Environmental Science Water Research & Technology PAPER Cite this: DOI: 10.1039/c9ew00955h Received 26th October 2019, Accepted 25th January 2020 DOI: 10.1039/c9ew00955h rsc.li/es-water Emerging investigator series: control of membrane fouling by dissolved algal organic matter using pre-oxidation with coagulation as seawater pretreatment† Bhaskar Jyoti Deka, a Jiaxin Guo, a Sanghyun Jeong,* b Manish Kumar c and Alicia Kyoungjin An * a 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 4 ) and sodium hypochlorite (NaOCl) was investigated. In addition, ferric (Fe 3+ ) and alum (Al 3+ ) 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–Fe 3+ is relatively high when compared to other combinations of oxidant and coagulant because of in situ ferrate (Fe 6+ ) generation, which was detected by the ABTS (2,2′-azino-bisIJ3-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 Fe 3+ 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–Fe 3+ treatment generated 1.31 mg L -1 of in situ ferrate (Fe 6+ ). 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. 1. Introduction Water stress has become a serious global issue due to the rapid increase in population, global warming, and climate change. 1–6 To address this issue, desalination of seawater via reverse osmosis (RO) has become a widely used approach, 7–9 however, fouling on RO membranes results in higher desalination cost. 10–17 Adoption of a low pressure microfiltration (MF) membrane unit prior to RO is a cost- effective method for removing materials that are responsible for scaling and fouling on RO membranes. 18,19 It has been observed that dissolved algal organic matter (AOM) and natural organic matter (NOM) consisting of humic substances (HS), biopolymers (BP), polysaccharides (PS), proteins, etc. in seawater play a dominant role in membrane fouling. 20–22 Moreover, AOM results in membrane pore blockage and perpetual flux decline. 23,24 Environ. Sci.: Water Res. Technol. This journal is © The Royal Society of Chemistry 2020 a School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. E-mail: alicia.kjan@cityu.edu.hk b Environmental Engineering, Pusan National University, Busan 46241, Republic of Korea. E-mail: sh.jeong@pusan.ac.kr c Department of Earth Sciences, Indian Institute of Technology Gandhinagar, Gujarat – 382355, India † Electronic supplementary information (ESI) available: Fig. S1 and S2; Table S1. See DOI: 10.1039/c9ew00955h Water impact Natural or algal organic matters composed of humic substance, polysaccharide, protein etc. can significantly foul the membranes during the seawater desalination. Formation of harmful disinfection by-products during conventional oxidation process is also a concerning issue. Dissolved air flotation with in situ generated green chemical, ferrate showed a great ability to mitigate the membrane fouling as demonstrated by a non-destructive monitoring technique. Published on 27 January 2020. Downloaded by City University of Hong Kong Library on 3/2/2020 3:35:10 PM. View Article Online View Journal