Solid—liquid separation of animal slurry in theory and practice. A review
Solid-liquid separation of manure is a method to produce nutrient and dry matter rich fractions with higher volumetric methane potential than that of the original liquid manure. Centrifugation and chemical precipitation and flocculation are efficient options for such separation. Centrifugation efficiency depends on factors such as manure type, G-force, and dewatering volume, while chemical precipitation and flocculation depend on the amount and type of chemicals and polymers used. We assess all these factors in this study. The methane yields of the solids from centrifugation and precipitation were assessed by batch digestion. Dewatering volume and G-force had great influence on separation efficiency and on the chemical composition of the solids. Centrifugation transferred increasing amounts of nutrients and dry matter to the solid fraction as gravitational force increased to approximately 2200G. However, increasing gravitational force beyond 2047G did not significantly improve separation efficiency. The quantity of solids and separation efficiency of dry matter and nutrients varied for different manure types. Separation efficiencies for total N and dry matter greatly depended on the manure’s dry matter content, while separation efficiency for total P was little affected by the same factors. Because different manure types were used for the tests with precipitation and flocculation, it was impossible to determine the effect of changing the chemicals. However, the separation efficiencies achieved for N and P were higher than those achieved with centrifuged manure of the same dry matter content. The methane yield from the solids separated by chemical precipitation and flocculation were significantly higher than the yield from centrifuged solids. The yields from the solids produced by centrifugation of pig manure were 161 to 186 L CH4/kg VS compared to 253 L CH4/kg VS from centrifuged dairy cow manure, while the yields from the solids produced by coagulation and flocculation were 392 to 404 L CH4/kg VS.