Honu. green sea turtle. Hawai'i.

Water Filtration
Christine L. Case

"Man is embedded in nature. The biologic science of recent years has been making this a more urgent fact of life. The new, hard problem will be to cope with the dawning, intensifying realization of just how interlocked we are"
-Lewis Thomas

My home page Skyline Biology


Getting Ready
Making a Water Filter
Treating Water

Bacteriologic Quality























When water is obtained from uncontaminated reservoirs fed by clear mountain streams or from deep wells, it requires minimal treatment to make it safe to drink. Many cities, however, obtain their water from badly polluted sources, such as rivers that have received municipal and industrial wastes upstream. Very turbid (cloudy) water is allowed to stand in a holding reservoir for a time to allow as much particulate suspended matter as possible to settle out. The water then undergoes flocculation-that is, removal of colloidal materials such as clay, which would otherwise remain in suspension indefinitely. A flocculant chemical, such as aluminum potassium sulfate (alum), forms aggregations of fine suspended particles called floc. As these aggregations slowly settle out, they entrap colloidal material and carry it to the bottom. Large numbers of viruses and bacteria are also removed by this treatment. Alum was used to clear muddy river water during the first half of the nineteenth century in the military forts of the American West, long before the germ theory of disease was developed. There were also observations at the time in European cities that people who used water filtered through sand to remove turbidity had a lower incidence of cholera during outbreaks. (See the figure.)

After flocculation treatment, water is treated by filtration-that is, passing it though beds of sand or diatomaceous earth. Some protozoan cysts, such as those of Giardia lamblia and Cryptosporidium, are removed from water only by such filtration treatment. The microorganisms are trapped mostly by surface absorption in the particulate beds. They do not penetrate the tortuous routing between the particles, even through the openings might be larger than the organisms that are filtered out. These filters are periodically backflushed to clear them of accumulations. Water systems of cities that have an exceptional concern for toxic chemicals supplement sand-type filtration with filters of activated charcoal (carbon). Charcoal removes not only particulate matter but also most dissolved organic chemical pollutants.

Before entering the municipal distribution system, the filtered water is disinfected. The most common disinfectant is chlorine. Because organic matter neutralizes chlorine, the plant operators must pay constant attention to maintaining effective levels of chlorine. There has been some concern that chlorine itself might be a health hazard, that it might react with organic contaminants in the water to form carcinogenic compounds. At present, this possibility is considered an acceptable risk when compared with the proven usefulness of chlorination of water.

One substitute for chlorination is ozone treatment. Ozone is a highly reactive form of oxygen that is formed by electrical spark discharges and ultraviolet light. (The fresh odor of air following an electrical storm or around an ultraviolet light bulb is from ozone.) Ozone for water treatment is generated electrically at the site of treatment. Use of ultraviolet light is also a possible alternative to chemical disinfection.

Getting Ready



1. Collect 500 mL or about 16 oz. of dirty lake or stream water in a clear beaker or jar.
2. Before treating the water:
a. Record the color, turbidity, pH, and any odor of the water.
b. Do a coliform count and a heterotrophic plate count.

Water Treatment





1. Add a small amount of polymer to the water and stir. Describe what happens.
2. When the water looks as clear as you think it is going to get, pour the water slowly over the filter unit you have made. Open up the bottom and collect the filtered water in a clean beaker or jar.
3. Optional. Add 2-3 drops of 5% hypochlorous acid (Clorox®) to the filtered water and stir.
a. If the water is clear check for chlorine after 5 minutes. If chlorine is present, the water is probably disinfected.
b. Chlorine can be detected by smell or with a chlorine test kit.
c. If your filtrate is turbid add 3-4 drops of chlorine and wait 30 minutes and test for chlorine again. If chlorine is not present, repeat step 3c.

After treating the water

1. Record the color and any odor of the water.
2. Determine the turbidity.
3. Record the pH.
4. Do a coliform count and a heterotrophic plate count.
Source; Jean Balisteros. Contra Costa County Water District

Back to Water Treatment Procedure

About the polymer: Your local water or wastewater facility uses cationic, anionic, or nonionic polymer as a flocculant.


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