Microfiltration hybrid systems in wastewater treatment for reuse
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Generally, the conventional wastewater treatment cannot remove all the effluent organic matter (EfOM) such as synthetic organic chemicals and natural organic matter etc. As a result, the biologically treated effluent from sewage treatment plant needs to undergo further advanced treatment processes. To obtain water of recyclable quality, initially physico-chemical processes such as flocculation, sedimentation, filtration and adsorption were normally used. However, with advanced technologies and ever increasing stringent water quality criteria, membrane processes are becoming more attractive in water reuse. Among different membrane processes, although microfiltration (MF) can be operated economically, it alone cannot remove organic matter. If MF is combined with an enhanced flocculation or/and adsorption, it will be able to reduce superior level of organic contaminants. The aims of this study are: (i) improving the dissolved organic removal and reduce membrane fouling of two membrane hybrid systems (crossflow microfiltration (CFMF) and submerged membrane adsorption hybrid system (SMAHS)) using different pretreatment methods (flocculation, adsorption and flocculation- adsorption); (ii) investigating the critical flux of a laboratory-scale CFMF with and without different pretreatments. The incorporation of powdered activated carbon (PAC) as pretreatment to CFMF resulted in high TOC removal efficiency (more than 80%) when the PAC-CFMF system was operated at a relatively high filtration flux of 250 L/m2.h. The incorporation of flocculation and PAC as pretreatments to CFMF process resulted in a very high TOC removal efficiency (99.7%) and a stable filtration flux during 5-hour filter run (less than 12% flux decline), when the hybrid system was operated at a higher filtration flux (270 L/m2.h). Application of membrane processes requires lower investment and operating costs. One of the ways is to operate system at a constant filtration flux below the critical flux. With both flocculation and adsorption as pretreatment to CFMF, the critical flux of biologically treated effluent increased dramatically (5-7 times increase). The preadsorption, PAC dose, aeration rate and filtration flux had effects both on organic matter removal efficiency and TMP development. The preadsorption of 1 hour prior to the membrane operation was important in mitigating the membrane fouling. The suitable aeration rate, filtration flux and initial PAC dosing were 16 L/min, < 24 L/m2.h and 5 g/L respectively for the wastewater used in this study. The long term SMAHS experiments conducted with regular PAC replacement indicated that the PAC replacement in PAC-MF reactor could stimulate both biological activity and adsorption, as well as optimize the operation of the hybrid system.
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