Application of potassium ferrate combined with poly-aluminum chloride for mitigating ultrafiltration (UF) membrane fouling in secondary effluent: Comparison of oxidant dosing strategies

Abstract

Coagulation has been widely applied as a pretreatment for ultrafiltration (UF) membrane in wastewater reclamation, however, it is unable to effectively ensure the removal of organic micropollutants (OMPs) and genotoxicity. To solve this problem, oxidant ferrate (VI) (FeVIO42−, FeVI) was combined with coagulant poly-aluminum chloride (PAC) as the pretreatment of UF to treat secondary effluent, and three oxidant dosing strategies (namely oxidation followed by coagulation (O–FeVI-PAC), simultaneous oxidation and coagulation (S–FeVI-PAC), and coagulation followed by oxidation (C-PAC-FeVI)) were compared at two oxidant doses. The results showed that C-PAC-FeVI pretreatment exhibited the best performance for the removal of DOC (35.9%), UV254 (33.7%), protein (71.8%), and polysaccharide (22.1%). Molecular weight and fluorescence analysis revealed that the removed organics were mainly humic substances. Both the direct UF process and PAC pretreatment showed limited removal of OMPs and genotoxicity, however, the combined pretreatments of FeVI and PAC dramatically removed them. The maximum removal efficiency of the fourteen selected OMPs and genotoxicity was obtained under S–FeVI-PAC (59.6% on average) and C-PAC-FeVI (84.1%), respectively. With respect to membrane fouling control, the normalized flux reduction showed an apparent regularity of C-PAC-FeVI > O–FeVI-PAC > S–FeVI-PAC, however, FeVI dose should be carefully determined. The addition of FeVI delayed the transition of membrane fouling mechanism from pore blockage to cake filtration, especially in C-PAC-FeVI pretreatment, which was confirmed by the fluorescence characterization of hydraulic reversible and hydraulic irreversible foulants. To sum up, C-PAC-FeVI dosing strategy seems to have more potential in membrane fouling alleviation and effluent quality improvement.