Process Improvement of a Two-Stage Upflow Anaerobic Sludge Blanket System by Micronutrient Supplement in Relation to Sulfur Transport

Abstract

The main purpose of this research was to elucidate the influence of the addition of iron alone (0–100 ppm) or with 2 ppm each of cobalt, copper, and nickel on the performance of a two-stage UASB system treating ethanol wastewater. The system consisted of two reactors connected in series. The system was maintained at a fixed chemical oxygen demand loading rate (16 kg/m3d) and 37 °C with a recycle ratio of 1:1. Both Co and Ni were found to be deficient in the ethanol wastewater, while Fe and Cu became inadequate in two-stage UASB. The addition of Fe provided dual benefits of a reduction in both sulfide toxicity and hydrogen sulfide content in produced gas, resulting in marginally improved hydrogen and methane productivities. When the Fe dosage was increased beyond the optimum value of 50 ppm, the process performance drastically declined, as a result of high total volatile fatty acid concentrations that inhibited acidogens and methanogens predominantly present in the 1st and 2nd reactors, respectively. The chemical precipitation of iron sulfide was responsible for the reduction of produced H2S in both aqueous and gaseous phases. The addition of 2 ppm each of Co, Cu, and Ni together with 50 ppm Fe resulted in an enhanced process performance, as indicated by the improved CH4 yield (mL/g COD applied) to about 42.3% compared to that without micronutrient supplement. Supplementation to fulfill the sufficiency of all micronutrients was vital for the maximization of anaerobic digestion (AD) performance, as generally indicated in methanogenic activity.