UASB treatment of wastewater with concentrated mixed VFA

The upflow anaerobic-sludge blanket (UASB) process consistently removed 97-99% of chemical oxygen demand (COD) from wastewater containing concentrated mixed volatile fatty acids (VFA) at 37°C at loading rates of up to 24 g-COD/(L·d), corresponding to a food/microorganism ratio of 0.78 g-COD/[g-volatile suspended solids (VSS)·d]. It suggested that, with preacidification, the UASB process can be effective for a wide variety of wastewaters. The COD removal efficiency deteriorated at higher loading rates; there was no butyrate in the effluent, suggesting that butyrate degradation was not a rate-limiting step. Of the COD removed, 92.6% was converted to methane; the rest was converted to granular biomass with an average yield of 0.054 g-VSS/g-COD. The granules had a size of 1−2 mm and settled satisfactorily. Each gram of granule in the reactor was capable of converting a daily maximum of 0.86 g of COD into methane. The granules had a fluffy surface mostly composed of interwound filamentous Methanothrix-Iike bacteria. Syntrophic associations between Methanothrix-. Methanospirillum hungatei-, and Syntrophobacter-Iike bacteria were prevalent in the granule interior. The syntrophic relation between these species was elucidated by thermodynamics.