The 2023 World Biogas Expo was held in Birmingham last month and attendees included biogas plant owners, operators and developers from various parts of the world. There was a significant presence of Irish exhibitors and attendees.

One of the main focuses at the expo was on nutrient recovery, driven by high fertiliser prices, the necessity to decarbonise agriculture and food production, and concerns over excess nutrients on land.

As a result, many are now examining how digestate, the byproduct of anaerobic digestion (AD), can provide a solution.

Although digestate itself is a beneficial fertiliser, there are now various systems available on the market that can remove water, concentrate nutrients, split N, P, and K, and pelletise the digestate.

The aim is that AD operators will be able to produce an organic fertiliser comparable to that of chemical fertiliser, either in pelleted or liquid form, which can be exported and sold anywhere around the country.

While many of these systems are designed to work on AD plants, some can also serve as standalone units on large, intensive livestock farms.

UK-based Esmil Process Systems specialises in industrial effluent treatment systems and had a strong presence at the show.

The company claims that its containerised system can be installed on any AD plant and offers digestate treatment using state-of-the-art membrane separation and dewatering technology.

Esmil’s digestate treatment system consists of three processes: dewatering, vibrating membrane filtration, and reverse osmosis membrane filtration.

1. First, dewatering presses remove solids from the digestate, which can be dried and pelletised.

2. The remaining liquid is then filtered through vibrating membranes, which separates water, creating a nutrient-dense concentrated liquid fertiliser which can be used to fertilise crops.

3. Esmil’s final stage is to clean the extracted water through reverse osmosis membranes, which can either be recycled back into the system or discharged into a watercourse, the company states.

Esmil suggests that, typically, for every 1,000t of digestate produced, between 650t and 700t of clean water are recovered, which can be discharged safely into the watercourse.

Between 300t and 350t of concentrated liquid fertiliser are recovered, which may be either used directly, sold, or dried to produce a pelleted fertiliser.

US-based Livestock Water Recycling (LWR) is aiming to make inroads on the European market, with its solution for dealing with excess manure stored in lagoons at large livestock farms and digestate from AD plants. The patented LWR system uses both mechanical membrane and chemical treatments to segregate organic nutrients.

As manure or digestate flows through the system, solids are sequentially removed. This process results in the extraction of significant amounts of phosphorus, potassium, ammonia, and organic nitrogen by separating the solids and fine particles.

The solids can be dried and pelletised or spread directly on land.

The liquid fraction is then further processed to separate the water and nutrients using membranes and chemical treatments.

The process can extract up to 75% of the water from manure, producing clean, drinkable water, the company claims. About 17% of the total volume forms a concentrated nutrient liquor that can be used as a fertiliser or dried and pelletised.

Various systems use evaporation technology to treat digestate. One such system, which was developed by Tyrone-based Colloide Engineering Systems, was showcased at the Expo.

Its system treats digestate through a combination of vaporisation, reverse osmosis, chemical dosing and multiple membrane treatments.

The system extracts up to 95% of ammonia from unseparated digestate and up to 95% of phosphate from the liquid fraction of digestate.

Digestate typically has high levels of these compounds, which can limit the rate at which it can be applied on land. This can become a significant factor in determining the application rate, particularly in areas where soil indices are high.

The phosphate and concentrated liquid ammonia sulphate solution produced in the process can be used as a liquid fertiliser or exported and sold off site. The system requires excess heat from the AD plant, however, and there is currently a pilot plant in operation in Northern Ireland.

Netherlands-based Tema Process has developed a new solution to turn digestate into a granulated fertiliser, using fluidised bed drying technology.

The dryer uses surplus heat from the AD plant’s combined heat and power engine, or occasionally from a dedicated biogas boiler, and can dry a mixed stream of thickened, solid and concentrated digestate liquid. The end result is a dry material which can be pelletised. The pelletised digestate can be sold and exported off site.

The author Stephen Robb is currently involved in a family/community proposal for an anaerobic digestion facility in Co Donegal.