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  • Writer's pictureBryn Power

How we fertilise our fields, and why digestate is best

anaerobic digester

Farmers have always fertilised their land to ensure the soil is healthy and rich in the nutrients needed to grow crops. Dairy farmers like us would traditionally spread cattle slurry because it is full of the nutrients the ground needs and is a great way to recycle the cow manure generated by the herd.

Modern farming practices has seen farmers diversified their operations, and one very common way is to process cattle slurry through an anaerobic digester to generate electricity. This further processing of the slurry also creates a rich and nutritious biofertiliser called digestate. Digestate is widely used as a fertiliser and is an environmentally sustainable alternative to spreading raw slurry or manufactured compound fertilisers.

That further processing of the slurry, which is broken down by bacteria inside a seal system called an anaerobic digester, has the added effect of removing huge amounts of methane from the slurry and reducing the potency of the main odour producing compounds – hydrogen sulphide, amines and volatile organic acids. By carefully balancing the temperature and acidity in the digester, nitrogen rich ammonia is also converted to ammonium – also nitrogen rich but odourless and less volatile.

In 2016, our sister company Bryn Power invested £4.6 million in the construction of a state-of-the-art anaerobic digester (AD) at Gelliargwellt Uchaf. We now process all our liquid cattle slurry and around 15,000 tonnes of food waste from Caerphilly residents and other commercial customers through the AD, to generate sustainable electricity and digestate.

By processing food waste through the AD, this removes thousands of tonnes of methane from waste that would otherwise go to landfill. Methane is a ‘greenhouse gas’ and is around 31 times more harmful to the atmosphere than carbon dioxide. (Source: UNECE)

what are the methane level comparisons

Does the AD process smell? No, because from the moment the food waste is delivered to us and unloaded inside our negative-pressure reception building and until it is spread on the fields as digestate biofertiliser, the entire system is sealed. There is no “venting of gases” or “open cooling tanks”; it is a carefully managed and strictly regulated process.

Does digestate smell? Absolutely, but the anaerobic digestion process reduces the quantities of the main odour producing compounds found in slurry. Some people have observed that it smells different to cattle slurry, and they are right. In the same way that cow slurry, chicken droppings, pig slurry and indeed human poo all smell different, the bacteria in an anaerobic digester create a distinct odour when they digest the slurry and food fed to it.

Research conducted into the different odour profile of raw slurry, crusted slurry and digestate found digestate to be less potent and that the odour lingers for a shorter time. (Source: Centro Ricerche produzioni animal S.P.A, Italy and Agri-food & Biosciences Institute)

Is digestate just rotting food? No, far from it. Anaerobic digestion it is a multi-stage, biochemical process, and not dissimilar to what happens in a cow's stomach. The food waste and slurry – which together is called ‘feedstock’ – is not rotting, like in a compost heap, but rather it is literally digested as if by an animal. Indeed, our bacteria have regular checks by a vet to ensure they are healthy.

The whole process and the resulting digestate biofertiliser produced in the AD at Gelliargwellt Uchaf is inspected against strict criteria set by the British Standards Institute, and our digestate has gained PAS110 accreditation as a fertiliser product.

That is why we not only use it as part of our sector leading nitrate management plan for the farm, but we also supply neighbouring farms too. By recycling our cattle slurry and local food waste to create nutrient rich, digestate biofertiliser, the carbon footprint of farming in South Wales is being reduced significantly. Farms using digestate do not need to use as much or any compound fertilisers, which are made from petrochemicals, often in Eastern Europe and create a huge carbon footprint in their production and transportation.

odour concentration graph

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