Effects of the reactor volumetric ratio and recycle ratio on the methane and energy productivity of a three-step anaerobic sequencing batch reactor (3S-ASBR) treating ethanol wastewater

Abstract

A new three-step anaerobic sequencing batch reactor (3S-ASBR) was developed to enhance methane productivity and energy yield by varying the reactor volumetric and recycle ratios. The pH of the first reactor (R1) was maintained at 5.5, whereas that of the others (R2/R3) was not controlled. The effluent from R3 was recirculated to R1 to amplify the alkalinity and lessen the NaOH quantity for pH regulation. The 3S-ASBR was continually operated to attain a steady state at a moderate COD loading rate (10 kg/m3d) with different reactor volumetric ratios under 37 °C, a fixed total liquid volume of 30 L and a constant recycle ratio of unity. A R1:R2:R3 volumetric ratio of 1:1.5:5 gave the highest COD reduction (90%) and yielded 343 mL CH4/g COD applied with an overall energy extraction efficiency of up to 66%. This volumetric ratio corresponded well to the generation time ratio of the microbes involved in the hydrolysis/acidogenesis, acetogenesis, and methanogenesis steps which can be applied to scale up both 3S-ASBR and three-step/stage upflow anaerobic sludge blanket (3S-UASB) systems. The low pH level (5.5) in R1 was responsible for maintaining adequate amounts of all micronutrients in the system, causing both high microbial concentrations and activities. This was because the lower the solution pH, the higher the fraction of produced H2S to be in the gaseous phase, causing the lower the amounts of micronutrients to be precipitated chemically.