Local study to put slurry nutrient separation to the test

A pioneering study at the new Nutrient Management Centre at the Agri-Food and Biosciences Institute (AFBI) is set to test ways of removing surplus phosphorus (P) from livestock manures.

It is hoped the centre will serve as a working nutrient processing unit for AFBI; and that it will also be used to demonstrate farm nutrient management to the agri-food industry – both locally and nationally.

Looking ahead, the need for enhanced nutrient management on-farm is expected to be critical as P levels in soil could potentially exceed permissible limits.

P levels in waterways are regulated under the EU Water Framework Directive.

Dr. John Bailey, head of AFBI’s agri-environment branch said: “More than 50% of grassland fields on dairy farms are oversupplied with P and have soil P indices greater than the agronomic optimum P index 2+.

“When soil P exceeds this agronomic optimum, there is an increased risk of some P being lost in run-off to water course; particularly where topography is steeply sloping and soils have a clay texture,” he said.

As a result, farms are limited to spread 170kg nitrogen/ha/year unless granted a derogation.

A strategy to reduce phosphorus losses from manures is to remove it prior to land application.

Nutrient separation

After separation, the liquid fraction with lower P, valuable nitrogen and potash could be spread as organic fertiliser without exceeding P limits for soil and with the potential to more exactly match plant nutrient supply to crop requirements.

Meanwhile, the separated solid fraction could be further processed – dried, composted or granulated – to produce a saleable, stable, low moisture product that could be exported off-farm if required.

Screw press separation and centrifugation are the two established technologies currently being investigated.

Separation of feedstock produces a solids fraction containing a high proportion of P which is more economical to transport off-farm for both agricultural and non-agricultural purposes.

It’s planned that separated solids will be exported off-farm to other agricultural land where P supply is low, and the separated liquid fraction – with its reduced solids and P content – will be applied to AFBI grassland as a bio-fertiliser.

The screw press separation technology has the potential to reduce total P from slurry or digestate by 20-40%.

Results during the commissioning phase of the screw press indicate that 25% of the total P, 15% of the total nitrogen and 10% of the total potash in digestate was removed in the separated solids fraction – with a dry matter content of 24.5%.

It is expected that greater removal of P should be possible once the system is fully commissioned.

The use of centrifugation should result in even higher levels of P removal; with the potential for more than 60% of P being removed in the solid fraction.