Turbidity
Testing for Turbidity
Turbidity is a description of the relative cloudiness of water. If water is very turbid, then one cannot see through it like the sample shown at left. Turbidity is caused by small particles suspended in the water that reflect light and make the liquid look "murky."
Large particles can settle-out in a short time, but some particles can remain in suspension leaving the water both unhealthy looking and also full of foreign matter. Water that is turbid looks "bad" and we are not inclined to drink it based on looks alone. It is important to know that even though water "looks" clean, it may contain highly toxic chemicals or disease causing bacteria. Getting the water to look clean is only one aspect of filtration we should consider.
Filtering Turbid Water
The plaster filters you have constructed can help to reduce turbidity by filtering the large particles out of the water. Since the plaster is embedded with tiny pores, some materials are too large to seep through the matrix.
For conducting turbidity tests, you will need a source of turbid water, your plaster filters, a screen and a clean cup.
Procedure
Note: Use the STEM Notebook for recording the procedure and results.1) Set up the filters you have created using labels so that you know the porousness of each pot you are using. Since time is not a factor in this experimental procedure, you can run all trials simultaneously.
2) Prepare turbid water by mixing river bottom mud or other silt into a sample of water. It is perfectly fine to let some sediment to settle as this would occur in nature.
3) Add a sample of turbid water to each plaster pot. Put the same amount in each pot. Allow time for the water to filter through the pot.
4) Examine and compare the water that comes out of each filter (the effluent) to the turbid water. In the sample below, the water is much clearer, but there is still some residue in the bottom. Finer filters may yield better results.
5) Apply a visual test to the liquid using a guide to indicate the relative clarity of the water with regard to personal preference. See an example below. Or, as an alternative or in addition, use the test described in #8 below to provide more accurate data.Suggested Rating System for Turbidity
5- This water looks really clean and if I knew it was bacteria free I would drink it without hesitation
4- This water looks pretty clean and if I knew it was bacteria free I would drink it if I were really thirsty.
3- This water doesn't look very clean, but if I was desperately thirsty I would drink it.
2- This water is definitely murky and I would not think of drinking it unless I would die without it.
1- I would never drink this water under any circumstances.
6) Set up a data table to receive your ratings:
Visual Ratings for Each Filter
7) Recommend the best filter considering turbity. If you had also noted the rate of flow of the water through the filter (percolation rate), you could consider both turbidity and percolation rate in your recommendation. What happens to turbidity when there is a fast percolation rate?
Alternative Turbidity Measure
8) The following process describes how to measure turbidity in a more precise manner using an optical secchi system.
Find a graduated cylinder with a glass bottom that is transparent.
Place the graduated cylinder on .25" rule graph paper. Confirm that you can look down the cylinder and see the lines on the paper from above.
Pour turbid water into the graduated cylinder slowly and watch the lines on the paper. When they disappear, make a note of how much water has been poured into the cylinder. The relative murkiness of the water can now be relayed as a function of distance and recorded as a number for each level of turbidity.