Submerged Microfiltration: An Energy Efficient Process for Water Reuse

Abstract

Membrane separation process is being emerged as an innovative wastewater treatment technology. However, its use at present is limited due to its high cost of installation and its long-term operational difficulty. Membrane fouling is a major obstacle to the successful operation of the membrane separation process. The membrane processes such as reverse osmosis and nano-filtration can remove most of the pollutants, including dissolved organics, but their operational costs are high because of high-energy requirements and membrane fouling. Micro or ultra-filtration is a cost-effective option, but they cannot remove dissolved organic matter due to their relatively larger pore sizes. The membrane fouling can be reduced by operating the membrane process under critical velocity and/or by combining the membrane processes with physico-chemical and/or biological processes. In this study, a submerged hollow-fibre membrane with powdered activated carbon (PAC) adsorption (adsorption-membrane hybrid system) was investigated for the removal of organics from biologically treated sewage effluent from a sewage treatment plant and a low strength synthetic wastewater. The main aim of adding PAC to the system was to reduce the direct organic loading to the membrane surface. In this hybrid system, the organics are adsorbed ento the PAC, and the organic laden PAC is eventually separated by the membrane. In long run, after the growth of microorganisms on PAC surface, the organic would be biodegraded by the microorganisms and thus the PAC can be used for long time. The membrane is also free from fouling (or very little fouling) and thus can be used for long time without cleaning. The membrane was submerged in a tank containing wastewater. A known dose of PAC was added to the tank. An air diffuser was used to keep the PAC in suspension and to provide dissolved oxygen for biological activity. The influent and effluent flows to and from the tank were maintained using pumps. The level of wastewater in the tank was maintained with the help of a level sensor. The membrane fouling was observed in a transmembrane pressure gauge. The hybrid system was tested for its long-term use in wastewater treatment. The use of PAC in the membrane system was found very effective, not only in removing refractory organics but also in reducing membrane clogging. The organic removal efficiency of the system was consistent for 47 days for the biologically treated sewage effluent. There was very little pressure drop, which meant the membrane fouling was negligible. This system led to an initial 85% organic removal with the low strength synthetic wastewater. However, the efficiency decreased to 55 % after 20 days operation, then after the efficiency remained consistent upto 30 days of operation. After 30 days of continuous operation, the membrane experienced significant blocking and the desired filtration flux of 0.288 m3/m2d could not be maintained. When the PAC dose was increased to 5 gIL and membrane backwashing provided (at 0.87 m3/m2d for 2 mins; once per day), the organic removal efficiency remained consistent at 80% even after 40 days of operation, and there was no pressure increase observed. Thus, with the higher PAC dose and/or simple backwash mechanism, the system could achieve higher organic removal efficiency, and longer periods of operation. This submerged membrane-adsorption hybrid system has many advantages. The PAC can be used for a long period. As adsorbed organics undergo biodegradation, more adsorption sites are created on the PAC surface. The submerged membranes do not become clogged as almost all organics are removed by PAC and the role of membrane is only to retain the PAC and other suspended solids. The energy requirement is very low (as low as 0.2 kwh/m3) and no sludge problem. These experimental results showed that the membrane separation process when used in conjunction with adsorption could be a viable long-term solution to the problems currently associated with the application of membrane.