HYDROGEN SULFIDE IN DRINKING WATER

Hydrogen sulfide is a gas present in some waters. There is never any doubt as to when it is present due to its offensive "rotten egg" odor. This characteristic odor is sometimes apparent in concentrations below 1 mg/l. Obnoxious as are the taste and odor of hydrogen sulfide, these are only two of the problems it presents. Hydrogen sulfide promotes corrosion due to its activity as a weak acid. Further, its presence in the air causes silver to tarnish in a matter of seconds. High concentrations of hydrogen sulfide gas are both flammable and poisonous. While such concentrations are rare, their presence in drinking water has been known to cause nausea, illness, and in extreme cases, death. High concentrations of dissolved hydrogen sulfide can also foul the bed of an ion exchange softener. Its continued presence will lead to lower and lower capacity and may finally necessitate replacement of the resin bed. Generally, hydrogen sulfide occurs in concentrations of less than 10 ppm (milligrams per liter). Occasionally the amount goes as high as 50 to 75 mg/l. Hydrogen sulfide is more common to well waters than to surface waters supplies.

There are several methods for removing hydrogen sulfide from water. Most of them involve converting the gas into elemental sulfur. This insoluble yellow powder can then be removed by filtration. Low to moderate concentrations of hydrogen sulfide can be eliminated through use of an oxidizing filter of the same type satisfactory for iron removal. Because the elemental sulfur precipitate tends to clog the filter material, it is usually necessary to replace this material from time to time.

Chemical treatment is recommended for medium to high concentrations of hydrogen sulfide. In such cases, solutions of household bleach or potassium permanganate serve as satisfactory oxidizing agents. When these oxidizing agents-such as household bleach and permanganate solution are used, a small chemical feed pump will serve to feed the agent into the water. A ratio of 2 mg/l chlorine per 1 mg/1 H₂S is suggested as a starting dosage. This level will normally provide a high enough chlorine residue to insure complete oxidation of the sulfide to sulfur. The feeding rate of the chlorine solution may be adjusted from the original settings to provide the most efficient operation. As in the case of iron, the chlorine solution should enter the water upstream from the mixing or storage tank to provide sufficient contact time. A contact time of at least 20 minutes should be allowed for complete reaction. After this contact time, the water should pass through an activated carbon filter to remove the now insoluble sulfur and excess chlorine

If potassium permanganate can be used as the oxidizing agent, an iron filter is recommended to remove the insoluble products from the water. (Theoretically, 6.2 mg/l of pure KMnO₄ are necessary to oxidize 1 mg/l H₂S) However, a slight excess of permanganate, as shown by a light pink color, should be fed to keep the filter in a "regenerated" state. In this way it acts as a reserve to protect against any unexpected increase in the hydrogen sulfide content of the water.

An activated carbon filter alone will remove, race amounts of hydrogen sulfide. In this process the carbon simply adsorbs the gas on its surface areas. The use of an activated carbon filter can be economical when extremely small amounts of the gas are present. Regeneration of the activated carbon is not usually practical. Periodic replacement is necessary. With moderate to high concentrations of hydrogen sulfide this becomes impractical from an economic standpoint.

Some large users of water depend on aeration to remove hydrogen sulfide from water. Although this is the simplest basic method, it's not normally used for household applications. It has the disadvantage of high initial cost and incomplete removal of the gas. There has been some use of the ion exchange process for removal of hydrogen sulfide. The ion exchange material for this purpose is a strong base anion substance which can be regenerated with salt or a mixture of salt and sodium bicarbonate. This technique has the advantage of simplicity in operation. On the other hand, it offers relatively low flow rate and an effluent water that has all chloride anions.