In order to check the sludge reduction capacity of metazoa in a membrane bioreactor (MBR), pilot-scale studies were conducted. Three MBRs had been set in a wastewater treatment plant at Tokyo, Japan and they were receiving real wastewater. Initially pH inside the three MBRs was controlled as pH 7, 6 and 5 respectively. Then metazoa population was monitored along with MLSS change. It was found that the presence or absence of the metazoa population did not have any significant effect on the increasing pattern of MLSS. In the MBR with pH 6 highest accumulation of sludge was observed though a high and steady level of metazoa (1,000-2,000 per ml) was present there. But in this MBR a lot of metazoa attached in the membrane was also observed and here the increase in transmembrane pressure was less than in the other two. So, metazoa population especially the attached one in the membrane plays an effective role in fouling control of the membrane. Presence of attached media may provide a suitable niche for metazoa in the process. So, attached media known as DB lace was also inserted in MBRs for testing its capacity along with inoculum of oligochaete worms. Accumulation of sludge was not satisfactory in the attached string and it seems that inoculated worm could not adjust to the environment as they were not sludge originated. So, in the next experimental stage, attached media was inserted in the form of a bundle and this time no inoculation of worm was used. A steady metazoa population was observed in the system but the accumulation of sludge in the attached media was the same as before.

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