Treating Hydrogen Sulfide and Iron with Hydrogen Peroxide Injection
Water Treatment Grade 7% Hydrogen Peroxide
Hydrogen peroxide (H2O2) is one of the most powerful oxidizers available for water treatment. Although it can be used to control bacteria, its main use is as pretreatment for filters removing iron and hydrogen sulfide.
Less hydrogen peroxide than chlorine is required to treat iron and hydrogen sulfide. When hydrogen peroxide reacts, oxygen is liberated and an oxidant potential 28 times greater than chlorine is produced. It is this large charge of liberated oxygen that makes hydrogen peroxide work so well.
Seven percent hydrogen peroxide (70,000 ppm) is the standard water treatment strength. At this strength liquid hydrogen peroxide can be transported through normal shipping methods and is not considered hazardous.
Thirty-five percent hydrogen peroxide (350,000 parts per million) is sometimes used. It is a hazardous material and must be handled with great care. It usually requires dilution with distilled water for residential use. For this reason, for most home applications 7% hydrogen peroxide is the product of choice.
A Filter Is Required
Like air, ozone, and chlorine, hydrogen peroxide prepares contaminants to be removed by a filter. The oxidizing agent is only half of the treatment. The filter that follows is necessary to remove the precipitated contaminants. Carbon is in most cases the filter medium of choice after hydrogen peroxide treatment. Manganese dioxide media like Birm, Katalox and Pyrolox can be destroyed by hydrogen peroxide. Carbon, both standard and catalytic, works well for both hydrogen sulfide and iron removal. Carbon also breaks down the residual peroxide, so there is usually no peroxide left in the service water. Mixed media filters, zeolite filters, and redox filters (KDF) have also been used successfully.
If the water is very clean and no iron is present, a carbon block filter alone can be used following H2O2 injection, but in most cases–in all cases, if iron is present–a backwashing filter is required. The backwashing process can also clear the system of gas pockets which can form, so backwashing filters are preferred in most cases, even if only odor is being treated.
Stability and Storage
Hydrogen peroxide is exceptionally stable, having around a 1% per year decomposition rate. Heat and sunlight can increase the rate of decomposition. Dilution of the peroxide should be done only with the best water possible. Distilled water is preferred. H2O2 reacts with impurities in the water and loses strength in the process.
If using 35% peroxide, the 35-percent solution should be diluted to 7%. To do this, add 5 parts distilled, reverse osmosis, or deionized water to 1 part 35% hydrogen peroxide. Seven percent hydrogen peroxide is usually fed without dilution although it can be diluted if the injection system will not feed it in small enough quantities.
Practical Treatment Limits
H2S2 can be used to treat up to 10 ppm iron.
There is virtually no limit for hydrogen sulfide. It is not uncommon to oxidize up to 70 ppm hydrogen sulfide with peroxide.
Dosage: Simple But Not So Simple
Figuring the dosage needed for your application could not be simpler.
Here’s the formula:
- Well pump output rate in gallons per minute, multiplied by
- Required dosage in parts per million, multiplied by
- 1440—the number of minutes in a day—divided by
- Solution Strength in parts per million, which equals
- Needed Metering Pump Output in gallons per day (GPD).
Just joking about the “could not be simpler” part. Actually, dosage calculations are impossible and only work in college chemistry classes. In the real world, there will always be parts of the equation that you don’t know. However, working the formula helps you make an educated guess so you will know which size pump to buy and it will give you a starting place. Understand that in the end, there will always need to be some trial and error, some adjustment to your settings, then more trial and error. The information and calculator on this page may help, but don’t expect the calculator to give you a pat answer.
Other Considerations in Sizing and Setup
Use 0.4 ppm peroxide for each ppm of iron. Hydrogen sulfide treatment is pH dependent. Use 1 ppm hydrogen peroxide for each ppm of hydrogen sulfide at pH 7.0. The more alkaline the pH, the greater the dosage required. Adjust dosage accordingly for higher pH. Some trial and error will be necessary.
Warm water also causes oxygen to dissipate more quickly, so a higher dosage may be necessary as water temperatures increase.
Dosage is determined by the same formula as with other oxidants: gpm x 1,440 x dosage/ % concentration of H2O2= chemical feed rate needed.
Never mix H2O2 with alkaline chemicals such as soda ash, limestone, or ammonia. This will cause the rapid decomposition of the hydrogen peroxide and might even result in a violent reaction.
If an alkaline chemical like soda ash is need to raise pH, feed hydrogen peroxide with one pump and soda ash with a separate pump.
Contact Time Required
One of the great advantages of using hydrogen peroxide rather than chlorine is that its reaction rate is much faster. Therefore, it is common to use hydrogen peroxide without a retention tank. A retention tank between the injection pump and the filter is a necessary part of the system with chlorine; with hydrogen peroxide, the reaction rate is so fast that a retention tank is usually not needed.
As stated, a holding tank is usually not needed with hydrogen peroxide. Inject the peroxide with a peristaltic pump. (Conventional pumps can be used, but they often require modification.) Non-electric pumps also work well with hydrogen peroxide. If 7% peroxide is fed undiluted, a very low delivery rate pump (< 3 gpd, for example) is usually best in theory, but since hydrogen peroxide dosage needs don’t always follow theory, a higher dosage rate pump often works best. If no holding tank is used, a static mixer at the injection point is recommended. Injection is normally done before the well’s pressure tank. The filter, of course, follows the pressure tank. A softener, if used, must be downstream of the filter. Injecting hydrogen peroxide directly in front of the softener with no filter is not a good idea.
Reference: Scott Crawford, “Residential Use of Hydrogen Peroxide for Treating Iron and Hydrogen Sulfide,” Water Conditioning and Purification, December, 2009 . See also online.