Phosphorus is an important nutrient for organisms. Many agricultural and industrial processes also use phosphorus, which has led to increased concentrations in runoff and effluents1. Higher phosphorus concentrations in water bodies can lead to eutrophication and algal blooms that harm aquatic species2. The ionic form of phosphorus, phosphate (POs4-), bonds with positively-charged ions (hydrogen) and metals to form acids and solids in water. Phosphorus and phosphates are often used interchangeably in water quality discussions.
Wastewater treatment facilities are often responsible for removing phosphorus from effluents3. Typical treatment approaches include physical treatments (sand and membrane filtration), chemical treatments (precipitation), and biological treatments (i.e. enhanced biological phosphorus removal). During chemical treatment, phosphate ions bond with metal additives and precipitate out of solution. Wastewater treatment facilities typically use iron (Fe3+) and aluminum (Al3+) as cost-effective additive compounds to facilitate precipitation4. Approximately 1.8 pounds of iron must be added to remove 1 pound of phosphorus from solution for moderate phosphorus concentrations. Aluminum coagulants are more efficient and require fewer additives per pound of phosphorus.
Phosphorus presents an increasingly pertinent issue for treatment plant managers and several issues are of concern. First, precipitating phosphorus using iron, which is affordable and abundant, can cause wastewater pH to drop by approximately 0.1 for every 10mg/L of iron added5. This pH drop necessitates further treatment to increase alkalinity. Second, as phosphorus concentrations in wastewater decrease, it becomes increasingly difficult to remove. Additional coagulants are necessary, which compounds associated pH problems. Third, in anoxic conditions, iron phosphate compounds are more likely to release phosphate, which can counteract treatment and harm downstream water quality6. Treatment systems that utilize anaerobic processes may find it difficult to control such effects. Finally, phosphorus removal increases sludge and disposal needs7.
Phosphorus removal in treatment facilities is likely to remain important to meet water quality requirements for rivers and estuaries. Phosphorus recovery for fertilizer use offers a potential avenue to supplement treatment costs8. Thus, phosphorus removal is likely to present both challenges and opportunities for treatment in future decades.
References
1. Lee, G. F., Rast, W. & Jones, R. A. Water Report: Eutrophication of water bodies: Insights for an age old problem. Environmental Science & Technology 12, 900–908 (1978).
2. Yeoman, S., Stephenson, T., Lester, J. & Perry, R. The removal of phosphorus during wastewater treatment: A review. Environmental Pollution 49, 183–233 (1988).
3. Perry, S., Barbon, J. & Lee, B. Phosphorus Treatment: Advanced Removal Mechsnisms and Amended Design for Stormwater BMPs. Proceedings of the 2009 Georgia Water Resources Conference (2009).
4. Bratby, J. Coagulation and flocculation in water and wastewater treatment. (IWA Publishing: London; Seattle, 2006).
5. The Water Planet Company Phosphorus Removal from Wastewater: Phosphorus Chemistry. (New London, CT, 2012).at <http://www.thewaterplanetcompany.com/docs/10pdf/Phosphorus%20Chemistry.pdf>
6. Shapiro, J., Levin, G. & Zea, H. Anoxically induced release of phosphate in wastewater treatment. Journal of the Water Pollution Control Federation 39, 1810–18 (1967).
7. Strom, P. Technologies to Remove Phosphorus from Wastewater. (Rutgers University: 2006).at <www.water.rutgers.edu/Projects/trading/p-trt-lit-rev-2a.pdf>
8. Tweed, K. Sewage Industry Fights Phosphorus Pollution. Scientific American (2009).at <http://www.scientificamerican.com/article.cfm?id=sewages-cash-crop>