Farmers reap benefits of managing crust on covered slurry stores

Current proposals under the clean air strategy state that all slurry stores must be covered by 2027.

The cost of covers and the crust that can form under them is often seen as a hindrance to the investment.

However, dairy farmer’s daughter and managing director of EnviroSystems, Liz Russell, believes it is a step in the right direction.

“This year has highlighted the benefits of covers as they stop rainwater from getting in, which maximises capacity and has been essential due to the late spring. 

“This extra capacity means farmers can target the slurry when the crop needs it, and they will have a better fertiliser value as it hasn’t been diluted by rainwater.”

Initially sceptical

Mrs Russell was sceptical of slurry covers when they first came out. 

However, a project to research biological treatments to improve slurry and remove ammonia emissions soon revealed limitations.

She received a grant of £250,000 from Innovate UK for the research.

Proof-of-concept studies explored a biochemical pathway which actively removed ammonia from the slurry.

While these were successful, when the study was scaled up to a practical and realistic size by the team, it showed up limitations, she said.

“However, this allowed the project to identify areas where emissions could be indirectly reduced, and slurry management improved,” she added. 

Redeveloping an existing product

The team at EnviroSystems and Myerscough College redeveloped an existing slurry inoculant product.

The product helps to reduce crust formation in slurry when stored under covers and, therefore, encourages the use of slurry covers to reduce ammonia emissions.

“That research highlighted that the only sustainable way to manage emissions from lagoons is to cover them. 

“However, management of the slurry under covered lagoons is vital, as crust issues are a real problem without mechanical agitation. 

“This can cause difficulties with slurry handling by blocking up pumping equipment and reducing storage capacity,” she added.

Challenges of covering slurry

Dairy beef finisher Will Hunt from Lydney, Gloucestershire, encountered crust formation in his 22,000 m3 lined lagoon when he covered it with a gas membrane two years ago.

Before the lagoon was covered, it was agitated using mechanical propellors, which were then changed to liquid mixers when the lagoon was covered. 

Liquid mixers work by sucking the liquid out of the lagoon on one end and pumping it back into the other end to try and agitate it. 

However, for Mr Hunt, the liquid mixer wasn’t enough to keep the crust at bay.

It was only by luck that he had to cut the cover when installing a new piece of apparatus that he became aware of the actual extent of the crusting problem.

“Covering a lagoon makes it impossible for farmers to know what is going on underneath the cover. 

“We couldn’t believe it when we saw a crust over 1.5m deep in some areas. 

“That got us looking at options to get on top of the issue,” he explained.

Expensive and damaging

Not only does a crust eat up valuable space in the lagoon, but if it is not removed, it can result in a very costly exercise for a farmer by cutting the cover fully open and employing a digger to extract the crust. 

“This would be very expensive and could potentially cause damage to the lining of the lagoon. It was something we were keen to avoid”, he said. 

Mr Hunt’s lagoon contains digestate from the family’s 1.5-megawatt biogas plant, fed with manure from the farm’s 1,500-head dairy beef finishing herd, crops, and some poultry manure from neighbouring farms. 

The dry matter of the digestate is slightly higher than that of pure slurry, up at about 7%.

New solution

Keen to get on top of the problem and fearing his whole lagoon, which at the time was 75% full, could turn to crust, he approached Liz Russell at EnviroSystems after hearing about their new slurry inoculant, SlurryBugs 2024. 

The new version of SlurryBugs was launched at DairyTech this year after two years of extensive research. 

It contains specialised strains of bacteria and fungi that have been found to reduce crust formation by 29%.

READ MORE: Are you ready for six months of slurry storage?

Treating a covered slurry lagoon

Mr Hunt worked with EnviroSystems to calculate the amount of product that was needed. 

The lagoon was treated to its full capacity with 44 x 1.5-kilo bags. 

The SlurryBugs were mixed evenly into 5 x 100-litre containers of warm water to form a solution and left for about 30 minutes for the bugs to ‘come alive’.

Once bubbles started appearing, the product was added to the lagoon through the hole in the cover and mixed in using the liquid pumping system to get an even spread. 

After only three weeks, there was already some evidence the bugs were working, with bubbles appearing on the surface of the lagoon and the crust breaking down. 

“Where we did have the visual assessment through the temporary cut in the cover, we could see activity with bubbles appearing on the surface and the crust cracking.

“We have now repaired the cut in the cover and left the bugs to work their magic under it to break down the crust”, he explained. 

Important to consider

Mr Hunt added: “It’s important that farmers consider how they will manage the crust when they invest in covering their lagoons. 

“SlurryBugs has helped the problem and we are closely monitoring this as we continue to spread. 

“Due to the wet spring, we still have 70% of the lagoon to spread and 80% of our maize to drill therefore the benefits of the slurry treatment will continue throughout the summer.”

In addition, Mr Hunt said his investment in the cover has been worthwhile because keeping the rainwater out has improved the quality of the slurry and capacity. 

“We have also seen an early positive effect on gas production which is now being circulated back into our AD system for energy and heat production.”

How does it work?

Explaining how the product works, Dr David Townsend at EnviroSystems said: 

“The five strains of bacteria and fungi contained within SlurryBugs are unique to our product and have been found to work most effectively on maximising the complex lignocellulosic biochemical pathways within slurry stores. 

“The bacteria secrete enzymes which break up the undigested fibre (cellulose and other plant matter), releasing the soluble nutrients and making them available for spreading onto land, producing a more homogenous liquid slurry. 

“Our research has also found increases in each of the following nutrients in the treated slurry – Ammonium N +21%, Phosphorus +48%, Potassium +17%, Sulphur +31%. 

“This is because organic materials are released into the slurry when the crust is broken down.

“Not only does our product make slurry handling easier, but it also reduces the need for bought-in fertiliser, helping farms to save costs and meet sustainability requirements,” he concluded.

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