Hydrogen Sulfide, H2S, is a gas which is best known for giving water a "rotten egg" odor. The objectionable odor makes water unfit for drinking, household use, and for industrial processes. It interferes with water tests and cannot be easily tested in a laboratory. Testing must be done on site because the gas goes away quickly when exposed to air.

According to Wikipedia: "Hydrogen sulfide (or hydrogen sulphide) is the chemical compound with the formula H2S. It is a colorless, very poisonous, flammable gas with the characteristic foul odor of rotten eggs. It often results from the bacterial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers (anaerobic digestion). It also occurs in volcanic gases, natural gas, and some well waters. The body produces small amounts of H2S and uses it as a signaling molecule."

Hydrogen sulfide can also blacken plumbing fixtures and the fumes can even blacken painted surfaces.

What is commonly called hydrogen sulfide actually results from a variety of causes. The most common explanation is that the smell is the result of the metabolism of bacteria which consume sulphur. It can also result from petroleum contamination of wells, especially natural gas. The smell is often most prevalent in hot water, and it is sometimes treated by changing the sacrifical anode of the home's hot water heater.

Treatment: Treatment of hydrogen sulfide usually involves oxidizing the gas with the standard water treatment oxidants--chlorine, aeration (pressure tank style) hydrogen peroxide, ozone, potassium permanganate--then filtering out the oxidant and the remnant of the oxidation process, elemental sulphur. Hydrogen sulfide can also be treated by open tank aeration in which the contaminant is allowed to escape into the atmosphere.

If the problem is slight, carbon, especially catalytic carbon, can be used as a stand-alone treatment, but if the hydrogen sulfide is present in a significant amount carbon filtration alone is insufficient.