LESSON 3 Activity 8

Treating Our Wastewater

Grades:

4th - 7th

Objective:

Discover two ways in which wastewater is treated.

Method:

A simple model provides a visual example of how wastewater arriving at a treatment plant or septic system is filtered and becomes usable again after it leaves the treatment facility.

Materials:

Muddy water, one clear plastic 2-liter bottle, two clear glass jars or clear plastic containers, fine sand, coarse sand, pebbles, a small piece of wire screen. The Treatment Plant and the Home Septic are useful diagrams you may wish to use will help students to better visualize the treatment process.

Time:

Preparation Time: 10 minutes
Class Time: 20 minutes

Overview:

"One third of the septic tanks in the U.S. are polluting water."
- National Wildlife Federation

Where does water go when it drains down our kitchen and bathroom sinks? It is surprising how many people today have no idea. To maintain safe ground and surface waters, wastewater from our homes and businesses must be treated before being released into the environment. Household sewage is a combination of wastewater from several sources: toilets - 40%, laundry - 15%, bathing - 30%, kitchen - 10%, and miscellaneous water uses - 5%. (Septic Systems, 1989, University of NH Coop. Ext.) Many rural homes, such as in small Catskill Mountain communities, have septic systems. Larger villages and cities use wastewater plants or other forms of treatment to deal with community wastewater.

Improperly or inadequately treated wastewater can contaminate surface and groundwater supplies of drinking water. When wastewater is treated correctly, dangerous contaminates are removed, insuring the safety of the surrounding environment and the communities involved. The following experiments can be used to familiarize students with the specific steps taken in wastewater treatment systems for treating water safely and efficiently.

The Steps of a Wastewater Treatment Plant

Wastewater treatment plants go through several steps in a treatment process in order to safely treat large quantities of wastewater. When the water first arrives at the treatment plant, it passes through a comminutor, which screens, grinds up, and removes large floating objects like rags, sticks, cans, etc. After this screening process, the water passes through a grit chamber, where sand, grit, cinders, and small stones settle to the bottom. Once the screening is completed and the grit is removed, the sewage then moves on to the sedimentation tank where most of the solids settle to the bottom as raw sludge. The sludge is mechanically removed from the tank when necessary. These steps all constitute the first, or primary, stage of treatment.

In the past, this was often enough treatment for most communities. With the amounts and types of pollutants now entering our waste streams, secondary measures are often needed. After the sedimentation tank, secondary treatment involves moving the sewage to an aeration tank, where the sewage is mixed with air and bacteria and allowed to remain for several hours. During this time, the bacteria in the sludge break down any organic materials left in the water. The resulting sludge mixture, now teaming with millions of bacteria, can be combined with new sewage in the presence of additional supplies of fresh air. The next treatment commonly used after the wastewater leaves the aeration tanks is secondary sedimentation, which occurs in the clarifier. At this point, the water is allowed to settle out any remaining particulates before the final step. Chlorination is the final step before water is discharged through a pipe into a nearby stream, river, or ocean. Very small amounts of chlorine gas are added to the water to kill any remaining bacteria.

There are several other methods for treating wastewater for reuse, but the above techniques are the most commonly used. Other secondary treatments include combinations of aeration tanks or trickling filters with sedimentation tanks, or the use of lagoons or wetlands with natural ecosystems to absorb the impurities present. Advanced treatments, when necessary, include nutrient extraction, further filtration, disinfection, and chlorine removal before the effluent is discharged or reused. The amount of processing a municipality uses on its wastewater is set by the Federal Clean Water Act and New York State effluent standards.

The Elements of a Septic System

These two-part systems generally consist of an underground, watertight receptacle called a septic tank and a soil absorption or drainage field. As wastewater goes down the drain, it travels through pipes to the septic tank. Inside the tank, beneficial bacteria begin to break down some of the solids. The remaining heavy solids settle to the bottom of the tank and the lighter solids and grease float to the surface. This settling process inside the tank is called sedimentation. To maintain proper functioning inside the tank, the sludge and scum are periodically pumped out.

From the tank, sediment-free liquid flows through a distribution box to perforated pipes that have been laid in trenches filled with crushed stone. There, the partially-treated water flows through the surrounding material toward the groundwater. During its travels, bacteria, oxygen, and soil help purify the water through chemical and physical reactions. This penetration process is a form of filtration. Because the soil is the most critical factor in cleansing the wastewater when it leaves the tank, soil type determines what kind of system can be installed.

Procedure:

  1. To demonstrate the primary stage treatment of sedimentation in a septic system and waste-water treatment plant, have students fill one of the clear containers with shaken muddy water. Allow the particles to settle, and observe the water every five minutes. Record the results.

  2. To illustrate the filtration step, have the students cut the bottom off of the 2-liter plastic bottle. Turn the bottle upside down and put a piece of wire screen inside the neck. Next, place layers of clean soil materials in the bottle beginning with the coarsest materials such as pebbles, and ending with the fine sand on top. Run some tap water through the filter to clean it.

  3. Slowly pour half of the settled water into the filter bottle. Record the results comparing filtered water with muddy water and the remaining settled water.
Assessment:
  1. How is your community's wastewater treated?

  2. What additional water treatment steps does your community take to ensure the safety of your drinking water supply? (e.g. testing, chlorination)
NYS Learning Standards:

Math, Science, and Technology
Standard 4 - Science: Physical Setting 2; The Living Environment 7
Standard 6 - Interconnectedness: Models


Sources: Activity adapted from Frost Valley YMCA's The Ways Of The Watersheds. Activity information obtained and lessons adapted from "Wastewater Treatment Worksheet" in Captain Hydro Water Conservation Workbook and Pollution Prevention Through Watershed Management in Sullivan and Ulster Counties, New York Handbook.