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

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 + 4 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 (Al 2 (SO 4 ) 3 ) or ferric chloride (FeCl 3 ) seem to be very efficient for improving the mechanical separators. Alternatively, the mineral struvite (MgNH 4 PO 4 ) 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.

Agronomic Values of Anaerobically Digested Cattle Manure and the Separated Solids for Barley Forage Production

“…This contrasting finding probably derives from the cattle manure in our study being in solid form prior to anaerobic digestion. Typically more total P is partitioned into the solid than liquid (slurry) fraction of manure (Hjorth et al, 2010), which was the substrate for the anaerobic digestion process in our study, rather than an animal manure slurry used by Vanden Nest et al (2015). The higher annual precipitation and temperature and the acidic soil in Belgium may have also contributed to differences with our study.…”

Phosphorus Use Efficiency of Bio-Based Fertilizers: Bioavailability and Fractionation

“…This phenomenon would be caused by the enhanced formation and precipitation of calcium phosphate, magnesium phosphate, and/or struvite through mineralization of N, P, and Mg during (co-)digestion in combination with a substantial increase of the manure pH (Hjorth et al, 2010;Le Corre et al, 2009;Möller and Müller, 2012). As a comprehensive example, Güngor et al (2007) showed that 43% of the mineral P species in dairy manure were struvite and 57% more weakly bounded dicalcium phosphate (CaHPO4:2H2O), whereas 78% struvite and 22% hydroxylapatite (Ca5(PO4)3(OH)) were detected in the digested manure sample.…”

Nitric oxide and nitrous oxide emissions from cattle-slurry and mineral fertiliser treated with nitrification inhibitor to an agricultural soil: A laboratory approach

“…The solid-liquid separation of cattle slurry has been referred as a good solution for manure management at farm scale and with potential to reduce the environmental impact of cattle-slurry (Hjorth et al, 2010). The derived solid fraction is generally exported out of the farm and the derived liquid fraction (LF) can be used to substitute mineral N application since it contains a high amount of ammonium (NH 4 + ) (Villar & Guillaumes, 2010).…”