By Grant Leslie, Co-founder & Chief Operating Officer, SEM

Digestate contains nutrients that agriculture needs. Making them useful and making recovery worthwhile for the biogas operator, is where the work begins.

For an anaerobic digestion operator, the value of digestate has to be weighed against the practicalities of storage, transport, treatment and finding suitable land for its use. Those arrangements affect how a site runs and what it can earn. They can also influence whether the business has room to grow.

At SEM, we are developing H2OPE to recover nutrients and return clean water from digestate.

Every site has a different starting point

The first conversation should be about the site.

How much digestate does it produce? What does it contain? Where does it go today, and what does managing it involve? Is storage becoming a constraint? Is there usable surplus heat?

An operator with nearby agricultural outlets may have very different priorities from one moving digestate over longer distances. A site planning to increase production may need to reconsider arrangements that have worked well until now.

These differences matter when deciding whether nutrient recovery makes sense although I would say that every operator should look to recover the nutrients regardless of location.

Depending on the site and commercial agreement, the benefits may include savings on digestate management or income from supplying usable surplus heat to us. We need to assess those opportunities against the operator’s current position and the requirements of the proposed system.

There needs to be a market for what we recover

H2OPE focuses on recovering nitrogen, phosphorus and potassium for use in fertiliser production.

Over the past 24 months, our testing at small and larger scales has shown nutrient recovery rates of up to 99%. Performance depends on the digestate being processed, the system configuration and operating conditions.

Recovering those nutrients gives us material to work with. Producing a fertiliser that buyers can use requires further attention to its composition, quality and practical performance.

A manufacturer or distributor needs to know what they will receive and how consistently it can be supplied. A farmer or agronomist needs to understand its nutrient value, suitability for the crop and how it should be applied.

That is why product testing and discussions with potential buyers need to happen alongside the engineering work. The incoming digestate affects the recovered material, while the intended agricultural use helps determine the final formulation.

The CiNURGi nutrient-recycling project in the Baltic Sea region offers a useful reference. Its recommendations address product safety, farmer knowledge and the conditions needed to develop markets for recycled fertilisers. It does not assess H2OPE, but it highlights questions that businesses working in this market need to answer.

Making it work on site

We have demonstrated H2OPE processing at throughputs of up to two tonnes per hour. Our first planned commercial plant is being designed to process 40,000 tonnes per year.

The planned annual capacity is a design specification and has not yet been demonstrated in operation. As we move H2OPE into commercialisation, customer contracts are progressing and procurement is underway for future deployment.

Throughput is only one consideration. The design also needs to account for feedstock variability, available space, existing infrastructure, storage, and how materials will move through the site.

The liquid output deserves as much attention as the recovered fertiliser. H2OPE incorporates filtration and treatment, with the treatment specification developed around the intended discharge, reuse or onward management route.

Our patented process also enhances the recovery of micronutrients from the liquid stream, capturing these alongside the main nutrients for use in fertiliser production. Testing helps establish the composition of the recovered material and how it can be used in the final product.

The required water quality and any permissions depend on the site, location and receiving environment. Establishing that route is an essential part of assessing the project.

Understanding the carbon benefit

There is also an opportunity to reduce emissions associated with fertiliser supply.

Our current modelling indicates a potential saving of 1.3–1.9 tonnes of CO₂ equivalent per tonne of finished fertiliser against the selected conventional fertiliser alternatives assessed.

This is an indicative modelled range. The result depends on the fertiliser formulation, energy and heat sources, transport distances, and the product being replaced. Any comparison needs to make clear which stages of production, transport and use it includes.

Reducing digestate storage may offer additional benefits, but those need to be assessed separately against the arrangements at each site.

For an operator considering H2OPE, the commercial and environmental assessments should help answer the same question: what would change for this business, and would the change be worthwhile?

Digestate already has agricultural value… our aim is to recover that value in a form that gives operators more options and meets the needs of our fertiliser customers.

If you operate an AD or biogas facility and would like to explore the opportunity for your site, feel free to reach out and speak to a member of our team.

Further reading: CiNURGi policy recommendations for nutrient recycling in the Baltic Sea region.