The study examines the constraints on thermal pretreatment of sludge at temperatures in excess of 100 °C and its effects on subsequent anaerobic digestion. The table below summarizes the maximum measured increase rates and the corresponding optimum pretreatment temperatures as a function of the type of sludge tested. The influence of the sludge type is considerable.

Sludge TypeGas Yield without/with PretreatmentMaximum increase RateOptimum Pretreatment Temperature
l/kg VSinfl%°C
Digested Sludge 60.4 223.0 + 269.2 180 
Excess Sludge SLR = 0.03 124.7 278.1 + 123.0 135 
Excess Sludge SLR = 0.15 314.7 546.7 + 73.7 170 
Excess Sludge SLR = 2.00 234.7 334.2 + 42.4 135 
Primary Sludge 259.0 326.6 + 20.3 170 
Excess Sludge SLR = 0.60 596.8 600.0 + 0.5 135 
Sludge TypeGas Yield without/with PretreatmentMaximum increase RateOptimum Pretreatment Temperature
l/kg VSinfl%°C
Digested Sludge 60.4 223.0 + 269.2 180 
Excess Sludge SLR = 0.03 124.7 278.1 + 123.0 135 
Excess Sludge SLR = 0.15 314.7 546.7 + 73.7 170 
Excess Sludge SLR = 2.00 234.7 334.2 + 42.4 135 
Primary Sludge 259.0 326.6 + 20.3 170 
Excess Sludge SLR = 0.60 596.8 600.0 + 0.5 135 

Calculation of detailed energy balances for various sludge treatment systems with and without thermal pretreatment revealed a clear economic advantage for thermal pretreatment. The excess heat for an overall system composed of thermal pretreatment, anaerobic stabilisation and digester-gas electricity generation is significantly higher than that for a system without pretreatment. Sludge pasteurisation represents an additional advantage of thermal pretreatment.

In general, it may be stated that, provided certain conditions are satisfied, thermal pretreatment of sewage sludge prior to anaerobic stabilisation can positively affect the energy balance of the treatment plant.

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