Over recent months, we have seen a flurry of activity around contractors separating cattle slurry on farms using mobile mechanical screw press systems. The primary driver for this appears to be guaranteed extra storage tank space (claims of up to 20%) by removal of some of the slurry solids.

An additional benefit is a separated liquid fraction with lower solids content, which is easier to spread using Low Emission Slurry Spreading Equipment (LESSE), helping to mitigate ammonia lost during field application compared to inverted splash plate technology.

The mobile separators being used appear to be high throughput units, setup to separate large slurry volumes over short time periods at high flow rates. Separation throughput is the rate at which feedstock can be separated.

The actual flow rate will largely be dependent on input dry matter (DM) content. High-DM slurry should have a lower flow rate than lower-DM slurry and this will determine actual productivity. So, how the separator is setup and operated is extremely important.

Screw presses separate slurry based on particle size using a central Archimedes screw. As slurry moves through a tube against a cylindrical screen, pressure builds due to a plug and resistance plate.

Solids are pressed out against the plate, while smaller particles and liquid pass through (see graphic). Flow rate affects solid separation and can be adjusted to control moisture content.

Typical design of a screw press separator.

AFBI research

Work at the Agri-Food and Biosciences Institute (AFBI) over the last number of years has focused on using a screw press separator to remove phosphorus (P) from slurry and digestate, to reduce the amount of P applied to the field and its consequent impact on water quality.

This work investigated the different operational parameters associated with screw press setup and running, to maximise solids removal and consequently the amount of P which can be removed to the separated solid fraction.

There are several variables that may be adjusted to alter the separation profile of a screw press, namely mesh size of the screen, flow rate and scraper/resistance plate tension. AFBI is concerned that unless static or mobile screw press units are setup and run correctly to enhance solids and also P offtake, then the amount of solids and P removed will not be optimal.

The temptation to achieve highest feedstock throughput and flow rates to separate as quickly as possible, usually means using larger mesh size screens and low scraper tension. This results in lower solids and P separation efficiencies with less solid material and P removed from the separated liquid fraction.

This is a sub-optimal way to run the separator and will also mean that less saving in tank storage space is achieved. The length of slurry storage time prior to separation is an important consideration in relation to efficiency of separation. To maximise solids and P removal, slurry should be processed as fresh as is possible.

Longer storage periods and increasing ambient temperature before separation will result in slurries with lower DM content and less solids to separate due to organic matter breakdown, resulting in lower levels of solids and P removal.

Better phosphorus management

While on-farm slurry separation, if carried out correctly, creates more tank storage space, it is also an opportunity to manage P better.

Livestock slurries are useful and valuable sources of plant nutrients; however, application of slurry P that is surplus to crop requirements can result in high losses to surface and ground waters, leading to significant water quality issues.

Much of our intensively managed land in Northern Ireland is running at high P surplus and farm P balances need to be reduced if we are to move towards sustainable agricultural production.

Farmers and contractors should not only be targeting tank storage space as a key driver for mechanical slurry separation, but they should also consider that by optimising separator setup, they can create a lower P separated liquid with reduced solids content which can be land spread by LESSE, reducing ammonia emissions by percolating into the soil quicker, with increased nitrogen use efficiency for crop growth.

The solid fraction containing the separated P should not be reused on the farm if not required but rather exported to areas of P need.

By field spreading the separated solids on the same farm as they originated from, there is no net reduction in farm P balance if this is necessary. There is also the uncertainty that if the separated solids are spread on grassland (rather than ploughed in) this may contribute to increased P loss to water through more particulate runoff.

Export options for separated solids

There are several options for exporting separated solids. In all cases, it’s best to move the solids off-farm as soon as possible after separation, for reasons outlined below.

1. Export to arable land:

  • This is a beneficial route, as the organic matter and P in the solids support soil structure and crop nutrition. However, given the limited area of arable land in Northern Ireland, this option is constrained.

    • 2. Horticultural substrates use:

  • The horticultural industry is showing interest in separated solids, particularly as composted material. With growing pressure to reduce peat use in substrate blends, separated solids present a promising alternative.

    • 3. Further processing:

  • Solids can be dried or pelleted and used in renewable fuel production or for making biochar. AFBI is actively researching these options. Each of these routes enables the transfer of excess P from the intensive grassland sector — where it poses an environmental risk — to sectors outside of agriculture where P management is less of an issue.

    • 4. Biogas production:

  • DAERA has supported several projects under the Sustainable Utilisation of Livestock Slurry (SULS) Small Business Research Initiative (SBRI). Two current Phase 2 projects are trialling mobile on-farm separation, with export of solids to biogas plants for renewable energy production. Solids from fresh slurry can retain 60–70% of the energy value of grass silage (by weight), making them an effective biogas feedstock. These projects are evaluating the scalability of replacing a significant proportion of silage with slurry solids, which could offer a strong local economic driver for renewable energy.

    • Separated slurry solids.

    Separation: an on-farm tool to reduce water pollution

    In July 2024, the Department of Agriculture, Environment and Rural Affairs (DAERA) minister launched the Lough Neagh Report and Action Plan, highlighting the need to reduce all forms of pollution entering waterways.

    It emphasised stakeholder consultation on a new regulatory framework for slurry processing to cut excess P land spreading, promote renewable energy and recycle organic nutrients — aiming to reduce P loading and improve water quality.

    Proper slurry separation can support sustainable policy development in intensive livestock sectors and enhance water quality through farm-level action.