Phosphorus Recovery From Surface Waters: Protecting Public Health and Closing the Nutrient Cycle.

Author:Jin, Guang

The Public Health Consequences of Phosphorus Pollution

Phosphorus (P) is essential for plant and animal life. Most of earth's ecosystems, however, evolved under P-limited conditions (Schoumans et al., 2014; Scinto & Reddy, 2003). Even relatively small increases in P can significantly alter an ecosystem. To increase agricultural output for a growing human population, the world has relied increasingly on production of mineral-based P fertilizers. Of the P fertilizer applied to crops, it is estimated that approximately

50% is lost through surface runoff and soil erosion (Rittmann, Mayer, Westerhoff, & Edwards, 2011). Most of the P taken up by crops and consumed by livestock and humans ultimately is excreted in wastes.

Sewage and manure treatment systems, even with modern technologies, represent a significant source of P into waterways (Karunanithi et al., 2015).

The public health implications of P pollution are significant. Toxins produced by harmful algal blooms (HABs) have been recognized for decades as a threat to human and animal health, but are increasing in frequency and severity, not only across the U.S. but also around the world (Berdalet et al., 2016; Carmichael, 2013; D'Anglada 2015; Erdner et al., 2008). The first recorded instance of HABs in the U.S. was in South Dakota in 1925 when more than 120 hogs were killed after consuming water from a lake that was undergoing an algal bloom (Carmichael, 2013). HABs on the Ohio and Potomac Rivers in 1931 were estimated to have sickened 5,000-8,000 people (Carmichael, 2013).

Seafood taken from waters contaminated by HABs can be harmful, causing numerous outbreaks of paralytic shellfish poisoning and other illnesses (Erdner et al., 2008). Another HAB, red tide in coastal areas, can harm not only aquatic life but also release toxins into the air, producing respiratory problems in humans, especially in individuals with asthma (Erdner et al., 2008). In 2014, HABs in Lake Erie interrupted the supply of drinking water to the city of Toledo (Ho & Michalak, 2015). HABs are caused by a number of algal species and can involve a variety of toxins, but nutrient pollution--particularly from phosphorus and nitrogen (N)--is estimated to be a significant contributing factor in many HABs (Heisler et al., 2008).

Pollution from P also has significant indirect implications for public health due to the need to restore soil fertility where P has been depleted. Production of mineral-based P fertilizer creates a waste product that can be mildly radioactive. Over 1 billion tons of this waste is stockpiled currently in Florida and a number of other states (Cordell & White, 2013). The recent opening of a sinkhole beneath one of these stockpiles resulted in the loss of over 200 million gallons of phosphate- and radionuclide-contaminated water (O'Donnell, 2016).

Phosphate fertilizer is made from phosphate rock, a limited resource that is found in relatively few places in the world. While the U.S. once had rich deposits, world supply is now dominated by Morocco and China (Scholz, Ulrich, Eilitta, & Roy, 2013). World population growth and a shift to greater percapita meat consumption is predicted to dramatically increase demand for P fertilizer in the coming decades, with higher prices and potential shortages anticipated (Desmidt et al., 2015). Unlike other limited resources, such as fossil fuels, there are no substitutes for phosphate rock. Current reliance on mineral-based P fertilizer is unsustainable and poses a significant food and national-security threat (Desmidt et al., 2015).

The Role of the Environmental Health Professional

The greatest sources of P pollution include runoff from crop production, livestock wastes, urban stormwater, and discharge from sewage treatment plants and septic systems (U.S. Environmental Protection Agency [U.S. EPA], 2017). Environmental health professional responsibilities can influence a number of these sources, including onsite wastewater treatment systems, land use planning, watershed and drinking water protection, stormwater control, and others.

Efforts to reduce nutrient pollution increasingly use watershed-based planning (U.S. EPA, 2013). The stakeholder groups assembled to participate in the planning process offer an opportunity for the environmental health professional to further public health goals and assure that nutrient control is performed through the most cost-effective means. To be most effective, environmental health professionals should be familiar with the range of methods available for P pollution control.

A number of excellent reviews have been published on methods to reduce the flow of P into waterways (Clary, Jones, Strecker, Leisenring, & Zhang, 2017; U.S. EPA, 2007, 2018). Relatively little has been published, however, on the need and methods to remove and recover P already present in surface waters. There are several reasons why P removal and recovery from surface waters is important (Rittmann et al., 2011). First, efforts to reduce P inputs to waterways have progressed slowly, and are likely to be insufficient to control HABs in the near term (Lurling, Mackay, Reitzel, & Spears, 2016).

Second, even if P inputs could be sharply reduced, sediments and vegetation have built up sufficient P stocks from prior contamination to keep aqueous P concentrations high for decades to come (Jarvie et al., 2013; Lurling et al., 2016). And third, recovered P can reduce reliance of commercial P fertilizer, reducing the public health and environmental impact of fertilizer production and improving food security.

In the remaining sections we explain the current technologies for P removal and recovery from wastewater, highlight the challenges of adapting these technologies to surface waters, and note the prospects for recovering P while attaining water quality sufficient to avoid environmental and public health harm. But first we provide a brief explanation of the behavior of P in the natural environment and its implications for removal and recovery.

Phosphorus in the Natural Environment

In the environment, P exists in a number of different forms (Figure 1). Labile P refers to the forms of P readily usable by plants, and is typically dominated by orthophosphate--the ionized forms of phosphoric acid ([H.sub.2]P[O.sub.4.sup.-], HP[O.sub.4.sup.2-], and P[O.sub.4.sup.3-]) (Karunanithi et al., 2015). Plants and bacteria can use P to produce organic compounds, which can then be converted back to labile P through a number of P-metabolizing enzymes.

Labile P can be readily adsorbed on soil and vegetation surfaces. In both surface water and wastewater, most P is associated with solids, and typically only a small fraction is present as dissolved P (Ibarra, 2011). Aluminum (Al), calcium (Ca), iron (Fe), and certain other metal oxides form positively charged surfaces when hydrated. The presence of these compounds in solids often determines the extent to which P is particle bound (Loganathan, Vigneswaran, Kandasamy, & Bolan, 2014). Adsorption on metal oxides is pH dependent...

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