Enhancing the production of butyric acid from sludge fermentation with an emphasis on zinc, cobalt, cuprum, ferrum and manganese

Abstract

Anaerobic microorganisms involved in butyric acid production have inherent requirements for trace metals. The inefficiency of trace metals in an acidogenic reactor can result in the poor yield of butyric acid. However, trace metal requirements for butyric acid production are not often reported in literature. Therefore, in order to enhance butyric acid production, a novel technique was adopted in this paper, i.e., trace metals including zinc (Zn), cuprum (Cu), cobalt (Co), manganese (Mn) and ferrum (Fe) were added into the fermentation substrate to favor the butyric acid-producing bacteria. Firstly, the effect of each trace metal on butyric acid production was investigated, respectively. And then, response surface methodology combined with Box–Behnken design was used to optimize the mixing conditions of the five trace metals. The results showed that each trace metal had the potential to enhance butyric acid production, and the influence was as follows: Fe > Mn > Co ≈ Zn > Cu. The optimal mixing conditions of these five trace metals were found to be 0.112 % Fe, 0.005 % Co, 0.0055 % Zn, 0.0042 % Cu and 0.112 % Mn, under which, the concentration of butyric acid reached 5102 mg L−1. The maximum yield of butyric acid arrived at 310 mg g−1 VS added, and its percentage in the total volatile fatty acids was 72.1 %. The fitting result came up to 99.6 % between the model prediction and the experiment value, which indicated that the established polynomial regression model was feasible. Protein was the main contributor for the butyric acid production. The terminal restriction fragment length polymorphism analysis combining with sequencing showed that the addition of Fe, Co, Cu, Zn and Mn with the optimal contents could induce the growth of Clostridium as the dominant bacterial genus in the process of butyric acid production.