Water Reduction–Meanings

Question

I have a couple questions about the CAEE 201 lab from this week. I am wondering what "Potable Water Reduction" and "Storm Water Reduction" mean conceptually. I have talked to a few students, and none of us seem to be able to figure out or find online what those phrases mean so that we can figure out how to do the calculations.

Response

Potable water = Drinkable water = water bought from the city
Storm water reduction = water that doesn’t go into the city sewers that would have without your prevention system, in this case the cistern and it’s associated engineering components.

Supplemental Question

In the lab, we were told that the potable water reduction value and storm water reduction value should be different numbers. However, I keep getting the same value (but different percentages) for both. My thought process was that potable water reduction is water we don't have to buy from the city (what we collected in the cistern), and that storm water reduction is water that doesn't go into city sewers because of the cistern (what we collected in the cistern). With that, it makes sense that I am getting the same value, but since we were told in lab that they should be different I assume that I am missing something. I would appreciate if you could clarify that they should/should not be the same value.

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4/22/2015 Update

I need to apologize to those students who asked this question.  Thanks to another student not being satisfied with the answers I'd given I checked and found that indeed the potable water purchase reduction produces exactly the same amount of storm water overflow reduction.  The percentages will be different, but the actual numbers ARE the same.

This does make sense since the water that goes into flushing the toilets does not go down the drain.

I've used this lab for multiple years and never been asked the question before so I never thought it through and never noticed the identity.

My suggestion below of using boundary conditions is still often helpful, but was misleading in this case.  That's why I've put lines through the answer. 

Response - See Above 4/22/2015

It’s often useful to think of a “boundary condition” example when trying to address this sort of problem.  Think of the following:

• There is no rain whatsoever.
• Would the city have to buy water in this case?
• Would there be any overflow in this case?

Hopefully that will help you think through the answer to your question.

Update 4/20/2015
It was pointed out after lecture that this boundary case isn't as illustrative as desirable.  So, think of another one.

• The amount of rain that falls every day is exactly what is needed by the toilets
• In this case there would be no water purchased from the city (Maximum Reduction) AND no overflow and therefore a maximum of storm water reduction.
• If the rainfall increases there will be no change in the water purchased for toilet flushing (purchase remains at zero).  There will, however, be an increase in the overflow and thus a decrease in storm water reduction.