Full-scale anaerobic co-digestion of organic waste and municipal sludge

Abstract

A full-scale experiment on the anaerobic co-digestion of organic waste from domestic refuse (swill) and municipal sludge is described. In a wastewater treatment plant of 50,000 population equivalents, two conventional mesophilic digesters with a combined volume of 2000 m3 and 20 days hydraulic retention time were used. The digesters’ usual influent is waste sludge from wastewater treatment plants (a mixture of primary sludge and waste activated sludge) with an average organic loading rate of 0.8 kg m−3 d−1 of volatile suspended solids. In the experiment, organic waste was added to the digester influent to increase the organic loading rate by 25% to 1.0 kg m3 d−1 of volatile suspended solids. Biogas quantity increased by 80% and specific biogas production increased from 0.39 m3 kg−1 volatile suspended solids inserted prior to the experiment to over 0.60 m3 kg−1 volatile suspended solids’ inserted, peaking at 0.89 m3 kg−1 volatile suspended solids inserted. The excess biogas was used in a boiler and a 50 kW combined heat and power engine. Electrical energy production increased by 130% and heat production increased by 55%. Volatile suspended solids degradation efficiency increased from 71% to 81% with no increase of volatile suspended solids in the digester effluent. Virtually all of the organic waste was degraded.