The theoretical work presented here analyses various process chains for the energetic utilisation of municipal sewage sludge in their energy and greenhouse gas balance taking into account the hydrothermal carbonisation (HTC), based on the operating data of an HTC pilot plant. In the comparison with reference processes for sewage sludge dewatering (centrifuge, chamber filter press) the HTC with this offers energetic advantages with the treatment of digested sludge through high energy credit notes in the incineration and relatively small additional expenditure if the process can be operated via waste heat. For raw sludges without digestion the HTC offers no advantages as the energe tic advantage of the high calorific value are balanced out through additional outlays (natural gas, increased return loading). Decisive factors with the energetic evaluation of the HTC process are here the internal heat management and the biogas yield from the HTC process water. To be noted is, however, that the refractory COD in the process water can lead, via the return loading of the wastewater treatment plant, to considerably increased COD discharge values, which the introduction of an HTC in many cases would prevent. Along with the energy balance the HTC technology for sewage sludge should therefore be comprehensively evaluated in large-scale trials in order to investigate more accurately the economic efficiency and environmental relevance of the process.

The goal of this study is to analyze whether the integration of a Hydrothermal Carbonization (HTC) process into sewage sludge disposal routes improves the holistic energy balance compared to state of the art technologies. Furthermore the decisive parameters for the improvement are identi ed. For this a static model is set up within the energy and material flow calculation software Umberto. Within the selected treatment scenarios without and with anaerobic digestion the Cumulative Energy Demand (CED) and Global Warming Potential (GWP) are determined per functional unit disposal of one kg TSsludge. The model is fed with full-scale data from state of the art sludge treatment and data of a pilot HTC plant. It comprises all relevant processes including their chemicals and energy demands as well as transportation of materials. Expenditures for infrastructure are excluded. The reference input flow is based on the annual sludge amount of a waste water treatment plant for 500,000 people equivalents. The final disposal options of the sludge or hydrochar are either co-incineration within a lignite power plant or mono-incineration. Some co-products such as electricity, biomass fuel (dried sludge, hydrochar) and nitrogen fertilizer are created during sludge treatment and accounted for as substitutes for production of equivalent resources. HTC distinguishes from the conventional sludge treatment by improved mechanical dewaterability of the products. It reaches dry matter contents of ~ 65%. Trade-offs are the significant process heat demand of 88kWh/m3 sludge at high temperatures > 220 °C and a decreased mass yield of 72 % for the undigested and 75 % for the digested sludge. The dry matter loss results in process liquor with multiple load compared to raw sludge liquor (80 x org. C, 60 x Ntot, 25 x Ptot). The CED and GWP results generally show good correlation. For the CED of raw sludges the net values range from savings of -11.7 to expenditures of +1.8MJ/kg TS. The GWP ranges from -1.07 to +0.43 kg CO2-eq/kg TS. The net values for the HTC scenarios exceed the reference scenarios for undigested sludge when the dry matter content after sludge dewatering is < 27% or if it is ~ 27% and the process heat demand of the HTC can be reduced by half e.g. via insulation. However, the best scenario for undigested sludge includes HTC with a small scale digester only for the liquor. The loads are largely reduced, saving energy for the return ow treatment and producing biogas for use in a CHP plant. The heat can be fed to the HTC reactor while grid electricity is substituted. In disposal routes including sludge digestion the CED ranges from -11 to -1.1MJ/kg TS and the GWP ranges from -0.73 to +0.22 kg CO2-eq/kg TS. The scenarios with HTC exceed the reference scenarios irregardless of the TS after dewatering. The HTC liquor is returned to the digester, reducing the load and yielding extra biogas as mentioned above. Also, with sludge digestion the HTC process benefits from the larger amount of CHP heat. It is sufficient to cover the heat demand within the analyzed scenarios. The reference sludge treatment is based on representative full-scale data, but the pilot plant data of HTC showed inconsistencies. The data has to be validated in full scale. Furthermore, important aspects such as refractory COD within the hydrochar liquor, pollutants such as heavy metals, legal aspects of the hydrochar incineration, nutrient recovery and economic aspects have to be addressed in future studies.