Influence of organic loading on the performance and microbial community structure of an aerobic stage reactor treating pharmaceutical wastewater

An unique up-flow anaerobic stage reactor (UASR) treating pharmaceutical wastewater at various organic loading rate (OLR) was investigated and showed efficient substrate removal at low OLRs (0.43–1.86 kg COD m-3 d-1) by promoting efficient chemical oxygen demand (COD) reduction (70–75%). However, increasing the OLRs to 3.73 kg COD m-3 d-1 by reducing the hydraulic retention time (HRT) (4–2 d) reduced the COD removal efficiency (45%). Fluorescent in situ hybridisation (FISH) analysis showed that the microbial community of the reactor stages was dominated by methanogenic archaea (56–79% of 4', 6 diamidino-2-phenylindole (DAPI)-stained cells in all stages) when the reactor was fed at OLR 0.86–2.98 kg COD m-3 d-1 but decreased in Stage 1 (30% of DAPI-stained cells) when the OLR was at 3.73 kg COD m-3 d-1. The two methanogenic genera detected were Methanosarcina and Methanosaeta. Generally, Methanosaeta dominated the reactor stages when the OLR was 0.86–1.86 kg COD m-3 d-1, while Methanosarcina dominated during the period of high OLR (2.98–3.73 kg COD m-3 d-1). Based on the number of Desulfovibrio cells detected (8–36% of the total eubacteria) in all stages during the entire operation of the reactor, it appears that sulfidogenic bacteria contributed to the overall degradation of organic substances in the pharmaceutical wastewater