Steve, your column in the September 2005 issue, non-hazardous waste has become hazardous, in which you describe how changes in process and wastewater pretreatment chemistry can have a profound impact upon the filter press cake’s status as a RCRA hazardous or non-hazardous waste reminded me of a situation we had about 10 years ago. At the time we had a captive metal finishing process and wastewater pretreatment system which had been producing a filter press cake that was non-hazardous. In fact, we tested it about every six months to make sure it remained non-hazardous. Out of the blue, the independent lab we used reported that chromium and lead were above their respective Toxicity Characteristic Leaching Procedure (TCLP) thresholds of 5.0 mg/liter. Needless to say we were in a panic and several loads had to be disposed as RCRA hazardous waste.
Fortunately, we had been using the lab for a number of years and had them go back to see if anything had changed. And, indeed, something had. For all the previous samples, the laboratory used TCLP extraction fluid #1 while for the filter press cake sample that flunked TCLP, they used extraction fluid #2. We then received a good education by the lab on the TCLP procedure (EPA test method 1311, Test Methods for Evaluating Solid Waste, Physical/Chemical Methods; EPA publication SW846).
When the filter press cake sample arrives at the lab, a portion is mixed with distilled/deionized water, and the pH of this slurry is checked. If the pH is less than 5.0, then the lab uses extraction fluid #1 which has a pH of 4.93.
If the pH is above 5.0, the lab takes another portion of the sample, mixes it with one normal (N) solution of hydrochloric acid (HCl), heats it up, allows it to cool, and again checks pH. If this pH is less than 5.0, the lab uses extraction fluid #1, but if the pH is greater than 5.0, the lab uses extraction fluid #2 which has a pH of 2.88, basically 100 times more acidic than extraction fluid #1. As expected, our filter press cake when mixed with the deionized water always had a pH above 5.0. In the past, after the acidification with hydrochloric acid, the pH would typically be between 4.5 and 4.9. Now, the sample exhibited a pH of 6.0 after acidification. What this told us was that, apparently, the alkalinity of our filter press cake had increased.
It did not take long to discover the reason. A few months before, our wastewater treatment operator found that by increasing the pH setpoints in our wastewater treatment system, the effluent became less cloudy and more clear although we had no compliance problems with the slightly “cloudy” effluent. We then restored the pH setpoints to their original values, waited several days to get the “old” sludge out of the system, and sampled filter press cake. Sure enough, its alkalinity had dropped so that the lab could use extraction fluid #1. We waited another week to make sure the process stabilized before taking another sample; it qualified for extraction fluid #1, and we had the TCLP performed on the sample, which passed just as in the past. Apparently, the increase in the filter press cake’s alkalinity was insufficient to overcome the more aggressive extraction fluid #2.
Feel free to pass this along to your readers. A.W.
Thanks, A.W., and I thought I was the only one to experience this problem. Because of this issue, whenever we have TCLP performed on a sample, we request the lab to also report initial pH, pH after acidification with hydrochloric acid, extraction fluid used, and TCLP extraction pH. This has solved some headaches as wastes change, and we become detectives to find out why.