[Pierre]

As someone that started his professional working life in the water industry, I can understand in general terms the issue at hand.

Sewage Treatment Works/Waste Water Treatment Works are designed to handle a certain flow rate.

When we have storms/ flash floods a lot of the excess water finds itself in the waste water system and overwhelms the treatment plant.

What should happen is that the excess flow should exit the plant before it enters the plant over a storm weir into storm tanks. The excess material will then be pumped into the treatment plant for treatment from the storm tanks after the excess flow has receded to manageable levels.

But the storm tanks are a finite volume, and when they are full the excess flow will then by diverted into whatever watercourse the treatments plants outfall is.

There are screens so condoms, tampons, nappies, wet wipes (all the stuff you shouldn't flush down the toilet anyway) should be stopped.

Also when you say "raw sewage" it is heavily diluted by the excess flow, and a large % of the solid matter will settle in the storm tanks.

The only way to alleviate the problem is to build more storm storage tanks or build more treatment plants. Which is a legitimate question of investment.

And/or stop storm water entering the waste water network. Storm water should enter drains and watercourses, not sewers (unless specifically designed to)