Wasted activated sludge dewaterability: comparative evaluation of sludge derived from CAS and MBR systems

Nowadays, sludge dewatering is one of the greatest operational cost to wastewater treatment cycle. Specifically, 1t of fresh sludge to be disposed is composed, on average, by 0.25 - 0.30t of suspended solids, with an average cost for treatment and disposal around 280 - 470 €/t of suspended solids. Despite several technologies have been developed with the focus to reduce also the specific sludge production, still mechanical dewatering represents a crucial step to limit the amount of sludge to be disposed. Many physical–chemical parameters influence the sludge dewaterability: floc structure, particle size, bound water content, surface charge and hydrophobicity, Extracellular Polymeric Substances content and sludge retention time. Many authors agree in identifying the sludge origin as one of the main aspect involved in sludge dewaterability. In order to elucidate the key factors influencing the dewaterability process, the present work is aimed to investigate the influence of the treatment plant lay-out on sludge dewaterability. The sludge samples were analyzed according to EN International Standards. The analyzed sludge samples were derived from 4 conventional activated sludge and 2 membrane bioreactor wastewater treatment plants (WWTPs). The results confirm the complexity of the inter-relationships between many factors affecting the sludge dewaterability (i.e., WWTP lay-out, physical-chemicals and biological factors).