Wastewater pH Control Tips
Tuesday, May 30, 2017
Many industries have to control the pH of their process wastewater in order to comply with local and state sewer discharge permit limits. Wastewater pH control can be very frustrating, costly, and even dangerous if it is not done properly.
The following tips will help ensure the success of any wastewater pH control system:
- A minimum retention time of 10 minutes is needed for the neutralization reaction.
- pH control should be done in a tank where the depth is greater than the width or diameter.
- Mixing is critical and must be as aggressive as possible (air or impeller-type mixer).
- Round pH control tanks need anti-swirl baffles for effective mixing.
- Stainless steel, fiberglass, and polyethylene are good materials for pH control tanks.
- Concrete tanks need an appropriate protective coating for the interior (i.e., epoxy).
- Influent and effluent piping should be located opposite from each other vertically and laterally.
- Neutralization chemicals should be injected near the influent piping location.
- 25% sodium hydroxide is the most powerful and flexible alkaline reagent.
- 93% sulfuric acid is a powerful acidic reagent but is extremely dangerous.
- Acid should never be injected close to the wall of a tank, even one with protective coatings!
- Carbon dioxide gas is a much safer alternative and can be more cost-effective than sulfuric acid.
- All chemical feed pumps should be located inside clear plastic enclosures to improve safety.
- All chemical feed pump tubing should be located inside PVC piping for containment.
- Chemical feed pumps should be sized for both fine (proportional) and coarse (high-demand) control.
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- The pH probe used for control should be located near the effluent piping.
- All pH probes should be mounted in a way that facilitates easy removal for cleaning/calibration.
- A pH transmitter should be located near the probe so one person can perform calibrations.
- Probes should be cleaned daily and calibrated weekly.
- Probes should be calibrated with 4.0 and 10.0 buffers at a minimum.
- Fats, oils, and grease (FOG) in a process can coat a probe and “blind” it very quickly.
- Toilet bowl cleaner is a good general-use cleaner for pH probes.
- A dilute muriatic acid solution is a good cleaner for removing FOG from probes.
- A separate pH probe should be used to verify compliance after the pH control tank.
- Circular chart recorders are the best choice for permanent recording of control and effluent pH.
denotes original PSARA content