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. Whole digestates from three plants (two food-based and one manure-based) were analysed for a wide range of chemical, physical, biological and microbiological properties. The analyses showed that digestate contains valuable quantities of major plant nutrients (e.g. nitrogen, phosphate, potash and sulphur), which are all essential for plant growth. In particular, digestate was shown to be a valuable source of readily available N (c.80% of total N for the food-based digestates) that can be used to replace manufactured fertiliser N use, as well as useful amounts of organic matter. Notably, the digestate samples had a significant biochemical oxygen demand, indicating that care is needed when applying these materials to land in order to minimise any water pollution risks and to maximise their fertiliser value.
Key words: Anaerobic digestate, biochemical oxygen demand, digestate, fertiliser, nitrogen, plant nutrient, organic matter, soil conditioner.
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 100,000 tonnes fresh-weight), when compared with livestock manures (around 90 million tonnes), biosolids (3-4 million tonnes) or compost (2 million tonnes). This quantity is expected to increase to over 2 million tonnes (fresh-weight) in the next 10 years, 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 (EC, 1999) 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). Defra’s Vision Statement identifies anaerobic digestion as making “a significant and measurable contribution to climate change and wider environmental objectives….in diverting organic waste, especially food waste, from landfill” (Defra, 2009).
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 dramatic (3 to 4-fold) increases 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.
BSI PAS110 (BSI, 2010) for the processing and production of digestate, and the Quality Protocol for Anaerobic Digestate (WRAP/EA, 2009) have recently been published. PAS110 requires producer’s to undertake Hazard Analysis and Critical Control Point (HACCP) planning to ensure digestate meets minimum requirements for microbial pathogens, heavy metals, stability and physical contaminants, and is fit for purpose. The Quality Protocol sets out criteria for the recovery/production of quality digestate from source-segregated biodegradable waste (which includes compliance with PAS110). Adherence with the Quality Protocol means that though ‘waste’ feedstocks are used as an input to the process, the resultant digestate can be used as a product, and hence is not subject to Environmental Permitting Regulations (SI, 2010) when applied to land in England, Wales and Northern Ireland.