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The HBOD Test
The traditional biochemical oxygen demand (BOD) test was invented
around the beginning of the 19th century
to determine the strength of wastewaters discharged into rivers. The
five-day time period was based on the time of flow of a river (the
Thames in England) between London and the sea. It was a simple, yet
primitive test that required ground-glass stoppered bottles, wet
chemical techniques to measure dissolved oxygen, and wastewater
dilutions.
Improvements of the BOD test have consisted
mainly in the development of probes to measure dissolved oxygen. The
BOD test is of course still used today. However, it is a
time-consuming test that lacks the sophistication of other modern
analytical techniques. Because of the need to dilute the wastewater
for a BOD test, the degradation rates of biological organic matter
is slow and often an accurate measurement of the BOD cannot be made
due to poor choice of a dilution volume. Everyone has used a BOD
test in the US because they have to, but now there are other
options.
Bruce Logan originally developed the headspace
BOD (HBOD) test when he realized that with modern improvements in
container caps (plastic wasn’t invented in 1906!) and instruments,
glass stoppered bottles and wet chemical measurements were no longer
necessary to determine a biological oxygen demand of a wastewater
sample.
The HBOD test consists of simply sealing a
volume of wastewater in a gas-tight tube and then incubating the
tube (on a shaker table) for given period of time. In only 2-3 days,
an HBOD value identical to that of the 5-day BOD is obtained. The
HBOD can be determined from measuring either the oxygen
concentration in the air—or in the wastewater-- and calculating the
total oxygen used from a mass balance. There is no need to dilute
the wastewater. The volume of wastewater added to the tube can be
varied to suit the strength of a specific wastewater. The HBOD range
of a single tube is much greater than a single BOD bottle dilution,
and therefore there is less chance of running the test outside the
measurable range. Tube-to-tube variations for the HBOD test are
smaller than those for the BOD test, making the HBOD test more
precise than a BOD test.
The most significant improvement in the HBOD
test recently occurred with the availability of the HBOD
Probe. Using this probe, it is now possible to measure the
oxygen in a tube headspace in just seconds. An HBOD test could be
run with equipment available in most modern laboratories, using a
dissolved oxygen (DO) probe or a gas chromatograph (GC). However,
the HBOD Probe analysis time is much faster than these other methods
(seconds, versus 2-5 minutes for the DO and GC tests) and probe
calibration takes only a few minutes using laboratory air.
The HBOD is a type of respirometric biochemical
oxygen demand (RBOD) test and the RBOD test that is now included as
a proposed method in Standard
Methods for the Examination of Water and Wastewater. In Europe,
however, RBOD tests are now routinely used for measurement of
wastewater strength instead of a BOD test. If you are interested in
finding out more about the HBOD test, read through this web site and
please contact us if you’d like additional information.
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