What you said is true if the drain system treats all fluids equally. But
consider the difference between these fluids.
It is much more important that sewage water, with its Constant Flow Rate, get to
its intended destination and not leech its pollutants into the ground. Storm
water, with a bursty Variable Flow Rate, will not cause environmental damage if
leeched into the ground during overflow cases. What is needed is a drain system
which can distinguish between storm water and sewage.
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