The bacterial communities of membrane-separation bioreactors (MBR) fed with raw sewage were analyzed by a pilot scale study. The community was analyzed by both Fluorescent in Situ Hybridization (FISH) and PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) techniques. Five rRNA targeted group specific oligonucleotide probes showed that the alpha- and beta- subclasses of proteobacteria were the most dominant groups among them. The identification of ammonia-oxidizing bacteria in MBR was confirmed by three probes: NEU, Nsv 443 and Nso 190. Mostly the ammonia-oxidizers were found in groups and present in the form of clusters or aggregates. The ratio of NEU/EUB was estimated by double hybridization and image analysis techniques as 6%. The Nitrobacter sp. was also identified inside the MBR with the help of a NIT3 probe and they were also found to be present in the form of a cluster. Usually the clusters formed by the Nitrobacter sp. were smaller than those of ammonia-oxidizing groups. After numerical analysis on the band pattern of DGGE, it was found that the MBR bacterial communities were different from that of conventional activated sludge (CAS) communities with dissimilarity indexes more than 0.6. The diversity of the microbial community was estimated by the Shannon-Weaver index of general diversity. It was found that the value of the diversity index for the CAS process was 1.61 while those for two MBR processes were 1.68 and 1.59.

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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.

In this study the role of higher organisms and the effect of their predation on bacterial size distribution as well as on floc size variation were investigated in the membrane-separation activated sludge system. The pilot plant study was conducted by setting a membrane-separation bioreactor (MBR) in a wastewater treatment plant in Tokyo, Japan. A large number of predator organisms were observed in the MBR dominated by ciliated protozoa. The dispersed bacterial size distribution was obtained by using the combination of florescent in situ hybridization (FISH) and image analysis technique. With the increase of predator species especially sessile ciliates and free-swimming ciliates, reduction of smaller bacterial population (1 μm or so) was significant. It was found that, bacteria and predators are negatively correlated i.e. with increasing number of predator organisms the number of dispersed bacteria decreased. This effect of predation was confirmed while observing the floc size distribution with an automatic particle distribution measuring instrument simultaneously. It was observed that when the higher predator organisms (protozoa and metazoa) were high in number, the percentage of floc groups less than 10 μm (optimum prey size for most of the ciliate species) were less and vice versa. From this study it can be concluded that, mainly the smaller flocs which mostly consist of small (<1 μm) dispersed bacterial species, were grazed heavily by the predator population in the membrane-separation activated sludge.