There is a fourth stage, which considers increasing production for an energy surplus – meaning, turning the treatment of sewage into a power generator for the community. There are various options available that can increase the energy production from the addition of low temperature incineration of the final solid waste to generate heat. This allows the move from the standard Mesophilic digestion to the faster and higher methane-producing Thermophilic digestion. The heat can also be used to warm up the treatment buildings in the winter and provide cooling in the summer months. This can be coupled with selective combined waste digestion, using industrial/domestic organic waste, which is mixed to optimize the biogas production.
Taking a similar simplified high-level view of GHGe savings, i.e. the annual saving in GHG emissions from just decoupling the aeration process:
- CO2 generated from biological oxygen demand (BOD) oxidation = 1.1 kg CO2/kg O2 BOD oxidized. The reduction in GHG for a 72 ML/d plant would be approximately 7,200 tons per annum using a GWP of 1.0.
- Nitrous oxide has been calculated as 15.8 kg/d, therefore, the reduction in GHG for a 72 ML/d plant would be approximately 1,790 tons per annum using a GWP of 310.
Moreover, the expected reduction created through the removal of indirect emissions from the consumption of purchased electricity and gas will yield additional savings (calculated using 0.84 kg CO2e/kWh). Assuming the reduction is caused by the decoupling of the aeration process and the mixers and pumps associated with the activated sludge process, which would be 1500 kW total power requirement less the 385 kW for the membrane process, the reduction in energy requirement is 1115 kW for 24 hours a day, giving 26,760 kWh or 8,200 tons of CO2 per annum. The total GHG reduction would be approximately 17,190 tons of CO2e per annum.
A final advantage of the ceramic membrane approach is the space required. Comparing the physical footprint below of the ceramic membrane approach with only the activated sludge process (excluding any primary and secondary clarifiers), it is clear that the membrane approach can fit inside or above the existing activated sludge process.