Lab-scale digesters were used to perform side by side evaluations of alternative digestion systems including TPAD, Acid/Methane Phased digestion, Mesophilic Digestion, and the performance of the thermophilic stage of TPAD alone. Total detention times in the systems evaluated varied
from 8 to 20 days. Temperatures in the mesophilic stage of TPAD were varied from 35 to 43.3° C, and temperatures in the thermophilic stage were varied from 55 to 58.9° C. The emphasis of the study was to evaluate volatile solids (VS) reduction and thickening properties of the different
systems. Parameters evaluated included solids, volatile acids, gas production and quality, various chemical constituents of the sludge, and thickening properties using polymer and ferric chloride plus polymer. The TPAD systems were found capable of higher VS destruction at total detention
times of 10 and 12.5 days than the other systems at 15 to 20 days. A lab scale thickening test was performed to evaluate thickening properties. Based on obtaining 95% capture in the test, the polymer demand of mesophilic and acid/methane phased digestion was lowest. The TPAD system
required approximately 180% the dosage of the mesophilic sludge, and thermophilic sludge approximately 310% of the dosage. Temperatures of 40.5 and 43.3° C in the mesophilic stage of TPAD were found to increase the polymer requirement and decrease the VS reduction of the
TPAD system. Higher temperatures (above 56.1° C) and lower detention times (<15 days) in the thermophilic digesters seemed to result in problems with degradation of volatile acids, particularly proprionic acid.
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