Possible agricultural use of digestate

The aim of this study was to evaluate the agricultural use of digestates obtained from laboratory-scale experiments of anaerobic co-digestion of different organic wastes (glycerol, compost from landfill, fish farm sludge, and catering waste and their mixes with sewage sludge) and from full-scale biogas plants (cattle slurry). The concentration of nitrogen, phosphorus, and heavy metals and presence of Salmonella spp. in digestates were monitored. The co-digestion trials were performed using laboratory-scale reactors. The microbiological analyses of digestate showed the presence of Salmonella spp. in both the laboratory-scale reactors and samples taken from full-scale biogas plants. Some digestate samples highlighted the importance of the microbiological quality evaluation of the digestate in studying the possible health risks for consumers. The heavy metals concentrations did not exceed the maximum levels permitted by the Estonian Minister of the Environment Regulation No. 78 of 01.02.2003 'Requirements for the application of sewage sludge in agriculture, landscaping, and recultivation'. Although Cd concentration showed values lower than 3 mg/kg TS and Hg was only found in catering digestate, environmental contamination would be possible if digestates were used for agricultural purposes. This work can be considered as a preliminary study in evaluating the possible agricultural use of the digestate obtained from the co-digestion of different organic wastes.