The Cornell Waste Management Institute
http://cwmi.css.cornell.edu

Sewage Treatment Tour

Adapted from Critical Issue: Water. You Can Make a Difference!, Water Resources Education Kit, Grades 4-6, by Cornell Cooperative Extension of Nassau County, NY, and Water Wise: Lessons in Water Resources, Cornell Cooperative Extension
 
Trash Goes To School
Cornell Waste Management Institute
 
GRADE LEVELS: 9-12
 
SUBJECT AREAS: biology, chemistry
 
CONCEPT: How do we clean up our wastewater?
 
OBJECTIVE: To understand how our wastewater is treated and to make a connection between what is put down the drain and where it goes.
 
MATERIALS:
- paper
- pen or pencil
- set up a tour of a wastewater treatment plant
 
KEYWORDS: leachate, permeability, aquifer
 
BACKGROUND: The following information gives teachers sufficient background knowledge to inform students about modern wastewater treatment. It is up to the discretion of the teacher to cover as much on this topic as he/she wants. Class ability and time available will be the deciding factors. The important concept is not the water treatment process itself, but rather the fact that most wastewater needs to get treated before it can be recycled or reused and that this treatment results in a sludge which must be disposed of.
 
In rural areas, wastewater exits the house and enters cesspools or septic tanks. These are tanks in the ground where sewage is partially purified through decomposition by bacteria. The water slowly percolates back to the groundwater supply which can then be pumped to the surface as it is needed.
 
In more densely developed areas, sewers are used. These are underground pipes connected to a wastewater treatment facility. Once in the treatment plant, wastewater goes through a series of actions which will help to clean up the water. A wastewater treatment plant's basic function is to quicken the natural processes by which water purifies itself.
 
Wastewater treatment is vital to the purification of our water and the health of the population. At present, two basic stages exist in the treatment of wastes: primary and secondary.
In the primary stage of treatment, solids are allowed to settle and are removed from the water. Here's how it works: as sewage enters a plant for treatment, it flows through a screen. The screen removes large floating objects such as rags and sticks that may clog pumps and small pipes. After the sewage has been screened it passes into what is called a grit chamber where sand, grit, cinders and small stones are allowed to settle to the bottom.
 
The unwanted grit or gravel from this process is usually disposed of by filling land near a treatment plant. With screening completed and the grit removed, the sewage still contains dissolved organic matter along with suspended solids. In a sedimentation tank the suspended solids will gradually sink to the bottom, forming sludge. Then, the sludge is mechanically removed from sedimentation tanks.
 
The secondary stage of treatment removes up to 90% of the organic matter by making use of the bacteria in it. Two techniques are used in this stage: trickling filters and the activated sludge process.
 
A trickling filter is a bed of stones from 3-6 feet deep through which sewage passes. Bacteria gather and multiply on these stones until they can consume most of the organic matter in the sewage. The cleaner water trickles out through pipes in the bottom of the filter, then flows to another sedimentation tank to remove the bacteria. To complete the process, the water gets chlorinated for disinfection purposes.
 
The other technique which is being used more today is the activated sludge process. After the sewage leaves the settling tank in the primary stage, it is pumped to an aeration tank where it is mixed with air and sludge loaded with bacteria. It is allowed to remain here for several hours. During this time, the bacteria break down the organic matter. The sludge can be reused by returning it to the aeration tank and mixing it with new sewage and an ample amount of air.
 
Meanwhile, the sewage flows from the aeration tank to another sedimentation tank to remove the bacteria. The final step, as with the first technique, is the addition of chlorine.
 
In some cases, tertiary treatment also is used. In this final step, chemical treatment is used to remove specific compounds such as phosphates. This allows the water to be in better condition before it is put back into the water cycle system.
 
As our water supply demands are increasing, tertiary treatment allows us to use wastewater to recharge the groundwater supply. However, most wastewater plants do not employ this procedure due to cost and feasibility.
 
PROCEDURE:
1. Take a tour of a wastewater treatment plant, following the path of the water as it gets treated. Find out how many gallons of wastewater are treated per day, and how many people the plant serves.
 
2. Discuss questions such as the following:
- What steps does the water go through in wastewater treatment?
- What is the role of microorganisms?
- Where does the water go after leaving the wastewater treatment plant? How often is it tested, and for what pollutants?
- How much sludge is produced? How is it treated or disposed of?
 
FOLLOW-UP:
Students may wish to study and contrast a leach field with a sewage treatment plant.
If sludge from the wastewater treatment plant is composted or spread on land, the class might also want to visit the site of these operations.

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