High methanogenic activity of a three-stage UASB in relation to the granular sludge formation

A new UASB (upflow anaerobic sludge blanket) system with three-stage concept and granular sludge formation was developed to provide a very high process activity, in terms of a high energy yield and a large optimum COD loading rate, from ethanol wastewater at 37 °C and a controlled pH of 5.5 in the first bioreactor with recirculation of the final overflow from the third to the first two bioreactors to lower the amount of sodium hydroxide used for pH control in the first bioreactor and to increase the total alkalinity of the whole system. The proper volumetric ratio of the three-stage UASB with flocculant sludge provided the dominance of the three sequencial steps of acidogenesis, acetognesis and methanogenesis to occur in the respective bioreactors, causing a higher process performance and giving the optimum COD loading rate of 15 kg/m³d. The presence of granular sludge in all three bioreactors, resulted from the slow increment in the COD loading rate over a long operation period with high levels of microbial concentrations in all three bioreactors under the continuous feed operation and steady sate condition. This further increased the optimum COD loading rate from 15 to 25 kg/m³d (calculated from the total liquid volume) with an extremely high energy yield of 11,740 kJ/kg COD applied.