Location: Memphis and Shelby Counties, Tennessee
Title: Drinking Water from Wells, Not From the River
Memphis is located in the southwest corner of Tennessee in the United States. The state of Arkansas is to the west and the state of Mississippi is to the south. Bordered by the Mississippi River on the west, the city of Memphis is a tri-county city in the mid-south portion of the United States. Although the Mississippi River and its tributaries supply this area with great amounts of surface water, this river does not supply drinking water for Memphis, nor is it used for any other purpose.
Memphis and Shelby counties rely on groundwater for their drinking water supply. Their drinking water supply is drawn from a natural reservoir deep in the earth. From geological formation, the Memphis area is located in the north-central part of the Mississippi Embayment, a broad structural syncline that plunges southward along the Mississippi River. Within this formation, four water-bearing layers of sand and gravel are situated from 50 to 2600 feet below ground. One saturated permeable aquifer, called the Memphis sand aquifer, lies from 350 to 1100 feet below ground. With the abundance of groundwater resources, Memphis' water supply comes mainly from the 500-foot sand layer of the Memphis sand aquifer. It is estimated this sand layer contains a trillion gallons of water. Since the first artesian well was drilled in 1887, the level of the Memphis sand aquifer has dropped only 125 feet. With an average aquifer thickness of 850 feet, that drop in the Memphis sand aquifer is an extremely small amount. Based on this fact, it is safe to say that Memphis has little fear of a water shortage.
The formation of groundwater tells the natural purification process. Rainfall enters underground sand and gravel as groundwater seepage at the elevated outcroppings of the formation. During more than 2000 years of travel through the sand layers, the water moves toward the Memphis aquifer or deeper aquifers. Because of this natural filtration process, Memphis' drinking water quality is considered to be one of the best in the United States.
Memphis Light, Gas, and Water Division (MLGW) is the primary utility operation agency in Memphis and Shelby counties. Ten water-pumping stations are operated by MLGW to supply drinking water to the city and some adjacent areas in Shelby County. All of these stations, except one, have ten to thirty large-capacity wells in their well fields. According to the water demand in the serving areas, there are 162 wells in the MLGW system. These wells are operated by electric motors that drive deep-well turbine pumps. Each well can produce between 1.5 million to 2.5 million gallons of drinking water per day.
The unit operations and processes of water treatment in the Memphis area include aeration, filtration, and disinfection (Figure 1). As the water pumps from the aquifer, it goes into a tray-type aeration tower for hydrogen sulfide and dissolved carbon dioxide removal (or odor removal). A typical tray-type aeration tower has a vertical flow channel that distributes water to the top of the trays, which allows the water to drip down the stack. The trays usually contain coke that can process adsorption for odor and iron removal. For iron removal, the iron oxidation process can easily oxidize soluble ferrous irons (Fe++) to insoluble ferric oxides (FeOx).
Fe++ (ferrous) + oxygen ® FeOx (ferric oxides) (1)
As ferric oxides appear in the water after the aeration, they are associated with the water dripping down from the top tray to the bottom. The coke in the tray uptakes the ferric oxides on its surface. Before the water goes into the next water treatment unit, the water is stored in a reservoir. Leftover ferric oxides can be settled and removed in the reservoir. The water is then pumped into a filtration facility. The dissolved iron that would stain textiles and clothing is converted into solids and filtered out in filtration. When the head loss might exceed the collection trough in the filtration bed, the filtration bed needs to be cleaned by backwashing.
After filtration, one part of fluoride, chlorine, and phosphate per million parts of water is added into the water. MLGW is required to add fluoride and chlorine in order to prevent tooth decay and kill any bacteria that might exist in the water. The phosphate is added to inhibit corrosion in the distribution system. Once the water has been treated, it flows into concrete reservoirs where it is stored until needed for distribution to the users. Most of these reservoirs are underground, with a concrete roof and 2 feet of sod covering the tanks to prevent contamination and the growth of algae. This procedure also keeps the water at a constant temperature (63-65 degrees Fahrenheit).
High-driven pumps transfer the water from the underground reservoir into the distribution system. MLGW is very careful to prevent impurities from getting into the water body during the transferring of water from the storage reservoir to the users. Each month, more than 3000 water samples are taken from different locations in the distribution system and the pumping stations are examined. The results must be reported to the U.S. Environmental Protection Agency. Table 1 shows the water quality in Memphis and Shelby counties.
|Iron||0.02 ppm||Manganese||0.004 ppm|
|Alkalinity||57.2 ppm||Hardness||51.9 ppm|
|Calcium||27.5 ppm||Magnesium||24.4 ppm|
|Sodium||8.12 ppm||Potassium||0.92 ppm|
|Sulfate||8.60 ppm||Chloride||6.00 ppm|
|Nitrate||0.13 ppm||Phosphate||1.41 ppm|
|Dissolved Solid||80.2 ppm||Silica||13.1 ppm|
Approximately 140 million gallons of water are treated and pumped daily to the users, including residential, commercial, and industrial customers. The water is not only good, but is also "cheap." Water is billed by MLGW in units called "100 cubic feet of water" (Ccf). One Ccf is equal to 748 gallons. Based on data from September 1999, 100 cubic feet of water (Ccf) costs only 0.90 cents in the city, or 9 cents per 100 gallons of water.