Many of the great engineering achievements beginning at the earliest times through the present have been associated with providing a reliable water supply and with disposing of wastewater. The Roman aqueducts that supplied Rome and other cities with drinking water between 300 BC and AD 300 are one example of this. The sewer system in London developed during the late 1800s is another example. Because of these engineering accomplishments, most people in the developed world have access to safe drinking water.
In this section, we investigate the relationship between engineering and water. We first describe the concept of water quality standards and how drinking water should meet these standards. We then describe the techniques engineers have developed to provide potable water and to treat wastewater so that it does not unduly pollute the environment.
Water Quality StandardsEdit
Most of us know that the water we receive from our local municipalities is treated. The treatment of water has to follow water quality standards. In the United States, the development of water quality standards began in the early twentieth century. Over time, these water quality standards have evolved and have become more rigorous. Laws and regulations such as the Clean Water Act govern water quality standards in the United States. The US Environmental Protection Agency (EPA) has the mission of protecting human health and the environment. The EPA has created water quality standards that define the goals for a body of water and specify its uses. They establish criteria to protect those water uses. They also establish requirements to protect water from pollutants.
In general, a water quality standard consists of four basic elements (EPA):
- Designated uses of the water body (e.g., recreation, water supply, aquatic life, agriculture).
- Water quality criteria to protect designated uses (numeric pollutant concentrations and narrative requirements).
- An antidegradation policy to maintain and protect existing uses and high-quality waters.
- General policies addressing implementation issues (e.g., low flows, variances, mixing zones).
State governments are required to identify the appropriate uses for water bodies. These uses are identified by taking into consideration the value of the water body for various uses. The uses for a water body can include any of the following: public water supply, protection of fish, shellfish, and wildlife, recreation, agriculture, industry, and navigation. In making these designations, the appropriate authorities must examine whether the water body is suitable for the intended use. This determination is made based on the physical, chemical, and biological characteristics of the water body, its geographical setting and scenic qualities, and economic considerations.
The quality of drinking water differs from one location to another. Quality is dependent on the condition of the water source and the treatment. Each local community water supplier is required to provide an annual report, sometimes known as a "consumer confidence report" to the public. This report usually provides information on the quality of the local drinking water, its source, and the contaminants found in the water (Figure 7).
Scientists report the contaminants in water as contaminants in parts per million (ppm), parts per billion (ppb), and parts per trillion. When water contaminant measurements are reported in this manner, you may wonder: How can amounts of contaminants that seem so very small cause health problems? Scientific research has shown that, for example, lead concentrations as small as 15 ppb can be harmful to infants and children. The smell of petroleum products in water can be detected by humans even when the amount of petroleum in water is as low as 10 ppb.
It is very important to note that there is no such thing as naturally pure water. All water in nature contains some sort of impurities. Since water is a good solvent, as it flows through various layers of soil and over rocks, it dissolves minerals. These minerals in the water may get the taste (good or bad). However, if these minerals are above a certain level, they could be harmful.
Have you ever thought about where your drinking water comes from? Many of us take the drinking water we get when we turn on our kitchen faucet for granted. The water in many urban areas comes from surface sources such as lakes, rivers, and reservoirs. These sources can be near the urban areas or far away. Water suppliers procure and treat the water they provide. It is important to consider the entire watershed that provides the water. A watershed is the land from which precipitation (usually rain or snow) flows into the river, lake, or reservoir.
In many urban and rural areas, water also comes from ground water that is pumped from a well. Wells provide water from aquifers, which are natural water reservoirs under the earth's surface. Aquifers may lie under parts of several states or may only be a few miles wide. Thus, water quality is affected not only by the water source such as a river or well that we can see, but also by activities that occur many miles away.
Before the water is delivered to our homes, the water is treated. The treatment of drinking water that is most common is disinfection. Water suppliers add chlorine or another disinfectant to kill bacteria and germs in the water. This commonplace drinking water treatment is considered to be one of the most important scientific advancements of the twentieth century.
Common water treatment methods: Before we examine a water treatment plant, list and describe some common ways to treat water or purify water for drinking that you can do at home or during emergencies.
Materials needed: Internet-accessible computer, science textbooks.
Suggested phrases for searching on the Internet: Water purification at home, Water purification during emergencies, Common water purification methods.
Directions: Working alone or in a group, brainstorm a list of ways in which drinking water might be bad. From your list, identify problems that might need to be treated. Brainstorm a second list of ways to obtain good drinking water in your home. Do any of these solve the problems identified on your first list?
Using the Internet, research news stories related to water supply and water problems in your local community. How do the results of your research compare to your first list of potential problems with water? Are there problems that you did not anticipate?
Research information on water purification techniques that can be used at home. Discuss in small groups what you learned from your research. Prepare a chart with a brief description of common water problems and the treatment/purification methods appropriate to each problem; include a list of materials you would need for each treatment method and illustrate each method by making your own drawing.
Hints: Filtration, Boiling, Distillation, Water Softeners, Reverse Osmosis
Extensions: What will be the cost of implementing these home-based water purification techniques if you were to need clean water for a family of four? How would you go about determining the amount of water that you will need? What is the most effective method in terms of removing impurities for the quantity of water you need for a family for four for drinking/food purposes?
Water Treatment BasicsEdit
Municipalities must perform two types of water treatment: treatment of water before it is used for drinking and other purposes, and treatment of wastewater such as sewage so that it is safe to return to the environment. Typically, drinking water is treated based on the quality of the source water. Ground water, for instance, usually requires less treatment than surface water from a river or a lake. Thus, the quality of the water that enters your community water treatment plant determines how the water will be treated prior to distribution.
Wastewater includes sewage as well as water used in industrial and agricultural processes. It flows out of homes and neighborhoods through sewage pipes to a wastewater treatment plant. Here, the wastewater is treated to remove solids and contaminants. The treated wastewater may be re-used for irrigation and landscaping. The treated wastewater is also returned to streams, rivers, and oceans, which can also be a source of pollution.
Treatment plants for drinking water and for wastewater have equipment and processes to remove or destroy harmful materials and organisms. A treatment plant uses tanks and mechanical parts such as valves and pumps to move the water through the different processes. These processes are designed and managed by water engineers. Other engineering specialties, including mechanical and construction engineering, play a role in the construction and maintenance of water treatment plants.
Drinking Water TreatmentEdit
Water from rivers, lakes, and streams or ground water is pumped and transported to a drinking water treatment facility. Then the water is processed through various units to prepare the water for distribution to homes. The water treatment involves the steps shown in Figure 8 and described below.
Coagulation. First, dirt and other particles must be removed from the water. Flocculants are chemicals such as alum (aluminum potassium sulfate) that cause the dirt and other particles to stick together; flocculants are added to the water, which creates larger particles called floc.
Sedimentation. As the water moves through the sedimentation tanks, the floc particles settle to the bottom of the tank. The clear water then flows to a filtration unit.
Filtration. Filtration removes small particles from the water by passing it through layers of sand, gravel and charcoal. The water then moves to disinfection before storage.
Disinfection. Water is disinfected with chlorine or other chemicals, called disinfectants, to kill any bacteria and other harmful organisms. The amount of disinfectants added to the water has to be carefully adjusted, because too much may be harmful to humans, but too little will not kill the harmful organisms.
Storage. After disinfection, the water is stored in storage tanks until it is needed for distribution to homes, businesses, and other water users.
What are the mechanisms that maintain water pressure during peak water usage times in urban areas? You can easily imagine that in most urban cities, water usage will peak during certain times of the day. Have you wondered how the water flow rate and the pressure at which the water flows out of your home's faucets is maintained during the mornings when most people are preparing to leave for school or work? This is a time when water use is high as people brush their teeth, shower, and use water for personal hygiene.
Materials needed: Internet-accessible computer, science textbooks.
Suggested phrases for searching on the Internet: Water pressure, water flow during peak water usage times
Directions: Research and review information on how water delivery systems or local municipalities or your city's water supply systems regulate water flow and water pressure during peak times. Discuss in small groups what you learned from your research. Prepare a brief essay that describes the mechanisms or techniques used by water suppliers to maintain water flow and pressure during peak times. Illustrate this technique by making your own drawing.
Hint: Have you seen water tower tanks in cities that are in flat areas? Research what purpose these water tower tanks serve. What do water pumps do? Research the various ways in which a city or municipal water supply system uses water pumps.
Before wastewater can be released into the environment, it is treated in a wastewater treatment plant. An aerial view of a wastewater treatment plant is shown in Figure 9. Figure 10 illustrates the steps involved in the treatment of wastewater. We describe the steps in more detail below.
Pumping. Wastewater treatment facilities are usually located on low ground so that gravity will move sewage from homes to the treatment plant. Usually, pumps are needed to lift the sewage as it enters the treatment facility. The treatment facility uses gravity to move the wastewater through the treatment process.
Bar screen. As it enters the treatment plant, wastewater may contain large items such as plastic bottles, cans, sticks, rocks, and even dead animals. These items are removed by the bar screen and sent to a landfill. If they are not removed, they will damage equipment in the treatment plant.
Grit chamber. After screening, wastewater enters the grit chamber in which larger particles (such as sand or dirt) settle out of the water. Often, the water is aerated (air is bubbled through it) to keep smaller particles from settling out. Aeration causes some of the gases that are dissolved in the water (e.g. hydrogen sulfide that smells like rotten eggs) to be released.
Sedimentation tank. In the sedimentation tank (also known as the primary clarifier), solids settle to the bottom as sludge and scum floats to the top. The sludge is pumped out of the primary clarifier and sent to the solids processing facility. The scum is composed of lighter materials such as grease, oil, soap, and so forth. Slow-moving rakes are used to collect the scum from the surface of the wastewater.
Secondary aeration and clarifier. The wastewater is exposed to air in an aerator, which provides oxygen for microorganisms that help break down contaminants in the water. This may be done by spraying the wastewater into the air or by bubbling air through the wastewater. The aerated effluent is passed into a secondary clarifier, which is a large tank or pond; in the clarifier, microorganisms decompose organic material and absorb nutrients such as nitrogen and phosphorus. The microorganisms and remaining solids settle out of the effluent as activated sludge. Most of the activated sludge is pumped to the solids processing facility, while the remaining sludge is pumped into the wastewater entering the aerator. This introduces additional microorganisms to the wastewater to hasten the breakdown of organic matter.
Filtration. Filtration may be used to further reduce the organic matter in the water. The water is filtered through a substance, usually sand and rocks. During this filtration process, most bacteria are removed, turbidity and color in the wastewater are reduced, odors are removed, the amount of iron content in the wastewater is reduced, and any other solids that may have remained in the water are also removed. This water may subsequently be filtered again through a carbon filter such as charcoal to remove organic particles.
Disinfection. To kill remaining harmful bacteria and other pathogens in the processed wastewater, chlorine and other chemicals are added in a disinfection tank. The chlorine can be harmful if added in excess quantities. (You may have noticed the smell of chlorine or have had irritated eyes when you were exposed to chlorine in a swimming pool.) Therefore, in some cases, the chlorine must be neutralized with other chemicals after it has killed the bacteria to protect marine organisms.
Solids processing. Solids include the sludge and scum removed in the sedimentation tank and the activated sludge removed from the secondary clarifier. These solids may be processed further in devices called digesters, which are heated and enclosed tanks. The solid wastes are kept in these tanks for 20–30 days to reduce the volume of the material, reduce odors, and also destroy any organisms that have the potential to cause disease. Depending on the source and composition of the wastewater, the digested solids are either sent to a landfill or used as fertilizer for crops. The use of the processed solid wastes as fertilizers is usually done only after careful testing for any potential dangerous contamination.
The treated water that is released by the plant is called effluent. The effluent is usually released into a local river or the ocean. In some places, this water may be used for landscaping (e.g. to water lawns or golf courses), but not for drinking purposes.
The following questions will help you assess your understanding of this section. There may be one, two, three, or even four correct answers to each question. To demonstrate your understanding, you should find all of the correct answers.
- b, d
- a, d
- a, c, d
- b, c, d
- body of water
- A significant amount of water either naturally (such as lakes, rivers, and oceans) or man-made (such as ponds, lakes, and harbors).
- A substance that may be harmful to humans or other forms of life when released into the environment.
- A widespread outbreak of a contagious disease.
- The force of attraction exerted between objects. Often, this is the force of attraction that the earth exerts on objects at its surface.
- A disease-producing organism.
- pH (potential of Hydrogen)
- A measure of the activity of hydrogen ions (H+) in a solution. This is a measure of the solution's acidity or alkalinity. pH is the logarithm of the reciprocal of the effective hydrogen-ion concentration and is a number between 0 and 14; the number has no units. A pH of 7 indicates neutrality, that is, the solution is neither acidic nor alkaline. pH numbers lower than 7 indicate acidity, while pH numbers higher than 7 indicate alkalinity. Each change of 1 pH represents a tenfold change in acidity or alkalinity.
- To make something dirty, foul, or unclean.
- Potable water is water that is clean enough to drink.
- Water that falls from the atmosphere to the earth's surface. The most common form of precipitation is rain. Snow, sleet, hail, and freezing rain are also forms of precipitation.
- Environmental Protection Agency. "Water Quality Standards". Downloaded March 4, 2008. Available on the web at http://www.epa.gov/waterscience/standards/about/
- George Constable and Bob Somerville. A Century of Innovation: Twenty Engineering Achievements that Transformed Our Lives. Joseph Henry Press, Washington, D.C., 2003.
- Henry Petroski. Pushing the Limits: New Adventures in Engineering. Knopf, New York, 2004.
- John Snow Archive and Research Companion. MATRIX: The Center for Humane Arts, Letters, and Social Sciences Online at Michigan State University. Downloaded May 28, 2008. Available on the web at http://matrix.msu.edu/~johnsnow/
- NRC (National Research Council). Our Common Journey: A Transition Toward Sustainability. National Academy Press, Washington, D.C., 1999.
- World Health Organization. "Cholera: Prevention and Control". Downloaded July 2008. Available on the web at http://www.who.int/topics/cholera/control/en/