Impact of HRT on propionate removal and response of propionate-oxidizing bacteria in an UASB reactor

To investigate the effect of HRT on propionate degradation and propionate-oxidizing bacteria, an upflow anaerobic sludge blanket (UASB) containing propionate as sole carbon source was introduced. The propionate oxidation with decrease of HRT was evaluated and propionate-oxidizing bacteria shift was analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). Under influent propionate 2000 mg·L^-1 condition, the shortening of HRT from 10 h to 8 h, 6 h, and 4 h would cause a transient accumulate in propionate and could achieve new steady states with high propionate removal after a certain period. When the HRT was gradually decreased from 10 h to 4 h, the specific COD_proremoval of anaerobic sludge in the UASB system was increased to 1.7 kg/(kg·d) from 1.2 kg/(kg·d). The decrease of HRT made the band 2-4 (Syntrophobacter fumaroxidans), propionate-oxidizing bacteria enriched gradually in UASB system, and the band 5 (S. sulfatireducens) reduced, while band 1 and band 6 (Pelotomaculum propionicicum) maintained a relatively stable level. The enrichment of band 2-4 with high specific degradation rate resulted in the propionate removal to 94.1% under organic loading rate of 18 kg/(m³·d).