pH neutralization is an essential wastewater treatment process in many industrial manufacturing environments. Because a chemical reaction is taking place and is usually being controlled by sensitive instrumentation, pH neutralization systems can be trouble-prone. Here are some tips to help diagnose the most common issues. This first thing to do when troubleshooting pH neutralization equipment is to clearly identify the problem. This involves three basic steps:
- Check the mechanical operation of the equipment
- Check the operation of the pH electrodes
- Determine if the process wastewater has changed
Mechanical Operation
The first step when troubleshooting pH neutralization equipment is to review the mechanical operation of the equipment. This is the easiest and normally the first thing that you should check. Check the following items:
Mixer:
- Proper Rotation: The direction of the rotation should be away from the point of discharge and the wastewater should be pushed down.
- Speed: Fast enough to provide good mixing, but slow enough to prevent splashing.
- Pumping Capacity: Determine if it is sufficient to provide a dead time of 0.05.
- Blades: Should be propeller type. Check to make sure that all of them are on the shaft.
- Power: If the mixer is not operating, make sure that there is power to the mixer.
Chemical Feed:
- Power: If the pump is not pumping, make sure there is power to the pump.
- Receiving Signal: Check to determine if the pump/controller valve is receiving a signal from the controller. Use a buffer solution or change the pH set point to determine if the controller is sending a signal to the pump/controller valve.
- Poor Pump Performance: If the discharge pressure is low, this may be an indication of a clogged inlet or outlet condition.
Control Valves:
- Respond to Signal: Test to determine if the control valve is receiving a signal from the controller. Adjust either the pH to above the existing measured pH if an alkali is being used (or below, if an acid is being used). Observe whether the control valves attempt to open.
- Leaking: Change the set point to completely close the valve and observe whether the valve is leaking. An obstruction in the valve or the corrosion of the valve may be the reason for the leak.
- Clogged: Another reason for the control valve not responding to a call for an increase in the amount of reagent is valve clogging. With the pump off and all the pressure upstream and downstream of the valve released, remove the control valve and inspect for debris. Corrosion may also be the cause of the valve not opening.
- Draining: The control valve should be as close to the delivery point as possible. Check the reagent delivery piping arrangement to determine if the end section is free draining.
Reagent:
- Inventory: If the pH is not being adjusted, there may not be any solution in the reagent tank. Check to determine if there is enough solution.
- Concentration: Check the concentration of the reagent to determine if it has been changed. If the solution was more dilute than normal, the chemical feed pump or control valve will operate at its highest capacity and will not be able to keep up with controlling the pH. If the solution is too concentrated, the pump will operate at the low end or nearly closed and will not be able to control the pH properly. Remember a small amount of reagent dramatically changes the pH.
- Proper Reagent: Check to make sure the reagent you are pumping is needed for the process. It is possible the wrong reagent was put into the tank.
PH Electrode/Sensor
Unless physically abused by handling or solids in the wastewater, the pH probe should last nine months to two years in most systems. To keep the handling to a minimum, you might use the following method for checking the calibration of the pH electrode:
- Take a sample of the wastewater noting the pH reading by the existing pH electrode.
- Then immediately measure the pH in the laboratory with an electrode that has been calibrated with two buffer solutions.
- Standardize the field pH using the differential between the previously noted measurement and laboratory reading.
The table below summarizes the different conditions and symptoms of the common sources of pH probe errors. Just the sheer quantity of sources shows you that pH measurement accuracy can easily deteriorate, and that pH measurement trouble-shooting can be difficult. By identifying the symptoms, the problems that do develop can be remedied.
pH Probe Errors:
| Source of Error: | Symptom: |
| Bulb broken | No response (4-6 pH) |
| Fill contamination | No response (4-6 pH) |
| Bulb abrasion | Slow, erratic shortened span & upscale pH |
| Bulb dehydration | Slow, erratic shortened span & upscale pH |
| Bulb etching | Slow, erratic shortened span & upscale pH |
| Partial bulb coating | Very slow |
| Complete bulb coating | No response |
| Low temperature: | pH increases with temperature |
| Probe wiring cut: | pH fixed at around 7 |
If a new electrode is needed, the same procedure can be used except that before calibration the new electrode should be immersed in the process liquid for no less than twenty minutes and up to two or three hours for solid state reference electrodes in a highly ionic solution. If you have a three-electrode system, it is easier to tell if there is a problem with an electrode. With three electrodes you would remove one only if it differed by more than 1.0 pH unit for more than 10 minutes after repeated process calibration. If one electrode is coated, it will lag behind the other two. If two or all three are coated, there will be a loss of movement of the readings with respect to each other.
Manufacturing Process Changes
Changes in chemicals, the amount of acid or base, changes in the amount of production, or changes to the manufacturing procedures can create a change in the characteristics of the wastewater and, therefore, a possible change in the titration curve(s) and controller. In this instance you should check with your contacts in the production department to see if any changes were made. One of your tools to determine if a change has been made or if the wastewater has changed in its characteristics is the titration curve. If different processes discharge different acids and bases or different ionic strength, then more than one curve will have to be created and compared. If the titration curve is different from the one that was originally used to program or set constants on the controller, you may need to tune the loop using the new titration curves.
Back to the Basics:
If all else fails when troubleshooting pH neutralization equipment, go back to the basics of what makes a good pH control system. Examine whether the equalization tank is correctly sized and is being mixed in the right direction. If you do not have an equalization tank, then you should have at least two reactors in series with mixing in the first and in the second. Checking the electrode, transmitter, and controller dead time determines if there is a built-in lag between the time of reagent addition brought about by a change in the influent pH and the time it is noted by the pH meter.
You may also check to see if the concentration of the reagent has increased or if it is a different chemical altogether. Either of these could require a longer residence time to react. Check to see if the electrode is being affected by its environment. Is the pH so high that it causes an alkalinity error or so low that it causes an acidity error? Has the temperature of the wastewater changed? Is the velocity past the pH electrode fast enough to provide a quick response time? A review of all these factors should reveal the problem. With the problem identified, you can proceed in a step-wise manner to correct the problem or modify the equipment to meet the new conditions.
Remember, since pH neutralization systems are very sensitive and trouble-prone, it is wise to invest in preventative maintenance to avoid costly breakdowns. An ounce of preventative maintenance is often worth a pound of breakdown maintenance.
For More Information:
If you are having trouble diagnosing an issue with your pH neutralization equipment, please contact us and we can help get you up and running.
