Application of biological activated carbon to mitigate algal organic matter fouling of ceramic microfiltration membranes

Ceramic membranes are being used increasingly in water treatment because of their excellence in service such as high chemical and thermal stability, long operational life. However, membrane fouling affects their efficiency and remains as a major challenge for their widespread application for drinking water treatment. Algal blooms in water reservoirs lead to the release of algal organic matter (AOM) which causes severe fouling during microfiltration due to the presence of high molecular weight (HMW) organic matter. Feed water pre-treatment may minimize membrane fouling, however information regarding the potential of biological pre-treatment processes for mitigating the AOM fouling is very limited. The core objective of this study was to determine the efficiency of biological activated carbon (BAC) process as a pre-treatment to mitigate the membrane fouling caused by AOM and also to acquire information on the fouling potential of AOM from the most common blooming algae species including Chlorella vulgaris and Microcystis aeruginosa. Fouling potential of soluble algal organic matter extracted from exponential phase (day 12) and stationary phase (day 25) of C. vulgaris (C-AOM) and M. aeruginosa (M-AOM) were examined in this study. For a better insight into the fouling effect of AOM present in surface water, the AOM was diluted either with pure water (Milli-Q) or natural surface water to prepare the feed solutions for the BAC and microfiltration (MF) tests. A constant DOC concentration of AOM was used in all feed preparation and significant flux decline was observed for both of the algal species at both of the growth phases. The results obtained in the fouling study of single species AOM indicated that membrane fouling was influenced significantly by algal species, their growth phase, AOM characteristics and also on background water. Solutions containing AOM extracted from stationary phase exhibited the worse normalized flux (0.15) during MF and C. vulgaris posed highest fouling potential. BAC process removed more DOC (4.3 mgL-1) than MF alone (3.1 mgL-1). MF and BAC process was found to reduce the concentration of DOC and UVA indicated the removal of organic matter and humic substances in samples. Considerably better UVA removal than DOC removal was observed after MF (49% cf. 34%) and BAC process (46% cf. 62%) due to removal of UV-absorbing substances such as humic substances. After BAC followed by MF, M-AOM day 12 samples achieved higher DOC (74%) removal whereas day 25 samples achieved higher UVA (76%) removal. In the case of C. vulgaris DOC (72%) removal was observed to be better than UVA (69%) removal after BAC followed by MF at both algal growth phases. C-AOM extracted from stationary phase lead to better organic removal after whole treatment process. According to the permeate analyses, AOM contained carbohydrate, protein and humic substances that deposited on/in membrane during MF and were considered as major foulants. More amounts of carbohydrate than protein as removed by MF whereas more protein than carbohydrate was removed by BAC pre-treatment. Alum coagulation was investigated to examine its performance as a pre-treatment to improve membrane permeate flux. An optimum dose of 5 mg Al3+ L-1 was found efficient to mitigate the membrane fouling. AOM extracted from C. vulgaris led to greater fouling than that of M. aeruginosa similar to findings of previous study. Better flux improvement was obtained for alum compared with the BAC pre-treatment, however DOC and UVA removal was slightly higher after BAC than alum pre-treatment. This suggested that the fouling potential was not only depending on DOC concentration but also the other factors such as AOM characteristics. The alum treated M-AOM samples achieved better DOC reduction than the alum treated C-AOM samples. After alum and BAC pre-treatment, UVA removal was higher than the DOC removal which is due to the removal of UV absorbing substances. According to the characterization of the permeate, the better membrane flux obtained after pre-treatment was mainly due to the removal of the biopolymers such as protein and carbohydrate, and the humic substances. The effect of mixed AOM derived from both algal species on the MF performance was then investigated and BAC was applied as a pre-treatment process for mitigating the fouling. A natural surface water was spiked with AOM extracted from the stationary phase of C. vulgaris and M. aeruginosa at the ratio of 1:1 in terms of DOC content. The flux for the untreated mixed AOM samples was comparable with the flux of stationary phase M. aeruginosa. The DOC and UVA removal by MF for the mixed species AOM was comparable with the DOC and UVA removal for single species AOM. Although having considerable removal in DOC and UVA, the poor flux for the BAC treated solution of the mixed AOM was attributed to the other characteristics of the organic matter rather than DOC level alone. BAC process provided a considerable reduction in carbohydrate and protein concentration from the solutions containing mixed species AOM. The EEM spectrum of the untreated water confirmed the presence of fulvic- like acid (FA) and humic-like acid (HA) substances in the samples, which were removed by BAC process may reduced membrane fouling. Two BAC columns packed with two different types of GAC were utilized to investigate the influence of carbon media on their pre-treatment performance. The performance of the two different BAC columns did not differ significantly in terms of flux profile and organic matter removal due to their similar GAC characteristics. Both of the BAC columns reduced fouling although less flux improvement was observed for mixed AOM species. Overall, the integrated process of BAC pre-treatment for the MF system has the potential to mitigate the fouling of the ceramic MF membrane. Given the inherent advantages of the bioloigcal pre-treatment such as chemical-free process and low energy requirement, this study recommends further research to optimize the process to enhance its effectiveness for its possible applications in membrane based drinking water treatment