The smell of sewage pipes is a common phenomenon especially in hot countries. Most foul-smelling substances come from the anaerobic breakdown of sulfur-containing organic matter.
The best known and most dangerous smelly gas is hydrogen sulfide (H2S). It is toxic, in high concentrations can lead to poisoning, and is corrosive to the concrete of sewers. Hydrogen sulfide is formed during the anaerobic decomposition of sulfur-containing organic material and by the degradation of sulfite and sulfate ions, in conditions where there is not enough oxygen concentration.
Various methods are used to treat the odors of H2S and other odorous compounds such as:

• Improving the oxygen balance in wastewater
• Addition of suitable chemical compounds
• Aspiration and passage of odorous compounds through appropriate capture systems

Chlorine and hydrogen peroxide are commonly used to oxidize hydrogen sulfide. Various minerals react with hydrogen sulfide to form insoluble sulfates which precipitate, while various nitrates in wastewater drastically reduce the risk of hydrogen sulfide formation because they provide the oxygen needed by the beneficial microorganisms to grow.