Anaerobic digestion (AD) may sound like something that belongs in the darker recesses of a school chemistry lab, but it is widely believed to be one of the few truly green forms of renewable energy.
The principle is simple. You take unwanted materials – such as slurry and food waste – and ferment them in a large tank. At the end of the process you get methane (which fuels a generator to produce electricity) and a low-odour by-product that can replace fertiliser. But putting all that theory into practice is surprisingly tricky. And doing it on a big scale calls for skill, determination and sheer pioneering spirit.
Which makes Bedfordia Farms’ achievement in building not one but two large-scale AD plants no mean feat. This Bedfordshire farming company, with its 2200ha (5500 acres) of arable land and 23,000-strong pig enterprise (so plenty of slurry to deal with), could have carried on simply farming. But chairman John Ibbett had a truly green vision. The farm would take 12,000t/year of pig slurry and mix it with 30,000t/year of food waste from local households, businesses and food manufacturers.
This would generate electricity from two different waste products, cut the amount of food going to landfill (where it bleeds copious amounts of methane to the atmosphere) and replace 250t/year of oil-based fertiliser.
The first Bedfordia AD plant used a lot of German technology (believed to be Europe’s leader in AD) and was finished in 2006. But as soon as they finished it, they realised they could build a better one more suited to the UK feedstock model.
With the help of UK AD expert Greenfinch (which they bought last year) the farm has recently finished what is probably the UK’s most advanced AD plant. And it was largely designed in-house using British know-how and equipment.
Getting the food waste (which varies from load to load) into a homogenous feedstock took a lot of thinking time, too. “It’s like working with a high-performance cow rather than a tractor,” says Mr Ibbett. “Dealing with food waste is difficult because it varies so much (baked beans one day, followed by Brussel sprouts the next, and raw meat the next). So it requires sophisticated monitoring, a high degree of management skill and daily fine tuning to make it perform.”
No one could accuse Bedfordia of half measures in any aspect of this project. The 34,000t/year of digestate (what is left at the end of the AD process) isn’t carted to the fields in an endless procession of tankers: it passes through a grid of 150mm (6in) pipes before being applied over 600ha (1480 acres) by an umbilical system.
The electricity produced at the plant is enough to power 2700 houses and the 1.6MW of heat produced by the generator engines is sufficient to heat a local leisure centre.
But Bedfordia isn’t just about generating power – it is putting a lot of effort into saving it, too. New buildings being built to accommodate the pig herd cut energy use by a remarkable 50%. Air for the buildings passes through an underground chamber first and because the underground temperature stays at a steady level, it warms the air in winter and cools it in summer.
There are other clever touches too. Sow plates (a sort of giant hot-water bottle) under the mother take the excess heat (sows like to be at 18C) and passes it to a similar plate that warms the piglets (who prefer 28C).
All this state-of-the-art design did not come cheap and Bedfordia had to dig deep into its coffers to finance it. It is a long-term investment, but with the UK saddled with 20m tonnes of food waste a year and a need for renewable energy, it is likely to be in strong demand – and will no doubt be a design beacon for others.
What the judges liked
- Impressively integrated approach that took in waste products and produced power and fertiliser
- Solved several environmental problems at once
- Learnt important technical lessons from the first plant and applied them to the second
- Owns/manages 2200ha (5500 acres) of mainly arable land and breeds and rears 23,000 pigs a year
- Stores 27,000 tonnes of grain
- First anaerobic digestion plant was completed in 2006, uses 12,000t of pig slurry and 30,000t/year food waste. Second 45,000t/year-capacity plant is generating electricity
- As well as using food waste that would otherwise go to landfill and generate methane, the plant produces a great source of green electricity and digestate, which is used as fertiliser