The State of our Tap Water Writing 340 [email protected](626)315 -4496 December 11, 2013 Monique Warren Author biography: Monique Warren is a senior at USC, and will be receiving her Bachelor of Science degree in civil engineering with an environmental emphasis in May 2014. She has taken numerous water courses at USC, of which includes: Water Chemistry, Water Treatment Design, Wastewater treatment design, and Water Supply and Sewerage System Design . She hopes to one day work in water treatment for developing countries.
Author biography: Monique Warren is a senior at USC, and will be receiving her Bachelor of Science degree in civil engineering with an environmental emphasis in May 2014. She has taken numerous water courses at USC, of which includes: Water Chemistry, Water Treatment Design, Wastewater treatment design, and Water Supply and Sewerage System Design. She hopes to one day work in water treatment for developing countries.
Abstract
The average human body is made up of about 65% water. About 70% of the earth’s surface is covered with water. Water surrounds us every day, and we can’t go a day without using it somehow. Yet most people do not know the intricacy of the stringent treatment that their water goes through before it comes out of their tap. As a result, there is a stigma in our society that tap water is low quality and may even be harmful to drink. However, upon further examination one may find that the treatment process that produces our drinking water is not only thorough but trustworthy.
Recommended media
A form of media that could accompany this article would be a video tour of a water treatment plant. This video could highlight key points in the article, and give readers a closer look at what these treatment units look like in real life.
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The State of our Tap Water
IntroductionWhether you turn on the shower, brush your teeth, or even do the dishes, there’s no
getting around the use of tap water throughout your day. However, there seems to be a stigma
surrounding the quality of water that runs from the faucet. Afraid that the water on tap is
unhealthy, people seek alternatives to drinking water straight from the tap. The sale of bottled
water increased by 6.7 percent in 2012, and now totals $11.8 billion [1]. People tend to turn to
bottled water and home filters as opposed to drinking water straight from the tap. Is this fear of
tap water justified? Well, before you can really decide that, you must take a closer look at what’s
coming out of the faucet. If you do, you will find that due to extensive treatment the tap water
typically received in homes around the U.S. is of high quality.
Importance of drinking water treatmentThe water that comes out of your tap goes through long, strenuous treatment. This is due
to the fact that you cannot simply pump water from a river or other water source directly into the
houses of a community. Even though the water from these sources can be of high quality, this
water is still are not suitable for drinking until it is treated. The water that is sent to the treatment
plants to be cleaned can contain different pollutants that have to be removed before the water can
be used. The treatment process generally used is known as the conventional drinking water
treatment process. The process can change slightly depending on the water that the drinking
water treatment plant is treating. For instance, if the source of water for a treatment plant comes
from groundwater that contains an excessive amount of nitrate (an ion that can cause blue baby
syndrome in infants if consumed), the conventional treatment process will need to be adjusted to
treat the nitrate in the water. Thus, it is important to look at the source of the water.
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The source of the drinking water being treated will depend on the area that the treatment
plant is serving. For instance, a family located in Southern California may receive water
originally from the Colorado River (a surface water source), or from Northern California brought
to Southern California by way of the State Water Project (surface water), or from a local
groundwater source. It is important to consider where the water comes from, because the
treatment plant needs to know what to treat the water for before they distribute it to the public.
The two most basic categories of water sources would be groundwater and surface water.
Groundwater being water beneath the ground, held in rocks and porous soils, and surface water
being water on the surface of the earth like rivers, lakes, and streams.
Evaluating the drinking water sources
GroundwaterCompared to surface water, groundwater is less likely to be contaminated; however,
chemicals like pesticides or fuels, or calcium or magnesium are all contaminants that could
potentially pollute groundwater sources [2]. This is due to the fact that groundwater is made up
of rain water and surface water that has moved from the surface to ground. If polluted surface
water infiltrates into the groundwater, then the groundwater can become polluted. For example,
in Florida, the Floridian Aquifer is infiltrated by water from the Alapaha River. As seen in Figure
1, the flow from the Alapaha River flows directly into the Floridian aquifer, so that all of the
contaminants and pollution in this surface water also flows into the groundwater. Figure 2,
shows a diagram of groundwater from which we can also see the how the proximity of a
groundwater source to a surface water source can lead to the infiltration of the groundwater
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source. Groundwater can be contaminated, but it’s not exposed to as many contaminants as
surface water.
Figure 1: Alapaha River flowing into the Floridian Aquifer [11]
Figure 2: Surface water and groundwaterModified from: [11] and [12]
Surface watersOn the other hand surface waters can be polluted by a number of things, because they are
more exposed. Runoff containing fertilizer, untreated sewage, runoff from parking lots and other
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impervious ground, and unprotected watersheds can all be potential sources of contamination [2].
These sources of contamination are of concern, because they pose the threat of the water
containing pathogenic bacteria like Giardia or Cryptosporidium, which are both harmful to
humans when consumed. In Figure 2, the Colorado River is shown. From this picture we can see
how this river is exposed to many different elements that could provide sources of
contamination; from animal waste to runoff containing pollutants from nearby areas.
Contaminants in water sourcesGiardia and Cryptosporidium are both contaminants that can be a result of fecal
contamination (animal or human). Since, surface waters are completely exposed to the elements
and animals it makes sense that there is a risk of fecal contamination. If water containing Giardia
were to be consumed, small intestinal problems like diarrhea could be a result. Comparably,
water-containing Cryptosporidium poses the threat of fever, diarrhea, nausea, and cramping upon
consumption [3]. To ensure that the water the public receives is of good quality, water treatment
engineers must take it through a long, extensive process of treatment. Surface waters aren’t the
only source of pollution for groundwater. Groundwater can also be polluted by seepage from
landfills, uncontrolled hazardous waste, septic systems, and gas or oil storage tanks to name a
few sources [14]. Each source of groundwater and surface water will be different depending on
the area, and what is nearby. A treatment plant will test the source of water they intend to pull
from to determine its quality and what contaminants need to be treated.
Treating the contaminants – The drinking water treatment processThe treatment process for drinking water contains several steps, by which different
contaminant types are removed. The water first enters the plant and must go through a
pretreatment process where it is screened. After the pretreatment, the water can then go through
the treatment process. The water goes through coagulation, flocculation, sedimentation,
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filtration, and disinfection before it is stored and then distributed to the community served.
Figure 3 shows the flow of the conventional treatment process. The steps in the process are
numbered in the figure based on the order they are completed, and are placed under the tank or
unit where they occur. As seen in the figure, the first step in this process is the screening of the
water.
Figure 3: Flow chart of a conventional water treatment plant
Screening
Generally, the water received by the plant is first put through pretreatment where a
simple screening is done so that larger materials, like sticks and other debris, don’t enter the
plant with the water. These larger objects cannot enter the treatment plant, as they would damage
treatment equipment. The screening process is comparable to straining pasta, in that the sizes of
the holes in the strainer are small enough to allow the hot water to pass through and keep the
cooked pasta in the strainer. After the water has been screened, the flow then moves to the rapid
mix tank for coagulation.
Coagulation and flocculation
In Figure 2 flocculation is grouped with coagulation in step 2. The processes of
coagulation and flocculation are typically referred to as one because they go hand in hand. In
order to have flocculation, you need coagulation; and they also both occur in the rapid mix tank.
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In the rapid mix tank (as shown in Figure 3), a chemical called a coagulant is added to the water
and then mixed rapidly, for thirty seconds or more depending on the plant size, and then slowly
mixed, for twenty minutes or more again depending on the size of the treatment plant. This
process is called coagulation. Coagulation is performed in drinking water treatment, to settle out
particles that won’t settle out on their own by gravity. The majority of these particles are
negatively charged so that they repel each other instead of coming together to make a clump.
Mixing the coagulant with the water changes the charge on some of the particles so that they no
longer repel each other [4]. Once the charge on some of the particles is changed, they can clump
together and form what are called flocs, which are able to settle out by gravity; this process is
known as flocculation.
If the water coming out of your tap were to be cloudy or murky, it would not only be
aesthetically displeasing but alarming. To us, if something looks funny (especially our water) we
are not going to want to consume it. This is what makes coagulation and flocculation so
important. These processes settle out the particles in the water that are responsible for making it
appear cloudy. The flow of the water then moves to the sedimentation tank where these flocs
can settle out of the water.
Figure 3: Rapid mix tank
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Sedimentation
The flocs that were formed by the processes of coagulation and flocculation now need to
be removed from the water, and the simplest way to do that is to let them settle. Imagine
dropping a golf ball into a pool, what does it do? Due to gravity, the golf ball will sink to the
bottom of the pool. The same concept applies here. Before coagulation and flocculation the
particles in the flow were too light to settle out of the water, but after they clumped together to
form flocs they became heavy enough to be removed by sedimentation.
In the sedimentation basins, the water is allowed to sit so that the flocs can sink to bottom
of the tank and be removed by scrapers. Figure 5 shows a typical drawing of a sedimentation
basin, that illustrates how the flocs settle to the bottom of the tank. Once these clumps gather
together at the bottom of the sedimentation basin, they are then referred to as sludge. The sludge
is scraped from the bottom of the tank and sent to be disposed of with other solid waste. Figure 6
shows the sedimentation basins at an actual water treatment plant. Though you cannot see the
cross section of the basin, the picture does show how great masses of water are allowed to sit
while the flocs settle. The cleaned effluent then leaves the sedimentation tank and moves on to
filtration.
Figure 5: Sedimentation basin design
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Figure 6: Sedimentation basins at Bollman Water Treatment Plant [9]
Filtration
The process of filtration is responsible for removing non-dissolved solids from the flow
of water. During filtration, the filter removes smaller particles that cause the water to be cloudy,
biological contaminants like Cryptosporidium and Giardia, and inorganic chemicals like mercury
and arsenic [2]. Any flocs that weren’t settled out by sedimentation will be removed here. As
seen in Figure 7, the flow from the sedimentation tank comes in to the top of the filter and then
flows through the layers of media in the filter. This part of the treatment process can be
accomplished with by a few different kinds of filters, like a slow sand filter or granulated
activated carbon filter (GAC). It all depends on what media is selected.
The point of the filtration process is for the particles in the water to become trapped in the
media of the filter, thus the selection of the media is important to filtration. There are a number
of key elements to the selection of filter media like the durability of the filter media, length of the
filter run, and ease of filter wash to remove suspended matter from the media [5]. It takes time
for the water to filter and go through the different layers in the filter. The filter fun time will
depend on how long it takes for a certain amount of water flow through the filter. The filters also
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have to be cleaned, which is done by using some of the filtered water to backwash the filter. To
back wash the filter, water is pumped up through the filter.
Sand and gravel are typically the media used in the conventional filtration process.
Activated carbon is another helpful media to use in filtration. Previously, activated carbon was
used in filtration as merely control for the taste and odor of water; however, as technology has
evolved pollutants like Disinfectant By-products (DBP), Synthetic Organic Compounds (SOC),
and Endocrine Disrupting Compounds have been discovered. The presence of these compounds
in water can be dangerous, so they must be regulated and treated for and activated carbon is one
means of doing this [6]. Activated carbon is very porous, which means it is good at adsorbing
particles and this is what allows it to be so beneficial in the filtration process. In Figure 7, once
the water flows through the layers of media the water then collects at the bottom of the filter and
then flows out to disinfection.
Figure 7: Filtration tank [10]
Disinfection
The most common process used for the disinfection of water is chlorination, the addition
of chlorine to the water to kill any microorganisms, pathogens, or other harmful pollutants in the
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water. Chlorine is effective against a variety of pathogens, so it is an ideal choice for treatment
[7].When the water reaches the public obviously it is important that it is still clean, which is why
some residual disinfectant must remain in the water. Before the water can come out of your tap,
it has to travel through many pipes. These pipes can be dirty, have buildup, or other harmful
pollutants that you don’t want to drink. This is why there must be residual disinfectant in the
treated water, as it treats any bacteria that the water may come across in the pipes during the
distribution process. The extra chlorine in the treated water that leaves the plant can kill any
bacteria it comes across, before it reaches your faucet. All the proper precautions must be taken
to ensure that the water is of the highest quality possible when it gets to the consumer.
Making sure the water is clean – regulating treated waterThe Environmental Protection Agency (EPA) has set in place regulations to ensure that
the public can trust the water they receive. The EPA gives regulations for the contamination
levels of water produced by treatment plants. Since a wide range of contaminants can potentially
contaminate our drinking water the EPA sets what is known as MCLs, maximum contaminant
levels. An MCL is the highest level of a contaminant that is allowed in drinking water and MCLs
are enforceable by law. MCLGs, maximum contaminant level goals, are also set. MCLGs give
the level of contamination in water that the EPA wants treatment plants to work toward, below
which there is no anticipated risk to health [8]. The water treated by a plant is bound to have a
small amount of contaminant left in it, but with these sorts of regulations in place the
contaminant level in the water is not at a level that should pose a threat to human health.
Trustworthy tap waterIf you still question the water that comes out of the tap, you can purchase a water testing
kit and check it yourself. If you find that there is something wrong with your water, it can be
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reported to your water distributer so that the problem is fixed. The conventional drinking water
treatment process is widely used by treatment facilities and has many different components. The
water that is distributed to households has high quality, because it is well treated, monitored, and
regulated. The regulations set in place, MCLs, are enforceable by law. So, if a treatment plant
were to send water out with contaminant levels higher than the set MCL they would be breaking
the law. Thus, treatment plants test the water they send out to make sure that it meets the
regulations set forth by the EPA. All of the effort that goes into treating the water (whether it’s
from the ground or surface) is well designed, planned, monitored, and enforced. So, the water
that comes out of the tap is water worth trusting.
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References
[1] International Bottled Water Association. U.S. Consumption of bottled water shows continued growth, increasing 6.2 percent in 2012; sales up to 6.7 percent. IBWA.http://www.bottledwater.org/us-consumption-bottled-water-shows-continued-growth-increasing-62-percent-2012-sales-67-percent
[2] Scott G. Curry. (2003). “Drinking Water Treatment.” Water: Science and Issues. [Online] (1) pp. 257-260. Available: http://go.galegroup.com/ps/i.do?action=interpret&id=GALE%7CCX3409400087&v2.1&u=usocal_main&it=r&p=GVRL&sw=w&authCount=1
[3] Chittaranjan Ray and Ravi Jain. Drinking Water Treatment: Focusing on Appropriate Technology and Sustainablility. Dordrecht: Springer, 2011. Print
[4] “Our Water Treatment Process.” Our Treatment Process. San Diego Public Utilities, http://www.sandiego.gov/water/quality/watersources/treatmentprocess/index.shtml
[5] Susan K. Burns, V. Dean Adams, and Steel B. Maloney, “Direct Filtration versus Conventional Water Treatment on the Intermountain Region,” Utah Water Research Laboratory (1984). Reports. Paper 580. http://digitalcommons.usu.edu/water_rep/580
[6] Ozgur Aktas and Ferhan Cecen. (2007, Sep.) Competitive adsorption and desorption of a bisolute mixture: effect of activated carbon type. Springer Science. [Online]. (13) 159-169. Available: http://download.springer.com/static/pdf/15/art%253A10.1007%252Fs10450-007-9017-5.pdf?auth66=1383998596_9877b5f34793fd3b9103ad8a170373a8&ext=.pdf
[7] Daniel Gerrity and Shane Snyder. “Wastewater and Drinking Water Treatment Technologies” in Human Pharmaceuticals in the Environment: Current and Future Perspectives Emerging Topics in Ecotoxicology 4. Springer
[8] “National Primary Drinking Water Regulations.” US EPA. EPA. Web.
[9] Contra Costa Water District. CCWD Photos & Maps of CCWD Facilities & People [Online].Available: http://www.ccwater.com/photopages/index.asp
[10] Michigan Environmental Education Curriculum. Drinking Water Treatment. [Online]Available: http://techalive.mtu.edu/meec/module03/Sources-Groundwater.htm
[11] USGS Surface Water Information. USGS Surface Water Photo Gallery. [Online].
