Lesson #22: Sewage Treatment
Introduction
We often say in environmental science that there is no such thing as "away." Garbage we "throw
away" has to go somewhere—it doesn't just disappear because the garbage truck takes it away from
our view. Chemicals we spill on the ground disappear beneath the soil but end up in the groundwater.
And the stuff we flush down the toilet, well, where does that end up?
That's the question you will answer in this lesson.
What You Should Know
Before you begin, you should:
 Understand the basics of Earth’s water pollution
Lesson Expectations
By the end of this lesson, you should be able to:




Describe methods for treating sewage and wastewater
Differentiate primary, secondary, and tertiary treatment
Explain the environmental advantages of secondary and tertiary treatment
Explain the main steps of the wastewater treatment process
Lesson Keywords
Primary Treatment
Secondary Treatment
Septic Tank
Sewage
Tertiary Treatment
Sewage and Wastewater
Domestic wastewater, or [Keyword: sewage], is the waste water that comes from human
communities. Its sources include toilets, bathrooms, laundries, and kitchen waste. So it includes the
water that goes down the drain after your shower, the water that flushes down the toilet and the stuff
that goes with it, and the water that is used to wash your dishes and your clothes.
You can also speak of agricultural wastewater, which is generated from farming activities, and
industrial wastewater, which is generated in a commercial or industrial process.
All of these forms of wastewater contain high levels of contaminants. So wastewater has to be treated
somehow to make it suitable for discharge or reuse back in the environment. It may be treated close
to its source or transported via pipes and pumps to a treatment plant.
How Do We Treat Sewage?
In rural areas where land space is adequate, sewage from each house can be discharged into a
[Keyword: septic tank]. In this treatment option, domestic wastewater is pumped into an
underground settling tank. Solids fall to the bottom of the tank and are decomposed by bacteria. The
water is then partially decontaminated and then seeps through holes in the tank downward through
the soil. The soil then filters out more pollutants, and soil bacteria continue the process of
biodegrading the pollutants.
You can view an online tutorial of how a septic tank system works here. How would you like a job as
the person who cleans out septic tanks? This video will show you what a day at work is like when it's
your job to clean out septic tanks. Click to view the video clip “Dirty Jobs-Septic Tank Technician."
In urban area, septic systems are not as frequent. Rather a network of pipes are installed
underground which lead to a wastewater or sewage treatment plant. You will look next at how those
work.
Wastewater Treatment Plants
Wastewater treatment plants treat sewage in three main steps. The first of these is called [Keyword:
primary treatment]. In primary treatment, the water that comes into the plant, called influent, is first
screened to remove large objects like rags, sticks, and garbage (like pieces of food that go down a
kitchen garbage disposal).
Primary treatment is a physical process. After large materials are screened out, the water flows into a
primary settling tank called a clarifier where solid waste (mostly human fecal material)
settles to the bottom. This settled out material is called sludge. Materials like oil and grease
float on the top and are called scum. After the settling out of sludge, the next step is secondary
treatment.
You can take a virtual tour of a wastewater treatment plant. Click on all of the processes to see
how water is treated and cleaned. The primary treatment is found by clicking numbers 1 and 2.
Secondary Treatment
Whereas primary treatment is a physical process, secondary treatment is a biological process. The
goal of secondary treatment is to reduce the biochemical oxygen demand of the wastewater.
Biochemical oxygen demand (BOD) is a measure of the amount of oxygen consumed in the water.
This measure is made higher by the presence of oxygen-demanding wastes (organic substances
like human waste or plant material) because these materials require microbial action in their
decomposition.
As the microbes eat the stuff, they deplete oxygen from the system. We don't want this process to
take place in a river or ocean, because the resulting oxygen depletion would kill the fish. So instead,
we make sure it takes place at the wastewater treatment plant before the wastewater is returned to
the environment (a local river or the ocean). Secondary treatment is the process used to accomplish a
large percentage of this task. In fact, after secondary treatment more than 90% of the oxygendemanding wastes have been removed.
Here are the main steps of the secondary treatment process.
1. The water flows into large tanks that have been seeded with bacteria. They are called aeration
tanks because they have air pumped through them to provide oxygen the bacteria need to live and
grow.
2. In aeration tanks, the water is constantly churned. The bacteria thrive in this environment, multiply,
and eat the waste materials in the water.
3. After spending time in the aeration tanks, the mixture of bacteria and water flows into a secondary
settling tank. This is also called a clarifier or settler. The still water in the settling tank makes the
bacteria and any remaining solids settle out to the bottom.
The cleaned water then flows out of the secondary treatment plant and can then be discharged back
into the environment. It now has a much lower BOD than it did when it first came in. The
microbes have done their job, and the oxygen-demanding wastes have been largely
removed from the water.
Most often the secondary treated water is piped to a body of surface water like a river or bay so it can
be returned to the environment. Water leaving a treatment process is called effluent.
You can now go back to our virtual tour and see how secondary treatment works. Click on numbers
3 and 4 to learn more.
Tertiary Treatment
Primary and secondary treatments combined can remove almost all of the solids and oxygendemanding wastes, most of the phosphates, and about half of the nitrates.
Why might it be desirable to carry out advanced treatment of the wastewater at this point and remove
even more nitrogen and phosphorus?
a.
b.
c.
d.
To reduce the risk of pathogens
To get more oxygen in the water
To settle out grit
To prevent algae growth in the receiving water
Answer: D. nitrogen and phosphorus are plant nutrients, and they are good in the soil
where plants grow, but in the water, they lead to harmful growth of algae, which can then
disrupt aquatic food chains and kill fish.
When it is necessary to remove more of these contaminants, tertiary treatment must be used. Tertiary
treatment can be implemented right after water flows out of the secondary settling tank and before
discharge back into the environment. One of the big advantages of tertiary treatment is that it further
removes the plant nutrient contaminants, namely nitrogen and phosphorus.
The removal of nitrogen is achieved by using bacteria to convert nitrogen compounds in the
wastewater (mostly ammonia) to nitrogen oxides and then to nitrogen gas, which can then
be released to the atmosphere. Phosphorus can also be removed by bacterial action.
Visit the virtual tour again to see how nitrogen is handled during tertiary treatment. Click on
numbers 5 through 7 on the interactive image.
Disinfection
Before discharge of primary, secondary, or tertiary treated water, one more step is usually added.
This step is disinfection to kill disease-carrying bacteria and some viruses. This is usually accomplished
by chlorination, or the addition of the element chlorine.
Disinfection can also be accomplished by UV light or by passing ozone through the water. UV light
alters the genetic makeup of the bacteria so that they cannot reproduce. Ozone is very reactive and
destroys most organic material that it comes in contact with.
You can now view the disinfection stage of the virtual tour. Click on number 8 and 9 on the
interactive image to learn more.
What To Do with the Sludge
Remember that first step of primary treatment—screening out of large materials? Those materials are
usually collected and either sent to a landfill or burned. And what about the sludge material, what
happens to it? It has to be discarded in a suitable way. The sludge contains toxins and pathogens that
have to be treated. This may be done by exposing the sludge to bacteria and letting them decompose
it or by composting the sludge. Composting involves mixing the sludge with a carbon source such as
sawdust or straw and then letting bacteria eat away at the mixture. What's left can then be safely
used as a fertilizer or burned as a fuel even.
ACTIVITY
Use this fill in the blank activity to review the ideas presented in this lesson. Capitalize the first letter
of your answer and be sure to click submit after entering each answer.
Quiz Statistics
Question:
Whereas primary treatment is a physical process, secondary treatment is a _____ process
Your Answer: v
Correct Answer: Biological
Explanation:
Question:
The goal of this treatment process is to use bacteria to decrease BOD
Your Answer: v
Correct Answer: Secondary
Explanation:
Question: This treatment process is used if it is necessary to remove plant nutrients from the
wastewater
Your Answer: v
Correct Answer: Tertiary
Explanation:
Question:
This is the most common method of_______disinfection
Your Answer: v
Correct Answer: Chlorination
Explanation:
Question:
This process kills pathogens in the wastewater
Your Answer: v
Correct Answer: Disinfection
Explanation:
Question:
This process kills pathogens in the wastewater
Your Answer: v
Correct Answer: Effluent
Explanation:
Question:
A plant nutrient removed by tertiary treatment.
Your Answer: v
Correct Answer: Nitrogen
Explanation:
Question: The solid waste material that settles to the bottom of a primary settling tank.
Your Answer: v
Correct Answer: Sludge
Explanation:
Question:
This process pumps oxygen into a tank so microbes can decompose waste material.
Your Answer: v
Correct Answer: Aeration
Explanation:
Question:
A tank that allows solid materials to settle out of the liquid
Your Answer: v
Correct Answer: Clarifier
Explanation:
Question: The first step of primary treatment, wherein large objects are removed.
Your Answer: v
Correct Answer: Screening
Explanation:
Field Trip Wastewater Treatment Plant
You can often schedule field trips and tours to local wastewater treatment plants. These tours are a lot
more interesting than you might think. If feasible, call your local plant and arrange for a tour. Find out
how your local treatment plant handles solids, where the effluent water is discharged, and how they
test the effluent water to make sure it is clean.
This video also gives you a glimpse of a wastewater treatment plant.
SUMMARY
In this lesson, you have covered:
 Sewage is the waste water that comes from human communities. It contains lots of contaminants
and must be treated before return to the environment
 In rural areas, homes can have septic systems for underground treatment of wastes. In urban
areas, wastes are piped to a treatment plant
 Waste water treatment plants use primary, secondary, and sometimes tertiary treatment on the
wastes
ASSIGNMENT
Complete your lesson assignment.