Little change, big Results

Now our wastewater treatment operator pays much more attention to the firmness quality of the sludge, and we send out a sludge sample for total solids analysis on a monthly basis. Over the last several years, our annual sludge analysis for TCLP chromium has given results within historical norms.


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Q: I just had the opportunity to read two of your articles: “Non-hazardous waste has become hazardous” from Sept., 2005 and “Non-hazardous waste becomes hazardous revisited” from Nov., 2005. These articles reminded me of an incident that occurred several years ago at our plating job shop where I’m the Production Manager and also wear the environmental compliance “hat.” At our facility, we perform several types of acid and alkaline zinc plating with trivalent chrome dips.

Because of our conversion to all trivalent chromates, we decommissioned our wastewater chrome reduction system. For many years, our wastewater sludge wasn’t a hazardous waste since it passed the Toxicity Characteristic Leaching Procedure (TCLP) for the eight regulated metals: arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver; only chromium would show up in the test results, usually around?0.5–2?mg/L as compared to its TCLP threshold of 5.0 mg/L.

As you described, we always requested information on the leaching solution used and, consistently, the more acidic and aggressive #2 solution was used by the testing laboratory. We typically sampled and analyzed our sludge once a year to provide ongoing evidence that is was not a hazardous waste. Also, the Subtitle D sanitary landfill into which it is disposed requires recertification every two years.

?Out of the blue, our annual sample exhibited a TCLP for chromium of >10 mg/L. We had the lab recheck its procedures and calculations, and they found no issues. We quickly sampled again and found about the same TCLP chromium concentration.
We then took another sample, mixed it well, and split it—one sample going to our normal lab and the other to a second lab. Both labs reported elevated TCLP chromium concentrations.

We even had both of the labs run a distilled water leaching procedure and check for the presence of hexavalent chrome to verify it wasn’t coming from the trivalent chromates. These results came back non-detectable for hexavalent chrome.

I had one of my staff go through our files, find as many of the sludge analyses as possible and place the results in a spreadsheet in chronological order; in addition to TCLP chromium, we also had the sludge samples analyzed for other parameters. We were attempting to evaluate the data to see if there had been any other changes. After assembling the data, we discovered that one parameter had changed significantly: total solids.

For many years, our sludge’s total solids ranged from 20–25%; beginning with the first sample to fail TCLP and the subsequent samples that also failed, the total solids ranged from 15–18%. We theorized: Could it be that with a lower amount of solids in the sample, there is more acidic leaching solution to attack the chromium hydroxide, since the sample is mixed with 19 times its weight of leaching solution? And second, what caused the total solids to drop significantly?

We reviewed our filter press operations and found?two changes. Our wastewater treatment operator stated that recently he noticed that the sludge had exhibited a mushy middle instead of the firm middle he had seen before. And, second, the time to fill the press had increased somewhat. On a monthly basis, we had cleaned the filter press clothes with acid and alkaline cleaner; however, no one could remember when we last purchased new clothes. Could the filter press clothes be worn out?
To make a long story short, we purchased new filter press clothes, ran several loads through it, and then sampled and tested sludge for total solids. After the lab reported a solids content of about 22%, we instructed them to proceed with the TCLP analysis. The result for?TCLP chromium was about 1 mg/L. We repeated this two more times over the next week, and both the total solids and TCLP chromium results returning to historical values.

Now our wastewater treatment operator pays much more attention to the firmness quality of the sludge, and we send out a sludge sample for total solids analysis on a monthly basis. Over the last several years, our annual sludge analysis for TCLP chromium has given results within historical norms.

Thought you would like to hear our experience. M.E.

 

A: Thanks so much for sharing this experience. It never ceases to amaze us how small changes to processes can result in significantly bad and costly results. I would like to ask our readers to tell us if they, too, have experienced surprising or unusual changes in the quality of their filter press cake (“sludge” is such a dirty word) so much so that it inadvertently became a RCRA hazardous waste or became difficult to dispose.

Also, did you ever consider what you would have done next if restoring the filter press cake to its previous and higher solids content?did not restore TCLP chromium to below its regulatory threshold? How about injecting a “soft” or weak alkali, such as sodium bicarbonate (baking soda), calcium carbonate, or magnesium hydroxide into the sludge transfer line from the clarifier to the sludge holding tank? This would increase total alkalinity of the sludge to help resist the acid leaching solution without significantly increasing the pH of the water fraction and, possibly, redissolving some of the zinc hydroxide. Also, you could consider partially drying the filter press cake to raise the solids content. I am sure our readers could also tell us of other feasible possibilities.

Again, thanks for your input. 

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