Hydrogen Sulfide Control and Removal
Hydrogen sulfide is probably the most obnoxious and troublesome compound to be dealt with in a potable water supply. It is an almost impossible task to produ ce a palatable water that is free of taste and odor at all times if hydrogen sulfide is present in the raw wa ter in significant concentra tions. Not only does the treatment process require continuous surveillance, bu t the distribution system and consumers hot water systems require monitoring. Hydrogen sulfide occu rs mainly in well waters. Occurrence in surface supplies is primarily by groundwater intrusion; however, with the rising pollution of natural waters by sewage and industrial wastes, surface waters may become contaminated with hydrogen sulfide. Sulfides in well water are probably produced through chemical and bacterial changes under anaerobic conditions far underground. Sulfates may be reduced to sulf ides by organic matter under anaerobic conditions, and the resultant metallic sulfide changed to hydrogen su lfide by the action of carbonic acid. The sulfate reducing bacteria (Desulfovibrio desulfuricans) are another source of hydrogen sulfide production. In anaerobic environments these bacteria convert sulfates and other sulfur compounds to H 2 S. They have a growth range of pH 5.5 to 8.5 and are found to exist in temperatures of 0 to 100 C, with an optimum range of 24 to42 C. Another group of bacteria also plays an important part in sulf ur bearing waters. These are the sulfide oxidizing forms. The most preval ent are Beggiatoa and Thiobacillus. Beggiatoa are filamentous white sulfur bacteria that obtain the energy necessary for their growth by oxidizing the sulfide ion to colloidal sulfur, which is then stored in their cells. Brief exposures, (30 minutes or less) to H 2 S concentrations as low as 0.1 percent by volume of air may be fatal. The gas is highly soluble in water to the extent of 4000 mg/l at 20 C and one atmosphere. The minimu m detectable concentration by taste in water is given as 0.05 mg/l. At pH 7, hydrogen sulfide is approximately 50 percent of the total dissolved sulfides; at pH 5, it is practically 100 percen t of the total; at pH 9, it is nearly all hydrosulfide ion. Therefore the existence of hydrogen sulfide in sulfur-bearing waters is pH dependent. This scientific fact has been well documented.
Whenever the equilibrium between the hydrosulfide ion and the hydrogen sulfide in solution is upset, as when H 2 S is removed by oxidation, the shift will be to form more H 2 S from the remaining dissolved sulfides to reestablish the equilibrium. This stored su lfur gradually disappears by metabolic action, being itself oxidized to sulfate to yield more energy. The primary objection to waters containing hydrogen sulfide is the offensive rotten-egg taste and odor. The secondary objection is the marked corrosiveness of these waters to both metals and concrete structures. Another serious problem is their ability to promote luxuriant blooms of the various types of filamentous sulfur bacteria that lead to a general degradation of water quality in the system. The presence of H2S w ill turn silverware black, discolor lead base paint, make bathing in tubs or showers extremely unpleasant, and stain all plumbing fixtures. The sulfides in the water react with iron from the mains to form a susp ension of iron sulfide that causes a discoloration of the water. This suspension makes laundering almost impossible. The presence of hydrogen sulfide is noticeable to the extent of 0.5 ppm, even in cold wa ter. If the pH is high, the odor may be slight.
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