Chemical options
Chemical options
This domain tests chemical selection for sulfide control, pH adjustment, oxidation, and odor neutralization. The exam asks for the right chemistry and the safety consequences of the wrong mix.
Core rules
- Oxidizers include sodium hypochlorite, hydrogen peroxide, potassium permanganate, ozone, and chlorine dioxide. They destroy sulfide and other reduced compounds.
- Precipitants such as ferrous chloride and ferric chloride bind sulfide as iron sulfide, reducing H2S release.
- pH adjusters shift equilibria: caustic keeps sulfide in solution; acid can strip H2S and is dangerous.
- Nitrates can suppress sulfate reduction in collection systems by providing an alternative electron acceptor.
- Masking agents and essential oils change perception; they do not remove toxic H2S.
Worked example
A force main generates H2S. Ferrous chloride doses iron to precipitate sulfide; sodium hypochlorite oxidizes sulfide; nitrate addition controls biological sulfide production. The choice depends on dose point, pH, safety, and byproducts. Never mix hypochlorite with acid, because chlorine gas can be released.
Traps
- Using a masking agent instead of controlling toxic H2S.
- Overdosing permanganate or hypochlorite and creating residuals.
- Ignoring pH when adding caustic or acid.
- Mixing incompatible chemicals in storage or dosing.