Chambers B1, Gale P2, Litterick A3, Longhurst P4, Taylor M1, Tompkins D5, Tyrrel S4, 1ADAS, 2AVHLA, 3Earthcare Technical, 4Cranfield University, 5WRAP(free)
Abstract This paper reports on the development of guidance for the application of BSI PAS110 digestates (biofertilisers) in agriculture. Evidence drawn from a comprehensive quantitative risk assessment of the potential for harm to crops, humans, animals and the wider environment was used to develop practical guidelines on the use of digestate. A matrix of differing crop types, aligned with existing agricultural guidance is presented, which takes account of the opportunities for hazard prevention from the feedstock source, pathway of application and point of exposure to sensitive receptors. Scenarios that considered the highest plausible combination of hazards arising from anaerobic digestion feedstocks, processing, land application rates and differing crop categories were used to prepare the guidance. The work provides practical, evidence-based guidance for the beneficial application and use of digestate as a sustainable nutrient source in agriculture.
Keywords Digestate, biofertiliser, best practice, recycling organic materials to land.
Introduction The recycling of organic materials to land is regarded as the best practicable environmental option in most circumstances, completing both natural nutrient and carbon cycles. Organic materials are valuable sources of major plant nutrients (i.e. nitrogen – N, phosphate – P2O5, potash – K2O and sulphur – SO3), which are essential for plant growth and therefore sustainable crop production. Organic materials also provide a valuable source of organic matter, which improves soil water holding capacity, workability and structural stability etc.
Anaerobic digestion (AD) involves the breakdown of biodegradable materials (such as household food waste, livestock slurry and waste from food processing plants) in the absence of oxygen. During the AD process methane is released that can be used to provide heat and power, and a digestate is produced. In the UK, the quantity of digestate (from source-segregated biodegradable materials) currently recycled to agricultural land is relatively small (around 1 million tonnes freshweight; WRAP, 2012a), when compared with livestock manures (around 90 million tonnes), biosolids (3-4 million tonnes) or compost (around 2 million tonnes). It is predicted that there could be up to 5 million tonnes of food-based digestate produced by 2020 (DECC/Defra, 2011), as the drive to remove organic materials from landfill increases and the need to generate gas/electricity from renewable sources grows. The EC Landfill Directive sets strict limits on the amount of biodegradable municipal waste that can be disposed of via landfill; the amounts must be reduced by 65% in 2020 compared with 1995 levels (EC, 1999).
In addition to the legislative drivers, there is increasing agricultural demand for organic materials (e.g. digestate, compost, etc.) as sources of crop available nutrients. This is largely due to recent volatility in the price and also availability of manufactured fertilisers, as a consequence of the growing demand for food as the world’s population continues to grow. As a result, farmers and growers are becoming increasingly interested in and reliant on organic materials to supply crop nutrient requirements. Additionally, as the importance placed on carbon footprinting increases, farmers and land managers will be encouraged to reduce the carbon footprint of their products, for example, through using organic materials to offset the use of manufactured fertilisers.