Technology▼Solutions Phosphorus removal from agricultural effluents
© 2003 American Chemical Society
The researchers claim that biofouling is not a problem with the synthetic gel; however, they used filtered samples in their experiments. “The filtering was very crude. It was done just to remove large particulates, so I do not expect any biofouling with unNATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION/DEPT. OF COMMERCE
Until recently, agricultural wastewater regulations focused primarily on nitrogen, but permitting requirements released by the U.S. EPA and the U.S. Department of Agriculture in December 2002 put a new emphasis on phosphorus. As a result, technologies that remove phosphorus from wastewater are needed to prevent surface waters from becoming further impaired. The new requirements, which apply to large-scale poultry, swine, sheep, and cattle operations, are expected to reduce the amount of phosphorus released into the environment by 56 million pounds. Although aquaculture effluents are not covered under the new rule, they too contribute large amounts of phosphorus into receiving waters. Guidelines proposed by EPA in August 2002 for large-scale aquaculture facilities are expected to reduce the discharge of nutrients by 8.7 million pounds per year. Various industrial waste materials, such as slag from the steel industry and fly ash, have been shown to adsorb phosphorus in laboratory column studies (Environ. Sci. Technol. 2002, 36, 4642–4648). Such materials, however, often raise the effluent pH, making it unacceptable for discharge into surface waters. Commercial ionexchange resins are also available for removing phosphorus from wastewater effluents, but most cannot be regenerated, suffer from biofouling, and break down after extended use. Peter Kofinas and Dimitri Kioussis at the University of Maryland–College Park have developed a synthetic polymer hydrogel that overcomes many of these limitations. Originally developed as a phosphate binding pharmaceutical for chronic kidney failure, the gel was shown to remove up to 99% of phosphate from aquaculture and poultry effluents in preliminary laboratory studies (Environ. Sci. Technol. 2003, 37, 423–427). The gels can be regenerated and used over multiple cycles.
New technologies are needed to remove phosphorus from wastewater generated by fish hatcheries, such as the one shown above.
filtered samples,” says Kofinas. Experts in the wastewater treatment field, however, disagree. “A very common problem with ion-exchange resins or hydrogels is that they plug up with solids and you can’t pass water through them,” says David Jenkins, emeritus professor of civil and environmental engineering at the University of California–Berkeley and consultant for the wastewater treatment industry. “I find the [hydrogel] method of phosphorus removal to be rather impractical for the treatment of wastewaters.” “Organic matter and suspended solids would certainly cause problems if they did not filter the effluents,” agrees Aleksandra Drizo, an associate researcher at Ecole Polytechnique of
Montreal in Canada, who has spent several years working in the area of phosphorus removal in both the United Kingdom and Canada. “The effluent of a typical fish farm has very high volumes. A medium-sized trout farm that produces 100 tons/year needs about 10,000 m3/day of fresh water to maintain a minimum ammonia concentration. Such a flow rate corresponds to what would be used in a community of 40,000 people,” she says. Primary treatment, such as a microscreen to filter out the organic particulate matter, is therefore recommended in the field, she adds. One advantage of a synthetic resin could be that it is easier to control its composition, says Drizo, who has worked with several industrial waste materials for removing phosphorus. “When we wanted to employ slag in a fish farm in Quebec, the [Canadian] Ministry of Environment was initially concerned about the metal content in slag material. However, after we carried out a leachate study and proved that the metals are locked within the material matrix, they approved the demonstration project,” she says. Still, most experts say they don’t see much advantage to using a synthetic gel over other existing technologies already on the market, unless there is an economic advantage, which there doesn’t appear to be. “There are other technologies for dealing with this problem already being used,” says Jenkins. “Where there are concerns with loading of the land with phosphate from the disposal of manures, people add a precipitant [such as ferrous or ferric salts and aluminum salts] that fixes the phosphate in an insoluble form so it can’t be leached.” Another common method involves manipulation of the wastewater treatment process so that microorganisms take up more phosphorus, says Jenkins. With regard to aquaculture and poultry waste, the precipitant method is likely to be the most promising, he says. —BRITT E. ERICKSON
MARCH 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 93 A
