Energy Recovery from Sewage Sludge: The Case Study of Croatia

• Main Authors: Dinko Đurđević, Paolo Blecich, Željko Jurić
• Format: Article
• Language: English
• Published: MDPI AG 2019-05-01
• Series: Energies
• Subjects: wastewater treatment; sewage sludge; solar drying; energy recovery; incineration
• Online Access: https://www.mdpi.com/1996-1073/12/10/1927

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 kg_DM/PE of sewage sludge. Biogas can be converted into 12−16 kWh_el/PE of electricity and 19−24 kWh_th/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 kWh_el/PE and 30−38 kWh_th/PE). However, energy for sewage sludge drying exceeds energy recovery, unless solar drying is performed. The annual solar drying potential is estimated between 450−750 kg_DM/m² of solar drying surface. The lower heating value of dried sewage sludge is 2−3 kWh/kg_DM 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. For the same amount of energy, sewage sludge emits 58% fewer emissions than natural gas and 80% less than hard coal and fuel oil. Moreover, this paper analysed the feasibility of sludge disposal practices by analysing three scenarios (landfilling, co-incineration, and mono-incineration). The analysis revealed that the most cost-effective sewage sludge disposal method is landfilling for 60% and co-incineration for 40% of the observed WWTPs in Croatia. The lowest CO₂ emissions are obtained with landfilling and mono-incineration in 53% and 38% of the cases, respectively.