Odor sources
Odor sources
This domain tests source identification: which compounds form under which biological and chemical conditions, and where they are released in a facility. The exam expects you to link an odor complaint to a process condition, not just name a chemical.
Core rules
- Anaerobic or septic conditions drive sulfate-reducing bacteria to produce hydrogen sulfide (H2S); organic sulfur compounds produce mercaptans and other reduced sulfides.
- Protein breakdown produces ammonia, amines, indole, and skatole. Fermentation of carbohydrates and fats produces volatile fatty acids such as butyric and propionic acid.
- Release depends on pH, temperature, turbulence, and Henry's law. Low pH shifts sulfide toward H2S gas; high pH shifts ammonia toward NH3 gas.
- Common wastewater sources include headworks, grit chambers, primary clarifiers, sludge thickeners, digesters, dewatering, lift stations, and force mains with long detention time.
- Non-wastewater sources include landfills, composting, food processing, rendering, and industrial tanks.
Worked example
A lift station has a long force main, low flow, and septic wastewater. The likely odor is H2S from anaerobic sulfate reduction. Because the wastewater is anaerobic, control options include preventing septic conditions with aeration or nitrate addition, precipitating sulfide with ferrous chloride, or oxidizing sulfide with hypochlorite. If the complaint is ammonia-like, check pH and protein load instead.
Traps
- Assuming every rotten-egg odor is H2S; mercaptans also smell sulfurous at very low concentrations.
- Ignoring pH when predicting which compound will be released.
- Looking only at the treatment plant and missing collection-system sources.
- Confusing anaerobic odor production with aerobic treatment.