The Effects of Change in Volatile Fatty Acid (VFA) Composition on Methanogenic Upflow Filter Reactor (UFAF) Performance

Abstract

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.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.

Keywords:

• Two-phase anaerobic digestion
• upflow anaerobic filter reactor
• methanogenesis
• volatile fatty acids