Insitu biofilm, incubation slide and freezing techniques were used to investigate the diversity of protozoan and metazoan populations inhabiting rotating biological contactor (RBC) biofilms in relationship to effluent quality during different seasons of the year at two treatment plants. The insitu biofilm techniques yielded the best results primarily because samples contained higher numbers of individuals per species and were easier to obtain. A protocol based on 2 microscopic scans (200x magnification) across a total width of a covership on each of 3 slides is proposed as a cost-effective procedure for discriminating among three classifications of effluent quality (Good: BOD5 ≤ 25 mg/l, Fair: BOD5 - 26 to 40 mg/l Poor: BOD5>40 mg/l) based on the populations of organisms inhabiting RBC biofilms.

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