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 pound 0.44 and pound 2.21/tonne p.a., respectively. (C) 2000 Elsevier Science Ltd. All rights reserved.