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Anaerobic digestion aid herd expansion


A substantial investment in an on-farm anaerobic digester has helped to justify a Wiltshire dairy herd’s expansion from 150 to 500 milking cows. Ann Hardy reports.

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When Wiltshire farm manager Gavin Davies was faced with the prospect of updating the 150-head dairy herd he managed as part of West Stowell Farms, he knew the cow numbers could not justify a significant investment.


Confronting a situation familiar to many farmers, he says the dairy unit was outdated. There were difficulties with slurry, the parlour was too small and too old and the comfort of the modern cow was compromised in the dingy, narrow buildings.


Having arrived on the 1,315 hectare (3,250 acre) mixed estate close to Pewsey in 2007, he says it was not long before he realised radical change was needed.


Keen to retain the mixture of enterprises because they fulfilled his own and the estate trustees’ farming philosophy, he says they also wanted to retain and use the skill-set of the existing farm staff.


“We definitely felt confident there was a future in dairying,” he says. “But the 150-cow unit was not adequate for legislative and welfare standards, and it just did not lend itself to being updated.”


Equally he adds, “It would be almost impossible to invest what was needed for that number of cows.”


Instead, Mr Davies turned his attention to a brownfield area on the farm, which he describes as having a ‘mish-mash of asbestos and flapping tin buildings’ which were used for beef and youngstock, and set about planning a brand new facility for 500 milking cows.


The facility would be designed with aesthetics, functionality and cow comfort as priorities. Wherever possible, it would be constructed using local tradesmen.


Alongside the livestock part of the operation, an anaerobic digester and the associated 499ksW combined heat and power unit (CHP) would generate more than 4,000 megawatts of electricity each year - capable of supplying about 1,000 homes - and 3.55 million kWh of surplus heat.


Despite the strong environmental credentials, the plan was driven by profitability, which Mr Davies says had to be his priority.


“My brief is to run a successful business and that comes down to profitability,” he says. “For the AD unit, that was very much the case and I am confident it will fulfil this objective.”


This confidence came in spite of the £7 million to be invested in the unit, £2.5m of which would be for the AD operation and all of which would be financed by the bank.


“We opted against using any capital grants for the installation because of the restrictions that would have applied,” he says.


Planning objections to a proposal of this scale and nature were inevitable, and Mr Davies says overcoming these involved addressing each concern which was raised and working with planners in a sensitive way.


“The major opposition was not really planning-based, but more biased towards objections to the system of farming,” he says, remarking the high yielding group would be mostly housed, with only lows and far-off dry cows given access to grazing.


“But there was a lot of misinformation,” he says. “One person even produced a report stating there would be 500 traffic movements a week - something it would have been impossible to generate.”


After a series of local meetings and some continued opposition, he says: “We were fortunate the planners saw the bigger picture and thought the objections were unfounded.”


Demolition of the old buildings began in September 2010 and the two main open-sided buildings - totalling 8,440sq.m and positioned side by side to ‘reduce the mass of development’ - began to be constructed.


These would be separated by a 10m feed passage sheltered by oversails, and would feature ‘fair faced’ brickwork, brick quoins and extensive timber boarding.


Included inside would be 402 cubicles, straw-bedded yards, a 38-point internal rotary parlour, a public viewing platform and an education room.


“We wanted to host groups to help increase knowledge about both anaerobic digestion and dairy farming in general,” he says, adding that school, youth and various interested groups and Open Farm Sundays have been hosted on the farm.


Mr Davies says: “I am proud of the high standards we produce under, whether it be our animal welfare or care for the environment.


“I feel it is something to shout about and get the public out to see in action, so the next time they go into a supermarket they buy into this higher standard by choosing British.”


Stocking the unit precisely 12 months after demolition began, the original herd’s numbers were bolstered with maiden heifers acquired through herd dispersal sales.


Today, as the dairy unit celebrates two years of operation, cow numbers are up to 460 and will reach 500 before 2014.


Milk sold stands at about 10,000kg/cow with the prospect of that rising to 11,000kg on the strength of this year’s forage. The AD unit is running to capacity.


“But the CHP [which converts the biogas produced by the feeds into electricity] was not commissioned until September 2012,” says Mr

Davies, describing the series of delays which beset the installation.


“We lost time because some tasks, which fell between individual contractors, were overlooked, with parties blaming each other for the oversight,” he says. “This included things like the installation of meters and physical connection to the grid.


“When you are used to running a business where you think on your feet and move when conditions allow, it is frustrating when you can’t,” he says.


“I am a lot wiser after the event, but if I were to do this again, I would have someone permanently kicking the backside of the network,” he says.


“We had a project manager on the construction side, but I would also include a role for someone to act as a go-between to pull all the companies together and ensure there was a working timetable that was adhered to.”


But with all obstacles overcome and the unit now having delivered power to the national grid for one year, Mr Davies admits to a deep satisfaction.


“We budgeted for the AD plant to run at 90 per cent efficiency, and with the exception of two months we have been in the very high 90s,” he says.


“It is never possible to get 100 per cent because of maintenance and breakdowns, but it did not surprise me to achieve this level of efficiency as I had seen units running at these levels in Germany.”


Now supplying 85 per cent of its production to the national grid, he is paid the Government’s Feed-in Tariff of 13.5-14.5p/kWh for all electricity generated, including what is used on-farm, and a further 3.8-5p/kWh for the portion exported to the grid.


“I am happy with its output efficiency,” he says, as he prepares for the next phase of development and the connection of 1.5km of pipework that runs between the farm, two schools and the leisure centre in Pewsey which will use surplus heat from the CHP.


“The plant stands up on the generation of electricity alone, but if we can combine that with using the heat, then it is a double winner,” he says.


With the AD plant running at capacity, it uses about 18 tonnes of maize and four tonnes of grass silage every day, and takes slurry from the dairy.


“The slurry is great for the digester’s health, but has a lower output than the silages, accounting for roughly 60 per cent of the input, but just 15-20 per cent of the biogas output,” he says.


But as an outlet for the farm’s waste, the plant serves an invaluable function, constantly accepting the slurry through gravity-flow channels, dispensing with the need for a slurry lagoon, and mixing the waste with the other feeds before sending it for biodigestion.


The resulting by-product of both liquid and solid digestate are useful farm fertilisers. The solid is taken to fields on an almost daily basis for storage and subsequent spreading and the liquid is stored in a tank, from which it is taken by umbilical system or tankers for spreading within NVZ rules and as conditions allow.


Mr Davies says: “It has reduced our fertiliser bill by £150,000, although the cost of application is significantly more than it was before,” says Mr Davies.


The ongoing benefits of the system continue to come to light, but so far are thought to include:

  • Better condition of crops and the good condition of soil
  • The chance to grow a catch crop of stubble turnips and short-term Westerwolds and Italian ryegrass between crops of cereals and maize
  • Generally more sustainable arable rotations, with little need for second cereals.

The dairy and AD unit have also created three new jobs and great deal of hope for the future.


“We’re very optimistic,” says Mr Davies. “There are frequent swings in all directions when it comes to the price of milk, but underlying this is the positive feeling that demand for dairy will increase worldwide as diets become more western.


“Of course, like any dairy farmer, I have my fingers crossed for milk prices going forward, but at least with the AD unit, we know the future’s secure.”

West Stowell Farm Facts

  • New 500-head dairy unit incorporating anaerobic digestion
  • Total acreage 1,315ha (3,250 acres)
  • Includes 769ha (1,900 acres) of arable and the remainder mostly grass
  • 260ha (643 acres) maize is grown for cattle feed and bio-fuel
  • 1,400 ewes graze permanent pasture and downland under Higher Level Stewardship
  • The farm has 14,000 tonnes of silage storage capacity, 7,000t each for the dairy and digester
  • £7m has been invested in both the dairy and anaerobic digester plant
  • All has been bank-financed without use of grants for installation

The anaerobic digester and 499kWh CHP unit

  • Approx 7-10,000 tonnes of crops/year (freshweight) go into the digester
  • Requires around 170-200ha (450-500 acres) of land for a self-sufficient system 
  • The feedstock comprises maize (18t/day), grass (4t/d) and a little wholecrop rye
  • All feedstock originates on the farm to minimise traffic and as a condition of planning
  • Surplus heat is used on site for the new-build farm house
  • The remainder will heat two Pewsey schools and the village’s leisure centre
  • 13.5-14.5p/kWh Feed-in tariff is guaranteed and index-linked for 20 years
  • All electricity generated (including that used) receives the FIT
  • The AD unit will pay back the investment within 7-9 years
  • Estimated lifespan is 25 years (some components are less)
  • CHP lifespan (cost 400,000 Euros) is around 7-8 years
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