In this research, three SRTs (about 10, 30 and 75 days (without wasting the sludge except for sampling)) were applied to three reactors equipped with non-woven and coarse pore filter modules. The flux was adjusted to about 1 m/d during operation. The main objective of the study was to compare the performance and microbial population dynamics under different SRTs in this process. The results of reactors with SRTs of about 10 and 30 days have shown very good effluent quality without any clogging problem for more than 4 months of operation. For the reactor with long SRT (75 days), the filter clogging was observed after about 80 days of operation and caused an increase in the operation pressure and deterioration in effluent quality on some days. Excessive abundance of filamentous bacteria was observed in the reactor with SRT of about 10 days, which had the best effluent quality. According to the FISH results, type 021N was predominant in the reactor with long SRT, which had the clogging problem. On the other hand, other reactors (with SRTs of about 10 and 30 days) did not contain much type 021N, but some other filamentous bacteria dominated. Maximum EPS concentration (as mg/L) was observed in the reactor with long SRT. Also the abundance of two types of metazoa (Pristina sp. and tardigrades) was observed in the reactor with long SRT, which had the clogging problem and poor effluent quality.

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