Scale-up of anaerobic digestion of the biowaste fraction from domestic wastes

In the City of Karlsruhe/Germany anaerobic digestion of 7200 t a(-1) of separately collected biowaste has proven its feasibility at an organic loading rate (OLR) of up to 8.5 kg COD m(-3) d(-1). An extension of biowaste collection over the whole city area would increase the amount of biowaste to 12,000 t a(-1), leading to an OLR of the existing anaerobic reactor of up to 15 kg COD m(-3) d(-1). To test, whether the increased amount of biowaste could be stabilized in the existing plant, biowaste suspensions were digested in a laboratory reactor at a maximum OLR, that exceeded the future OLR of the full-scale plant. The laboratory reactor was started with effluent of the full-scale biowaste digester. Like in full-scale, biowaste suspension from the hydropulper was added in a fed-batch mode. The elimination of organic material (measured as COD, chemical oxygen demand) and the volumetric gas production were linearly increasing with the OLR from 4.3 to 19 kg COD m(-3) d(-1). Thus, safe operation of the full-scale plant at an OLR of 15 kg COD m(-3) d(-1) should be possible, leaving still some reserve capacity. To determine the metabolic reserves for fatty acid degradation during digestion at an OLR of 10 kg COD m(-3) d(-1), digester effluent was supplemented with either 40 mmol l(-1) acetate, propionate, i-butyrate or n-butyrate. Results of these batch assays indicated a rapid degradation of all fatty acids and fatty acid conversion rates, that would allow a stable anaerobic fermentation at 15 kg COD m(-3) d(-1) OLR. On the basis of the laboratory results the OLR of the full-scale methane reactor was increased to 15 kg COD m(-3) d(-1). After 7 months, results of full-scale digestion were still consistent with the previously obtained laboratory results. (C) 2002 Elsevier Science Ltd. All rights reserved.