Hydrogen Peroxide for Water Treatment: Treating Hydrogen Sulfide and Iron with Hydrogen Peroxide Injection.
In my thirty-plus years of experience, I have personally tried every method known to man to remove iron. Today, I primarily use hydrogen peroxide, as it is a much better oxidizer of iron than either chlorine or potassium permanganate and does not leave excess air in the water like oxygen systems. Unlike chlorine, hydrogen peroxide is simply hydrogen and oxygen and produces no harmful chlorination byproducts. A hydrogen peroxide system consists of a chemical injection pump, solution tank, in-line static mixer, and a backwashing filter to remove the oxidized iron.--The Water Doctor.
Hydrogen peroxide is one of the most powerful oxidizers available.
Less hydrogen peroxide than chlorine is required to treat iron and sulfide. When hydrogen peroxide reacts, oxygen is liberated and an oxidant potential 28 times greater than chlorine is produced. It is this liberated oxygen that is the key to how hydrogen peroxide works.
Seven percent hydrogen peroxide (70,000 ppm) is the standard water treatment strength. It 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.
After hydrogen peroxide treatment, a filter is necessary to remove the precipitated elements. Carbon is almost always the filter medium of choice after hydrogen peroxide treatment. Catalytic carbon works wonderfully. Manganese dioxide media like Birm 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 and redox filters 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.
Stability and Storage:
Hydrogen peroxide is exceptionally stable (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. H2O2 reacts with impurities in the water.
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.
Practical Treatment Limits:
10 ppm for iron.
There is virtually no limit for hydrogen sulfide. It is not uncommon to oxidize up to 70 ppm hydrogen sulfide with peroxide.
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.
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. (The easy way is to use the calculator at http://www.pwgazette.com/feedpumpsizeandset.htm.)
Use with Other chemicals:
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 with separate pumps.
Contact Time Required:
“It is unusual to use a contact tank with peroxide. The reaction rate is so much faster than other oxidants that it is usually not necessary.”
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.) If 7% peroxide is fed undiluted, a very low delivery rate pump (< 3 gpm, for example) is usually best. If no holding tank is used, a static mixer at the injection point is recommended. Injection is always before the well’s pressure tank. The filter, of course, follows the pressure tank. A softener, if used, must be downstream of the filter.
Reference: Scott Crawford, “Residential Use of Hydrogen Peroxide for Treating Iron and Hydrogen Sulfide,” Water Conditioning and Purification, December, 2009.