On 04/08/2018 12:57, T i m wrote:

8

I though most surface water goes into the same drains as the grey
(particularly in urban areas) and why the *sewage systems* can become
overloaded when there are storms?

That's only in poorly planned areas like London, the sewage and rain
water run off are in different drains here.
Although there are idiots that still connect them to the wrong ones.