Investigation of beneficial use alternatives of products obtainedby hydrothermal carbonization of sewage sludge

The reuse of the products obtained from hydrothermal carbonization of biomass has recently become a field of study that has attracted attention. The fact that hydrothermal carbonization process does not require pre-drying of biomass indicates that it may be a suitable method for sewage sludge. In this study, the beneficial use alternatives of the products obtained after hydrothermal carbonization of municipal wastewater treatment plant sludges were evaluated in order to contribute to the management of sewage sludges. In the study, firstly, the optimization of the parameters affecting the hydrothermal carbonization process (sludge/water ratio, temperature, time) was carried out according to the BoxBehnken Statistical Design of Experiments Method for maximum thermal value and energy efficiency. Four different beneficial use alternatives (increasing methane formation efficiency in anaerobic digester, phosphorus recovery from hydrochar, use as adsorbent in heavy metal removal by adsorption and use as soil amendment) were evaluated for the obtained hydrocoals and the most suitable hydrochar properties for each application area were tried to be revealed. The upper heating values (HHV) were found to vary between 15.97-18.22 MJ/kg. The percentage increase in the heating value of the hydrocoals was found to be between 95.29% (1 h, 215°C, v/w: 0.1) and 108.71% (3 h, 250°C, v/w: 0.07). In adsorption studies, heavy metals were removed. According to the optimization results: zinc yield ≥99.51%, lead yield ≥99.98%, nickel yield ≥87.79%, iron yield ≥99.75%, copper yield ≥99.45%, cobalt yield ≥87.00%, cadmium yield ≥99.34%. The effects of temperature, time and Mg(OH)2 were analyzed in the phosphorus recovery study. Phosphorus was recovered between 23.96-97.58% in hydrocoal. In this study, time and Mg(OH)2 amount are the most important parameters. Significant results were also obtained above the methane formation efficiency in the anaerobic digester. Cumulative methane yields during the anaerobic digestion experiments were 15.75-221.67 %LEL CH4. In the study, high methane yields were observed in reactors with the addition of hydrochar liquid phase obtained at high temperatures and high times.