Study On Membrane Fouling Control And Cleaning Method

Membrane Bioreactor (MBR) is a comparably new type of biological reactor that has attracted great attention in recent years. The advantages offered by MBR over conventional activated sludge process (ASP) include a small footprint, reduced sludge production and so on. In the experimental procedure, submerged MBR was used to treat municipal wastewater. During nearly one year operation period, from May 2006 to April 2007, the operation effects were as below. The average removal efficiencies of CODCr , BOD5 and ammonia were respectively 96.9%,95.0% and 84.7%.Membrane fouling is one of the most important factors that influence the development of MBR. The increases of membrane fouling under different conditions were monitored and membrane fouling increase showed a two-stage trend, which were stable increase stage and accelerated increase stage, under sub-critical flux. Membrane fouling was accelerated with the rise of sludge concentration. Furthermore, the multinomial regression of membrane fouling increase trend was made and the exact decomposition time was obtained to give theoretical background to on-line backwashing. On-line backwashing was made every five days and then the time that membrane fouling entered the second stage was delayed. The energy consumption loss due to over-great membrane fouling speed was also reduced. But the reagent introduction during on-line backwashing could have adverse influence on microorganism activity. The influence that reagents had on microorganism activity was reviewed in terms of dehydrogenase activity and OUR and the two methods were compared. Experimental result showed when NaClO concentration was 250ppm, the slight microorganism activity change would not influence system operation, but NaClO concentration more than 1000ppm would lead to considerable microorganism activity decline. Membrane fouling speed was so great under high sludge concentration that guided boards were added to both sides of the membrane to reduce membrane fouling.Membrane cleaning is the focus of current study. Through analysis of membrane resistances under different conditions, it was found that cake layer, which would be eliminated by high speed stream, was the most important factor of membrane fouling. The analysis of the substance content on membrane surface before cleaning showed that the amount of carbon was great and mental ion was comparably less in surface gel layer. Chemical cleaning was usually used to eliminate gel layer. Comparing single reagents, the cleaning effect of NaClO was the best, and when multiple reagent was used, 2%NaOH, 1% NaClO and 0.1%EDTA was the best combination. Temperature had significant influence on cleaning effect and higher temperature led to better effect. However, the acceleration of flux resume rate after cleaning was decreased and high temperature would easily result in membrane damage. So the optimum temperature used in this experiment was within 25 and 35 centigrade.