去似春归无觅处

The lost within water distribution system, according to the city I worked before was 5% in summer, and 3% in winter. The lost is due to the leak or break in the pipe.
My suggestion is to create water distribution system hydraulic model. The steps are: 1 buy software(it is too expensive to create the software), 2 creat and calibrate the model either using SCADA data or field testing data. The software should tell you the flow in the pipe, and which source is the water from.
When I design water treatment plant in the US, the intake Q = 1.1 x Treatment Q. The water use in the treatment plant is about 10% (5% for coagulation/sedimentation, and 5% for filter backwashing). The lost in the distribution system is very limited.
It is so expensive to waste the treated water in the system.

这个问题简单，你们算不错的，建设部规定的网损是18%，有的地方，例如东北的一些城市是40%以上呢。至于你说的15%左右的损失，不知道是按进厂水还是按出厂水计算的。另外，在你们城市的管网中，肯定有漏失的，就是你们没找到了吧，技术还不过关。