Influence of initial pH on the production of volatile fatty acids and hydrogen during dark fermentation of kitchen waste

The purpose of this work was to determine the effect of initial pH on the production of volatile fatty acids (VFA) and hydrogen (H₂) in the dark fermentation processes of kitchen waste. The study was conducted in batch bioreactors of working volume 1 L for different initial pH in the range from 5.5 to 9.0. The dark fermentation processes were carried out for 4 days at 37°C. Initial organic load of the kitchen waste in all bioreactors amounted to 25.5 gVS/L. Buffering of pH during the fermentation process was carried out with the use of ammonia contained mainly in digested sludge. The optimal conditions for the production of VFA and H₂ were achieved at the initial pH of 8. Production of VFA and H₂ in these conditions was, respectively, 13.9 g/L and 72.4 mL/gVS. The main produced components of VFA were acetic and butyric acids. The production of ethanol and lactic acid was at very low levels due to the high ratio of the volatile fatty acids to total organic content of 0.86. With the optimal initial pH of 8 the yield of CO₂ production was 0.30 gC/gC. High initial pH value (above 8) extended the lag phase duration in the course of H₂ production. The dominant groups of micro-organisms at the most favourable initial pH of 8 for the production of VFA and H₂ were Bacteroidetes, Firmicutes, Spirochaetes and Waste Water of Evry 1 (WWE1) at the phylum level.