A farm in Gloucestershire has significantly reduced its monthly energy costs by integrating a farm-scale anaerobic digester with solar panels for electricity generation. This innovative approach at Court Farm, Kempley, not only yields electricity but also generates heat as a beneficial secondary product while enhancing the quality of slurry.
This investment, aimed at a seven-year return, complements several initiatives by Richard and Sarah Carter, who operate the farm alongside Sarah’s parents, Calvin and Rachele Pugh, to boost operational efficiency and ensure the welfare of their cows.
The family maintains a housed population of 260 pedigree Holsteins, which are milked by four robotic units installed in 2017, in addition to establishing 60 cubicles for a distinct group of first-lactation heifers. Enhancements in housing ventilation have been achieved through the installation of helicopter and boxed fans, and the replacement of automatic scrapers with two robotic slurry collectors has led to a cleaner herd.
Key information about Court Farm includes:
A herd comprising 260 pedigree Holsteins, with an in-house breeding program.
A diet that mixes grass and maize silage supplemented with commercial concentrate.
An average milk production of 12,800 litres per cow annually, with a butterfat content of 4.16% and protein at 3.43%.
Utilization of 4 Lely milking robots.
Operations spanning 100 hectares of grass and maize cultivation, in addition to 20 hectares of leased maize land.
The management of 150 beef cattle.
Slurry management
Regarding slurry management, the farm installed a 33kW Biolectric mono digester in 2019, designed to process solely slurry without the need for additional feedstock required by larger digesters.
Richard was captivated by the concept of converting waste into energy, leading to an improved end product, stating, “Seeing a headline claiming one could ‘Make money from sh*t’ was certainly eye-catching. The idea of generating energy from a by-product and enhancing its value was clearly advantageous.”
The digester seamlessly integrated into the farm’s existing slurry management system, necessitating the construction of an additional reception pit and the update of underground piping. The slurry undergoes collection in a reception pit and is then transferred to the digester multiple times a day.
To ensure efficient operation, the system uses a pressure sensor and computerized calculations to monitor and adjust the volume of slurry being processed. The processed slurry exits the system via gravity, regulated by a valve that opens for two minutes to allow outflow before the process repeats with additional slurry input.
Using a mechanical screw press, the slurry undergoes division into solid and liquid components. The solid fraction is repurposed for stall bedding, while the liquid is utilized as a natural fertilizer.
Before the adoption of this advanced system, the farm operated with a basic method for slurry separation. The desire for an upgraded system sparked the initial investment. Richard expressed a preference for using recycled manure solids (RMS) over sawdust for bedding to reduce expenses and enhance cow comfort by utilizing a freely available resource.
Slurry separation
The process of slurry separation is carried out daily, with the solid portion, constituting 85%, being recycled as RMS for use in the cattle housing. This recycled material is then reintroduced into the digester in a cyclical process, contributing to the digestion process due to its bioactive nature.
The cow mattresses are refreshed with a thin layer of RMS applied using a JCB equipped with a bedding dispenser attachment. The excess is accumulated and saved for use as a high-organic matter soil enhancer.
Richard notes that the processed liquid exhibits a lower dry matter content compared to untreated slurry. Analysis revealed that untreated slurry had a 10.2% dry matter content with a total nitrogen level of 4%, compared to the treated slurry’s 9.5% dry matter content and 4.3% total nitrogen level.
For grassland application, the slurry is distributed using a dribble bar connected to an umbilical system, aiming for a minimal application to allow for silage cutting within 28 days.
Expenditure and savings
Regarding financial aspects, the Biolectric system was acquired for £229,000, with the overall expenditure, including the separator, reaching £300,000. Richard states, “Together with our 50kW solar installation, this setup fulfills 85% of our power requirements.”
Richard aims for the digester to generate 20,000kW per month. Given its operational energy needs of about 3,000kW per month, this leaves 16,000-17,000kW of electricity available for use each month. This significantly offsets their potential monthly electricity bill of £4,500, covering approximately 85% of it.
Additionally, the operation of the digester’s engines generates excess heat, utilized for various heating needs around the farm, including frost protection for milking robot rooms, warming water for calf feed and tank cleaning, as well as heating the farmhouse.
The digester commenced operation just before the closure of Feed-in Tariff payments, securing a rate of 4.5p/kW for the energy produced over the next 20 years. An application for the Renewable Heat Incentive is currently in progress.
Maintenance
Maintenance routines require servicing the digester engines every 800 hours, translating to approximately 20-30 minutes of maintenance monthly, conducted by the farm’s own staff. Richard likens this to the maintenance needs of the robotic milking machines, emphasizing the importance of interest and basic mechanical and electronic skills for successful operation.
Post-Brexit and amidst the COVID-19 pandemic, sourcing parts for the digester has been challenging due to the necessity of importing them from Belgium under the service contract, which includes quarterly maintenance visits, breakdown support, and spare parts.
Richard adopts a practical stance towards potential issues, summarizing that the primary consequence of operational problems would be the need to purchase electricity.
Key performance metrics include:
Milk production from forage stands at 4,650 litres.
The farm harvests silage six times annually.
Perennial ryegrass leys show a persistency of five years.
The herd’s replacement rate is 26%.
