Removal of Hydrogen Sulfide by Aeration

Task

Determine the theoretical amount of air required to oxidize hydrogen sulfide in sewage. This air volume can be estimated very accurately, allowing the condition and type of ejector to be used to be determined.

A. Chemical Properties of Water

The pH of the water is not as important as the oxidative precipitation of iron and manganese ions. For hydrogen sulfide, the pH value should be maintained at 6.8 to 7.5. At this pH, hydrogen sulfide decomposes into its ionic form: H2S → 2H+ + S2-. S ions are free to react with oxygen.

B. Other Factors

Oxidation of hydrogen sulfide does not occur immediately. Therefore there must be a residence time, or contact tank providing sufficient time for reaction and settling to occur. Depending on actual conditions, the contact time is 5 to 15 minutes.

C. Reaction Equation

2H2S + O2 → 2H2O + 2S0
注：S0 是单质硫。

D. Reaction Ratio

The atomic weight of sulfur is 32.06. When one oxygen molecule reacts with 2 sulfur atoms, the molecular weight of oxygen is 31.999. The reaction ratio is 31.999 / 32.06 = 0.9981 or 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. Provide a buffer when hydrogen sulfide fluctuates.
2. Improve water quality.
3. Improve mixing, making the reaction between hydrogen sulfide and oxygen more complete and faster. The residual oxygen in the water should be 5 mg/L. If there is initial oxygen in the water, the residual oxygen value should be 5 mg/L minus the initial oxygen.

F. Theoretical Oxygen Demand

The theoretical oxygen demand calculation formula for oxidizing hydrogen sulfide is:

理论需氧量 = [Xs · (S)] + R
X = 硫化氢反应系数
(S) = 硫化氢浓度，mg/L（以硫计）
R = 最终残留氧 = (5.0 - 初始氧) mg/L
示例：(S) = 25 mg/L，初始氧 = 0.0 mg/L
需氧量 = (1.0) × (25) + (5.0 - 0.0) = 25 + 5 = 30 mg/L 水流量

G. Theoretical Air Requirement

The density of air at 20°C and 1 atmosphere is 1.2047 g/L. The oxygen content of air under this condition is 20.95%, so 1 liter of air contains (1.2047 g/L) × (0.2095) = 0.2524 g/L = 252.4 mg/L of air oxygen. For the calculation of the theoretical amount of air required to oxidize hydrogen sulfide, it is necessary to know the water flow rate; if the hydrogen sulfide content level is known, it is more convenient to use kiloliters as the unit of measurement for water flow rate.

示例：以上数据为例，并且水流量为 100 L/min。
理论空气需要量 = (100 L/min) × (30 mg/L) / 252.4 mg/L = 11.89 L/min 空气

H. Actual Air Requirement

The oxygen transfer efficiency of an aerator is as low as 5% and as high as 25 to 35% for an ejector. A conservative value of 25% can be used for GW jet ejectors. The actual air requirement is therefore 4 times the theoretical air requirement. Therefore, the actual air requirement is 47.6 L/min. Depending on the special circumstances, a certain safety factor can be adopted, and the actual air requirement can be increased by another 10 to 20% on this basis.