Solid–liquid separation of livestock slurry: efficiency and cost

Environmental problems may arise on specialized livestock farms, which produce a surplus of nutrients in relation to crop requirements. Separation of this slurry into a liquid fraction and a dry-matter and nutrient-rich fraction, followed by transporting the nutrient-rich fraction to farms with fewer animals, may alleviate the problem. This study showed that the most informative index of separation is the reduced separation efficiency index; where an index value of 0 indicates that the nutrients are distributed equally between the solid fraction and the liquid fraction, and a value of 1 indicates that the nutrients are concentrated in the solid fraction. The reduced separation efficiency values obtained when using simple mechanical screen separators were 0.07–0.5 for dry matter (DM), 0.01–0.1 for total N (TN) and 0–0.2 for total phosphorus (TP). In comparison, decanting centrifuges were very efficient in removing DM (index: 0.36–0.74) and TP (index: 0.4–0.82), but not in separating TN (index: 0.11–0.19). The costs of a treating 4000 tonnes of slurry with a mechanical screen separator or with a decanting centrifuge are £0.44 and £2.21/tonne p.a., respectively.