• Energy Research

Abstract Biogas production is increasing as a sustainable energy supply, with digestate resulting as a by-product of biogas plants. As a result, the high concentration of Clostridium spp. in digestate became a concern in dairy farming areas. Clostridium spores can contaminate soils and crops when digestate is used as fertilizer, causing a conflictual cohabitation of biogas with traditional cheese productions. In order to solve the problem, this study aimed to search for a technical solution enabling either a drastic reduction or the elimination of the content of Clostridium spp. within digestate. Results showed a complete elimination of Clostridium spp. in pelleted stored solid digestate; in addition, pelleting caused a reduction of pH and water mass fraction in terms of fresh weight, and a concentration of mineral nutrients compared to stored solid digestate. Pellet can represent a possible sustainable solution both in reducing potential risks linked to the presence of Clostridium spp. in digestate and in improving the transportation and distribution of high-value fertilizer. Hence, pelleting of solid digestate could offer a simple and efficient method to allow cohesistence of biogas plants and dairy farming.

An agronomic strategy to mitigate climate change impact can be the build-up of soil organic carbon. Among agronomic management approaches, the administration of organic fertilizers like livestock manure represents an effective strategy to increase soil organic carbon. However, livestock manure usually contains a high amount of water, reducing its sustainable delocalization and impacting on greenhouse gas emissions and nutrient leaching. Furthermore, the possible presence of weed seeds and harmful microorganisms could reduce the agronomic value of the manure. To overcome these issues, the combined effects of dewatering, composting and pelleting were investigated on livestock manure to produce sustainable organic fertilizers. Our results showed that composting and pelleting can represent a feasible and sustainable solution to reduce the potential risks related to the presence of weed seeds and harmful bacteria, concentrating nutrients and allowing a sustainable valorization and delocalization of the livestock manure. In addition, the processed manures were assessed as fertilizers in the growing medium (GM), displaying an increase in water retention and nutrient availability and a decrease of GM temperature and weed seed emergences. However, further study is needed to validate, both in open field and greenhouse productions, the effects of the proposed fertilizers in real cropping systems.