Chapter 6. Ensuring Safe Drinking Water - Water in the Tucson Area: Seeking Sustainability

Water in the Tucson Area: Seeking Sustainabliity

ch. 6, pp. 77 - 80

Chapter 6: ENSURING SAFE DRINKING WATER

Different toxic substances require different kinds of treatment. Removal of TCE through aeration is discussed above. Because neither CAP water nor Tucson groundwater generally contain problem amounts of other toxic substances, other treatment methods will not be discussed.

Figure 6-5 Out of the aborted effort to introduce CAP water to the community arose a brand of CAP humor. Above is a "Fitz" cartoon that appeared in "The Arizona Daily Star" in the spring of 1994. Used with permission from David Fitzsimmons, The Arizona Daily Star.

REDUCING HARDNESS AND SALINITY

Ion exchange processes are used to reduce hardness and also can be used to remove all types of ions, such as arsenic, chromium, excess fluoride, nitrates, radium and uranium. Different ion exchange systems are available, from those useful at the commercial level to small in-home systems. Sodium or potassium is used as the exchange agent in water softeners.

Salinity can be reduced through thermal systems (distillation) or through membrane processes. Thermal processes have been used since 4 B.C. when Greek sailors used an evaporative process to desalinate seawater. The thermal systems use heat to produce water vapor that is condensed to produce fresh water. Approximately 60 percent of the desalting systems used in the world today are thermal systems. Only between 25 percent and 50 percent of the source water is recovered, with the rest left in a highly saline brine. These systems are most useful along the coast where a water supply is not a problem, and the brine can easily be returned to the ocean.

Membrane processes include electrodialysis (ED), reverse osmosis (RO) and nanofiltration (NF). ED uses electricity to move salts selectively through a membrane, leaving fresh water behind. Because most dissolved salts are ionic (either positively or negatively charged) and the ions are attracted to electrodes with the opposite charge, membranes that allow selective passage of either positive or negative ions can accomplish the desalting. The advantage of this type of system in water-short areas is that 80 to 90 percent of the water is recovered and only 10 to 20 percent is lost to the brine.

RO and NF systems physically force water through membranes. Larger suspended solids must be removed to avoid clogging the membranes. The primary difference between RO and NF is in the size of the pores and the energy needed for pressuring the water. The smaller the pores, the more energy is needed.

Table 6-5 How some southwestern communities treat their drinking water.

In any desalting process, a saline concentrate is produced which must be disposed of in some manner. Various options have been suggested for CAP water. These include a pipeline to move the brine concentrate to the Gulf of Mexico, the Salton Sea in California or the Colorado River at Yuma. Another option is to evaporate the brine locally, leaving a solid salt which could be disposed of in landfills. Another possible option would be to join with other Arizona cities to build a desalting plant near the Colorado River. The concentrate could then be more readily taken to the ocean or Salton Sea. Since Phoenix is not interested in participating in building such a desalting plant, this may not be a feasible option. If Tucson adopts desalting as a treatment method, the community would have to accept water losses of between 15 and 25 percent as part of the process. If 130,000 acre-feet of water were desalinated, between 25,000 and 40,000 acre-feet could be lost as brine.

OTHER COMMUNITIES CHOOSE TREATMENT STRATEGIES

The treatment strategies of three towns are described below to illustrate the importance of analyzing community circumstances and needs when choosing a suitable treatment method.

Buckeye, Arizona

Because of a long history of irrigated agriculture, the town of Buckeye’s water supply was too salty to drink, having TDS (total dissolved solids) ranging from 1,500 to 4,000 ppm. The water ruined pipes and appliances and “just plain tastes bad.” In 1962, the town became the first community in the United States to treat all its municipal water supply with an electrodialysis desalting plant, with a capacity of 65,000 gallons per day (gpd). In 1988, the town constructed a new 900,000 gpd electrodialysis reversal system in place of the old system. The brine from the plant is put into evaporation ponds. When the water evaporates, a saline sludge remains which is released to irrigation canals.

Glendale, Arizona

The rapidly growing City of Glendale needed to build another treatment plant to treat Salt River Project water and CAP water. Officials had four concerns:

Land scarcity necessitated that the new water treatment plant take up as little space as possible;
Turbidity needed to be reduced by removing particles in the water, which at times is clear and at other times murky, especially in the rainy season;
Water had to be disinfected to meet anticipated new EPA standards for THMs;
Taste and odor problems occurring occasionally due to algae growth in the canal had to be confronted.

Glendale officials decided to use ultrafiltration, a method to remove pathogens, particles and 20 to 30 percent of the organic matter that is a precursor of THMs. Since salinity was not a concern, nanofiltration or reverse osmosis were not considered. A pilot plant producing one million gallons a day will open this spring. If the process works well, a much larger plant will be built. The water is pretreated with chlorine, treated with alum and allowed to settle so large particles drop out. The water will again be dosed with chlorine, then put through membrane filters and finally dosed once more with chlorine in the distribution system. The use of powdered activated carbon helps to reduce occasional taste and odor problems. Glendale does not treat water for corrosivity. Construction and operating costs are acceptable to the city, especially since the cost of ultrafilters has been decreasing as this method becomes more popular, and filters last seven years or more. This type of filtr ation requires a smaller facility than most other methods, so land use needs were minimized.

Las Vegas, Nevada

Las Vegas gets 85 percent of its water from the Colorado River at Lake Mead and 15 percent from groundwater. The Las Vegas Valley Water District considers Colorado River water to be very high quality water needing little treatment. The treatment process in use today involves disinfection with chlorine, aeration, flocculation, filtration and more chlorine treatment to produce a residual effect in the system. Although the water readily meets EPA standards for THMs, a new treatment plant under construction will use ozone disinfection instead of chlorine to minimize the THM content and meet anticipated new EPA standards. Las Vegas’ Alfred Merritt Smith Plant will be retrofitted with five 4,000 pound-per-day ozone generators, and the new River Mountain facility will use three 2,000 pound- per-day ozone generators. A small amount of zinc orthophosphate is added at the end of the process to retard corrosion and chloramine is added for residual disinfection in the pipes. Ozonation wil l be implemented in the year 2000 at the existing Alfred Merritt Smith Treatment Facility and at the new River Mountain Treatment Facility which will be constructed by 2002. This is the same treatment method used at Tucson Water’s Hayden-Udall Treatment Plant.

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