OPERATION OF ANAEROBIC DIGESTERS AT INCREASED SOLIDS CONCENTRATIONS

Conventional anaerobic digesters treating municipal sewage usually fun ction with a hydraulic retention time of 30 to 60 d and at a total sol ids (TS) concentration of 1.5 to 3% (m/v). A cross-flow microfiltratio n unit was constructed at Northern Waste Water Treatment Works, Durban to concentrate sludge from a conventional anaerobic digestor and to f acilitate operation with a higher solids concentration. Semi-continuou s digesters were, therefore, operated with a 30 d retention time and a t an optimum temperature to investigate the efficacy of digesters supp lied with concentration digested sludge. The results showed hat the ra te of gas production from a digester supplied with a 3.8% (m/v) TS was higher than the control (2% (m/v) TS) and a digester supplied with 3% (m/v) TS. Increased total solids concentrations (4.7% (m/v)), however , effected slightly lower gas production rates that the digester with 3.8% (m/v) TS. The volatile solids concentrations of all four digester s were similar, indicating neither favourable nor unfavourable effects from increased solids concentrations. The digesters operated at total solids concentrations 3.8% (m/v) TS produced higher concentrations of volatile acids than the control. The alkalinity concentrations (ca. 4 000 mg . l(-1)) were similar for all four digesters.