Odor-producing substances found in domestic wastewater and sludge are small, relatively volatile molecules having molecular weights between 30 and 150 g/mole. Most of these substances result from anaerobic decomposition of organic matter containing sulfur and nitrogen. Inorganic gases produced from domestic wastewater decomposition commonly include hydrogen sulfide, ammonia, carbon dioxide, and methane.
Of these gases, only hydrogen sulfide and ammonia are malodorous. Often, odor-producing substances include organic vapors such as indoles, skatoles, mercaptans and nitrogen-bearing organics.
Hydrogen sulfide is the most commonly known and prevalent odorous gas associated with domestic wastewater collection and treatment systems. It has a characteristic rotten egg odor, is extremely toxic, and is corrosive to metals such as iron, zinc, copper, lead and cadmium. Hydrogen sulfide is also a precursor to sulfuric acid formation, which corrodes lead-based paint, concrete, metals and other materials.
The conditions leading to H2S formation generally favor production of other malodorous organic compounds. Thus, solving H2S odor problems can often solve other odor problems as well. Many of the odors detected in wastewater collection and treatment systems result from sulfur-bearing compounds being present. For example:
Substance Molecular Weight
- Allyl Mercaptan 74.15
- Amyl Mercaptan 104.22
- Benzyl Mercaptan 124.21
- Crotyl Mercaptan 90.19
- Dimethyl Sulfide 62.13
Sources of Sulfur in Domestic Wastewater
Sulfur is present in human and livestock excreta and sulfates are found in most water supplies. Sufficient sulfur is normally available in domestic wastewater in the form of organic sulfides such as mercaptans, and disulfides for the production of odorous gases by anaerobic and facultative bacteria.
The sulfate ion (SO42-) is one of the most universal anions occurring in rainfall, especially in air masses that have encountered metropolitan areas. Sulfate concentrations in wastewater can vary from only a few milligrams per liter (mg/L) to hundreds of milligrams per liter. Generally, for domestic wastewater, the main source of sulfide is sulfate.
Physical and Chemical Properties of Hydrogen Sulfide
Hydrogen sulfide is a colorless gas that has a foul odor (rotten egg smell) and is slightly heavier than air. Human exposure to small amounts of hydrogen sulfide in air can cause headaches, nausea, and eye irritation.
Higher concentrations can cause respiratory system paralysis, resulting in fainting and possible death.
Hydrogen sulfide is moderately soluble in water, possible maximum concentrations ranging between 3,000 and 4,000 mg/L at the normal temperatures found in wastewater. Hydrogen sulfide solubility decreases with increasing temperature.
See Hydrogen Sulfide Hazards in Municipal & Industrial Accounts and the Toxicity Chart, page 8
Sulfide Production Rate
The rate at which sulfide is produced by a slime layer on collection pipes depends on the following environmental conditions:
- Concentrations of organic matter and nutrients
- Sulfate concentration
- Dissolved Oxygen (DO)
- Stream Velocity
- Surface area
- Retention time