energies Article Energy Recovery from Sewage Sludge: The Case Study of Croatia Dinko Đurđevi´c 1,* , Paolo Blecich 2 and Željko Juri´c 1 1 Energy Institute Hrvoje Požar, 10000 Zagreb, Croatia;
[email protected] 2 Faculty of Engineering, University of Rijeka, 51000 Rijeka, Croatia;
[email protected] * Correspondence:
[email protected] Received: 26 April 2019; Accepted: 16 May 2019; Published: 20 May 2019 Abstract: Croatia produced 21,366 tonnes of dry matter (DM) sewage sludge (SS) in 2016, a quantity expected to surpass 100,000 tonnes DM by 2024. Annual production rates for future wastewater treatment plants (WWTP) in Croatia are estimated at 5.8–7.3 Nm3/people equivalent (PE) for biogas and 20–25 kgDM/PE of sewage sludge. Biogas can be converted into 12–16 kWhel/PE of electricity and 19–24 kWhth/PE of heat, which is sufficient for 30–40% of electrical and 80–100% of thermal autonomy. The WWTP autonomy can be increased using energy recovery from sewage sludge incineration by 60% for electricity and 100% of thermal energy (10–13 kWhel/PE and 30–38 kWhth/PE). However, energy for sewage sludge drying exceeds energy recovery, unless solar drying is performed. 2 The annual solar drying potential is estimated between 450–750 kgDM/m of solar drying surface. The lower heating value of dried sewage sludge is 2–3 kWh/kgDM and this energy can be used for assisting sludge drying or for energy generation and supply to WWTPs. Sewage sludge can be considered a renewable energy source and its incineration generates substantially lower greenhouse gases emissions than energy generation from fossil fuels.