CASE STUDIES FOR POLLUTION ENGINEERING. Degassing of dissolved Hydrogen Sulfide from Drinking Well Water

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1 CASE STUDIES FOR POLLUTION ENGINEERING Degassing of dissolved Hydrogen Sulfide from Drinking Water Drinking water wells that were being used to supply water to a rest area on the Florida Turnpike at Fort Drum had moderate level of hydrogen sulfide (1-4 ppm) in the well water, making the water smell and taste bad. The water was pumped from the well to a water storage tank, and in the past the client had used a tray aerator, installed at the top of the tank to remove the hydrogen sulfide from the water before he water entered the storage tank. The water was being chlorinated for disinfection purposes before flowing into the storage tank. Hydrogen sulfide exists in the water in the dissolved and ionized form, and the amount that is ionized depends on the water ph, as shown in Figure 1. At low ph (ph < 5), all of the hydrogen sulfide is present as dissolved gas and hence can be removed by degassing. However, at higher ph conditions, only a fraction of the total hydrogen sulfide is present as a dissolved gas and only this fractional amount can be removed by degassing. % Hydrogen Sulfide as Dissolved Gas Figure 1. Percent Hydrogen Sulfide as Dissolved Gas Liquid ph Hydrogen sulfide in drinking water can be removed by four main methods: (1) aeration, such as contacting the water with air, so that the dissolved hydrogen sulfide can transfer from the water into the air; this is the approach used in Tray Aerators, in which the water trickles down trays with holes in them, and this allows the water to contact ambient air, thereby removing the hydrogen sulfide; Forced aeration can also be used with blowers bubbling air through the water; (2) chemical oxidation using hypochlorous acid, which is formed when chlorine is dissolved in water; however, this approach consumes more chlorine than needed just for disinfection purposes, and also forms disinfection by-products; A recommended dosage is 2.0 mg/l 1

2 chlorine for every 1.0 mg/l hydrogen sulfide; (3) Membrane Degassing in which a membrane separates the water, containing dissolved hydrogen sulfide, from a vacuum condition, thereby allowing the hydrogen sulfide gas to transfer across the membrane; and (4) Chemical methods such as Manganese greensand, in which the hydrogen sulfide reacts with the coating on the sand, or chemical oxidizing agents which oxidize the hydrogen sulfide to sulfate. Tray aerators that are generally installed on the top of water storage tanks suffer from several disadvantages: (1) they are not very efficient, since air bubbling is not a very good method of contacting the water with air, unless the air bubbles are very small; (2) the trays get fouled by biological growth and the sulfate reducing bacteria in these biofilms convert the sulfate in the water to sulfide, and this results in the tray aerator having a negative removal efficiency, i.e., they generate hydrogen sulfide rather than removing it; (3) the tray aerator sump can be easily accessed, thereby jeopardizing the security of the drinking water supply to entire towns; (4) the ambient air contacting the water also contains air pollutants, such as smog, dust, etc. At one location, a cloud of dust was rising during grass mowing around the storage tank that was directly going past the tray aerator, located at the top of the tank, which allowed al this dust to contact the drinking water; and (4) the weight of the tray aerator is significant and many tanks are unable to support this weight, requiring a separate support structure to be built next to the tank. While chlorination oxidize the hydrogen sulfide to sulfate the expense of the additional chlorine needed to oxidize the sulfide and the formation of disinfection by-products makes this approach unacceptable. At the Fort Drum location, the degassing system was designed to operate under the conditions shown in Table 1. Table 1. Design Operating Conditions for the Membrane Degassifier. Parameter Maximum Value Average Value Inlet water flowrate 300 (gpm) 150 (gpm) Inlet Dissolved Hydrogen sulfide 1.2 (ppm) 0.9 (ppm) concentration Outlet Dissolved Hydrogen Sulfide concentration < 0.1 ppm < 0.1 ppm Inlet water temperature < 50 (deg C) 30 (deg C) 10 psi maximum 40 psia maximum Pressure drop across system Maximum Inlet water line pressure as specified 2

3 Membrane Degassing, developed by PRD Tech, Inc. is a compact system in which water, under pressure from the well pump, is contacted with a vacuum condition in a membrane cartridge, where the only material that can transfer across the membrane is the dissolved hydrogen sulfide gas, as shown in Figure 2. Multiple cartridges are housed in a single chamber, and the water simply flows past the membrane surface and does not cross across the membrane. Particulates, such as silt, sand in the inlet feed water flow through the system, past the membranes, and since water does not cross the membrane, there is no issue with membrane clogging, as in a membrane filter. Figure 3 shows a photograph of the Fort drum Membrane Degassifier. The system is highly efficient compared to a tray aerator, without the biofilm growth issues, introduction of air pollutants and does not jeopardize drinking water security, since the system is under pressure. Figure 2. Principle of Membrane Degassing. DISSOLVED HYDROGEN SULFIDE/ WATER SIDE WITH DISSOLVED HYDROGEN SULFIDE/ GAS GAS PHASE HYDROGEN SULFIDE/ VACUUM SIDE POROUS MEMBRANE DENSE MEMBRANE 3

4 Figure 3. Photograph of the Fort Drum Membrane Degassing System. Table 1 shows the performance of the system at Fort Drum, FL. The hydrogen sulfide concentration was measured in the field using a colorimeter test. Samples of water taken from the sump of the existing tray aerator showed higher values of hydrogen sulfide than the inlet water, indicating that the sulfate in the water was being reduced to hydrogen sulfide by the biofilms growing on the surface of the tray aerators and on the inside walls of the water sump. 4

5 Table 1. Performance Results of the Fort Drum Membrane Degassing System. Inlet w ater ph Inlet water Temperature (deg C) Inlet Total Sulfide (ppm) Outlet water ph Outlet water temperature (deg C) Outlet Total Sulfide (ppm) Parameter Measured % Inlet Hydrogen Sulfide (Table 1) % Outlet Hydrogen Sulfide (Table 1) % Hydrogen Sulfide Removal Efficiency (BDL = 0.1 ppm) West West Membrane Degassifier West West East East BDL 0.25 BDL BDL BDL 0.19 Note: BDL is Below Detection Limit = 0.1 ppm % Hydrogen Sulfide Removal Efficiency (BDL = 0 ppm) Outlet Hydrogen Sulfide Conc. (ppm) Average Removal Efficiency (%) taking BDL = 0.1 ppm Average Removal Efficiency (%) taking BDL = 0.0 ppm Average Outlet Hydrogen Sulfide Conc. (ppm)