This Research work is about one of many applications of Programmable Logic Controllers (PLCS) for the water treatment plants. The authors have developed PLC software for control of a water treatment system which is called “Backwash Water Treatment Process”. PLC is regarded as heart of the PLC based automatic control system. With the designed software, the PLC constantly monitors the state of the water treatment system through the field input devices and feedback signal. Based upon the developed program logic, it then determines the course of action to be carried out at the field output devices. In order to operate the backwash water treatment plant, the filter backwash sequence is formalized into nineteen steps. The condition which must be met before jumping from one step to next is listed in each step. Developed software is based on the filter backwash sequence which determines the time to backwash and the tie to stop backwashing the filter and the quantity of Aluminum sulfate AI2 (SO4)3 to be added. The decision is based on data acquired from limit switches, level switches/sensors, counters and timers. The logic sequence is designed to minimize production down time when a unit was off line due to backwashing and ensured that the primary sludge tank has sufficient capacity to accept the dirty backwash water. The algorithm is developed by considering filter backwash sequence, the I/O table developed for all input and output devices obtained by assigning the corresponding INPUT and OUPUT number to these devices considering the particular programmable controller, developed timing diagram showing the length of occurrence of process (I/O) signals, flow chart developed for the control process and using CX-Programmer software supporting predominant ladder programming language for CQMI PLC used as Master PLC and CPM2A -60 CDR PLCS used as slave PLCS for control of a water treatment system “backwash water treatment process. COMI is meant for advanced control system operations requiring 16 to 192 I/O points per PLC. CX-simulator software was used for simulating software developed by CX- Programmer.