The effects of change in volatile fatty acid (VFA) composition on methanogenic upflow filter reactor (UFAF) performance.

The effects of changes in composition of a volatile fatty acid mixture (namely acetic, propionic and butyric acids) fed to a methanogenic upflow filter reactor (UFAF) were investigated in terms of reactor chemical oxygen demand (COD) removal efficiency and biogas production level. A mixed substrate consisting of differing proportions of VFAs (on a COD basis) was fed to the reactor in semi-continuous mode under constant organic loading rate (OLR) and hydraulic retention time (HRT). At an OLR of 1.2 kg COD m(-3) x d and HRT of 4 days, increasing propionic acid concentrations of about 312 mg l(-1), 617 mg l(-1), 951 mg l(-1) and 1242 mg l(-1) fed to the reactor provided COD removal rates and biogas production levels of, 84.70%-1301 ml d(-1), 80.80%-1214 ml d(-1), 73.65%-1071 ml d(-1) and 65.53%-942 ml d(-1), respectively, indicating inhibitory effects of propionic acid on the methanogenic ecosystem. Subsequent introduction of butyric acid (247 mg l(-1)) in addition to propionic acid (891 mg l(-1)) increased the performance of the UFAF, yielding 76.10% COD removal and 1075 ml d(-1) of biogas production. The instability observed in the system towards the end of the study probably originated from operating the reactor at relatively high VFA concentrations for a long period of time. Change in VFA composition did not seem to have significant effects on the effluent MLVSS/MLSS ratio. The upflow anaerobic filter reactor adapted to variations in feed composition in a short period of time.