In order to determine the potential indicator value of protozoa and metazoa for control of sequencing batch reactors, mixed liquor solids were examined microscopically for 200 complete cycles from start-up; a total of 100 days. Over the first 40 cycles a healthy, diverse, protozoal population was established which comprised free-swimming, crawling and attached ciliates. During this period the reactor demonstrated efficient removal of COD and suspended solids but did not nitrifiy. With the onset of complete nitrification there was a drastic change in microfauna which became dominated by flagellated protozoa. At around the 80th cycle rotifers appeared and at cycle 100 attached ciliates became the dominant group. The protozoal composition of the mixed liquor responded to reactor shocks such as pH variations and was shown to be a good indicator of the nitrification efficiency of the reactor.

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