Effect of the addition of conditioning agents and of Inverted Phase Fermentation pretreatment on the biochemical methane potential of sludge

Abstract

Biodegradability tests were performed on sewage sludge to assess the effect of conditioning additives (FeCl3 and cationic polyelectrolytes) on the methane yield. In addition, enzymic hydrolysis pretreatment (42 ºC, 48 hours), called “Inverted Phase Fermentation” (IPF), was also applied to the sludge as a pretreatment and biodegradability tests were performed on the concentrated solid phase obtained. The specific methane potential (SMP) of untreated (raw) sludge was 246 LCH4/kgVSo. The addition of high amounts of FeCl3 (5.6 g/L sludge) caused a minor change in the SMP, yielding 242 LCH4/kgVSo. When a cationic polyelectrolyte was also added, increases of 11% and 25% in SMP were obtained depending on the flocculant. When applying IPF as pretreatment, the concentrated solid phase produced a 20% increase in SMP if the sludge did not contain FeCl3, whereas a decrease of 6.8% was obtained if the sludge contained this coagulant. As IPF concentrates solids in an upper thickened layer, the addition of a cationic polyelectrolyte to the sludge inhibited this pretreatment.