Production of volatile fatty acids through co -digestion of sewage sludge and external organic waste: Effect of substrate proportions and long- term operation

•Co-digestion of sewage sludge and organic waste for VFA production was studied.•Acetic acid dominated in short-term operation in batch mode.•An increasing proportion of organic waste increased valeric and caproic acid.•Caproic acid dominated during long-term operation in semi-continuous mode.•VFA-rich liquid achieved higher denitrification rate than acetate and methanol.