In my last newsletter I stated something incorrectly and the EMC kindly pointed me to the correct information. I stated “There has still not been an official change to adjustment formula in the CFR to use either O2 or N2 for the adjustment under the method,” which was incorrect.
In fact, there had been a correction made to the adjustment formulas in the August 30, 2016 Federal Register and I had missed it. There is still a small problem with the new formula criteria for use. Although it corrected the worst cases, it still does not affect the more common but lesser biased samples.
The new criteria for use of the formula is stated in two sections,
12.5.1 NMOC Concentration with nitrogen correction. Use Equation 25C-4 to calculate the concentration of NMOC for each sample tank when the nitrogen concentration is less than 20 percent.
12.5.2 NMOC Concentration with oxygen correction. Use Equations 25C-5 to calculate the concentration of NMOC for each sample tank if the landfill gas oxygen is less than 5 percent and the landfill gas concentration is greater than 20 percent.
This wording only seems to allow the correction for oxygen when the nitrogen exceeds 20 percent and not if the ratio is significantly different from that in ambient air. I was initially told the purpose of this correction to the NMOC was to adjust for the intrusion of ambient air via leakage during sampling, which would make the ratio a critical aspect under those circumstances. If you have ~20 percent nitrogen in ambient air you will have ~5% oxygen based on the known ambient air ratio. The potential bias this creates for those samples with less than 20 percent nitrogen is less than the samples above 20 percent, but it is still significant. Using these criteria there would not be a different adjustment for oxygen concentrations if the nitrogen concentration was 20 percent, even though there could actually be anywhere from 0 to 5 percent oxygen in the sample.
To put this bias into perspective for the effect on the NMOC concentration, if you have a sample with ~1100 ppmC NMOC with no oxygen and 20% nitrogen, it would be adjusted to ~1480 ppmC NMOC. This would be the same adjustment for 5 percent oxygen if only the oxygen concentrations were considered for the adjustment. If there was no oxygen in the sample there would not have been nitrogen added from ambient air and no correction would be needed. If the oxygen concentration were any other concentration up to 5 percent the adjustment would be more correct and the bias would be lessened, but there would still be a positive bias.
This would be a benefit for those landfills in arid conditions where we see almost no oxygen and 40 percent nitrogen, which are the absolute worst case examples. However, it does not presently address the more commonly seen concentrations in the landfills elsewhere. As a result, we will continue to rely on the instructions in the original email and using the ratio of the oxygen and nitrogen in ambient air as the determination. Whichever component gives the least adjustment will be the one we use thereby preventing any potential bias from that adjustment.
In looking up the information I noticed from the dates on the email, it was one month shy of exactly 14 years before the new changes were published. I can only imagine how many opportunities there were over that time frame for there to be a hiccup in the application of the formula especially since other than myself, everyone discussing it at the time has retired since then.
On an associated note, the question of method specific hold times often comes up. As there are no hold times listed for either Method 25-C or Method 3-C we generally try to use a default 30 day limit as would be applied to other types of canister samples. This is not to say the samples would be invalid after that time, but that it is a general target for hold times. To my knowledge, there have not been any significant studies published on either hold times. The Method 3-C should not have any difficulties in maintaining sample integrity over time due to it only dealing with higher concentrations of the fixed gases, which are not considered reactive and should be stable for longer periods. The Method 25-C should be more stable than the normal canister sample since it really does not matter if there are any reactions as long as the carbon is not taken out of the gas phase. That is the beauty of a carbon counting method over time. The only real concern would be the integrity of the valves on the sample canisters and possibility of ambient air leakage over time. Since we have documented the ability of these tanks to maintain a specific vacuum for six months or more, this is not as much of a concern, The small amount of vacuum generally left in the canister after sampling should make leaks even less likely due to a decreased pressure differential. Any leakage should not have a major impact on the Method 3-C results before the pressure reached atmospheric. The Method 25-C is also adjusted to an air free basis so any small amount of leakage from any source should not cause the sample to be invalid or prevent the adjustment offsetting this leakage in the reported concentration of NMOC.
Triangle Environmental Services, Inc.
P.O. Box 13294 122 US Hwy 70 E
Research Triangle Park, NC 27709 Hillsborough, NC 27278
(919) 361-2890 (800) 367-4862 Fax: (919) 361-3474