Impacts of HRT on UASB performance and propionate-oxidizing bacteria

Propionate-oxidizing bacteria community can oxidize propionate to acetic aid and H₂/CO₂ which can be used by methanogens, and they occupy a unique niche in anaerobic digestion systems. An understanding for the shift of propionate-oxidizing bacteria in anaerobic wastewater treatment system is of important significance for regulating and operating the process. Impacts of hydraulic retention time(HRT)on performance of an upflow anaerobic sludge bed reactor(UASB)and shift in propionate-oxidizing bacteria community were investigated at mesophilic condition. The results showed that shortening HRT within certain range of 24 h to 8 h, at an influent COD of 1000 mg·L^-1, would cause shocks to UASB, resulting in a transient accumulation of propionate and acetate, and after maintaining run for a certain period UASB could achieve new steady states again and keep higher removal efficiency of COD, which were 94.9%, 93.9%, 92.5%, 91.8% and 87.9%, at HRT 24, 20, 16, 12 and 8 h, respectively, during these steady states. An obvious change of propionate-oxidizing bacteria community in anaerobic activated sludge was observed when HRT decreased. Syntrophobacter pfennigi was appeared at HRT 24 h and disappeared completely at HRT 8 h, while S. sulfatireducens, Pelotomaculum and Desulfotomaculum were dominant bacteria at HRT 8 h. Smithella propionica maintained a relatively high richness in whole operational process, indicating its existent in a wide niche. The pollutant removal and running stability of the UASB should be improved by the enrichment of propionate-oxidizing bacteria.