The two most used coagulants to chemically remove phosphorus from wastewater are aluminum sulfate, commonly known as alum, and ferric sulfate.
The following equations show the predominant chemical reactions involved during the chemical phosphorus removal from wastewater:
Between the “Al3+” cation of aluminum sulfate and the “PO43-” anion of orthophosphates to form “AlPO4” aluminum phosphate
Al2(SO4)3 + 2 PO4→ 2 AlPO4 ↓ + 3 SO4
Between the ferric sulfate cation “Fe3+” and orthophosphate anions “PO43-” to form iron phosphate “FePO4“
Fe2(SO4)3 + 2 PO4→ 2 FePO4 ↓ + 3 SO4
Theoretical Phosphorus Removal values for the two most common coagulants, alum and ferric sulfate, based on chemical reaction stoichiometry, are as follows.
Theoretical dosage kg metal/kg P
% metal in coagulant
12.5% Iron in ferric sulfate AQ-8810
4.3% Al in alum AQ-8924
Theoretical dosage kg coagulant/kg P
14.5 kg ferric sulfate/kg P to remove
20.9 kg of AQ-8924/kg P to remove
In theory, if the coagulant had to react only with the phosphorus contained in the untreated water, it would require 0.9 kg of aluminum to remove 1 kg of total P or 1.8 kg of iron to remove 1 kg of P. Unfortunately, the aluminum contained in alum, or the iron contained in ferric sulfate does not react only with phosphorus, but also with all the other negative charged particles contained in the untreated water. Coagulation aims to destabilize them. It involves neutralizing their opposite electrostatic charges to enable them to agglomerate. When inorganic coagulants, such as alum or ferric sulfate, are added, the metal ions (Al3+ or Fe3+) are hydrolyzed to form a precipitate (floc or pinfloc) of aluminum hydroxide or iron hydroxide and H+ hydrogen ions, in accordance with the following chemical reactions:
This means that the dosages required to chemically “dephosphate” wastewater will inevitably be higher than the theoretical dosages resulting from chemical reactions, as phosphorus will have to compete with all the other contaminants contained in the water to be removed. The required coagulant dosages will also depend on the characteristics of the wastewater to be treated, the initial phosphorus concentration and the phosphorus removal target conditional on the discharge limits set by the city sewer system or the environment. Furthermore, a low initial phosphorus concentration will require a high ratio of coagulant injection/kg phosphorus removed to achieve an even lower P concentration.
Be aware that in some instances, the use of a pre-hydrolyzed coagulant that consumes less alkalinity in your wastewater would be preferable to remove phosphorus from your wastewater. You can rely on AQUASAN’s technical team to determine which coagulant is best suited to “dephosphate” your wastewater, considering the characteristics of your effluent and treatment process. We have extensive technical expertise in wastewater treatment.
We are dedicated to providing you with an innovative, straightforward-to-use chemical program that delivers the performance you need to meet your performance objectives. We have outstanding training and technical expertise, and we know how to provide innovative solutions to ensure that your treated water remains of the highest quality.