Digestate is a useful source of nutrients, and storage on-farm is an increasingly popular way of ensuring convenient supply, reports Jane Carley.
Contractor AWSM Recycling spreads 325,000 tonnes of digestate a year from AD plants in Yorkshire, aiming to find a land bank for application within 10 miles of each plant.
Adam Metcalfe takes charge of transport, storage and application, covering an area up to 70 miles from the business base near Richmond.
Storage of digestate is an ongoing challenge for plant operators and farmers, brought into sharp focus with the wet conditions this spring which delayed spreading.
Constructing suitable stores is a developing issue, as new regulations were brought in during 2017 which place stringent requirements on the type of store and the licences required to build and utilise it.
Recently, the Government’s Clean Air Strategy has proposed all slurry and digestate stores will need to be covered by 2027.
“Some plants will build six months’ storage on site, but the logistics of moving the material to the field where it is to be applied are complex, especially where there is the need to transport through built–up areas,” explains Mr Metcalfe.
“We use lorries which can carry greater loads and reduce the amount of traffic, but it is more practical to store on the farm where it is to be applied. Finding suitable storage sites is a service we now offer to operators as part of our package.
“Having satellite stores in place on-farm also makes a huge difference to the efficiency of applications, especially if a plant has movement hours restrictions. The lorries might be at a standstill by 5pm, but if the digestate is stored on-farm we can carry on spreading.”
The company offers a number of solutions through its AWSM Storage division.
“We can build lagoons, concrete tanks, metal tanks, but cost and permanency are issues,” he adds.
“A steel tank can cost £70,000, and you would be looking at 10 years for a return on investment. Many farms can’t plan that far ahead, or don’t wish to commit to using digestate for a long enough period.”
Lagoons are generally only justified for large volumes. He adds: “We built a couple of 6,000-tonne capacity lagoons, – but it’s rare to have a parcel of land that can use 6,000 tonnes of digestate.
Generally you only need about 1,250 tonnes capacity, and you can’t afford to move digestate twice. We came to the conclusion it was better to have a number of smaller satellite tanks, and started looking for a solution.”
He came across the Silo System from the Netherlands, which comprises a range of tanks, lagoon liners and covers.
The Mesh Silo consists of a galvanised mesh exterior built onto tiles. Inside is a 1.5mm thick HDPE liner to protect the rubber bag, which, along with its floating cover is specified according to the materials to be stored. Filling and emptying is carried out at a single point either beneath the silo or over the edge.
“The top soil is stripped off the site, and 34 steel grid panels are bolted together before the plastic liner and rubber bag are inserted, offering 1,250 tonnes of storage,” Mr Metcalfe says.
There is no requirement for foundations or a concrete floor, making the tank quick and easy to build or remove, he adds.
For new AD plants, timescales and outputs for digestate can be uncertain, so the ‘semi-permanent’ tanks offer flexibility, he says.
“They can be built under ‘permitted development’ with just 28 days’ notice and meet the Environment Agency’s requirements for digestate storage,” he says.
“If the land use or farming policy changes, the tanks can easily be dismantled and rebuilt elsewhere.”
A further benefit is that as a ‘removable item’, asset finance, normally offered for mobile equipment, can be used to purchase the tanks.
For more details: www.awsmstorage.co.uk
Clean air strategy: consult.defra.gov.uk/environmental-quality/clean-air-strategy-consultation/
Digestate storage licencing: