Towards energy positive wastewater treatment plants

Abstract

Energy requirement for wastewater treatment is of major concern, lately. This is not only due to the increasing cost of electrical energy, but also due to the effects to the carbon footprint of the treatment process. Conventional activated sludge process for municipal wastewater treatment may consume up to 60% of the total plant power requirements for the aeration of the biological tank. One way to deal with high energy demand is by eliminating aeration needs, as possible. The proposed process is based on enhanced primary solids removal, based on advanced microsieving and filtration processes, by using a proprietary rotating fabric belt MicroScreen (pore size: 100–300 μm) followed by a proprietary Continuous Backwash Upflow Media Filter or cloth media filter. About 80–90% reduction in TSS and 60–70% reduction in BOD5 has been achieved by treating raw municipal wastewater with the above process. Then the partially treated wastewater is fed to a combination low height trickling filters, combined with encapsulated denitrification, for the removal of the remaining BOD and nitrogen. The biosolids produced by the microsieve and the filtration backwash concentrate are fed to an auger press and are dewatered to about 55% solids. The biosolids are then partially thermally dried (to about 80% solids) and conveyed to a gasifier, for the co-production of thermal (which is partly used for biosolids drying) and electrical energy, through syngas combustion in a co-generation engine. Alternatively, biosolids may undergo anaerobic digestion for the production of biogas and then electric energy. The energy requirements for complete wastewater treatment, per volume of inlet raw wastewater, have been calculated to 0.057 kWh/m3, (or 0.087 kWh/m3, if UV disinfection has been selected), which is about 85% below the electric energy needs of conventional activated sludge process. The potential for net electric energy production through gasification/co-generation, per volume of inlet raw wastewater, has been calculated to 0.172 kWh/m3. It is thus obvious, that the proposed process can operate on an electric energy autonomous basis.