Production of carboxylates from high rate activated sludge through fermentation

•pH of 7 and 35 °C were found as optimal parameters for A-sludge fermentation.•Fermentative inoculum presence enhances carboxylate production from A-sludge.•A-stage sludge produces more VFA compared to waste activated sludge values.•Iron content is directly proportional to CH4 but not to carboxylate production.

The aim of this work was to study the key parameters affecting fermentation of high rate activated A-sludge to carboxylates, including pH, temperature, inoculum, sludge composition and iron content. The maximum volatile fatty acids production was 141 mg C g−1 VSSfed, at pH 7. Subsequently the potential for carboxylate and methane production for A-sludge from four different plants at pH 7 and 35 °C were compared. Initial BOD of the sludge appeared to be key determining carboxylate yield from A-sludge. Whereas methanogenesis could be correlated linearly to the quantity of ferric used for coagulation, fermentation did not show a dependency on iron presence. This difference may enable a strategy whereby A-stage sludge is separated to achieve fermentation, and iron dosing for phosphate removal is only implemented at the B-stage.