. No significant VFA accumulation existed after 40 days, where the concentration of total VFA decreased from 4.98 to 1.45 g/L in digester PPW-FR-1, while digester PPW-1 had a slight increase from 1.86 to 2.54 g/L. The highest VFA concentrations were observed at 14.95 and 10.84 g/L on day 20 and 16, respectively for digesters PPW-1 and PPW-FR-1. The produced acetic acid was consumed quickly, while other VFAs were converted slowly and exhibited a slight substrate inhibitory effect