Substrate degradation of propionate-utilizing sludge

Abstract

A propionate-utilizing sludge was enriched in a chemostat reactor for over 90 days. Its methanogenic degradation characteristics were investigated in batch reactors using various substrates. The suspended-growth sludge degraded propionate at 0.65 mg-COD (mg-VSS d)-1, which is comparable to the rate reported in the literature for the granular sludge in a UASB reactor. The propionate-degrading rate was not affected by the presence of butyrate as a co-substrate initially; but it was lowered substantially as propionate concentration became below 100 mg l-1. The sludge was also able to degrade butyrate at 0.52 mg-COD (mg-VSS d)-1, which is about 30% of the rate reported for granular sludge in a UASB reactor treating butyrate as the sole substrate. The butyrate-degrading rate was lowered when propionate was present at high concentrations. In the interim of butyrate degradation, several higher-molecular weight acids were detected in the mixed liquor, including i-butyrate, i-valerate and caproate; valerate was, however, detected only in the batch when the P(H2) was as high as 10-1.8 atm. The presence of formate lowered the degrading rates of both propionate and butyrate.