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BOD vs nBOD
Mark Milne
Posted: Friday, June 18, 2010 12:55 PM
Joined: 10/7/2009
Posts: 7


We are having some problems with our BOD limit. It has been over 30mg/l in the effluent, but our cBOD is running around a 9mg/l. We seed with our co-settled primary effluent but I was thinking of switching to poly-seed.

 

It is my understanding that nBOD isn't supose to kick in for 5-7 days, so it usually isn't included in the BOD results. But if the seed has a high population of n-bacteria that the nBOD will show up sooner. Switching to poly-seed would mean less n-bacteria and probably keep the nBOD out of our BOD results.

 

The big question is would this be cheating.

 

Mark


Anonymous
Posted: Friday, June 18, 2010 1:17 PM

"The big question is would this be cheating"


That is the funny thing about test methods and regulations isn't it.  As long as you follow the method, no one can accuse you of any wrongdoing.  The method allows you to use any seed that demonstrates acceptability according to your G:Ga solution and seed correction factor range.  I say give the polyseed a try.  It could help you with your reporting numbers.  Of course, if it bothers you to know that something is going wrong and it just isn't being addressed because of some technicalities, you might want to try to solve the problems.


dsmith
Posted: Friday, June 18, 2010 1:17 PM
Joined: 12/31/2009
Posts: 40


That post was mine.  I hate that I get logged out at random times.


Anonymous
Posted: Friday, June 18, 2010 2:26 PM

 "Of course, if it bothers you to know that something is going wrong and it just isn't being addressed because of some technicalities, you might want to try to solve the problems."

 

The solution is the new plant that is being built. The current plant struggles through nitrification each spring and fall. New plant is designed to nitrify all the time.

 

There are some other things going on this year; State highway that was supposed to be done last November isn't paved yet and the manholes are lower than the storm drains. They get raised when the asphalt comes; Asphalt hasn't come because it is the 2nd wettest May on record and June beat its monthly rain average by the second day; Rain events turn the primary clarifier into a ballasted floc tank because the grit chamber was removed so the new one could be built. Smooth operation isn't going to happen this year.

 

My preference for testing would be to run BOD and cBOD because it shows the problem and the likely number if we were just trying to produce good lab numbers.

 

Mark


Perry Brake
Posted: Friday, June 18, 2010 3:37 PM
Joined: 12/16/2009
Posts: 69


Mark, I don't think anybody could claim you are "cheating" if you use a commercial seed, which are designed to contain no nitrifying bacteria, because the method says you can use them.  But I guess I am wondering why you WANT to exclude nBOD from you results.  If your regulatory agency had wanted you not to include nBOD in what you report to them, they would require to to report only CBOD, and not BOD. 


If you do BOD with Polyseed or Biosystems seed, you are essentially doing a CBOD, and although allowed by the method, it probably isn't what your permit people are looking for if your permit says to do BOD.  If your primary goal is to use a commercial seed to lower your effluent BOD result, technically you are not cheating, but if I were your permit manager, I would write you up for violating the intent of the permit.  I'm a pretty easy going guy and would probably just tell you to quit doing in and instead work on getting the plant's effluent BOD numbers down.  A not-so-easy-going permit manager might start using terms like data integrity and falsification of data.  Not good!!

    



Anonymous
Posted: Friday, June 18, 2010 4:42 PM

Shouldn't there be a GGA type check for nBOD if nBOD is to be included? I would say nBOD is an interference to the BOD test unless there is some QA/QC to say the seed was good enough for a nBOD test. Winter time the plant is out of nitrification(low temp.) and while the nBOD food is there, there are no bugs in the seed to eat it. Winter ammonia limit is a big part of our up-grade. 

 

Mark


Perry Brake
Posted: Saturday, June 19, 2010 7:27 PM
Joined: 12/16/2009
Posts: 69


If you want to estimate the nBOD contribution of the overall BOD result for GGA, you could analyze a sample for both BOD, and for CBOD.  Do that several times and calculate an average BOD and average CBOD.  You could assume that the average nBOD contribution is the difference between those two.  I have never heard of anybody doing that, however.


James Royer
Posted: Monday, June 21, 2010 8:44 AM
Joined: 9/21/2009
Posts: 98


The use of seed without nitrifiers is not "cheating" but good lab practice. If you read the 21st ed. of Standard Methods it defines BOD as "the test measures the molecular oxygen utilized for the biochemical degradation of organic material and inorganic material such as sulfides and ferrous iron."

 

It also states that it can oxidize reduced forms of nitrogen. Oxidation of reduced forms of nitrogen, such as ammonia, have historically been considered an interferenceand including ammonia in the dilution water contributes an external source of demand. The interference from oxidation of ammonia can be prevented by an inhibitor.

 

Measurement including nitrogenous demand are not useful unless corrections are made for the oxidation of ammonia in the dilution water, which is cumbersome for each dilution.

 

If you have raw sewage there should be few nitrifiers to affect the results. If you have disinfected effluent there should be few nitrifiers. It is the intermediate samples that have nitrifiers. Seeding with fresh raw sewage or commercial seed material should introduce few nitrifiers to cause interference. Using a mixture of seed material with nitrifiers will cause problems because you can not differentiate between oxidation of ammonia in the sample and ammonia in the dilution water.

 

If you put a liltte glucose in the dilution water you would recognize that as an interferent, so a little ammonia in the dilution water would be an interferent if nitrifiers are persent!!!


Elizabeth Lawrence
Posted: Monday, June 21, 2010 10:33 AM
Joined: 9/22/2009
Posts: 7


I haven't found the poklyseed to work very well.  If the concerns nitirfying bacteria in the seed use settled influent which should not have any nitrifying bacteria in it.

 

 

Sue


Anonymous
Posted: Monday, June 21, 2010 5:57 PM

Hey Sue,

 

Thats what we are going to do. We just have to go a few manholes outside the plant. We had to divert the WAS to the old septage dumpers manhole for construction.

 

Mark

 

Nice to see you all!


GSain
Posted: Wednesday, August 11, 2010 3:46 PM
Joined: 8/4/2010
Posts: 6


By any chance, do you have WAS coming back upstream of your primary clarifier?  Have you tried raw sewage upstream of your plant as seed?  Its possible something could be putting a high level of nitrifying bacteria in your seed source.  If you can get more reasonable/representative results by just changing your seed source, I'd say you would be good.

 

War -story:  I once had a lagoon with a polishing pond that ran below capacity such that we had 25+ days HRT.  It held the water so long, we quit disinfecting the effluent.  Didn't need to; fecals were consistently in spec by the time the water reached the end of the system. 

 

Downside of this, is we really had nitrification taking place in the system, with a disproportionally high number of nitrifiers in the effluent (and the low pH's to prove it, but that's another story).  Rather than start back chlorinating to knock out the nitrifying bacteria, we went to the state and got a cBOD limit (change in analytical method) using nitrification inhibition in the analysis.  The cBOD limit was slightly more restrictive, but we never challenged it.

 

Moral to this story, if you change methods, run it by your permit writer first.  They may already have the regs set up to allow monitoring cBOD only, but you may have to get your permit changed to allow you to do it.


watertech
Posted: Tuesday, August 17, 2010 11:20 PM
Joined: 7/26/2010
Posts: 8


Hello Mark,

 

Perhaps it would be worth it to explore the possibility of changing your NPDES permit from BOD to cBOD values.

 

M. Sanchez

 


Anonymous
Posted: Thursday, August 19, 2010 3:49 PM

I am Back,

 

Yes, the WAS is entering above the primary, in fact for construction it is entering before the headworks. Our lab tech read SM 20 a bit closer and it sounds like we should have been using an inhibitor. The plant has converted into full nitrification mode so the difference between a BOD and a cBOD is about 2 mg/l.

 

Mark


Thomas Jenkins
Posted: Friday, August 20, 2010 8:25 AM
Joined: 1/13/2010
Posts: 8


I don't know current or local policy, but (as GSain indicates), some plants obtained approval to use nitrification inhibiting chemicals in their BOD samples when the results were obviously being distorted by unintended nitrification. I would run a series of parallel standard and inhibited tests and verify the difference is coming from nitirification, then go to your regulators.


Keith Chapman
Posted: Wednesday, August 25, 2010 10:32 AM
Joined: 10/2/2009
Posts: 35


Don't overlook one other source of nitrifying bacteria:  your sampler.  Nitrifiers prefer attached growth and will easily and quickly grow on the sampling hoses, pipes, etc.  We found that chlorinating the sampler hoses, etc. once or twice a week removed them and our effluent BODs and CBODs returned to being only a few mg/l different from each other.  It was a simple fix and we did not have to find another source of seed.