Control refers to any mechanical, electrical or electronic system that activates, regulates or monitors a process. The degree of human interaction for controls can be divided as the following:
-Manual: When a human interaction commands a simple electrical or electronic device such as a manual a switch. (E.g. turning on/off a pump)
-Semiautomatic: When a more complex electrical or electronic device performs some logic control on the process but still some human interaction is required. (E.g. activating a cable logic system for a chemical panel)
-Automatic: A minimal human interaction is required and a device such as a programmable logic controller (PLC) performs the majority of the events. (E.g. a PLC system controlling a pressure filtration system)
Water treatment plants can operate completely manual, semiautomatic and automatic. Defining the right control strategy is often one of the most important decisions. The type of process will often dictated the type of control algorithm; these can be sequential (E.g. an SBR process) or continuous (E.g. an extended aeration process)
Continuous controls can be:
-On/off: The simplest automatic feedback control mode. Only requires two fixed set points.
-Feedforward Control: This type of control calculates the output based on one or more input variables. The type of calculation is always fixed and simple. This type of control generates excellent results when the proportion for the inputs remains constant.
-Feedback Control: This control calculates and then adjusts based on the difference between the real outcome and the expected outcome of a process. These can be Proportional, Proportional plus Integral (PI) and Proportional plus Integral plus Derivative (PID).
The system software for most water treatment plants is often composed of a supervisory control and data acquisition system (SCADA) and/or distributed control system (DCS). As for the hardware, programmable logic controllers (PLC), remote terminal units (RTU) and plant networks are selected and installed depending on the application
A well-designed control system should be properly engineered and tailored to the customer needs and requirements. With over 300 years of combined experience in the water and wastewater treatment industry, let Napier-Reid be your single solution for all you water treatment plant control needs.