Microfiltration of activated sludge using submerged membrane with air bubbling (application to wastewater treatment)

Abstract

A membrane bioreactor is one of the applications of membrane technology to wastewater treatment. In this study we present the results obtained using hollow fiber microfiltration membranes directly immersed in the bioreactor. Two sizes of equipment were used: laboratory scale with a specially designed set of microfiltration hollow fibers (filtration area 0.016 m 2) and pilot scale with a bundle of hollow fibers (filtration area 0.5 m 2). Filtered liquid was pumped at a constant flow rate by suction through the membranes at a fairly low transmembrane pressure (less than 300 kPa). The bubbles produced by aeration of the biomass were essential for cake removal. The study was performed with short-term and long-erm experiments successively. In the short-term experiments, the critical conditions, which were the limit of steady operation, were identified. In our conditions of operation the value of critical flux was about 30 l/h.m. 2. The influence of air flow rate on critical flux was investigated. It was demonstrated that aeration was a significant factor governing the filtration conditions and decreasing fouling resistance. With application of optimal air flow rate, it was possible to perform in quasi-steady conditions with a permeate flow rate which was about 30% of the initial pure water flux. During long-term trials, the influence of the impact of sludge residence time (10, 20 and 30 days) was compared in three bioreactors operated in parallel, considering sludge production, sludge concentration and the fouling potential of the different fractions of activated sludge (solid fraction and liquid fraction). Neglecting the negative effect of the anti-foaming agent, very high efficiency of COD removal could be obtained (up to 90%) at a volumetric load of 3.2kg COD/m 3/d and a sludge residence time of 20 days.