We are having an ongoing “battle” with our local sewer district over wastewater monitoring cyanide results. Our facility is regulated under the Metal Finishing Pretreatment Standards, and we are required to sample and analyze for total cyanide at the exit of our cyanide treatment tank. Consistently, our own self-monitoring and split sample results for total cyanide are well below the 1.20 mg/liter daily maximum and 0.65 mg/liter 30-day average, in fact, more often than not it is non-detectable. However, the sewer district’s samples usually show total cyanide concentrations well above our results, and some of the results are even above the limits. Based upon their results, we are now facing a proposed fine of several thousand dollars. We have thoroughly examined our instrumentation and chemical feeds and everything checks out. Do you have any recommendations?
You are experiencing what I believe is the most frustrating and troubling aspect of compliance with the Electroplating and Metal Finishing Pretreatment regulations and will become even more so if the Metal Products and Machinery (MP&M) regulations for cyanide are finalized as proposed. Unlike metal analysis, cyanide analysis requires a distillation step that is wrought with potential positive interferences, that is interferences that could give a false elevated reading; potential positive interferences include sulfides, fatty acids, aldehydes, carbonates, glucose, nitrates, color and turbidity. Also, cyanide problems are like ghosts, here this minute and gone the next.
Since you have checked your instrumentation and chemical feeds, I will confine my comments to treatment tank design, sampling and analysis. First, I would change your relationship with the sewer district from “battle” to “problem solving.” That is your desire to work with them to solve the cyanide monitoring problem so that you and the sewer district are confident of the results and solve any cyanide compliance issue that may really exist.
Second, I would carefully review your treatment tank’s design. Are the tank’s inlet and outlet pipes located so as to minimize short-circuiting? For example, is the inlet pipe located near the bottom of the tank while the outlet pipe is located on the opposite side and at the top of the tank? Are the chemical feeds located near the inlet pipe while the pH and ORP probes are located near the outlet pipe? Is the mixer providing vigorous agitation? Do you have at least 20 min of retention time at peak flow? Assuming all this checks out, I would then focus on the sampling and analysis.
Next, I would compare notes with the sewer district on how the samples are taken and preserved. Are they taken at the cyanide treatment tank’s exit or just nearby? You may want to install a labeled sampling tap on the exit piping before the next treatment tank to provide a consistent sampling point. What preservatives are used? Sodium hydroxide to raise pH above 12 and refrigeration are the EPA approved preservatives, but the sewer district may also be using sodium thiosulfate or citric acid in order to “kill” any residual chlorine in the sample. This residual chlorine could continue to react with any residual cyanide in the sample bottle as it waits for analysis. Is the sewer district and your laboratory analyzing the sample within the EPA approved holding time of 14 days?
Last, I would have the sewer district’s laboratory personnel compare notes with your laboratory personnel. Is there QA/QC data that exists that can be reviewed? What EPA approved methods are the labs using? If different, can they agree on the same method for the sample matrix? Are the samples being treated for possible interferences? If not, can the labs agree upon an approved pretreatment method to remove possible interferences? Are the sample sizes used for distillation the same or different? If different, can the labs agree on the same sample size? In summary, the goal is to have both labs performing the analysis in the exact same way in order to eliminate as much variability as possible.
Whether or not you are able to obtain the sewer district’s cooperation in this strategy, we would also recommend that you use two separate independent laboratories to perform the cyanide analysis using the exact same method. Assuming that these labs obtain the same results as you had in the past, it will give you greater leverage for the sewer district to change their method or to document that the agreed upon method is achieving consistent results.
If you still discover that your total cyanide is periodically near or above your limitation due to cyanides that will not be removed by chlorination, such as iron or nickel cyanides, you have the option under the Metal Finishing Pretreatment standards (40CFR 433) to request an alternative cyanide amenable to chlorination (aka cyanide-A) limit of 0.86 mg/liter daily maximum and 0.32 mg/liter 30-day average. Cyanide-A analysis is typically more expensive than total cyanide and is arrived by subtraction; the total cyanide concentration of a sample chlorinated by the laboratory is subtracted from the total cyanide concentration without chlorination. Again, having the sewer district and your laboratories performing the analysis the exact same way is still crucial; however, we have found that the cyanide-A limit is easier to consistently achieve.