Degradation of phenol in an upflow anaerobic sludge blanket reactor (UASB) at ambient temperature

Abstract

In this study, a synthetic wastewater containing phenol as sole substrate was treated in a 2.8 liter upflow anaerobic sludge blanket (UASB) reactor at ambient temperature. The operational conditions and phenol removal efficiency were discussed, microbial population in the UASB sludge was identified based on DNA cloning, and pathway of anaerobic phenol degradation was proposed. Phenol in wastewater was degraded in an UASB reactor at loading rate up to 18 g COD/l.d, With a 1:1 recycle ratio, at 26±1°C, pH 7.0-7.5. An UASB reactor was able to remove 99% of phenol up to 1226 mg/l in wastewater with 24 hours of hydraulic retention time (HRT). For HRT below 24 hours, phenol degradation efficiency decreased with HRT, from 95.4% at 16 hours to 93.8% at 12 hours. It further deteriorated to 88.5% when HRT reached 8 hours. When the concentration of influent phenol of the reactor was 1260 mg/l (corresponding COD 3000 mg/l), with the HRT decreasing (from 40 hours to 4 hours, corresponding COD loading increasing), the biomass yields tended to increase from 0.265 to 3.08 g/d.l. While at 12 hours of HRT, the biomass yield was lower. When HRT was 12 hours, the methane yield was 0.308 l/(g.COD removed)and it was the highest. Throughout the study, phenol was the sole organic substrate. The effluent contained only residual phenol without any detectable intermediates, such as benzoate, 4-hydrobenzoate or volatile fatty acids (VFAs). Based on DNA cloning analysis, the sludge was composed of five groups of microorganisms. Desulfotomaculum and Clostridium were likely responsible for the conversion of phenol to benzoate, which was further degraded by Syntrophus to acetate and H2/CO2. Methanogens lastly converted acetate and H2/CO2 to methane. The role of epsilon-Proteobacteria was, however, unsure.