By: Rosa Lau
Phone: (626) 456-0781
Email: rosalau@usc.edu
12/5/2011
Biography: Rosa Lau is currently a 2 nd
year at the University of Southern California, majoring in Civil Engineering with an emphasis on the environment. After graduation, she hopes to be able to complete her master’s degree at USC.
Key Words: Wastewater Treatment, civil engineering, environment, water, recycle
Abstract: People talk about recycling different items such as bottles, cans, magazines, newspapers, phonebooks, plastics, and even electronics. However, the one recyclable that is hardly talked about in everyday life is wastewater. Recycling wastewater is a process that happens at wastewater treatment plans. The treated wastewater can be further purified by allowing it to perforate through the soil which allows it to combine with natural sources of water. This process, groundwater recharge, is one that is essential and has many benefits for cities that do not have a constant or reliable drinking water source. Recycled water is one solution that has been proven effective and beneficial and should be further explored by people like me and you alike.

By: Rosa Lau
Introduction:
What goes in must come out. Whether you like to talk about it or not, it is a fact of nature.
Everyone needs to go the restroom. This may be a strange topic to talk about, but did you ever wonder what happens to what we flush down the toilet? Where does this waste go? What happens to those toy figures that children flush down the toilet? What happens to the soup that we pour down the drain or trash we dump down the garbage disposal? Did you ever wonder what happens to all of this? All of this waste leads to one place, the sewer. The sanitary sewer is fascinating piece of infrastructure that consists of a system of pipes used to transport human waste to waste-water treatment plants. What if I told you that we actually re-use the contents that flow in the sewers? We have all heard about recycling bottles, cans, magazines, newspapers, plastics, electronics and even rechargeable batteries, but have you ever heard about recycled water? Recycled or reclaimed water is wastewater that has been treated extensively and as a result, made reusable for a variety of applications.
Why is recycling water necessary?
As much as 75% of Earth’s surface is covered by water. However, only 2.5% of it is freshwater and less than “1% of the fresh water is accessible for direct human uses” [1]. This statistics tells us one thing: water is precious. We use water every day for a variety of applications. We need it to hydrate ourselves and our pets. We need it to be able to clean the food we prepare as well as use it to make food. We need water to water the trees, plants, and grass. Water is the key to life and is one of the most important resources that make life on earth possible. It is what
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differentiates us between other planets in our universe and is the key to our existence. So the most important question is: how can we protect this scarce resource? This is where recycling water comes into play. In many developed countries, there is a reliance on the wastewater treatment plants. At these plants, wastewater coming from homes and commercial buildings are treated through a wastewater treatment process. This process mimics the water cycle, but does so in a shorter amount of time. Treating wastewater results in a product that can be re-used for landscape irrigation and other commercial purposes. This treated or reclaimed water can and is already being used as an indirect drinking source.
How does a wastewater treatment plant work?
Figure 1: Conventional Wastewater Treatment Plant [2]
The wastewater treatment system generally consists of three types of treatment processes. These systems always include primary, secondary, and tertiary treatment. In primary treatment, the raw sewage from the sewers travels to the treatment plant and all solids are removed from the water.
As shown in Figure 1, during primary treatment, the sewage is first passed through a bar screen
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which catches all of the large objects such as solids, rags, and even toy action figures. It is then passed through the next tank, the grit chamber, which removes sand particles, egg shells, coffee grounds, etc. After this chamber, it is passed through the primary clarifier. In this tank, the particles settle by gravity which forms something called sludge. The sludge is transported to a
“sludge digester” where microbes work to break down the sludge. The water left over from the primary clarifier is then sent to the aeration tank where settled sewage and microorganisms are mixed together. Oxygen is provided in this tank for the microorganisms by pumping air through perforated pipes. The microorganisms (bacteria) are able to break down the settled wastewater and use it as a food source [3].
The next part of the treatment system is secondary treatment where the particles that cannot be seen or are too light to settle are removed. The wastewater then goes into the secondary clarifier where microorganisms settle by gravity and forms more sludge. Half of this sludge is sent to the same “sludge digester” mentioned before and half of it is sent back to the aeration tank. The purpose of this is to make sure that the bacteria, which is acclimated to breaking down the waste, remains in the system to breakdown more of the waste in a continuous cycle.
Following secondary treatment is tertiary treatment. There are several types of tertiary treatment and how advanced the system is depends on what the water is going to be used for. The first type of tertiary treatment is simply chlorination and de-chlorination. After chlorination, the reclaimed water is already ready to be discharged to the oceans or lakes. Chlorination is often the last step of treatment for traditional wastewater treatment plants. The second type of tertiary treatment is a process in which water is passed through a filter to remove unsettled particles.
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Many plants use sand filters which may or may not consists of anthracite coal (carbon), sand and gravel. These dual media filters are also found in drinking water filtration systems which illustrates that the treated water is already at drinkable quality. Reverse osmosis is another type of advanced treatment which is currently the most advanced technology. “Reverse osmosis uses pressure to force water through a semi-permeable membrane that filters out virtually all impurities, even tiny viruses. Reverse osmosis enable[s] [us] to produce high-quality, potable water at a lower cost than could be obtained from any other source” [4]. Groundwater
Replenishment System (GWRS) of Orange County also add hydrogen peroxide and use ultraviolet (UV) light to oxidize and degrade remaining contaminants. The water can then be piped to settlement ponds where it can slowly percolate into the ground and aquifer. An aquifer is a natural underground layer of water-bearing porous rock which transports groundwater to springs and wells. Allowing the recycled water to flow into the aquifer is a process known as groundwater recharge or indirect potable reuse. During groundwater recharge as shown in
Figure 2, the water blends into river water and rain water that is already present in the aquifer.
Allowing percolation is another form of treatment because as the water travels through the soil and rocks, the water is naturally filtered. The soil also adds back minerals which have been filtered out during the treatment process which returns the natural taste to the water [4].
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Figure 2: Process of Groundwater Recharge, Water Replenishment District of Southern California [5].
Is the water drinkable?
When the recycled water leaves the wastewater treatment plant, the water must meet drinking water standards. This ensures that the water is safe and can be used in the process of groundwater recharge. The water is also well monitored and managed. Despite the high quality of the water, the reclaimed water is still not being used directly as drinking water. “It is blended with other water and also must be retained in the ground for additional purification and as a safeguard before the blend becomes a water supply source” [5]. The purified water is as good as water from natural sources and even has the natural taste after it is allowed to perforate through the soil. The blending of water with natural sources is implemented in many systems because drinking our own treated wastewater is an unacceptable or inconceivable concept for many. As a result, there have been many project plans that have been shot down by critics. “The public still
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has a strong psychological block against drinking it straight from the plant. But if that same water is delivered indirectly by injecting it into the ground and then drawing it from wells, public acceptance could be forthcoming” [4]. Fortunately, because of the last part of the process, people are more open to the idea. Some cities also currently use recycled water for “golf course irrigation, industrial process water, habitat restoration, [and] groundwater recharge” [6].
Where is recycled water being used as a drinking source?
Unknown to many, there are actually many places in the United States that utilize the process of groundwater recharge and use the treated wastewater in the aquifers as a drinking source. Areas in the U.S. that uses treated wastewater for ground water recharge include: El Paso, Texas;
Tucson, Mesa, and Phoenix, Arizona; Ventura County, California, etc. [6]. The cities that recharge their aquifers using this method are located mostly in the Southeast, Southwest and
Western states of the U.S. This is primarily due to the lack of a natural reliable source of water.
In 2005, “five Arizona towns are developing a plan to conduct groundwater recharge on a large scale. Known as the Agua Fria Linear Recharge Project, the plan calls for recharging depleted aquifers with up to 60000 acre-ft of wastewater per year” [7] which can serve up to 360,000 people.
The county that first used this technique and technology is Orange County, California, home to three million people and Disneyland. Their GWR System is the “largest planned indirect potable reuse (IPR) project in the world… The GWRS produces 70 million gallons (265,000 cubic meters) per day of high-quality water that exceeds all state and federal drinking water standards for nearly 600,000 residents in north and central Orange County” [5]. This system was put into
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use starting January 2008 and has already produced 74,344,000 gallons of high-quality water to date. Before GWRS, the Orange County Water District developed Water Factory 21 (WF-21) in
1976 which was aimed to prevent seawater intrusion into their aquifer. “WF-21 was a first-of-its kind, water recycling facility that helped pave the way for many international projects” and
GWRS [5].
Benefits of Groundwater Recharge
Based on the statistics mentioned before, only 1% of freshwater on earth is available for 6.9 billion people living today. As time passes, this percent gradually decreases because of droughts, pollution and the increased use of water due to population growth. Especially in Southern
California, where most of our water is imported, we need a way to ensure that we have a reliable source of water for our survival. As our population increases, our demand for water increases as well. Replenishing and making sure that our aquifers are filled with enough water is extremely important, especially during droughts or natural disasters such as earthquakes. Groundwater recharge also saves cities money because it is more expensive to pipe the treated wastewater hundreds of miles in order for it to be released at an outfall in the ocean or a lake. It is necessary to find solutions to preserve the scarce freshwater that is available to us. Recycled water is one solution that has been proven effective and beneficial and should be further explored by people like me and you alike.
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References:
[1] University of Michigan. “Human Appropriation of the World’s Fresh Water Supply.”
[Online]. Available: http://www.globalchange.umich.edu/globalchange2/current/lectures/freshwater_supply/fresh water.html
[Accessed: Oct. 5, 2011].
[2]
Professor Mike Pirbazari. Class Lecture, Water and Wastewater Engineering: “CE 210,
Wastewater Engineering.” University of Southern California, Los Angeles, CA, date.
[3]
“Recycled Water.” [Online]. Available: http://www.westbasin.org/water-reliability-
2020/recycled-water/about-recycled-water [Accessed: Oct. 5, 2011].
[4] P.S. Wenz, "Spigot to Spigot," Planning , vol. 74, no. 8, pp. 6–12, Aug/sep 2008. [Online].
Available: MasterFILE Premier. [Accessed: Oct. 18, 2011].
[5] “Groundwater Recharge with Recycled Water Frequently Asked Questions.” [Online].
Available: http://www.wrd.org/water_quality/recycled-water-groundwater-recharge.php
[Accessed: Oct. 5, 2011].
[6] “Engineering Reports: Using Recycled Water for Groundwater Recharge in the Central and
West Coast Basins - A Successful History,” WRD Technical Bulletin, vol 16, Summer 2008.
[Online] Available: http://www.wrd.org/engineering/recycled-water-groundwaterrecharge.php
[Accessed: Oct 18, 2011].
[7] Landers, Jay, "Arizona Cities Move Forward with Groundwater Recharge Plan," Civil
Engineering , vol. 74, no. 2, pp. 20–21, Feb 2004. [Online]. Available: MasterFILE Premier.
[Accessed: Oct. 18, 2011].
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