Characterisation of sludge produced by the agri-food industry and recycling options for its agricultural uses in a typical Mediterranean area, the Segura River basin (Spain).

Agri-food sludge is both an environmental and economic problem, since companies must bear the costs of proper management as required by current disposal regulations. The aim of this work was to quantify the sludges in a typical Mediterranean area like the Segura River basin in Spain, which has a high concentration of agri-food businesses and intensive farming operations. Ultimately, we wanted to offer options for converting this "waste" into a "resource." The agri-food companies studied were classified according to their final product: juices, frozen vegetables or canned products. The total volume of sludge ranged between 12,895 and 22,520 tonnes per year, with an average cost of around 350,000 euros. In characterising the agri-food sludges, we identified a high organic load and nutrient content. Besides, the heavy metal levels were well below those legally permitted in sewage sludge, fertilisers or similar products for agricultural use. Furthermore, we only detected pesticide residues occasionally, and we did not detect Salmonella spp. or Listeria monocytogenes in the samples analysed. E. coli was only found in about 20% of the samples. This paper identifies the most suitable technologies for the specific agri-food industries in the area, which are surrounded by intensive agriculture operations. We have analysed direct soil application, sludge drying, composting and the use of anaerobic digestion to produce energy and have shown the main advantages and disadvantages of each. We found that composting and its advanced modifications for making tailored composts are among the most adequate strategies for obtaining organic amendments.