Steam-explosion pretreatment of municipal sludge to enhance anaerobic digestion

This thesis presents results for the steam-explosion pretreatment of sludge, which is a promising alternative for enhancing anaerobic digestion by solubilizing organics before digestion. The steam-explosion process consists of the application of high-pressure steam (150--600 psi) at high temperature (180--260°C) in a reaction vessel. The sludge is release through a small orifice causing an explosion that further aids the disintegration of the organics.; In this study, samples containing a mixture of thickened waste activated sludge (TWAS) and anaerobically digested sludge cake (biosolids) and samples of biosolids from the City of Ottawa wastewater treatment facility were treated at different steam-explosion conditions and anaerobically digested in batch and semi-continuous flow reactors at mesophilic temperature. The effect of pretreatment on the TWAS was estimated from the digestion of the TWAS/Biosolids mixture and the biosolids alone.; Results of this study show that the steam-explosion solubilizes the organics present in the sludges, which concomitantly enhanced the subsequent anaerobic digestion. For example, pretreatment at 300 psi of TWAS/biosolids mixture and biosolids alone increased the degree of solubilization (SCOD/TCOD) from 7 to 41% and by 3 to 35%, respectively. Batch digestion of the TWAS/biosolids mixture after treatment at 300 psi resulted in a 43% increase of biogas yield over the control and the biosolids treated at the same pressure resulted in a 69% increase biogas yield over the control. Digestion in semi-continuous reactors was very stable even at the relatively low SRT of 8-days, which indicated that the organics solubilized during pretreatment at 300 psi were easily converted to biogas during the digestion without signs of inhibition. Additionally, biogas production of samples pretreated at 300 psi in the semi-continuous reactors also showed an improvement over the controls for both the TWAS/biosolids mixture and the biosolids.