Clean Drinking Water
A Discussion of the Water Treatment Process
A typical day in a typical American household will see several trips to the kitchen sink for a glass of water, a pot of water to cook with, water to make iced tea, or a variety of other uses. The process of going into the kitchen, grabbing a glass from the cupboard, and turning on the faucet is a set of actions that take place almost without a conscious effort. In fact, there are millions of people right this moment pouring a glass of water from their household tap, without even thinking about where this water came from and how it became consumable. For something that is considered essential for human life, clean drinking water is used and taken for granted by many consumers who are unaware of the process behind creating their consumable water.
The human body is comprised of about 60% water (The water in you). Water is perhaps the first most important element for human survival and health. Water is a vital resource and provides many benefits to the human body, including maintaining constant temperature, lubricating and cushioning joints, protecting the spinal cord and other sensitive tissues, and ridding the body of waste (Water & Nutrition). It is possible to live for several days to a few weeks without food, but a person will die within just a few days without water. Considering the vital importance of water to human health, one would think that the treatment process would be common knowledge, but although many people are unaware of it, there is a great deal of work performed to provide clean drinking water to American consumers.
The vast majority of Americans, about 90%, receive their household drinking water from public drinking water systems (Water: Public Water Systems). The remainder acquire their water from other sources, such as wells. The United States Environmental Protection Agency (EPA) defines a public water system as a system that provides “water for human consumption through pipes or other constructed conveyances, if such system has at least fifteen service connections or regularly serves at least twenty-five individuals” (Water: Public Water Systems). The infrastructure that provides water to the majority of American households is a system of local public water utilities. These public water systems are regularly monitored for quality by the EPA. Private household wells are not regulated by the EPA, but more information can be found on their website.
Clean, safe drinking water has been a public health concern for decades. In 1974, the Safe Drinking Water Act (SDWA) was passed by Congress in an effort to protect public health (Water: Safe Drinking Water Act). Protection involves regulating the drinking water supplies for the nation. The laws have been amended a few times in order to protect the water sources, which includes lakes, rivers, reservoirs, springs, and ground water wells (except private wells). The Safe Drinking Water Act enables the United States EPA and other regulating agencies to monitor the nation’s water supplies, while protecting them from natural or human induced contaminants. Prior to the SDWA, and dating back to 1948, the first law passed in the United States to address water pollution was the Federal Water Pollution Control Act. As the public became more aware of pollution issues, the law has been amended, and in 1972 it became known as the Clean Water Act. The Act provides the basis for regulation of pollutants that are discharged into source waters, funding for sewage treatment plants, authority for the EPA to implement pollution control programs and setting water quality standards (History of the Clean Water Act). The histories and summaries of these laws can be found by visiting epa.gov, as only a brief introduction to these laws providing for one aspect of the public health of the United States is given here.
The source and quality of the water prior to treatment can determine the exact processes used. Drinking water comes from a variety of sources such as lakes and rivers. Salt water can be treated for human consumption, and has been throughout history. However, the desalination process can get quite expensive, which has hindered its use to some degree (Saline water: Desalination). Wastewater is treated in many communities for use in categories such as irrigation, but it is not currently used for drinking water in spite of adherence to drinking water standards in its treatment. This may be largely due to human abhorrence to using treated wastewater as drinking water.
There are a number of processes and steps used in treating water to make it safe for human consumption. The most common steps used in treating drinking water include coagulation and flocculation, sedimentation, filtration, and disinfection (Drinking Water: Water Treatment). Coagulation and flocculation can typically be considered one step in the process. A coagulant is added to the water, causing dissolved particles within the water to clump together; this new larger particle is called floc. Once the floc has formed, the next step is sedimentation, where the floc settles to the bottom. After the floc has settled—formed sediment—it is filtered to remove the floc. Filters vary depending on the water treatment system and source of the water, but are usually comprised of sand, gravel and coal. Once filtration has been accomplished the water is treated with a disinfectant to remove any remaining harmful microorganisms and to protect the water as it travels to its customers (Drinking Water: Water Treatment). This is a significantly generalized description of the water treatment process. It is important to note that the concept of treating water is simple but the entire process becomes quite complicated when applied to the large scale processes used to supply entire cities and counties with clean water.
Every public water system in the U.S. is regulated by the laws and standards set forth via agencies such as the EPA and Centers for Disease Control and Prevention (CDC). Each step along the treatment course is monitored for quality. Proper maintenance of equipment and the use of quality control are two key components in ensuring that the public drinking water supplies are safe. Additionally, real-time monitoring of the entire distribution system is perhaps most crucial in preventing illness by water-borne pathogens (Riley, Gerba and Elimelech). The public drinking water supplies in the U.S. are among the cleanest and safest in the world (Drinking Water: Public Water Systems). Perhaps this fact is what drives the ongoing oblivion to the work that goes into providing this high quality, clean drinking water. Americans tend to only notice public utilities when some catastrophe occurs, and do not pay much attention to the daily activities performed to keep them safe and healthy.
One public drinking water supplier, the Southern Nevada Water Authority (SNWA), recently received the “15 Year Directors Award of Recognition” from the Partnership for Safe Water (Southern Nevada Water System receives national award of recognition). This award is given in recognition of the water supplier surpassing federal water quality guidelines for its supply of drinking water to the community. The SNWA provides quality drinking water to the Las Vegas, NV, area, including the surrounding communities such as Henderson and Boulder City. Continuously providing clean, safe drinking water to the two-million-plus individuals in the Las Vegas area for this length of time is commendable, especially considering this area is surrounded by desert. The source for this water is the Colorado River, by way of Lake Mead. The SNWA currently has two water treatment facilities, the Alfred Merritt Smith facility located on Lake Mead and the River Mountains facility located in Henderson, NV.
A recent visit to the River Mountains Water Treatment Facility has provided valuable insight into the water treatment process. Included in the visit was a personal tour and interview with Richard Giltner, a treatment manager for the SNWA. The facility is located about 17 miles from Lake Mead just outside of Henderson, and faces the Las Vegas valley. The treatment process employed by the River Mountains Water Treatment Facility is called direct filtration. This process utilizes coagulation and flocculation along with rapid mixing and filtration. The sedimentation step is not generally used in this process, as the water quality prior to treatment must be of high quality, low turbidity and low contaminant levels (Direct Filtration).
Beginning at the intake level for the River Mountains Water Treatment Facility, chloramines are added to the water prior to its entering the infrastructure that will take it to the treatment facility. In this instance, chloramines are useful for removing quagga mussels and preventing them from entering and harming the pipes and infrastructure that make up the water treatment process (Giltner). The water is pumped to the treatment center, but once it enters the facility it is gravity-driven through the treatment process. This facility treats approximately 80 million gallons per day during the daytime hours, and approximately 200 million gallons during the night when the facility can take advantage of lower utility costs. Upon entering the treatment facility, it is first disinfected using ozonation. Ozone is a very potent disinfectant that kills waterborne pathogens with the best efficiency (Akin, Hoff and Lippy). This facility produces the ozone it uses, and the water is completely contained within a closed system while the ozonation process takes place. Nitrogen is removed from air, leaving almost pure gaseous oxygen, about 95% O2. This is then pumped into the water, which is then hit with about 4100 volts of direct current electricity. This creates the ozone, O3, which in turn provides disinfection. Prior to releasing the water into the next step, calcium thiosulfate is added to quench the remaining ozone, as it is considered a pollutant can cannot be released into the air at ground level.
Fig. 1. The pictures above depict one of the tanks inside which ozonation takes place at the River Mountains Water Treatment Facility (Giltner).
Fig. 2. (Left) Mr. Giltner explains how the DC electricity is passed through the ozonation tanks (right). (Giltner)
After leaving the closed tanks, the water moves into a flash mixer, where a coagulant, ferric chloride, is added. Along with rapid mixing, this coagulant causes the small particles still left in the water to clump together, forming floc. (See Fig. 3 below.) The floc particles are filtered in large filter tanks using layers of coal and sand. A six-inch layer of anthracite coal, crushed to 1 to 1.2 mm particles, and an eighteen-inch layer of sand particles, 0.45mm – 0.55mm, are what make up the filter.
Fig. 3. Filter (left) and mixing tank (right). Below is a motor that operates a mixing paddle in the mixing tank. (Giltner)
Once the floc has been separated from the water, the filtered water enters the clearwell, where it is treated and ready to be distributed to customers. A corrosion inhibitor is added to prevent corrosion of customers’ pipes. A proprietary compound of zinc orthophosphate, Zn3(PO4)2, is added as the corrosion inhibitor, at about 0.1mg/L. Fluoridation takes place by adding fluorosilicic acid, H2SiF6. And post chlorine is added to maintain the disinfection in the form of sodium hypochlorite, NaClO. The solution of NaClO is around 0.8%, as compared to over-the-counter bleach sold at about 15% solution (Giltner).
Each step along the process is monitored in real time, testing for quality. This real-time monitoring carries over to the entire distribution system. Depending on the particular item being tested for, these testing methods are quite specific and/or sensitive. For example, the turbidity of the raw water from Lake Mead as it enters the facility is around 0.7 NTU, and contains approximately 3000 particles per mL of water (of size greater than 2 microns). When the water leaves the facility to be distributed to consumers, it has less than 12 particles/mL and 0.033 NTU turbidity.
The River Mountains Water Treatment Facility is only one of many treatment facilities in the U.S., but it is a great example of how thorough and effective the treatment process is. And considering the award the SNWA just received for exemplary quality for the last 15 years, there is no need to worry about the quality of the drinking water in the Las Vegas area.
Perhaps the reason so many Americans are unaware, or oblivious, to the work being performed to bring them clean drinking is due to the fact that the U.S. has the highest quality drinking water in the world. As previously mentioned, most people tend not to notice their utilities until something bad happens. Most people methodically and unconsciously pour water from their household taps without even considering where it came from or how it came to be clean and consumable. And as long as water in the U.S. remains at this level of quality, there should be no reason for concern and people can continue their lives without worry for their health—at least with respect to clean drinking water.
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Giltner, Richard. Treatment Manager Tara Ginn. 6 August 2014. Personal interview.
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 Chloramines are disinfectants added to drinking water to remove microorganisms. They are made up of chlorine and ammonia. (Stanford University)