Hydrothermal carbonisation of sewage sludge and resulting biofuels as a sustainable energy source

Sewage sludge is nowadays a permanent element of our lives. One promising idea is to turn these wastes into fuels. Special scientific attention is given to the possibility of applying the low temperature thermochemical method commonly referred to as hydrothermal carbonisation (HTC) in the context of sewage sludge (SS) utilisation. While many scientist suggest the use of HTC products in agriculture, this review focuses on the energetic use of HTC products as solid fuels and co-substrates in the anaerobic digestion (AD) process of a biogas plant. HTC is a very promising technology, however, a potential barrier to its implementation may be the lack of authoritative data on long-term economic and environmental benefits and the legal restrictions associated with the further use of the products. In addition, incomplete knowledge in the field of subcritical water effects on the complex and irregular structure of SS may not provide a guarantee for the appropriate application of this method. This article attempts to systematise the information, insights and research deficiencies related to the HTC process of various forms of sludge towards the energetic possibilities of using the products obtained.