The two main sources of off-farm recycled N are sewage and anaerobically digested food waste. The availability of sewage sludge is primarily driven by the need to dispose of the material, the volume of which depends on, and varies with, the human population in a given area. As the processing considerations are the same as for the P in the sludge, these are discussed in the page on phosphorus. 

The availability of food waste, processed in an anaerobic digestion (AD) plant, is primarily driven by the growth, or not, of suitable processing systems. Given the current desire in many societies to reduce the volume of food waste that is disposed of in land fill sites, the ability to process food waste using AD will grow, and thus so will the availability of N from this source for use in crop nutrition.

Anaerobic digestion is the controlled biological decomposition of biodegradable materials (e.g. food waste, livestock manures or purpose-grown crops) in the absence of oxygen, releasing biogas that can be used as a source of heat and/or power. The process also produces a nutrient-rich digestate which is a potentially valuable source of crop available nitrogen together with phosphate and potash. Approximately 90% of the total N content of digestate derived from food waste is present in a readily available N form (i.e. ammonium-N), compared to c.65% for digestate derived from livestock manures and purpose grown crops, and 45% for a typical cattle slurry. The main stages of the AD microbial process are illustrated in simplified form in Figure 1.

Due to the constraints of organic residual materials referred to on the Recycling Overview page, it may be that AD evolves to be used as a pre-treatment process, as is incineration, rather than producing a final product. Digestates are normally mechanically dewatered (or should be), which results in a splitting of the N and P, and potentially K, leaving the N and K dissolved in the liquid phase and most of the P in the solid phase. The liquid can only be used locally to the AD plant, while the P containing solid residue is more economical to transport and allows more technical downstream processing options to be used.

Because of the inherent potential variability in the feedstock materials being used, and the potential for pathogens to be transmitted, quality assurance (QA) is particularly important for AD processes, and many countries have developed QA regulations and regimes. At present digestate is used by being applied directly to adjacent land, due to high transport costs. Processes and systems to incorporate this material into mainstream fertiliser products have not yet been developed.

References

Hobson, P.N. and Richardson, A.J. (1985). The microbiology of anaerobic digestion. In ‘Anaerobic Digestion of Farm Wastes’ Eds. B.F. Pain and R.Q. Hepherd. NIRD Technical Bulletin 7, pp 15-28. ISBN 0-70840-320-4.

Links to related IFS Proceedings

409, (1998), Agricultural Use of Biosolids (Sewage Sludge), T D Evans

632, (2008), Anaerobic Digestion of Farm Manures and Other Products for Energy Recovery and Nutrient Recycling, J Morgan, B F Pain

790, (2016), Crop Available Nitrogen Supply from Food-based Digestate, A Bhogal, F Nicholson, M Taylor, A Rollett and J R Williams

Links to external resources

WRAP (2008). Anaerobic digestate. A technical report for the production and use of quality outputs from anaerobic digestion of source-segregated biodegradable waste.