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THERMOPHILIC-MESOPHILIC DIGESTION: OCCURRENCE AND PREVENTION OF HIGH VFAS

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A temperature-phased anaerobic digester (TPAD) and batch, staged-temperature anaerobic digester (B-STAD) were maintained in parallel with a conventional mesophilic control to compare VS destruction, VFA concentrations, and biomass activity. The TPAD system consisted of sequential 8-L thermophilic (55°C) and 16-L mesophilic (35°C) digesters. The B-STAD was similar, except that 0.4 L/day of mesophilic biomass was recycled to the thermophilic digester. The mesophilic control digester was a 24-L, single-stage unit. All systems were fed 1.6 L/d of municipal wastewater sludge (70/30% v/v blend of primary and thickened waste activated sludge), and had total system SRT values of 15 days. Daily batch feeding was employed to preclude short-circuiting that reduces fecal coliform inactivation. The VS destruction values of the systems were not statistically different, and averaged 58%. The TPAD and B-STAD achieved higher fecal coliform inactivation than the control. The TPAD system required nutrient addition to increase the pH in the thermophilic digester and reduce final VFA concentrations which may have initially been high due to toxicity from long-chain organic acids produced in the thermophilic phase. Recycling of mesophilic biomass to the thermophilic phase, as performed in the B-STAD configuration, and trace nutrient supplementation are described as potential methods of maintaining neutral thermophilic pH values and reducing VFA concentrations in batch fed systems. Alkalinity addition to the TPAD system also maintained neutral pH values in the thermophilic phase, but did not reduce final VFA concentrations.
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Document Type: Research Article

Publication date: 2004-01-01

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