As far as I have understood it, this is mostly a result of overbuild infrastructure. They planned the capacity of some central pipes and sewers in a time where

As far as I have understood it, this is mostly a result of overbuild infrastructure. They planned the capacity of some central pipes and sewers in a time where modern water saving technology wasn't yet used as much or even invented, because such projects are supposed to last a few decades. They build for growing usage and population based on the historic growth, but then water saving became a big thing, appliances became much more efficient, and people moved to other parts of the country and the bigger cities etc. So now that infrastructure is underutilized and flow is too low in some parts. But this is managed by the water companies, which know best where and when to increase flow. This is pretty localized, for example when they planned for a future housing development that never came, and just flushing twice won't put a dent in it anyways. Building a papermill in a suburb or countryside is also probably not the most efficient way to solve it.