FROM SEWAGE SLUDGE TO RENEWABLE FERTILIZER: EFFICACY AND ENVIRONMENTAL RISKS IN OPEN FIELD EXPERIMENT.

The increasing of human population worldwide set, among others, two challenges: food provision, which lead to fertilizers production and waste disposal. Sewage sludge, that can be harmful if not-correctly managed, if properly treated with anaerobic digestion can be used as worthy fertilizer/soil conditioner, while producing energy through biogas combustion. This thesis focusses on a full-scale anaerobic digestion plant treating mainly sewage sludge at thermophilic condition (55°C) for relatively long time (HRT about 50 days) leading to the production of a highly stabilized digestate. Furthermore, the plant has a stripping system which recover ammonia from the substrate, producing ammonium sulphate that can be used as N fertilizer. The recovered fertilizers produced (digestate and ammonium sulphate) were characterized and monitored along three years. Moreover, an experimental maize field was set up for three crop seasons to study the use in agriculture of the recovered fertilizers produced (digestate in pre-sown and ammonium sulphate in topdressing), in comparison with parallel use of synthetic fertilizers (urea in pre-sown and ammonium sulphate in topdressing). The digestate was injected in soil at a depth of 15 cm, urea was spread on soil surface, and ammonium sulphate given as fertigation. During the pre-sown fertilizations were measured ammonia, and odour emissions. The emission of greenhouse gases from the experimental fields was also determined for the ten months following the spreading in pre-sowing. Furthermore. Along the three years yield were quantified and grain quality was tested. At the beginning and at the end of the period were run complete soil analyses. Finally, an LCA study was conducted comparing the environmental impacts of the recovered fertilizers studied and the traditional synthetic urea fertilization. Digestate characterization showed a high N (47% in ammonia form) and P concentration, while a lower K content. All organic and inorganic pollutants (emerging pollutants among the others) were far below the legal limit for its use in agriculture and microbiologic analyses showed an almost complete sanitation of the substrate. Ammonium sulphate (7% N) was free from pollutant, showing a high purity level. Ammonia emissions in open field were similar for digestate and urea, and odour were slightly lower for digestate than urea, being ammonia coming from urea hydrolysis responsible for odour productions. The emission of greenhouse gases throughout the crop season was equivalent between fields fertilized with recovered fertilizers and synthetic fertilizers. Furthermore, soil analyses showed no significant differences in organic or inorganic pollutants. Digestate ensured an ammonia availability for plant equal to urea fertilization and agronomic yield were similar for digestate and urea treatment, while significantly higher than untreated plot, as expected. Finally, in the LCA study, digestate showed a better environmental performance than urea, due the impact of synthetic urea production and considering the renewable energy production of biogas. In conclusion the high stabilized digestate and ammonium sulphate can effectively and safely substitute the traditional chemical fertilization on crop, bringing the advantage of being renewable and helping waste management.