Recycling is important for sustainability and the recycling of plant nutrients to agriculture from organic wastes is a long established practice and has been recognised as beneficial long before the Circular Economy was invented.
Phosphorous is a key plant nutrient and significant amounts are required in agriculture as fertiliser. Western Europe however has no natural mineral sources of P and relies on imports of natural inorganic phosphate. .. However P is found in most organic materials including wastes such as biosolids and manures, and hidden within food (including imported food and landed sea fish). These organic materials may contain quantities of phosphate which might supply the inorganic phosphate fertiliser requirements.
This paper explores the amounts, sources and fates of phosphate in the agricultural arena and how close we may be to fulfilling a circular economy for phosphate in the UK when this organic P resource is considered, and if not where focus might be put to move towards this aim. Overall the results suggest that the inputs and outputs of P into UK agriculture are roughly balanced, but that the inputs still include a large percentage of inorganic P fertiliser of about 60%. Therefore there is significant scope through recycling more P derived from agricultural products in order to move toward a more circular economy for P in agriculture.
Keywords: Phosphorous, phosphate, organic waste, circular economy, biosolids, fertiliser
The “Circular Economy” is a topical theme that may present a useful new angle in some areas. On the other hand a cynic may consider it re-branding of recycling with much taken from sustainable development. It has six key principles:
Applying the circular economy to biogenic organic matter is certainly on the agenda in the EU. Recent initiatives to incorporate organic fertilisers into the fertiliser regulations and develop of end of waste criteria for compost and digestate products derived from biodegradable organic wastes have however been put on hold, but are likely to be included within revised EU circular economy proposals (www.mrw.co.uk/news/relief-as-new-fertiliser-regulations-are-dropped-inbrussels/8680264.article).
Organic wastes are part of a carbon cycle between CO2 and complex organic matter, which includes an energy component, i.e. simply sunlight energy is used to make complex organic matter from CO2 and is effectively then stored in the complex organic matter which can then be released as the organic matter is oxidised back to CO2. . We may harness some of this renewable energy from burning organic matter, either directly by incineration or indirectly by anaerobic digestion and production of CH4. Therefore organic matter can have a clear linkage to the energy circular economy component.
The organic wastes will however contain other elements, which are vital nutrients that will be translocated associated with the organic matter in some but not all parts of the carbon cycle, for example the nutrients will obviously become disassociated from C on combustion to CO2. Nevertheless a circular economy type concept might be applied to embedded elements within organic matter in an agriculture-food-sewage/food waste-fertiliser cycle. Nitrogen fertiliser is applied in UK agriculture in huge quantities (about 1,060,000 t N as inorganic N in 2014, (Defra 2015). Significant amounts of N fertiliser also applied to agriculture from organic wastes, biosolids and manures.
Nitrogen fertilisers in the forms of NH3 and nitrates are biologically labile and may be lost through microbial transformation by nitrification of NH3 to soluble nitrate and then by denitrification of nitrates to nitrogen gas. Therefore a N fertiliser circular economy concept based on organic wastes as N source is unlikely to get close to a perfect closed circle as additions of inorganic N must surely always be required to replace such losses.
Phosphorous, however, is a stable element in its common forms, (usually inorganic or organic phosphate compounds), and is therefore potentially amenable to total capture and recycle. Phosphorous is an essential nutrient and P fertilisers are often added to agricultural soils, and, in the UK significant amounts of this is sourced from natural raw mineral phosphate rocks. However, in Western Europe there are no natural sources of inorganic phosphate minerals, and significant imported inorganic P fertiliser occurs. Additionally the world resources of high quality natural rock mineral phosphate is declining and that being used is of poor quality, e.g. contaminated with heavy metals such as Cd (FEI 2000).
Yet, hidden within food (including imported food and sea fish) sewage and other biowaste are huge quantities of P which might fill the requirements for importing P fertiliser if harnessed appropriately. Although the element of importing P hidden in our imported food may be questioned in considering a UK focused circular economy.
Phosphorous is partially soluble in the environment and together with N is a major concern for causing pollution in surface waters through eutrophication. Therefore there are aspects of the circular economy principle of ecosystem protection to be considered within a P circular economy.
Recycling of treated sewage sludge (biosolids) to agricultural land as a source of fertiliser is a long established practice, as is the practice of recycling animal manures. Processing of other organic materials, such as green, catering, food, and farm residues, via composting and aerobic digestion, has increased significantly in recent years reducing landfilling of many of these waste. Most of the compost and digestate products derived from these materials are also applied to agricultural land. There is consequently the potential to consider a P circular economy for the UK in a broader food, organic waste, sewage, manure, agriculture context.
This paper explores this concept and how close the UK might be to a P circular economy in agriculture and what might be key areas for improvement to concentrate on in order to close the circle. The approach taken is is to attempt to develop a mass balance of the major inputs and outputs of P to and from UK agriculture. It is useful however to discuss a simple model to illustrate some of the facets of P as a fertiliser
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