Effect of previous coagulation in direct ultrafiltration of primary settled municipal wastewater

Direct ultrafiltration of primary settled municipal wastewater can be a feasible option for water reuse in several applications by avoiding biological treatment. This paper discusses the effect of previous clarification by coagulation/sedimentation of raw wastewater on ultrafiltration performance and its relationship with the main operation parameters in dead-end mode (flux and backwashing duration). Reversible and residual membrane foulings in a hollow-fibre bench-scale unit were determined over a broad range of filtration parameters and coagulant doses used in the previous clarification of domestic wastewater. Moreover, results were also compared with those obtained by ultrafiltration of biologically treated effluent from a conventional WWTP. Reversible membrane fouling, which can be described by the cake formation model, seems mainly caused by the colloidal fraction of the wastewater. In fact, under optimal coagulant dosage, the cake resistance values (expressed by αω, in m−2) were similar to those obtained with the secondary effluent. Direct observation of the membrane surface suggests a solid accumulation in the vicinity of the membrane after backwashing, which could justify the increase in cake resistance during the initial filtration/backwashing cycles until steady-state conditions are reached. Residual fouling resistance decreased exponentially with the duration of backwashing, obtaining a limit resistance value which was independent of the coagulant dose. Moreover, high efficiency of chemical oxygen demand (COD) removal was achieved by the tested treatment train, regardless of the coagulant dose and the initial quality of the primary effluent. ► Direct ultrafiltration of municipal wastewater can be a feasible option for water reuse. ► Pre-clarification by coagulation permits a significant reduction in cake resistance. ► Cake was detached during backwashing, but only partially dispersed in the suspension. ► High efficiency of COD removal (81–95%) was achieved, regardless of the coagulant dose.