Science

EXERCISE #2 WATER FILTRATION

Background

Surface waters such as rivers, streams, and lakes, together with underground aquifers represent possible sources of potable water. Outbreaks of waterborne diseases such as cholera and typhoid fever in the 19th and early 20th centuries led to widespread development of water and wastewater treatment technologies to enhance overall water quality. Philadelphia draws its water from the Delaware and Schuylkill Rivers and until 1905, distributed it to the public without treatment. During this period, the city suffered from periodic outbreaks of e.g. typhoid fever (see Figure 1). In 1906, Philadelphia began filtering source water before distribution, which led to an immediate reduction in the number of outbreaks. The addition of a disinfection stage in 1913 further lowered the number of reported cases of typhoid fever.

Figure 1: Typhoid fever cases per 100,000 population from 1890 to 1935 in Philadelphia. Source: Davis and Cornwell (2013) “Introduction to Environmental Engineering,” McGraw-Hill, New York.

Groundwater from deep-well boreholes has a low risk of being contaminated with pathogenic organisms as it has passed through the soil on its way to the aquifer; the soil acts as a natural filter. However, groundwater may have high concentrations of dissolved minerals which can lead to e.g. hard water, and as such also requires treatment before entering the distribution system.

In the United States, all water drawn from surface sources must undergo filtration and disinfection before it can enter the distribution system and our homes. Such treatment of surface waters works to remove the threat of pathogenic microorganisms and additionally improves aesthetic properties such as taste, color, and odors. A typical municipal water treatment plant for a surface water supply includes the following treatment steps: Aeration, Coagulation, Sedimentation, Filtration, and Disinfection.

Objective

To demonstrate the steps of coagulation, sedimentation, and filtration that municipal water treatment plants use to purify surface water to meet the EPA’s standards for drinking water.

1

Materials

• 1x 500 mL of turbid surface water (contains 600 mg/L clay)

• 1x 500 L glass beaker

• 1x 10 mL glass beaker

• 1x 10 mL graduated cylinder

• 6 mL concentrated ferric chloride (FeCl3, 4000 mg/L)

• 1x stirring rod

• 1x turbidimeter

Procedure

1. Collect 500 mL of turbid surface water and add it to the 500 mL glass beaker.

2. Measure 6 mL of the concentrated FeCl3 using the graduated cylinder and add it to the 10 mL glass beaker.

3. Return to the bench and measure the pH and turbidity of your turbid surface water. Record your obser- vations of the appearance of the water.

4. Add the 6 mL of ferric chloride to the 500 mL beaker containing the turbid surface water. Ferric chloride acts as a coagulant.

5. Stir the mixture rapidly for approximately 10 seconds to evenly distribute the ferric chloride within the solution.

6. Continue stirring the solution, however do so very slowly. You should begin to see particles in the water clump together (coagulate) to make larger flocs. Be careful not to stir too vigorously. You risk destroying the flocs. Stir slowly for approximately 5 minutes, and record your observations of the water/flocs.

7. After stirring for 5 minutes, measure the pH and turbidity and record this.

8. The flocs which formed during coagulation can now be removed more easily by gravity (sedimentation). Allow the beaker of water to stand undisturbed for a total of 5 minutes. Measure the turbidity and record your observations about the appearance of the water.

9. Finally, carefully pour the top two-thirds of the water through the filtration setup provided by your TA and collect a small sample (10 mL) as it exits the filter. Avoid pouring the settled solids through the filter.

10. Measure the turbidity of the filtered sample. Compare the treated and untreated water. Has treatment changed the appearance/turbidity of the water? Record your observations. Do not drink water!

Discussion Questions

1. What role does coagulation play in water treatment? Why is it necessary to add ferric chloride?

2. During the mixing step, briefly discuss why gentle mixing of the solution is necessary after the addition of ferric chloride? How does this impact the next step of sedimentation?

3. Although the water which has been filtered looks clean, why do we need to take the extra step of disin- fecting it?

4. Most of us have had a glass of water from a Brita® pitcher. What is inside of a Brita® filter cartridge and what does it filter out? What doesn’t it filter out?

5. Last year, the average American used 167 disposable water bottles (16.9 oz or 0.132 gallons). If the cost of 1 gallon of tap water in Philadelphia is approximately 7 cents, how much money could be saved by drinking tap water if we assume the cost of 1 bottle of water is $1.00?

2

Observations

St ep

Tu rb

id ity

pH A

pp ea

ra nc

e (N

TU )

3 (in

iti al

)

6 (d

ur in

g co

ag ul

at io

n)

7 (a

ft er

c oa

gu la

tio n)

7 (a

ft er

5 m

in s

ed .)

10 (a

ft er

g ra

ve l-s

an d

fil tr

.)

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