Assessment of pre-treatment to seawater reverse osmosis

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Membrane based desalination is widely used process to produce fresh water either from wastewater or seawater. However, membrane fouling on the reverse osmosis is a major hurdle. It increases the energy consumption as well as operating cost of reverse osmosis. A pre-treatment before reverse osmosis (RO) desalination can sign ificantly reduce the membrane fouling. The main objective of this study was to assess the relative merits of different pretreatment processes in terms of membrane fouling reduction, and removal of organic matter in terms of molecular weight distribution and dissolved organic carbon (DOC). Different fouling indices (such as silt density index (SDI), modified fouling index (MFI) and cross-flow samp ler modified fouling index (CFS-MFI)) were used to study the pretreatment efficiency of different process such as flocculation, adsorption, microfiltration and biofiltration. The effectiveness of different pretreatment on the fouling propensity of the feed was studied using synthetic waste water. The fouling potential of the feed was characterized by standard modified fouling index (MFI) and cross-11ow sampler modified fouling index (CFS-MFI). In CFS-MFI, a cross-flow sampler was used to simulate the condition of a cross-flow filtration. The results indicated that the pretreatment such as flocculation with an optimum dose of 68 mg/1 FeCb and adsorption with powdered activated carbon (PAC) of 1 g/I substantially reduced the fouling propensity of the feed. The standard MFI of flocculated wastewater was reduced by around 99% compared to that of the untreated wastewater. The effect of molecular weight distribution (MWD) of the foulants in the wastewater on the fouling propensity of the feed was also investigated. The MWD of pretreated effluent was correlated well with the MFT and CFS-MFI indices. Different processes such as flocculation with ferric chloride (FeCb) and deep bed filtration (sand filtration and dual media filtration) as a pre-treatment to microfiltration (MF) were used for seawater desalination. The performance of these pre-treatments was determined in terms of silt density index (SDI) and modified fouling index (MFI) and flux decline in MF. Flux decline of MF with seawater was 45% without any pre-treatment, 42% after prexvt treatment of FeCb flocculation , 24% after pre-treatment of sand filtration with in-line coagulation and 22o/o after pre-treatment of dual media filtration (sand and anthracite), respectively. MFI and SDI also indicated that deep bed filtration with in-line flocculation was better pre-treatment than flocculation alone. Detailed molecular weight distribution (MWD) of seawater organic matter was examined after different pretreatments. MWD of the initial seawater mainly ranged from 1510 Da to 130 Da. Deep bed filtration with in-line flocculation removed relatively large molecular weight of organic matter (1510 - 1180 Da), while the small molecular weights (less than 530 Da) were not removed. The removal of particulate matter and dissolved organic matter from seawater by the use of biofiltration was investigated through long term on-site operation of biofilters. Granular activated carbon (GAC) and anthracite were used as biofi lter med ia at two different filtration velocities. Filtrate quality was measured in terms of silt density index (SDI), modi fied fo uling index (MFI) and turbidity removal. Reverse osmosis (RO) was used as a post treatment. Both biofilters demonstrated similar fouling reduction behavior in terms of SDI and MFI. Fouling potential in terms of MFI values decreased to 10 s/L2 within the first 10-15 days of operation and kept constant up to the remaining experimental period of 55 days of operation for both GAC and anthracite biofilter. The filtrate turbidity was steady after 10 days and remained low at a value of 0.2-0.3 NTU and 0.28-0.31 NTU for anthracite and GAC biofilter respectively. Furthermore, the headloss development was low and within 20 em for biofilter operated at a low velocity of 5 m/h. A post treatment of reverse osmosis after a pretreatment of GAC and anthracite biofilters showed a reduction in normalized flux decline (J/Jo) from 0.22 to 0.12 and 0.35 to 0.21 during the first 20 hours respectively. The RO flux for seawater declined at a faster rate and continued even after 3 days when no pretreatment was provided. Based on the experiments, it was found that both med ia fi ltration (dual media) and biofi ltration are appropriate pre-treatment before RO. In patticular, Biofilter led to a consistent removal of organic matter over a long period of time.
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