Chemical treatment using ferric chloride or aluminum sulfate in combination with calcium hydroxide has been studied for the purpose of removing phosphorus from ortho-phosphate, ortho-and pyro-phosphate and ortho- pyro- and tripoly-phosphate solutions in the pH range 7.5 to 9.5.
The data showed that pyro- and tripoly-phosphate could be more easily removed than ortho-phosphate in the experimental pH range. The data also showed that the combination of ferric chloride and calcium hydroxide had better settling ability than the combination of aluminum sulfate and calcium hydroxide.
With a molar ratio of 1:1 between iron or aluminum and phosphorus as ortho-phosphate and 150 mg/1 of calcium hydroxide, a supernatant of about 1 mg/1 P was obtained from a phosphorus solution of 10 mg/1 P as ortho-phosphate, 4 mg/1 P as pyro-phosphate and 6 mg/1 P as tripoly-phosphate near pH 9. After filtration through a sand column, the residual phosphorus was reduced to about 0.6 mg/1 P. The residual phosphorus could be further reduced to less than 0.2 mg/l P during the first eight hours and about 0.3 mg/1 P even after 24 hours by using a mixed column of sand and soil to replace the sand column.