Mesophilic aerobic digestion is an autoheating digestion process that includes sludge thickening followed by two or three stages of treatment in aerobic reactors. To achieve the balance of heat in the process, sludge has to be pre-thickened to 4 to 5% solids. In the first stage of digestion, the detention time is from 8 to 16 days, the dissolved oxygen is kept between 0.2 and 1.0 mg/L, and the temperature reaches 20 to 35°C. In this stage, ammonium bicarbonate alkalinity keeps the pH between 6.8 and 9.0. In the second stage, the detention time is from 10 to 17 days, the dissolved oxygen is kept between 0.2 and 1.0 mg/L, the temperature is kept between 15 and 30°C, and the pH is 6.5 to 7.5. A third stage may also be added with a detention time of 10 to 17 days. Detention time in all stages can be reduced by attaining temperature in the range 30 to 35°C in the first stage. Mesophilic aerobic digestion can qualify as a process to significantly reduce pathogens (PSRP); however, compared to conventional aerobic digestion, use of this process is limited, due to high capital and operating costs.
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