WEF Discussion Forums
Laboratory Management and Technical Issues
We've been getting low GGAs (130-160) for the past 2 weeks.
Can't figure out the causes.
We have good initial D.O.(8.6-9.0)
Blank of < 0.2 mg/L.
Consistent Polyseed range; seed = 50-60 mg/L, using 6 mL seed/bottle.
What are your depletions for the seed and for the seeded GGA? Is this a new lot of Polyseed? Or have you had good results with this lot?
I always have another manufacturer's seed in house that has worked properly so as I can compare different seeds if there is a problem. I also seed with some fresh raw sewage as another known seed source for comparison. The problem can then be pinpointed rather quickly and corrected.
Andy, you haven't given us much information to work with. For example, you say your GGAs are 130 - 160 for the last 2 weeks, but what were they before that? Your blanks are <0.2, but what were they before the low GGAs kicked in? If they used to be higher than 0.2, that alone could be a big factor in the suddenly low GGAs.
Why don't you take a little time and fill out a chart containing the info below for 20 or so batches before the problem, and another chart for the period when the low GGAs have been experienced. Also not any change that happened in you procedure at the beginning of the problem period. Here's what would help, and the info should be for each bottle...not for an average where duplicates or multiple dilutions were done. Include wastewater samples only if you want to...there is not much to be learned from wastewater results.
Sample Volume Seed Vol D-1 DO D-5 DO
That may seem like a lot of work, but the information is critical to discovering the problem unless the cause is something simple, like "I used to use primary effluent for seed, and now I use Polyseed." If you already have digitized data and can covert it to a Word doc or photo file, you could simply attach the files on a posting at this Technical Discussion page.
Thank you Perry and sorry to Perry and everyone for not getting back sooner.
I've been frustrated about the low GGAs and tried to solve the problem. I used the primary effluent as seed and the GGAs worked out great. The reason we abandoned the sewage seed b/c of the 0.6-1.0 corrective factor method restriction. Now our auditor does not abide by the corrective factor any more as long as PT passed. Andy
If your like me I used get hung up on the .6 to 1.0. If your read standard methods closely, it states, "should be between .6 and 1.0. Some regulators dont care if it comes out to 1.7 and you GGAs come out, then OK.
The seed correction of .6 to 1.0 is not a requirement but it is still a good guideline. If our seeding material does not meet this I would check out other seeding materials until I found one that does and still meets the proper GGA value.
I always worry about nitrification when the GGA value is only in range when I use sewage for a seed but can not get the proper value with a purchased seed that I know contains no nitrifiers.
James correctly said:
"The seed correction of .6 to 1.0 is not a requirement but it is still a good guideline."
The reasoning behind this is that a seed that depletes too much Oxygen will leave little left for the actual sample depletion.
Having as much range as possible will ensure that at least 2 bottles deplete within the 1.0 residual and 2.0 depletion limits.
If you only get one good bottle, that is ok, but getting 2 good bottles ups the confidence level greatly. Getting 3 good bottles hits the jackpot and you get the bonus of detecting toxicity.
I'm not an expert but my experience is and have been told by people who sell the commerical seed, you will eventually fail a GGa with that type of seed. I've been threw it a lot in the last couple of years. Yes, even if your seed factor comes out, try uping the 6 to 8 in a bottle or two. I was also to by several experts, use your own seed from supernanat from influent or mix liquor, you will get better results.