Stench is a common phenomenon, especially in countries where the climate is warm, and it originates in municipal sewage drains.
Most smelly substances emitted at a sewage treatment plant are the result of anaerobic decomposition of organic substances containing sulfur.
Hydrogen sulphur (H2S) is the most known and dangerous smelly gas. It is toxic, it poses a high poisoning risk (when present in high concentrations in the atmosphere) and is very corrosive to the concrete in sewers.
Hydrogen sulfide is formed during the anaerobic decomposition of organic material containing sulfur, and the degradation of sulphites and sulphate ions. It cannot be formed when there is a high concentration of oxygen.
In addition to hydrogen sulfide that creates odors, other volatile constituents like indoles, skatoles, and mercaptans, also formed during the anaerobic decomposition, can also create odors which are much more unpleasant than that of hydrogen sulphide, but not as dangerous.
The oxidizing substances commonly used to prevent the phenomenon are chlorine and hydrogen peroxide, which oxidize sulfide to sulfur and sulfate. The metal salts like divalent or trivalent iron, react with hydrogen sulfide to form insoluble sulphates that precipitate. Using nitrates in industrial effluents drastically reduces the risk of formation of hydrogen sulphide because they provide the necessary oxygen for the development of microorganisms. The addition of nitrates is a preventative method. But they cannot bind the hydrogen sulfide produced at the stages before their infusion.
The nitrates used to fight hydrogen sulphide are calcium nitrate salts, sodium, magnesium, aluminum and iron.
It is important, however, to add to the sewage as much as is necessary, so that it gets completely consumed by the time it reaches the treatment facilities, otherwise, the nitrates will be added to the nitrogen load.
Specifically, to address the odor of H2S, and the other malodorous compounds, per case, three methods apply:
a) Improvement of the balance of oxygen in the industrial effluent
b) Addition of appropriate chemical bonds
γ) Uptake and then crossing the smelly associations with appropriate systems of bonding ( e.g. absorbent and adsorption materials).
Removal of sulfur compounds, such as hydrogen sulfide, mercaptans and acid compounds from gases is crucial in several cases. For this reason, special filtration products such as activated carbon are designed and used for the treatment of polluted air. When using these products in your system the air that gets emitted in the atmosphere is clean, since the filter captures any undesirable elements.