Phosphorus Removal

Phosphorus (usually in the form of phosphate) is a normal part of the environment. It occurs in the form of phosphate containing rocks and as the excretory and decay products of plants and animals. Human contributions to the phosphorus cycle result primarily from the use of phosphorus-containing detergents and fertilizers.

The increased load of phosphorus in the environment as a result of human activities has been a matter of concern for more than four decades. The primary issue has been to what extent additional phosphorus has contributed to the eutrophication of lakes, ponds, and other bodies of water. Scientists have long recognized that increasing levels of phosphorus are associated with eutrophication. But the evidence for a direct cause and effect relationship is not entirely clear. Eutrophication is a complex process involving nitrogen and carbon as well as phosphorus. The role of each nutrient and the interaction among them is still not entirely clear.

In any case, environmental engineers have long explored methods for the removal of phosphorus from wastewater in order to reduce possible eutrophication effects. Primary and secondary treatment techniques are relatively inefficient in removing phosphorus with only about 10% extracted from raw wastewater in each step. Thus, special procedures during the tertiary treatment stage are needed to remove the remaining phosphorus.

Two methods are generally available: biological and chemical. Bacteria formed in the activated sludge produced during secondary treatment have an unusually high tendency to adsorb phosphorus. If these bacteria are used in a tertiary treatment stage, they are very efficient in removing phosphorus from wastewater. The sludge produced by this bacterial action is rich in phosphorus and can be separated from the wastewater leaving water with a concentration of phosphorus only about 5% of its original level.

The more popular method of phosphorus removal is chemical. A compound is selected that will react with phosphate in wastewater, forming an insoluble product that can then be filtered off. The two most common substances used for this process are alum, aluminum sulfate and lime, or calcium hydroxide. An alum treatment works in two different ways. Some aluminum sulfate reacts directly with phosphate in the wastewater to form insoluble aluminum phosphate. At the same time, the aluminum ion hydrolyzes in water to form a thick, gelatinous precipitate of aluminum hydroxide that carries phosphate with it as it settles out of solution.

The addition of lime to wastewater results in the formation of another insoluble product, calcium hydroxyapatite, which also settles out of solution.

By determining the concentration of phosphorus in wastewater, these chemical treatments can be used very precisely. Exactly enough alum or lime can be added to precipitate out the phosphate in the water. Such treatments are normally effective in removing about 95% of all phosphorus originally present in a sample of wastewater.