PHORPHORUS REMOVAL

Phosphorus is found in waste water in three principal forms, orthophosphate, polyphosphates, and organic phosphorus compounds.  Typically testing is done for total phosphorus so the actual forms of the 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 waste water.  As the bacteria use the phosphorus it converts typically to ortho phosphate.  Ortho phosphate is the easiest form of phosphate to remove so this is good.  Industrial waste water 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, the mole ratio for Al:P is 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 95 lbs of PO4 to form 122 lbs of AlPO4.  since each 95 lb (1 lb-mole) of PO4 contains 31 lbs (1 lb-mole) of P, the weight relationship between Al and P is 27 lbs of Al to 31 lbs of P or 0.87 for this reaction.

We have products that contain aluminum in various different forms.  Here is a list of products with their approximate amounts of aluminum

AQ 60             5%
AQ 64             5%
AQ 66             8%
AQ 100           12.2%
AQ 120           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 ration Al:P = 1:1
Weight ratio  Al:P = 27 : 31 = 0.87 :1
AQ 100 contains 12.2% Al
Therefore, AQ 100 required per lb of P = 0.87/.122 = 7.13 lb
AlPO4 the compound formed 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 typically is unrealistic and not attainable in waste waters due to cost.  You can get removal at higher pH’s but the efficiency drops.  Additionally the acidity of the iron compounds requires pH neutralization using caustic or lime.  

Our Iron products are AQ 126 and AQ 129.  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 what precipitant is the least expensive.