Energy recovery from waste activated sludge in a circular economy approach Dr. Roy Posmanik Agricultural Research Organization (ARO), Volcani Center, Israel

Wastewater treatment plants (WWTPs) produce large amounts of sludge – a key parameter of the plant's carbon footprint and operating costs. Anaerobic digestion (AD) is frequently adopted to produce biogas that is utilized for heat and power, which helps offset the carbon footprint. However, waste activated sludge is generated continuously and must be removed at a cost, while ignoring its energy value. Hydrothermal carbonization (HTC) allows the degradation of waste activated sludge in an aqueous medium of subcritical water, while producing hydrochar – a carbon-rich solid material. To move towards sustainability in WWTPs, HTC can be integrated as a platform for on-site sludge treatment. In this talk we will cover the aspects of energy recovery from hydrothermal co-products (i.e., hydrochar and process water) for their use as a solid fuel and a feedstock for AD, respectively. Yields and quality of the hydrochar, produced at different operational conditions (i.e., time and temperature) will be demonstrated by its elemental composition, higher heating value and combustion behavior.  Digestibility of the hydrothermal process water will be illustrated by its biochemical methane potential. An overall energy balance suggests that mild reaction temperature can serve as an appropriate platform returning more than 70% of the energy from waste activated sludge, using the HTC-AD process integration. This process integration is expected to reduce: 1) the overall energy consumption; 2) the operational costs; and 3) the carbon footprint of WWTPs.