In this study, novel submerged low pressure membrane (microfiltration) systems coupled with physico-chemical treatment such as coagulation/flocculation or adsorption, termed in this study, as “submerged membrane hybrid systems (SMHS)” were investigated as pretreatment for seawater reverse osmosis (SWRO).
Coagulation as pretreatment: In recent years, coagulation and subsequent media-filtration as well as low pressure membrane-based system such as microfiltration (MF) and ultrafiltration (UF) have been used as pretreatment methods for seawater reverse osmosis desalination. Coagulation can remove colloidal matters and dissolved organic compounds which cause irreversible membrane fouling. In this study, a commonly used ferric chloride (FeCl₃) was used as coagulant for removing organic compounds from seawater. The coagulation by FeCl₃ at optimum dosage removed a majority (95%) of hydrophobic compounds. In addition, the ultrafilter modified fouling index (UF-MFI) decreased considerably from 15,848 s/L² with raw seawater to 3,025 s/L² with seawater after coagulation. It was observed that critical flux was increased from 20 L/m²-h in the conventional submerged membrane system to 55 L/m²-h in submerged membrane coagulation hybrid system (SMCHS). The SMCHS led to a high DOC removal efficiency (more than 70%) while keeping the development of trans-membrane pressure (TMP) significantly lower than that of conventional submerged membrane system (SMS).
Prediction of DOC removal by FeCl₃ coagulation using mathematical modeling: Coagulation removes colloidal matters and dissolved organic carbon (DOC) which can cause irreversible membrane fouling. However, how DOC is removed by coagulant is not well-known. Jar test was used to study the removal of hydrophobic and hydrophilic DOC fractions at various doses (0.5-8.0 mg-Fe⁺³/L) of FeCl₃ and pH (5-9). Natural organic matter (NOM) in seawater and treated seawater were fractionated by liquid chromatography-organic carbon detector (LC-OCD). Compared to surface water, the removal of DOC by coagulation was remarkably different. Majority of DOC could be easily removed with very low coagulant dose (<5.0 mg-Fe⁺³/L) and the removal efficiency did not significantly deteriorate with pH, but remaining DOC composition has changed. Hydrophobic fraction (HB) is better removed at high pH while hydrophilic fraction (HF) is better removed at low pH. A model which assumes that the removal occurs by adsorption of un-dissociated compounds onto ferric hydroxide was formulated and successfully validated against the jar test data.
The effect of flocculation on the performance of MF in SMCHS was also investigated with modified poly ferric silicate (PFSi-δ). Both PFSi-δ and FeCl₃ were found to be suitable in removing organic compounds. The results show that PFSi-δ was better than FeCl₃ in terms of removing turbidity and DOC, particularly in removing hydrophilic compounds. It was observed that PFSi-δ (1.2 mg Fe³⁺/L) and FeCl3 (3.0 mg Fe³⁺/L) led to an increase of critical flux from 20 L/m²•h to 35 L/m²•h and 55 L/m²•h, respectively.
The removal of different fractions of organic matter in seawater was investigated using titanium tetrachloride (TiCl₄) as flocculant and compared with that of FeCl₃. The hydrophobic compounds removal was predominant by both flocculants. However, the removal of hydrophilic organic compounds, such as humics and low-molecular weight neutral compounds of seawater was superior with TiCl₄ compared to FeCl₃.
This study also investigated the effect of powder activated carbon (PAC) dosed in a submerged membrane adsorption hybrid system (SMAHS) in removing organic matter from seawater. The addition of PAC into submerged microfiltration reactor not only improved critical flux from 20 L/m².h to 40 L/m².h but also helped reduce the TMP of the system. The analyses of organic matter fraction showed that PAC was able to remove most of hydrophobic compounds (more than 96%) and a significant amount of hydrophilic compounds (78%).
Submerged membrane adsorption hybrid system with flocculation as pretreatment: Biofouling on RO membranes is the most serious problem which affects RO process efficiency and increases its operation cost. The biofouling cannot be effectively removed by the conventional pre-treatment traditionally used in desalination plants. SMS coupled with adsorption and/or coagulation can be a sustainable pre-treatment in reducing membrane fouling and at the same time improving the feed water quality to the seawater reverse osmosis. The addition of PAC of 1.5 g/L into SMAHS could help to remove significant amount of both hydrophobic compounds (81.4%) and hydrophilic compounds (73.3%). When this SMAHS was combined with FeCl₃ coagulation of 0.5 mg of Fe³⁺/L, dissolved organic carbon removal efficiency was excellent even with a lower dose of PAC (0.5 g/L). It should be noted that PAC addition was only at the start and no further PAC addition was made during experiment. The SMAHS and the submerged membrane coagulation–adsorption hybrid system (SMCAHS) can significantly remove the total bacteria which contain also live cells. As a result, microbial adenosine triphosphate (ATP) concentration in treated seawater and foulants was considerably decreased. These led to a significant reduction of assimilable organic carbon (AOC) during the initial stage of RO operation. In this study, AOC method was modified to measure the growth of bacteria in seawater by using the Pseudomonas P.60 strain.
Application of fouling model in SMAHS: The application of three different membrane fouling models namely pore blockage, pore constriction, and cake formation models showed that cake formation was the predominant fouling mechanisms causing fouling in SMHSs.
Characterization of SMAHS effluent: Organic matter in seawater before and after pretreatment was characterized in terms of XAD fractionation, molecular weight distribution (MWD) and fluorescence. A detailed study on the seawater organic matter (SWOM) structure was made through ¹H-nuclear magnetic resonance (¹H-NMR),pyrolysis-gas chromatography mass spectrometry (Py-GC/MS) and liquid chromatography mass spectrometry-ion trap-time of fright (LC/MS-IT-TOF). The three dimensional-fluorescence emission-excitation matrix (3D-FEEM) showed a removal of humic-like materials by SMHSs. In addition, a humic-like relative to protein-like compounds was reduced significantly but aromaticity of humic-like materials was increased. After pretreatment by SMHSs, humics and biopolymers of over 900 Da. were found to be reduced and their structure associated with element composition was also changed.
RO membrane foulant characterization: The organic and biological foulants on RO membrane operated with seawater pretreated by SMHSs were characterized. Organic foulants on RO membrane were characterized in terms of MWD, fluorescence and extracellular polymeric substance (EPS) analyses. The organic foulants were mainly composed of high molecular weight matters representing biopolymers in the foulants. The 3D-FEEM analysis showed that protein-like materials were dominant with samples pretreated by SMHSs. Humic-like materials which have lower aromaticity were also present in the foulant. Biological foulants were investigated in terms of total direct cell (TDC) count, cell viability and biomass activity (in terms of ATP). Biological fouling was found to be reduced by organic removal with SMHSs. The fouled membranes were characterized using environmental SEM/EDX, attenuated total reflection-Fourier transform infrared spectrometry, zeta-potential measurement, atomic force microscopy, and contact angle measurement.
Development of a rapid AOC test: One strategy to minimize biofouling is the pre-treatment of seawater prior to RO application. In this regard, there is a need for tools that can be used to assess the influent water which allows for the subsequent selection of the optimum pretreatment methods. One parameter that is directly linked to biofouling potential is the concentration of AOC in the feed-water, where high nutrient levels are associated with increased growth potential of the microbial fouling community. A rapid and accurate of AOC method was developed for marine (sea) waters. This method is based on quantifying the bioluminescence response of the marine bacterium Vibrio fischeri MJ-1. Compared to previous methods, this new V. fischeri method was rapid (within 1h), sensitive (detection limit=0.1 μg-C glucose equivalents/L) and highly suitable for seawater samples. V. fischeri method was evaluated using real seawater samples. The results showed positive reproductive AOC values. The new V. fischeri AOC method developed has a highly promising potential to be practically adopted as a rapid indicator of AOC concentration and hence biofouling potential of influent marine water.
Submerged membrane adsorption bioreactor (SMABR) as sustainable pretreatment was investigated. SMABR removed organic matter by adsorption and biological degradation. At a PAC residence time of 66 d (1.5% of daily PAC replacement), higher organic removal was achieved with a high removal of biopolymers, humics and hydrophobic organics. A continuous MBR operation with the optimal PAC residence time of 66 d was conducted and compared with MBR with no PAC replenishment in terms of the removal of organic and microbes. High removal of organics of up to 72% was maintained with only a marginal increment of trans-membrane pressure and stable bioactivity (TDC and ATP) during the 50 d of operation. The SMABR was found to be a sustainable biological pretreatment to RO with only a small amount of PAC requirement (2.14 g of PAC/m³ of seawater treated).
Contact flocculation filtration as pretreatment: Deep bed filtration has traditionally been used as a pretreatment in seawater desalination. The performance of contact flocculation–filtration (CFF) as pretreatment of SWRO was evaluated in terms of pressure drop through the filter and removal of organics and turbidity. The performances of CFF were experimentally evaluated with different flocculant doses (0.5–3.0mg Fe³⁺/L) and rapid mixing times (1.7–14.4s). The headloss also significantly decreased when the flocculant dose was reduced from 3.0 to 0.5 mg Fe³⁺/L. However, the organic matter removal was lower at a lower dose of ferric chloride.
In this study, it was also investigated the potential of CFF acting as a biofilter in addition to its major function of flocculation and particle/floc separation. Two different media (sand; S-CFF and anthracite; A-CFF) were tested. Bacterial activity in the filter bed was assessed in terms of cell number, ATP measurement and microbial community over the filter run of 90 d. With the growth of an active microbial population on the filter bed medium, significant removal of organic compounds, especially low molecular weight (LMW) organics, from the seawater was achieved. It was found that CFF functions both as flocculation and separation unit and also as biofilter with moderate efficiency in reducing biofouling potential. A-CFF needed longer time to achieve bio-stabilization but it showed more effective biofiltration potential than S-CFF.