Wastewater Treatment - taken from Mountain Empire
Community College
Water is naturally cleaned by micro-organisms (bacteria) that use
our waste as food. In the human world, water is also cleaned and
reused but human towns and cities concentrate our waste to a level
greater than what can be cleaned by nature. Our wastewater is
channeled to a septic system or wastewater treatment plant where it
is cleaned – usually by these same bacteria – and released back
into lakes and rivers. This water re-enters the hydrologic cycle and
will eventually be pumped back up by another drinking water
treatment plant to be purified and released to customers.
Sources of Wastewater
You can classify wastewater as domestic, industrial, or storm,
according to its origin.
1. Domestic sources include water used for normal activity in
homes and stores. Domestic wastewater is fairly easy to clean and
return to the water cycle. More on that below.
2. Factories and industry can often add really nasty chemicals to
water. These may need to be removed and handled by the factory
before their waste water can be sent to a treatment facility. No
more about this.
3. Storm water can go to a treatment plant, although it is usually
low in pollutants. Often, storm water just flows in the street, down
the storm drain and into a creek or river. This is what happens in
Telluride. Great amounts of storm water, if it is sent to a treatment
facility, can interfere with treatment efficiency in two ways: Storm
water may cause too much dilution of the wastewater. This starves
the bacteria of their food. At the same time, it may cause
overloading of the plant – too much water after a big rain for the
facility to treat. No more about this, except to say:
DO NOT EVER PUT ANYTHING DOWN A STORM DRAIN,
EXCEPT WATER!!!!!!
Domestic Waste Water
1. The septic system
Domestic waste water is all of the water that leaves a house or a
store – kitchen, toilet, shower, dishwasher, sink, etc. This water is
often cleaned using a septic system. Sepsis is a Greek word that
means rotten or decayed. A septic system is one very basic way to
deal with the water that has been used by humans. It cannot be
used for large populations. A septic system consists of two main
parts: a septic tank and a drainage field.
The septic tank is where all the water goes when it leaves a house water from the shower, the toilet, the sinks, the dishwasher, the
washing machine, etc. In the tank, heavy "stuff" sinks to the
bottom, micro-organisms eat everything the can, and clear water
escapes out in the drainage field. Look at the pictures.
In the drainage field, the water re-enters the water cycle and is
further cleaned by natural micro-organisms. The septic tank
basically contains the process and lets the solids settle down so
they can be eaten before the water re-enters the water cycle. On a
regular basis the sludge - dead micro-organisms built up in the
septic tank - needs to vacuumed out.
In the Treatment Plant - a "septic system" for many
houses.
In towns or cities, there are too many people and not enough room
for separate septic systems. Often a treatment plant is needed to
treat the domestic wastewater from so many people. In the
treatment plant there are many steps involved in treating
wastewater, but it is basically the same as the septic system. Below
is a quick overview of the possible steps involved.
Influent
Influent is the raw material that has been collected and sent to the
plant for treatment. It includes all the water and debris that entered
the collection system. This may include the trucks that service the
Porta-Potties at festivals, for example.
Pre Treatment
The influent is passed through a big screen. This traps all of the
large items that may have made it into the wastewater: rags, toys,
cans, etc. These inorganic items cannot be digested by the
bacteria.
After passing through the bar screen, the influent enters the grit
chamber. Here, the wastewater passes into a wide basin, which
slows the wastewater's speed. The slower flow causes grit to settle
out. Grit is the heaviest material in wastewater and includes
substances such as sand, coffee grounds, eggshells, gravel, and
cinders. Grit cannot be broken down by biological processes in the
facility, so it is hauled to a landfill.
Next, the remaining wastewater reaches a grinder. The wastewater
is passed through a rotating, cutting screen. This cutting screen
shreds any large chunks of organic matter in the wastewater into
smaller pieces. This makes it easier for the microorganisms to use
the organic matter as food.
In the final stage of pretreatment oxygen is added to the water. In
addition, microorganisms are seeded (added to the wastewater)
here, and they begin to use the oxygen to break down organic
matter. As the microorganisms eat, they multiply rapidly and
consume their “food” very quickly. The microorganisms usually
only require two hours to consume all of the organic matter.
The air which is forced into the wastewater serves another purpose.
It keeps the microorganisms suspended in the water so that they
do not settle on the bottom.
TREATMENT
The lighter organic solids remain suspended in the water and flow
into large tanks, called clarifiers.
The clarifier is a big tank in which the heavier solids sink to the
bottom and the lighter materials float to the surface.
Here, the heavier organic solids settle by gravity. These settled
solids, called primary sludge, are removed along with floating
scum and grease and pumped back into the system for further
digestion.
One of the primary purposes of the clarifier is to remove the
microorganisms from the water. After digesting organic matter in
the aeration basin, these microorganisms now have food, grit, and
other particles stuck to their outer coating. So they are heavy and
sink to the bottom in the clarifier. The cleaner water rises to the
top and is allowed to flow out of the clarifier.
The sludge at the bottom of the clarifier contains a great deal of
microorganisms. Some of the sludge is reused to seed the aeration
chamber with microorganisms. In Telluride, the sludge (mostly
dead bacteria) is gradually removed and spread on a large field the
town rents. Here it acts as a natural fertilizer and cows eat the
grass in the field.
Disinfection
After leaving the clarifier, the water is chlorinated and allowed to
sit in a contact chamber while the chlorine reacts with
microorganisms in the water. This process disinfects the water,
killing the disease-causing microorganisms. In Telluride, we also
send the water past ultraviolet lights that kill any remaining
organisms. Now the water has been thoroughly treated and can be
released into natural bodies of water. In Telluride, this happens at
the Lawson Hill fields. The facility is right across the river, and
our clean wastewater is returned directly into the San Miguel
River.