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Occurance of hydrogen sulphide

Sulfates are a combination of sulfur and oxygen and are a part of naturally occurring minerals in some soil and rock formations that contain groundwater. The mineral dissolves over time and is released into groundwater.

Sulfur-reducing bacteria, which use sulfur as an energy source, are the primary producers of large quantities of hydrogen sulfide. These bacteria chemically change natural sulfates in water to hydrogen sulfide. Sulfur-reducing bacteria live in oxygen-deficient environments such as deep wells, plumbing systems, water softeners and water heaters.

Hydrogen sulfide gas also occurs naturally in some groundwater. It is formed from decomposing underground deposits of organic matter such as decaying plant material. It is found in deep or shallow wells and also can enter surface water through springs, although it quickly escapes to the atmosphere. Hydrogen sulfide often is present in wells drilled in shale or sandstone, or near coal or peat deposits or oil fields.

Treatment methods for removal of hydrogen sulphide

The removal methods vary according to the concentrations of hydrogen sulphide present in the well.

Using activated carbon filters (For Hydrogen sulphide trace amounts of 0.05-0.3 mg/l)

Activated carbon is used as a granular form in tank-type filters (Figure 1) and as finely divided powder in a cartridge. The hydrogen sulfide is adsorbed onto the surface of the carbon particles. A granular filter must be backwashed periodically; a cartridge filter must be cleaned or replaced periodically. Cleaning frequency depends on the amount of hydrogen sulfide in the water and the volume of water treated. Moderate to high levels of hydrogen sulfide will require frequent filter replacement.

Aeration Method (less than 2mg/l hydrogen sulphide concentration)

Aeration (adding air to the water) is a treatment option that is commonly used by city water treatment systems. Oxygen in the air reacts with hydrogen sulfide to form an odorless, dissolved form of sulfur called sulfate. Several types of aeration systems are available. Type 1: compressed air is injected into the water system. The air then must be removed from the water to prevent knocking or air-blocks in the system and to reduce the corrosion potential caused by dissolved oxygen. Type 2: water is sprayed into a non-pressurized storage tank. 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 storage tank and aerator must remain secure to prevent contamination of the water supply, or the system must be chlorinated. This process usually produces a strong hydrogen sulfide odor near the aerator, and may not always reduce the hydrogen sulfide to non-detectable levels. In such cases, a carbon filter can be used to remove some of the remaining trace amounts of hydrogen sulphide.

Using Iron removal filter with manganese greensand pressure filtration system

An iron removal filter containing manganese greensand can remove low to moderate levels of hydrogen sulfide in addition to iron and manganese. This process oxidizes hydrogen sulfide into sulfate; iron and manganese form precipitates that are filtered out (Figure 2). Manganese greensand filters must be recharged with a solution of potassium permanganate when the oxygen is depleted. This process is very similar to the regeneration process used in water softeners, and must be performed at regular intervals of 1 to 4 weeks depending on the chemical composition of the water, size of the unit and amount of water processed. Water with a pH below 6.7 could require acid neutralization, increasing the pH to 7.5 to 8.3, before iron removal will be effective.

Figure 2 Manganese greensand pressure filtration system for removal of hydrogen sulfide, iron and manganese

Continuous chlorination method (Up to 75 mg/l)

Continuous chlorination using an automatic chemical feed pump can effectively remove medium to high levels of hydrogen sulfide (Figure 3). Chlorine quickly oxidizes hydrogen sulfide into a tasteless, odorless form. Continuous chlorination also effectively removes iron and manganese that can occur in association with hydrogen sulfide. Chlorine demand (concentration) and contact time are very important to successful chlorination. The amount of chlorine needed to react with organic and inorganic materials in the water can range from 1 mg/l to more than 10 mg/l if the water is high in sulfide, ammonia, iron or manganese. For removal of hydrogen sulphide, the chlorine dosage needs to be 2.2 times H2S concentration (mg/l). Contact time (exposure) depends on the concentration of chlorine in the water, water temperature and pH. Oxidation of iron and hydrogen sulfide is instantaneous, while manganese oxidizes more slowly. Chlorine should be introduced into the system before it reaches the storage tank. The tank must have sufficient volume to provide a 20-minute contact time between the water and the chemical. In addition, a fine-retention sediment filter can be used to remove any oxidized iron and manganese. Excess free chlorine is undesirable and can produce an objectionable taste in the water. An activated carbon filter can be used to obtain chlorine-free water for cooking and drinking.

Figure 3 Chlorine injection system for removal of hydrogen sulphide.

Source: Texas A&M AgriLife Extension Service http://AgriLifeExtension.tamu.edu