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Recovery and Recycling of Phosphates
Phosphorus is one of the more common elements on earth and is essential to all living organisms. It is found combined with other elements in the earth's crust in the form of phosphate rock. The major commercial deposits are in the United States, China, the former Soviet Union, Morocco, Finland, South Africa, and some Pacific Islands. It is estimated that there are 40 billion tons of reserves of phosphate rock or a 250-year supply at current usage. This may seem like an abundant supply, but some of this material is not accessible or of poor quality. In addition, we are using this material at an increasing rate.
Over 80% of the ore currently mined is used in fertilizer products. As pressure for greater agricultural yields intensifies, more fertilizer is applied to replenish the nutrients removed by crops. In earlier times, food was consumed near the place of production, and the animal and human wastes were returned to the land. This natural recycling loop for nutrients has been broken due to societal changes.
Sewage treatment plants are potential point sources of phosphorus for reuse. Presently, most sewage treatment facilities in Europe and North America remove phosphorus prior to further processing. This has been typically done by precipitation with iron or aluminum salts. Unfortunately, phosphorus in this particular form is not readily usable as a nutrient. There is work underway in Sweden and other countries to convert the iron salts to iron sulfide by biological processes. The phosphorus would then be released in a more soluble form.
Treatment facilities in the Netherlands have begun recovering phosphorus as a calcium salt, used further as a raw material for other phosphate products. Japan and Australia are removing phosphorus in the form of struvite, a magnesium ammonium phosphate compound, which is a valuable fertilizer containing these three nutrients. None of these options are technically or commercially feasible in every site or country, so further research is underway.
Large feedlots are another point source of recoverable nutrients. To date this resource has not been exploited to any great extent. Some hog farms in Holland are experimenting with recovery technologies. In the United Kingdom, chicken litter is incinerated for energy and the ash recycled as slow-release fertilizer. Recovery of nutrients from animal waste is an area of great potential, but much more research is required to develop a form that can be stored during non-fertilizing seasons and is economical to transport to nutrient-deficient areas.
Many industries, such as chemical processing, pharmaceutical production, metal finishing, food processing, and others, produce waste streams that could be rich sources of phosphorus. Recovery of this material in a form that is useful as a fertilizer or another feedstock is a laudable goal. As each site and process is unique, so will be the solutions.
The Phosphate Forum of the Americas has sponsored two studies to examine the economics of phosphorus recovery at sewage treatment facilities and at poultry and potato processing sites. While technically feasible, the present low cost of landfilling waste does not make recovery economically attractive.
The Centre Europeen d'Etudes des Polyphosphates (CEEP), the European Chemical Industry Council’s (CEFIC) sector group representing European producers of polyphosphates, has been instrumental in sponsoring research in the recovery and recycling of phosphates. They have sponsored two conferences on this subject.
Further information may be obtained at the following websites:
US Environmental Protection Agency study “Phosphorus Recovery from Sewage”:
Phosphorus & Potassium, Issue No. 216, July-August 1998.
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