Henderson's New Membrane Bioreactor Water Reclamation Facility – Choosing and Applying the Control System Architecture

The City of Henderson, Nevada is planning a new water reclamation facility that will serve the needs of its growing population. For this new satellite 8 million gallon per day (mgd) water reclamation facility, a membrane bioreactor (MBR) treatment alternative was selected. The configuration and operation of MBR facilities require careful selection of the control system architecture, including integration of the membrane equipment supplier controls with overall treatment plant processes, key data emulation methods, and a high reliability and redundancy configuration.The system configuration has been based on MBR control system experience and was integrated into the procurement documents for the membrane equipment supplier, including requirements for support in the control system data emulation and integration throughout the plant control system.The main concepts for the control system architecture are:Programmable Logic Controller (PLC) System DesignOperator Interface Modules for Monitoring and ControlNetwork Communications System DesignAlarm and Response ManagementStandby PowerThis type of 5th generation MBR system requires multiple PLCs, and the configuration of the PLCs, I/Os, and control logic within each is important. The design includes redundant master PLCs as well as train PLCs, with the coordinating logic and control of utility services (air supply, chemical feed) resident in the masters while each train PLC acts as a slave and provides control of one or more individual trains of equipment.Network communications between PLCs and with peripheral devices such as Operator Interfaces and plant SCADA systems must be maintained, since the master PLC must be able to initiate and coordinate certain train PLC events such as backpulsing, relax, flow setpoint adjustment, and recovery clean events. Fully looped redundant communications networks are required to ensure that there cannot be a single point of failure that would disrupt communications between the master and train PLCs.