Economic–environmental analysis of handling biogas from sewage sludge digesters in WWTPs (wastewater treatment plants) for energy recovery: Case study of Bekkelaget WWTP in Oslo (Norway)

Abstract

This paper outlines a methodology for a systematic economic–environmental analysis of realistic and realisable options for recovering and utilising energy from biogas produced in sewage sludge digesters in WWTPs (wastewater treatment plants). Heat, electricity and transport fuel can be produced from biogas, consumed in-plant or even sold to external end-users. The paper initially considers global warming as the environmental impact of concern, but later also stresses on the necessity of avoiding problem shifting by factoring in other environmental impact categories as well. The methodology is subsequently applied to the Bekkelaget WWTP in Oslo (Norway). Five different options for handling biogas are considered, in addition to the status quo – the business-as-usual in year-2012, and a baseline case, where it is assumed that all biogas generated is flared completely and not utilised for energy recovery of any kind. Seven different cost scenarios – for electricity, natural gas, wood pellets, bio-methane and diesel – are constructed. This gives a total of 49 combinations, for each of which the net costs and net environmental impacts (global warming, eutrophication and acidification) are determined for the 10-year period 2012–2021. The changes (in percentages) with respect to the corresponding values for the baseline case, are recorded; suitable weighting factors are considered after interaction with experts and personnel associated with the plant, and the options are evaluated using this double-bottom-line approach (economic and environmental).