The off-farm recycling of nutrients back into crop nutrition has been historically modest. (NOTE: as FertInform has been developed for use by those involved with the production of fertilisers, on-farm recycling of nutrients in manure and slurry is not covered in this section.) However it has been growing in recent years, and will continue to do so as environmental protection and supply security considerations drive increasing interest and development activity. There is considerable debate around the desirability, benefits and practicalities of off-farm nutrient recycling, which is ongoing. The focus of this information resource, however, is purely on the existing techniques, technologies and processes involved in handling recycled nutrients to produce a material suitable for use in crop nutrition. Many of these processes are currently at an early stage of development; this information resource will evolve as they do.
Of the three primary crop nutrients, potash is deliberately recycled on only a small scale, using K2SO4 from vinasses as a liquid fertiliser and K2SO4 from bio-gas plants as a solid fertiliser. Nitrogen (N) is off-farm recycled, primarily though the anaerobic digestion of food waste and in sewage sludge, the drivers being a desire to avoid resource wastage, reduce the need for land fill, the need to find productive uses for ‘waste’ materials, and to reduce the need for mineral fertilisers. More attention is being paid to the recycling of phosphorus (P), the drivers here being a desire to avoid resource wastage and find productive uses for ‘waste’ materials, the need to reduce the leakage of phosphate into ground and surface water, causing eutrophication, and the need to find alternative sources of P to rock phosphate, which is a finite resource.
Figures 1 (for N) and 2 (for P) (from Schroder, 2019) provide a numerical context for the current situation of recycled nutrient flows, using the European Union as an example. (Data on such flows can only be estimates, which can be variable. But they portray the overall situation.) They indicate that, in 2014, cropland in the EU received 11.2 M tonnes of N via mineral fertilisers, and 0.5 M tonnes (4.3% of the total of the two sources) from off-farm recycled materials. For P the figures were 1.2 M tonnes via mineral fertilisers, and 0.3 M tonnes (20% of the total of the two) from off-farm recycled materials. For both nutrients, 30-40% of the recycled nutrient came from sewage.
Recycling organic matter into modern agriculture is seen as valuable since soils are increasingly depleted in organic matter. The limiting factor in doing this is the content of N and P in the manure which limits the quantity of manure per ha that is allowed. A solution to this could be to separate organic and nutrient fractions, where the organic substrate can be applied on-farm in unlimited quantities, while the nutrients in the form of ammonium sulphate solution and Ca-phosphate or struvite are processed off-farm in industrial units.
For both N and P, off-farm recycling materials can be categorised into organic recyclates such as biosolids and digestates, and dry materials with a more concentrated nutrient content. This latter group of materials is at a less advanced stage of development. However, there are several disadvantages or constraints with the organic, bulk materials:
- The nutrient content of bulk organic residual materials can vary significantly, with wide variations around quoted average figures. While this characteristic is manageable for on-farm recycling, the situation is different for off-farm recycling, especially if the aim is to produce and market a fertiliser that is compliant with EU and national standards.
- Organic residues can contain pollutants, such as micro plastics, pharmaceutical residues and bacteria / viruses, and as such their use is becoming less acceptable.
- Organic matter is more difficult to handle and process off-farm because it is never fully stable.
- The high water content makes it costly to transport, per unit of nutrient. The impact of this is worsened as most of this material is produced either in rural areas of high livestock density, or in urban areas of high human density. In both cases, such areas are likely to be some distance from a) where processing plants are located and, b) where the nutrient hungry arable cropping areas are.
- The ‘production’ of these organic residuals material tends to be relatively continuous throughout the year. However, the usage of fertilisers is seasonal, with peaks in the autumn and spring. Consequently, the off-farm recycling of these organic materials would require significant long term storage facilities.
These characteristics may mean that the long-term future of recycling will be a combination of on-farm application of organic matter and off-farm recycling of nutrients. To achieve this on a significant scale will require the development of processes to produce materials that have been dried and have a more consistent nutrient content with low or no contaminants.
834, (2019), Circular Agriculture: Easier Said Than Done, J J Schroder
Links to related IFS Proceedings
219, (1983), Utilisation of Organic Wastes as a Fertiliser, J H Voorburg
222, (1983), Environmental Aspects of the Use of Organic Farm Wastes and Sewage Sludge, R J W Dight
293, (1990), Agro-Industrial Waste Composting and its Agricultural Significance, J M Lopez-Real
342, (1993), Opportunities and Constraints for Recycling Nutrients from Organic Wastes, G W Searle
372, (1995), Opportunities and Constraints in the Recycling of Nutrients, J Lammel, Prof. H Kirchmann
409, (1998), Agricultural Use of Biosolids (Sewage Sludge), T D Evans
432, (1999), Speciality Mineral and Organo-Mineral Fertilisers – Products and Markets, A Rainbow
623, (2008), Food, Fertilisers and Footprints – An Environmental Essay. 25th Francis New Memorial Lecture, C J Dawson
624, (2008), Relationships Between Nutrient Recycling, Environmental Impacts and Agricultural Production, J J Schröder, J F F P Bos
630, (2008), Resource or Waste: the Reality of Nutrient Recycling to Land, A E Johnston
632, (2008), Anaerobic Digestion of Farm Manures and Other Products for Energy Recovery and Nutrient Recycling, J Morgan, B F Pain
633, (2008), Sustainability Aspects of Biofuel Production, W J Corré, J G Conijn
634, (2008), Marine Algal Biomass for Energy Regeneration and Possible Phosphorus Recovery, W A Brandenburg, B L Smit, J J Neeteson
635, (2008), Nutrient and Carbon Recovery from Household and Food Biowastes, T D Evans
636, (2008), Industrial Symbiosis: Nationally Co-ordinated By-Product Use and Nutrient Recycling, M R Bailey, P D Jensen, H Hitchman, A Gadd
637, (2008), Policies to Encourage Integrated Nutrient Management and Recycling, Å E Sjöström
640, (2008), Non-Metallic Contaminants in Domestic Waste, Wastewater and Manures: Constraints to Agricultural Use, B Vinnerås, J Clemens, M Winker
641, (2008), Plant Nutrients from Organic Industrial By-Products, P L Graziano, R Calzavara, M Perelli, G Roccuzzo
688, (2011), Nutrient and Carbon Cycling in Agro-Ecosystems and their Interactions with Ecosystem Services. 27th Francis New Memorial Lecture, J J Neeteson
701, (2011), Role of Mineral Fertilisers in Optimising the Use Efficiency of Manure and Land, J J Schröder, P Sørensen
716, (2012), Agronomic Potential of Mineral Concentrate from Processed Manure as Fertiliser, G L Velthof, P Hoeksma, J J Schröder, P A I Ehlert, G Holshof, G Klop, J P Lesschen, W van Geel, J C van Middelkoop
790, (2016), Crop Available Nitrogen Supply from Food-based Digestate, A Bhogal, F Nicholson, M Taylor, A Rollett and J R Williams
846, (2020), Exploring Variations in the Demand for Fertilisers Derived from Recycling in North West Europe, R. Postma, I. Harms, N. Power, A. Egan, L. van Schöll
Links to external recourses
Peter Fisk Associates. (2019). Digestate and compost as fertilisers: Risk assessment and risk management options. Report commissioned by the European Commission.
WRAP (2008). Anaerobic digestate. A technical report for the production and use of quality outputs from anaerobic digestion of source-segregated biodegradable waste.
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