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Solid–liquid separation of animal slurry in theory and practice. A review

M. Hjorth¹, K. V. Christensen², M. L. Christensen³ and S. G. Sommer²

¹ Department of Agricultural Engineering, Aarhus University, Schüttesvej 17, 8700 Horsens, Denmark
² Department of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, University of Southern Denmark, Niels Bohrs Allé 1, 5230 Odense, Denmark
³ Department of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark

Corresponding author: sgs@kbm.sdu.dk

Accepted: 22 April 2009

Abstract

Animal slurry contains plant nutrients that are essential for crop production. However, intensive livestock production may lead to a surplus of plant nutrients on farms and, as a consequence, discharge or emission to the environment. In order to ensure that the slurry applied to fields matches the nutrient requirements of the crops, techniques have been developed to reduce the nutrient content of slurry by means of separation. This review discusses the separation technologies used for animal slurry treatment and the physical and chemical processes involved in separation. These processes need to be understood before efficient, reliable and cheap separation technologies that take into account the actual properties of slurry and the likely end-use of the separation products can be developed. A simple separation efficiency expression can be used to assess the efficiency of slurry separation. It is indeed important to measure the amount and composition of the slurry before treatment, the dry-matter-rich fraction and the liquid fraction. The separation efficiency of mechanical separators for the removal of dry matter and phosphorus (P) is ranked as follows: centrifugation > sedimentation > non-pressurized filtration > pressurized filtration. In general, the separation of total N and NH follows the same pattern, but the separation efficiency is lower than for dry matter and P. Treatment with a flocculant before separation improves separation efficiency significantly. Of the polyacrylamide polymers tested, high-molecular-weight, linear cationic polymers with a medium charge density (20–40 mol%) were found to be the most efficient flocculants. The best mechanical separation techniques for flocculated slurry are screens or filter belts. The separation efficiency of polyacrylamide-treated slurry can be improved by adding a multivalent ion to coagulate particles and for precipitation of phosphorus. Aluminium sulfate (Al₂(SO) or ferric chloride (FeCl₃) seem to be very efficient for improving the mechanical separators. Alternatively, the mineral struvite (MgNH₄PO₄) may be formed by changing the slurry characteristics, such as by the addition of magnesium (Mg) or by increasing the pH to 9. The struvite crystals are removed during solid–liquid separation. The products of the solid–liquid separation may be further treated by evaporation, membrane filtration or ammonia stripping in order to obtain the desired end-products; however, low-maintenance and/or cost-efficient operation of these post-treatments has not yet been demonstrated. The separation should be developed as a whole-system approach, paying attention to parameters such as the value of end-products, environmental consequences and economy.