Chapter 6 summary
Opportunities to recycle phosphorus-rich organic materials
Chapter authors: Will J. Brownlie, Ruben Sakrabani, Geneviève S. Metson, Martin S.A. Blackwell, Bryan M. Spears • 10min read
Chapter highlights
Recycling phosphorus-rich organic wastes and manures is critical for phosphorus sustainability and a transition to a more circular economy for phosphorus. Beyond agronomic benefits, the win-wins are numerous, with benefits to society, environment, economy, and business growth. However, to significantly increase phosphorus recycling, education, awareness-raising, investment in technology and infrastructure, and policy support are urgently needed.
Introduction
Why we should increase the recycling of phosphorus-rich wastes and manures
Currently, the main input of phosphorus to the anthropogenic phosphorus cycle is phosphorus mined from phosphate rock. Around 85% of mined phosphorus is used for fertilisers applied to soils to grow crops, which are either consumed directly by humans or used to feed livestock. Significant phosphorus losses occur throughout the food production system. However, much of this ‘lost’ phosphorus remains on land. This phosphorus can be considered misplaced, and therefore potentially recyclable within the agricultural system, where it can be used to improve soil fertility and optimise crop yields. Here, we define phosphorus recycling as the use of phosphorus from waste streams (e.g. manure, biosolids, food wastes) in the production of food (e.g. crops and vegetables) and non-food agricultural products (e.g. fibre and timber). This definition highlights a key role phosphorus recycling plays in transitioning to a more circular economy, which is to decouple economic growth from the consumption of finite resources. Increasing the relative proportion of recycled versus mineral phosphorus (i.e. derived from phosphate rock) is essential to redress the global anthropogenic flow of phosphorus, which was identified as having passed its planetary boundary over a decade ago. Without implementing sustainable phosphorus management to reduce phosphorus losses and increase phosphorus recycling, environmental damage will continue to increase with potentially irreversible consequences.
The most direct method of phosphorus recycling is the application of manures and biosolids to cropland, in which phosphorus is returned to soils, where plants can assimilate it back into agricultural products. Alongside manures and slurry, many other phosphorus-rich waste streams can support phosphorus recycling, including crop residues, food processing wastes (including from aquaculture), abattoir wastes (especially bone meal which is high in phosphorus), domestic food wastes, sewage derived biosolids, and wastewaters. However, the recycling of phosphorus-rich organic materials does not come without challenges. Phosphorus-rich organic materials must be appropriately processed to enable storage (time of production and use often do not match), transport (such as reducing water content and ensuring the stability of nutrients), to avoid decomposition (odour, greenhouse gas emissions), ensure sanitary safety (pathogen elimination and removal of contaminants), and to supply reliable and consistent agronomic characteristics (e.g. good plant availability of phosphorus).
Nonetheless, beyond the agronomic benefits, the wins-wins of recycling phosphorus-rich organic materials are also numerous, with benefits to society, the environment, and the economy. Organic fertilisers, when available in sufficient quantities, provide beneficial soil organic matter that improves soil health, fertility, structure, and water retention capacity, and adds micronutrients essential for plant growth and for boosting the nutritional value of crops. Phosphorus-rich organic materials can also be used to make bioenergy, producing phosphorus-rich residues as a by-product. Biochar and biogas residues can be applied directly to soils and are effective slow-release phosphorus fertilisers under certain conditions. Furthermore, mineral and organic fertilisers can play complementary roles, with mineral fertilisers supplementing the nutrients provided by organic fertilisers with concentrated nutrients that are immediately available for plant uptake.
Increasing the recycling of phosphorus-rich organic materials will help to deliver on the objectives of multiple United Nations Sustainable Development Goals (SDGs), including poverty alleviation (SDG 1), zero hunger (SDG 2), water/sanitation (SDG 6), responsible consumption (SDG 12), life on water (SDG 14) and life on land (SDG 15). In the following section, we discuss the challenges and solutions for recycling phosphorus-rich wastes and manures.
Key issue 6.1
Organic wastes and residues are often treated as pollutants and not nutrient resources
The challenge
Organic materials are often managed as pollution rather than as a valuable nutrient resource. Consequently, improvements in the management of phosphorus-rich organic materials are necessary including collection and storage, processing, and application practices. Farmers and stakeholders may reject recycling some organic materials as fertilisers because of negative perceptions over the safety of their use in food production; these concerns must be overcome.
The solution
A paradigm shift in how we view phosphorus-rich waste streams is needed; from pollutant to valued nutrient resource. Key actions in delivering this shift include raising awareness of the costs of phosphorus losses and benefits of phosphorus recycling, providing education and extension services to encourage stakeholders to recycle phosphorus, and mobilising investment in infrastructure and technology to make phosphorus recycling safe, easy, and efficient.
Key issue 6.2
Manure and waste production is often ‘decoupled’ from croplands where it can be recycled
The challenge
In many regions, the distances between the production of phosphorus-rich organic materials and arable land are increasing, driven by the expansion of specialised and intensive farming, urbanisation, and globalised trade. This can make transporting such materials to areas where they can be recycled prohibitively expensive. Decoupling of livestock and arable farming systems is particularly problematic for farmers producing organic foods and feeds. This is because ‘conventional’ mineral phosphorus fertilisers, and in some cases manures from confined animal feeding operations, cannot be used to fertilise organic crops.
The solution
Landscape planning can integrate arable and livestock farming to maximise nutrient recycling. Whilst efforts should be made to ensure animal densities in livestock farming do not exceed nutrient needs, some farming systems must rely on disposal/utilisation contracts. Arable-livestock farming partnerships can support the exchange of crops, grains, and manures, and coordinate land-use to support more regionally closed feed-manure loops.
Key issue 6.3
The reliability of phosphorus-rich organic materials is often lower than mineral fertilisers
The challenge
The concentrations of phosphorus in organic materials are variable, not easy to determine quickly and lower than mineral phosphorus fertilisers, representing a challenge for farm-scale nutrient management. The bioavailability of phosphorus in organic materials varies and influences their performance as fertilisers, and can be affected by soil type, pH, and crop breed. The bulky nature of many organic materials can make them difficult to spread consistently, affecting their reliability as a fertiliser.
The solution
The reliability of phosphorus-rich organic materials as fertilisers can be improved by processing to improve fertiliser quality, and developing better systems to help farmers assess the phosphorus content and phosphorus bioavailability of the materials. Furthermore, farmers can be better supported to optimise the application of recycled phosphorus products and other nutrients in order to maximise phosphorus uptake by plants. However, critical to this is a sufficient understanding of farm- and local-scale nutrient budgets.
Key issue 6.4
Some phosphorus-rich organic materials can contain contaminants
The challenge
Pathogens, hormones, antibiotics, potentially toxic elements, and microplastics can be present in some phosphorus-rich organic materials. It is important to ensure contaminants are removed, destroyed or concentrations reduced to safe levels in any phosphorus-rich organic materials to be used as fertilisers. In some cases, contaminants can accumulate in soils and may pose a risk to human and animal health and environmental quality.
The solution
Most phosphorus-rich organic materials need some processing to reduce contaminants and pathogens to safe levels for use in food production. Reducing livestock dietary intake of potentially toxic elements and imposing strict limits on the non-therapeutic use of antibiotics in livestock, will reduce levels of these contaminants in manure and biosolids. Assurance that fertiliser products derived from phosphorus-rich organic materials are safe for their intended use should be provided to end-users.
Key issue 6.5
There is a lack of policy, infrastructure, and financial support for phosphorus recycling
The challenge
There is a lack of coordinated policy and regulation to support an increase in the recycling of phosphorus-rich organic materials. In some regions, there is little economic incentive for farmers to switch from mineral phosphorus fertiliser to phosphorus-rich organic materials. Some farmers can face legal and certification barriers stopping them from recycling certain phosphorus-rich organic materials.
The solution
Improved coordination between relevant government bodies and relevant stakeholders is required to develop coherent, holistic policies and create markets for recovered phosphorus fertiliser. Investment in infrastructure and technologies supported by cross-sectorial innovation, co-creation and sharing of knowledge can help to make phosphorus recycling simple and efficient. The economic benefits for society of recycling phosphorus need to be better quantified and used to encourage stakeholders to recycle phosphorus more efficiently. The value of recovering phosphorus can be maximised by selecting methods to process organic materials that produce additional co-benefits.
Conclusion
Recycling the nutrients in organic materials to grow foods instead of allowing them to pollute waterbodies or to be lost to the environment is pivotal to achieving global phosphorus sustainability. Sufficient knowledge and technology are already available to make significant increases in recycling phosphorus-rich materials globally. Furthermore, the win-wins are numerous, with benefits to society, environment, economy and business growth. The solutions listed above cannot be addressed by the end-users alone (i.e. the farmers); multiple stakeholders throughout the food value chain, policymakers and consumers can all play a critical role. The challenge moving forward is finding the investment, resources and policy support to make the transition to a circular economy for phosphorus.
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The full chapter contains references to the evidence provided above and acknowledgements of images.
Suggested citation for this chapter: W.J. Brownlie, R. Sakrabani, G.S. Metson, M.S.A. Blackwell, B.M. Spears. (2022). Chapter 6. Opportunities to recycle phosphorus-rich organic materials, in: W.J. Brownlie, M.A. Sutton, K.V. Heal, D.S. Reay, B.M. Spears (eds.), Our Phosphorus Future. UK Centre for Ecology and Hydrology, Edinburgh. doi: 10.13140/RG.2.2.33143.29605