AD down on the farm

16th July 2012


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A unique anaerobic digestion system in Wales is producing a surfeit of renewable energy and a rich fertiliser. the environmentalist investigates

Richard Tomlinson and his extended family run two farms near Wrexham in North Wales. Combined, the two farms cover 1,100 acres, have 650 milking cows and sell four million litres of organic milk annually.

When Tomlinson wanted to grow his family’s organic farm business a few years ago, introducing an anaerobic digestion (AD) system that would strengthen the existing farming operation, by providing a source of energy and fertiliser, was the obvious solution.

It is not easy in today’s economy to run a lucrative dairy business, which is why many farmers choose to diversify. One option is developing an AD process to optimise productivity of the existing farm business and provide an alternative, non-food income stream, independent of supermarkets.

A key advantage of AD is that it does not require re-skilling. As Tomlinson points out: “If you understand cow nutrition you can already feed a digester.”

The formation of Fre-Energy by Tomlinson and his two co-founders – brother Jonathan and engineer Chris Morris – follows several years spent developing and improving the AD process. Fre-Energy is the only company in the world to offer patented de-gritting and foam alleviation technology as part of the AD process.

The company is about to undergo considerable expansion as commercial interest grows in its unique product, from farms and businesses both at home and overseas.

Conventional AD falls short

AD is a process whereby microorganisms break down biodegradable materials such as animal slurry and food waste in the absence of oxygen, producing a biogas that can be used to generate electricity and heat.

Instead of sending waste to landfill where it breaks down, releasing methane, a potent greenhouse gas, AD helps to tackle climate change by trapping the methane and using it as biogas. As an added bonus, the leftover digestate is a rich bio-fertiliser that can be put back on the land.

Identifying the right kind of AD process for the farm proved far from easy, however, as Tomlinson soon discovered that an appropriate model was not yet in existence. After a trip to Germany to look at the systems used there, Tomlinson decided that he wanted a waste digester capable of processing farm waste, to complement the core business of milk production, rather than growing plants to feed a large crop digester – essentially turning the farm into a rural power station.

One major problem with conventional anaerobic digesters is that they struggle to remove grit such as sand, ash and soil from the AD process. In an ideal world these materials would not be allowed to enter the AD process but in reality they inevitably do, as waste is not clean.

It also makes it impossible to load the AD with certain types of waste, such as chicken litter. Chicken litter is an excellent feed for AD because it has a high nutrition content – but it also has a high grit content. Furthermore, as the cows on Tomlinson’s farm bed down on ash, another gritty substance, any type of conventional AD system would not be able to cope with the cow slurry from his farm.

Another serious difficulty Tomlinson quickly identified is the build-up of heavy foam in most waste digesters. This can happen if the diet of the AD is changed significantly over a short period of time, not allowing the bacteria time to adjust, a common occurrence in digesters that process a high percentage of unpredictable food wastes. Foaming is extremely problematic for several reasons.

If the foam reaches the methane offtake pipe in the digester roof it will shut down the generator – if it reaches the generator, it will destroy it. Furthermore, digesters with gas storage above that also have inflatable fabric roofs can be seriously damaged by excessive foaming.

The Fre-Energy way

Tomlinson and his team set out to develop a new AD process that could overcome the grit and foaming problems and therefore cope with the on-farm materials that needed to be loaded into the system.

“Fortuitously, we discovered that some very old, local digesters had been built or designed by James Murcott – who already had ideas about designing a process that would deal with these issues,” says Tomlinson. Fre-Energy employed Murcott’s services as technical director to oversee the development of a prototype AD on Tomlinson’s farm.

Helped by a £45,000 innovation grant from the Welsh Assembly government, a digester was designed and built on Lodge Farm, with 1,000m3 capacity.

One of its unique design elements is the inclusion of a de-gritting arm that very slowly rotates and removes any grit from the waste material by depositing it in a grit trap.

Environmental scientist Denise Nicholls, a new member of the Fre-Energy team, explains why this design factor is crucial: “Most conventional digesters comprise a fully mixed system which involves using a lot of energy to continuously agitate the material inside the tank, to try and keep any grit from entering the system.

“In a Fre-Energy digester, reducing the agitation means a reduced parasitic load, allowing stratification to occur in the tank. This permits predominantly digested material to be selectively extracted.

“Reduced mixing encourages bacterial growth, improving digestion, resulting in more throughput and more gas, hence more output and profit.”

The Fre-Energy AD system is also designed to combat the foaming problem inherent in other AD processes. It has a solid, thermally insulated fibreglass roof and the system automatically shuts down the generator if foaming occurs, preventing any foam entering the methane gas offtake pipe. It then allows the foam to be removed from the gas space without the need to reduce the digestate level in the tank.

“The system is a waste disgester that can process crops and handle real-world organic wastes that contain ash, soil, grit, plastics and metal, whereas a conventional digester is a crop digester that can’t process waste,” explains Tomlinson. “This means that our AD can operate 24/7 as there is no more silting up and expensive down-time needed to empty the grit and sludge.”

A balanced diet

The de-gritting capability of Fre-Energy’s AD system means that it can function effectively on animal slurry alone, as part of a slurry management regime. When the environmentalist visited Fre-Energy, representatives from an American environmental regulatory body and the chicken farm industry in Maryland were also visiting Lodge Farm.

The Fre-Energy AD’s capacity to process chicken litter represents a major breakthrough for the chicken farmers, who collectively have signed an agreement with Fre-Energy to buy a minimum of 11 of its anaerobic digesters and up to 100 over the next three years.

Tomlinson recommends that the Fre-Energy AD be fed a diverse diet. As he says: “You don’t want to give the digester indigestion and so a stable, balanced diet is the best approach.”

The farm’s own digester is fed 100 tonnes of chicken litter over a seven-week period, as well as around five times that amount of cow slurry. There are also plans, following protracted negotiations with the local council, to import food waste from a Wrexham industrial estate, diverting it from landfill.

The big advantage of adding organic wastes such as crops, spoiled silage, grass or food processing waste is that, unlike animal slurry, they still have most of their energy available. Tomlinson explains that, tonne for tonne, organic wastes typically deliver between four and 10 times as much energy, depending on the material. And by bringing in other wastes, it is possible to create another revenue stream.

There is also the significant environmental benefit of the nutrients of these wastes being used instead of wasted in composting or landfill. “Potentially, farms using brought-in wastes through AD will negate the need to purchase expensive, unsustainable chemical fertilisers,” adds Tomlinson.

The digester itself requires no more than two hours of human labour a day spent loading the feedstock and simple maintenance of the various pumps and mixers. On the Fre-Energy digester, all serviceable parts are external, eliminating the need to enter the gas space to maintain any of the digester parts, making this system much safer than conventional digesters.

Energy security

Once the feedstock is agitated and heated to around 38°C in the huge sealed tank, the bacteria present break down the organic parts, releasing biogas comprising around 60% methane and 40% carbon dioxide. This gas is then used to generate renewable energy.

The outputs from the Lodge Farm plant are typically 160kW of electricity and 200kW of heat per hour. The electricity needed to power the engineering business, the Fre-Energy office and a large farmhouse is less than one-fifth of this. Around 60kW of the heat generated is used to heat the cow slurry and chicken litter in the digester, while the remainder heats the offices and farmhouse.

As well as ensuring energy security for the farmhouse and business, the renewable energy produced by Fre-Energy’s AD generates an additional income stream for the farm. The surplus electricity is exported to the National Grid (with sufficient excess to supply around 80 houses) and is eligible for renewables obligation certificates and feed-in tariffs.

In future, the heat energy generated and used on the farm will also be eligible for income under the renewable heat incentive, which will pay a fixed fee for every kilowatt of heat the AD system produces. Additional payments will be received for exporting any surplus heat. When Lodge Farm starts adding the food processing waste from the industrial estate to the feed mix, the potential for generating renewable energy will increase significantly.

Back to the land

The digestate produced by Fre-Energy’s AD caters for the 1,000 acres of farmland without the need to import other nutrients or soil conditioners. The farm’s business plans are consistently demonstrating a 20% return on investment without taking into account the uplift from the fertiliser value of the digestate over and above undigested slurries. Aside from the financial saving this “closed loop” process represents, the rich quality of this natural fertiliser brings with it a range of other benefits to the farm and the environment.

The digestate that comes out of the digester goes through a separator, and the liquid is stored in a lagoon from where it is spread onto the grassland. The solid digestate, which contains a higher proportion of phosphate and potash, is transported by road to land that is used for growing winter crops to feed the dairy herd. Trial work carried out by Bangor University has demonstrated that the BOD (biological oxygen demand) of both the solid and liquid products is reduced by up to 90%.

Another benefit is that any weed seeds that enter the digester are no longer viable when returned back to the land. And the digestate has around 80% less odour than straight cow slurry, thus making the spreading of manures a far more socially acceptable activity.

“The digestate also supports nitrogen-fixation in plants and so crop yield is increased,” adds Nicholls. “It is of such high grade that the fertility of the land has improved to the extent that the farmers have reduced the quantity of cereal imported to supplement the cattle’s diet. And nitrate run-off is virtually non-existent. All of these claims are supported by the data from the comparative university-led trials.”

Future plans

There is no shortage of interest in Fre-Energy’s AD system from a diverse range of businesses. In addition to the contract with the American chicken farmers, the company has also received several UK orders. The only reason it is not being taken up more rapidly is the lack of easily accessible funding from financial institutions, says Tomlinson.

Fre-Energy’s ground-breaking AD system not only brings with it extensive environmental benefits and additional income for the farm, the new business is also generating much-needed employment opportunities in the region.

“Future plans for Fre-Energy are to team up with one or several strategic civil engineering partners that banks and venture capitalists would be comfortable lending their money to,” says Tomlinson. “This would see the roll-out of this technology for on-farm or municipal waste AD expanding rapidly in the UK.”

“The potential for this AD system is phenomenal: carbon reduction, employment opportunity, investment, food security, the list is endless,” says Nicholls.

“Our land is a finite resource and we cannot afford to sacrifice food production for energy production. Organic wastes contain valuable energy and fertiliser that must be recycled. Anaerobic digestion is the most efficient way of recovering this valuable resource, and it’s a win-win solution.”


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