Combined sewers collect blackwater, greywater and stormwater runoff in one ant the same single pipe system. Combined sewers provoke the pollution of little polluted water by mixing it with heavily polluted water. This particularly results in pollution when sewers overflow during heavy rain periods. As combined sewers need to be large, they can also be more expensive to install. Today, combined sewers are perceived as obsolete and in newer construction separated sewer systems are installed instead, although many systems remain combined as the replacement of an existing systems would be too expensive.