Performance of a bioreactor with submerged membranes for aerobic treatment of municipal waste water

Aerobic treatment of municipal waste water in a membrane bioreactor was studied for 535 d. Apart from sampling, sludge was retained completely by a submerged hollow fibre membrane with a pore-size of 0.2 μm. The pilot plant comprised an anoxic zone to enable denitrification. The maximum liquid hold-up of the plant was 3.9 m3. In this study the reactor performance and the stability of the process and the membrane capacity were investigated. A stable flux of 18 l m−2 h−1 could be realised with a mean transmembrane pressure difference of 0.3 bar with air-bubbling and backflushing the membrane and cleaning it in place every two months for one or two hours. For about 140 d, a flux of 27 l m−2 h−1 was achieved, but cleaning became necessary more often. The hydraulic retention time (HRT) varied between 10.4 and 15.6 h. Accordingly the volumetric loading rate was between 1.1 and 1.7 kg COD m−3 d−1. No inoculum was used. The mixed liquor suspended solids (MLSS) concentration gradually increased to 18–20 g MLSS l−1. The feed to microorganism (F/M) ratio varied according to the operation conditions but decreased against a value of 0.07 kg COD kg−1 MLSS d−1. Treatment performance was very stable and on a high level. The COD was reduced by 95%. Nitrification was complete and up to 82% of the total nitrogen could be denitrified.