Mitigation of inorganic fouling on pressure retarded osmosis (PRO) membranes by coagulation pretreatment of the wastewater concentrate feed

Abstract

Utilization of wastewater or wastewater concentrate as the feed solution in pressure retarded osmosis (PRO) via thin film composite (TFC) membranes causes rapid and significant fouling within the porous substrate and onto the polyamide selective layer facing the substrate. Scaling of calcium phosphate salts has been identified as the predominant cause for the severe PRO membrane fouling. In this study, an acidic coagulant, AlCl3, and a caustic coagulant, NaAlO2, are employed to remove the phosphate from the wastewater concentrate. Their optimal dosages are investigated in order to most effectively remove phosphate and maintain a mild pH environment. 1000 ml of AlCl3 and NaAlO2 pretreated and untreated wastewater concentrates are then used as the feed solutions in the PRO fouling studies. The untreated wastewater concentrate that is saturated with calcium phosphate causes a dramatic flux reduction of 45.2% during the first 360 min of the tests and further reduces to 30.7% after a 25% recovery of the wastewater concentrate. Results show that both AlCl3 and NaAlO2 pretreatments can increase the initial water flux to 25.5 LMH and 24.8 LMH at 20 bar respectively, and increase the normalized water flux to 66% and 64% respectively upon a total permeation volume of 250 ml. However, the AlCl3 pretreatment shows a lower water flux recovery after a hydraulic backwash and a faster flux reduction in the repeated fouling tests. Elemental analyses reveal that AlCl3 is less effective in silica removal from the wastewater concentrate, which causes silica condensation underneath the selective layer of the TFC membrane. NaAlO2 appears to be the more effective coagulant in both phosphate and silica removal for a prolonged PRO operation. Moreover, a critical water flux framework is developed to analyze the PRO calcium phosphate fouling, in order to determine the stable water flux at a given phosphate concentration or the maximum allowable phosphate concentration to achieve the target PRO flux and power density.