Q: We have a wastewater pretreatment system for the treatment of chrome, nickel and zinc, so we have chrome reduction with acid and metabisulfite followed by precipitation. After our clarifier, the wastewater then discharges to the city.
Over the last several months, we have had difficulty with calibration of our pH sensors, particularly the pH adjustment sensor that controls pH in the range of 9.1– 9.5 for precipitation. Every week we remove both sensors, rinse with water, and then calibrate with buffers 7 and 10. Lately, it has taken many repeated attempts to get the controller to calibrate with these buffers; sometimes when we thought we had the sensor calibrated, we would check it with buffer 7 and the reading would be off by more than 0.5.
We then thought we had a bad sensor and ordered a new one; however, we experienced the same problem. We have checked the electronics and everything is OK. Somehow the sensor still works well enough for us to achieve metal precipitation to maintain compliance, although our effluent’s metal concentrations are a little higher than in the past. We feel very uncomfortable continuing this way. Any ideas to help? A. J.
A: Our first task is to check the quality of the pH buffers. All pH buffers have a shelf life. Once opened, the buffers begin to degrade. This is especially so for buffer 10. When buffer 10 is exposed to the atmosphere, carbon dioxide in the air dissolves into solution, forms carbonic acid and neutralizes the weak alkali in the solution; over time, the pH of the buffer solution drops. This phenomenon does not occur with buffer 4 because it is already acidic, and is extremely slow to occur with buffer 7.
When using pH buffer solutions, follow these recommendations:
A second key element to peak pH sensor performance is its cleaning, and your current practice of using water alone is certainly inadequate. pH sensors operate by allowing hydrogen ions (H+) to flow to the sensor; anything that interferes with this flow, such as oil or polymer films, scale, and metal precipitants, will slow down the sensor’s response to the changing pH conditions in the wastewater. If this condition is allowed to deteriorate, it will cause the pH to go out of control and/or excessive chemical (acid and caustic) usage.
For the pH sensor in the pH adjustment tank for metal precipitation, we recommend at least daily cleaning with water to remove the likely collection of loose precipitated metals (zinc hydroxide, iron hydroxide, etc) around the sensor.
At least on a weekly basis we recommend a vigorous cleaning as follows:
Instead of using a cleaner and then acid, we have also found success in using janitorial strength inhibited acid-based toilet bowl cleaner as a one-step chemical that can remove oily films at the same time as scale. The bottom line is to create a thoroughly clean surface for maximum ion transfer.
A third key element to peak pH sensor performance is calibration. Recognizing that pH sensors are in reality batteries with a “life,” the purpose of calibration is not only to keep the sensor accurate, but also to discover when it is nearing the end of its life so that it is replaced before it “dies.”
For pH sensor calibration, we recommend the following general procedure:
I am confident that if you follow our recommendations regarding buffer integrity, pH sensor cleaning, and pH sensor calibration, it will solve your problem and help you achieve maximum performance and efficiency from your wastewater pretreatment system. Let me know your results