System Controls Variables In RO Water Treatment

A multifunctional control system, such as Burkert's MXControl 8620 multi-parameter controller, automates the control of process variables in RO water treatment systems. In some cases, the controller is able to reduce control and measuring instrumentation costs to almost half. Simple monitoring and control functions, as well as the gathering of data, are integrated into one compact and easily configurable control unit, with its integrated display and five soft-touch keys for the setting and display of all important variables and parameters.

Through parameterisation of a number of control values, via an SD card slot and a USB or Ethernet port, the 8620 is especially adapted for process monitoring and chemical control systems. A configurable PC software helps adapt the controller to many types of process requirements and applications. Data such as flow rate, pressure, pH/ORP, conductivity, O2, Cl2, water level and temperature can be displayed, controlled, recorded and transmitted. The multi-parameter controller can be connected to up to six analogue and eight digital inputs - four of which can be flow - such as conductivity, pH/ORP, temperature and pressure.

It features up to four analogue and nine digital outputs for a multitude of conditions, from simply retransmitting a process value to pulse or frequency dosing a metering pump. In addition, the controller can transmit analogue outputs from one of the eight PID control blocks. An example of using the MXControl to determine a key parameter in RO system health is a brackish water system with a typical recovery of between 75 and 85 per cent. If this value decreases during the course of the process, the control unit monitors and data logs every single parameter that is necessary for fault analysis.

The initial focus is on pH, then on conductivity, temperature and pressure. If all parameters are within the control range but the performance of the membranes is still falling short of the nominal value, this may be a result of a scaling or fouling event. This can also be quickly differentiated by appropriate monitoring points. If a higher pressure drop occurs at the first stage, the reason is most often a result of colloidal or bio-fouling. A pressure drop increase at the second stage generally indicates that an elevated mineral level or scaling has occurred.

It is this ability to precisely track and log multiple process points simultaneously that enables the MXControl to reduce costs and improve efficiency in reverse osmosis systems. The MXControl process makes sense for more complex RO systems, but a chemical pump combined with a compact pipe-mounted dynamic pH transmitter/controller such as Burkert's 8205 is sufficient for simpler RO systems where only the feed-water pH needs to be lowered. In these applications, the combination of Element transmitters that can transmit process values and temperature can also perform simple on/off control or alarm functions to reduce overall system costs.

The Element units are field-mounted devices that enable the quick refitting and upgrading of simple RO processes. Apart from measuring the conductivity, pH and ORP, Element transmitters also have options that enable the transmission and on/off control of temperature. The transmitters are programmed via a removable module that could be left with the unit if local display is required. If values deviate from the originally programmed set value, the transmitter triggers a transistor switch that opens or closes a valve or starts a pump, allowing the influx of fresh water or chemicals to regulate the pH, conductivity, temperature or ORP.

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