Hydrogen Sulfide in Drinking Water Causes and Treatment Alternatives

Transcript

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2 Sometimes hot water will have a “sour” smell, bacteria. If the bacteria are detected, treatment to similar to that of an old damp rag. This smell often control the bacteria should eliminate the problem. develops when the thermostat has been lowered to Other contaminants (sulfate, organic compounds save energy or reduce the potential for scalding. and metals) can cause taste and odor problems in Odor-causing bacteria live and thrive in warm drinking water. Water tests should always be water and can infest the water heater. The problem conducted on new wells or when problems are frequently can be corrected by returning the suspected in an existing water supply. This can be thermostat to its recommended operating tempera- particularly important if more than one contami- ture (approximately 160 degrees F) for 8 hours. nant is involved because it will affect proper Caution: This will kill the bacteria. ( Be sure the diagnosis of the problem and selection of the most water heater has an operable pressure relief valve effective and economical water treatment system. before increasing the temperature, and warn Treatment options for occupants of the scalding hazard.) If a lower tem- perature is preferred for routine operation, periodic hydrogen sulfide high temperature flushing can be used whenever The recommended treatment to remove hydro- the odor returns. gen sulfide from a water supply depends largely on the gas concentration. Testing for hydrogen sulfide Trace amounts of hydrogen sulfide (0.05 - 0.3 mg/l) The cause of the hydrogen sulfide problem must be determined before proper testing and appropri- Activated carbon adsorbs soluble organic com- ate treatment can be applied. If the problem is pounds and certain gases, such as chlorine and caused by hydrogen sulfide in the groundwater, hydrogen sulfide, that contribute tastes and odors then the concentration of the gas in the water must to a water supply. Very small amounts of hydrogen be measured in order to select and properly size a sulfide can be removed from water with activated treatment system. Hydrogen sulfide gas dissolves carbon filters (Fig.1). Activated carbon is used as a readily in water and can readily escape from it granular form in tank-type filters and as finely (volatilize). Thus, water samples must be tested on- divided powder in a cartridge. The hydrogen sulfide site, or immediately stabilized for laboratory is adsorbed onto the surface of the carbon particles. analysis. Sample bottles with stabilizing chemicals should Faucet Mount In-Line be obtained from the labora- tory that does the analysis. Careful sampling according Filter to laboratory recommenda- tions is critical to ensure accurate and reliable results. In general, the requirements Hot will involve delivery of a Hot refrigerated sample within Cold 48 hours of collection. Sulfide Filter Cold concentrations are reported by laboratories in units of milligrams per liter (mg/l) or parts per million (ppm). Line Bypass Point-of-Entry (POE) These two terms are used Nonhousehold interchangeably. Plumbing If the gas is being pro- duced by an active colony of sulfur-reducing bacteria, this To Household Plumbing must be determined. Active colonies could be located Hot anywhere within the plumb- Water Meter Cold Filter ing system, well casing or in Filter the aquifer. Selected commer- cial water testing laborato- ries can determine the Figure 1. Point-of-use and point-of-entry placements of activated carbon presence of sulfur-reducing filters. 2

3 Up to 10 mg/l hydrogen sulfide Some filters will improve taste, but will not elimi- nate unpleasant odors. A granular filter must be An iron removal filter containing manganese backwashed periodically; a cartridge filter must be greensand can remove low to moderate levels of cleaned or replaced periodically. Cleaning frequency hydrogen sulfide in addition to iron and manga- depends on the amount of hydrogen sulfide in the nese. The process oxidizes hydrogen sulfide into water and the volume of water treated. Moderate to sulfate; iron and manganese form precipitates that high levels of hydrogen sulfide will require frequent are filtered out (Fig. 3). Manganese greensand filter replacement. These filters also can remove filters must be recharged with a solution of potas- sium permanganate when the oxygen is depleted. tannins and other dissolved organic compounds. This process is very similar to the regeneration Less than 2 mg/l hydrogen sulfide process used in water softeners, and must be Aeration (adding air to the water) is a treatment performed at regular intervals of 1 to 4 weeks option for this concentration range, and is commonly depending on the chemical composition of the used by city water treatment systems. Oxygen in the water, size of the unit and amount of water pro- cessed. Water with a pH below 6.7 could require air reacts with hydrogen sulfide to form an odorless, acid neutralization, increasing the pH to 7.5 to 8.3, dissolved form of sulfur called sulfate. Several types before iron removal will be effective. These filter of aeration systems are available. In one type, systems are very specialized and the installation compressed air is injected into the water system. and operation instructions of the manufacturer The air then must be removed from the water to should be followed precisely. prevent knocking or air-blocks in the system and to reduce the corrosion potential caused by dissolved From Well oxygen. In another type, water is sprayed into a To House nonpressurized storage tank (Fig. 2). A second pump is needed to re-pressurize the water. The storage tank provides 6 to 8 hours holding time for oxidation of hydrogen sulfide, iron and manganese. The Regeneration Unit storage tank and aerator must remain secure to Manganese Treated Greensand prevent contamination of the water supply, or the KMnO Feed Tank 4 system must be chlorinated. This process usually KMnO Solution 4 produces a strong hydrogen sulfide odor near the Gravel aerator, and may not always reduce the hydrogen Filter sulfide to nondetectable levels. In such cases, a Figure 3. Manganese greensand pressure filtration carbon filter can be used to remove some of the system for removal of hydrogen sulfide, iron and remaining trace amounts of hydrogen sulfide. manganese. These complex systems are not recommended for household use. Aerator/Degasifer Check Valve Well Second Pressure Pressure Tank Tank Gate/Ball Valve Product Water Second Pump Well Pump To Well Figure 2. Nonpressurized aeration system from hydrogen sulfide removal. 3

4 Pressure Tank Check Valve Well Chlorinator Pump Pipe Coil Chlorine Solution Figure 4. Chlorine injection system for removal of hydrogen sulfide. Table 1. Approximate Chlorine Dosage for Oxidation Up to 75 mg/l hydrogen sulfide of Selected Minerals Based on Concentration (mg/l). Continuous chlorination using an automatic chemical feed pump can effectively remove medium Manganese 1.3 times Mn concentration (mg/l) to high levels of hydrogen sulfide (Fig. 4). Chlorine 0.64 times Fe concentration (mg/l) Iron quickly oxidizes hydrogen sulfide into a tasteless, S concentration (mg/l) 2.2 times H Hydrogen Sulfide 2 odorless form. Continuous chlorination also effec- tively removes iron and manganese that can occur in association with hydrogen sulfide. single high dose of chlorine placed in direct contact Chlorine demand (concentration) and contact with water and the plumbing system. Chlorine, if time are very important to successful chlorination. used in sufficient concentration and with adequate The amount of chlorine needed to react with contact time, is an excellent disinfectant that kills organic and inorganic materials in the water can range from 1 mg/l to more than 10 mg/l if the water most disease-causing bacteria, viruses and cysts of is high in sulfide, ammonia, iron or manganese. protozoans. It also kills nonpathogenic iron, manga- Table 1 presents typical dosages required to treat nese and sulfur bacteria. specific problems. In some cases, iron and sulfur bacteria are more Contact time (exposure) depends on the concen- resistant to chlorine because they occur in thick tration of chlorine in the water, water temperature layers and are protected by a slimy secretion. If and pH. Oxidation of iron and hydrogen sulfide is problems with hydrogen sulfide persist after two instantaneous, while manganese oxidizes more attempts at shock chlorination, continuous chlori- slowly. Chlorine should be introduced into the nation could be required. system before it reaches the storage tank. The tank Concentrations of chlorine used in shock chlori- must have sufficient volume to provide a 20-minute nation are 100 to 400 times the amount found in contact time between the water and the chemical. treated “city water.” The amount of chlorine In addition, a fine-retention sediment filter can be needed is determined by the amount of water used to remove any oxidized iron and manganese. standing in the well. Table 2 lists the amount of Excess free chlorine is undesirable and can ordinary household bleach (5.25 percent hypochlo- produce an objectionable taste in the water. An rite) needed for shock chlorination based on casing activated carbon filter can be used to obtain chlo- diameter and water depth in the well. If the depth rine-free water for cooking and drinking. of the water in the well is unknown, use a volume of bleach equal to two times the 150-foot depth for Controlling hydrogen sulfide the appropriate casing diameter. For example, an 8- bacteria in wells inch casing diameter with unknown water depth Shock chlorination can effectively eliminate would require 3 gallons of household bleach. Do not hydrogen sulfide when the problem is caused by the use bleach in excess of the recommended amount presence of sulfate- and sulfur-reducing bacteria in because it is not necessary and will require addi- the plumbing system. Shock chlorination uses a tional flushing before household use. 4

5 Table 2. Amount of chlorine needed for shock chlorination. Depth of Casing diameter water 4-inch 10-inch 12-inch 6-inch in well 8-inch 1.5 cups 2 pints 1 cup 10 feet 1/2 cup 1 pint 1 pint 3 pints 4.5 pints 1 cup 25 feet 2 pints 1 quart 2 quarts 3 quarts 1gallon 50 feet 1 pint 1 quart 1 gallon 1.5 gallons 2 gallons 100 feet 2 quarts 3 quarts 1.5 gallons 2 gallons 3 gallons 150 feet 3 pints levels to lawns, gardens or other sensitive plants The procedure for treatment is as follows: because injury can occur. 1. Dilute the bleach with water in a 5-gallon bucket. This increases the volume. Pour the 7. Finally, turn on the indoor faucets until the system is completely flushed. solution around the sides of the well casing. 2. Connect a garden hose to a nearby faucet and Summary wash down the inside of the well. Hydrogen sulfide in drinking water is a common 3. Do not operate the water system for 2 hours. nuisance contaminant. Although it is generally not 4. Next, open each faucet one by one and allow a health hazard, the offensive odor and corrosivity water to run until a strong odor of chlorine is of water containing hydrogen sulfide usually make detected. If the odor is not detected, check the treatment necessary. Various treatment alterna- rate from Table 2 and add more chlorine to the tives exist; however, chlorination is the most well, repeating steps one through three. common and effective method. Iron and manganese frequently occur in association with hydrogen 5. Do not operate the water system for at least 12 sulfide, and chlorination is an effective treatment to 24 hours. for removing all three of these nuisance contami- 6. Next, flush the system of remaining chlorine. nants. Table 3 presents a summary of treatment Begin by turning on outside faucets and letting options with estimated cost ranges and associated the water run until the chlorine smell dissipates. benefits. Always compare installation and mainte- Let the water run on the ground to reduce the nance costs and efficiencies of appropriate treat- load on your septic system. High loads of chlo- ment alternatives and use qualified professionals rine in the septic tank can kill beneficial bacteria for installation and service. and require re-inoculation of the septic system. Also, do not apply water with high chlorine Table 3. Treatment options for removal of hydrogen sulfide.* Treatment Type Treatment Range Comments Cost $25-100 <1 ppm Disposable carbon cartridge Requires regular replacement. Air purge systems +$600 <20 ppm Requires sizeable equipment placed away from residence. Chlorination of water could be required in some cases. Not recommended for hard water. Oxidizing carbon filter <2-4 ppm Can be used in conjunction systems with $800-1,200 chlorination systems to remove all hydrogen sulfide greater than 2 ppm in water. Greensand/Potassium Permanganate $1,200-1,400 <10 ppm System requires regeneration of oxidizing media. Chemicals can be messy and unpleasant. Chlorination systems $900-1,800 <75 ppm Upper cost range assumes removal of chlorine using a carbon filter system. *(Based on informal survey of water treatment/equipment suppliers in November 1998) 5

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