In today’s post, we’ll learn how to calculate detention time. It’s a common question on the operator certification exam, so be sure to understand this concept. This applies to both water and wastewater treatment.
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So what is detention time? It’s the amount of time it takes for a drop of water to pass through a basin or a tank. For example, if someone asked you to find out how long it takes the flow to reach the outlet of a rectangular clarifier, they’re asking you to calculate the detention time.
So, how do you calculate detention time?
Here’s the formula for detention time:
Detention time is calculated by dividing the volume of the tank or basin by the flow rate. It’s fairly simple and straight-forward.
The most challenging part of calculating detention time is making sure the units on the top and bottom of the formula cancel out.
Most exam questions will give you the dimensions of the clarifier in feet, but the flow rate is usually given in MGD or gallons/day. This means that the volume you calculate will initially be in cubic feet, which you’ll convert to gallons. You want to make sure that the gallons cancel out to obtain the answer in hours.
When you’re converting units on the exam, be very careful. It’s easy to make mistakes when you’re rushing to complete the exam. Often, when I check my students’ math problems, I see mistakes that were made, probably because they were in a rush to complete the problem. The bottom line is to be very careful when you’re solving any math problem.
For both water and wastewater treatment, the detention time for clarifiers is usually in the range of 2 to 3 hours. The reason why the flow spends a long time in the clarifier is to enable the solids to settle to the bottom. Otherwise, you’ll see more solids in the effluent of the clarifier.
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