Pig Slurry Treatment Strategy in a High Livestock Concentration Area: Anaerobic Digestion as the Key Process

A pig slurry treatment strategy must include processes with the aim of reducing volatile organic compound emissions, controlling odours, mineralising nutrients, improving its fertilising proprieties, and recovering energy, when it is possible. In this sense, anaerobic digestion fulfils all this requirements. However, in areas with high animal farming density and a structural nutrient surplus, it is necessary to include other processes, which favour nutrient redistribution. The objective of this paper is to study the importance of including anaerobic digestion process in the treatment strategy when the main objective is to recover nitrogen by means of ammonia air stripping at 80oC, or by vacuum evaporation in order to reduce volume and to favour redistribution. Two types of pig slurry (fresh and anaerobically digested slurry) were used and the initial pH was chosen as the control variable. Apart from producing part of the required thermal energy, previous anaerobic digestion presented several clear advantages: The consumption of volatile fatty acids and volatile organic compounds during anaerobic digestion reduced the volatilisation of organic matter in the stripping and vacuum evaporation processes. This fact resulted in high quality condensate when vacuum evaporation is applied and an ammonia salt with low organic matter contamination when nitrogen is recovered with stripping / absorption process. In this case, high ammonia removal efficiency is possible, over 96%, without modifying the pH.