Self-sufficient system ensures the future of Scottish pig unit

IT was his entrepreneurial spirit which allowed Andrew Rennie to take a step into the unknown and turnaround the fortunes of his business. Erika Hay visits Aberdeenshire to meet the man behind the methane magic.

Pioneering Aberdeenshire farmer, Andrew Rennie is in a good mood. Having taken the step to diversify in 2006 - with very little external funding - and continued with much success, he has recently been awarded the Future Farmer of the Year award for his hard work and efforts.

Andrew has slashed his input costs by creating a self-sufficient system revolving around cereals, pigs and an on-farm anaerobic digester. The result is enough electricity is generated to take care of all the farm’s power needs as well as using digestate as a replacement for inorganic fertiliser, ultimately making the farm carbon neutral.

“I was shocked to win the award,” says Andrew, who was presented with £4,000 and a package of practical support to help further develop the business. “It is important in the currentclimate to drive your business forward and to make it more sustainable for the future.”

Andrew farms in partnership with his father John, and mother Monica, at Gask on the outskirts of Turriff. He describes the 263-hectare (650-acre) farm as a traditional, North East, mixed farm growing cereals and producing pork, but that is where any similarities end.

In 2006, after years of research and red tape, the family built the first anaerobic digester in Scotland and the second in Britain on their farm.

Fed with slurry from the pig unit and waste from a local abattoir and fish processing plant, the digester not only provides an income from electricity sales, but has completely done away with the need to buy fertiliser. The cycle is completed with all the cereals grown on the farm fed to the pigs.

Winter wheat, winter barley and oilseed rape are grown in equal measures (around 40ha (100 acres) of each), with the remainder of the farm down to spring barley. The rape is sold but all the cereals are dried and stored for feeding to the pigs. Any surplus is sold the following spring.

Pig farmers in Scotland have not had an easy time of it in the last few years, with production in the North East reduced by two-thirds, and Andrew is the first to admit that, if it was not for his self-sufficient system, he too would have been unable to maintain the pig unit.

“If I had to buy all my compound feed, I could not afford to have pigs,” he says.

The 280-strong hybrid breeding sows are on a straw-based system and an average of 10 piglets are weaned per sow. These are moved in batches through slatted sheds until they reach their finishing weight at 25 weeks old, when they are marketed in batches of 250 every fortnight through Scottish Pig Producers to Vion and Woodhead Brothers.

Farm facts

  • Average yields on the heavy, loamy clay are four tonnes for wheat, three tonnes for winter and spring barley and up to 1.75 tonnes for OSR
  • 280 hybrid breeding sows
  • All pigs are finished at around 79kg deadweight
  • Diversified in 2006 with the creation of an anaerobic digester (AD)
  • It was the first AD plant to be built in Scotland
  • It uses waste streams from the local abattoir and blends this with the pig slurry to create a rich fertiliser
  • The process produces 15,000 tonnes of digestate each year and offsets the carbon footprint by making 340kW/hour of renewable electric
  • The State Veterinary Service monitors the anaerobic operation on a regular basis

Breeding unit

The straw and dung from the breeding unit is ploughed in during the autumn and winter, while the slurry from the finishing unit is recycled through the anaerobic digester before being either injected into the soil or applied using an umbilical system and dribble bar.

“After the slurry has passed through the digester, it has exactly the same nutrients but is in a more liquid form, which doesn’t scorch the growing crop. The analysis is 4:1:1 but the nutrients are in a more readily available form for uptake by the plants.”

Five years into the project, they now just have to buy a few tonnes of fertiliser to spread on the steeper parts of some fields where they are unable to apply digestate. By using digestate in this way, they have found yields are maintained and in some cases even increased. The only significant input he buys for the land is lime.

The Rennie’s have been at Gask since 1988, but it was in 2002, when regulations for injecting slurry and abattoir waste became prohibitive, that John and Andrew began to look into processing alternatives.

A trip to Germany convinced them an anaerobic digester was the best option for their farm, but it took four years of patient dealing with Scottish Environment Protection Agency, environmental agencies and the State Veterinary Service before they could put their plan into action.

“It was a steep learning curve for all of us,” explains Andrew. “There was nothing like it in this country so all the legislation was in draft form and kept changing. But we got the go-ahead in 2005 and were in operation by early 2006.” There are now 55 such plants in the UK, mostly for processing by-products from supermarkets and households.

Biogas plants

While Andrew points out there are now around 6,000 biogas plants in Germany, many are used as commercial, electricity producing enterprises as opposed to being run hand-in-hand with a farming system like his.

“On the continent they grow maize to feed the plant. Every 1cu.m of maize yields 200 to 300cu.m of gas, while every 1cu.m of pig slurry yields only 20cu.m gas, but we have not taken land out of food production and we are offering a service to local businesses in taking their waste.”

The digester was purchased from and built by a German company, but there were also intake pits for the raw materials to consider, holding tanks for the resulting slurry and a whole new system of fertiliser spreading involved in the cost. The total investment was £2 million, helped by a 1 per cent diversification grant from the Scottish Government.

The raw material is pumped into three 100cu.m capacity underground reception pits which feed the 2,500cu.m digester every four hours. Newly introduced material, which has a dry matter of 15 to 18 per cent rises to the top of the digester during the heating process.

The slurry has to reach an optimum temperature of 38degC for the bacteria to work and it stays in the digester for 50 days before being drawn off the bottom of the tank at 2 per cent dry matter. This is not the end of the procedure for the slurry though as it has to be pasteurised by heating it to 70degC to kill pathogens and E.coli. The resulting liquid fertiliser is held in either a 5,000cu.m tank or a 7,500cu.m covered lagoon on the other side of the farm.

Methane and other gasses rise to the top of the digester and are drawn off.

“The higher the methane content the better,” says Andrew. “Around 67 per cent is about average and this is used to power the two engines, which in turn power a generator from where the electricity goes directly into the national grid at a rate of about 400kW per hour.” The main other gas produced is carbon dioxide which naturally dissipates.

About 90 per cent? of the electricity generated goes into the National Grid, the rest being used to meet the needs of the biogas unit itself.


The system is very efficient, with water circulating throughout the system, cooling the engine before being passed through a heat exchanger in the pasteurisation plant.

“The whole operation is fully computerised and needs only about half an hour each morning checking everything is working well.

“If anything does go wrong, it can take a long time for the good bacteria to build back up to the required levels as we discovered in 2007 when we got the balance of ingredients wrong and the plant took seven months to get up to full operating speed again.”

To counteract this, he now pays around £15,000 per year for a bio-chemistry contract for the plant which sees him sending a sample every fortnight to Germany to be analysed and checked for levels of good and bad bacteria.

At the time of installing the plant he was also offered a maintenance contract for the engines which he turned down as it was too expensive, however the biogas produced can be a very dirty gas and his annual spend on maintaining the engines can vary from £80,000 to £120,000.

Despite this however, he believes the plant will break even within 10 years, although he has no plans to develop or increase the capacity of the plant as it produces just enough fertiliser for his farm.

“We would have to buy more land or sell the fertiliser to make it worthwhile expanding in the future. The fertiliser is of good nutritional value but we have to apply a lot of it (between 40 and 60m3 per hectare, depending on the plant requirements).

“Based on today’s fertiliser prices, the nutritional value works out at £6 per m3 so it does not make sense to transport it long distances in a tanker.”

As well as the direct income from electricity which averages out at about 4.5p per kW there is also income from Renewable Obligation Certificates at about 8.8p per kW.

“We built this plant before the Scottish Government brought in its Feed-in Tariff for renewable energy, so we have these ROC’s which we can sell each month to the highest bidder. Generally the coal-fired power stations buy them to offset their carbon footprint.”

Another direct source of income is from the abattoir and fish processors who pay to get rid of their waste, however it is the indirect benefits which make the system fit so beautifully with the farming enterprise at Gask - the easy disposal of pig slurry and the resulting fertiliser.

Looking to the future, Andrew has already accounted for the recent £4,000 prize won from the award and plans to use the award to host an open day next summer to promote and explain his system.”The Scottish Government says there is a need for 100 new plants and we are hopeful an open day might encourage more farmers to invest in a bio-digester,” he says.

Readers' comments (2)

  • Anyone interested in entering the Future Farmer Award 2012 (Scotland) and following in Andrew's footsteps should visit

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  • Very pleased to find this well-written article about an excellent project. Contact information would be appreciated; we'd like to cover the story in the future in Scotland is now poised to advance renewable energy dramatically, and this pioneer certainly deserved praise, University back-up and funding support.

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