Hydrothermal Carbonization as an Energy-Efficient Alternative to Established Drying Technologies for Sewage Sludge: A Feasibility Study on a Laboratory Scale

Hydrothermal carbonization (HTC) of stabilized and non-stabilized sewage sludge was investigated in a 25 L vessel as a pretreatment for sewage sludge before incineration, and the composition and properties of the obtained HTC coal and process water were studied. The observed values for H/C and O/C in HTC coal from stabilized and non-stabilized sewage sludge were shown to be higher than in natural coal and rather close to typical values for cellulose. The upper heating value of the stabilized sewage sludge was increased from 3.4 to 6.5%, and the upper heating value of the non-stabilized sludge was increased from 5.8 to 11.0%, after carbonization. The carbon efficiency ranged from 62 to 71% for stabilized sewage sludge and from 60 to 66% for non-stabilized sewage sludge, and the dry matter (DM) loss after carbonization was 31 and 42% for stabilized sludge and 34 and 44% for non-stabilized sludge. After carbonization, the mechanical dewaterability was increased from 30 to 70% DM content for non-stabilized sludge and from 37 to 52% for stabilized sludge. The drying process of sewage sludge including HTC needs a clearly lower energy input than established drying techniques to produce a fuel. For the drying process of 1 ton of non-stabilized sewage sludge with 9% DM, the calculated energy consumption was lowered by 99.6 kWh of thermal energy and 8.5 kWh of electric energy by introducing HTC. The results of these experiments show the feasibility of the HTC process as part of the drying process of sewage sludge and the fate of key elements in the process on a laboratory scale. However, the process has to be further optimized and developed on an industrial scale. Further important development steps include recovery steps for the carbon in the process water and adapted process water treatments.