Aeration Oxidation To Remove Iron and Manganese

A. Chemical Properties of Water

The pH value of water is a critical parameter for the oxidation and settling of iron and manganese.

For the aeration oxidation of iron, the pH value of water should be at least 7.2, and ideally maintained between 7.5 and 8.0.

If manganese is present, the minimum recommended pH value is 9.5; below this value, the air oxidation rate of manganese is very low.

If the pH value or alkalinity is low, it may be necessary to add an alkaline material, such as caustic soda (sodium hydroxide), to raise the pH value.

B. Other Factors

The air oxidation of iron and manganese is not completed immediately. For this reason, it is recommended to use a contact tank or retention tank to provide enough time to complete the oxidation and settling. Depending on actual conditions, the contact time ranges from 5 to 15 minutes.

C. Reaction Equation

Iron ion oxidation equation:
4Fe(HCO₃)₂ + O₂ + 2H₂O → 4Fe(OH)₃ + 8CO₂

Manganese ion oxidation equation:
2Mn(HCO₃)₂ + O₂ + 2H₂O → 2Mn(OH)₄ + 4CO₂

D. Ratio

The atomic weight of iron is 55.847. When one oxygen molecule reacts with four iron atoms, the reactive weight of the iron atoms is 4 times this value, i.e., 223.39. The molecular weight of oxygen is 31.999, so the reaction ratio is 31.999/223.39 = 0.1432. This means that oxidizing 1 mg/L of iron requires 0.1432 mg/L of oxygen (measured as iron).

The atomic weight of manganese is 54.938. When two manganese atoms react with one oxygen molecule, the reactive weight of the manganese atoms is 2 times this value, i.e., 109.88. The molecular weight of oxygen is 31.999, so the reaction ratio is 31.999/109.88 = 0.2912. This means that oxidizing 1 mg/L of manganese requires 0.2912 mg/L of oxygen (measured as manganese).

E. Residual Oxygen

Sufficient air must be injected into the water to maintain the required residual oxygen, in order to:

• Provide a buffer (flexibility) to cope with fluctuations in iron and manganese levels.
• Improve water quality.
• Create a mixing effect between oxygen and water, making the reaction between iron, manganese and oxygen faster and more complete.

F. Theoretical Oxygen Demand

Theoretical oxygen demand = [Xf·(Fe)] + [Xm·(Mn)] + R

Xf = iron reaction coefficient
(Fe) = iron concentration, mg/L
Xm = manganese reaction coefficient
(Mn) = manganese concentration, mg/L
R = final residual oxygen = (5.0 − initial oxygen) mg/L

Example: (Fe) = 10 mg/L, (Mn) = 2.5 mg/L, initial oxygen = 0.0 mg/L
Oxygen demand = 0.1432 × 10 + 0.2912 × 2.5 + 5.0 − 0.0
= 1.432 + 0.728 + 5
= 7.16 mg/L of water flow

G. Theoretical Air Requirement

At 20°C and 1 atmosphere, the density of air is 1.2047 g/L. The oxygen content of air is 20.95%, so the oxygen content per liter of air is (1.2047 g/L) × (0.2095) = 0.2524 g/L = 252.4 mg/L.

Example: The oxygen demand is 7.16 mg/L, and the water flow rate is 100 L/min.
Theoretical air requirement = (100 L/min) × (7.16 mg/L) / 252.4 mg/L = 2.84 L/min of air

H. Actual Air Requirement

The efficiency of jet aerators is between 25% and 30%; a conservative value of 25% is used. The actual air requirement is 4 times the theoretical air requirement.

So the actual air requirement is 11.36 L/min. Depending on special circumstances, a safety factor of 10–20% can be added on this basis.