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^-1OLR. 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.