Comparison of CSTR and AnMBR for anaerobic digestion of WAS and lipid-rich flotation sludge from the dairy industry

Process stability and limitations of loading capacity of anaerobic membrane bioreactors (AnMBRs) and continuously stirred tank reactors (CSTRs) for the treatment of WAS and a lipid-rich flotation sludge from the dairy industry were investigated in pilot scale. The digesters were operated with increasing shares of lipids up to 85% at low sludge retention times (SRT) down to 10 days. High shares of lipids are favorable for treating wastewater sludges from industrial WWTPs on-site instead of their disposal, but often lead to long chain fatty acid (LCFA) accumulations and digester failures. In this study, high COD removals were obtained in all digesters resulting in methane yields over 250 L/kg CODin. Methane production and COD removal followed substrate first-order kinetics. High lipid shares of CODlipid > 80% did not inhibit COD degradation. Process stability was determined based on the ratio of specific methanogenic activity and mean acetate conversion in the digesters. Process stability depended mainly on SRT and lipid shares in the substrate as both parameters affected the degradable lipid content in the anaerobic sludge. Shorter SRTs and higher shares of lipids in particular reduced process stability as observed by inhibitions of acetoclastic methanogenesis. The OLR was not a decisive factor. When treating COD shares of lipids higher than 80%, SRT should exceed 15 d. COD shares of lipids below 80% were also successfully treated at SRT of 10 d. Residual degradable lipid content of anaerobic sludge negatively influenced process stability starting from 50 mg lipid/g TS. The performance of an AnMBR in terms of COD degradation and process stability corresponds to a CSTR with equal SRT, but can be operated with higher OLRs. At equal HRT compared to a CSTR, the AnMBR offers a higher process stability by extending the SRT.