Aeration Oxidation for Iron and Manganese Removal

A. Chemical Properties of Water

Water pH is a critical parameter for the oxidation and precipitation of iron and manganese. For aerated oxidation of iron, the pH of the water should be at least 7.2, and ideally should be maintained between 7.5 and 8.0. If manganese is present, a minimum pH of 9.5 is recommended, below which the air oxidation rate of manganese is very low. If the pH or alkalinity is low, it may be necessary to add an alkaline ingredient such as sodium hydroxide to raise the pH.

B. Other Factors

The air oxidation of iron and manganese is not completed immediately. Based on this consideration, it is recommended to use a contact tank or retention tank to provide sufficient time to complete oxidation and sedimentation. According to actual conditions, the contact time is in the range of 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 iron atom reaction weight is 4 times this value, or 223.39. The molecular weight of oxygen is 31.999 and the reaction ratio is 31.999 / 223.39, or 0.1432. This means that 0.1432 mg/L of oxygen is required to oxidize 1 mg/L of iron, measured as iron.

The atomic weight of manganese is 54.938. When two manganese atoms react with one oxygen molecule, the manganese atom reaction weight is 2 times this value, or 109.88. The molecular weight of oxygen is 31.999 and the reaction ratio is 31.999 / 109.88, or 0.2912. This means that 0.2912 mg/L of oxygen is required to oxidize 1 mg/L of manganese, measured as manganese.

E. Residual Oxygen

Sufficient air must be injected into the water to maintain the required residual oxygen for the purpose of:

1. Provide a buffer to cope with fluctuations in iron and manganese levels.
2. Make water quality more ideal.
3. Create a mixing effect of oxygen and water, making the reaction between iron, manganese and oxygen more rapid and 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 water flow

G. Theoretical Air Requirement

The density of air at 20°C and 1 atmosphere is 1.2047 g/L. The oxygen content of air is 20.95%, and 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 for iron is 7.16 mg/L and the water flow is 100 L/min.
Theoretical air requirement = (100 L/min) × (7.16 mg/L) / 252.4 mg/L = 2.84 L/min air

H. Actual Air Requirement

The efficiency of the ejector is between 25% and 30%, taking a conservative value of 25%. The actual air requirement is 4 times the theoretical air requirement. Therefore, the actual air requirement is 11.36 L/min. Depending on special circumstances, a safety factor of 10% to 20% can be added to this basis.