Water: Section D: Water Purification and Treatment
D.1 Natural Purification Through the Hydrologic Cycle
 If we, as a society want to act responsibly so that generations to come may have ample
supplies we must know how to clean water, and how it can be brought up to required
quality.
 First, where does water come from? The faucet is not the correct answer…
 Develop a list of three or four possible resources.
1.
2.
3.
 Until the late 1800’s, Americans got their water from ponds, lakes, streams, rivers,
wells, and rainwater holding tanks.
 We got rid of waste into holes, dry wells, or leaching cesspools (pits lined with broken
stone) and some waste was simply poured on the ground.
 In the late 1800’s citizens started to see a need for other ways to get rid of waste,
because of the increases in our population as a nation.
 So, flushing toilets and municipal water systems were developed. But these systems
were quite primitive. Waste was often released into lakes and other bodies of water
without treatment.
 As a result, waterborne diseases increased as the concentration of intestinal bacteria in
drinking water rose. Today, we have refined this process.
 Nature, to a certain extent, can purify water very well.
 As energy from the sun heats the water, it begins to evaporate into the atmosphere.
This leaves behind any pollutants that may be in the water, items such as heavy metals,
molecular substances and other insoluble compounds.
 Rainwater strikes the ground and picks up impurities and organic compounds that may
be on the ground.
 As rainwater travels through the hydrologic cycle, these compounds may accumulate in
certain areas, causing pollution throughout the area.
 As the water continues through the cycle and passes through gravel, sand, and even
rock, it can become purified.
 This removes bacteria and other suspended matter.
 Evaporation, condensation, bacterial action, and filtration are just some of the steps of
the hydrologic cycle.
D.2 Municipal Water Purification
 Water that is cleaned before it is used is called water treatment and occurs at a
municipal filtration and treatment plant.
 There are 7 major steps to water treatment:
1. Screening - metal screens prevent large objects from entering the treatment plant.
2. Pre-chlorination - Chlorine is used to kill disease-causing organisms.
3. Flocculation - Al2(SO4)3 and Ca(OH)2 are added to remove suspended particles such as
clay from the water.
4. Settling - Al(OH)3 and other substances are allowed to settle to the bottom of the tank.
5. Sand Filtration - Any remaining suspended material that did not settle out are removed
here.
6. Post-chlorination - The [Cl] is adjusted to protect it from bacterial infestation.
7. Optional further treatment - Depending on the community, there may be more
treatment at this point.
 These include:
a. aeration - spraying water into the air to remove odor and improve its taste.
b. pH adjustment - acids or bases are added to the water to bring it to a pH level
of 6.5 - 8.5.
c. Fluoridation - 1 ppm of F- may be added to the treated water. This helps
prevent tooth decay and helps reduce osteoporosis and hardening of the
arteries.
- Water is also cleaned post-use at a sewage treatment plant.
- The major goal of wastewater treatment is to prevent bacteria and viruses in human
waste from infecting the public. There are also other undesirable materials. (food
particles, used wash water, organic solvents, etc.) that must be removed.
- There are 5 major steps in sewage treatment:
1. Screening and grit removal - sand and gravel are allowed to settle out and large
objects are removed.
2. Primary settling - Floating grease and scum are skimmed off and solids are allowed to
settle out as sludge.
3. Aeration - Most of the sludge is held on baseball-sized rocks in an aeration tank. Air
circulates between rocks and aerobic bacteria digest complex substances.
4. Final settling - Most of the sludge is aerated, chlorinated, dried, and sent to an
incinerator or landfill, the rest goes back to the aeration tank.
5. Disinfection - Chlorine or other substances are added to kill germs.
6. Optional further treatment - The pH of the water is adjusted and heavy metal ions may
also be removed.
- Gas is produced during sludge aeration and digestion. The sludge gas usually contains
about 65% CH4 and 25% CO2.
- Chlorine is the best known and most common water treatment substance.
D.4 Chlorination and THMs
- Chlorine has been used in water disinfection for many years because it kills disease
causing microorganisms.
- Chlorination (adding chlorine) occurs in one of 3 ways:
1. Chlorine gas (Cl2) is bubbled into the water.
2. A water solution of sodium hypochlorite, NaOCl, is added to the water.
3. Calcium hypochlorite, Ca(OCl)2, is dissolved in the water.
- Scientists believe that when added to water, HOCl, hypoclorous acid is formed.
- When chlorine reacts to some organic compounds, trihalomethanes (THMs) are
formed. There is an EPA limit of 100 ppb of THMs in municipal water supplies.
D.6 Water and Water Softening
- Hard water can cause some problems in the home. When hard water reacts with soap,
the ions in the hard water form insoluble precipitates.
- These insoluble compounds often appear as soap scum in your shower or bathtub. Hard
water can also cause soap to curd up and make deposits on your clothing. This is hard
to remove.
- So what do we do? Sometimes bicarbonate ions are added to water to react with the
calcium ions in hard water. This causes the CaHCO3 to form and precipitate out.
- Unfortunately, this sometimes forms a rock-like scale that can form in items like tea
kettles and hot-water heaters. Why do we care? This scale acts as a thermal energy
blocker. This prevents water from heating efficiently, causing energy bills to rise.
- Many ways to eliminate hard water have been developed in the past. Some chemicals,
such as early detergents, did not bind with the ions in water. They cleaned clothing
efficiently, but the problem was they were non-biodegradable. Bacteria could not break
them down.
- Streams and other waterways would often have foamy layers on them, polluting our
environment.
- These detergents were also rich in phosphates, which caused the growth of green algae.
This lowered the dissolved oxygen levels in the water, and killed off some larger water
dwelling species.
- What can we do about it?
- One of the ways you can soften water is through an ion-exchange resin. This is a
common method used in modern water softeners.
- Hard water (containing Ca, Mg, and Fe) flow into the water softener. It is then filtered
through the ion exchange resin.
- The resin reacts with the water and exchanges the hard water ions with Na. The
resulting softened water flows out, with the sodium in it, into your house for you to
use.
- Why use an ion-exchange resin? It lasts for a long time and reacts very quickly to
remove hard water ions.
- After some time, the resin must be regenerated. Water softener salt, which contains the
Na+ ion, is flushed through the resin which contains Ca, Mg, and Fe.
- The reaction is reversed, and the hard water is then collected to be used again.
- This is called the resin-regeneration cycle.