Compost versus biogas treatment of sewage sludge dilemma assessment using life cycle analysis

Abstract

The establishment of the Urban Wastewater Treatment Directive resulted in an immense increasing quantity of sewage sludge produced from wastewater treatment plants. With most up to date the “Ex-post evaluation of certain waste stream directives”, various legislation and strategies were derived from fulfilling the requirements resulting from this directive appropriately, in an economical and environmentally acceptable way. This paper intends to assess and compare the environmental impact of exploiting the sewage sludge in open windrow facilities for compost production versus using an anaerobic digester for the production of biogas. The two alternative post-processing pathways' environmental assessment is conducted with the use of the Life Cycle Assessment methodology. The investigation presented that the energy savings for managing sewage sludge for biogas production than producing compost are estimated at 27060 MJ per 1 t of input (SS), and the avoided carbon dioxide emissions are 697 kg/t of digested sludge. The scientific value of this study is that it provides a justified answer to the compost versus biogas dilemma, based on a quantified assessment of both methods' environmental aspects. Life Cycle Assessment's employment brings forth in-depth information regarding the energy embodied in compost and biogas production. The selection among the most ecological method relevant to sewage sludge exploitation is provided by thoroughly sophisticated decision-making. The life cycle stages of the analyzed mechanisms that have the most damaging consequences on the ecosystem include the composting and anaerobic digestion stage, whereas the least energy-consuming stages are the thickening, and drying process, in both scenarios. The Life Cycle Assessment outcomes indicated that the environmental impact of the anaerobic digestion process is lower than the composting process, in the range between 6.5% and 7.5%, across all environment effect levels.The exploitation of waste materials to produce useful products such as compost and biogas gives the final product added environmental value.