Dewaterability of CAS and MBR Sludge: Effect of Biological Stability and EPS Composition

The dewaterability of sludge from two conventional activated sludge (CAS) and three membrane bioreactor (MBR)-based wastewater treatment plants is investigated prior to and after anaerobic digestion. The concentration and composition of extracellular polymeric substances (EPS) mostly affect the dewaterability of all raw sludge samples. Better sludge dewaterability is observed when the concentration of proteins, carbohydrates, uronic acids, and humic acids is below approximately 400, 250, 200, and 40 mg/L, respectively. In contrast, the specific resistance to filtration (SRF) increases in the sludge samples with a higher EPS concentration. The MBR results in a lower EPS production and a uronic acid-dominating EPS composition. This especially affects the dewaterability of one MBR sludge, also characterized by high salinity and a smaller particle size. Anaerobic digestion results in a higher SRF for both CAS and MBR sludge, with the particle-size distribution having the preponderant effect on the digested sludge dewaterability.