Emerging investigator series: Control of membrane fouling by dissolved algal organic matter using pre-oxidation with coagulation as seawater pretreatment

Marine algae produce organic matter, namely algal organic matter (AOM), especially during a harmful algal bloom. AOM has been recognised as a key cause for the formation of organic fouling on membranes in seawater desalination applications. In this study, pre-oxidation of AOM by potassium permanganate (KMnO<inf>4</inf>) and sodium hypochlorite (NaOCl) was investigated. In addition, ferric (Fe³⁺) and alum (Al³⁺) coagulants were used for subsequent coagulation. Two different operational modes, conventional coagulation-flocculation-sedimentation (CFS) and coagulation-flocculation-dissolved air flotation (CF-DAF) processes, were used to evaluate pretreatment performance using synthetic AOM with an initial dissolved organic carbon (DOC) of around 4.8 mg C L^-1 (turbidity ≈ 4.47 NTU, pH ≈ 8). Pre-oxidation with coagulation removed more AOM, compared to oxidation or coagulation alone. The removal of DOC by NaOCl-Fe³⁺ is relatively high when compared to other combinations of oxidant and coagulant because of in situ ferrate (Fe⁶⁺) generation, which was detected by the ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid))-ultraviolet visible (UV-vis) method. Pre-oxidation with 1.5 mg L^-1 NaOCl followed by coagulation with 2.5-3.0 mg L^-1 Fe³⁺ achieved a maximum DOC removal of 65-76% during the CFS treatment; while, the DOC removal could further increase up to 83-85% by introducing CF-DAF. Particularly, the NaOCl-Fe³⁺ treatment generated 1.31 mg L^-1 of in situ ferrate (Fe⁶⁺). Finally, pre-oxidation and coagulation coupled with DAF successfully reduced fouling and lowered flux decline in a microfiltration (MF) membrane. Non-invasive optical coherence tomography (OCT) was performed to monitor the fouling development on the MF membrane before and after pretreatment.