How To Guide
How To Measure Turbidity

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What is Turbidity and Why is it Important?

Turbidity is one of the more important water quality metrics we use in the wastewater field. It not only gives us a clear indication about how clean our water is, but is fundamental for our third stage of treatment, UV light disinfection. The lower our turbidity, the more easily light can penetrate our wastewater. It is only then that we can ensure our water has been sterilized and is safe to discharge to Suisun Bay.

 

How Do We Currently Measure Turbidity in the Classroom?

The typical way we use to measure turbidity in a classroom is by using a colorimeter. I hate this machine for several reasons. The main reason is that it is a literal black box! Students have no idea how to use it, what the tool is telling them or what the information it gives them means. They are given a number, in a unit measurement they have never heard of and will most likely never use again. If our goal is to help students understand what turbidity is, how we measure it, and why its important. The current classroom tools we use fail on all counts.

 

Whats a Better Way to Measure Turbidity?

I have developed a much easier way to get students to start thinking about turbidity. I have also invented a scale that we can use to measure the turbidity of our water. PLEASE NOTE that this scale does not in any way shape or form mirror a scale used by chemists or water quality engineers. It is simply a way to help students begin to understand and quantify what they are seeing and what it means.

 

  • Start with a laser. You can find cheap ones at any pet supply store.

 

  • Fill a clear plastic cup or beaker with a solution

 

  • Shine your laser through the side of the cup/beaker while looking down from above. See the turbidity scale below. 

 

And yes, you may have noticed that I have a toy shark with a frickin laser beam on its head. I like to have fun too.

 

Turbidity Level One

This is clear water. Light can penetrate fully through it. As a result, we cannot see the laser beam at all in our liquid. This gives us an indication of how clean our water is.

 

Turbidity Level Two

This is water with dissolved solutes in it. In this case, table salt. Once there are solid in our water, even if we cannot see them with our naked eye, we know they are there if we shine a light through them. This works because our light now has something solid to bounce off of.

 

Turbidity Level Three

This is water with solutes that are not fully dissolved. In our case, used coffee grounds. Notice that you can see the water is a brown color. But also notice that the beam of light is much thicker and brighter than in level 2. This is because there are far more solids in our water.

 

Turbidity Level Four

The solids in this water are so dense that light cannot even penetrate. Notice how the beam of light stops before making it fully through the cup. The solids in this case are half and half.

 

All Four Levels Together

These four cups represent the four levels of turbidity that we use to measure our water quality. Most household items can be used to replicate these levels.