Water Education - Water and Health

HYDROGEN SULFIDE IN DRINKING WATER Page 2

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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 ensure 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 a 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 the 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 gas are present. Regeneration of 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 the 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 the removal of hydrogen sulfide. The ion exchange material for this purpose is a strong base anion substance that 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 a relatively low flow rate and effluent water that has all chloride anions.

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