Bioindicator value of flagellates in urban wastewater treatment using membrane bioreactors

Two experimental submerged membrane bioreactors (MBR) at the industrial scale with different membranes (microfiltration and ultrafiltration) were used to analyse the influence of the small flagellates on the lack of significance of the sludge biotic index (SBI). Also, with these systems, flagellates were analysed as bioindicators in the activated sludge of MBRs. Both facilities were operated in parallel with a pre-denitrification system for a period of nearly two years and fed with real pretreated urban wastewater. During the experimental period, the sludge-retention time in the facilities was from 16.5 to 36.5 days, the organic loading ranged between 1.8 Kg COD/d m³ and 0.14 Kg COD/d m³, and the temperature of the activated sludge was between 32.8 and 12.8 °C. Under these conditions, the concentrations of total solids in the activated sludge reached 21.6 and 0.3 g/L, and the ratio of food:microrganims (F:M) remained stable at around 0.1 Kg BOD₅/Kg VSS day, while the trans-membrane pressure (TMP) followed an increasing trend. Both facilities presented a high capacity for organic matter elimination and complete nitrification. The small flagellates were present throughout the experimental period in the activated sludge and showed no significant bioindicator role in the MBR systems which limited the application of SBI. Of the large flagellates studied, only Peranema sp. could be considered to be a bioindicator of MBR systems, indicating the effectiveness of denitrification for systems with complete nitrification and correlating with the time course of the TMP.