Abstract Due to recent requirements by the U.S.E.P.A. towards the standards on chlorine residuals for chlorination and dechlorination processes, together with the commitment to optimize the process operating conditions for chemical cost savings and to let the plant operator have more time working on other important projects, the City of Houston has been utilizing automatic control systems (Programmable Logic Controllers) at over twenty WWTPs to meet the above tasks. This paper will present one of the City's WWTPs which implemented the auto-control system. It also discusses how to solve the problems of noises in instrumentation signals and dead time which are common in auto-control systems. Finally, it will show how much success the system has complied with the U.S.E.P.A.'s requirements and effectively reduced the chemical costs.