Ask the Expert Question-and-Answer Archive (Wastewater Treatmnt)

by Mike McGinness, EcoShield Environmental Systems, Inc.
October, 2006

Metal in Wastewater

Q1: We mix different of metal compounds in 0.5 m3 reactor. When we clean reactor with 0.2% NaOH solution, the metal in waste water increase after sedimentation:

Cr 840 mg/liter
Cu 2.0 mg/liter
Mo 1.7 mg/liter
Se 0.65 mg/liter
Zink 110 mg/liter
COD 10000 mg/liter

If we use microfilter, can we reduce metal% ? or have you another suggetions ( Precipitation with SDDC /neuteriluization/ anionic polymer)!!!!!?

A1: If I understand your question, you can simulate a full sized microfilter with out buying one, by taking a sample of the waste water you intend to filter, split the sample into two samples, use a 0.5 micron lab filter to filter one of the two samples, then have both samples analyzed for total metals. Your local analytical lab can run the entire test including splitting the sample and filtering one of the two samples.

Why are you cleaning the reactor with NaOH? What pH are you using to settling out or precipitate the metals? Each of the metals you are dealing with has a different optimal pH for precipitation. Are you decanting, separating the rinse waste water from the sedimentation (precipitant) before final pH adjustment?

What other compounds are in the waste water mix, such as EDTA or other chelating agents? Is the Cr hexavalent? If so, it must be reduced to trivalent Cr with a chemical reducing agent at an acidic pH, before raising the pH. If this is not done first, the Cr will not precipitate.

Q2: I shall make the experiment and sent the samples to Lab. We mix different of metal compounds in stainless steel reactor (0.5 m³) for produce multivitamine tablet.

Compounds are: Sodium selenite, Sodium molybdate, CrCl3, Soppersulfat, MnSO4, Ferrous furmat, ZnO... mix with ethanol, shellac and sometimes with citric acid and Stearic acid .... depends on the product. This material builds up on the wall of reactor, so when we change the product shall clean the reactor. From experience the best with 2% NaoH (pH about 13) We wait for precipitate the metals then separation (about 2% solid material) We reduce pH to 9 (with HCl) and we wait for sedimentation We use Citric acid (crystal)!!! but not always

A2: Have you considered saving (storing) the waste water after sedimentation and using it the next time you clean the reactor to make the 2% NaOH solution?

Or even better, just filtering and reusing the 2% NaOH solution on the next clean out!!! If you filtered and reused the 2% NaOH solution you could add a smaller amount of NaOH to get back to the 13 pH before reusing it.

I am assuming that the NaOH cleaner is followed by a rinse.

Q3. It is good idea, but Quality Assurance QA, refuses to use solution two times (SOP for cleaning reactor and for production tablet).

A3. I understand, but I would try an convince them to justify their reasoning (other than blind fear) for not reusing it. At that pH, 13, it still has plenty of life as far as cleaning and removing solids off the walls goes and it is certainly going be sterile at that pH. The 2% NaOH is only used to loosen the solids. The final (hot?) rinse(s) is what it gets the final QA/QC cleaning job done. Ask them to consider the fact that when they drain the cleaner from the reactor, even after it its first and currently only use, how dirty it is with solids. Then compare that to a filtered, used cleaner that has enough NaOH added to it to restore the 13 pH. In other words they are already using a used cleaner moments after the new cleaner loosens some of the solids.

Additionally QA/QC testing of the reused cleaner can be set up. Other industries reuse cleaners this way all the time. They typically dumped the used cleaner when the M and P alkalinity ratio reaches a certain point.

Please let me know how the filter works.

Regards,

Mike McGinness