WASTEWATER: Understanding hydrogen sulfide
Which statement about hydrogen sulfide is correct?
- A. Hydrogen sulfide causes corrosion due to the development of high pH conditions.
- B. The lower the pH of the sewage, the higher the potential for hydrogen sulfide to be present.
- C. Using acidic chemicals in lift stations is a way to prevent hydrogen sulfide formation.
- D. Hydrogen sulfide is lighter than air and will accumulate in the upper parts of manholes.
Answer: B. Hydrogen sulfide (H2S) is a deadly gas commonly found in sewage collection systems, manholes, vaults, wastewater influent treatment structures, empty treatment tanks that contain decomposing organic matter, and other confined spaces. H2S is about 2.5 times heavier than air and can usually be found near the bottoms of the spaces listed. This colorless gas has a rotten egg odor and in high enough concentrations can deaden a person’s sense of smell, giving a false sense of safety. Methods of controlling H2S in wastewater include adjusting the liquid pH with a basic chemical like sodium hydroxide, magnesium hydroxide or sodium hypochlorite (bleach). The lower the pH of the sewage, the more likely H2S will be present. H2S gas in moist environments like manholes and lift stations creates sulfuric acid, which is very corrosive to metal and concrete structures.
WATER: Where’s that pink color coming from?
The operator of the permanganate-greensand-filter water treatment plant sees a light pink color in the water leaving the filters. What could cause the pink water and what can be used to remove it?
- A. An overdose of chlorine is the cause, and sulfur dioxide can remove the color.
- B. An under-dose of potassium permanganate is the cause, and soda ash can remove the color.
- C. An overdose of chlorine is the cause, and sodium bisulfate will remove the color.
- D. An overdose of potassium permanganate is the cause, and powdered activated carbon can remove the color.
Answer: D. Potassium permanganate is a strong oxidizing agent used to convert soluble iron and manganese to an insoluble particulate form that can be caught on filter media and removed from the water. When overdosed, potassium permanganate can turn the water a slight pink color, and if dosed too high it can turn the water purple or brown-purple. Powdered activated carbon (PAC) can be used to remove the pink color, but could in itself turn the water a dark color. Dark-colored water containing PAC must be returned to the plant for retreatment and filtration. Some operators may believe the pink water is due to chlorine overdose, since the reagent used for chlorine residual (DPD) also creates a pink color. However, this is a false answer.
About the author
Ron Trygar, a certified environmental trainer, is the senior training specialist for water and wastewater programs at the University of Florida’s TREEO Center. He has worked in the wastewater industry for more than 30 years in a variety of locations and positions. He holds a Florida Class A wastewater treatment operator license and a Florida Class B drinking water operator license.