A microbial additive developed for use in slurry has successfully increased the available nitrogen content of digestate and made it easier to handle at a Lincolnshire anaerobic digestion (AD) plant.

Digestate has always been seen as a valuable by-product of the digestion process, and soaring fertiliser prices mean it is being especially prized in recent months as a fertiliser replacer.

However, its value could be increased if more of the nitrogen was available to crops.

See also: How cover crops can help growers get more from digestate

With spring applications, Agrii national technical manager Tom Land is seeing about 55% of the nitrogen being available to the growing crop for digestate from AEL Biogas’ 1.2MW anaerobic digestion plant.

“So if we can increase the readily available part and get closer to bagged fertiliser, it may help mitigate some fertiliser costs,” he says.

Furthermore, AEL Biogas AD manager Mark Wallace says the liquid by-product can be difficult to handle.

AD plant

The nine-year-old plant on the disused East Kirkby RAF base complements the family firm’s (AE Lenton Group) two farms.

Maize and rye is grown to feed the digester, while the arable area benefits from the digestate.

Spent digestate is put through a separator, with the solids used as a soil conditioner on the organic farm near Boston.

The liquid is pumped into the lagoon, where it is stored until the spring, then used on the main farm after the nitrate vulnerable zone closed period.

The plant produces 18,000cu m of liquid a year.

AEL farm manager Richard Hubbert says one problem seen in recent years is the sediment settling out in the covered lagoon.

“We have been seeing pockets of solid matter in the lagoon.”

His concern is that it will have to be eventually dug out, as the plastic liner prevents the use of a stirrer.

The floating roof is an added complication, as removing the solids would involve having to cut the roof at great expense.

Another problem is the solids make it difficult to handle, slowing down pumping and occasionally bunging up the spreader.

So Mr Hubbert and Mr Wallace concluded that something had to be done.

Slurry additive

Origin Fertilisers believes it has the solution. Speciality sales manager Callum Norman points to a new slurry additive launched by Origin Fertilisers in the past year.

It is aimed at increasing the ammonia N content of slurry and reducing crusting on the top of stores, making it easier to pump.

Mr Land, who also does all the nutrient plans and fertiliser advice for AEL farms, says it’s known that slurry and digestate are similar in terms of nutrient content and being anaerobic.

Therefore, a trial was carried out at the Lincolnshire plant – the first time the product had been used in digestate.

Mr Norman explains that the Digest-it product contains a mix of food for existing microbes as well as dormant aerobic bacteria.

They feed on and break down the organic matter in slurry and use the ammonia gas as a source of nitrogen to grow, thus turning it into ammonium N.

The product works in slurry within 8-12 weeks, so the trial started on 6 December 2021 and ran over a 13-week period until February 2022.

The additive was applied at a rate of 20 litres/455cu m of liquid digestate, and the lagoon contents were sampled and tested every fortnight.

Results

Results (see “Economic value of extra nitrogen”) show that using the product resulted in a 20% increase in ammonia N in digestate that is available to crops, says Mr Land.

The total nitrogen content also fell from 4.3kg/cu m to 3.8kg/cu m, enabling higher application rates.

“It means you need 13% less land to dispose of the digestate,” says Mr Norman. This could prove valuable where land is limiting.

Looking at the value of the extra available nitrogen in the digestate, Mr Norman calculates that it equates to an extra £36.47/ha.

Mr Wallace says it is easier to handle, having noted that the fill time for the tanker has fallen. “The consistency was nearer to water and with virtually no solids.”

This is backed up by the test results revealing 29% less solids.

Mr Hubbert adds that this spring was the emptiest they’ve seen the lagoon, with just 15-20% remaining that could not be pumped out.

Hopefully, this will decrease with time with routine use of the additive.

In fact, Mr Norman says the benefits may be greater next time, as they plan to apply the additive earlier in October.

This will give it more time to work before spreading starts in spring 2023.

Want to know more about the new manure rules and how they affect digestate use? See A guide to the updated autumn manure spreading rules