representing land areas and on land-use databases used for the inventory
8.3 Wastewater handling [6B] 1 Source category description
This source category covers emissions released from wastewater handling and includes emissions from industrial, commercial and domestic wastewater and septic tanks.
In 2012, the mixture of domestic, commercial and industrial wastewaters was treated aerobically in 343 public wastewater treatment plants (WWTP). The
treatment of the resulting wastewater sludge was accomplished mainly by using anaerobic processes. During wastewater treatment, the biological breakdown of degradable organic compounds (DOC) and nitrogen compounds can result in CH4 and N2O emissions, respectively. The discharge of effluents, as well as other direct discharges, subsequently results in indirect N2O emissions from surface waters due to the natural breakdown of residual nitrogen compounds. The source category also includes CH4 emissions from a total of 53 anaerobic industrial wastewater treatment plants (IWWTP). Moreover, as 0.6% of the resident population is still connected to a septic tank, CH4 and N2O emissions from septic tanks are also calculated, but these are small compared with those from public WWTP.
N2O emissions from wastewater treatment (see Table 8.1) accounted for approximately 5 per cent of total N2O emissions in 2012 and 0.3 per cent in total CO2-equivalent. N2O emissions from wastewater handling and effluents decreased by 5 per cent during the 1990–2012 period. This small decrease was the result of two counteracting trends. Improved biological breakdown of nitrogen compounds at public WWTPs (see Table 8.4) has led to a gradual increase in N2O emissions. Improved nitrogen removal and lower untreated discharges, however, has resulted in lower effluent loads (see Table 8.4) and a subsequent decrease in (indirect) N2O emissions from human sewage.
The contribution of wastewater handling to the national total of CH4 emissions in 2012 was 1.3 per cent. Since 1994, CH4 emissions from public WWTPs have decreased, due to the introduction of a new sludge stabilization system in one of the largest wastewater treatment plants in 1990. Because the operation of the plant took a few years to optimize, venting emissions were higher during the introductory period (1991–1994) than they were under normal operating conditions. CH4 emissions from wastewater handling decreased by 31 per cent during the period 1990–2012. The amount of wastewater and sludge being treated does not change much over time. The interannual changes in methane emissions, therefore, can be explained by varying fractions of methane being vented incidentally, instead of flared or used for energy purposes. It should be noted that non-CO2 emissions from the combustion of biogas at wastewater treatment facilities are allocated to category 1A4 (Fuel combustion – other sectors) because this combustion is partly used for heat or power generation at the treatment plants.
Table 8.3 shows the trend in greenhouse gas emissions from the different sources of wastewater handling.
8.3.2 Methodological issues
Activity data and emission factors
Detailed information on activity data and emission factors can be found in the monitoring protocol 14-035 on the website http://english.rvo.nl/nie.
Most of the activity data on wastewater treatment are collected by Statistics Netherlands (CBS. 2012) in yearly questionnaires that cover all public WWTPs, as well as all anaerobic IWWTPs; see www.statline.nl for detailed statistics on wastewater treatment. Table 8.4 shows the development in the key activity data with respect to domestic and commercial wastewater treatment, as well as industrial wastewater treatment and septic tanks. Due to varying weather conditions, the volumes of treated wastewater and of the total load of DOC of domestic and commercial wastewater can fluctuate from year to year depending on how much run-off rainwater enters the sewer systems. In the method developed for calculating
methane emissions, the DOC is based on an organic load in terms of the chemical oxygen demand (COD).
From Table 8.4 it can be concluded that the DOC of treated wastewater and sludge does not significantly change over time. The interannual changes in CH4 emissions, therefore, can be explained by varying fractions of CH4 being vented, instead of flared or used for energy purposes. The source ‘Septic tanks’ has steadily decreased from 1990 onwards. This can be explained by the increased number of households connected to the sewer system in the Netherlands (and therefore no longer using septic tanks; see Table 8.4).
A full description of the methodology is provided in the monitoring protocol 14-035 (see the website http:// english.rvo.nl/nie) and in the background document (Oonk et al. 2004). In general, emissions are calculated according to the IPCC Guidelines, with country-specific parameters and EFs used for CH4 emissions from wastewater handling Table 8.3 Wastewater handling emissions of CH4 and N2O (Units: Gg/year).
Parameter 1990 1995 2000 2005 2010 2012
CH4 industrial wastewater 0.25 0.33 0.34 0.36 0.34 0.34
CH4 domestic & commercial wastewater 9.07 7.90 7.96 8.20 8.60 8.36
CH4 septic tanks 4.47 3.25 2.20 1.47 0.77 0.78
Net CH4 emissions 13.79 11.48 10.50 10.03 9.70 9.48
CH4 recovered and/or flared 33.0 39.2 40.4 41.9 45.0 47.5
Recovery/flared (% gross emission) 70.5 77.4 79.4 80.7 82.3 83.4
N2O domestic & commercial wastewater 0.66 0.75 0.88 0.99 1.12 1.16
N2O from human sewage 0.85 0.65 0.53 0.43 0.32 0.30
N2O septic tanks 0.052 0.043 0.029 0.019 0.010 0.010
Total N2O emissions 1.55 1.45 1.44 1.44 1.45 1.47
Table 8.4 Activity data of domestic and commercial wastewater handling (WWTP), Industrial anaerobic wastewater handling (IWWTP) and septic tanks.
Unit 1990 1995 2000 2005 2010 2011 2012
Wastewater DOC1) WWTP Gg/year 933 921 921 943 953 965 972
Sludge DOC1) WWTP Gg/year 254 269 281 298 320 325 347
Nitrogen removed in public WWTP
Gg/year 42.0 47.7 55.8 63.1 71.3 74.0 73.8
Treated volume WWTP Mm3/y 1,711 1,908 2,034 1,841 1,934 1,917 1,989
Wastewater DOC2) IWWTP Gg/year 181 233 244 261 239.7 238.3 245.0
Nitrogen in effluents3) Gg/year 53.8 41.5 33.8 27.8 20.5 19.4 19.3
Resident population 1,000 14,952 15,459 15,926 16,320 16,615 16,694 16,756
% Inhabitants with septic tanks % 4.0 2.8 1.9 1.2 0.62 0.62 0.62
Annual per capita protein uptake
kg 34.86 39.97 38.69 38.03 38.62 38.62 38.62
1) DOC = degradable organic component. in terms of chemical oxygen demand (COD)
2) For anaerobic industrial wastewater treatment plants; this is reflected by the design capacity in terms of the COD. 3) Total of industrial. domestic and commercial effluents
(including sludge). The calculation methods are equivalent to the IPCC Tier 2 methods.