Water Smell Like Rotten Eggs? Here Is Some
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Hydrogen Sulfide Hydrogen sulfide
(that rotten egg smell) in
household drinking water is a common nuisance contaminant. Although it is
not hazardous to human health, the offensive odor of sulfur water makes
treatment desirable. Fortunately, various physical and chemical treatment
options are available. Often, the treatment for hydrogen sulfide is the
same for iron and manganese so that all three contaminants can be removed
in one process. |
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contaminants and more.
Sources
Water containing hydrogen sulfide, commonly called sulfur water, has a
distinctive "rotten egg" or swampy odor. Hydrogen sulfide is a
gas formed by the decay of organic matter such as plant material. It is
typically found in groundwater containing low levels of dissolved oxygen
and a pH less than 6.0. If the pH range of the water is higher (7.0-12.0),
the water may contain other forms of sulfur (sulfide or bisulfide). Sulfur
problems occur less frequently in surface waters because flowing water is
aerated naturally so that the hydrogen sulfide reacts with oxygen and
escapes as a gas or settles as a solid.
Sulfur bacteria are also found in many drinking water wells and
household distribution systems. These harmless bacteria use natural sulfur
compounds in water as a food source, thereby producing hydrogen sulfide.
Although sulfur bacteria pose no health risk to humans, their presence in
drinking water may cause objectionable tastes and odors.
A water heater may also produce a sulfur odor. The magnesium rod
present in many water heaters can chemically change naturally occurring
sulfates in water to hydrogen sulfide. If this occurs, the magnesium rod
can be replaced with an aluminum rod or simply removed, although removing
the rod may nullify the warranty on the water heater.
Drinking water standards and testing
Many impurities are regulated by drinking water standards set by the
U.S. Environmental Protection Agency (EPA). Hydrogen sulfide is not
regulated, however, because it is considered a nuisance chemical and does
not pose a health risk at concentrations typically present in household
water. Concentrations high enough to be a health risk also make the water
unpalatable. Conversely, concentrations as low as 0.5 milligrams per liter
(mg/L) can add objectionable taste and a rotten egg odor to drinking
water.
Since hydrogen sulfide is a gas that can easily evaporate when
dissolved in water, the water sample must be stabilized before a
laboratory analysis for the chemical can be done. Sample bottles
containing a stabilizing chemical should be obtained from a laboratory
certified by the New York State Department of Health (contact your local
health department or Cooperative Extension office for a list of certified
labs). Stabilized water samples cannot be analyzed for other contaminants.
Finally, if laboratory tests show the absence of hydrogen sulfide in
drinking water that has a characteristic sulfur odor, the water should be
tested for sulfur bacteria.
Water treatment options
It is important to know the origin of hydrogen sulfide so the problem
can be treated properly. To determine if the water heater is involved, run
cold water, such as from the shower, inside the house. If no odor is
detected, turn off the cold water and run the hot water. The presence of
sulfur in hot but not cold water indicates that the problem is most likely
caused by bacterial activity. In many areas, sulfur water is prevalent in
groundwater at certain well depths or because an old dug or abandoned well
provides a food source for sulfur bacteria. In these situations, it may be
necessary to locate a new water supply or install a home water treatment
system.
Most methods for treating sulfur water rely on the oxidation of
hydrogen sulfide gas into elemental sulfur, a solid. Oxidation is the
process by which soluble or dissolved contaminants are converted to
soluble byproducts or insoluble products that can be filtered. This
process changes the chemical and physical properties of the reactants.
Hydrogen sulfide can be oxidized by several methods. If concentrations
exceed 6.0 mg/l, chemical oxidation such as chlorination is recommended.
If concentrations do not exceed 6.0 mg/l and water pH is above 6.8, an
oxidizing filter such as manganese greensand can be used.
Regardless of the method, good initial design and maintenance of the
treatment system are important. For example, sulfur bacteria produce slime
that may cause clogging problems. Shock chlorination of the entire water
system, including storage tanks, hot water tanks, and distribution lines,
may be necessary to
kill the bacteria.
Chlorination
Continuous chlorination is a very common and effective method for
oxidizing hydrogen sulfide, especially if the water pH is 6.0-8.0.
Chlorine has the secondary advantage of being lethal to bacteria so it
typically follows other treatment options to maintain a chlorine residual
and prevent bacterial activity. Chlorine is usually administered as sodium
hypochlorite, which reacts with sulfide, hydrogen sulfide, and bisulfide
to form compounds that do not cause foul taste or odors in drinking water.
Although the amount of hypochlorite to be administered depends on the
concentration of hydrogen sulfide in the water supply, a recommended
dosage is 2.0 mg/l chlorine for every 1.0 mg/l hydrogen sulfide. Chlorine
should be added before the water reaches the mixing tank, and sufficient
storage must be provided so that the water is in contact with the chemical
for twenty minutes. Treated water may have lingering tastes or odors
caused by the formation of certain harmless by-products or residual
chlorine. After the required contact time, therefore, the water should be
passed through a depth filter or activated carbon filter to remove final
suspended sulfur or excess chlorine.
Chlorination systems are available as a pellet-drop unit or a
liquid-chemical feed, both of which include an activated carbon filter to
remove excess chlorine. The pellet-drop system automatically dispenses a
measured amount of chlorine down the well casing or into the retention
tank during the pumping cycle. The chemical feed system features a liquid
feeder connected to the well pump. Some disadvantages of chlorination
systems include the complexity of chemical reactions and maintenance of
the system. Chlorination systems can be difficult and expensive to operate
because they require continuous addition of chemicals.
Aeration
Another common treatment for sulfur water is aeration. Hydrogen sulfide
is physically removed by agitating the water via bubbling or cascading and
then separating or "stripping" the hydrogen sulfide in a
container. The undesired hydrogen sulfide is removed as a volatile gas by
venting it into a waste pipe or to the outdoors. Aeration is most
effective when hydrogen sulfide concentrations are lower than 2.0 mg/l. At
higher concentrations, this method may not remove all of the offensive
odor unless the air is used to oxidize hydrogen sulfide chemically into
solid sulfur, which is then filtered.
In a typical aeration system, ambient air is introduced into the water
using an air compressor or blower. Well-designed aeration tanks maintain a
pocket of air in the upper third or upper half of the tank. If the tank
does not maintain an air pocket, sulfur odor may return. Most household
water supplies contain less than 10 mg/l of sulfur, in which case an
aeration tank about the same size as the filter tank (10'' x 54")
works fine. When sulfur levels exceed 10 mg/l, larger aeration tanks,
repressurization systems, chlorination systems, or a combination may be
needed.
Aeration has many advantages for removing hydrogen sulfide, including
its effectiveness at low levels and lack of chemical additives. Also,
maintaining a properly designed aeration system is less costly than many
chemical-based treatment systems. But the introduction of oxygen to the
water may cause problems if some hydrogen sulfide is oxidized to sulfide,
bisulfide, or solid sulfur particles, all of which are not air-strippable
and need to be filtered from the treated water. In addition, aeration
tanks, spray nozzles, and trays can accumulate bacterial slime and other
substances that must be removed periodically.
Aeration is not always practical for home water treatment, especially
if hydrogen sulfide concentrations exceed 10 mg/l, because it requires
very acidic conditions (pH 4.0-5.0), long contact times for the air and
water to mix, and usually large space requirements. In addition, treated
water may need to be repressurized for distribution within the house and
objectionable odors must be removed by venting the gas outside. Aeration
can actually accentuate bacterial sulfur problems if the bacteria are not
removed first. Some water treatment specialists prefer to install
chlorinators -to kill bacteria and lower extreme sulfur levels before
aeration. Aeration systems may be installed following a water softener
when there are low sulfur levels.
Catalytic carbon
Although several methods have always been available to treat hydrogen
sulfide in drinking water, advancements in catalytic carbon now provide
another alternative to chemical treatment. Essentially, catalytic carbon
is activated carbon with a modified carbon surface. Activated carbon is
typically associated with adsorption, a physical process in which
dissolved molecules adhere to the surface of the carbon filter. When used
alone, activated carbon filtration removes very small amounts of hydrogen
sulfide, generally concentrations below 0.3 mg/l. Activated carbon,
however, has a limited capacity to adsorb hydrogen sulfide. Once the
filter is saturated, the activated carbon must be replaced, not
regenerated. As a result, activated carbon is not effective for removing
moderate or high concentrations of hydrogen sulfide in drinking water.
Catalytic carbon retains all of the adsorptive properties of
conventional activated carbon, but it combines them with the ability to
promote or catalyze chemical reactions. During the treatment process,
catalytic carbon first adsorbs sulfides onto the carbon surface. Then, in
the presence of dissolved oxygen, it oxidizes the sulfides and converts
them to non-objectionable compounds. In this capacity, catalytic carbon is
similar to manganese greensand and chlorination systems that remove
sulfides through oxidation. It differs in that it maintains consistent
catalytic activity (oxidation) that can be controlled and enhanced to
treat sulfur water without use of chemical additives.
Several design considerations affect the performance of catalytic
carbon, including empty bed contact time (the amount of time it takes for
water to travel from the top of the carbon bed to the bottom, typically
three to five minutes), backwash capability (as with manganese greensand,
backwashing with treated water is recommended to remove any solid or
filtered material such as elemental sulfur), and the concentrations of
hydrogen sulfide and dissolved oxygen in water. A minimum dissolved oxygen
level of 4.0 mg/l is necessary for complete oxidation of hydrogen sulfide
to elemental sulfur, which is filtered. In water containing less than 4.0
mg/l dissolved oxygen, levels can be increased by aeration or the addition
of chemical oxidants. Higher levels of dissolved oxygen are required to
remove higher concentrations of hydrogen sulfide. Overall, catalytic
carbon systems are relatively easy to operate, and they have lower space
and maintenance requirements than other systems.
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