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 produce a palatable water that is free of taste and odor at all times if hydrogen sulfide is present in the raw water in significant concentrations. Not only does the treatment process require continuous surveillance, but the distribution system and consumers hot water systems require monitoring. Hydrogen sulfide occurs 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 sulfides by organic matter under anaerobic conditions, and the resultant metallic sulfide changed to hydrogen sulfide 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 H2S. 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 sulfur bearing waters. These are the sulfide oxidizing forms. The most prevalent 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 H2S 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 minimum 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 percent 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.