Feedback on Question About Neutralization

In the May 25 issue we ran a reader question from Frances Greenspan, ISO/TL administrator, Viking InterWorks:

"I need to come up with a way to test the pH level for a 100 gallon tank and instructions on how to neutralize the pH level before it is sent down the drain. Test strips make sense to me, but correcting the levels with the measurements of baking ash (sodium carbonate) vs baking soda (sodium bicarbonate) is the kind of area that I am thinking. I need to include this in an SOP for the manufacturing floor maintenance crew so it has to be pretty simple (i.e. if the strip is this color, add 1/4 cup baking soda, wait 5 minutes, retest). Thanks for your help."

Following are responses received:

Robert J. Lee, PE, Environmental and Waste Management Services, Brookhaven National Laboratory: Since there are many factors that impact the neutralization process, I recommend taking a small sample of the wastewater, doing a bench top neutralization noting the amount of neutralizing agent added during the process then scaling up the amount of neutralizing agent needed to the remaining tank volume. All neutralization reactions should be done slowly to prevent heat build-up, minimize off gassing and potential foaming. Staff also should be equipped with appropriate PPE such as goggles or face shield, protective gloves and apron, and also be performed with appropriate ventilation.

Derreck R. Copsey, PE, Sr. process engineer, Church & Dwight Co. Inc.: Assuming that "neutralization" is meant as 6.0<pH<9.0, sodium bicarbonate would be a better fit than sodium carbonate in a "simple" application. The reason that sodium bicarbonate may better serve you in this application is that the maximum pH of a saturated solution (at 25°C) is 8.2 std units while the maximum pH of a saturated (at 25°C) sodium carbonate (soda ash) solution is near 12.0 Std units.

Martin Graham, manager maint. support, U.S. Postal Service: I have a swimming pool and I test the pH every week using a pool test kit. The more advanced test kits allow you to add additional chemicals to your original sample to determine how much soda ash or acid is required to give you your required results. Stop by a swimming pool store.

Robert J. Schoenewe, CPG, REP, environmental department manager, ATC Associates Inc.: Based on experience, you will be chasing the titration curve all over the place. The adjustment of pH is not as simple as adding a base compound to an acid. You could easily turn the acidic liquid to a caustic. Additionally, mixing of the liquid is necessary to insure a homogeneous pH.

Randy Putnam, environmental engineer, Intertape Polymer Group: Rather than using sodium carbonate or sodium bicarbonate, magnesium hydroxide has the advantage that, no matter how much you add, the pH will not rise above 9. It can be added to excess in the tank, and if kept well mixed will continue to neutralize added acidic material. It buffers out a pH 9, so it meets the Clean Water act limits of pH 6 to 9. Magnesium hydroxide is also relatively safe (it is milk of magnesia) so handling it is generally not a problem. The downsides are that it is more expensive than the soda products and it forms a slurry, like baking soda, rather than dissolves.

Michael J. Zimmerman, PE, REA, senior project manager, Clayton Group Services: It sounds like you have a simple batch system and want a simple solution. Using color strips and manually adding caustic agents such as baking soda may seem simple but this system could be a real headache. Typically, we'll recommend that a facility install a pH meter and a small metering pump that is connected to a 5-gallon container or 55-gallon drum of caustic. Your tank is small, but we also recommend that a facility allow as much material as possible to enter the tank if different pHs are present because this can neutralize the solution and require less caustic addition. In the long run, installing a pH meter and metering pump for $500 or so will save you labor costs and minimize potential violations due to human error.

Perry E. Mayer, City of Le Roy Water Department: The adjustment for your application is going to be very dependent on the accuracy of the pH testing. If your margin of error is high I believe that test strips are the way to go. If the margin of error is low the more expensive equipment may be necessary. The concentration of the sodium carbonate is another consideration that must be taken into account, in order to correctly add the appropriate amount. I have included a link to one site that talks about these factors -- there are many others, http://www.fmcchemicals.com/Content/CPG/Images/Caustic.pdf.

Bill Bahr, Department of Environmental Quality, State of Montana: I suspect that a relatively inexpensive pH meter would be far better and give instantaneous readouts. The pH could be readjusted and pH values monitored in real time to accomplish neutralization before discharge.

Jackie McHugh, manager, Environmental & Safety Compliance, Sempra Energy: There is not a simple answer to your question because what is not known is the buffering capacity of the solution that you are trying to neutralize. You can have two tanks with exactly the same pH but one tank might take twice as much sodium carbonate or bicarbonate than the other because it has a stronger buffering capacity. The tank is small (100 gallons) but you don't mention what the typical starting pH is or what the target pH is. Also, how will the tank be mixed? With so many unknowns my suggestion would be to add the sodium carbonate or bicarbonate in small increments (2 oz.), mix well, and wait about 10-15 minutes to recheck with the test strips until you reach the desired pH.

Don W. Young, PhD, WESTWATER LLC: It would be helpful to know what constitutes the waste material. Neutralization of an acid metal salt could cause precipitation, with sludge disposal problems of their own. Perhaps you could correlate conductivity (meters are cheap) to pH and simply add carbonate or bicarbonate until the needle reads right.


*The publisher is not responsible for the contents of the comments herein, and any person following the advice or procedures does so at his or her own risk. Neither the publisher nor any contributor will be responsible for any error of omission in the information provided and/or for the results obtained from the use of such information.

This article originally appeared in the 06/01/2005 issue of Environmental Protection.

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