Regarding emissions from all kinds of reservoirs, this exclusion had the effect of increasing the relative CH4 flux
(CH4-C/(CH4-C + CO2-C)) to 27 percent (first data row in Table 6A.3) which is higher than previously thought.
For an estimate of this effect on the other data, it is noted that about 52 percent of the data listed by Deemer andcolleagues included ebullition, and for this about 40–60 percent of the total ebullitive and diffusive flux was due
to ebullition. To illustrate the potential effect of adding ebullition to the reservoir studies a scaling factor of 1.48
= 0.52/1 + 0.48/0.5 is applied for reservoirs and hydroelectric reservoirs in the table. Deemer et al. (2016) used
(Bastviken et al. 2011) for an estimate of methane emissions from lakes and the original data (Table 1 in
Bastviken and colleauges) suggests a smaller correction factor for that lake data of 1.23. For rivers, Stanley et al.
(2016) reports a relationship similar to Deemer et al’s observation – that of total flux, 46 percent is ebullitive
among those studies reporting both kinds of fluxes (see Table 1 in Stanley et al. 2016). Correcting the average ofthe larger dataset in Stanley et al. (2016) with this figure, suggests a range of riverine yields of 0.4–6 percent.
Note that the riverine CO2 figure in Deemer et al. (2016) of 7,954 mg C/m2/day was based on original 1,800 Tg
C/y datum in Raymond et al. (2013), which has been reduced using the Lauerwald et al. (2015) updated estimateof 659 Tg C/y figure and is thus 2,872 mg C/m2/day.
Considering the data in Table 6A.3, the average yield for reservoirs and lakes is 19percent (8–27 percent) while
the average for rivers is much lower at 3.5 percent (0.4–6 percent). Data ranges rather than standard deviations
are given owing to the potential for overlap between the underlying datasets. These CH4 yield data are used toestimate the MCF. On the same basis as the Bo calculation, 1.067 kg of O2 are required to oxidise one kg of
glucose. Considering that 40 percent of glucose is carbon, this implies the COD of glucose on a carbon basis is2.67 kg COD/kg C. So 0.19 kg CH4-C/kg (CH4-C + CO2-C) corresponds to 0.071 kg CH4-C/kg COD. Taking a
global perspective, Tranvik et al. (2009) state that approximately 1.4 Pg of carbon is emitted from inland waters
at CO2 and a further 0.1 Pg as CH4, the remainder being sequestered in sediments or lost to the ocean. Thus of
3.0 Pg of carbon inputs, 50 percent is emitted to the atmosphere. Correcting the previous figure for this yield andconverting to kg CH4 suggests 0.071 [kg CH4-C/kg COD] × (12.01 + 1.00797×4)/12.01 [kg CH4/kg CH4-C] ×
0.5 = 0.048 kg CH4/kg COD. For use in conjunction with Bo then MCF = 0.048/0.25 = 0.19 (0.08–0.27) for lakes
and reservoirs. In the same way, MCF = 0.035 (0.004–0.06) for rivers and streams.

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TABLE 6A.3 (NEW)SUMMARY OF LITERATURE INVESTIGATING METHANE EMISSIONS FROM WASTEWATER DISCHARGE
Type of
WaterbodyReservoirs
Reservoirs

Hydroelectricreservoirs
Lakes
Rivers

6.60

CH4 flux (mgC/m2/day)n (reservoirs,
lakes or rivers)

CH4 flux corrected
for ebullition(mg C/m2/day)

CO2 flux (mgC/m2/day)Yield

Approx.
revised yield

120

75

120

330

27%

27%

82–96

161

121–142498

14–16%

20–22%

CH4: Bastviken et al. (2011), St. Louiset al. (2000); CO2: St Louis et al.
(2000)

24–11285, 104

48–224

386–660

6–15%

8–20%

Barros et al. (2011); Li and Zhang(2014)

40

66

80

216

16%

19%

CH4: Bastviken et al. (2011); CO2
Raymond et al. (2013)

6–98

21, 26

11–1832872

0.21–3.3%

0.39–6.0%

CH4: Bastviken et al. (2011), Stanley
et al. 2016; CO2: Lauerwald et al.
(2015)

Reference
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Chapter 6: Wastewater Treatment and Discharge

Annex 6A.5 Estimation of default emission factors for N2O indomestic wastewater treatment plants
We reviewed scientific literature to find emission factors from full-scale domestic wastewater treatment plants.
Although much literature exists regarding N2O emission from wastewater treatment processes, relatively few
studies relate to full-scale treatment plants and provide key information such as influent nitrogen load. We
collected 30 data and found that N2O emission correlated with influent nitrogen load (Figure 6A.1). Emission
factors and references are listed in Table 6A.4 and Table 6A.5. Specific and relatively new/uncommon treatment
processes such as membrane bioreactors were intentionally excluded from this list in order to develop emission
factors for the most typical and widely used treatment processes globally. Although N2O emissions vary by thetype of nitrogen removal process used, more on-site exhaustive monitoring data are required to develop different
N2O emission factors for different treatment processes.
Figure 6A.1 (New) Correlation between influent total nitrogen (TN) loading and N 2 O emissions

TABLE 6A.4 (NEW)DEFAULT N2O EMISSION FACTORS FOR DOMESTIC WASTEWATER TREATMENT PLANTS

N2O emission factor (kg N2O-N/kg N)

SD

Maximum

Minimum

0.016

0.012

0.045

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TABLE 6A.5 (NEW)N2O EMISSION FACTORS IN FULL-SCALE DOMESTIC WASTEWATER TREATMENT PLANTS

Type of treatment
process1

Categories

References

N2O emission factor(kg N2O-N/kg N)

AO

Biological Nitrogen
Removal (BNR)

Daelman et al. (2015)

AO

BNR

Foley et al. (2010)

0.021

AO

BNR

Foley et al. (2010)

0.045

A2O

BNR

Foley et al. (2010)

0.013

SBR

BNR

Foley et al. (2010)

0.023

OD

BNR

Foley et al. (2010)

0.0080IA

BNR

Kimochi et al. (1998)

0.0005EA

BNR

Foley et al. (2010)

0.015

A2O

BNR

Wang et al. (2016)

0.013

CAS

BNR

Aboobakar et al. (2013)

0.00036AO

BNR

Rodriguez-Caballero et al. (2014)

0.00116OD

BNR

Masuda et al. (2018)

0.00016

AO

BNR

Masuda et al. (2018)

0.0013AO

BNR

Masuda et al. (2018)

0.0049Separate-stage BNR

BNR

Ahn et al. (2010)

0.00019

Bardenpho

BNR

Ahn et al. (2010)

0.0036

Step-feed BNR

BNR

Ahn et al. (2010)

0.011

MLE

BNR

Ahn et al. (2010)

0.0007MLE

BNR

Ahn et al. (2010)

0.0006OD

BNR

Ahn et al. (2010)

0.0003

Step-feed BNR

BNR

Ahn et al. (2010)

0.015

Step feed, plug flow

BNR

Ni et al. (2015); Pan et al. (2016)

0.019

SBR

BNR

Bao et al. (2016)

0.029

SBR

BNR

Rodriguez-Caballero et al. (2015)

0.038

Plug flow

Non-BNRAhn et al. (2010)

0.0041Plug flow

Non-BNRAhn et al. (2010)

0.0036

Step-feed non-BNR

Non-BNRAhn et al. (2010)

0.010

Plug flow

Non-BNR

Masuda et al. (2015)

0.023

AO

Non-BNR

Bao et al. (2016)

0.013

IA

Non-BNRde Mello et al. (2013)

0.0016

0.028

AO; Anaerobic-oxic activated sludge process, A2O; Anaerobic-anoxic-oxic activated sludge process, SBR; Sequencing batch reactor, OD;
Oxidation ditch, IA; Intermittent aeration process, EA; Extended aeration process, CAS; Conventional activated sludge process, MLE;
Modified Ludzack-Ettinger.
1

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Chapter 6: Wastewater Treatment and Discharge

Annex 6A.6 Estimation of default emission factors for N2O in
effluent
Table 6.11 in the 2006 IPCC Guidelines was based on expert judgment. For the refinement of these guidelines,
literature was reviewed for reported yields of N2O from environments with known oxygen saturation conditions.
A total of 62 data points for well-oxygenated environments and 59 for low-oxygen environments were obtained
from the following literature: Kaplan et al. (1978); McElroy et al. (1978); Goreau et al. (1980); McCarthy et al.
(1984); Seitzinger et al. (1984); Kaplan and Wofsy (1985); Seitzinger (1988); Yoshida (1988); Mantoura et al.
(1993); Kester et al. (1997); Seitzinger and Kroeze (1998); Punshon and Moore (2004); Frame and Casciotti(2010); Beaulieu et al. (2011); Yan et al. (2012); Zhu et al. (2013); Ji et al. (2015); Wang et al. (2015); He et al.
(2017).
These data suggested a mean yield of 0.019 kg N2O-N/kg-N for low oxygen environments (for use in Tier 3
methodologies, if needed) and 0.005 kg N2O-N/kg-N for well-oxygenated environments (default Tier 1 factor).
Calculation of a simple standard deviation is inappropriate for this data, since it is a proportion. Therefore,
confidence intervals for these averages were calculated using the adjusted Wald method for small proportions(Bonett & Price 2012). The resulting 95percent (two tailed) limits are 0.0041–0.0912 and 0.0–0.0753
respectively.

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Annex 6A.7 List of countries by region included in Table 6.10a
Table 6.10a in Section 6.3.1.3 presents default factors for protein consumed as a fraction of protein supplied,fraction of food not consumed and disposed in sewer, and a fraction to represent additional nitrogen introduced
to the sewer from household chemicals. The information is provided be geographic region. The list of countries
by region can be found in Table 6A.6.
TABLE 6A.6 (NEW)LIST OF COUNTRIES BY REGION INCLUDED IN TABLE 6.10A

RegionCountry

EuropeAlbania, Armenia, Austria, Azerbaijan, Belarus, Belgium, Bosnia andHerzegovina, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark,Estonia, Finland, France, Georgia, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxemburg, Macedonia, Moldova,
Montenegro, Netherlands, Norway, Poland, Portugal, Romania, RussianFederation, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Ukraine, United Kingdom

North America and Oceania

Australia, Canada, New Zealand, United States of America

Industrialized Asia

Japan, China, South Korea

Sub-Saharan Africa

Angola, Benin, Botswana, Burkina Faso, Burundi, Cameroon, Central
African Rep., Chad, Congo-Brazzaville, Congo-Kinshasa, Cote d’Ivoire,
Equatorial Guinea, Eritrea, Ethiopia, Gabon, Gambia, Ghana, Guinea,
Guinea-Bissau, Kenya, Lesotho, Liberia, Malawi, Mali, Mauritania,
Mozambique, Namibia, Niger, Nigeria, Rwanda, Senegal, Sierra Leone,
Somalia, South Africa, Sudan, Swaziland, Tanzania, Togo, Uganda,
Zambia, Zimbabwe

North Africa, West and Central Asia

Algeria, Egypt, Iraq, Israel, Jordan, Kazakhstan, Kuwait, Kyrgyzstan,
Lebanon, Libya, Mongolia, Morocco, Oman, Saudi Arabia, Syria,
Tajikistan, Tunisia, Turkey, Turkmenistan, Utd Arab Emirates,
Uzbekistan, Yemen

South and Southeast Asia

Afghanistan, Bangladesh, Bhutan, Cambodia, India, Indonesia, Iran, Laos,
Malaysia, Myanmar, Nepal, Pakistan, Philippines, Sri Lanka, Thailand,
Vietnam

Latin America

Argentina, Belize, Bolivia, Brazil, Chile, Colombia, Costa Rica, Cuba,
Dominican Rep., Ecuador, El Salvador, Guatemala, Guyana, Haiti,
Honduras, Jamaica, Mexico, Nicaragua, Panama, Paraguay, Peru,
Suriname, Uruguay, Venezuela

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