Fouling characterisation in membrane bioreactors

Abstract

Compared with conventional wastewater treatment processes, membrane bioreactors offer several advantages, e.g. high biodegradation efficiency, smaller sludge production and compactness. However, membrane fouling is the main limitation to faster development of this process. An experimental study has been performed using hollow fibers (pore size 0.1 μm) immersed in an aerated tank for treating synthetic wastewater representative of dairy effluent. For the same organic load (5.7 kg COD/m3 per day) the COD removal efficiency, the sludge production and fouling ability were compared in three reactors operated at different sludge ages (10, 20 and 30 days). COD removal was high: 95–97.5%. The sludge production decreased from 0.31 to 0.16 kg MLSS/kg CODremoved when the sludge age increased from 10 to 30 days. Concerning the fouling ability of the sludge, a specific experiment (measurement of the specific resistance and hydraulic resistance during filtration) was designed to determine the influence on membrane fouling of the three fractions of the sludge: suspended solids, colloids and solutes. All the experiments confirm the importance of the interstitial matter — colloids and solutes — in membrane fouling. Consequently, bubbling can be expected to be only partially efficient, as bubbles are efficient for limiting particle deposition and polarisation phenomena, but not for internal fouling. Increasing the air flow rate from 1.2 to 3.6 m3/m2membrane area per hour, it was possible to decrease the total resistance — thus increasing the filtrate flux — by a ratio of 3. However, for given conditions of aeration, periodic backwashing gave an additional efficiency by decreasing internal fouling. In optimal conditions of backwashing (15 s every 5 min) the resistance could be decreased by 3.5-fold.