Book 25, Question 58
Why do some sewers overflow when it rains hard?
In some older parts of cities, one pipe carries both rainwater and household wastewater.
When a storm brings lots of rain, the pipe can fill up faster than the treatment plant can handle. Extra water may overflow into a river or lake. Cities are building big storage tanks and tunnels, separating pipes, and adding rain gardens and green roofs to soak up rain.