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No. 193: Wastewater treatment and discharge

Thomsen, M. 2014. Wastewater treatment and discharge. Aarhus University, DCE – Danish Centre for Environment and Energy, 79 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 193 http://www.dmu.dk/Pub/SR193.pdf



The Danish emission inventory for wastewater treatment and discharge is based on plant level monitoring data in the influent and effluent wastewater, reported data on flaring and methane loss at plant level and reported energy recovery data. For this reason, a country-specific methodology for calculating the national emissions from wastewater treatment and discharge have been developed (Thomsen & Lyck, 2005) as the default IPCC methodology do not fit countries having input-output monitoring data (IPCC, 2006, Chapter 5). The focus of the present sector report is to verify the country-specific methane (CH4) emission factor calculated at 1.3 % of the reported methane recovery from biogas production at Danish wastewater treatment plants with anaerobic sludge treatment.

This report presents the status of methodological development within the sub-sector 5.D Wastewater treatment and discharge. Focus of the report is to present a COD (Chemical Oxygen Demand) mass balance for the Danish wastewater treatment plants, verifying the country-specific methane emission factor, and the resulting level of methane emission from anaerobic sludge digestion, at the Danish wastewater treatment plants (WWTP). The latter requested for by the UNFCCC expert review team.

Varying plant design and sludge management strategies at the individual WWTPs results in varying methane production efficiencies and emissions, which has initiated an ongoing process of collection and combining activity data at plant level with the aim of documenting a country-specific emission inventory – optimally at plant level.

The present report presents status of COD mass balance for verification of the country-specific methane emission factor based on available information according to the status on the development of a plant level database. Plant level monitoring data extracted from reports published by: 1. The individual WWTPs, i.e. Environmental Reports, 2. Yearly reports published by the Danish Nature Agency, i.e. results obtained from National Monitoring and Assessment Programme for the Aquatic and Terrestrial Environments (NOVANA) and 3. Energy Producer Account data received by the Danish Energy Agency.

Plant and National level COD mass balances and methane emission calculations is presented with the aim of data gap filling and verification of the emissions factor for methane emissions from anaerobic sludge digestion.

Another aspect that has been addressed by the UNFCCC expert review team is improved documentation of the fraction of the population, which are not connected to the collective sewer system. For this reason, the report includes a verification of the fraction of the population living within the scattered settlements, i.e. not connected to the collective sewer system, and the associated septic tank modelling approach used to calculate the methane emission from scattered settlements.

Lastly, a first evaluation of the possibility to include direct N2O emissions from separate industries in future emission inventories to EU and the UNFCCC for the sub-sector 5.D Wastewater treatment and discharge is presented.

The main objective of this report is to document the accuracy of the country-specific emission factor (EF) value for methane emissions from anaerobic treatment of sludge, i.e. 1.3% of the recovered methane. The latter is obtained by setting up COD mass balances and associated methane budgets at national and plant level based on Danish monitoring data.

There is a general tendency for the methane recovered as calculated from the Energy Producer Account data to be lower than the reported methane recovery data in the Environmental Reports published by the individual WWTPs. There may be several reasons for this tendency. One reason may be that the methane conversion efficiency is lower than the IPCC default value of 80 % of the maximum CH4 producing capacity (Bo). The error of propagation is within the range of uncertainty reported at tier 1 and 2 (Nielsen et al., 2014) justifying the country-specific EF of 1.3 % of the recovered methane.

The value of the country-specific methane emission factor for WWTPs with anaerobic sludge digestion is sufficiently justified at plant level. However, national and plant level COD balances indicate that it is important to take into account external carbon when calculating the EF value from COD mass balances and associated CH4 budgets at plant level.