WEF Discussion Forums
Laboratory Management and Technical Issues
Effluent BOD from Membrane Bioreactor
We have recently started up a membrane Bioreactor. Usually we like the idea of very low BOD's, but my Effluent BOD's are so low they are less than 1 mg/l and less than 2 mg/l depletion. GGA's, seed, and influent numbers are good. We have UV disinfection so no chlorine. I don't know about all regulators but they really don't like to see nondetects on the reports. I am getting less than 1 mg/l on TSS, non detect on ammonia, non detect on fecal coliform. Is anyone else having this issue? What dilutions are you using?
It is possible that you really do not have any BOD in your water. As long as you are using the maximum possible volume of sample and report your non-detect as less than your reporting limit, there isn't anything that can be done about it. If anyone raises a stink about the non-detects, ask if they want you to make your effluent less clean.
There are many thousands if not a few million people out there that wish they had your "problem", Elizabeth!
Since you are analyzing more than 200 mL of effluent in the 300-mL bottle (I assume), you must make sure to use the "special" nutrient/buffer pillows made especially for such samples where the chemicals are added directly to the bottle. If you don't use "pillows", you can add 1.1 mL of each of the reagents directly to the BOD bottle. I'll assume you are doing one of those.
You must add sufficient seed to give you a depletion (seed contribution) of 0.6 - 1.0 mg/L (so says the method and EPA). Let's say you add enough seed for a 0.6 mg/L depletion, and there is no additional depletion for the bottle caused by the effluent. You will have to check with your data user on how they want you to report such a result. They might tell you to report it as a "non-detect", or they might say to report "<1.4". An easier solution would be to use the reporting rules in a pre-21st Edition of Standard Methods and report "<2.0 mg/L", the minimum reporting level in the 18th, 19th, and 20th Editions.
I really think the best advice anybody can give you is to "report as directed by your data user" for such cases.
Thanks for cleaning up the environment!!
Although this won't solve your "less than" problem, why not let the samples incubate longer than 5 days? If you find more depletion you will at least know that the "bugs" are doing their job. Another option would be to incubate a full 300 ml (no seed, no dilution water) to see what sort of depletion occurs. This "full volume" method isn't really equivalent to 5120B, but we use it on river samples that have BOD's less than 2 mg/l.