Substrate degradation of propionate-utilizing sludge

A propionate-utilizing sludge was enriched in a chemostat reactor forever 9 0 days. Its methanogenic degradation characteristics were investigated in b atch reactors using various substrates. The suspended-growth sludge degrade d 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 con centration became below 100 mg l(-1). The sludge was also able to degrade b utyrate at 0.52 mg-COD (mg-VSS d)(-1), which Is about 30% of the rate repor ted for granular sludge in a UASB reactor treating butyrate as the sole sub strate. The butyrate-degrading rate was lowered when propionate was present at high concentrations. In the interim of butyrate degradation, several hi gher-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 format e lowered the degrading rates of both propionate and butyrate.