Phosphorus is found in wastewater in three principal forms, Orthophosphate,Polyphosphates, and organic phosphorus compounds. Typically testing is done for total phosphorus so the actual forms of th phosphorus is not very important. Bacteria in a municipal waste treatment plant need a certain amount of phosphorus to grow and digest the contaminants that are in the wastewater. As the bacteria use the phosphorus it converts typically to orthophosphate. Orthophosphate is the easiest form of phosphate to remove so this is good. Industrail wastewater can have more polyphosphate that has not been digested. All the chemistries work in varying degrees on all the forms of phosphorus.
To reach the levels of removal of phosphorus that is required for discharge chemistry needs to be used. The chemicals that are typically used are aluminum based, iron based or calcium based.
ALUMINUM CHEMISTRY FOR REMOVING PHOSPHORUS
Aluminum ions can combine with phosphate ions to form aluminum phosphate as follows
Al 3+ + PO43- ------>AlPO4
The above equation indicated that the mole ration for the Al:PO4 is 1:1. in as much as the mole ration of P:PO4 is also 1:1 or Al/P = 1 when both aluminum and phosphorus are expressed in terms of gram-moles or lb moles. On a weight, rather than a mole basis, this means that 27 lbs of Al will react with 95lbs of PO4 to form 122lbs of AlPO4. Since each 95lb (1 lb-mole) of PO4 contains 31 lbs (1lb-mole) of P, the weight relationship between Al and P is 27 lbs of Al to 31lbs of P or 0.87 for this reaction.
We have products that contain aluminum in various forms. Here is a list of the products with their approximate amounts of aluminum.
- AQ60 5%
- AQ64 5%
- AQ100 12.2%
- AQ120 4%
With these percentages you can calculate the amount of product required to precipitate out the phosphorus. It should be noted that these calculations are only a beginning step and can vary from the real world due to variations in the types of phosphorus and other factors.
Mole ratio Al:P = 1:1
Weight ratio Al:P = 27 : 31 = 0.87 : 1
AQ100 contains 12.2% Al
Therefore, AQ100 required per lb of P = 0.87/.122 = 7.13 lb
AlPO4 the compound forned when combining aluminum and phosphorus is least soluble at a pH of approximately 6.0. At higher pH's phosphorus will still precipitate out of solution, but dosages can be higher.
IRON COMPOUNDS AS PHOSPHORUS PRECIPITANTS
Both ferrous and ferric ions can be used in the precipitation of phosphorus. With Ferric the reaction is very similar to that of Aluminum chemistry. It is a 1 : 1 mole ration of FE:PO4. Since the mole weight of iron is 55.85, the weight reaction of Fe:P is 1.8.1 Just as in the case with Aluminum chemistry potentially a larger amount of iron chemistry is required to precipitant the phosphorus than is predicted by the equation.
Iron salts are most effective for phosphorus removal at certain pH values. For Ferric the optimum pH range is 4.5 to 5.0. This is typically is unrealistic and not attainable in wastewaters due to cost. You can get removal at highter pH's but the efficiency drops. Additionally the acidity of the iron compounds requires pH neutralization using caustic or lime.
Our Iron products are AQ126 and AQ129. Both are approximately 12% iron in solution.
From all this information a dosage can be calculated and cost of materials applied to the information to determine whtat precipitant is the least expensive.