The Anaerobic Digestion and Bioresources Association’s annual Northern Ireland (NI) conference took place this month in Belfast, where the major theme of the day was tackling the nutrient problems facing the region.

Nutrient problems in NI have gained a lot of attention in recent years due to the visible impact of algal blooms on Lough Neagh. In reality, however, this problem has been building for decades and agriculture is partly responsible.

Cue anaerobic digestion (AD). As explained in last week’s article, NI has been a leader in AD, having developed over 80 farm-based plants in a short space of time. The next step is to develop a biomethane industry, and NI’s unique nutrient challenges will be key in achieving this.

The scale of the challenge

Lough Neagh acts like a natural bowl for much of NI, with a catchment area of over 400km². Excess phosphorus and nitrates entering Lough Neagh, combined with higher temperatures and the spread of the invasive zebra mussels, have resulted in the green-blue algal blooms we are all familiar with. It is estimated that 62% of the phosphorus in NI’s waterways comes from agriculture, with much of the rest coming from homes and sewage treatment plants. For agriculture, the increased imports of concentrate feedstuffs and the consequent rise in the NI agricultural phosphorus surplus have resulted in higher losses to waterways. At present, approximately 40% of agricultural land has excess soil phosphorus and can be a significant source of diffused nutrient pollution. Increased rainfall and soil temperatures have also been factors.

Despite some improvements, a major reduction in the phosphorus surplus in the NI agricultural sector is needed to improve water quality, and may take decades.

Slurry separation

A potential solution is processing slurry so that the high-phosphorus solid fraction can be exported away from land that is already high in phosphorus. To demonstrate that this could be a viable solution, last year, the Department of Agriculture, Environment and Rural Affairs (DAERA) awarded £100,000 to six companies under phase one of the Small Business Research Initiative (SBRI), to develop models to economically separate nutrients by processing livestock slurry, particularly from cattle and pigs.

The next step of the initiative will be phase two, in which three of the initial six projects will be awarded £12m to demonstrate their solutions at scale. At the time of writing, the successful companies had not yet been announced.

Speaking at the conference, Jonathan McFerran, deputy director of DAERA, said that the results from phase two would see the separation of slurry at scale on hundreds of cattle and pig farms across NI, with the aim of removing 1,000t/year of agricultural phosphorus from the system. This will effectively remove one-sixth of the excess phosphorus in animal manures in the region.

AD potential from nutrients

A key aim of DAERA’s strategy to tackle excess phosphorus is to efficiently recycle the nutrients through AD plants. The solid fraction will be separated on the farm and transported via trailer of HGV to be used as feedstock in AD plants in NI and possibly in the Republic.

Richard O’Loughlin, biogas technical consultant with Vogelsang, spoke at the conference and explained that slurry separator technology is well established and can provide up to 20% extra storage capacity in tanks. The liquid fraction will remain on the farm to be spread. So far, from testing done by AD operators, the solid fraction appears to have a biogas yield similar to that of grass silage, but further analysis is needed.

As reported last week, the potential from NI’s housed livestock manure plus silage is significant, and could replace up to 90% of its distribution gas needs. This would require wide scale upgrading of existing AD plants, or the construction of new plants to produce biomethane. The SBRI phase two demonstration projects are set to provide a blueprint on how slurry separation and biomethane production can be developed in tandem at scale across NI.

Digestate management

However, just because the separated slurry is processed in an AD plant, doesn’t mean the nutrient problems go away as the phosphorus-rich digestate will still likely have to be land spread somewhere. Indeed, ammonia emissions from the use of digestate can also be higher when not handled correctly, creating more problems. Dr Gary Lyons, senior scientific officer at AFBI, spoke at the conference and said that digestate will likely need to be processed further to either reduce water content or recover, separate and concentrate nutrients. He said there are a number of biological, chemical and physical processing technologies available to do that. By upgrading digestate, the material will become easier and cheaper to transport, and will likely need to be transported to an area where it can be spread, either in NI, the Republic or further afield.

Once upgraded, the digestate may also have a sale value, but until then, digestate management will likely remain a cost for the AD operator, according to Jack Blakiston Houston, managing director of BH Estates, who spoke at the conference. As far as he was concerned, digestate processing was going to be an essential part of plant management going forward.

The challenge, however, is that the investment cost of digestate processing systems currently on the market cost millions. Without knowing the sale value of the upgraded product, securing finance for these processing systems will remain difficult, as pointed out by Richard Kennedy, CEO of Carbon AMS, in the audience.

Making the problem worse

DAERA Minister Andrew Muir also spoke at the conference and explained that while he is keen to support indigenous biomethane production, he needs to ensure it happens in a way that also protects water quality and biodiversity.

“The excess nutrient issue could obviously be exacerbated if increased biomethane production leads to the unsustainable management of digestate from AD plants,” he said. Minister Muir added that, as well as protecting the environment, he is keen that the agri-food sector helps to deliver affordable renewable energy in the form of biomethane and by doing so, support the agri-economy and our rural society.

In short