Anaerobic codigestion of municipal solid waste and biosolids under variousmixing conditions - I. Digester performance

The feasibility of codigestion of the organic fraction of municipal solid w aste, primary sludge, and waste activated sludge was evaluated in mesophili c (37 degreesC), laboratory-scale digesters. In a first experiment, differe nt startup strategies were compared using foul digesters, operated under co ntinuously mixed conditions. After two weeks, the experiment was continued under minimally mixed conditions. Results demonstrated that reducing the le vel of mixing improved digester performance. Therefore, in a second experim ent, six digesters were operated to compare performance under continuous mi xing and reduced mixing levels at various loading rates and solids levels. The continuously mixed digesters exhibited unstable performance at the high er loading rates, while the minimally mixed digesters performed well for al l loading rates evaluated. In a third experiment, it was demonstrated that an unstable, continuously mixed digester was quickly stabilized by reducing the mixing level. These experiments confirmed that continuous mixing was n ot necessary for good performance and was inhibitory at higher loading rate s. In addition, reduction of mixing levels may he used as an operational to ol to stabilize unstable digesters. (C) 2001 Elsevier Science Ltd. All righ ts reserved.