The Impact of Slurry Separation on Nutrient Content of the Separated Fractions

Slurry separation is the mechanical division of livestock slurry or digestate, to create a liquid and solid fraction.
Three common methods of separation used on farm include:
Screw press
Roller screen press
Decanting centrifuge
The efficiency of the separator to remove dry matter (DM) into the solid fraction underpins the association of nutrients into the relevant fractions. Typically, the liquid fraction is characterised by low DM, high readily available nitrogen (N) and low phosphorus (P) and carbon content. The solid fraction has a higher DM, and P content, and has been acknowledged for its potential benefits to improve soil quality indicators such as water holding capacity, aggregate stability and drainage. Typical UK values for separated organic materials are presented in the AHDB Nutrient Management Guide (RB209).
The partitioning of P into the solid fraction can benefit the farmer in supporting the potential use of separated solids to maintain or build up soil P levels without over-applying N, and the potential for P-rich solids to be removed from an area with excess soil P.
Ultimately, farmers are faced with a trade-off between the cost and availability of separation methods. Centrifuge separators are reportedly the most effective at creating a phosphate (PO4-) rich solid material. However, this technique is the most expensive per tonne of separated material, has the highest electricity demand, is the most expensive to install, and has a lower throughput of slurry compared to a screw press. It is for farmers to assess which technology is best suited for their slurry management based on their business and regulatory requirements.