The key to proper wastewater management is the bacterial decomposition of undesirable substances contained therein. If, however, oxygen levels are allowed to become low, the bacteria must seek other sources. If nothing else is provided, they obtain needed oxygen by devouring and chemically breaking down sulfate. One breakdown product is bad smelling and highly toxic is hydrogen sulfide gas. Treatment is essential!
Common Methods of Treatment
When a problem of insufficient oxygen is determined, naturally the method of first choice is to aerate the wastewater. This is not always a viable method of dealing with the problem. In that case, methodologies may utilize chlorine gas or hypochlorite, nitrate, permanganate, or hydrogen peroxide.
Treatment by Chlorine
Although direct oxygenation by bubbling is one solution, oftentimes it is not practical. In that instance, chemical additions are made to eliminate the hydrogen sulfide. Chlorine gas is one of the most commonly used and cost-effective treatments. In addition to the use of chlorine in gas form as supplied by cylinders, chlorine can be added as sodium hypochlorite in solid form or in solution. The reaction eliminating hydrogen sulfide is
H2S + 4 NaOCl —> H2SO4 + 4 NaCl
This says that the hydrogen sulfide reacts with the sodium hypochlorite to produce one molecule of sulfuric acid and four molecules of ordinary table salt.
Possible problems with chlorination include the formation of chloramines and the destruction of desirable bacteria.
Treatment by Nitrate
The processing of wastewater treatment includes a variety of oxidation and reduction cycles, which oxidize ammonia to nitrate that eventually reduces to harmless nitrogen gas. To settle the problem of hydrogen sulfide production, nitrate can be added to provide bacteria with a substitute for sulfate.
As an example of success in this methodology, the City of Santa Cruz utilizes “a… sulfide ion-selective electrode analyzer… [that] continuously monitors… sulfides and controls the addition of a proprietary nitrate chemical… [that] oxidizes the sulfides present… and significantly reduces corrosion and odor.” (See references)
Treatment with Permanganate
Potassium permanganate is a powerful oxidizing agent that reacts with hydrogen sulfide according to one of two reactions. Under acid conditions, the reaction is
3 H2S + 2 KMnO4 —> 3 S + 2 H2O + 2 KOH + 2 MnO2
Under alkaline conditions, the reaction is
3 H2S + 8 KMnO4 —> 3 K2SO4 + 8 MnO2 + 2 KOH + 2 H2O
There are a number of disadvantages in this procedure. A high dosage of permanganate is needed and the exact dosing necessary is difficult to determine.
Treatment with Hydrogen Peroxide
Hydrogen peroxide reacts with hydrogen sulfide to produce either sulfite and water or sulfur and water, depending upon pH. However, disadvantages are the need for special safety equipment, since concentrated hydrogen peroxide can inflict serious burns, and there is the threat of spontaneous combustion.
References and Resources:
University of Idaho – “Hydrogen Sulfide Odor Control in Wastewater Collection Systems”
Excel Water Technologies, Inc. – Hydrogen Sulfide Removal Methods
University of Wisconsin-Madison – Anaerobic Respiration
US Peroxide – Sodium Hypochlorite
Alken-Murray – “Solving the Hydrogen Sulfide Odor Problem”