by Bob Dabkowski
Ion selective electrodes are known for the ease of use, but also known for their troublesome inaccuracy at low concentrations. Hach has recently released the NH4Dsc online ammonium probe that overcomes the challenges of conventional ion selective electrodes to deliver an accuracy of 5% of measured value or 0.2mg/L (whichever is greater) and a detection limit of 0.2mg/L. The solution is contained in the CartricalTM technology.
The CartricalTM system contains three electrodes: the ammonium electrode, a potassium electrode, and a reference pH electrode; along with a temperature sensor. The most significant potential interference is from potassium ions (K+) which is compensated through the use of an integrated potassium ISE to correct the ammonium value. The reference pH electrode is a robust pHD differential pH sensor.
In one step, the system calibrates each electrode individually, and then calibrates all three together through a process calibration.
As part of the proving ground that all new Hach products must go through, one NH4Dsc sensor was tested in an Aqua-Aerobic Systems sequencing batch reactor in Illinois. The municipal facility has two SBR tanks with a total design flow of 300,000 gallons per day. Each tank was outfitted with a dissolved oxygen, suspended solids, and pH sensor integrated into the Aqua-Aerobics IntelliPro® process management system. For the test, the NH4Dsc sensor was added to the network and immersed into the SBR tank.
“The probe was really simple to set up,” said Manuel de los Santos, a senior engineer with Aqua-Aerobics. “A Hach SC1000 module was already in place and connected to our PLC via ModBus® communication, we plugged the sensor to the module and it began giving us numbers. The data was used in our IntelliPro® software.”
“During the testing period, we took a few samples and tested in the lab. The online data was consistent with the grab samples,” de los Santos said. “We used the optional air-blast cleaning system and it kept the sensor clean.”
The test proved successful, but too short - de los Santos and the Aqua-Aerobic Systems team want to continue to test the sensor under different operating conditions to determine if it should be included as a part of the IntelliPro® package.
Another proving ground test was performed in an aeration train of an extended aeration activated sludge municipal wastewater treatment plant. The NH4Dsc system was able to track the diurnal variations in ammonia loading over a three-week period at this particular facility and closely matched laboratory values measured with the salicylate method (See Graph). This level of transparency within one train is not typically affordable with online reagent based analyzers, and offers another justification for tapered aeration and quality aeration control.
As technologies develop, common knowledge and conventional wisdom continue to change. Ion selective electrodes, once thought to be sensitive, delicate laboratory devices, are becoming robust, reliable online process systems at an affordable cost.
About the author:
Bob Dabkowski is a wastewater specialist with Hach Company. As a licensed Colorado Wastewater Operator he has consulted with hundreds of different facilities about process control and optimization, and authored several papers and articles. He can be reached at [email protected].
