Determination of the dewatered digestate amounts and methane yields from the co-digestion of biowaste as a basis for a cost-benefit analysis

Abstract

Co-digestion is the simultaneous digestion of two or more substrates and a common practice at wastewater treatment plants (WWTPs). The amounts of methane and digested sludge produced are key parameters for evaluating the economic efficiency of co-digestion. However, the share of dewatered digestate produced from co-substrates is not known. Synergistic effects in co-digestion, i.e. a better biodegradability compared to the mono-digestion of each substrate, might reduce the amounts of digested sludge and increase methane yields. However, these effects might also influence the calculation of methane and digestate quantities from co-substrates. The main objective of this work was to provide a basis for the cost-benefit analysis of biowaste (BW) co-digestion at WWTPs for this data. Therefore, continuous and batch experiments with sewage sludge (SS) and BW co-digestion were conducted and evaluated for methane and digestate production, and possible synergistic effects. BW co-digestion led to an additional production of 0.35 t total solids (TS) of dewatered sludge per ton TS_added in continuous and 0.23 t TS of dewatered sludge per ton of TS_added in batch experiments. The methane yield from BW was 441 L/kg TS_added in continuous experiments and 482 L/kg TS_added batch test. No synergistic effects were observed in both batch and continuous co-digestion experiments. Batch tests were found to be suitable for a rough estimation of the co-digestion economic efficiency key parameters. Continuous experiments are recommended to obtain more robust data. A cost-benefit analysis found that electricity production from co-digestion can generate savings of 88-170 €/t TS_added compared to grid purchase.

Keywords: Anaerobic co-digestion; Biomethane potential; Dewatered sludge; Sewage sludge; Synergistic effects; Wastewater treatment plant.