Treatment of high-strength wastewater with coupled flocculation and membrane filtration prior to ammoniacal nitrogen enrichment by osmotically assisted reverse osmosis

Abstract

Membrane scaling poses significant challenges in membrane processes involving high-strength feed streams. In this work, a treatment of high-strength NH4-N-containing wastewater comprising the flocculation of Ca and Si and membrane filtration is introduced. The pretreated wastewater underwent further dewatering and enrichment through a hybrid osmotically assisted reverse osmosis and reverse osmosis (OARO-RO) process. During the treatment, the molar ratio of the acid flocculant FeCl3 to Si was adjusted, as well as the molar ratio of the alkaline flocculant Na2CO3 to Ca. Furthermore, the molar ratio of FeCl3 and Na2CO3, as well as the order of flocculant addition, was optimized in terms of Si and Ca removal and treated water pH necessary for NH4-N recovery. Membrane filtration was afterwards conducted to remove the floc from the treated wastewater prior to the OARO-RO operation. The optimized pretreatment process resulted in removal of approximately 70 % Si and 60 % Ca content in the wastewater. OARO-RO was operated using pretreated wastewater to demonstrate the effectiveness of the coupled flocculation and filtration pretreatment in enrichment of highly concentrated feed streams. Two scenarios of OARO-RO operation were performed by manipulating the operational parameters. Successful continuous operation of the OARO-RO system was observed despite the operational conditions and the wastewater was enriched more than 11 times, without operation interruption and occurrence of scaling, resulting in an enriched wastewater containing around 50,000 ppm T-N. Therefore, coupled flocculation and membrane filtration treatment process is effective to mitigate membrane scaling when concentrating highly concentrated feed streams.