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.

The management and disposal of sewage sludge is becoming a growing concern at the global level. In the past, the main goal was to completely eliminate sewage sludge since it was deemed a threat to humans and the environment, but recently different possibilities for energy generation and material recovery are emerging. Existing technologies such as incineration or direct application in agriculture contribute to quantity reduction and nutrient recovery but are unable to fully exploit the potential of sewage sludge within the frameworks of circular economy and bioeconomy. This paper developed a model within the PROMETHEE method, which analyses technologies for the sustainable management of sewage sludge, which could make the most from it. For the empirical part of the study, the Republic of Croatia was used as a country in which sewage sludge is increasing in quantity as a result of recent upgrades and expansions in the wastewater system. Incineration, gasification, anaerobic digestion, and nutrient recovery were analyzed as treatment concepts for the increased amounts of sewage sludge. The model reveals that the best solution is the material recovery of sewage sludge, using the struvite production pathway through analysis of selected criteria.

Sewage sludge management has gained significance in the last several years, due to its nutrient and energy content. However, technology selection is one of the greater challenges because it is not possible to implement a technology that covers all the requirements of the considered environments. Consequently, this paper shows an example of the utilization of an analytical hierarchy process, as a decision-making tool in terms of technology selection, for sewage sludge management in Rijeka, Croatia. The criteria structuring and evaluation process with the description of several possible alternatives for thermal treatment technologies are defined within this research. For the case of Rijeka, the best and most suitable technology for sewage sludge treatment is gasification, which coincides with the results obtained from the analysis of the literature review. According to the results in this paper, the possibilities of the use of this scientific method on the national level for the selection of sewage sludge treatment technology should be considered, due to the simplicity of its use and capability of its adaptation to various situations and areas.

Agriculture is one of the leading sectors on the global level contributing to greenhouse gas (GHG) emissions increase. With the utilization of biogas production technology within the agriculture sector, ecological benefits could be achieved, with immediate economic profit. Therefore, to retain economic profit and environmental sustainability, implementation of bioeconomy principles is of key importance. This paper examines four options of digestate treatment, which is identified as one of the highest operational cost for the biogas plant. A simple and robust model in Excel Solver was developed to determine the best solution for minimising GHG emissions and maximise profit for the biogas plant operator, through an upgrade of the plant with digestate treatment technologies. The model was implemented on a case of a Croatian biogas plant and the best solution in terms of GHG reduction and profit increase proved to be fertilizer production (Option 1), through a crystallization process of struvite within the digestate. This option obtains a significant reduction in GHG emissions compared to standard biogas production without additional upgrades (Option 4), by over 90%, and increase of profit for the biogas plant operator, which diversifies the income source and creates multiple positive impacts on the environment.

The number of biogas power plants increased significantly in the recent years. A lot of countries developed competitive national biogas markets after decades of intensive research and technology innovation. Biogas power plants make use of advanced anaerobic digestion processes to increase the production of biogas as a principal source of income. On the other hand, digestate is still considered a by-product and usually disposed of in landfills. This article proposes energy generation from digestate as a financially viable alternative. Before energy generation, digestate is separated into solid and liquid fractions and dried by thermal energy. Calorimetric measurements showed that the heating value of digestate is 13 MJ/kg, which favors its energy generation potential, especially if the excess heat from cogeneration is used for the drying. The digestate produced in biogas power plants can be used as a low-cost alternative fuel for electricity generation in biomass power plants. It is estimated that digestate-generated electricity could contribute with 4.2% of the total electricity generation in Croatia. The economic analysis revealed that the price of electricity generated from digestate, under the present feed-in tariff system in Croatia, is two to three times higher than the total cost of digestate.