Sewage sludge as a fuel and raw material for phosphorus recovery: Combined process of gasification and P extraction K. Gorazda a,⇑ , B. Tarko a , S. Werle b , Z. Wzorek a a Institute of Inorganic Chemistry and Technology, Cracow University of Technology, Warszawska 24, 31-155 Kraków, Poland b Institute of Thermal Technology, Silesian University of Technology, Konarskiego 22, 44-100 Gliwice, Poland article info Article history: Received 6 March 2017 Revised 3 October 2017 Accepted 22 October 2017 Available online xxxx Keywords: Sewage sludge Gasification Phosphorus recovery Fertilisers Secondary raw materials abstract Increasing problems associated with sewage sludge disposal are observed nowadays. As the thermal con- version of sewage sludge (combustion, co-combustion, gasification and pyrolysis) appears to be the most promising alternative for its management, the solid residues left after gasification were examined. The present study evaluates the potential of this waste as an alternative phosphorus source in the context of phosphorus recovery. The obtained solid gasification residues were characterised (chemical and phase composition, thermal properties, surface properties and technological parameters used for phosphorus raw materials) and compared to commercial phosphate raw materials. It was revealed that gasification residue is a valuable source of phosphorus and microelements, comparable to sewage sludge ash (SSA) considered nowadays as secondary phosphorus raw materials. Chemical properties as well as technolog- ical parameters characteristic for natural phosphate ores are different. Solid gasification residue was lea- ched with mineral acids (phosphoric and nitric) according to the patented method of phosphorus recovery - PolFerAsh, developed by Cracow University of Technology. It was revealed that phosphorus can be selectively leached from solid gasification residue with high efficiency (73–82%); moreover, most of the iron and heavy metals stay in the solid phase due to the low concentration of acids and proper solid to liquid phase ratio. The obtained leachates are valuable products that can be considered for the produc- tion of fertilisers. Combining the gasification process with nutrient recovery provides the opportunity for more environmentally efficient technologies driven by sustainable development rules. Ó 2017 Published by Elsevier Ltd. 1. Introduction Thermal conversion (combustion, co-combustion, gasification and pyrolysis) appears to be the most promising alternative for sewage sludge management in the future. Gasification is the pro- cess of converting a solid material into a gaseous fuel by treating the solid feedstock in a generator (gasifier) with O 2 , air and water vapour or by mixtures of these (Muzyka et al., 2015; Wasielewski et al., 2013). Gasification has several advantages over a traditional combustion process, such as higher energy recovery and lower- cost atmospheric emission control (Pinto et al., 2007). As a conse- quence of the reducing atmosphere, gasification prevents the emis- sion of sulphur and nitrogen oxides, heavy metals and the potential production of chlorinated dibenzodioxins and dibenzofurans. A smaller volume of gas is produced compared to the volume of flue gas from combustion because gasification is characterised by the environment containing low levels of gasification agents. Due to the reducing conditions used for gasification, most of the sulphur, nitrogen, chloride and fluoride in sewage sludge will be released as H 2 S, NH 3 , HCl and HF. The presence of these compounds is undesir- able as they may be converted into the respective oxides during gas utilisation. Therefore, their formation should be monitored and controlled. Sewage sludge also contains relevant quantities of heavy metals. Some may be volatilised to the gas phase at high temperatures, while other elements may be retained in the solid residue, trapping some of the sulphur, nitrogen and chloride intro- duced by the feedstock (Werle and Dudziak, 2013). Gasification may become an alternative for small sewage treatment plants, due to the smaller and less expensive installation of gas cleaning and lower investment costs. Gasification of sewage sludge leads to a high-quality flammable gas that can be used for the generation of electricity or support processes such as the drying of sewage sludge (Marrero et al., 2004). The lower heating value (LHV) of the gasification gas, using air as a gasification agent, varies around 4 MJ/m 3 (Werle, 2015a; Werle and Wilk, 2011). An important aspect of the process of sewage sludge gasification is producing hydrogen by means of a high-temperature gasification process https://doi.org/10.1016/j.wasman.2017.10.032 0956-053X/Ó 2017 Published by Elsevier Ltd. ⇑ Corresponding author. E-mail address: gorazda@chemia.pk.edu.pl (K. Gorazda). Waste Management xxx (2017) xxx–xxx Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman Please cite this article in press as: Gorazda, K., et al. Sewage sludge as a fuel and raw material for phosphorus recovery: Combined process of gasification and P extraction. Waste Management (2017), https://doi.org/10.1016/j.wasman.2017.10.032