This paper introduces the role of a PCS in a critical infrastructure, and especially addresses the processing flow and control of a water system. By leveraging advanced IT technologies, a modern process control system (PCS) can help monitor and control physical processes in a water system. Conventionally, a water system is composed of a water treatment subsystem and a water distribution subsystem. To provide better water quality and steady water flow, PCSs should be adopted in these subsystems to more precisely control the concentration of chemicals in water and the outlet valve of water tanks. We investigate the security issues and possible attacks in a water system adopting a close-loop control mechanism. Taking the concentration control of chlorine in water as an example, we formally explain how an integrity attack leads a water system into a harmful state by adding improper amount of chlorine into water. In the future, we will further develop a formal model describing a n-tank water system, evaluate the impact of simultaneous more attacks and discuss the potential impacts. We will also try to design a detection and defence system to resist against known attacks.