"清洗污水管" can be translated into English as "cleaning the sewage pipes" or "cleaning the sewer pipes".
Both expressions are commonly used and convey the same meaning. "Sewage" and "sewer" both refer to waste matter or the system that carries it away, but "sewer" is more specifically associated with the underground pipes or channels for draining off waste water and sewage. In this context, either word is appropriate, depending on the specific nuance you wish to convey or the regional preference for terminology.