Energy recovery of screening waste from wastewater treatment plants as solid recovered fuel

Abstract

In recent decades the principles of the circular economy have established themselves as an unavoidable field of action in wastewater treatment plants (WWTPs). Increasing energy costs and environmentally sustainable obligations in waste management are changing the focus on wastewater management. The biorefinery concept redefines WWTPs by implementing processes for energy production and moving towards zero waste. As part of wastewater management, in addition to sand and grease recycling processes, waste to energy (WtE) technologies development lies mainly in sludge. However, only the screening waste from pre-treatment still has no recovery process and is generally disposed of by landfill disposal. Although there are studies on the possible alternatives to landfill disposal of the screening waste, these are focused on anaerobic digestion processes. This is the only type of recovery route for this waste that has yet to be studied. Consequently, the main objective of this research has been the in-depth analysis of the possibility of energy recovery of the screening waste through its transformation into solid recovered fuel (SRF). In order to establish the state of the art, the scientific evolution of WtE technologies in municipal WWTPs was analyzed using a bibliometric review. Tests leading to the physical and chemical characterization of the screening waste from the Biofactoría Sur in Granada (Spain) were carried out concerning the laboratory work. Then, at an experimental level and having as main stages the drying, crushing and densification phases, SRF was produced without densification and densified in the form of pellets. The fuel quality was determined by utilizing its characterization based on a new SRF classification proposal developed based on several existing national and international regulations. Finally, laboratory tests and energy balances were carried out for the combustion, gasification, and pyrolysis processes to evaluate the use of the SRF produced through its valorization in thermochemical processes. In addition, the SRF production process, both densified and non-densified, has been subjected to environmental and economic feasibility studies. The results of the scientific mapping show an exponential increase of WtE publications in WWTPs, identifying at the same time the lack of study on the waste from desludging. The analyzed waste mainly comprises sanitary textiles (52.1%) and other fractions such as paper, plastics and vegetables. This composition could be similar to the rejection fraction from municipal solid waste (MSW). Furthermore, its calorific value and Cl and Hg contents make it viable for conversion into SRF according to ISO 21640:2021. The technical feasibility of SRF production, both nondensified and densified, was tested. For pellet production, the input variables were residue moisture and press size, obtaining that the optimum palletization conditions were for a moisture content of 10% and with matrices with compression ratios of 6/20, 6/24 and 8/32. Together, the determination of the characteristics of the SRF obtained showed that the requirements of the proposed classification were met, with the most favourable destination for recovery being waste-to-energy plants. For the environmental and economic study, four SRF production scenarios were established, differentiating the drying type and final product as an alternative to landfill disposal of the screening waste. The cost-benefit analysis, carried out by obtaining the Net Present Value (NPV) using Monte Carlo (MC) simulation, concluded that landfill disposal is the most pessimistic scenario in economic terms. Furthermore, including the monetization of the CO2 emissions generated in this analysis concluded that it is not a viable solution. The environmental impact study also ratified this conclusion, developed using Life Cycle Analysis (LCA) and for which the SimaPro 9.2 software was used. This tool certified that the landfill has the most negative impact in 6 of the 11 environmental categories analyzed according to the CML-IA baseline v3.08 methodology. Furthermore, from these studies, it was obtained that the most viable scenario in economic and environmental terms would be the generation of SRF without densification and by using thermal drying in the production process. From the perspective of energy recovery, the thermochemical processes of combustion and gasification were compared. The theoretical combustion analysis was carried out through energy balance, having the SRF with different moisture levels as the input stream. Laboratory-scale tests were carried out for gasification to determine the output gases produced. In this case, another energy balance was carried out for the combustion of these gases. Solid SRF combustion was the most effective process, with a maximum energy benefit of 178.63 MJ per 100 kg of raw SRF (at 77.3% moisture). In comparison, gasification gave maximum results of 42.48 MJ for the same amount of SRF. From a non-energy point of view, pyrolysis, also analyzed based on the laboratory-scale experimental design, reflected the feasibility of generating value-added products, such as char or pyrolysis liquid.