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How
Modern Wastewater Treatment Changed our World
Without wastewater treatment,
diseases and infections would ravage our society
By Trevor
English
Wastewater treatment is often an overlooked
necessity of civilization.
Without proper sewer systems, wastewater
treatment plants, and overall regulation, our cities would be ripe with disease
and human waste everywhere.
Believe it or not, much of the modern
wastewater management technology we consider standard in any 21st century home,
things like toilets and sewer pipes, are actually relatively new in the grand
scheme of history.
The history of wastewater treatment
That's not to say that sewer systems haven't
been around for ages.
After all, the ancient Romans had a complex
system of sewers at the peak of their empire.
 |
| Roman sewer in Cologne |
The Romans had a centralized sewage
management system, although it was fairly rudimentary by today's standards.
Open and closed ditches and pipes would carry
away excrement and trash, primarily using rainwater runoff.
The contaminated water would then flow into
large concrete tanks that let the sewage settle out before the water was
allowed to flow into the nearby rivers.
There was indoor plumbing, and public
latrines were also built over the sewers.
In medieval Europe, closed sewers, stone
conduits, or ditches were used to drain sewage away from residential areas,
often in conjunction with septic tanks, but chamber pots were often dumped
directly onto the streets.
Between 1858 and 1859 the Thames in London
was chock full of untreated wastewater, which combined with very hot weather to
cause what became known as "the Great Stink".
The 17th and 18th centuries saw a rapid
expansion in waterworks and pumping systems, but the Industrial Revolution led
to even more rapid growth of cities and pollution, which acted as a constant
source for the outbreak of deadly diseases like cholera and typhoid.
As cities grew in the 19th century,
increasing public health concerns led to the development of municipal
sanitation programs and the construction of sewer systems in many cities.
These systems often discharged sewage
directly into rivers without treatment, but by the late 19th century, chemical
treatments and sedimentation systems were in use in many cities.
The construction of centralized sewage
treatment plants began between the late 19th and early 20th centuries.
These systems passed sewage through a
combination of physical, biological, and chemical processes to remove
pollutants.
Also beginning in the 1900s, new
sewage-collection systems were designed to separate storm-water from domestic
wastewater, to prevent treatment plants from becoming overloaded during heavy
rains.
In the 1910s and 20s, engineers developed
more sophisticated systems to treat drinking water before it was supplied to
residents in cities.
Stepping back for a moment and examining the
timeline here, we can begin to understand just how recent effective wastewater
treatment on a grand scale appeared.
Roughly 150 years ago was the first few
centralized instances of water treatment for cities. It would take decades for
more rigid practices to emerge.
In 1972, the Clean Water Act was passed in
the United States. Up until this point, sewage treatment for some cities still
relied on chemical treatment and filtration, and the treated sewage was often
dumped into rivers and streams.
There was little in the way of pretreatment
of industrial wastewater to prevent toxic chemicals from interfering with the
biological processes used at sewage treatment plants.
After the passage of the Clean Water Act,
cities started a process known as secondary treatment, which removes all the
pollutant organic materials from the effluent.
Wastewater with high concentrations of
organic materials and nutrients being dumped into rivers was causing algal
blooms and the bacteria growth, which created dead zones in rivers.
The secondary treatment essentially
eradicates the effluent of microorganisms and organics so that when it's
discharged, it has little effect on the surrounding environment.
To think, just 50 years ago many communities
in the world were dumping mostly untreated sewage into rivers.
Wastewater treatment processes have really
experienced their most rapid growth in the last 30 or so years, now with every
planned municipality in the world having some form of a centralized wastewater
management system.
It's all at a hefty cost too - on the scale
of billions and billions of dollars.
Now, however, we can flush our toilets and
shower without really having to worry about what's happening to all that dirty
water.
It gets handled by trusty wastewater
treatment plant operators before being discharged into local rivers and lakes.
"Oh, and what happens to all the solids
from wastewater?" you might wonder.
Well in some cases, wastewater treatment
plants will let it dry, package it up and sell it as fertilizer to help
supplement the hefty costs of running a treatment plant.
In other cases, some plants will use the
sludge to produce methane, which they will then burn for power or sell.
Wastewater treatment today uses science and
engineering, though it is still a little bit smelly. We suppose it comes with
the territory.
Now that we understand just how recently our
knowledge of sanitation when it comes to human waste has emerged, let's take a
closer look at exactly how wastewater treatment plants work.
How modern wastewater treatment works
When you flush a toilet, your waste flows
through the sewers to a wastewater treatment plant that treats it.
Sewer systems are a topic all their own, so
we'll mainly focus on how your wastewater goes from one of the dirtiest
substances on the planet back into water that's safe for the environment, and
in theory, safe enough to drink.
Some wastewater plants known as full-cycle
reuse plants will even take wastewater and treat it all the way back to
drinking water, which will then be pumped to city inhabitants.
This may sound gross, but today's level of
engineering and chemistry allow full-cycle reuse plants to output drinking
water chemically identical to what's in your tap right now.
Before we dive into the specific process of
wastewater treatment, let's put things into a scale.
New York City has an array of 14 wastewater
treatment plants that handle 1.3 billion gallons of wastewater per day (4.9
billion liters).
That is enough wastewater to fill the dead
sea with sewage in 8 years, just from one large city.
So, society produces a lot of waste. Let's
see what happens first when it arrives at a wastewater treatment plant.
How Modern Wastewater Treatment Changed our
World
Pre & Primary Treatment
When wastewater arrives at a treatment
facility, it first gets all the large chunks filtered out through a screen, a
rather large one.
These screens are generally called bar
screens, and their main job is to make the sewage more homogenous so it can
flow through pumps and pipes in the plant.
The waste removed from bar screens is sent
off to the landfill, and the slightly less chunky sewage heads to the next
step, the grit chamber.
Grit chambers are essentially just big pools
that you definitely don't want to swim in, they allow the larger particles in
the sewage to settle out to the bottom.
These larger particles, things like dirt,
sand, and large food particles, are called grit. Again, this process aids in
making the sewage more homogenous than when it came in. The grit is also
trucked off to landfills.
After the sewage gets pretty homogenized in
these first few processes, it moves onto the primary clarifiers.
Primary clarifiers function as giant settling
basins that allow particles larger than 10 μm (0.01 mm), referred to as
suspended solids, to settle out to the bottom of the basin.
A giant skimming arm also scrapes away fat
and grease that rise along the surface of the water.
These primary clarifiers are based on a
principle called settling velocity, essentially just the speed at which
particles settle.
Engineers make sure that the inflow of the
water to the primary clarifier isn't more than the settling velocity of the
particles, which ensures that particles still settle out and the sewage keeps
on flowing.
Upon leaving the primary clarifiers, the
sewage is free of solids bigger than 10 μm and at this point, is mostly
contaminated with organic matter.
The sewage then moves on to aeration basins,
beginning the secondary treatment processes.
Secondary wastewater treatment
Aeration basins are essentially bubbly hot
tubs for sewage.
They bubble up air through the bottom of the
sewage, which invigorates the sewage with dissolved oxygen.
|
| A view of 3 clarifiers |
This activated sludge raises the oxygen
content of the water and the bacteria go on a feeding frenzy, eating up all of
the organic matter.
After the aeration basins, the sewage is
going to look a lot clearer and it will head onto the secondary clarifiers.
This is the final filtering process, where
all the remaining particles settle out.
The stuff that settles out is that activated
sludge just mentioned, and a part of it is reused to make the aeration basins
run smoothly.
What isn't used is left to dry out before
it's disposed of or used as fertilizer.
How Modern Wastewater Treatment Changed our
World
By the time the sewage leaves the secondary
clarifiers, 85 percent of all organic matter has been removed and it will look
fairly clear.
It might also be safe to drink too, but
you're probably not going to want to. The final process before discharge is disinfection.
This process kills off all the bacteria still
left in the water and makes sure there aren't any diseases being discharged
into rivers.
This is typically done through chlorine,
ozone, or ultraviolet disinfection (or a combination of these).
Ozone disinfection involves discharging
electricity into the water to cause oxygen gas molecules to turn into ozone
molecules, which oxidizes the bacteria, causing their cell walls to break, and
kills them.
Chlorine treatment kills the bacteria in a
similar manner but is a liquid chemical added to the water, and the treatment
plant operators will generally remove the chlorine before releasing the
effluent so the chlorine doesn't damage the environment.
Lastly, engineers can also use ultraviolet
light to scramble the DNA of the bacteria, making it impossible for them to
reproduce.
All three of these processes have different
pros and cons and are used fairly interchangeably across the world.
In most cases, after disinfection, the water
is released into rivers and streams.
In regions where water is scarce, sometimes
the treated wastewater will head back for another round of treatment to be made
into drinking water.
Chemically, this is very safe and could
probably be used in many more places around the world if it wasn't for the
stigma surrounding the closed-loop process of turning wastewater back into
drinking water.
The entire process takes around 24 to 36
hours for a molecule of water to make it through the treatment plant.
And that's the magic of wastewater treatment.
It's an essential process that allows us to live our lives without having to
think about our own waste.
Be sure to thank all the wastewater treatment
plant operators around you, because they have to deal with what you don't want
to, 24/7.
Trevor
English
Author
Trevor
is a civil engineer (B.S.) by trade and an accomplished writer with a passion
for inspiring everyone with new and exciting technologies. He is also a
published children’s book author and the producer for the YouTube channel
Concerning Reality.
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