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Alice Han
Writing 340
Professor Townsend
December 9, 2013
Abstract
With the diminishing resource of freshwater, our world is looking for alternative sources
that can provide enough water for our growing populations. One of the largest sources is treated
wastewater, which has been widely implemented throughout the nation for industrial and
agricultural purposes. This article provides an overview of how wastewater is treated to reusable
water and the significance of knowing the wastewater treatment process.
About the Author
Alice Han studies Civil Engineering at the University Of Southern California Viterbi
School Of Engineering. Her studies focus on water resources, and she has work experience in
the wastewater treatment field.
Contact Information
Alice Han
alicejha@usc.edu
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Waterland: Water’s Happiest Place on Earth
Would you believe me if I told you that the water you flushed down the toilet this
morning had been turned into clean, reusable water within a span of eight to ten hours? Would
you drink the cup of water I gave you if I told you that it was your morning waste a few hours
ago? I don’t think anyone needs any time to let the question settle in their minds—it’s an
automatic no! That’s gross! But the truth is— that water is completely safe for human contact,
and it meets the federal drinking water standards. The wastewater that we flush and drain daily
is treated in Water Reclamation Plants, and the treated water is recycled and reused within our
very own communities. However, more than half of the produced recycled water is discharged
into the ocean, becoming useless. With the current state of worldwide water shortages, recycled
water is an excellent alternative source to reduce the impact we have on decreasing freshwater.
But the public’s automatic negative response to the mere idea of treated sewage makes it difficult
to take full advantage of this water source. However, with knowledge of the travel route of
wastewater from dirty to clean, the public’s mindset can be changed. Follow me through this
travel route, and we can better understand how wastewater is treated to safe standards and how it
can be beneficially reused.
The wastewater treatment system is a human-designed system modeled after the natural
treatment process in rivers. The main sources of freshwater to cities are rivers and groundwater
[1], in which both become available for safe human consumption through this natural river-flow
process. By modeling the Water Reclamation Plants after rivers, engineers have been able to
transform sewage wastewater into a safe and reusable water source. There are three main stages
of the treatment process—primary treatment, secondary treatment, and tertiary treatment (also
known as Advanced Treatment), and all stages have been designed based on natural processes.
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The major component of the primary treatment stage is settling. Heavy solids settle to
the bottom of concrete tanks by gravitational forces, and lighter particles float to the top of the
water. To achieve maximum settling, the two sub-steps of rapid mixing and flocculation is
implemented into the system.
When raw sewage first enters the treatment concrete basin, coagulants and auxiliary
chemicals are added to the fluid.
This is the first sub-step of primary settling as a means of
preparing the water contents for the settling process. The most commonly used coagulant is
aluminum sulfate, and the most commonly used auxiliary chemical is a synthetic polymer. The
aluminum sulfate is useful in removing turbidity, including biochemical oxygen demand,
suspended solids, and total organic carbon [2], and the synthetic polymer destroys the
filamentous branches that naturally grow out of bacteria and disturb solid-liquid separation [3].
Ultimately, the additions of these chemicals assist the clumping of
waste materials that will undergo settling. After the raw influent
receives its dosage of chemicals, the water goes through rapid
mixing, where the purpose is to evenly distribute the chemical
contents throughout the water. The mixing is generally achieved
Figure 1: Vertical Shaft Impeller
Source: www.denizmuhendislik.com.tr
through mechanical mixing using vertical shaft impellers [4],
shown in Figure 1 to the left.
After rapid mixing, the water, now with dispersed elements of coagulants and chemicals,
enters the second sub-step in the flocculation tank. In flocculation, slow and gentle mixing takes
place in order to create large flocs that are heavy enough to finally settle [4].
Different from rapid mixing, in which the purpose is to disperse added chemicals in the
water, flocculation uses slow rotational mixing to create “clumps” of waste and organic particles.
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The chemicals that had been introduced in the rapid mixing are the agents that cling onto the
waste and form the clumped material in flocculation. The masses will become heavy enough to
settle to the bottom of the tank.
The heavier solids formed from flocculation will sink to the tank base and lighter density
particles will float to the surface. The settled wastes are scraped away from bottom of the
settling tank and the floating materials are skimmed off, and they are returned to the sewers for
further treatment at a different treatment plant. For example, the Los Angeles County Sanitation
Districts’ Water Reclamation Plants send their solids and sludge to their biggest treatment plant
called the Joint Water Pollution Control Plant (JWPCP). At this facility, the solids are chemical
and physical processed for land application (such as fertilization and landscaping), composting,
energy (from methane gas), or landfill disposal [5].
The components of rapid mixing, flocculation, and settling characterize the primary
treatment stage, which altogether represents the occurring preliminary treatment steps in a
natural river! When runoff first flows into the natural river, the heavy solids sink to the river
bottom as the material with density lighter than water rise to the surface and are carried
downstream. The concrete tanks in a Water Reclamation Plant simply replace the river [6],
shown in Figure 2 below. By the end of this stage, the water quality is still poor but is free of
suspended solids as a result of the described chemical and physical treatments.
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Figure 2: Primary Treatment Stage-- Rapid Mixing, Flocculation, and Settling
Unfortunately, there is still a concentration of dissolved organic materials (BODBiological Oxygen Demand) in the flow. Settling through gravitational forces is not enough to
remove the organic wastes and solids still suspended in the water. Therefore, in a natural river,
naturally-occurring microorganisms use naturally-provided oxygen to feed on the dissolved
organic material [6]. The secondary treatment stage is modeled after this, which contains
biological treatment and secondary settling.
Since oxygen cannot be naturally supplied to the water in a treatment plant as it is in a
river, engineers have designed aeration tanks for the secondary treatment process. Directly
following the river example, the raw wastewater already containing the organic matter (BOD) as
food supply enters the aeration basin, where naturally-existing microorganisms from wastes feed
on the organics and stimulate new growth [4]. Through the air bubbled into the wastewater,
microorganisms breathe and thrive to ultimately eat the suspended organic matter. As a result,
the water becomes filled with contents of BOD-heavy microorganisms and flows to the
secondary settling tank so that the masses can settle to the bottom and be scraped away from the
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system. Therefore, the water is now reduced in the organic matter content levels and the
microorganisms are removed and recirculated. They are recirculated so that they can be reused
to feed on organic material in future wastewater influents.
Figure 2 shows the secondary
treatment stage and the recirculation route of the microorganisms, which are also known as
“activated sludge.”
Figure 3: Secondary Treatment Stage
Source: water.me.vccs.edu
Sometimes, the amounts of naturally-existing microorganisms are not enough to digest all the
suspended organic matter in the flowing water. Therefore, the activated sludge is used as an
extra supply source.
The final stage is the tertiary treatment stage, which is filtration and chlorination.
Filtration is also the last step in the natural-river cleansing process. Much of our clean-water
supply is from groundwater sources, largely from percolation in rivers and lakes. Percolation is
a form of natural filtration. As the water infiltrates the layers of soil [7], remaining bacteria,
viruses, protozoa, and waste particles are “caught” between the pores of the soil. By the time the
water joins the main underground water supply, it has passed through multiple layers of soil and
is clean of the small harmful particles [6].
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Filters at Water Reclamation Plants are designed to imitate the structure of natural soil
compositions at the bottom of rivers and lakes. The most commonly used filter is the granularmedia gravity filter [4]. This filter is typically composed of a bottom layer of gravel, a middle
layer of sand, and a top layer of anthracite coal. The gravel layer ranges from 8 to16 inches and
both the sand and anthracite layers range from 24 to 30 inches, which are all contained within a
concrete tank about 9 feet deep [4]. Figure 4 below shows a cross section of the structure.
Through gravitational forces, the water flows downward through the filter and the remaining
unsettled flocs that were not treated by chemical coagulation and sedimentation are finally
removed [4]. Once the water passes through the three layers, it no longer contains harmful
bacteria and viruses, allowing it to become safe for human contact, groundwater recharge, and
other human-related uses [6].
Figure 4: Cross section of granular-media gravity filter
The filtration process is extremely important in finalizing the wastewater treatment
process. After the primary and secondary treatment stages, the water appears to be relatively
free of the dirty gunk. Nevertheless, the water still contains harmful particles that are unsafe for
contact or use. By the completion of filtration, 84 to 96 percent of turbidity, 97 to 99.95 percent
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of coliform bacteria, and 100 percent of Giardia Cysts are removed [8], in which the previous
stages had been unable to manage. This is important because fecal coliform bacteria and Giardia
Cysts are the top causes of waterborne diseases [12,13].
The final disinfection of the cleaned water in the tertiary treatment stage is most
commonly achieved through chlorination. Based on the amount of water being flowed through
the system, a calculated amount of chlorine gas is added to the water to remove pathogens.
Chlorination is used not only for disinfection of treated wastewater but also for disinfection of
drinking water! Once the water is chlorinated with the correct dosage, the water is now safe and
ready for storage in a clear well, distribution to communities, or discharge to the ocean.
It is important to note that water pushed through the filters must have been previously
treated with chemicals. If it has not been properly treated in the primary and secondary stages,
two major problems can occur—1) the influent may contain large unsettled flocs that clog the
filter, or 2) the influent may contain microscopic un-coagulated particles that pass through all
layers of the bed that is discharged in the effluent. This also requires proper design of the filter
media. The gravel and sands must be big enough to create large pores that can contain large
quantities of floc but also small enough to prevent suspended solids from passing through all the
soil layers. The depth needs to be properly designed for sufficient flow time through the filter
system [4]. In addition, to maintain the full efficiency of the filters, they are backwashed once
every 24 hours for about 5 to 10 minutes [4] to flush out excess turbidity and particles that are
stuck within the grains of the filter media.
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Let’s now take a look at the whole big picture:
Figure 5: Schematic Flow of a Water Reclamation Plant
The process shown above is a condensed overview of the engineered system that can reduce the
days-long natural process into a ten-hour human-operated process. This process, a pure imitation
of the natural treatment process that occurs in rivers, takes contaminated raw sewage and
produces an effluent that is free of turbidity and harmful bacteria.
All three stages of the wastewater treatment process are dependent on one another. Each
stage of the wastewater treatment process must successfully execute its own responsibility in
order to allow proper performance of the other stages. Figure 6 below reviews the main tasks of
each stage:
Figure 6: The results of each treatment stage
Source: Frost & Sullivan
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Today, the water produced from Water Reclamation Plants is used for irrigation,
industrial/commercial applications, construction, toilets, landscaping, and countless more [9].
Every application has its own level of water-quality requirements, but all are satisfied within the
boundaries and capabilities of human wastewater treatment facilities. In fact, according to the
Los Angeles County Sanitation Districts, the product water is safe to drink [6]. The treatment
process is extensive to the point that it satisfies state and federal drinking water standards.
However, more than half of reclaimed water produced by the Districts is discharged to the ocean
rather than being used for more city-implications. Much of the reason for this is because of the
“yuck factor”, the psychological issue of people being uncomfortable with using water that has a
history of being sewage [10]. People are unaware of how their water is treated and are therefore
reluctant to accept the idea that it’s actually harmless to use treated sewage [11], making it
difficult for the government and engineering society to aggressively push forward wider
implementations of reclaimed water. But by using the engineering secrets of treated wastewater
revealed to you, the general public can become more knowledgeable about this water source and
become open-minded of its implications in our communities.
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