One of the most common fumes is hydrogen sulfide, which smells like rotten eggs. Although chlorine and caustic soda once were sound economical choices to handle it, they have issues: chlorine is a regulatory problem; caustic soda has a high heat of mixing and liberates odors downstream; strong agitation, adding the caustic to the water — not the other way around — and not using hot or cold water during dilution are critical for avoiding accidents. Bleach or an iron chloride (FeCl3) solution are the best choices to use in a scrubber. Of the two options, I recommend FeCl3 because bleach can release chlorine if mixed with acids. However, FeCl3 is not without its problems. Heat tracing is required because of freezing. In addition, a small amount of hydrochloric acid usually is needed to keep the FeCl3 in solution; fiberglass, therefore, is the construction material of choice.
Another common source of odor problems is ammonia (NH3), which has a disagreeably pungent smell. Although water sparging or caustic (pH>11) will eliminate the vapor, you’re stuck with mulch for algae. The best option for water-bound wastewater with >100 mg/L NH3 concentration is an aerobic spray tower followed by anaerobic beds (settling ponds). The NH3 is converted to nitrogen and water. BP applied this approach successfully at its refinery in Lima, Ohio, in the 1990s.
To learn more about fumes, read “Fume Control: Stop the Stench!” from Chemical Processing.
