Wastewater Discharge—Cyanide Excursions

Q. Our plant is experiencing wastewater discharge permit excursions on its limits for Total Cyanide (daily maximum of 0.32 mg/liter, monthly average of 0.14 mg/liter) as regulated by our very small town. Sometimes it is the daily maximum, while at other times it is the monthly average that is exceeded.

The only source of the process wastewater is our five-stage washer where metal parts are cleaned and phosphated prior to spray painting. Our washer is described as follows:

          Stage 1 is the initial wash using a low- temperature caustic detergent,
          Stage 2 is a water rinse with a dilute phosphoric acid feed,
          Stage 3 is a cleaning step plus the iron phosphate coating,
          Stage 4 is a water rinse,
          Stage 5 is an additional wash tank that includes a surface sealing agent; at the exit of this stage is a fine water spray rinse.
The well water supply has been checked for possible cyanide content and none was detected. The washer’s chemicals have been tested, and some do show cyanide concentrations; the chemical supplier suggests that there are components in these chemicals that may result in a “false positive” test result.

The laboratory insists that they use the approved EPA analytical protocol for total cyanide and can include a provision for adjusting for possible interferences.

I have about run out of places to look or things to test for and would appreciate any suggestions you have. B.G.

 

A. B.G., looks like you have joined the huge crowd of metal finishers who have experienced the elusive and very troublesome cyanide “ghost,” that is, finding cyanide when it is totally unexpected. The other typical challenge as I reviewed your data is that the cyanide “ghost” appears then disappears, making its source difficult to detect and solve.

Since you did not mention it, we assume that there is no wastewater pretreatment system for metal removal, which one could expect for iron phosphate wastewaters.

As a reminder, make sure that you are using new, clean sample bottles (glass or plastic is acceptable) and that the samples are preserved with caustic and ascorbic acid.

After looking over your cyanide results of your washer chemicals, even assuming they are accurate and not caused by false positive interference and considering the likely dragout rates and rinse water flow rates, these concentrations are not high enough to cause your noncompliance.

Another issue to pursue is to verify with your chemical supplier that ferrocyanides are not contained in their products; these will show up as total cyanide and are very difficult to treat.

The likely source of your cyanide “ghosts” are positive interferences regarding the analytical method. Your lab is very likely using EPA Method 9010B - Total and Amenable Cyanide by Reflux-Distillation and then followed by either EPA Method 9014 - titrimetric and manual spectrophotometer determination of cyanide or EPA Method 9213 - ion selective electrode. In essence, during method 9010B, the sample is acidified and boiled, driving off hydrogen cyanide gas; the hydrogen cyanide is captured and absorbed into a caustic solution, and the caustic solution is analyzed for cyanide. Known positive interferences include sulfides and sulfur compounds that form sulfides during the reflux-distillation, nitrates, and nitrites. In addition, fatty acids, detergents, surfactants, and some organic compounds can cause foaming during distillation that may cause some difficulty in detecting the titration endpoint of method 9014. Many, if not all, of these substances could be present in your wastewater. Fortunately, your laboratory can take steps to eliminate many of these interferences.

Another possibility is that your laboratory is not conducting the analysis as carefully as you need it to be. The monthly average limit of 0.14 mg/liter is not very much above the methods’ detection limits; this presents a huge challenge to you and the analyst in that any hiccup during the analytical procedure can have a profound and negative compliance issue. The only way to check this out is to split samples among two to three labs and compare results.

If after your lab employs methods to reduce or eliminate the positive interferences and you are still experiencing total cyanide concentrations that are causing non-compliance, I suggest you approach the town and request if you can use cyanide amenable to chlorination (aka CATC or CNA) as an alternative parameter. This will likely require you to perform extensive analysis for both total cyanide and CNA. For CNA analysis, a portion of the sample is run through the reflux distillation to obtain total cyanide while the other sample portion is first chlorinated then processed through reflux distillation; the difference in results is reported as CNA. Hopefully, the positive interferences cancel out as the subtraction is made to calculate the result. Also, for CNA analysis, the laboratory can use EPA Method OIA-1677 - flow injection by liquid exchange which is much less susceptible to interferences with sulfides that only major concern.

As I said before, you have a great challenge with very low cyanide limits. As a reference, the metal finishing pretreatment standards (40CFR433) for total cyanide are 1.20 mg/liter and 0.65 mg/liter and cyanide amenable to chlorination are 0.86 mg/liter and 0.32 mg/liter, daily maximum and monthly average, respectively. Good luck and good hunting.

If readers have any other ideas, we would be happy to hear them and pass them along.