Where there's muck...

An innovative anaerobic digestion plant at Borders Estate offers a new use for farm manure and promises green heat and electricity to fuel the estate business.

Where there is farming, there is muck. And for many farm managers the ongoing disposal of tonnes of manure can be a headache. Although manure is a good fertiliser, the need to spread tonnes of wet material across farmland takes continual management.

However by using manure and waste straw to fuel an anaerobic digestion (AD) plant, this abundant by-product can be converted into green heat and electricity.

In 2016 the Estate commissioned a 200kW anaerobic plant built by German specialist firm Agrikomp. The groundworks were completed by local contractors Davidsons, whilst the control house and engine sheds were built by local builder Thorburns.

Now fully operational, the green energy plant on Carterhaugh Farm produces enough methane to generate 200kW of electricity around the clock, plus sufficient heat to dry woodchip and grain on the farm.

“It is easiest to think of the plant as a concrete cow,” explains Farm Manager Sion Williams.  “The anaerobic digestion process mirrors what occurs in nature. Anaerobic digestion is the decomposition of organic material in an oxygen-free environment which produces methane – a natural process that occurs in the stomach of cows.”

But whereas a cow feeds on grass and silage, the AD plant consumes up to 12 tonnes of manure per day. All of this will come from farms managed in-hand on the estate. The majority will be from cattle, with a smaller proportion from hens and sheep.

The system comprises two main tanks – a 16 metre diameter primary digester and a 24 metre secondary digester – together enough to hold more than four million litres of slurry. Both tanks are six metres high, although only the top third will be visible.

In the first tank, the slurry is heated to 50 degrees centigrade – in order to trigger the AD process – and this is where the majority of gas is produced. After 60 days, the part-digested fuel passes to the second digester, where it spends a further 100 days.

“This gives the system sufficient time to break down the straw which we deliberately add at the outset to aid the AD process,” Sion says.

And of course what goes in must come out. “The end-product is pasteurised, odour free slurry which actually has a better nutrient content than the original manure,” Sion explains. “This can then be used as fertiliser, or can be dried and mixed with straw to provide hygienic bedding for livestock.”

But the real big plus is the energy. The new AD plant provides enough methane to drive a 200kW gas-powered generator 24 hours a day throughout the year. The green electricity attracts a feed-in tariff (FIT) provided by the UK Government which will deliver a valuable stream of income for 20 years.

“We use a small amount of the electricity to power the plant and the rest is all sold on to the GB grid,” Sion says.

The gas engine also produces a significant amount of heat and none of this goes to waste either. Around half goes to warm the AD digesters with the remainder used in the farm’s drying floor, which is geared up to dry woodchip, grain or the pasteurised by-product of the AD process.

“To me the AD plant makes perfect sense,” Sion says. “It fits in perfectly with our existing livestock operation, it will reduce our reliance on bought-in straw for bedding and will provide a stable long-term income stream for the estate.

“The plant also helps us reduce the environmental impact of the farm and will make a decent contribution to Scotland’s low carbon goals,” Sion concludes.

AD plant case study.