10.3.3 Wastewater and human sewage CH4 and N2O: regional trends
CH4 and N2O can be produced and emitted during municipal and industrial wastewater collection and treatment, depending on transport, treatment and operating conditions. The resulting sludges can also microbially generate CH4 and N2O, which may be emitted without gas capture. In developed countries, these emissions are typically small and incidental because of extensive infrastructure for wastewater treatment, usually relying on centralized treatment. With anaerobic processes, biogas is produced and CH4 can be emitted if control measures are lacking; however, the biogas can also be used for process heating or onsite electrical generation.
In developing countries, due to rapid population growth and urbanization without concurrent development of wastewater infrastructure, CH4 and N2O emissions from wastewater are generally higher than in developed countries. This can be seen by examining the 1990 estimated CH4 and N2O emissions and projected trends to 2020 from wastewater and human sewage (UNFCCC/IPCC, 2004; US EPA, 2006). However, data reliability for many developing countries is uncertain. Decentralized ‘natural’ treatment processes and septic tanks in developing countries may also result in relatively large emissions of CH4 and N2O, particularly in China, India and Indonesia where wastewater volumes are increasing rapidly with economic development (Scheehle and Doorn, 2003).
The highest regional percentages for CH4 emissions from wastewater are from Asia (especially China, India). Other countries with high emissions in their respective regions include Turkey, Bulgaria, Iran, Brazil, Nigeria and Egypt. Total global emissions of CH4 from wastewater handling are expected to rise by more than 45% from 1990 to 2020 (Table 10.3) with much of the increase from the developing countries of East and South Asia, the middle East, the Caribbean, and Central and South America. The EU has projected lower emissions in 2020 relative to 1990 (US EPA, 2006).
The contribution of human sewage to atmospheric N2O is very low with emissions of 80–100 MtCO2-eq/yr during the period 1990–2020 (Table 10.3) compared to current total global anthropogenic N2O emissions of about 3500 MtCO2-eq (US EPA, 2006). Emission estimates for N2O from sewage for Asia, Africa, South America and the Caribbean are significantly underestimated since limited data are available, but it is estimated that these countries accounted for >70% of global emissions in 1990 (UNFCCC/IPCC, 2004). Compared with 1990, it is expected that global emissions will rise by about 20% by 2020 (Table 10.3). The regions with the highest relative N2O emissions are the developing countries of East Asia, the developing countries of South Asia, Europe and the OECD North America (Figure 10.6b). Regions whose emissions are expected to increase the most by 2020 (with regional increases of 40 to 95%) are Africa, the Middle East, the developing countries of S and E Asia, the Caribbean, and Central and South America (US EPA, 2006). The only regions expected to have lower emissions in 2020 relative to 1990 are Europe and the EIT Countries.
Figure 10.6a (upper): Regional distribution of CH4 emissions from wastewater and human sewage in 1990 and 2020.
Figure 10.6b (lower): Regional distribution of N2O emissions from human sewage in 1990 and 2020. See Table 10.3 for total emissions.