Thesis (M. Eng.) - Asian Institute of Technology, 1996

A thesis submitted in partial fulfillment of the requirements for degree of Master of Engineering.

In this study the possibility of application of air backflushing technique through hollow fiber microfiltration was investigated. The process employed direct solid-liquid separation by immersed two membrane modules with pore size of 0.2 om directly in the activated sludge aeration tank of 80 L volume. This study was conducted with high concentration of activated sludge and divided into short-term and long-tenn experiments. In short term experimental runs, the optimum air backflushing and filtration cycle was investigated. 15 minutes filtration and 15 minutes air backflushing provided the best result in term of flux improvement and stability. Due to the membrane module stability limitation, the applied compressed air pressure of 1 bar was not sufficient to remove the clogging completely. However this cyclic operation provided higher flux stability compared to operation without air diffusion. In long-term experiments, the initial sludge concentration was 13,000 mg/L. Three different hydraulic retention times (HRT) of 26, 18 and 10.5 hour which corresponds to the permeate flux of 3.08, 4.44 and 7.62 L/m2 . h were investigated. Here, it was noted that the filtration pressure related to the MLSS concentration. Whereas the stable operation could be obtained at 26 and 18 hours. All experimental runs provided more than 90% removal of COD, BOD and TKN with final MLSS of 40,000 mg/L in the reactor. Although the operation with daily sludge draining (1.6 Lid), the ML VSS/MLSS values seem slightly decreased. However, such conditions could not effect significantly to the process performance in term of physical, chemical, biological and bacteriological qualities of membrane bioreactor effluent.