PlugBug:

You've already mentioned two ways of lowering DO and they are increasing temperature of sample to ambient temperature as well as gently mixing to release any dissolved oxygen.  A DO of 9.8 mg/L is unusual for typical wastewater unless the sample is extremely cold, not frozen, but cold--what was the temperature of the sample when DO was taken?  One thing that might artificially increase DO is peroxide being present in sample.  Any chance of industrial flow adding to DO?

Double check DO calibration?

Plugbug:

I had to read SM to see how they addressed this issue.  I know that it was explicitly detailed but it has been years since I've done BOD's that I had to refresh my mind.  SM addresses the issue of samples supersatured with DO--"Samples containing more than 9 mg DO/L at 20°C may be encountered in cold waters or in water where photosynthesis occurs.  To prevent loss of oxygen during incubation of such samples, reduce DO to saturation at 20°C in partially filled bottle while agitating by vigorous shaking or by aerating with clean filtered compressed air." (SM 17th Edition).  Nowhere does it state that one can use sodium sulfite other than for dechlorination purposes so I would have to say that your idea would not be acceptable other than what SM states in terms of adjusting sample temperature and shaking dissolve oxygen out of sample.  Get the lab tech to put the cold sample in a large water bath (fill up the sink) and bring temperature to about ambient temperature--as close to 20°C as possible.

All of our wastewater samples are cold when brought to the lab as they are refrigerated while being sampled. We run warm water into the sink and place the sample container, about 80 ml in a liter bottle, in the warm water. The sample is shaken vigorously and the temperature checked. When the sample is at 20 C we take the sample out of the warm water and shake vigorously. The DO will be about 8.8 to 9.0 at that point. The sample is ready for analysis.

Thanks for all the input, I tried warming the sample and it brought the D.O. to acceptable levels.  We will soon see how the BOD results turn out.  Thanks again folks!

Thst does not aid in dealing with effluent samples where you need, say, 250 mls of effluent in a 300 ml bottle.  The temperature needs to be adjusted and also the D.O. of the sample without cutting it with anything else.  

~Eric

Adding sulfite would not be acceptable as it adds to the BOD value as it is oxidized. To deoxidize dilution water you would need to heat up the dilution water to remove DO and then cool the sample as not to injure your seed bacteria while adding it to the sample.

The simplist method it to quickly warm the sample to 20 to 23 oC in hot water in the sink, mixing the samples by shaking, remove the sample at 23 oC and shaking vigorously until excess DO is gone.

This is more time consuming with large volumes of samples but you need only to warm effluent and stream samples that have low BOD values. Influent samples already have low DO.

Plugbug wrote on Jan 4^th, 2010, 10:11am:


I like to think I mastered the BOD ....actually love running the dang test. I dealt with DO saturation years ago and through experimenting with time/temp/stirrer speed I've found that pouring 800mls effluent in a one liter beaker (initial temp - 3C, Initial DO 9.5 to 10ppm) place on stirring hotplate so that the vortex draws to near the bottom (vigorous stirring) of the beaker and hotplate pre-warmed to allow temp to reach 20C in approximately 7 minutes. When temp is reached continue stirring at same speed on a non-heated stirrer for a remaining 15-20 min. Whalla!!! D.O.'s at 20C will be 8.5 - 9.0 mg/l - you can work on other things while this is being prepped.