Additional Techniques and Hints for
Accurate BOD Results
(2nd of 3 BOD articles)
by Tim Loftus
The previous BOD article reviewed what
quality control measures indicate a “good” BOD run. These measures include
test-defined limits for the blank, standard, and seed, as well as limits on
dissolved oxygen (DO) residuals at the end of the analysis. This article
will cover additional techniques and hints to get accurate and valid BOD
results.
As with any biological system, pH affects the efficiency of the
bacteria breaking down organic matter in the sample. Adjust the pH of all
samples for BOD analysis to between 6.5 and 7.5 SU using 1 N sulfuric acid or 1
N sodium hydroxide.
Any sample that has been chlorinated, even if no chlorine residual is left,
must be seeded with viable bacteria so that the organic strength, or BOD, of the
sample can be measured. Samples that show chlorine residual must also be
dechlorinated using sodium sulfite (see Standard Methods for the recipe).
But be careful, excess sodium sulfite in the sample will exert an oxygen demand
giving false high BOD readings. It’s important to remember that the
dechlorinating agent for coliform/E. coli analysis cannot be used for BODs. It
is not the same chemical.
Most of us use electronic dissolved oxygen probes to measure the DO in the
BOD bottles. These probes usually calibrate to an air setting rather than DO
saturated water. If your probe is an air calibration type, calibrate to the
barometric pressure in your lab rather than to 760 mm (sea level) or to a
calculated air pressure based on your topographic elevation (which is commonly
done). Air pressure often changes daily and sometimes hourly. Most likely the
air pressure is not the same the day of a BOD setup and five days later when the
BODs are read again. This will be important when measuring the BOD blank. Since
the DO change of the blank should not exceed 0.2 mg/L, you can see where
calibration accuracy would aid in validating the analysis.
Bubbles in a BOD bottle also invalidate that bottle’s DO measurement. Algae
in a BOD sample and left out on a lab bench exposed to sunlight can be a source
of bubbles. Always put the BOD bottle in a dark incubator soon after the initial
DO is measured and the bottle sealed. But a more common source of bubbles is
from dirty glassware. Even though we should try to fill BOD bottles with sample
and dilution water as bubble free as possible, there seems to always be tiny
bubbles generated. If the glassware is not thoroughly cleaned, then the bubbles
stick to the side of the glass and will eventually collect near the bottle’s
seal during the five-day incubation period.
Another source of bubbles can come from aerated dilution water or from
samples that are at a lower temperature than 20 degrees C. Since cold water
will hold more dissolved air, aerating cold dilution water will give a higher
oxygen content than if the dilution water was aerated at 20 C. After placing
the samples in an incubator at 20 C, the water will warm and not be able to hold
as much DO. As a result, bubbles may form in the bottles. This can also happen
with a low dilution sample, such as an effluent composite sample that was
collected at 4 C and not warmed to temperature. It’s important to always warm
samples to 20 C, then shake the sample to remove excess dissolved oxygen before
setting up for BOD. If your laboratory has heating problems, as they all seem to
have, try storing the dilution water in your incubator overnight to stabilize
the temperature to 20 C. This will help remove excess dissolved oxygen from the
dilution water.
As with all analyses performed in your lab, always record the actions of
what you do to samples on the data sheet or in a bound notebook specifically for
that analysis. For these BOD examples, record sample temperature, pH of BOD
sample (before and after adjustment), chlorine residual and amount of
dechlorinating agent used (if needed), and the barometric pressure that the BOD
probe was calibrated to.
BODs are a lot of work to do. And it’s often harder to get them to come out
right. With the right techniques and some foresight of potential problems, your
results will not only be accurate, but will be valid as well.
The information in this article is based on an EPA accepted test method for
NPDES monitoring. As usual, check your federal, state, and local regulations.
You may have additional regulations or reporting requirements that you must
meet.
If you have any questions, suggestions, or comments, please contact NEWEA
Lab Practices Committee Chair Phyllis Arnold Rand 207-782-0917 (prand@gwi.net)
or Tim Loftus at (508) 949-3865 (timloftus@msn.com).
