Domestic wastewater 6221 Choice of method

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A decision tree for domestic wastewater is included in Figure 6.2.

Figure 6.2 Decision Tree for CH4 emissions from domestic wastewater

Figure 6.2 Decision Tree for CH4 emissions from domestic wastewater

Policy Factors

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.

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.

The steps for good practice in inventory preparation for CH4 from domestic wastewater are as follows:

Step 1: Use Equation 6.3 to estimate total organically degradable carbon in wastewater (TOW).

Step 2: Select the pathway and systems (See Figure 6.1) according to country activity data. Use Equation 6.2 to obtain the emission factor for each domestic wastewater treatment/discharge pathway or system.

Step 3: Use Equation 6.1 to estimate emissions, adjust for possible sludge removal and/or CH4 recovery and sum the results for each pathway/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 good practice to refer to national statistics (e.g., from regulatory authorities). If these data are not available, wastewater associations or international organisations such as the World Health Organization (WHO) may have data on the system usage.

Otherwise, consultation with sanitation experts can help, and expert judgement 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 sludge separation is practised and appropriate statistics are available, then this category should be separated out as a subcategory. If default factors are being used, emissions from wastewater and sludge should be estimated together. Regardless of how sludge is treated, it is important that CH4 emissions from sludge sent to landfills, incinerated or used in agriculture are not included in the wastewater treatment and discharge category. If sludge removal data are available, 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 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, a factor U is introduced to express 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 a spreadsheet, as shown in Table 6.5 below.

The general equation to estimate CH4 emissions from domestic wastewater is as follows:

Equation 6.1 Total CH4 emissions from domestic wastewater

CH4 Emissions =

Where:

CH4 Emissions = CH4 emissions in inventory year, kg CH4/yr TOW = total organics in wastewater in inventory year, kg BOD/yr organic component removed as sludge in inventory year, kg BOD/yr fraction of population in income group i in inventory year, See Table 6.5.

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

income group: rural, urban high income and urban low income each treatment/discharge pathway or system emission factor, kg CH4 / kg BOD

amount of CH4 recovered in inventory year, kg CH4/yr

6.2.2.2 Choice of emission factors

The emission factor for a wastewater treatment and discharge pathway and system (terminal blocks with bold frames in Figure 6.1) is a function of the maximum CH4 producing potential (Bo) and the methane correction factor (MCF) for the wastewater treatment and discharge system, as shown in Equation 6.2. The Bo is the maximum amount of CH4 that can be produced from a given quantity of organics (as expressed in BOD or COD) in the wastewater. The MCF indicates the extent to which the CH4 producing capacity (Bo) is realised in each type of treatment and discharge pathway and system. 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 system

Where:

EFj = emission factor, kg CH4/kg BOD

j = each treatment/discharge pathway or system

Bo = maximum CH4 producing capacity, kg CH4/kg BOD

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

Good practice is to use country-specific data for Bo, where available, expressed in terms of kg CH4/kg BOD removed to be consistent with the activity data. If country-specific data are not available, a default value, 0.6 kg CH4/kg BOD can be used. For domestic wastewater, a COD-based value of Bo can be converted into a BOD-based value by multiplying with a factor of 2.4. Table 6.2 includes default maximum CH4 producing capacity (Bo) for domestic wastewater.

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)

Table 6.3 includes default MCF values.

Table 6.3

Default MCF values for domestic wastewater

Type of treatment and discharge pathway or system

Comments

MCF 1

Range

Untreated system

Sea, river and lake discharge

Rivers with high organics loadings can turn anaerobic.

O.1

O - O.2

Stagnant sewer

Open and warm

O.5

O.4 - O.8

Flowing sewer (open or closed)

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

O

O

Treated system

Centralized, aerobic treatment plant

Must be well managed. Some CH4 can be emitted from settling basins and other pockets.

O

O - O.1

Centralized, aerobic treatment plant

Not well managed. Overloaded.

O.3

O.2 - O.4

Anaerobic digester for sludge

CH4 recovery is not considered here.

O.8

O.8 - 1.O

Anaerobic reactor

CH4 recovery is not considered here.

O.8

O.8 - 1.O

Anaerobic shallow lagoon

Depth less than 2 metres, use expert judgment.

O.2

O - O.3

Anaerobic deep lagoon

Depth more than 2 metres

O.8

O.8 - 1.O

Septic system

Half of BOD settles in anaerobic tank.

O.5

O.5

Latrine

Dry climate, ground water table lower than latrine, small family (3-5 persons)

O.1

O.O5 - O.15

Latrine

Dry climate, ground water table lower than latrine, communal (many users)

O.5

O.4 - O.ó

Latrine

Wet climate/flush water use, ground water table higher than latrine

O.7

O.7 - 1.O

Latrine

Regular sediment removal for fertilizer

O.1

O.1

1 Based on expert judgment by lead authors of this section.

6.2.2.3 Choice 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 in terms of biochemical oxygen demand (kg BOD/year). The equation for TOW is:

Equation 6.3

Total organically degradable material in domestic wastewater

Where:

TOW = total organics in wastewater in inventory year, kg BOD/yr P = country population in inventory year, (person)

BOD = country-specific per capita BOD in inventory year, g/person/day, See Table 6.4. 0.001 = conversion from grams BOD to kg BOD

I = correction factor for additional industrial BOD discharged into sewers

(for collected the default is 1.25, for uncollected the default is 1.00.)

The factor I values in Equation 6.3 are based on expert judgment by the authors. It expresses the BOD from industries and establishments (e.g., restaurants, butchers or grocery stores) that is co-discharged with domestic wastewater. In some countries, information from industrial discharge permits may be available to improve I. Otherwise, expert judgment is recommended. Total population statistics should be readily available from national statistics agencies or international agencies (e.g., United Nations Statistics, see http://esa.un.org/unpp/). Table 6.4 includes BOD default values for selected countries. It is good practice to select a BOD default value from a nearby comparable country when country-specific data are not available. The degree of urbanization 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. Table 6.5 includes default values of U and Ty for selected countries.

Table 6.4

Estimated BOD5 values in domestic wastewater for selected regions and countries

(g/person/day)

Range

Reference

Africa

37

35 - 45

1

Egypt

34

27 - 41

1

Asia, Middle East, Latin America

4G

35 - 45

1

India

34

27 - 41

1

West Bank and Gaza Strip (Palestine)

5G

32 - 68

1

Japan

42

4G - 45

1

Brazil

5G

45 - 55

Canada, Europe, Russia, Oceania

6G

5G - 7G

1

Denmark

62

55 - 68

1

Germany

62

55 - 68

1

Greece

57

55 - 6G

1

Italy

6G

49 - 6G

Sweden

75

68 - 82

1

Turkey

38

27 - 5G

1

United States

85

5G - 12G

4

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

4. Metcalf and Eddy (2003).

Table 6.5

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

Table 6.5

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

Urbanization(U) 1

Degree of utilisation of treatment or discharge pathway or method for each income group (Ty)

Fraction of Population

U=rural

U= urban high

income

U=urban low income

Country

Rural

urban-high2

urban-low2

Septic Tank

Latrine

Other

Sewer4

None

Septic Tank

Latrine

Other

Sewer4

None

Septic Tank

Latrine

Other

Sewer4

None

Africa

Nigeria

0.52

0.10

0.38

0.02

0.28

0.04

0.10

0.56

0.32

0.31

0.00

0.37

0.00

0.17

0.24

0.05

0.34

0.20

Egypt

0.57

0.09

0.34

0.02

0.28

0.04

0.10

0.56

0.15

0.05

0.10

0.70

0.00

0.17

0.24

0.05

0.34

0.20

Kenya

0.62

0.08

0.30

0.02

0.28

0.04

0.10

0.56

0.32

0.31

0.00

0.37

0.00

0.17

0.24

0.05

0.34

0.20

South Africa

0.39

0.12

0.49

0.10

0.28

0.04

0.10

0.48

0.15

0.15

0.00

0.70

0.00

0.17

0.24

0.05

0.34

0.20

Asia

China

0.59

0.12

0.29

0.00

0.47

0.50

0.00

0.3

0.18

0.08

0.07

0.67

0.00

0.14

0.10

0.03

0.68

0.05

India

0.71

0.06

0.23

0.00

0.47

0.10

0.10

0.33

0.18

0.08

0.07

0.67

0.00

0.14

0.10

0.03

0.53

0.20

Indonesia

0.54

0.12

0.34

0.00

0.47

0.00

0.10

0.43

0.18

0.08

0.00

0.74

0.00

0.14

0.10

0.03

0.53

0.20

Pakistan

0.65

0.07

0.28

0.00

0.47

0.00

0.10

0.43

0.18

0.08

0.00

0.74

0.00

0.14

0.10

0.03

0.53

0.20

Bangladesh

0.72

0.06

0.22

0.00

0.47

0.00

0.10

0.43

0.18

0.08

0.00

0.74

0.00

0.14

0.10

0.03

0.53

0.20

Japan

0.20

0.80

0.00

0.20

0.00

0.50

0.30

0.00

0.00

0.00

0.10

0.90

0.00

0.10

0

0

0.90

0

Europe

Russia

0.27

0.73

0.00

0.30

0.10

0.00

0.60

0.00

0.10

0.00

0.00

0.90

0.00

NA

NA

NA

NA

NA

Germany5

0.06

0.94

0.00

0.20

0.00

0.00

0.80

0.00

0.05

0.00

0.00

0.95

0.00

NA

NA

NA

NA

NA

United Kingdom

0.10

0.90

0.00

0.11

0.00

0.00

0.89

0.00

0.00

0.00

0.00

1.00

0.00

NA

NA

NA

NA

NA

France

0.24

0.76

0.00

0.37

0.00

0.00

0.63

0.00

0.00

0.00

0.00

1.00

0.00

NA

NA

NA

NA

NA

Italy

0.32

0.68

0.00

0.42

0.00

0.00

0.58

0.00

0.04

0.00

0.00

0.96

0.00

NA

NA

NA

NA

NA

North America

United States

0.22

0.78

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

Canada

0.20

0.80

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

Latin America

and Caribbean

Brazil

0.16

0.25

0.59

0.00

0.45

0.00

0.10

0.45

0.00

0.20

0.00

0.80

0.00

0.00

0.40

0.00

0.40

0.20

Mexico

0.25

0.19

0.56

0.00

0.45

0.00

0.10

0.45

0.00

0.20

0.00

0.80

0.00

0.00

0.40

0.00

0.40

0.20

Oceania

Australia and

New Zealand

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

Urbanization projections for 2005 (United Nations, 2002).

Suggested urban-high income and urban low income division. Countries are encouraged to use their own data or best judgment.

Ty values based on expert judgment, (Doom and Liles, 1999).

Sewers may be open or closed, which will govern the choice of MCF, see Table 3.3

Destatis, 2001.

Urbanization projections for 2005 (United Nations, 2002).

Suggested urban-high income and urban low income division. Countries are encouraged to use their own data or best judgment.

Ty values based on expert judgment, (Doom and Liles, 1999).

Sewers may be open or closed, which will govern the choice of MCF, see Table 3.3

Destatis, 2001.

Note: These values are from the literature or based on expert judgment. Please use national values, if available.

Example

Table 6.6 includes an example. Categories with negligible contributions are not shown. Note that the table can easily 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 values

FOR DEGREES OF TREATMENT UTILIZATION (T) BY INCOME GROUPS

Treatment or discharge system or pathway

T (%)

Notes

Urban high-income

To sea

10

No CH4

To aerobic plant

20

Add industrial component

To septic systems

10

Uncollected

Urban low-income

To sea

10

Collected

To pit latrines

15

Uncollected

Rural

To rivers, lakes, sea

15

Uncollected

To pit latrines

15

To septic tanks

5

Total

100%

Must add up to 100 %

Reference: Doorn and Liles (1999)

6.2.2.4 Time series consistency

The same method and data sets should be used for estimating CH4 emissions from wastewater for each year. The MCF for different treatment systems should not change from year to year, unless such a change is justifiable and documented. If the share of wastewater treated in different treatment systems changes over the time period, the reasons for these changes should be documented.

Sludge removal and CH4 recovery should be estimated consistently across years in the time series. Methane recovery should be included only if there are sufficient facility-specific data. The quantity of recovered methane should be subtracted from the methane produced as shown in Equation 6.1.

Because activity data are derived from population data, which is available for all countries and all years, countries should be able to construct an entire time series for uncollected and collected wastewater. If data on the share of uncollected wastewater treated onsite vs. untreated are missing for one or more years, the surrogate data and extrapolation/interpolation splicing techniques described in Chapter 5, Time Series Consistency, of Volume 1, General Guidance and Reporting, can be used to estimate emissions. Emissions from wastewater typically do not fluctuate significantly from year to year.

6.2.2.5 Uncertainties

Chapter 3, Uncertainties, in Volume 1 provides advice on quantifying uncertainties in practice. It includes guidance on eliciting and using expert judgements which in combination with empirical data can provide overall uncertainty estimates. Table 6.7 provides default uncertainty ranges for emission factor and activity data of domestic wastewater. The following parameters are believed to be very uncertain:

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

• 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 is very difficult to quantify.

Table 6.7

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 the uncertainty range is also technology dependent. The uncertainty range should be determined by expert judgement, bearing in mind that MCF is a fraction and must be between 0 and 1. Suggested ranges are provided below.

Untreated systems and latrines, ± 50%

Lagoons, poorly managed treatment plants± 30%

Centralized well managed plant, digester, reactor, ± 10%

Activity Data

Human population (P)

± 5%

BOD per person

± 30%

Fraction of population income group (U)

Good data on urbanization are available, however, the distinction between urban high income and urban low income may have to be based on expert judgment. ± 15%

Degree of utilization 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 Ty = 100%

Correction factor for additional industrial BOD discharged into sewers (I)

For uncollected, the uncertainty is zero %. For collected the uncertainty is ± 20%

Source: Judgement by Expert Group (Authors of this section).

6.2.2.6 QA/QC, Completeness, Reporting and Documentation

It is good practice to conduct quality control checks and quality assurance procedures as outlined in Chapter 6,

Volume 1. Below, some fundamental QA/QC procedures are included.

Activity Data

• Characterize all wastewater according to the percentages flowing to different treatment systems (aerobic and anaerobic), and the percentage of untreated wastewater. Make sure that all wastewater is characterized so that the wastewater flows sum to 100 percent of the wastewater generated in the country.

• Inventory compilers should compare country-specific data on BOD in domestic wastewater to IPCC default values. If inventory compilers use country-specific values they should provide documented justification why their country-specific values are more appropriate for their national circumstances.

Emission Factors

• For domestic wastewater, inventory compilers can compare country-specific values for Bo with the IPCC default value (0.25 kg CH4/kg COD or 0.6 kg CH4/kg BOD). Although there are no IPCC default values for the fraction of wastewater treated anaerobically, inventory compilers are encouraged to compare values for MCFs against those from other countries with similar wastewater handling practices.

• Inventory compilers should confirm the agreement between the units used for degradable carbon in the waste (TOW) with the units for Bo. Both parameters should be based on the same units (either BOD or COD) in order to calculate emissions. This same consideration should be taken into account when comparing the emissions.

CH4 Recovery and Sludge Removal

• A carbon balance check can be used to ensure that the carbon contained in the inflow and outflow (effluent BOD, methane emission and methane recovery) are comparable.

• 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.

Comparison of emissions estimates using different approaches

• For countries that use country-specific parameters, or Tier 2 or higher methods, inventory compilers can cross-check the national estimate with emissions using the IPCC default method and parameters.

COMPLETENESS

Completeness can be verified on the basis of the degree of utilization of a treatment or discharge system or pathway (T). The sum of T should equal 100 percent. It is a good practice to draw a diagram similar to Figure 6.1 for the country to consider all potential anaerobic treatment and discharge systems and pathways, including collected and uncollected, as well as treated and untreated. Any industrial wastewater treated in domestic wastewater treatment facilities should be included in the collected category. If sludge is removed for the purpose of incineration, disposal in landfills or as fertilizer on agricultural lands, the amount of organic material removed as sludge should be consistent with data used in the relevant sectors (see text under Section 6.2.2).

REPORTING AND DOCUMENTATION

It is good practice to document and report a summary of the methods used, activity data and emission factors. Worksheets are provided at the end of this volume. 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 judgement, etc.) have been derived (measurements, literature, expert judgement, 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, SWDS, or agricultural categories, respectively.

Where CH4 is recovered for energy use, then the resulting greenhouse gas emissions should be reported under Energy Sector. As discussed in Section 6.2.1, good practice in the Waste Sector does not require the estimation of CH4 and N2O from CH4 recovery and flaring. However, if it is wished to do so emissions from flaring should be reported under the Waste Sector.

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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