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Lesson 1 introduction to water treatment
- 2. 19/05/2015 Lesson 1: Introduction to Water Treatment
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Since the cycle is continuous, there is no actual beginning or end, so you can begin
anywhere. However, there are five basic processes that make up the hydrologic cycle:
Condensation, precipitation, infiltration, runoff, and evapotranspiration. Water vapor
condenses to form clouds, which result in precipitation when the conditions are suitable.
Precipitation falls to the surface and infiltrates the soil or flows to the ocean as runoff.
Surface water evaporates, returning moisture to the atmosphere, while plants return water to
the atmosphere by transpiration.
Condensation is the process of water changing from a vapor to a liquid. Water vapor in the
air rises mostly by convection. This means that warm, humid air will rise, while cooler air
will flow downward. As the warmer air rises, the water vapor will lose energy, causing its
temperature to drop. The water vapor then has a change of state into liquid or ice. You can
see condensation in action whenever you take a cold soda from the refrigerator and set it in a
room. Notice how the outside of the soda can "sweats"? The water doesn't come from inside
the can, it comes from the water vapor in the air. As the air cools around the can water
droplets form.
Precipitation is water being released from clouds as rain, sleet, snow, or hail. Precipitation
begins after water vapor, which has condensed in the atmosphere, becomes too heavy to
remain in atmospheric air currents and falls. Under some circumstances precipitation
actually evaporates before it reaches the surface. More often, though, precipitation reaches
the Earth's surface, adding to the surface water in streams and lakes, or infiltrating the soil to
become groundwater.
A portion of the precipitation that reaches the Earth's surface seeps into the ground through
the process called infiltration. The amount of water that infiltrates the soil varies with the
degree of land slope, the amount and type of vegetation, soil type and rock type, and whether
the soil is already saturated by water. The more openings in the surface (cracks, pores,
joints) the more infiltration occurs. Water that doesn't infiltrate the soil flows on the surface
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as runoff.
Precipitation that reaches the surface of the Earth but does not infiltrate the soil is called
runoff. Runoff can also come from melted snow and ice. When there is a lot of precipitation,
soils become saturated with water. Additional rainfall can no longer enter it. Runoff will
eventually drain into creeks, streams, and rivers, adding a large amount of water to the flow.
Surface water always travels towards the lowest point possible, usually the oceans. Along
the way some water evaporates, percolates into the ground, or is used for agricultural,
residential, or industrial purposes.
Evapotranspiration is water evaporating from the ground and transpiration by plants.
Evapotranspiration is also the way water vapor reenters the atmosphere. Evaporation occurs
when radiant energy from the sun heats water causing the water molecules to become so
active that some of them rise into the atmosphere as vapor. Transpiration occurs when plants
take in water through the roots and release it through the leaves, a process that can clean
water by removing contaminants and pollution.
As you can see, many processes are at work to give you the water you need. And these
processes are always at work. Just because Antarctica is frozen doesn't mean that
evaporation is not taking place (ice can turn directly to water vapor by a process called
sublimation). And because the Sahara Desert is so dry doesn't mean that precipitation is not
happening (it evaporates before it makes it to the ground).
Introduction to Water Treatment
Now that you know how nature treats water, let's see how it's done in the water plant before
we consume it in a glass of water.
Water treatment in a typical water treatment plant is shown in the picture below. Based on
the characteristics of the raw water and on other factors, this treatment process may vary
considerably from place to place.
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As water is pumped from the source (a well, spring, river, or lake) it is screened to remove
debris. Then, at the water plant, various characteristics of the raw water are tested.
The water may be prechlorinated to kill microorganisms, control odors and taste, and aid in
coagulation and setttling. The water may also be aerated, which removes carbon dioxide
(CO2) and raises pH, oxidizes iron (Fe) and manganese (Mn), removes hydrogen sulfide
(H2S), and removes organic contaminants. Potassium permanganate (KmnO4) may be added
to the water in the collection tray of the aerator in order to remove iron and manganese from
the water. Ozone may be added to the water to treat iron and manganese, remove algae,
neutralize hydrogen sulfide (H2S), and aid in flocculation. Many of these are dependent
upon the plant and the amount of water being distributed daily.
In most cases, the water then enters a flash mix chamber. Here, various chemicals are added
and are mixed into the water. Coagulants cause fine particles to clump together into larger
particles. Alkali are added to adjust the pH as well as to oxidize iron and manganese.
Hexametaphosphate may be added to prevent corrosion of pipes.
After flowing out of the flash mix chamber, the water goes through a chamber which causes
coagulation and flocculation to occur. Here, the fine particles of contaminants gather
together into large clumps called floc. When the water flows into the sedimentation basin,
some of the floc settles out of the water and is removed. Next, the water is passed through
filters which remove particles too small to settle out in the sedimentation basin.
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Finally, chlorine is added to the water. The water may also be fluoridated to reduce tooth
decay in the consumers. The water is left in the clear well for a period of time to allow the
chlorine to kill bacteria in the water and to oxidize hydrogen sulfide. The water is now
treated and ready to be distributed.
That was a quick overview of the water treatment process and what happens to water as it
goes through the plant step by step. We will get into greater detail in later lessons so that you
will understand each step of the process, why it is done and how to make sure each step is
being done efficiently.
Review
Nature treats water in its own way through the hydrologic cycle, though we still need to treat
the water before we drink it due to all the pollution and bacteria in the environment. The
hydrologic cycle provides the supply of water for us to use for consumption, continuously
cycling through over and over. The five basic processes that make up the hydrologic cycle
are condensation, precipitation, infiltration, runoff, and evapotranspiration.
In the water treatment plant the water comes in from the source, is aerated and the addition
of chemicals to aid in coagulation and flocculation occurs in the flash mix chamber, it is then
allowed to go through coagulation and flocculation, and settle out in the sedimentation basin.
The water will pass through a filtration system after the sedimentation basin, removing
partilces that were too small to settle out. Chlorine is added as the final step and then the
water is stored until it is distributed to the consumer.
Assignments
Work the following crossword puzzle that comes from definitions in your textbook. You
may either print the puzzle out, complete it and mail or fax back to the instructor or you may
send an email with the correct answers numbered accordingly.
Quiz
Answer the questions in the PreQuiz and the Lesson 1 quiz. When you have gotten all the
answers correct, print the page and either mail or fax it to the instructor. You may also take
the quiz online and submit your grade directly into the database for grading purposes.
