Slurry separation

Area of Expertise:

Under the Nitrates Action Programme regulations, farmers are restricted to a total farm limit of 170 kg/ha/year of organic nitrogen which limits the number of animals farms can carry, particularly pig units. One possible solution is to separate the slurry and transport the nutrients in the solids to e.g. arable farms where required. The liquid fraction is easily utilized for irrigation, injection or trailing-shoe application.

Research at Hillsborough

AFBI Hillsborough is instigating research into mechanical slurry separation, to investigate the management of separated fractions, evaluating their nutrient content, fertilising value and optimum usage strategies.

Future plans are to extract the phosphate and nitrate content and investigate alternative off-farm uses for novel products.

The process

Mechanical separation of slurry is not a new technology, but has come back into focus, mainly due to the implementation of the EU Nitrates Directive. The Guidance Booklet from DAERA and EHS for Farmers, on the Requirements of the Nitrates Action Programme (Northern Ireland) Regulations 2006 and the Phosphorus (Use in Agriculture) (Northern Ireland) Regulations 2006 allow for a maximum volume reduction of 20% for separated animal manures (except pig). 

A mechanical separator separates animal slurry into liquid and solid fractions. The solid fraction of separated cattle manure can be stored as farmyard manure, and can be applied to land throughout the year, provided soil and weather conditions are suitable.

The solid fraction from separated pig slurry is subject to the same restrictions as raw slurry in regard to when it can be land spread. The liquid fraction, often termed supernatant, is lower in volume and dry matter concentration than the original slurry and should not require mixing before being applied to the land. It is suitable for a number of methods of application such as irrigation, injection or application by trailing-shoe tanker. 

This is because a mechanical separator removes the larger fibre particles from the liquid fraction that might otherwise block delivery pipes. The supernatant will generally have a higher N : P ratio compared to the raw slurry and may therefore be better matched to crop requirements. Due to the lower dry matter concentration of the supernatant, the efficiency of use of the ammonia-N concentration in the supernatant should be improved, even if applied by splash-plate. This is because the supernatant will percolate into the soil more readily than raw slurry, thus decreasing the amount of time exposed to the atmosphere and as a consequence, volatilisation of ammonia should be reduced. 


Under the Nitrates Action Programme regulations, farmers are currently restricted to a total farm limit of 170 kg/ha/year of organic nitrogen. In reality, this means that a significant minority of livestock farmers in Northern Ireland do not have enough land for the amount of organic nitrogen produced on their farms.

One possible solution may be to separate the slurry produced and transport the separated solids (and the nutrients they contain) to another farm, where there is a requirement for these nutrients, e.g. arable farms. In this situation, the ability to differentially partition plant nutrients (nitrogen and phosphorus) into the solid fraction could be of benefit.  In the light of these possible scenarios, AFBI, Hillsborough instigated a research programme on mechanical slurry separation, in order to provide the industry with relevant information. Although cattle slurry comprises 88% of the total slurry produced by housed livestock in Northern Ireland (Frost 2005), pig slurry was the primary focus of this experiment, as many pig farmers have insufficient land to meet the 170 kg/ha/year of organic nitrogen limit stipulated in the Nitrates Action Programme.