Membrane hybrid system in high quality water reuse
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Although membrane filtration treatment for water reclamation is becoming more widespread, the challenges such as membrane fouling, high cost, management of concentrate, and incomplete removal of organic micro pollutants still prevail. This study presents technical alterations to minimize such issues via the development of pre-treatments techniques where fluidized bed contactors and membrane hybrid systems were used. Granular activated carbon (GAC) and an ion exchange resin (Purolite A502PS) were used as adsorbents for the above mentioned pre-treatments respectively. Biologically treated sewage effluent (BTSE) collected from a water reclamation plant, Sydney was used as a feed water for these experiments. The operational conditions such as fluidization velocity and adsorbent dosage of fluidized bed columns strongly influenced the removal of dissolved organics (DOC). GAC was found to be more effective in removing organics compared to Purolite A502PS. This could be due to the competition provided by other inorganic anions present in BTSE for Purolite exchange sites. Plug flow model was successfully used to predict the impact of the amount of adsorbent and of the flow rate on the removal of organic matter. A similar trend was observed when another pre-treatment technology of membrane hybrid system was used. Micro filtration (MF) – GAC hybrid system effectively removed hydrophobic organics, hydrophilic organics and organic micro pollutants, whilst, the removal of inorganic ions was minimum. Comparatively, the performance of MF-Purolite hybrid system was less efficient in DOC removal; however the removal of sulfate and nitrate ions was good. In both membrane hybrid systems, the addition of adsorbents directly into the membrane reactor reduced membrane fouling by membrane surface scouring and adsorption. Overall, the performance of membrane-GAC adsorption hybrid system was more effective than membrane-ion exchange hybrid system. Based on this, MF-GAC was suggested to combine with nano filtration (NF) system. Here second membrane filtration of NF was to further polish pre-treated BTSE in terms of dissolved organics, organic micro pollutants and for some divalent ions removal. This treatment system is referred as ‘dual membrane hybrid system’ i.e., the combination of MF-GAC adsorption hybrid system followed by NF. Traditionally RO is used as a polishing step in dual membrane systems in waste water reclamation plants. The use of NF instead of RO is found to be cost effective in terms of energy requirements. This dual membrane hybrid system is suggested to produce high quality water reuse where the removal of monovalent ions is not necessary; however the selection of treatment system depends on the requirements of recycled water for end purposes. For example, the recycled water used for irrigation requires sodium adsorption ratio of 3-9 for wide range of salt tolerant crops. Therefore, a study was conducted to determine whether the BTSE can be treated using nano filtration (NF) and reverse osmosis (RO) to bring these risk parameters within safety limits, because the NF treated BTSE could bring SAR levels only up to 14. As per the results, it was suggested to blend NF and RO permeate in equal proportions to produce a product quality suitable for irrigation with SAR value below 10. Utilizing NF prior to RO reduced the RO membrane fouling and both NF and RO removed most of the organic micro pollutants from BTSE and this may subsequently protect soil and ground water from potential hazards.
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