YDS Deneme Soru=3391

We can understand from the passage that ----.

Sewage systems collect wastewater and treat it
before discharging it back into the environment.
These systems consist of intricate networks of
underground conduits or sewers that convey the
sewage through the treatment process to the point
of disposal. Sewage systems also handle the flow
of rainwater, either separately or as part of a single
system. Separate systems are generally preferable
because, in single systems, heavy rainfall can
overload treatment plants, with the result that
untreated overflow can become a source of
pollution. In separate systems, rainwater is often
allowed to flow into streams untreated because it is
assumed to be relatively clean.