Effect of volatile fatty acid composition on upflow anaerobic sludge blanket (UASB) performance

The effects of organic loading rates (OLRs) and volatile fatty acid (VFA) composition on upflow anaerobic sludge blanket (UASB) reactors were investigated. Four 2.5 L UASB reactors were fed by mixed VFA of 20 g of chemical oxygen demand (COD) L-1 initially and subsequently fed with different VFA compositions. In the initial stage of the experiment where the reactors were operated at an OLR between 0.82 and 10.4 g of COD L-1 day(-1) with the substrate composed of a mixed acetate/propionate/butyrate composition, the COD removal efficiencies were in the range of 83 and 99% and the calculated methane yield ranged between 112 L of CH4 and 322 L of CH4 per g of COD removed. The result indicated that a higher methane yield corresponded with the increase of the OLR in the UASB treatment of the mixed VFA substrate. Mixed VFA-fed sludge showed a rather high activity in pure acetate and butyrate than pure propionate in a specific methanogenic activity (SMA) test. As the substrate composition changed from the mixed VFA (32:28:40 acetate/propionate/butyrate) to pure acetate, propionate, or butyrate at the OLR of 10.4 g of COD L-1 day(-1), respectively, in the UASB reactors, the acetate- and butyrate-fed reactors showed higher gas production and COD removal than the pure propionate-fed reactor as expected. The result reviewed that the composition of VFAs has a significant influence on the reactor performance. The propionate substrate exhibited severe inhibition, leading to system failure at a feed concentration of 20 g of COD L-1, while the butyrate substrate can be efficiently degraded compared to the acetate substrate because of its high energy gain via degradation.