Agri-Food and Biosciences Institute (AFBI) research is looking at the feasibility of physically extracting phosphorous (P) from animal slurries.

The work is ongoing at AFBI Hillsborough in Co. Down. Four of the scientists involved in these projects – Gary Lyons, Chris Johnston, Michael Wills and Ashley Cathcart – have recently published a summary of the results achieved up to this point.   

Livestock slurries are an important source of plant nutrients for crop growth in our mainly grass-based agriculture sector.

However application of slurry to agricultural land can lead to phosphorus (P) loss to watercourses through run-off, causing subsequent water quality and ecosystem problems.

Phosphorous from animal slurries

Courtesy of a Department of Agriculture, Environment and Rural Affairs (DAERA) funded project, AFBI evaluated the effectiveness of two mechanical separation technologies (screw press and decanter centrifuge) to reduce P levels in land-spread slurry and anaerobic digestate.

In areas with intensive livestock farming systems like those in the dairy sector, the quantities of nutrients available in slurries on a particular farm can exceed the nutrient requirements of grass grown on that farm.

Applications of P that are surplus to crop requirements, can result in high losses to groundwater and surface water, where eutrophication can cause reduced functioning and biodiversity of aquatic ecosystems and a decline in surface water quality.

Many intensive dairy farms have high soil P levels throughout their holding and need to move to a more sustainable position, as they make a vital contribution to agricultural productivity.

Technologies are available to separate P from livestock slurries, reducing the P-load applied to agricultural land from these sources and helping to lower environmental risks to water quality.

Such treatment technologies, allied with separated solids processing and P export to areas where it is required, would improve the environmental sustainability of dairy farm nutrient management.

Review of separation technologies

AFBI has undertaken a comprehensive review of published scientific literature on the effectiveness of these separation technologies to reduce the P loading of slurry and digestate before land application.

Staff at AFBI have also carried out trial work using the screw press and decanter centrifuge technologies installed at their Farm Nutrient Management Centre at Hillsborough.

Both technologies partition a proportion of the insoluble materials (including the P) in slurry into a separated solids fraction and also produce a separated liquid fraction with lower solids and P content.

The research examined capital and operating costs, separation efficiency and throughput, the use of chemical additives, management and processing of separated fractions including transport costs, environmental impacts and the biosecurity of separated solids for export.

Of the two technologies reviewed, screw press separation is a more cost effective option when lower amounts of export of P off-farm are acceptable. For farms and those with anaerobic digesters managing larger volumes of manure/digestate, screw press separation is possible.

However if higher levels of P removal are required, the use of decanting centrifugation is a viable option.

Centralised processing facilities could also make use of decanter centrifuge technology to act as processing hubs for a number of local farms within a distance that makes it economical for transport of slurry / treated slurry to/from the processor.

Both separation technologies could be integrated into agricultural slurry and digestate management systems, to provide a more sustainable approach to managing agricultural P loss and its associated impact on water quality.

The research also highlighted that screw press and decanter centrifuge separation could reduce phosphorous content in slurry and digestate separated liquid fractions by up to 34% and from 30% to 93% respectively, depending upon the type of technology deployed and operating conditions.

The separated solid fraction could be further processed (anaerobic digestion, composting, drying, pelleting, soil amendment, fuel source) improving the economic feasibility of export distances, or potentially to create a product that could be used locally.

However, such processing could be associated with high volatile nitrogen (N) losses and greenhouse gas (GHG) emissions which may involve the use of abatement technologies to avoid pollution swapping.

N losses can also be significantly reduced by acidification of slurries/digestates to pH 5.5.

The export of separated solids for better P management is a possible biosecurity issue and precautions may need to be taken to reduce the risks of cross infections caused by pathogenic microorganisms depending upon end use.