- [Narrator] This is my cat, Rubiks.
One of the many amazing things about Rubiks is
that he naturally works to keep himself clean.
His barbed tongue is really good
at getting rid of the dust and dirt
that he gets in his fur every day,
but sometimes he needs a little help getting clean.
Healthy lakes and rivers are kind of like cats.
They naturally clean themselves.
They have lots of spaces
where contaminants from water can settle
and to be filtered out by the soil
and organisms like plants.
The clean water produced by these systems
is important for irrigation, drinking water,
and the ecosystem.
But when humans add sewage
and storm water pollution into rivers too quickly
natural ponds and streams can't keep up.
The pollution can then build up
and become a danger to wildlife and people.
To prevent this from happening
many urban areas in developed countries
use water management systems to treat their water waste.
Let's say I'm giving Rubiks a bath one day,
when I unplug the drain
where will the water go?
First, it will travel through a long network of sewer pipes
where it will be joined by water waste
from homes and businesses
and maybe some storm water
from catch basins along roads.
Eventually, it will reach the water treatment plant
where it'll go under primary sewage treatment.
Primary sewage treatment,
which is also called physical sewage treatment
is where the treatment plant physically takes out
large objects from the water.
This prevents the objects from clocking up the pipes
and pumps in future treatment steps.
During primary treatment,
the water goes through a series of screens
with progressively smaller and smaller openings.
The screens sift out objects like sticks, rocks, rags,
plastic bottles, and Rubik's hair.
Even though the screens take out many of the larger objects
there's still a lot of grit in the water afterwards.
Grit is made up of small loose particles of sand, gravel,
coffee grounds, food waste,
and other pieces of material.
To get rid of the grit,
the treatment plant pumps the water
through long narrow tanks
that slow down the flow enough
that the grit has time to settle to the bottom.
All that thick soft mud
that gathers at the bottom of the grit tank
is called sludge,
and it's collected and treated.
The water then moves on to secondary
or biological treatment.
During this stage, the wastewater is aerated
or mixed with air
to create perfect conditions
to help increase the population of aerobic bacteria.
Aerobic bacteria are important
because they quickly feed on and break down
the organic waste in the water.
Sometimes bacteria loaded sludge called activated sludge
is also added to help grow even more bacteria.
When the bacteria have plenty of oxygen
they metabolize the organic material into carbon dioxide
and yes, more sludge.
The sludge, again, settles to the bottom of the tank
and is collected for treatment.
I like to think of secondary treatment
as kind of like being a good caretaker
to millions of microbial pets
when we provide our microscopic pets with oxygen and food
then they help us out by doing the cleaning work.
That's why secondary treatment
is also called biological treatment
because it uses living organisms.
Like primary treatment,
secondary treatment generates lots of sludge.
Some of the sludge may be recycled
and added to water entering secondary treatment.
This increases the population
of aerobic bacteria in the wastewater
so the bacteria remove the organic waste faster.
The sludge collected from primary and secondary treatment
can be dried and buried as a waste product,
or dehydrated and heated
to make a soil like material called biosolids
that can be used in agriculture.
By the time the primary
and secondary treatment steps are completed
about 85% of the organic pollution has been removed
from the wastewater,
but there are still some disease causing bacteria
and viruses that are present in the water.
So most treatment plants add another step
to secondary treatment, disinfection.
Some treatment plants
may use ultraviolet light treatment
to prevent disease carrying bacteria
from being able to multiply
or treatment may use chlorine
or ozone treatment processes
to kill bacteria and viruses.
Chlorination will kill more than 99%
of the harmful bacteria.
In some places, the wastewater treatment plant directly
releases cleaned water into drinking water sources,
agricultural water sources,
or recreational water bodies.
In these cases,
the water may go through yet another treatment process
called tertiary treatment or advanced treatment.
Tertiary treatment looks different
at different water treatment plants.
Sometimes it uses a more targeted approach
to the removal of specific wastewater components.
Some examples include additional biological treatments
capable of removing nitrogen and phosphorus
or physical chemical separation techniques
such as filtration, carbon absorption, distillation,
and reverse osmosis for removing a variety of chemicals.
Despite being effective,
these techniques are expensive and not widely used.
After treatment, the newly cleaned water
will go out into the world,
perhaps to an ocean or to a river
or maybe even into Rubik's next bath.