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

There are two ways we typically have students measure turbidity in a classroom. Here’s why both of these methods are terrible. 

The first is a tool called a colorimeter. I hate this machine for several reasons. Most importantly because 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 most importantly, they will most likely never use again. I have watched countless students give up while using this machine.

The second tool is a graduated cylinder that you place over different grey circles with values on them. Students are supposed to look down the cylinder and determine the value based on whether they can see the grey disk underneath. In essence, you are asking students to choose, with little to no criteria whatsoever, between literal shades of grey!

If our goal is to help students understand what turbidity is, how we measure it, and why its important, then we need a fundamentally new kind of tool. 

 

A Better Way to Measure Turbidity

I have developed a much easier way to get students to start thinking about turbidity and a scale so they can easily measure it. I call it My Morning Coffee Turbidity.

 PLEASE NOTE: 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, Sugar.

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

This is water with solids so dense that light cannot even penetrate. The solids in this case are Milk.

Notice how the beam of light stops before making it fully through to the other side of the cup. 
 

 

 

All Four Levels Together

These four cups represent the four levels of turbidity that we use to measure our water quality.

Most household items (like the ingredients to your morning coffee) can be used to replicate these levels. 

 

 

How Might We Use This in the Classroom

Measuring Turbidity is valuable in any investigation where students are working closely with water quality.

Turbidity is a useful tool for students to be able to identify between clean water and water with dissolved solutes in it. This can be powerful in an investigation like the Mystery Solutions.

Turbidity can also be used as a tool to measure the effectiveness of tools to clean our water. In the DIY filter investigation, we typically only ever ask students to design a filter that can turn brown water into slightly less brown water.

If we want students to be able to effectively determine which kinds of filter designs work best at cleaning our water, Turbidity can be an easy and powerful tool to help students evaluate their results.