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What is your phosphorus footprint?
A. Shaw1* and J. Barnard1
1 Black & Veatch, 8400 Ward Parkway, Kansas City, Missouri, USA
*To whom correspondence should be addressed. Email: ShawAR@bv.com.
ABSTRACT
In this paper, the case is made that, of all environmental impacts, the removal and reuse
of phosphorus is the most significant consideration for wastewater treatment facilities.
Phosphorus is non-renewable, is a vital nutrient for crops and is an important chemical
used in many products.
The concepts of global and local phosphorus cycles are introduced and the manner in
which anthropogenic activity shifts the mass fluxes in these cycles is discussed. The net
increase in the global flux due to human activity is estimated to be somewhere in the
range 2 – 14 Trillion g/yr which may be considered the global phosphorus footprint. It is
estimated that wastewater contributes approximately 1.5 Tg/yr to the global phosphorus
footprint which is at least 10% of the total based on the largest estimate for the global
phosphorus footprint. In contrast, wastewater treatment contributes less than one percent
of the global carbon footprint.
Life cycle assessments (LCA) can be used to determine the net flow of material into or
out of any system. Thus it can be used to assess the environmental “footprint” of any one
of a number of indicators. This technique can be used to assess any number of
phosphorus footprints for process options for treatment, reclamation, or biosolids
processing. An example is presented to show that the phosphorus footprint of a
wastewater treatment plant is reduced as it removes more phosphorus from the effluent
and that using phosphorus recovery can reduce the footprint still-further.
KEYWORDS: phosphorus, phosphorus footprint, nutrient recovery, carbon footprint
INTRODUCTION
Forget your Carbon Footprint or your Water Footprint, what is your Phosphorus
Footprint? In this paper, the authors make the case that, of all environmental impacts, the
removal and reuse of phosphorus is the most significant consideration for wastewater
treatment facilities. Phosphorus is a vital nutrient for crops and is an important chemical
used in many products. The proven reserves of phosphorus in the US will last an
estimated 35 years. Of the estimated 16m tons of phosphorus reserves 9.4 m is in China
and Morocco while 90% of supplies are found in the above, the USA, South Africa and
Jordan. (USGS , 2010)
PHOSPHORUS CYCLES
The Global Phosphorus Cycle
In a similar manner to the carbon and water cycles, the global flow of phosphorus can be
represented as a cycle (See Figure 1). Unlike the carbon and water cycles, however,
phosphorus does not occur naturally as a gaseous compound and so the natural short-term
cycle is extremely localized and the natural movement of most phosphorus from the
oceans back to the land takes many millennia. Some small return from the oceans is seen
as migratory species such as salmon which travel back up rivers to their spawning
grounds in pristine lakes, where they die and provide phosphorus to grow algae for the
survival of their off-spring. Filipelli (2002) estimated that the net movement of
phosphorus from the land to the oceans due to natural activity is 1 – 2 Tg/year.
Figure 1: Natural phosphorus cycle (from Filipelli, 2002)
Local Phosphorus Cycles
Similarly the movement of phosphorus within a local area can be considered as a system
with internal cycles, plus mass flows into and out of the system. For example, a tree will
draw phosphorus nutrients from the soil. The tree will then drop its leaves or branches
will fall and through natural decay processes the phosphorus will become reincorporated
into the soil and the net exchange will be zero. Rain and groundwater flow gradually
move some of the phosphorus out of the soil producing an overall net flux for the system
that is slightly negative.
Anthropogenic Impacts
Introducing anthropogenic activity into these cycles shifts the mass fluxes considerably
(see Figure 2). In the example of the tree, if instead of it falling back into the soil from
which it extracted phosphorus it is harvested and the lumber used to make paper then the
phosphorus will leave the forest with the timber and there will be a net loss of phosphorus
from the area. The net movement of phosphorus into or out of this area is the phosphorus
footprint. The net increase in the global flux due to human activity is estimated to be
somewhere in the range 2 – 14 Tg/yr (Bennett, 2002, Filipelli, 2002, Cordell 2009) which
may be considered the global phosphorus footprint. The US and Canada produces 67% of
surplus food in the world which means it is a virtual export of phosphorus. Wood, paper
and other agricultural products exhibit the same effect.
Figure 2: Anthropogenic impact on global phosphorus fluxes (from Filipelli, 2002)
Cordell (2009 and Figure 3) estimates that wastewater contributes approximately 1.5
Tg/yr to the global phosphorus footprint which is at least 10% of the total based on the
largest estimate for the global phosphorus footprint. The percentage contribution is
obviously greater if the global footprint is smaller. Wastewater is a significant
contributor to the global phosphorus footprint. In contrast, wastewater treatment
contributes less than one percent of the global carbon footprint.
Figure 3: Global phosphorus balance (from Cordell, 2009)
LIFE CYCLE ASSESSMENT (LCA) AND PHOSPHORUS FOOTPRINT
Life cycle assessments (LCA) can be used to determine the net flow of material into or
out of any system. Figure 4 is a simple diagram that shows the general principle of a life
cycle assessment. For each material used in a product or for a process, it is first mined
from the natural environment, then used in manufacturing, used and finally disposed. At
each step in the life cycle, some material is lost and goes back to the environment in the
form of solid, soluble, liquid or gaseous waste. Some materials can be reused or recycled.
Figure 4: Material Flow in a Life Cycle Assessment
The LCA technique is often used to assess the carbon footprint of a system due to direct
and indirect greenhouse gas emissions (e.g. see Flower, 2007). When doing so,
greenhouse gas emissions to the atmosphere are usually only considered for
anthropogenic activity and anything associated with the natural carbon cycle is termed
“biogenic” and not included. Another way to express these different emission types are
to consider carbon emissions that are “short-cycle” and generally move within the
biosphere (i.e. biogenic emissions) over a few to several-hundred years, and the “long-
cycle” emissions for carbon that include fossil fuels that may be burned in a matter of
minutes but take thousands to millions of years to reform. In this sense, anything that
moves carbon from the short-cycle to the long-cycle constitutes the carbon footprint and
the atmosphere is the ultimate sink for this extra carbon.
In a similar way, the movement of phosphorus within the natural environment (Figure 1)
has both a short cycle and a long cycle. The long cycle is also depicted on Figure 3
around the edge of the diagram and labeled “Natural biogeochemical P Flows.” This
figure shows the natural environment encompasses the “anthroposphere” which contains
the mass flows of phosphorus due to, mostly food-based, human activity. In this
diagram, the movement of phosphorus within the anthroposphere can also be considered
part of a short cycle, but any phosphorus that ultimately ends up in the water becomes
part of the long cycle of biogeochemical phosphorus that takes millions of years to
replenish. In this, water is the ultimate sink for phosphorus and flows to it from the
anthroposphere constitute the phosphorus footprint.
Figure 5: Phosphorus Mass Flows Showing Phosphorus Footprint
Figure 5 considers the more significant phosphorus mass flows into and out of a
wastewater treatment plant (WWTP) that does not have a tight limit on its phosphorus
discharge. In this facility, roughly half of the phosphorus from the wastewater that goes
into the WWTP is discharged to the effluent and passes to an inland or coastal water, the
ultimate sink for phosphorus. The remainder of the phosphorus is captured in the sludge,
biosolids or ash (depending on the sludge treatment) which is disposed or reused in some
beneficial manner (e.g. land application to agricultural land). This keeps the majority of
the phosphorus within the anthroposphere but a fraction will wash out in run-off or
leachate and eventually end up in a waterway. The sum of the mass load of phosphorus
in the effluent discharge and the run-off or leachate is the overall phosphorus footprint for
the WWTP in this figure. In a full LCA, the phosphorus mass flows associated with
energy use, chemical use and equipment production are also included in the calculation,
though these are usually insignificant in comparison to the effluent and sludge mass
loads. In an LCA, the term “eutrophication potential” is synonymous with phosphorus
footprint. (In a similar manner that “global warming potential” is synonymous with
carbon footprint).
An added benefit of re-using phosphorus is that it replaces the need for inorganic
phosphorus fertilizers. If the quantity of phosphorus captured and re-used can be
maximized, this reduces the phosphorus load in the effluent discharge and, though it may
increase the mass flow in the run-off or leachate, the overall phosphorus footprint will be
reduced significantly. If the captured phosphorus is in the form that can replace
inorganic phosphates, an additional net benefit can be applied to the overall phosphorus
footprint. This is exemplified in the results shown in Figure 6 which are the outputs from
an LCA used to develop phosphorus footprints for several treatment alternatives to
investigate the potential reduction in the Global phosphorus footprint if all facilities were
to implement phosphorus removal. In this example it can be seen that the phosphorus
footprint with a limit of 1 mg/L is considerably less than a non-phosphorus removal
facility. Reducing the limit to 0.1 mg/L reduces the phosphorus footprint and using
phosphorus recovery can reduce the footprint still-further.
Figure 6: Results of LCA Analysis for Different P removal options
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Eutrophication P
Global P Footprint from Wastewater Treatment (Tg/yr)
No P Removal
P Removal 1 mg/L
P Removal 0.1 mg/L
P Recovery
In another study Shaw et al (2011) investigated the trade-off between reducing the
phosphorus footprint and increasing the carbon footprint for effluent discharges near the
limit of technology (see Figure 7). They showed that the method for normalizing the
environmental impact scores has a strong influence on the decision to go to ultra-low
limits, but in some cases, almost complete (100%) phosphorus removal could be justified
owing to the significant contribution that WWTPs have on the global and local
phosphorus footprint.
Figure 7: Eutrophication potential (phosphorus footprint) and global warming potential (carbon
footprint) for different effluent P limits
Provided that sufficient data can be proved to develop an accurate inventory of material
flows, including phosphorus, an LCA can be used to assess any number of phosphorus
footprints for process options for treatment, reclamation, or biosolids processing. In
addition it can be used to assess carbon footprints, water footprints and other
environmental “footprints.”
SUMMARY
The authors have made the case for considering the phosphorus footprint for wastewater
treatment options. In their opinion, based on the opportunity to impact the global
footprint of various environmental impacts, LCA should be used to estimate the
following environmental footprints, as a minimum:
1. Phosphorus footprint
2. Water footprint
3. Carbon footprint
These environmental footprints are important for wastewater treatment but should also be
considered for other anthropogenic activities. Other environmental impacts, such as land
use, may also be significant for specific situations.
REFERENCES
Elena Bennett and Steve Carpenter (2002) “P Soup (the global phosphorus cycle),”
WORLD WATCH, March/April 2002, 24-32, Worldwatch Institute, Washington DC
Dana Cordell, Jan-Olof Drangert, and Stuart White (2009) “The story of phosphorus:
Global food security and food for thought,” Global Environmental Change, 19 (2009)
292–305.
Flower D.J.M., Mitchell V.G. and Codner G.P. (2007) “Urban Water Systems: Drivers of
Climate Change?” Paper presented at the 13th International Rainwater Catchment
Systems Conference & 5th International Water Sensitive Urban Design Conference,
Sydney, Australia.
Gabriel M. Filippelli, (2002) “The Global Phosphorus Cycle,” Reviews in Mineralogy
and Geochemistry; January 2002; v. 48;1; p. 391-425, 2002. Mineralogical Society of
America
Shaw,A., deBarbadillo, C., Tarallo, S., and Kadava, A. (2011) “Life Cycle Assessment
of the Relative Benefits of Meeting Ultra-Low Nutrient Limits at WWTPs”; Proceedings
of WEF - IWA Nutrient Recovery and Management 2011: Inside and Outside the Fence,
January 9 – 12, 2011, Miami, Florida, USA
USGS, U.S. Geological Survey, Mineral Commodity Summaries, January 2010,
http://minerals.usgs.gov/minerals/pubs/commodity/phosphate_rock/mcs-2010-phosp.pdf,
accessed July 10th 2011