Biological stability and dewaterability of CAS and MBR sludge

This paper investigates and compares the characteristics of sludges produced by membrane bio-reactor (MBR) and conventional activated sludge (CAS) systems. Stability and dewaterability of full-scale MBR and CAS treatment plants are measured and compared. Obtained results show that specific methane production is higher in CAS sludge compared to MBR sludge, although one of the CAS sludges investigated had a sludge retention time (SRT) higher than the two MBR sludges investigated. Nonetheless, MBR sludge results to be characterized by a non-negligible biometanation potential (BMP). Methane production measured during BMP tests is around 200 NmL/gVS for MBR sludge, equaling 2/3 of methane production obtained, in similar condition, for CAS sludge. Dewaterability of the sludge resulted to be linearly correlated with the SRT and the sludge extracellular polymeric substances (EPS) concentration. The higher the SRT and the lower the sludge EPS concentration, the lower the specific resistance to filtration.