February 2011, Vol. 23, No.2

About that phosphorus shortage...

news

Prices for phosphorus — the prime ingredient in fertilizer and an essential nutrient for life — spiked as much as 800% in recent years. Experts around the globe have predicted a shortage is looming. Among them are scientists from the Global Phosphorus Research Initiative (GPRI), who estimate global phosphorus production will peak around 2034, and readily available supplies will be inadequate to meet demand within 30 to 40 years.

Then again, maybe not. A study released in September by the International Fertilizer Development Center (IFDC; Muscle Shoals, Ala.) estimates that at current production rates, there are sufficient phosphate rock reserves to produce fertilizer for the next 300 to 400 years. 

IFDC’s findings are based on the agricultural sustainability organization’s own assessment of phosphate rock reserves, nearly 90% of which are concentrated in five countries: Morocco, China, South Africa, Jordan, and the United States. The IFDC report claims these countries contain approximately 60 billion Mg of reserves. That’s nearly four times the most recent estimate by the U.S. Geological Survey.

“Dire consequences for world agricultural production and food security are linked to peak phosphate,” said Steven Van Kauwenbergh, the IFDC principal scientist who authored the report. “However, there is no indication that a ‘peak phosphorus’ event will occur in 20 to 25 years.”

 

Who’s right, and does it matter?

Is Van Kauwenbergh correct? And if so, what do IFDC’s findings mean to those championing efforts to remove and recover phosphorus from agricultural and other waste flows so that it can be reused?

Not much, they say.

“If accurate, this geological survey suggests we have more phosphate in the ground,” GPRI leaders wrote in a response to the IFDC study. “More megatons does not change the fact that we are fundamentally shifting to an era where cheap fertilizers will be a thing of the past.”

James Barnard, a global practice and technology leader at Black & Veatch (Overland Park, Kan.), agrees. “There may be [more] phosphorus out there than we thought, but there’s still the question of getting to it,” he said.

The United States and Canada, Barnard explained, supply two-thirds of all the world’s surplus food and, therefore, use the most fertilizer. But with 85% of the world’s phosphorus reserves in Morocco, it will eventually come at a high cost.

“It may be true that as prices go up, more [difficult-to-reach] phosphorus will be mined,” Barnard said. “But phosphorus is already out of reach to half of the world’s population. If it becomes more expensive, that means food will also become more expensive.”

Whether the world has enough phosphorus to sustain life for 30 years or 300 may be beside the point. “We’re still working with a limited resource, and reserves are being depleted at an unsustainable rate,” Barnard said. “We should still be talking about using fertilizer more sparingly and taking advantage of opportunities to remove and recover phosphorus from the wastestream.”

 

Wastewater treatment plants play a role

While not the largest source of “leaks” in the human phosphorus cycle, wastewater contains a significant amount of phosphorus — approximately 4 to 6 mg/L, Barnard said. Using biological methods, it is possible to reduce the load to 1 mg/L or less — recovering as much as 90% for reuse.

By selling the recovered phosphorus as fertilizer, wastewater treatment plants (WWTPs) can play an important — and potentially profitable — role in helping to forestall a shortage. “A large plant that treats 550 mgd [2 million m3/d] could potentially recover up to 110,000 pounds [50,000 kg] of phosphorus a year,” Barnard estimated.      

Some WWTPs are already doing it. The Milwaukee Metropolitan Sewerage District, for example, has been selling its own brand of fertilizer — Milorganite® — since 1925. In Portland, Ore., and elsewhere, smaller WWTPs are using biological methods to recover phosphorus, magnesium, and ammonia and convert it into struvite, a slow-release fertilizer sold to golf courses and nurseries. Still others that discharge into the Great Lakes and Potomac River are removing phosphorus from their effluent — per U.S. Environmental Protection Agency mandate — but then discarding it.

Barnard believes it’s only a matter of time before more U.S. WWTPs recognize the value of the phosphorus flowing through them and begin recovering and selling it. The mineral’s volatile pricing may drive these efforts.

“The price of phosphates rose dramatically in 2007 and 2008 and then retreated,” IFDC’s Scott Mall noted. “While [phosphate prices] are higher now than their historical average of the past 50 years, they are cheaper than in the 2007–08 period.”

GPRI does not want to wait for prices to rise again to take action. “There is a pressing need to develop a coordinated response to global phosphorus scarcity,” its leaders said in their response to the IFDC study.

“For the past 150 years, the mining of phosphorus has been a one-way street,” Barnard said. “We discover it, we mine it, we use it and waste it. We need to break that pattern now, before it’s too late.”

Mary Bufe, WE&T

 

Arsenic stymies aquifer storage and recovery

 

Storing excess water underground for later use is a rapidly growing practice across the globe that cuts costs and stretches a finite resource in the face of population growth and climate change. But when the water is drawn out again for use, it sometimes picks up naturally occurring contaminants in the aquifer. Researchers in Florida have taken the lead on the issue and say they are close to solving the problem of arsenic mobilization in their stored water.

Aquifer storage and recovery (ASR) wells inject treated water 75 to 900 m underground into water-bearing rock aquifers. “The majority of ASR wells inject potable drinking water drawn from surface supplies,” said Mike Annable, an environmental engineer at the University of Florida (Gainesville). But storage of reclaimed wastewater also is a growing trend.

ASR systems enable utilities to serve more customers because excess water is stored in wet months and then is withdrawn through the same well during dry periods or when demand is high. In the past decade, ASR wells near the Peace River in Florida were essential to keeping the water flowing during two drought seasons, avoiding the costly measure of desalinating brackish groundwater, said Joe Haberfeld, a geologist at the Florida Department of Environmental Protection.

The introduced water pushes aside the native water in the aquifer, forming a “bubble” that ranges in size from 49,000 to 9.5 million m3 (13 million to 2.5 billion gal), said David Pyne, president of ASR Systems LLC (Gainesville).

The number of ASR wells has quadrupled since 1999, and there are now more than 542 wells across the southeastern and western U.S., according to the U.S. Environmental Protection Agency (EPA). The United Kingdom, Canada, Australia, South Africa, and Israel also have launched ASR systems, Pyne said. He added that the Netherlands, New Zealand, Thailand, Taiwan, and Kuwait are developing ASR wells.

ASR wells can be built at less than half the cost of traditional structures, such as surface reservoirs, Pyne said, and in some cases, they can help restore aquifers that have been depleted by thirsty cities and farms. Because ASR technology sidesteps the need to construct dams or reservoirs that flood important habitats, it is beneficial to ecosystems, according to Annable. The water is tucked securely underground, where there is no evaporative loss and no chance of becoming tainted by polluted runoff or sabotage by terrorists.

 

Mobilizing arsenic

EPA regulates the ASR process through its underground-injection control program and requires injected water to meet drinking water standards. “But the geochemistry of the introduced water is different from the native groundwater, and it can interact with the native water and rock to mobilize metals,” Annable said. Most notably in Florida, mobilization of arsenic from limestone aquifers has squelched further development of ASR wells, he said.

Even though native groundwater may present arsenic concentrations of 2 to 3 µg/L, Florida researchers have reported arsenic concentrations in ASR wells more than 10 times higher than the federal drinking water standard of 10 µg/L, according to the Florida Department of Environmental Protection. “We don’t know the complete answer to why the injected water elevates arsenic concentrations,” Annable said. But scientists are coming to a consensus that high levels of oxygen in the injected water are liberating arsenic bound up in pyrite deposits in the limestone.

The Floridan aquifer — which underlies Florida and parts of Mississippi, Alabama, Georgia, and South Carolina and provides water for several large cities — is a strongly reducing environment, with no measurable dissolved oxygen, said Don Ellison, a geologist at the Southwest Florida Water Management District. In contrast, treated surface water injected into ASR wells is saturated with oxygen at a concentration of 8 mg/L. The oxygen “rusts” the pyrite, releasing dissolved arsenic.

Lab tests have borne out the hypothesis that arsenic is mobilized by oxidizing substances, Ellison said. “But the strongest evidence came when Tampa [Fla.] switched from chloramines to ozone to disinfect their water, and arsenic levels jumped three times in their ASR wells,” he said.

 

Degasification or management?

Although there is no consensus on the best way to address the arsenic mobilization, a lot of attention is being paid to removing oxygen from water prior to storing it underground, Annable said. The City of Bradenton, Fla., is wrapping up a pilot program to remove oxygen so as not to mobilize arsenic from the Floridan aquifer, according to Seth Kohn, city engineer.

The Bradenton test well can inject or withdraw a volume of 3785 m3/d (1 mgd). Before injecting water, the researchers used membrane technology to cut dissolved-oxygen levels in the treated surface water from 8.4 mg/L down to between 9 and 10 µm/L, Kohn said. To date, the scientists have recovered about half of the degasified water from the well and have not been able to detect any arsenic, said Stuart Norton, an ASR project specialist for the City of Bradenton. “This is the first known application of degasification technology applied to ASR in the world,” he added.

The Bradenton facility cost roughly $1.6 million, and the pilot degasification system cost about $700,000, Norton said. Once all the kinks are worked out, a full-scale gasification system operating at 3785 to 7570 m3/d (1 to 2 mgd) could be built for less than $1 million. The cost is still well below the price tag for a reservoir, Norton said.

Meanwhile, others advocate for a simpler and cheaper solution. After four to eight cycles of injection, storage, and recovery, arsenic concentrations usually decline to acceptable levels in ASR wells, said Pieter Stuyfzand, a hydrogeologist at the University of Amsterdam. Arsenic attenuates because over time it leaches out of the pyrite and there is less to become mobilized.  In addition, a buffer zone consisting of a mixture of stored water and ambient groundwater forms around the core of injected water, preventing remobilization of arsenic. The recovered water from the first few cycles can be treated or blended with other water to meet drinking water standards until the well is sufficiently conditioned, he said.

“A more efficient approach is to form the buffer zone initially, prior to beginning cycle testing,” Pyne said. When water is injected, it forms a bubble of stored water extending as far as 300 m (1000 ft) away from the well. A buffer zone separates the stored water from the native groundwater and consists of a mixture of groundwater and stored water, Pyne said. Dissolved arsenic reprecipitates onto ferric hydroxide within the buffer zone and remains tied up there as long as the buffer zone is maintained intact. He recommends that if utilities want to pump 7570 m3/d (2 mgd) for 90 days, they should store at least 1.4 million m3/d (360 million gal) in the ASR well, twice the volume that they will pump out.

But adopting Pyne’s approach would require changing federal regulations that say drinking water standards for arsenic cannot be exceeded when injecting water into aquifers, Haberfeld said. Currently, the state has issued temporary enforcement documents that allow ASR operations to continue cycle-testing of wells, even if testing trips the arsenic standard for groundwater. The hope is that continued testing will mitigate arsenic levels. “However, we haven’t seen too many cases of continued cycle-testing resulting in cessation of arsenic mobilization,” Haberfeld said.

Ellison confirms that arsenic levels at one ASR well near Tampa dropped from a peak of 180 mg/L to 30 mg/L during a 5-year period. “The levels attenuate because pumping out the water removes arsenic and because remaining arsenic in the pyrite becomes sealed in with an armored coating of rust,” Ellison explained. Meeting the old arsenic standard of 50 µg/L would be no problem for most ASR wells, but meeting the new 10 µg/L standard is a challenge, he added.

Pyne responded that well managers in Florida are making a mistake by recovering all of the injected water. He said that if they left a buffer zone in place, they would be more likely to achieve the drinking water standard for arsenic.

Taming arsenic is a critical challenge for the Everglades restoration program, which aims to build more than 300 ASR wells; combined, these wells could store and recover as much as 7.6 million m3/d (2 billion gal/d) of water. “But the water management district and the U.S. Army Corps of Engineers have said that if the restoration program has to pretreat water to primary drinking water standards, the wells won’t be economical,” Haberfeld said. “We’ll need more than what we’ve got now from the EPA to allow expansion of the Everglades restoration program.”

The Florida Department of Environmental Protection is working with EPA to come to an understanding of how utilities can go forward with ASR, a process that is helping them save water resources and reduce costs, Haberfeld said. “Everyone hopes the EPA will be able to issue guidance in the future on how we can live with the regulations the way they are and still allow these projects to go forward,” he said. The state would like EPA to allow facilities to operate ASR systems as long as they don’t endanger people’s health and don’t put any drinking water supplies or well fields at risk. “However, without this flexibility from EPA, a change in the federal regulations will be needed,” he said.

 

Janet Pelley, WE&T

 

©2011 Water Environment Federation. All rights reserved.