Classroom Activity
Assessing the Effectiveness of Wastewater Treatment
Outline of Page
Click here for a PDF handout of the laboratory
Background Information
Water
treatment occurs in many ways, many of which can be modeled in the
classroom. Preassembled kits are
available from several suppliers (see resources), however it is also possible
to produce the systems using less expensive materials. The exercise described
below can be easily modified based on the availability of testing materials and
other laboratory equipment. Students
will explore two or three different types of treatment and investigate the
relative benefits and efficacies of each of the technologies. The simplest system involves 24 hours of
settling, the second adds filtration through a soil bed and the third
filtration through a vegetative bed.
None of these technologies depend on constant power or technology that
may be beyond the reach of a developing country.
This
activity draws on the material presented in the The Ganga Action Plan 20 Years Later and Varanasi and Kanpur broadcasts with the
supplementary materials described on the Water Quality Home Page.
Equipment
Needed (per group)
·
2
Oil Drain Pans or other small bucket
·
20
cm Plastic Tubing
·
Nitrate/Phosphate/Iron
test kit
·
Sterilized
Potting Soil (no nutrients added)
·
Optional:
planting tray, grass seed
Procedure
3 - 4 weeks prior to beginning lab:
Prepare
a planting tray by drilling four holes at one end. Fill the planting tray with
1.0 cm of gravel and fill to within 1.0 cm of the top with a sterilized potting
soil. Plant the grass or sedge seed as
directed on the packet and cover to retain moisture (Figure 1)
Figure 1. Construction diagram
for the planting tray.
Within one week of the laboratory in class:
Target
pollutants
Phosphate
Nitrate
Iron
'Wastewater" production – To achieve greater
consistency in results, a 20 liter batch of "wastewater" should be
mixed for group use prior to beginning the lab (assuming about 3 liters are
needed per group). The material should
consist of both dissolved and suspended material, and should contain levels of
material of sufficient concentration to allow for the test kits in use to
determine any change. Organic
fertilizers contain nutrients that will provide an excellent source of the
necessary materials. Dissolve approximately
three tablespoons of fertilizer per liter of water; this should form a cloudy
solution with high concentrations of nitrate and phosphate. If desired, iron can be added with iron
filings. Test the concentrations of the
three pollutants prior to beginning the lab!
Adjust as necessary to achieve the desired concentrations.
Preparation of Materials
Settling
Tank - Drill a hole in the side wall of one of the oil pans or buckets, one cm
above the bottom (Figure 2) that will allow the latex tubing to pass through it
snugly. Place rubber washers on either side of the tubing and affix with a
waterproof adhesive. Seal the area
around the washers with silicone caulking material. Place a clamp on the latex tubing that will
allow for the slow release of the water.
Figure 2. Construction diagram
for the settling tank.
Soil Filtration – The structure for the soil filtration
chamber is similar to that of the settling tank, with the discharge located at
the bottom of the bucket. Approximately 4
cm of sterilized potting soil should be placed at the bottom of the bucket and
a layer of paper towel placed over the soil (Figure 2).
Figure 3. Construction diagram
for the soil filtration tank.
Vegetative Filtration (tertiary treatment) – flow through
an artificial wetland: The planting tray
should be elevated about 2 cm from one end to the other. Make sure that this system is full saturated
with distilled water prior to beginning the experiment, otherwise much of the
sample water will be absorbed leaving little for analysis. It is also important to select inert potting
soils as many seed starting and other mixes contain fertilizers that will
confound the results.
Student Procedure
1. Settling
Begin
by stirring the "wastewater" to suspend the soil particles. Add three liters of water to the primary
settling tank and record parameter values for all pollutants. Allow this tank to settle for 24 hours. Being careful not to disturb the sediments,
remove as much water as possible above the drainage hole, reserving a
sufficient amount of water to allow for sampling (50 ml should be adequate) and
retest pollutant levels. Set the
remainder of the collected water aside to for use in the next step of the
experiment.
2. Soil Filtration
Take
the remaining water collected after settling and slowly pour it into the soil
filtration tank (with the drainage tube clamped closed) taking care not to
disturb the paper towel covering the soil.
Open the clamp sufficiently to allow the water to trickle out at a slow
rate. After 24 hours, most of the water
will have been collected in the receiving vessel. Once again, collect 50 ml of water for
testing, setting the rest aside for the vegetative filtration.
3. Vegetative
Filtration
24
hours prior to beginning treatment, the planting tray should be watered to
saturation and allowed to drain. Failure
to do this may result in excessive absorption of the water leaving too little
to test. Connect the drainage tube to
the distribution system and open the clamp to allow the water to slowly trickle
into the planting tray. Restrict the
flow such that there is no significant surface flow visible.
Evaluation of Results
1. Calculate the percentage of material removed
in each of the treatments using the formula:
(Initial Concentration – Final
Concentration)
Initial Concentration
2. Do the results suggest that an Advanced
Integrated Wastewater Pond System (AIWPS) system of ponds that allows for the
treatment of wastewater at a lower cost than conventional wastewater treatment
facilities would be an effective strategy for handling wastewater in developing
countries?.
3. The time that the water remains in each of
the treatments is critical to the extent of removal of the pollutants. However, more time of residence requires
greater area for the storage of the water and therefore greater expense. How would scientists determine the optimal
area for treatment?
For additional information or assistance with this
activity please contact David Black at dblack@groton.org.