Optimization of coagulation step in membrane treatment of municipal secondary effluents

Secondary effluents of municipal wastewater are the major pollutant and the most significant irrigation reservoir at the same time. Although secondary effluents are widely used for irrigation of some crops, the unrestricted irrigation and groundwater recharge require implementation of membrane technologies. A combination of coagulation, ultrafiltration (UF) and nanofiltration (NF) stages has a potential to turn secondary effluents to treasure from waste. The current study focuses on the optimization of the coagulation of secondary effluents from Sede Teiman wastewater treatment plant (Southern Israel). The study was performed with ferric chloride, and the optimal dose was determined in jar tests by parallel measurement of total suspended solids (TSS), UV absorption at 254 nm (UV₂₅₄) and conductivity. The optimal dose of 130 mg/l FeCl₃•6H₂O (equivalent to 27 mg/l Fe³⁺) was then applied for pretreatment of secondary effluents prior to UF membrane. The combined coagulation-UF treatment reduced the total organic carbon (TOC) by more than 50%. Moreover, it increased the average flow rate during 30 min filtration cycles by up to 90% when compared without coagulation. The most significant improvement in UF performance due to coagulation was observed in membranes with a molecular weight cut-off (MWCO) greater than 30 kDa.