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

The feasibility of codigestion of the organic fraction of municipal solid waste, primary sludge, and waste activated sludge was evaluated in mesophilic (37 degrees C), laboratory-scale digesters. In a first experiment, different startup strategies were compared using four digesters, operated under continuously mixed conditions. After two weeks, the experiment was continued under minimally mixed conditions. Results demonstrated that reducing the level of mixing improved digester performance. Therefore, in a second experiment, six digesters were operated to compare performance under continuous mixing and reduced mixing levels at various loading rates and solids levels. The continuously mixed digesters exhibited unstable performance at the higher loading rates, while the minimally mixed digesters performed well for all 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 not necessary for good performance and was inhibitory at higher loading rates. In addition, reduction of mixing levels may be used as an operational tool to stabilize unstable digesters.