Performance and sludge characteristics of UASB process treating propionate-rich wastewater

UASB process consistently removed 97–99% of chemical oxygen demand (COD) in wastewater containing concentrated propionate at 37°C in 12 h for loading rates up to 23 g-COD/1•day. Of all the COD removed, 95% was converted to methane and the rest was converted to biomass with an average sludge yield of 0.040 g-VSS/g-COD. Each gram of propionate-degrading granules in the reactor had a daily maximum capacity of converting 0.60 g of COD into methane. The granules were densely packed but did not exhibit any patterned microstructure. A typical propionate-degrading granule was composed of microcolonies of Methanothrix and several syntrophic microcolonies with hydrogen-producing acetogens in juxtaposition with hydrogen-consuming M. hungatei and Methanobrevibacter-like bacteria.