Supercritical gasification of wastewater treatment plant sludge for energy production

The number of wastewater treatment plants and accordingly the amunt of sewage sludge produced increased rapidly due to the rapid increase in population, industry and technology in the last decades. Sewage sludge has high water content such as 95% and organic matter content of around 70-80%. Disposal of sewage sludge have become a major problem worldwide for many reasons including the limited landfill area, increased environmental awareness, more stringent environmental standards managing the sludge disposal and dewatering challenges. High organic content of sewage sludge makes it an alternative biomass source that can be used for energy production. Thus, resarches on alternative methods for energy production from sewage sludge continues due to the increase in energy demand and the scarcity of available resources. Due to concerns arising from conventional treatment methods (accumulation of heavy metals in the soil, odor formation, leakage into the ground, etc.) as well as high organic matter and energy potential, interest in energy recovery from sewage sludge by supercritical gasification is increasing day by day. However, the high water content of the sludge is one of the biggest obstacles to the use as an alternative energy source. Supercritical water gasification is a new and promising technology which has been investigated for the energy recovery from wet biomass. It has been the subject of many researches for the evaluation of sewage sludge without any need for pre-treatment especially in recent years. The purpose of this study is to investigate the gasification of sewage sludge via supercritical water gasification, gas production with high H2 and CH4 content, and the feasibility of energy production from sewage sludge. Within the scope of this study, the effects of temperature, solids content, catalyst addition and raw material properties (basic components, protein and inorganic compounds) on the gasification of domestic/urban wastewater treatment plant sludge by SCWG process were determined by the pilot scale continuous-flow reactor. In this context, the gasification of the sewage sludge at supercritical conditions has been investigated for the first time in Turkey by continuous flow pilot scale reactor. As a result of the study, it was determined that the gasification efficiency and the H2 content of the gas increased by increasing the temperature and the addition of the catalyst, and decreased the content of H2 when the catalyst was not added by increasing the solids concentration. At 650 °C temperature, 2% solids concentration and 2% KOH catalyst addition, H2 yield has obtained at above 60%. The heating value of the gas produced under these conditions has determined to be 7,5 MJ/kg sewage sludge. High TOC removal efficiency (99%) has been obtained and all parameters except nitrogen in the resulting liquid products have been determined to provide discharge standards. According to the obtained results, it can be concluded that the supercritical gasification process is an efficient method that can be used for the energy recovery and safe disposal of sewage sludge. However, long-term field scale studies must be performed before the set-up of the industrial scale systems in order to improve system performance and economical disadvantages.