Abstract

The main purpose of this research was to clarify the influence of the addition of iron (Fe) alone (0–100 mg/L) or 50 mg/L of Fe with 2 mg/L each of cobalt (Co), copper (Cu) and nickel (Ni) on the methanogenic activity of a mesophilic two-stage UASB system treating ethanol wastewater at a fixed chemical oxygen demand (COD) loading rate of 16 kg/m³/day under a continuous mode of operation and steady state condition. The addition of Fe provided the dual benefits of a reduction in both the dissolved sulfide and the hydrogen sulfide (H<inf>2</inf>S) content in produced gas, resulting in marginally improved hydrogen (H<inf>2</inf>) and methane (CH<inf>4</inf>) productivities. When the Fe dosage was increased beyond the optimum value of 50 mg/L, the process performance drastically declined, as a consequence of the high total volatile fatty acid (VFA) concentrations that inhibited both the acidogens and methanogens predominantly present in the 1^st and 2^nd reactors, respectively. The chemical precipitation of iron sulfide was responsible for the reduction of produced H<inf>2</inf>S in both the aqueous and gaseous phases as well as the minimization of added amounts of all other micronutrients to fulfil the sufficiency of all micronutrients for anaerobic digestion (AD). The addition of 2 mg/L each of Co, Cu and Ni together with 50 mg/L Fe resulted in the greatest enhancement in process performance, as indicated by the improved CH<inf>4</inf> yield (mL/g COD applied) to about 42.3%, compared to that without micronutrient supplement.