Heterogeneity and contaminants in dairy processing waste sludge in the north of France

Background The production of dairy is accompanied by the generation of large quantities of dairy wastewater and dairy processing wastes (DPW). DPW are materials of substantial potential as fertilizers. Yet, these materials may be highly heterogeneous and may contain organic and inorganic pollutants that could prevent their use in agriculture. Methods We characterized DPW from 10 different dairy factories in the North of France. We analysed their elemental composition, their physical structure and the presence of pathogens (Coliforms and Salmonella). We compared the amount of pollutants and nutrients to the current fertilizer norms in the European Union (EU 2019/1009). Results We observed that DPW’s chemical and physical properties are very variable between the sites visited. Our results show that DPW may be a good source of nutrients for crops, containing large concentrations of phosphorus (P) (up to 77.91 g kg −1 ), nitrogen (N) (up to 82.48 g kg −1 ) and potassium (K) (7.65 g kg −1 ). Yet, only half of the samples complied with the current legislation as many pollutants and pathogens were found to be above the legal threshold. Conclusions We recommend that DPW should be always treated before its direct application as fertilizer and to reduce the input of chemical contamination in the wastewater treatment process. Further research should be undertaken to identify and reduce the sources of contamination in DPW.