A PLC is an industrial grade microcomputer primarily designed to substitute relay logic. PLCs have an input/output (I/O) subsystem that easily adapts to most of the water treatment plant process requirements. Instruments and sensors are easily connected in order to gather information for inputs to the PLC, these inputs are processed within the PLC and outputs are generated. (I.e. an upper level sensor sends an input signal to the PLC, the information is processed according to the program and the PLC turns the system off). For a small water treatment plant, one medium size PLC is enough, however, for a large water treatment plant, it may be necessary to use several interconnected PLCs.

A supervisionary control and data acquisition system (SCADA) often represents the human machine interface in a water treatment controly system. While a PLC effectively provides an effective control of a process they are oftenly scattered around the water treatment plant. The SCADA is installed in order to collect and analyze input from all these PLCs and to provide an interface for the plant operator to interact with the control system. A SCADA system gather the information as hard points (raw data from the PLC or other device) and soft points (processed data from hard points) and store them in a data base, therefore reports and other supervisionary information is easily created and accesed by the operators.

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