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Have you thought about the digestate?

During the past two years, the anaerobic digestion (AD) debate has been running long and hard. Much has been made of the benefits of using AD to process organic wastes, and there is little doubt it is one of the most carbon-positive solutions available.

The financing of AD plants is dominating the current conversation, with the level of Renewable Obligation Certificates and Feed-In Tariffs central to the debate. Following the ‘credit crunch’, financing represents one of the greatest hurdles that are preventing the Government’s aspirations from materialising. But those already operating plants will say that securing funding is only the start of the process: managing the digester and its outputs has its own set of challenges.

Management of digestate, the final output of the AD process, is a topic rarely discussed yet it poses one of the biggest challenges for the operator. Without a secure and sustainable outlet for the product, an AD plant will soon be full to the brim and will have to halt operations. Many people do not appreciate the sheer scale and involvement required to recycle digestate in a successful and sustainable way.

As any dairy farmer will tell you, slurry manage-ment requires significant resource and land space. With a medium-sized AD plant producing three times the volume of digestate as a large dairy herd would produce slurry, the resource requirement only increases.

Similarly to animal slurries, the application of digestate to land is heavily regulated. There are strict application rates, and with much of the country classed as a ‘nitrate-vulnerable zone’, there are seasonal restrictions that even prevent digestate from being spread at all during some winter months. This restriction aims to prevent liquid digestate from being applied, leaching into water courses and increasing nitrogen to unacceptable levels.

But the AD process cannot simply be turned off during the winter; large volumes require storage for up to six months. The alternative is to pass the digestate through a dewatering process to produce a solid ‘cake’ fertiliser and a dirty leachate-type water that needs further treatment. If this water is further processed to a condition where it can be discharged to a watercourse, most of the carbon benefits of AD are lost because of the intensive energy consumption of the process.

Unlike compost that can be strategically stockpiled all year round close to the end market, digestate needs to be stored in tanks or lagoons and transported to its market when required. Typically, this is just after harvest and could involve hundreds of vehicle movements during just a few weeks of the year. This presents not just logistical challenges but also planning challenges, with the risk of testing the patience of the local population for a few weeks each year.

The differences with compost do not stop as the product arrives at its end market. Compost is easily stockpiled and is simple for a farmer to apply using a standard muckspreader. But digestate should be applied ideally through injection or drop-hose booms. Injection reduces leaching and odour during application. Few farmers own this type of capital- intensive equipment and so specialist contractors may be needed to help apply the product. The complexities of application shrink the accessible market.

Nevertheless, digestate is a valuable product that is likely to increase in value as fertiliser prices rise and phosphorus resources run low. The agricultural community may be familiar with some of the operational challenges that digestate presents, but many fresh from the waste industry might not have yet appreciated what is in store.

Harry Waters is commercial director at Agrivert

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