Membrane bioreactor for the drinking water treatment of polluted surface water supplies

A laboratory membrane bioreactor (MBR) using a submerged polyethylene hollow-fibre membrane module with a pore size of 0.4 (mu)m and a total surface area of 0.2 m2 was used for treating a raw water supply slightly polluted by domestic sewage. The feeding influent had a total organic carbon (TOC) level of 3–5 mg/L and an ammonia nitrogen (NH3-N) concentration of 3–4 mg/L. The MBR ran continuously for more than 500 days, with a hydraulic retention time (HRT) as short as 1 h or less. Sufficient organic degradation and complete nitrification were achieved in the MBR effluent, which normally had a TOC of less than 2 mg/L and a NH3-N of lower than 0.2 mg/L. The process was also highly effective for eliminating conventional water impurities, as demonstrated by decreases in turbidity from 4.50±1.11 to 0.08±0.03 NTU, in total coliforms from 105/mL to less than 5/mL and in UV254 absorbance from 0.098±0.019 to 0.036±0.007 cm^-1. With the MBR treatment, the 3-day trihalomethane formation potential (THMFP) was significantly reduced from 239.5±43.8 to 60.4± 23.1 (mu)g/L. The initial chlorine demand for disinfection decreased from 22.3±5.1 to 0.5±0.1 mg/L. The biostability of the effluent improved considerably as the assimilable organic carbon (AOC) decreased from 134.5±52.7 to 25.3±19.9 (mu)g/L. All of these water quality parameters show the superior quality of the MBR-treated water, which was comparable to or even better than the local tap water. Molecular size distribution analysis and the hydrophobic characterisation of the MBR effluent, in comparison to the filtered liquor from the bioreactor, suggest that the MBR had an enhanced filtration mechanism. A sludge layer on the membrane surface could have functioned as an additional barrier to the passage of typical THM precursors, such as large organic molecules and hydrophobic compounds. These results indicate that the MBR with a short HRT could be developed as an effective biological water treatment process to address the urgent need of many developing countries that are plagued by the serious contamination of surface water resources.