Effects of free nitrous acid and freezing co-pretreatment on sludge short-chain fatty acids production and dewaterability.

Short-chain fatty acids (SCFAs) as recoverable carbon source from waste activated sludge anaerobic fermentation process have aroused wide concern. In this study, free nitrous acid (FNA) and freezing pretreatments were combined to enhance SCFAs yield and fermented sludge dewaterability in the anaerobic fermentation system. The effects of different FNA concentrations and different freezing conditions (with or without curing stages) were analysed and compared. The results indicated that combining 1.07 mg N/L FNA with 48 h continuous freezing at -5 °C, raised SCFAs production from 6.7 mg COD/g volatile suspended solids (VSS) for the blank (no pretreatment) up to 124.0 mg COD/g VSS. In addition, the minimal water content of the treated fermented sludge cake was 78.11%, which was less than that of the blank (81.22%). SCFAs production and dewaterability enhancement could be attributed to sludge disintegration induced by the co-pretreatment, which led to sludge solubilisation, organics release, methanogenesis inhibition and particle size variation. This study implied that FNA and freezing co-pretreatment has the potential to enhance SCFAs production and sludge dewaterability in wastewater treatment plants.