Evaluation of greenhouse gas emissions from municipal UASB sewage treatment plants

A mathematical model was developed to estimate the greenhouse gas (GHG) emissions due to the treatment of municipal sewage. The model took into account emissions from upstream, on-site and downstream activities. The emissions for three different sewage treatment configurations were estimated, consisting of an activated sludge system preceded by a primary settler, and by anaerobic pre-treatment (Upflow Anaerobic Sludge Blanket, UASB): with and without recovery of dissolved CH4 from the anaerobic effluent. The UASB-based configuration with dissolved CH4 recovery had the lowest direct emission of 6900 ton CO2 eq/y and the highest avoided emissions (4200 ton CO2 eq/y). In contrast, when dissolved CH4 recovery was not applied, the UASB-based configuration had the highest direct emissions: 25,500 ton CO2 eq/y. The primary clarification based configuration had similar emissions to the UASB-based configuration without CH4 recovery from the effluent. In addition to having the lowest GHG emissions, the UASB configuration with CH4 recovery was also the cheapest configuration to operate. This configuration produced excess electricity from the combustion of CH4 produced during the process. In comparison, the combustion of CH4 from primary sludge digestion provided only 48% of the electrical needs of the primary clarifier configuration. Finally, the excess sludge production from the UASB configuration with CH4 recovery was 56% less than that produced by the primary clarification configuration.