Aeration To Remove Hydrogen Sulfide

Task: Determine the theoretical air requirement for the oxidation of hydrogen sulfide in wastewater. This air requirement can be estimated very accurately, which determines the operating conditions and model of the jet aerator to be used.

A. Chemical Properties of Water

The pH value of water is not as critical as for the oxidation and settling of iron and manganese ions. For hydrogen sulfide, a pH value between 6.8 and 7.5 is sufficient. At this pH value, hydrogen sulfide dissociates into ionic forms: H ₂S → 2H⁺ + S^2-. The S ions can then react freely with oxygen.

B. Other Factors

The air oxidation of hydrogen sulfide is not immediate. Therefore, a retention time or a contact tank is needed to provide enough time for the reaction and settling to take place. Depending on actual conditions, the contact time is 5 to 15 minutes.

C. Reaction Equation

2H₂S + O₂ → 2H₂O + 2S⁰
Note: S⁰ is elemental sulfur.

D. Stoichiometric Relationship of Sulfur Oxidation

The atomic weight of sulfur is 32.06; one oxygen molecule reacts with 2 sulfur atoms, and the molecular weight of oxygen is 31.999. The reaction ratio is 31.999/32.06 = 0.9981, or approximately 1.0, which means that 1 mg/L of oxygen can oxidize 2 mg/L of hydrogen sulfide (measured as sulfur).

E. Residual Oxygen

1. Provides a buffer for fluctuations in hydrogen sulfide.
2. Improves water quality.
3. Improves mixing so that the reaction between hydrogen sulfide and oxygen is more complete and faster. The residual oxygen in the water should be 5 mg/L. If the water already contains initial oxygen, the residual oxygen value should be 5 mg/L minus the initial oxygen.

F. Theoretical Oxygen Demand

The formula for calculating the theoretical oxygen demand for the oxidation of hydrogen sulfide:

Theoretical oxygen demand = [XS·(S)] + R

X = hydrogen sulfide reaction coefficient
(S) = hydrogen sulfide concentration, mg/L (as sulfur)
R = final residual oxygen = (5.0 − initial oxygen) mg/L

Example: (S) = 25 mg/L, initial oxygen = 0.0 mg/L
Oxygen demand = (1.0) × (25) + (5.0 − 0.0) = 25 + 5 = 30 mg/L of water flow

G. Theoretical Air Requirement

The density of air at 20°C and 1 atmosphere is 1.2047 g/L. Under these conditions, the oxygen content of air is 20.95%, so 1 liter of air contains (1.2047 g/L) × (0.2095) = 0.2524 g/L = 252.4 mg/L of oxygen. To calculate the theoretical air requirement for oxidizing hydrogen sulfide, the water flow rate must be known; if the hydrogen sulfide concentration is known, it is convenient to use kiloliters as the unit of water flow.

Example: Using the data above and a water flow rate of 100 L/min.
Theoretical air requirement = (100 L/min) × (30 mg/L) / 252.4 mg/L = 11.89 L/min of air

H. Actual Air Requirement

The oxygen transfer efficiency of aerators is as low as 5% and as high as 25–35% for jet aerators. For a GW jet aerator, a conservative value of 25% can be used. Therefore, the actual air requirement is 4 times the theoretical air requirement.

So the actual air requirement is 47.6 L/min. Depending on special circumstances, a safety factor may be applied, and the actual air requirement can be increased by a further 10–20%.