by Dale Baker
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 Treatment
Figure 3.8
Illustration of a typical drinking water treatment process.
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.
Activity
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.
Wastewater Treatment
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.
Figure 3.9
An aerial view of a wastewater treatment plant in Dresden, Germany.
Figure 3.10
Steps in a typical wastewater treatment process.
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.
Review Questions
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.
The United States government agency that is responsible for water quality stand
ards is the Environmental Protection Agency
the national air and space administration
the president of the United States
the forest service
Urban water supplies come from fire hydrants
surface sources such as lakes, rivers, and reservoirs
the ocean
groundwater
Two types of water treatment are treatment of wastewater
freezing and melting
adding a slice of lemon
treatment of drinking water
The design and management of processes in a water treatment plant involved these engineering specializations water engineering
social engineering
mechanical engineering
construction engineering
The purpose of disinfection is to further pollute water
kill fish and other marine animals
kill bacteria
make water taste good
The purpose of wastewater treatment is to make the water clean enough to drink
remove solid and organic matter from water
kill harmful bacteria
reduce pollution caused by wastewater
Review Answers
Water and Engineering
a
b,d
a,d
a,c,d
c
b,c,d
Vocabulary
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).
Contaminant
A substance that may be harmful to humans or other forms of life when released into the environment.
Epidemic
a widespread outbreak of a contagious disease.
Gravity
The force of attraction exerted between objects. Often, this is the force of attraction that the earth exerts on objects at its surface.
Pathogen
A disease-producing organism.
(potential of Hydrogen)
A measure of the activity of hydrogen ions in a solution. This is a measure of the solution’s acidity or alkalinity. is the logarithm of the reciprocal of the effective hydrogen-ion concentration and is a number between and ; the number has no units. A of indicates neutrality, that is, the solution is neither acidic nor alkaline. numbers lower than indicate acidity, while numbers higher than indicate alkalinity. Each change of represents a tenfold change in acidity or alkalinity.
Pollute
To make something dirty, foul, or unclean.
Potable
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.
Precipitation
References
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.
NRC (National Research Council). Our Common Journey: A Transition Toward Sustainability. National Academy Press, Washington, D.C., 1999.
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/
World Health Organization. “Cholera: Prevention and Control.” Downloaded July 2008. Available on the web at
http://www.who.int/topics/cholera/control/en/
Instructor Supplemental Resources
Standards
ASEE Draft Engineering Standards. This chapter is focused on “Dimension 5: Engineering and Society” of the ASEE Corporate Members Council Draft Engineering Standards; these draft standards will serve as input to the National Academy of Engineering process of considering engineering standards for K-12 education. This dimension includes the following outcomes:
Students will develop an understanding that engineering is an ethical human endeavor that addresses the needs of a global society.
Students will be able to investigate and analyze the impact of engineering on a global society.
Common Preconceptions
Students have a number of preconceptions about engineering, technology, and the topic of this unit—water—which can affect their understanding of the reading material and activities. There are preconceptions relating to Dimension 5: Engineering and Society in general and others related specifically to the standards subsumed by Dimension 5.
Engineering and Engineers
Students have little to no knowledge about what engineers do or to the range of engineering careers open to them. They rarely know anyone who is an engineer unless that person is a relative. Perceptions of what engineers do are limited to planning, designing, building, fixing and repairing things. Engineers are also perceived as male and never female. Engineers who work with computers are viewed as hackers. All engineers are viewed as lacking social qualities.
Technology
Students also have preconceptions of technology. They see technology as limited primarily to computers and related to electronic devices. They do not see such simple artifacts as zippers or forks as technological innovations that were groundbreaking in their time. Nor, do they see the built world as filled with engineering innovations that have served the needs of society.
Addressing the Needs of a Global Society
Among female students in particular, the strongest preconception is that engineering does not meet the needs of society and as a consequence students do not choose engineering careers. This naïve conception is strongly linked to the lack of knowledge about what engineers do and the range of engineering careers available to them. Furthermore, since conceptions of engineering are limited to building, fixing, and repairing things, as well as designing and planning, students’ views of engineering and its reach is local rather than global. Female students are also more likely than males to describe the products of engineering as having just as many negative impacts on society, such as bombs, as positive impacts.
Investigate and Analyze the Impact of Engineering on a Global Society
Most people in the United States do not recognize the role of engineers in developing new forms of energy or drugs or even working in space. These activities are seen as the work of scientists. Furthermore, they do not understand that engineers work with scientists to create new technologies. In a survey of the International Technology Education Association, only 36% of respondents chose “changing the natural world to satisfy our needs” when asked to select what comes to mind when they hear the word technology.
When students look at large-scale problems such as those relating to the environment, they tend to focus their analysis on the scientific aspects of such problems and ignore the ethical, economic, legal, and social components. A narrow focus in analyzing problems that impact a global society, attributing the work of engineers to scientists and misunderstanding the role of technology must first be addressed before students can investigate and analyze the impact of engineering on a global society.
Water
When environmental problems such as water pollution are presented to students they make distinctions between local and global problems rather than seeing them as one and the same. Local problems are solved from an anthropocentric perspective based on personal views in contrast to global problems that are solved using scientific knowledge. Ethical, legal, and economic factors are rarely seen as important in solving environmental problems.
Furthermore, students do not understand that water is part of a complex dynamic sys
tem within which water is conserved. They do not understand the physical and chemical process of the water cycle. They do understand that the water system has an atmospheric component but not that it has a groundwater component. Nor, do they understand the water table. In addition, ground water is believed to be captured in impervious rock or to be contained in large static underground lakes.
Only % of students believe that industries play a role in water pollution by dumping industrial waste into streams. Students who live near polluted rivers had a limited sense of the extent of water pollution despite their proximity to polluted rivers. These students believed that rivers in rural areas were not polluted. And, because students do not see the connection between groundwater and atmospheric water they do not make connections between the hydrosphere and other components of various earth systems which limits their ability to once again think globally.
Bibliography and References