City phosphorus, country phosphorus: can we mitigate P in different environments?

Reference: Katrina A. Macintosh, Brooke K. Mayer, Richard W. McDowell, Stephen M. Powers, Lawrence A. Baker, Treavor H. Boyer, and Bruce E. Rittmann. Managing Diffuse Phosphorus at the Source versus at the Sink. Environmental Science & Technology Article ASAP. DOI: 10.1021/acs.est.8b01143.

Phosphorus and Dead Zones

Eutrophication and its colleagues, algal blooms and dead zones, are not a new topic for Envirobites. Eutrophication, an overloading of nutrients in an aquatic system, provides fuel for an explosion of plant life; as the plants die they are consumed by bacteria, along with all the available dissolved oxygen. Without oxygen, fish and other marine cannot survive and the overall value of the ecosystem declines (see figure 1).

Figure 1. How Dead Zones Form. Source: Image Credit: Dan Swenson | Staff Research, Flickr.

One of these excessive nutrients is phosphorus. An essential nutrient for all living organisms, phosphorus has been largely depleted from natural soils and today is mined from rock to support agriculture. There are no substitutes for phosphorus in agriculture; globally, we mine approximately 260 million tons a year to feed our population of 7.6 billion people (approximately 68 pounds or 31 kg per person!).

Figure 2. Mining of Phosphate rock in Nauru. Source: Lorrie Graham/AusAID, Wikimedia Commons.

Some phosphorous is lost in the mine to farm to fork pathway and cannot be reused (see the left side of Figure 3 below).  A recent review paper by Katrina A. Macintosh et al. at the Queen’s University of Belfast examined phosphorus recovery and mitigation cases, starting with the big question: how does phosphorus enter the environment?

There are two ways that describe how phosphorus enters the environment: point sources and nonpoint sources. Point sources are fixed and tend to be an “end of pipe” problem; think of a wastewater treatment plant. We have mostly limited emissions from these sources through improved technology and regulations. Non-point sources, however, are more difficult to identify and treat.  They tend to be periodic, diffuse, and are not as simple as managing what comes out of a pipe! Some of these examples, like crop run off or leaky septic systems, can be found in the right side of Figure 3 below. It is these challenging non-point sources that are the focus on this review paper.


Figure 3. Tracking phosphorus in the global food system on the left, emissions of phosphorus from diffuse sources into aquatic systems on the right. Reprinted (adapted) with permission from Macintosh et al. 2018. Copyright (2018) American Chemical Society.
First: Setting Boundaries

Like in many review papers, the authors first set up the rules for comparing phosphorus management strategies by setting standardized metrics of quantity and form. For quantity, they chose to use annual surface loading, i.e., kg phosphorus per hectare per year.

The second metric is form. Phosphorus can be dissolved in water or be solid particulates, and can be inorganic or organic. The form reflects how the phosphorus is chemically structured, which is important because it determines first, whether living organisms like algae and bacteria are able to absorb it from the water, and second, which strategies will work best for removal and recovery.

City phosphorus, country phosphorus

Current best practices vary by land use: agricultural vs. urban. While highly dense urban areas have higher rates of phosphorus input and output than agricultural areas (3.6% of land in the United States), agricultural areas account for larger surface areas (61% of land in the United States), and account for a large amount of phosphorous use and loss into the environment.

Figure 4. Major sources of phosphorus can be broken down into agricultural and urban groups. Reprinted (adapted) with permission from Macintosh et al. 2018. Copyright (2018) American Chemical Society.
Pet excrement is one way phosphorus enters the environment, as well as a way to ruin a pair of shoes. Source: Wikimedia Commons.

Katrina Macintosh and her team covered different agricultural cases in Ireland, New Zealand, and Minnesota, United States. In each scenario, policy makers and farmers seek to maximize yield but limit excess phosphorus application by prohibiting phosphorus fertilizer and manure application on already phosphorus-rich areas, remediating “critical source areas” instead of entire farms, and using watershed-level mass balance calculations.

Urban sources of phosphorus also include fertilizer application but expand to composting, septic systems, atmospheric deposition, and even pet excrement. That last part is no joke: after St. Paul, MN, banned lawn fertilizers, the major input of phosphorus was pet urine or feces not picked up by owners. Prohibiting lawn fertilizers and using street sweepers and strainers in urban storm water systems, and monitoring septic systems were all determined to be helpful strategies to reducing phosphorus emissions in urban systems.

Water World

Macintosh et al. also covered several cases where phosphorus treatment was performed at the sink, or endpoint: in aquatic systems. Two of these treatments are designed as wetlands: the Everglades, a natural system, and the Dixie Drain system in Idaho. This system, not yet constructed, will contain a presedimentation pond, a manmade wetland, and an aluminum-based coagulation step followed by another sedimentation pond. (Check out this cool video about it here!)

Aluminum was also used in the third case the author covered: lakes in the Midwest U.S. and Europe. In these systems, internal loading must also be addressed, as sometimes, sediments in the lake naturally contain phosphorus, which can throw off the calculation of added phosphorus.


Low hanging fruit
Dead zones along the Gulf of Mexico. Source: Wikimedia Commons.

Whether we address eutrophication or not is not the question; like these authors suggest, the question is how, not if.

The reason for that is that the cost of *not* taking any treatment steps is high: freshwater eutrophication costs the US a minimum of $2.2 billion every year; in England and Wales, this number is $100-$160 million; one assessment in China in the late nineties of Lake Tai was $6.5 billion.

Treatment systems can be expensive, but as technology improves, cost decreases. Other ways to reduce cost are to focus on the low hanging fruit, such as treating losses at the source where they are more concentrated, or limiting discharge in the first place (e.g., limiting fertilizer application). Mapping the best fitting solution onto each case is an important part of reducing cost.

Why it matters

Phosphorus management is critical to keeping our water clean and our bodies of water healthy. While recovery might not be economically viable – mined phosphate rock is less expensive, though we are facing a phosphorus potential shortage! – mitigation is still necessary.  Understanding the pathways phosphorus takes from its source (urban, rural, or aquatic) to sink (aquatic) and how we can effectively prevent phosphorus loss, or recover it at each step, is critical for determining the best management strategies and how much it is going to cost.

Thanks to this review by Macintosh et al., we know we can address phosphorus variability in diverse systems; no one-size-fits-all approach needed. 



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Laura Mast

Laura Mast

I'm a PhD student in environmental engineering at Georgia Tech. Broadly, I study resource recovery: how we can look at waste products as untapped mines of valuable materials. Specifically, I develop methods to extract rare earth elements (required for everything cool/high tech/green we've made in the last 20 years, like LED screens, batteries, permanent magnets) from coal fly ash- a byproduct from burning coal for electricity. In my spare time, I run a start up called Populy, which provides electronic judging for STEM competitions, swing dance, and hang out with my dog.

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