Closing the phosphorus loop
A developing green technology removes phosphorus from wastewater streams, yielding phosphate rocks that can be reused in the phosphate and fertilizer industries.
Sources of high-grade phosphate ore deposits could disappear within the next 100 years at current use rates, analysts say. In the hunt for alternatives to mining this nonrenewable, unique resource, which is necessary for agriculture and manufacturing consumer products, researchers have found one possible surprising new source: sewage treatment plants.
About 90% of the phosphate dug out of the ground goes into fertilizer; the remainder is used in products, including food additives, glues, flame retardants, and detergents, according to Chris Thornton, a research coordinator for the Centre Européen d’Etudes des Polyphosphates (CEEP), the phosphate industry research association housed at the European Chemical Industry Council. “If it runs out, then there’s no more agriculture as we know it,” Thornton points out. “The amount that can be reasonably recycled from sewage is not a huge percentage of the total amount being used, but it’s still significant.”
Most wastewater facilities in Europe and North America remove phosphorus to prevent algal blooms in receiving waters. Biological and chemical treatment methods concentrate the phosphorus into a residual sludge that is typically applied to agricultural fields as a fertilizer. But around big cities, particularly in Europe, less and less farmland is available for spreading the sludge. Concerns over contaminants, such as heavy metals, hormones, and pharmaceutical residues, contained in the sludge have led to more of it being placed in landfills or incinerated. The demand for other disposal outlets has led researchers to pursue technologies to recover the phosphorus contained in sewage in a form that can be recycled by the phosphate and fertilizer industries.
One of these technologies uses calcium silicate hydrates (CSHs), which are byproducts of the building material industry, to remove phosphorus through crystallization. It’s a simple, one-step process that removes phosphorus directly from the wastewater stream and recovers it without the addition of further chemicals, says Ute Berg, a geoecologist with the water and geotechnology division of the Institute for Technical Chemistry at the Karlsruhe Research Center (Germany). She presented results from long-term lab and pilot-scale experiments at the Water Environment Federation’s conference last year in Dallas, Texas.
The hydrates do not require pretreatment steps that other crystallization methods under development need, such as major pH adjustments and later removal of CO2, Berg explains. “We just add this starting material and have ready precipitation of calcium phosphate,” she says. In this way, Berg and her colleagues have been able to produce a secondary phosphate rock containing 12–13% phosphorus. “We’re nearly at the 13–15% levels that raw, high-quality phosphate ores contain,” adds Peter Weidler, chief physicist at the Karlsruhe Research Center.
Other research projects in Germany and elsewhere focus on the recovery of phosphorus from sewage-sludge ashes and from the sludge itself. The processes under investigation potentially can recover 90% of phosphorus, compared with 40–50% recovery for the CSH process, Weidler notes. The disadvantage is that chemical or chemical–thermal treatment steps are required to redissolve the phosphorus to separate it out. These additional steps translate into higher chemical and energy costs and larger initial volumes of sludge.
“The optimal recovery method is still an open question,” says Christian Schaum of Technische Universität Darmstadt (Germany), and will depend on local requirements for sludge disposal. Both Germany and Sweden have announced national objectives for phosphorus recovery from recycled sewage, but recovery rates have not yet been set.
The uncertainty remains partly because none of the recovery options is as yet technically or commercially feasible at every sewage treatment plant, Thornton says. The main problem is economic: phosphate prices on the world market still fall below the costs of any of the recovery methods currently used at sewage treatment plants. “The economic driver is less the actual value of the fertilizer you recover but more the money you save in having to dispose of” the sludge, Thornton explains, because all the recovery methods reduce the amount of leftover sludge.
On an industrial timescale, dwindling phosphate reserves “are not of immediate concern to us,” Thornton admits, but CEEP nonetheless has sponsored research in recycling the phosphorus recovered from sewage. “We used to be using ores that were 18% phosphorus, and now we’re down to 15%. If we can move to using recycled phosphate rather than a raw material from nature, then that’s a move toward sustainable development,” he says.
Known phosphate reserves (63KB PDF) stand at approximately 50 billion tons, according to the U.S. Geological Survey (USGS). The largest deposits are concentrated in northern Africa, China, the Middle East, and the U.S. Although no global shortages are expected in the near future, USGS is conducting a cooperative international effort known as the Global Mineral Resource Assessment Project (623KB PDF) begun in 2002 to identify land areas with the potential to contain undiscovered nonfuel mineral resources, including phosphate.
