Valorization of treated sewage sludge for Arundo donax production in a field experiment

Non-food crops provide an alternative renewable energy source. The high yield of Arundo donax makes this crop a suitable candidate for this purpose. Sewage sludge can be repurposed as an organic amendment to recover soil fertility and improve crop yield. This study sought to evaluate the effect of treated sewage sludge (SS) amendment on the production and heating value of A. donax as a source of biomass for energy production. The study was developed in experimental plots in Central Spain. As organic amendment, sewage sludge composted with pruning waste (CP) and treated by thermal drying (TD) were applied at a dose of 50 t ha- 1, in comparison to a mineral fertilizer treatment (F) and a control soil without treatment (C). After two growing seasons, plant biomass, production parameters and heating value were examined, as well as the effect of the amendments on soil properties. Most parameters increased in plants grown in sewage sludge-treated plots, including heating value reaching 19.19 MJ kg- 1. Moreover, the leaves of these plants had a higher chlorophyll (from 0.39 to 0.52 mg cm-2) and nitrogen content (2%) and a lower ash content (0.9 % reduction). This reduction in ash and the increase of heating value favor the use of this biomass for thermochemical applications. Moreover, the increase in soil organic carbon (around 40 %) due to the organic treatments contributes to increase the carbon stock in soil. The findings support that the application of treated sewage sludge emerges as a suitable strategy not only for enhancing the production of A. donax for energy uses but also for maintaining soil fertility in Mediterranean conditions.