New technology aims to turn slurry into fertiliser and reusable water

Farm slurry and digestate could become a more valuable source of fertiliser and reusable water as nutrient-recovery technology aims to help farmers cut input costs and make better use of resources already on their holdings.

A partnership involving Dynameau Ltd in the UK and Ireland-based Crowley Engineering and Agrigas Bioenergies Ltd is developing systems designed to recover nutrients and water from livestock slurry and anaerobic digestion digestate.

The approach can separate slurry and digestate into three potentially useful streams: recovered nitrogen, a phosphorus- and potassium-rich solid fraction, and treated water.

Ammoniacal nitrogen contained in slurry or digestate can be captured as liquid ammonium sulphate fertiliser, potentially allowing farms to replace some bought-in mineral nitrogen with nutrients already produced within the agricultural system.

A second output is a stackable solid material containing phosphorus and potassium which, depending on its composition, crop requirements and relevant regulations, could potentially be used as fertiliser on grassland, arable and tillage farms.

The process can also produce treated water. Where the required quality and regulatory standards are met, this could be stored and reused on farms, including for irrigation.

With drought and water availability becoming increasingly important concerns for British agriculture, recovering water during slurry and digestate treatment could also contribute to farm resilience.

The approach is not limited to livestock farms. Anaerobic digestion plants also produce substantial volumes of digestate, creating opportunities to recover ammonium sulphate and other nutrients from material that would otherwise require storage and management.

Where a farm or AD plant produces more recovered fertiliser than it needs, there could also be scope to supply nearby growers, subject to product standards and regulation.

Surplus liquid ammonium sulphate, for example, could potentially be supplied to vegetable, grassland or arable farms, creating value locally while reducing reliance on imported mineral fertiliser.

Recovering the nutrients is only part of the process, with the way they are applied also playing an important role.

Precision subsurface systems such as the Duport Liquiliser place liquid fertiliser below the soil surface and closer to the root zone, rather than broadcasting it across the top of the crop or field.

The approach is designed to reduce opportunities for ammonia volatilisation and nutrient loss while positioning nitrogen where growing plants can access it. It could be particularly relevant in nutrient-sensitive areas or where growers want greater control over fertiliser placement.

For vegetable and salad growers, subsurface application also reduces direct contact between liquid fertiliser and edible plant material.

However, the source cautions that the method should not be presented as eliminating food-safety risks, as microbial contamination can occur through several routes and appropriate controls remain necessary.

Taken together, the system aims to recover nutrients already present in slurry and digestate, turn them into usable products and return them to crops through more targeted application, while recovering water where treatment standards allow.

For farmers, the potential benefits extend beyond environmental compliance. The technology could reduce exposure to volatile fertiliser prices, improve nutrient-use efficiency and retain water for future use.

Where surplus recovered fertiliser can legally be marketed, it could also provide an additional source of income.

For anaerobic digestion operators, the system could create a closer link between renewable energy production and local fertiliser supplies, while arable and vegetable growers could gain access to locally recovered nutrients applied with greater precision.

The companies behind the project say rising input costs, pressure on water resources and tighter nutrient management are changing the way slurry and digestate are viewed.

Rather than focusing only on how the materials should be stored and managed, the question may increasingly be “how much value agriculture can recover from them”.