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Many practical design and operating decisions on wastewater treatment plants can have significant impacts on the overall environmental performance, in particular the greenhouse gas (GHG) emissions. The main factor in this regard is the use of aerobic or anaerobic treatment technology. This paper compares the GHG production of a number of case studies with aerobic or anaerobic main and sludge treatment of domestic wastewater and also looks at the energy balances and economics. This comparison demonstrates that major advantages can be gained by using primarily anaerobic processes as it is possible to largely eliminate any net energy input to the process, and therefore the production of GHG from fossil fuels. This is achieved by converting the energy of the incoming wastewater pollutants to methane which is then used to generate electricity. This is sufficient to power the aerobic processes as well as the mixing etc. of the anaerobic stages. In terms of GHG production, the total output (in CO2 equivalents) can be reduced from 2.4 kg CO2/kg COD(removed) for fully aerobic treatment to 1.0 kg CO2/kg COD(removed) for primarily anaerobic processes. All of the CO2 produced in the anaerobic processes comes from the wastewater pollutants and is therefore greenhouse gas neutral, whereas up to 1.4 kg CO2/kg COD(removed) originates from power generation for the fully aerobic process. This means that considerably more CO2 is produced in power generation than in the actual treatment process, and all of this is typically from fossil fuels, whereas the energy from the wastewater pollutants comes primarily from renewable energy sources, namely agricultural products. Even a change from anaerobic to aerobic sludge treatment processes (for the same aerobic main process) has a massive impact on the CO2 production from fossil fuels. An additional 0.8 kg CO2/kg COD(removed) is produced by changing to aerobic sludge digestion, which equates for a typical 100,000 EP plant to an additional production of over 10 t CO2 per day. Preliminary cost estimates confirm that the largely anaerobic process option is a fully competitive alternative to the mainly aerobic processes used, while achieving the same effluent quality.

The aim of this work was to demonstrate at pilot scale a high level of energy recovery from sewage utilising a primary Anaerobic Migrating Bed Reactor (AMBR) operating at ambient temperature to convert COD to methane. The focus is the reduction in non-renewable CO(2) emissions resulting from reduced energy requirements for sewage treatment. A pilot AMBR was operated on screened sewage over the period June 2003 to September 2004. The study was divided into two experimental phases. In Phase 1 the process operated at a feed rate of 10 L/h (HRT 50 h), SRT 63 days, average temperature 28 degrees C and mixing time fraction 0.05. In Phase 2 the operating parameters were 20 L/h, 26 days, 16 degrees C and 0.025. Methane production was 66% of total sewage COD in Phase 1 and 23% in Phase 2. Gas mixing of the reactor provided micro-aeration which suppressed sulphide production. Intermittent gas mixing at a useful power input of 6 W/m(3) provided satisfactory process performance in both phases. Energy consumption for mixing was about 1.5% of the energy conversion to methane in both operating phases. Comparative analysis with previously published data confirmed that methane supersaturation resulted in significant losses of methane in the effluent of anaerobic treatment systems. No cases have been reported where methane was considered to be supersaturated in the effluent. We have shown that methane supersaturation is likely to be significant and that methane losses in the effluent are likely to have been greater than previously predicted. Dissolved methane concentrations were measured at up to 2.2 times the saturation concentration relative to the mixing gas composition. However, this study has also demonstrated that despite methane supersaturation occurring, micro-aeration can result in significantly lower losses of methane in the effluent (<11% in this study), and has demonstrated that anaerobic sewage treatment can genuinely provide energy recovery. The goal of demonstrating a high level of energy recovery in an ambient anaerobic bioreactor was achieved. An AMBR operating at ambient temperature can achieve up to 70% conversion of sewage COD to methane, depending on SRT and temperature.

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