Coagulation for SWRO pretreatment : Eliminating SWRO biofouling potential

The coagulation of transparent exopolymer particles (TEP) produced using Chaetoceros affinis (CA) in synthetic seawater (SSW) was investigated in this study. TEP found in most natural waters, the most neglected gel-like particles have important implications for biological fouling in many filtration applications. The study is focused on the new concerns and an increased attention on ad-hoc pretreatment systems due to the operational problem caused by algal bloom episodes in SWRO plants during the last 5-10 years. TEPs are a planktonic type of EPS and have been identified as a major component of polysaccharide substances in seawater and other aquatic systems. (Passow, 2012). In polysaccharides, nucleic acids, proteins and trace elements may be associated with these gel-like particles. TEP may initiate or enhance biofilm development in RO membranes by creating a ""conditioning layer"" on the membrane surface. TEP may serve as ""food packages"" for microorganisms and stimulate ""bacterial growth"". Therefore, TEP has to be removed from RO feed water to reduce (bio)fouling potential in RO. The performance of RO relies upon the production of high quality water from pretreatment. The study targeted to the objective of the Kemira Project ""Eliminating Biological and Organic Fouling in SWRO with Coagulation Pretreatment"" with proposed pretreatment scheme of conventional coagulation (coagulation/flocculation/sedimentation) followed by filtration for the product of PIX-111 (ferric chloride). The aim of the experiment is to assess impact of coagulation on removal of TEP in terms of biopolymers (LC-OCD), TEP, residual iron, fouling potential (MFI-UF), DOC characterization (FEEM and UV254) from SSW on laboratory scale. The feed water was prepared using SSW and TEP cultured using CA by mixing at the specific ratio using biopolymer concentration of 0.5 mg C/L, which was the highest measured concentration in North Sea during bloom in spring (March-June). Feed water was diluted based on LC-OCD, which is relatively simple and precise method compared to TEP0.4, TEP10kDa, MFI-UF and FEEM. The feed water of 0.5 mg C/L as biopolymer has lower concentration of humic, building blocks and LMW neutrals but more LMW neutrals are mainly from the SSW. Protein like fluorescence peak could be clearly discriminated from humic-like peak in the FEEM spectra of the feed.Trial tests were performed to establish the experimental protocol and fix the conditions for rapid mixing intensity and time, flocculation intensity and time, settling time, filter pore size, vacuum or pressure filtration and interference of Fe in TEP measurements. From the experimental result, it was confirmed that the iron and TEP reacts each other and both are removed from the SSW by the coagulation sedimentation process. This was achieved by ensuring the availability of higher iron residual in the absence of TEP compared to that in the presence of TEP. Therefore, colloidal shielding of TEP on iron colloids does not occur. The influence of iron coagulant was investigated on TEP0.4 measurements by measuring TEP at known iron dose without settling and filtering. Iron coagulant contributes to more agglomeration of TEPs and higher retention on the 0.4µm filter during TEP0.4 measurements (physical effects). However, Iron coagulant does not react with alcian blue and give raise to higher retention on the 0.4µm filter during TEP0.4 measurements (chemical effects). The coagulant used for the experiment was KEMIRA's PIX-111. The supplier did not recommend dilution of the coagulant. Therefore, low dosing of coagulant was to be very precise. Hence, the volume dosed was confirmed by measuring volume against weight of water for micropipette. Further, the actual dose of the coagulant was verified by measuring the concentration of the Fe against the residual iron. The pHs wereadjusted to keep the final pH to the required pH. This was achieved by verifying the volume of acid or base from the stoichiometry calculation through titration test prior to real experiments. From the study of coagulation of TEP using iron chloride at pH 5 and 8, TEP removal measured as biopolymers (by LC-OCD) with coagulation followed by 0.45µm filtration is effective. The higher the coagulant dose the higher the effect, particularly at pH 8. The Effect of coagulation/filtration on TEP0.4 is high. This result was expected since TEP0.4 is collected by filtering through 0.45µm filters. In contrast, the effect of coagulation/filtration on TEP10kDa is substantial. However doubt about the blanks due to release of polymers from different membranes used in the analyses prevent firm conclusions. Hence, testing protocol for TEP10kDa is to be revised as proposed considering salinity effect and the biopolymer release from the membrane used for filtration. Filtration through 0.45µm significantly improves water quality for most parameters tested, even at low coagulant concentrations. Fouling potential (MFI-UF10kDa) of TEP solutions with biopolymer concentration of 0.5mgC/L is high. Coagulation with iron (III) followed by 0.45µm filtration reduces the fouling potential (MFI-UF10kDa). The higher the coagulant dose the lower the fouling potential.The cleaning frequency of RO membranes (estimated using the theoretical UNESCO-IHE model), can be significantly reduced with coagulation/filtration. However, the results are to be validated through existing operation of the RO system. Studying the efficiency of conventional coagulation/UF, inline coagulation/UF, etc. can reveal more information on the most suitable pretreatment option for SWRO in algal blooms