Environmental Engineering Reference
In-Depth Information
oxygen in bottom waters and the potential for a positive feedback accounts for the fact
that bottom water aeration has been used to try to manage eutrophication.
MANAGING HUMAN INTERACTION WITH
THE PHOSPHORUS CYCLE
Human activity dominates the P cycle (
Figure 8.2
). It is up to us to manage this cycle in
a sustainable manner, one that protects critical aquatic resources while promoting food
security worldwide. Currently, only one-fifth of all mined P makes it to our fork. Luckily,
promising strategies exist to slow the one-way flow of P to the ocean by using it more
efficiently and by keeping it cycling in the human food system (
Childers et al. 2011
). Some
solutions require advanced technologies, but many are quite simple and readily available,
even in poor countries.
By far the largest losses occur from farms. Over 7 Mt P is lost annually from animal
manure (
Childers et al. 2011
). Strategies to recover P from livestock production (i.e., to
return manure to the land that produced the feed used to grow the livestock) have the
potential to retain a sizeable percentage of this P. Large farms with high livestock densities
and little cropland, common in industrial agriculture, make recovery of manure relatively
easy. However, cropland on which manure could be used as fertilizer may be distant and
manure is heavy and expensive to move over long distances. Livestock can be genetically
modified to increase P uptake and reduce P in manure. Likewise, researchers have
recently engineered some crop plants to increase their ability to scavenge nutrients, includ-
ing P, from poor soils (
Gaxiola et al. 2011
). Similarly, the Enviropig
has been engineered
to produce phytase in its salivary gland, allowing use of P in otherwise indigestible P-rich
phytate molecules (
Golovan et al. 2001
). The result is a pig that can grow well on feed
with lower P content and, in doing so, generate 30% to 65% less P in its manure.
Dietary and other food use changes may also help increase the efficiency with which
we use P or reduce losses. Because the conversion of P inputs to dietary P is so much high-
er for crops than for livestock, shifts to a less meat-intensive diet can reduce demand for
mineral P. For example, a vegetarian diet requires about one-third the amount of mined P
of a meat-based diet (
Cordell et al. 2009
). Switching to grazed livestock, which requires
less P inputs than livestock fed grain produced with fertilizers, can also be part of the
solution. Food waste (distribution, retail, household, or institutional loss) comprises 1 Mt P
annually (
Cordell et al. 2009
). Producing food closer to areas of high demand such as cities
is one method to help reduce waste. Composting unavoidable wastes can allow P to be
recovered for local reuse, especially for nearby urban farming.
We can even reduce losses and increase recycling at the very end of the food system.
Human production of P in waste is about 1.8 g person
2
1
day
2
1
; trapping all of this P glob-
ally would result in 4.5 Mt P available for reuse, an amount that is nearly one-third of
global annual fertilizer consumption. Currently, only about 10% of the P in human waste
is recovered. Recent technical innovations allow struvite (magnesium ammonium phos-
phate) recovery from sewage treatment pipes and development into a pellet form that can
be used as fertilizer. And urine-separating toilets or those that allow urine to be recovered
can not only return these valuable nutrients to the soil, but also help improve sanitation.
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