By Sara Heger, Ph.D.
Hydrogen sulfide problems are most common in wells drilled into acidic bedrock such as shale and sandstone; produced by certain “sulfur bacteria” in the groundwater, well or plumbing system; or chemical reactions inside of water heaters. Sulfur bacteria produce a slime and can help other bacteria grow, such as iron bacteria. The slime can clog wells, plumbing and irrigation systems.
Hydrogen sulfide in water is an aesthetic concern that causes a disagreeable taste and odor to the water. While the gas is poisonous and flammable, the human nose can detect it well before it causes health concerns. Most people can detect hydrogen sulfide levels well below 0.5 mg/L. Hydrogen sulfide can also cause corrosion of metals in a plumbing system, and it can cause yellow or black greasy stains on fixtures or inside pipes when it forms metallic sulfides.
Hydrogen sulfide does not have a drinking water standard because it makes the water aesthetically undrinkable long before it reaches harmful concentrations. Testing the water to determine the concentration of hydrogen sulfide may be helpful when choosing between water treatment devices. Testing for hydrogen sulfide must be done in the home or the sample must be chemically stabilized before being sent to a commercial testing lab.
Sources and solutions
Sometimes hydrogen sulfide may be noticeable only in the hot water in the home. In this case, chemical reactions within the water heater may be the source of the rotten egg odor. Water heaters are fitted with a magnesium rod to inhibit corrosion of the heater. The magnesium rod can chemically reduce sulfates to form hydrogen sulfide. The production of hydrogen sulfide can often be simply treated by removing the magnesium rod from the hot water heater. However, removal could cause increased corrosion and reduced life of the hot water heater and will likely void the manufacturer’s warranty. Replacement of the magnesium rod with an aluminum rod should eliminate the rotten egg odor while maintaining corrosion protection for the heater. Disinfecting and flushing the water heater also may eliminate this issue.
In rare cases, the addition of water treatment equipment, like a water softener, may cause the production of hydrogen sulfide. In this case, the softener provides a favorable environment for sulfur-reducing bacteria to grow. Disinfecting and flushing the water softener may eliminate this issue.
If the source of hydrogen sulfide is naturally occurring, it can be effectively removed from water using a number of treatment processes. The most efficient and cost-effective treatment option will depend primarily on the concentration of hydrogen sulfide. Most treatment processes are designed to treat all of the water entering the home (known as point-of-entry or POE treatment) since hydrogen sulfide is an aesthetic odor problem.
When the water contains a small concentration of hydrogen sulfide, less than about 1.0 mg/L, activated carbon filtration may be effective. Activated carbon removes a variety of water contaminants, including hydrogen sulfide, by adsorbing the gas on the surface area of the carbon particles. A large unit (usually 1.0 to 1.5 cubic feet of carbon) capable of treating all of the water entering the home is required. Cost and maintenance requirements are low, but the carbon needs to be replaced periodically.
Oxidation is the most common form of treatment used to eliminate hydrogen sulfide. In this process, a chemical is used to convert the dissolved hydrogen sulfide gas into forms of sulfur that can be easily filtered from the water. Chlorine or potassium permanganate is often used as an oxidizing chemical to convert hydrogen sulfide gas to insoluble sulfur (a yellow solid). There are also chemical-free oxidation filtration units that are typically used to remove iron, sulfide and magnesium.
In all cases these filters create a solid that may be concern to the septic system. It is advisable to not plumb this backwash water into the septic system but discharge to the surface or into a separate system. There is the risk that the sulfur solids will not settle out and could damage and/or plug downstream components. There is also a concern with elevated sulfur in concrete septic tanks related to hydrogen sulfide production and related corrosion, so this is another good reason to plumb this backwash out of the septic system.