Differences in soluble COD and ammonium when applying inverted phase fermentation to primary, secondary and mixed sludge

Primary, secondary and mixed sludge were treated by inverted phase fermentation. This treatment results in solid–liquid separation of sludge after endogenous enzymatic hydrolysis (anaerobic conditions: 42°C, 48 hours). The soluble chemical oxygen demand (sCOD) was increased in the solid phase up to 1,800%, 21,300% and 260% in primary, secondary and mixed sludge, respectively. The corresponding increase in sCOD in the liquid phase accordingly reached values of up to 440%, 5,100% and 140%. Phase separation led to an enrichment of volatile solids in the solid phase (89–358% primary sludge, 80–102% secondary sludge and 29–133% mixed sludge). The NH₄⁺-N values increased notably after the endogenous enzymatic hydrolysis itself. To investigate the short-term evolution following the treatment, the variation in sCOD, NH₄⁺-N and solids was also monitored after keeping the hydrolysate at 37°C under anaerobic conditions for 24 hours. This stage showed no generalized pattern in terms of sCOD.