Man's Filters: Cleaning Up Wastewater

Purpose: This laboratory or classroom activity introduces students to wastewater treatment and teaches the steps involved in basic wastewater treatment.

Summary: Students will learn the components of wastewater, how wastewater is treated at a treatment plants, and the basic steps in the treatment process. They will then apply some of the steps to attempt to purify a contaminated water sample.

Background: All the water that goes down the drain, whether in homes or businesses, is considered wastewater. This includes water from sinks, baths and showers, dish washing, washing machines, and toilets and well as water used in manufacturing and industry. Some businesses may collect and treat their own wastewater. Others will discharge the wastewater into the municipal sewer system.

Wastewater from domestic and industrial uses contains a variety of substances, including human wastes, kitchen debris, toxic chemicals, and inorganic materials such as plastics. This water can be high in nutrients, pathogens, and toxic chemicals, and poses a risk to both drinking water and aquatic habitats.

For this reason, wastewater must be treated before it can be discharged to rivers and lakes. In nature, water filtration occurs when rainfall and snowmelt seep into the ground, where the soil materials act as filters, and microorganisms in the soil help degrade pollutants. Manπs wastewater processing systems incorporate some of the same elements.

There are two main methods of wastewater treatment in use today. On-site septic systems, in which wastes enter a tank where settling and biological breakdown occur before effluent is discharged to a leachfield in the soil, are primarily used for single homes or small clusters of homes. Most cities and counties rely upon municipal wastewater treatment plants. The effluent from these plants is considered a point source discharge, and levels of pollutants are carefully monitored.

The Truckee Meadows Water Reclamation Facility (TMWRF) is the primary wastewater treatment plant for the area. Another treatment plant has been built in the South Meadows area. TMWRF has the capacity to treat 40 million gallons per day (MGD), and on average, treats 30 MGD.

There are two basic phases to all municipal wastewater treatment: primary treatment and secondary treatment. During primary treatment, which is basically a physical process, the wastewater is screened to remove large objects such as stones or sticks that could plug lines or block tank inlets. A grit chamber is used to allow sand, grit, and small stones to settle to the bottom. The sewage than moves to a sedimentation tank, where most of the solids settle to the bottom as raw sludge, and oils and grease float to the top and are skimmed off. Substances such as alum may be used to help flocculate, or stick together particles, so they can be removed from the water. The raw sludge is removed from the tank when necessary for further processing.

The wastewater resulting after primary treatment still contains suspended and dissolved pollutants in the form of organic matter. During secondary treatment, an activated sludge containing microorganisms is used to break down organic material with aeration and agitation. Solids are then able to settle out. As much as 95% of wastewater pollutants can be removed during secondary treatment. The resulting sludge mixture, which is full of millions of bacteria that feed on organic wastewater pollutants, can be combined with new sewage in the presence of lots of fresh air to continue the process. Secondary settling tanks are used to separate the water from organisms. After these steps are complete, municipal wastewater is usually disinfected using chlorine or other disinfecting compounds.

Tertiary treatment may be used to remove nutrients such as nitrogen and phosphorus, and carbon adsorption can be used to remove chemicals. The sludge byproduct can be treated and landfilled or applied to land to improve soil properties, and the treated, clean effluent is discharged to a local waterbody.

TMWRF discharges its effluent into Steamboat Creek, which then flows into the Truckee River. By law, since TMWRF is a point source of pollution, it must have a permit with limits on the amount of pollutants that can enter the river. For example, TMWRF is permitted to discharge up to 500 pounds per day of nitrogen. The plant uses tertiary treatment to meet this requirement.

Materials: (one setup per group of students)

1. Muddy water with added grass clippings or orange peels, green food color and a few drops of detergent (Prepare in advance and let it sit in the sun for a few days to "ripen".)
2. 20 ounce clear plastic soda bottle cut in about ¼ of the way from the bottom (ask students to collect the bottles)
3. Alum (potassium aluminum sulfate; available at a pharmacy)
4. Flexible nylon screen (pantyhose works well)
5. 20 ounce large-mouth bottle with a lid
6. 2-3 plastic cups
7. Washed fine sand
8. Washed coarse sand
9. Washed small pebbles
10. Large beaker or jar
11. Chlorine bleach
12. Medicine dropper
13. Teaspoon
14. Rubber band

Procedure: Students will build a filter and use it to clean "swampy" or muddy water.

1. Pour about 1 cup of the muddy water into the 20 oz. large-mouth bottle with lid. Keep a sample of water to use for comparison at the end of the exercise.
2. Ask the students describe the appearance and smell of the water on their worksheet.
3. Have the students place the lid on the bottle and shake the water vigorously for 30 seconds. This will aerate the water. Pour the water into a plastic cup, and pour it back and forth at least 10 times to continue aeration. Ask the students to describe any changes they note on the worksheet.
4. Next, have students add one heaping teaspoon of alum crystals to the aerated muddy water. Explain that the alum will help glom the particles together so they will settle out during sedimentation. Slowly stir the mixture for 5 minutes.
5. Allow the mixture to sit for 20 minutes so sedimentation can occur. Have students check the solution every 5 minutes and write their observations of the changes in appearance on the worksheet. While they are waiting, have them construct a filter as described in the next step.
6. Have students build one filter per group.
1. Use the cut part of the soda bottle that includes the capped end.
2. Remove the cap. Attach a nylon screen to the neck of the bottle using a rubber band.
3. Turn the bottle upside down and place it in the beaker or jar to support it.
4. Pour a layer of fine sand on top of the nylon screen in the inverted bottle.
5. Pour a layer of coarse sand on top of the fine sand.
6. Pour a layer of pebbles on top of the coarse sand. The filter is complete. There should be enough room above the pebble layer to pour in the 1-cup water sample.
7. Pour clean tap water through the filter until it drains clean from the bottom. Be careful not to disturb the pebble layer too much. This step prepares the filter for use with the sample.
7. After the 20-minute settling period, carefully pour the top two-thirds of the water sample through the filter. Be careful not to disturb or pour out the sediment. It should stay in the jar. Let the sample filter into a clean jar or beaker.
8. Next, add one drop of the chlorine bleach to the filtered water and observe any changes.
9. Compare the treated and untreated water, and have the students complete their worksheet. Do not allow them to drink the water!

Steps in Wastewater Treatment

Step 1: Preliminary treatment

• Screening - removes large objects and allows sand and dirt to settle out
• Smaller objects are ground into even smaller pieces, and sand and dirt are allowed to settle out

Step 2: Primary treatment

• Primary settling - floating grease and scum are skimmed, and solids settle out.

Step 3: Secondary treatment

• Aeration - aeration tanks add water and allow bacteria to digest organic substances.
• Final settling - sludge continues to settle out, and is removed to a solids treatment process for stabilization. The final product, called biosolids, can be disposed on cropland, in landfills, or used for compost or other soil additives.
• Disinfection - additional chlorine is added to kill disease-causing organisms, or other methods of disinfection such as ultraviolet radiation are used.

Step 4: Tertiary treatment

• Carbon adsorption - used to remove chemicals
• Other methods used to remove nutrients such as nitrogen and phosphorus

Truckee Meadows Water Reclamation Facility Schematic

Student Worksheet - Wastewater Treatment

Record your observations during each step of the process.

	What changes do you see? Describe appearance, color, odor, clarity
Step 1: Aeration
Step 2: Sedimentation
Step 3: Filtration
Step 4: Chlorination

1. What changes in the liquid did you observe after aeration?
2. Did aeration remove any of the odor?
3. What was removed by sedimentation?
4. What was removed by the filter?
5. Do you think your purified water is safe to drink? Why or why not?
6. What else would you need to do to make the water clean enough to drink?

Resources

Water Environment Federation, www.wef.org

Truckee Meadows Water Reclamation Facility, www.tmwrf.com

Vocabulary Words

Activated sludge	Sludge particles produced by the growth of microorganisms in aerated tanks as a part of the activated sludge process to treat wastewater.
Aeration	Process of stirring or bubbling air through a liquid. This adds oxygen to the wastewater and allows other trapped gasses to escape.
Chlorine	A chemical element, abbreviated Cl, that is used as a disinfectant in drinking and wastewater treatment processes.
Contamination	The addition of any substance to water that makes it unfit for use.
Disinfection	Process by which most microorganisms in or on a substance are killed.
Filtration	A mechanical process which involves moving water through a material, usually sand, designed to catch and remove particles.
Flocculation	Process by which dirt and other suspended solid particles are chemically stuck together to they can be removed from water by sedimentation.
Point source pollution	Pollution that can be traced to a single, identifiable source, such as a pipe or culvert.
Pollutant	Something added to water that causes an undesirable change in the physical, chemical, or biological characteristics of the air, water, or land that may harm or affect humans or other living organisms.
Primary treatment	The first stage of wastewater treatment in which settleable or floating solids are removed.
Organic material	Material derived from living things and containing carbon compounds.
Secondary treatment	A type of wastewater treatment in which biological treatment processes such as activated sludge are used to convert dissolved and suspended pollutants into a form that can be removed.
Settling	The process of a substance, such as heavy organic solids or sediment, sinking to the bottom.
Sludge	Any solid, semisolid, or liquid waste that settles to the bottom of sedimentation tanks or septic tanks.
Surface water	All water on the surface of the earth, including lakes, rivers, streams, ponds, oceans, and runoff.
Tertiary treatment	Any level of treatment above secondary treatment, which could include filtration, nutrient removal, and removal of toxic chemicals and metals.
Wastewater	Water that has been used for domestic or industrial purposes.

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