Fouling characterisation in membrane bioreactors

Compared with conventional wastewater treatment processes, membrane bioreac tors offer several advantages, e.g. high biodegradation efficiency, smaller sludge production and compactness. However, membrane fouling is the main l imitation to faster development of this process. An experimental study has been performed using hollow fibers (pore size 0.1 mum) immersed in an aerat ed tank for treating synthetic wastewater representative of dairy effluent. For the same organic load (5.7 kg COD/m(3) per day) the COD removal effici ency, the sludge production and fouling ability were compared in three reac tors operated at different sludge ages (10, 20 and 30 days). COD removal wa s 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 t he specific resistance and hydraulic resistance during filtration) was desi gned 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. Consecluently, bubbling can be expected to be only par tially efficient, as bubbles are efficient for limiting particle deposition and polarisation phenomena, but not for internal fouling. Increasing the a ir flow rate from 1.2 to 3.6 m(3)/m(2membrane area) per hour, it was possib le to decrease the total resistance - thus increasing the filtrate flux - b y a ratio of 3. However, for given conditions of aeration, periodic backwas hing rave an additional efficiency by decreasing internal fouling. In optim al conditions of backwashing (15 s every 5 min) the resistance could be dec reased by 3.5-fold. (C) 2001 Elsevier Science B.V. All rights reserved.