Improved Bioconversion of Volatile Fatty Acids from Waste Activated Sludge by Trace Element Requirement

Abstract

Anaerobic bacteria involved in the bioconversion of volatile fatty acids (VFAs) have inherent requirements for trace metals. It is inferred that waste activated sludge contains inadequate amounts of bioavailable trace metals for the high-rate generation of VFAs, resulting in the poor yield of VFAs without any other obvious reason. Therefore, minimum requirements for trace metals including Zn, Co, Cu, Fe, Mn and Mo in the generation of VFAs from waste activated sludge were investigated. The experimental results showed that all of the metal elements used in this study had the potential to improve the generation of VFAs despite variation in the yield of total VFAs. The improved influences were as follows: Fe > Co > Mn > Cu > Zn > Mo. When increasing the content of Fe from 0 to 0.10%, the yield of total VFAs rapidly raised from 0.204 g/g-VS to 0.318 g/g-VS. The maximum yield of total VFAs was achieved with Fe, followed by Co, and they increased 56% and 42% more than the control, respectively. Acetic acids were the most prevalent product no matter what metal element was. Its percentages in total VFAs were more than 50%, which were much higher than any other individual VFAs. Butyric acids were the second major individual VFAs with a percentage of more 13% accounting for total VFAs. Adding Co could obviously improve the generation of acetic acids, and the maximum yield of acetic acids reached 0.206 g/g-VS. Also, n-butyric acids were significantly produced by the addition of Fe, and its maximum value was 0.100 g/g-VS, that is to say 325.8% higher than the yield of the control (0.040 g/g-VS).