Phosphorus Supply and Sustainability in the 21st Century
Introduction
The security of clean water, fuel and food are
obvious necessities to human life but behind these, mineral phosphorus is often
overlooked. It is a finite resource that will be unable to meet future demand
unless its use and management are drastically changed. For the sake of its
citizens, economy and future, the British Parliament should ensure that this
valuable resource is correctly stewarded. Britain’s reliance should be managed,
to minimize any adverse impacts brought about by phosphorus scarcity.
Furthermore, the wisdom gained from research and experience should be
disseminated to developing nations, to help prevent them following the same
myopic and unsustainable strategies as the developed nations.
Background
The chemical element phosphorus is a
cornerstone for many biochemical processes and is often a limiting factor for
agricultural output. The Green Revolution in the 1900s initiated the exploitation
of finite mineral phosphorus, to boost crop production and feed the increasing
world population. Recent efforts, to secure sustainable energy production from biofuels,
have put additional pressure on phosphorus-intensive agriculture. The US
Geological Survey has estimated that global phosphate rock reserves total
71,000 Mt, of which only 16,000 Mt are considered economically viable with
existing technology 1. With annual extraction rates nearing 200 Mt, peak
phosphorus is expected to occur as early as 2030, followed by a terminal
decline of mining output 2. Government policy must ensure that
Britain retains access to a secure supply of rock phosphate and moreover, that phosphorus
is used in a more sustainable manner, reducing our net national demand.
In contrast to the cycles of water, carbon and
nitrogen through the environment, phosphorus exists only in solid and liquid
forms, making it susceptible to loss. Once phosphorus is discharged or eroded
into waterways and ultimately the oceans, its fate is irreversible, the cycle
is broken and the finite solid reserves become the sole source. Eventually, if
the phosphorus cycle is not maintained, the finite reserves will be depleted
and agricultural production will diminish.
Historically, phosphorus levels in soils have
been maintained by direct disposal of undiluted human and livestock waste, to
the land from which they were fed. This labour-intensive approach, combined
with a lower intensity of agriculture meant that the cycle of phosphorus was
kept intact. Since the Industrial Revolution, urban populations have increased.
The subsequent demand for higher crop yields and the decreased availability of
human waste in proximity to farmland, started to break the cycle. This was
exacerbated by the Sanitation Revolution in the 1800s; using water-based
sewerage systems to remove human waste and introducing disease-prevention as a
priority over nutrient recycling. The greatest breach of the sustainable phosphorus
cycle can be attributed to the Green Revolution in the 1900s. This saw crop
yields boosted by application of artificial fertilisers manufactured from
finite reserves of phosphate rock.
These three revolutions have allowed the
world’s industrialized economies to develop without limitation by water-borne disease
or food shortages. However, these economies are now reliant on finite
phosphorus reserves and are exposed to their price volatility. Security of
supply is further compromised by 85% of the world’s reserves being located in
just five countries: China, Morocco, Jordan, South Africa and the US 1.
The effects of supply issues have already been felt with fertiliser prices
rising by 350% between 2003 and 2008 3. This contributed towards
food riots in Asia, and China imposing a temporary 135% tariff on phosphorus
exports, to secure supply for its own economy. Such events are likely to be
repeated in response to growing populations and increased demand for food and
biofuels.
Outlook
Although mineral reserves of phosphorus are
non-renewable and there is no alternative to phosphorus in its biological
roles, it can be recovered and reused. If this is done on a sufficient scale,
the phosphorus cycle can be repaired and net demand can be reduced to more
sustainable levels.
Current phosphorus recapture is primarily
driven by environmental regulations to prevent eutrophication (the EU Water
Framework Directive 4) but the value of nutrient recovery is increasingly
being realised. A number of technologies and approaches are available to allow
phosphorus recovery from wastewater and to prevent its loss from agricultural
land in the first instance:
·
Enhanced biological phosphorus removal.
This involves adapting traditional biological treatment to remove a greater
concentration of phosphorus from wastewater. The phosphorus can then be
recovered from the resulting biomass. Algae also show potential in this role.
·
Chemical adsorbents. By dosing iron or
aluminium coagulants to wastewater, insoluble phosphorus precipitates are
formed, allowing effective removal and recovery.
·
Ion exchange. Phosphorus in wastewater
will bind to suitable exchange media. Once saturated, the media can be rinsed
and regenerated, providing a concentrated and pure phosphorus product.
·
Anaerobic digestate. After wastewater
sludge has been digested and the energy value recovered as biogas, the
resultant nutrient-rich biomass can be heat-treated and marketed as a
fertiliser. The drive for increased biogas production from the codigestion of
animal and food wastes also increases the output of recovered nutrients from
this process.
·
Struvite (a phosphorus-rich mineral).
This valuable product can be produced from anaerobic digestion by optimising
the conditions. Its density and purity allows for easy transport and application
to crops.
While these innovations show great promise in
alleviating the scarcity of phosphorus supply, the scope of their benefit
should be noted. Of the 20 million tonnes per year of pure phosphorus used,
only 3 million tonnes are excreted by humans 5. Currently 46% of
mined phosphorus is lost to agricultural run-off 6. By improving
farming efficiency and catchment management, the loss of phosphorus from crops
can be minimised, reducing the demand for fertiliser. The second greatest loss
of phosphorus (40%) is in animal wastes. Greater efforts are required to
reapply these directly to agricultural land or ensure that they are effectively
diverted into wastewater treatment plants and recovered.
These phosphorus recovery strategies would be
a stride in the right direction and would greatly reduce the dependency on
phosphate rock reserves. However, high-tech recovery processes are not the sole
solution and a more holistic approach is required. High-tech solutions are also
unsuitable for countries with less-developed water-treatment and agriculture, from
which Britain sources some of its 40% of food imports. Accordingly, these
technological solutions will need to be augmented by behavioral changes
(curbing our phosphorus-intensive, meat-rich diets), education, management of
global population growth, and a paradigm shift in the role of drainage and
wastewater treatment. Many of these changes would have synergies with existing
government policies such as combating obesity, protecting aquatic ecosystems,
stopping world poverty and balancing Britain’s trade deficit.
References
1.
Ashley, K. et al. (2011) Chemosphere 84, 737-746
2.
Jasinski, S.M. (2010) Mineral Commodity Summaries, US Geological
Survey, January
3.
http://www.foreignpolicy.com/articles/2010/04/20/peak_phosphorus
4.
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:2000L0060:20090625:EN:PDF
5.
Cordell, D. et al. (2009) Global
Environmental Change 19, 292-305
6.
Rittman, B.E. et al. (2011) Chemosphere
84, 846-853
