Hydrothermal processes applied to waste water treatment plants

Conventional Activated Sludge systems have been widely implemented to treat wastewater. CAS systems produce huge amounts of waste sludge and its subsequent treatment represents up to 50% of the operational costs of Waste Water Treatment Plants. The final disposal of sludge is usually performed by landfilling or incineration, involving severe environmental issues. In order to reduce sludge amount, many studies have been conducted, developing new technologies. One of these is Hydro Thermal Carbonisation (HTC), where sludge is heated up to 180-250 °C at water vapor pressure producing a solid product enriched in carbon for possible energy exploitation [1,2]. The aim of this work is to apply HTC to different kinds of sludge such as thickened sludge, digested sludge and dewatered sludge and compare the behavior of the solid and liquid phases produced by the process. For the purpose, experimental tests were performed at different operating conditions in a lab batch reactor capable to withstand high pressure (140 bar) and temperature (300 °C). In order to compare the HTC products of the different kinds of sludge, Higher Heating Value (HHV) and solid yield of hydrochar and some pollutants of the liquid phase were measured. HTC allows sludge reduction producing hydrochar and allows phosphorous transfer from the liquid phase to the solid phase. HHV of hydrochar can either slightly increase or decrease due to an increment in the ash content.