Water footprint of low-tech digesters for bioenergy recovery from waste

This study aimed to assess the water footprint of plastic tubular digesters to treat organic waste and recover bioenergy. Different scenarios in a small-scale dairy farm in Colombia were considered: i) scenario without the digester in which cow manure is piled up, and dairy wastewater is directly discharged into the environment, ii) scenario with a plastic tubular digester treating cow manure, and iii) scenario with a plastic tubular digester co-digesting cow manure with dairy wastewater. The ReCiPe 2016 – midpoint (H) method was used to assess the degradation of water bodies (i.e. eutrophication and ecotoxicity impact categories), while the AWARE 2.0 method was applied to evaluate the water scarcity impact category. Results showed that, in terms of water quality degradation, the co-digestion scenario exhibited impacts 10% to 45% lower than those of the other scenarios, depending on the impact category. In terms of water scarcity, the co-digestion scenario exhibited approximately 10% lower water scarcity than the other scenarios, which showed a similar water scarcity footprint. This indicates that implementing plastic tubular digesters, even under mono-digestion conditions, does not worsen water availability. These benefits are greater if co-digestion of different agri-food waste is applied, since it helps preserve water quality by valorising the intrinsic water content of food wastewater. It also decreases environmental impacts by reducing synthetic fertiliser use through the production of a higher-quality digestate. Overall, plastic tubular digesters provide a low-water footprint alternative to the direct discharge and uncontrolled storage of livestock waste and food-production wastewater in small-scale farms.