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The primary objective of this experiment was to assess the impact of different retention times in a thermophilic reactor on hydrolysis and acidification of sludge. The secondary goal of this study was to provide a comparison between various anaerobic digestion processes and their effect on the stabilization of sludge and pathogen reduction. Three lab scale anaerobic digestion systems were assembled. The main system – two-phase anaerobic digestion (TPAD) – consisted of two reactors: a thermophilic reactor followed by a mesophilic reactor. Two other systems included single phase thermophilic digester (TAD) and single phase mesophilic digester (MAD). Hydrolysis and acidification rates in the first reactor of the TPAD were the highest at 1 d solids residence time (SRT) and the lowest at 5 d SRT. It was found that the two processes increased exponentially with the decrease of SRT. Hydrolysis was a faster occurring reaction than acidification. Therefore, at short retention times acidification could be the limiting step in the first stage reactor of the TPAD. Acidification index, AI, was used as an optimization parameter for the retention time in the first reactor in TPAD. No significant difference in VS destruction between digestion systems was observed. Increase in loading rates resulted in higher methane production but it also resulted in less methane produced per gram VS destroyed. The overall efficiency of the TPAD was offset by very low methane production in the first reactor and no significant difference in the overall methane production between digestion systems was observed. The single thermophilic reactor and the TPAD system achieved Class A biosolids standards with respect to fecal coliforms and spiked Ascaris suum eggs inactivation. The single MAD often failed to produce Class B biosolids using fecal coliforms as an indicator organism. It also showed very low level of destruction of Ascaris eggs. Clostridium perfringens destruction was very low for all systems: 1-log reduction was not achieved. Apparent regrowth of these organisms was observed in mesophilic conditions. There were no overt benefits of higher hydrolysis rates in the first reactor of the TPAD system in terms of higher VS destruction and/or higher methane production when compared to the single digesters. The two-stage anaerobic digestion system was the least sensitive to changes in loading rates and retention times and exhibited the least variability in capillary suction time (CST).

Document Type: Research Article


Publication date: January 1, 2003

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