Laboratory‐scale digesters were used to perform side‐byside evaluations of alternative digestion systems including thermophilic– mesophilic‐phased digestion (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 purpose 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 compared to single‐stage mesophilic or thermophilic digestion at 15 to 20 days. A laboratory‐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 the lowest. The TPAD system required approximately 180% of the dosage of the mesophilic sludge and thermophilic sludge required approximately 310% of the dosage. Temperatures of 40.5 and 43.3 8 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 (less than 15 days) in the thermophilic digesters seemed to result in problems with degradation of volatile acids, particularly propionic acid. Water Environ. Res., 76, 360 (2004).

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