Slurry separation
Gordon Campbell, Dairying Development Advisor, CAFRE, Ballyclare
Many farmers are considering their options under the Farm Nutrient Management Scheme. Slurry separation is not a common practice in Northern Ireland but is used widely in other EU countries. This brief article gives an outline of some of the issues to be taken into account at farm level.
Slurry separation is where slurry is divided into a liquid and a solid fraction. This can either be done slowly by a weeping wall system, or more quickly by mechanical separation. There are a number of different types of mechanical separators including rotary screens, roller presses, screw presses, inclined screens and vibrating screens. A typical slurry separator and gantry system (including pumps) similar to the one in operation at Greenmount Campus can cost between £25,000 and £30,000. At Greenmount both the dairy and pig slurry are separated using a Carier separator. The separated liquid is stored in above ground stores whilst the solid fraction is collected and spread along with farmyard manure.
The liquid produced by separation has a Dry Matter of approximately 4-5 percent. Therefore it is subject to SSAFO regulations and cannot be spread during any proposed closed winter period. The table below indicates the nutrient value of the separated liquid.
Table 1 - Nutrient value of separated liquid (MAFF Recommendations RB209)
| Total (kg/m3) | Available to next crop (kg/m3) | |
|---|---|---|
| Nitrogen | 3.0 | 0.6 |
| Phosphate (P2O5) | 1.2 | 0.6 |
| Potash (K2O) | 3.5 | 3.2 |
(to convert kg/m3 to units/1000 gallons multiple by 9)
Advantages of separating slurry
- The separated liquid is more easily absorbed by the soil surface than conventional slurry, which makes better use of available nutrients.
- Opportunity to apply separated liquid to grazing areas during the grazing season as the risk of taint is greatly reduced.
- Lower risk of leaf contamination in silage swards – liquid travels easily down the plant leaf. Therefore there is a wider window to spread separated slurry to silage swards.
- Separated liquid can be readily spread without prior mixing.
- Potential reduction in slurry storage capacity requirements of 10 percent to 25 percent.
- Separated solids can be spread during the proposed winter closed period.
Disadvantages of separating slurry
- Separate storage (above ground store) is required for the separated liquid in order to optimise the benefits of a separator.
- Solids must be spread with different equipment, which may not be available on the farm.
- Increased levels of potash could be built up in the soil if frequently applied to the grazing ground. This could increase the risk of grass staggers to the grazing animals.
- An additional midden may be required to store the separated solid material.
- Potential underutilization of a slurry separator’s capacity at individual farm level. There may be greater benefits for contractors or a share agreement between farmers.
Slurry separation allows greater flexibility in spreading times and application and thus can optimize the full nutrient potential of slurry. In order to get maximum return from the investment, a separator must integrate easily into the existing farm setup with little extra expense and there must be sufficient slurry produced on the farm to justify the outlay.