Parameters to Be Monitored by the SCADA System

Posted by MRB

Oxidation reduction potential (ORP), dissolved oxygen (DO), pH, and alkalinity are parameters that should be monitored by the Supervisory Control and Data Acquisition (SCADA) system. Manufacturers determine what parameters can be monitored and controlled by the SCADA
system. Monitoring of certain parameters is important, and the ability to adjust these parameters from a remote location is ideal. The operator needs to be able to add chemicals to raise the alkalinity and subsequently the pH. The set point should be an alkalinity value rather than pHbased. The operator should have the ability to fully control (i.e., modify) the plant-operating parameters, such as (but not limited to) cycle times, volumes, and set points.

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Alkalinity monitoring and addition ensures that a pH of less than 7.0 does not occur. Nitrification consumes alkalinity, and with a drop in alkalinity, pH also drops. If a plant has adequate alkalinity, pH does not change, so it does not need to be raised. Chemicals that raise alkalinity,
such as sodium bicarbonate and soda ash, are recommended over sodium hydroxide. Sodium hydroxide does not raise alkalinity; it does raise pH. See section for a discussion of the pros and cons of various chemicals used to increase alkalinity.

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For plants that nitrify and denitrify, ORP monitoring is desirable. ORP is the measure of the oxidizing or reducing capacity of a liquid. DO varies with depth and location within the basin. ORP can be used to determine if a chemical reaction is complete and to monitor or control a

Operators need the ability to make changes that will modify these readings to achieve appropriate nutrient removal. ORP readings have a range and are site specific for each facility. General ranges are: carbonaceous BOD (+50 to +250), nitrification (+100 to +300), and denitrification (+50 to -50).

On-line dissolved oxygen meters are very useful in SBR operation. They allow operators to adjust blower times to address the variable organic loads that enter the plant. Lack of organic strength reduces the react time during which aeration is needed to stabilize the wastewater. DO probes can be used to control the aeration-blower run time during the cycle, which in turn reduces the energy cost of aeration.

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It is desirable to locate DO, pH, and/or ORP probes in a place that can be reached easily by operators. These probes often clog or foul and need cleaning and calibration. If they are not easily accessible, proper maintenance may not occur.

The plant operator should have the knowledge and the ability to program the SCADA system to increase or decrease blower speed. Allowing the operator to adjust the blower speed, through the SCADA system, gives the operator much more control over the DO in the SBR.