The design of a drain or sewer is influenced by a number of factors. Firstly, the likely volume of sewage to be carried has to be assessed, allowing for possible water from other sources flowing into the pipes.

• 下水道和排污管的设计受到若干因素的影响。首先要估计下水道系统可能的容量,使管道能够容纳其它来源的水流。