Granular Fertiliser from AD Liquor

Anaerobic digestion (AD) is rapidly expanding worldwide due its numerous benefits covering both sustainability and energy generation. Reports have indicated that waste-to-energy facilities could produce enough electricity and heat to power a large amount of the UK's renewable energy. Looking beyond the UK, the European biological waste to energy market alone is estimated to reach $3.6 billion in 2016 highlighting the increasing value of this industry.

This use of AD for waste treatment and energy production is expected to increase in the UK (and globally), and there is a serious problem of disposing the current volume of waste produced or liquid effluent called digestate. The digestate has a negative impact on the sustainability of AD since it costs money to treat before being discharged into natural water bodies or disposed of via wastewater treatment plants. AD processes produce a large amount of waste liquor, which is commonly used as a raw liquid fertiliser on land, with associated problems of run-off, leaching and eutrophication of water courses. In addition, there are other problems linked to storage of the digestate and this is further compounded by regulations which govern storage as well as the direct application of the digestate onto fields. It has been forecasted that this issue of disposing the increasing amounts of anaerobic digestion waste produced is likely to worsen with the increased capacity in AD technology forecast. These simple disposal techniques are unsustainable, placing further pressure on the industry and diminishing its green credentials.

Researchers at Queen’s University, Belfast (QUB) have developed methods to convert the waste liquor from anaerobic digestion (AD) into an organic granular fertiliser. The nutrients contained within the AD waste liquor are concentrated onto solids to higher levels which can be stored and transported easily, and later used as a fertiliser. This could provide an alternative to the energy intensive, expensive synthetic fertilisers. The synthetic fertiliser represents a significant input cost to the international agricultural sector. The cost of synthetic fertiliser production is directly linked to fossil prices and these costs have almost doubled in the last two years due to high phosphate and natural gas prices. The world demand of fertiliser is expected to increase due to increase in the demand for food.

The technology has undergone proof of concept funding and the results were very optimistic. The next stage is to undergo a range of testing using larger volumes of AD liquor on a commercial and realistic scale. The team is in the process of scaling up the system to handle the potential volumes of AD liquor at an AD plant that would eventually be used in order to produce the fertiliser. Once this is completed the team will be set to develop the prototype device to bring to market. We hope to eventually have one of these devices fitted to every AD plant in the world.