Theory Of Operation; Oxygen Transfer To Sample - Hach BODTrak II User Manual

General information

2.2 Theory of operation

Respirometric Biochemical Oxygen Demand (BOD) is a test done at 20 °C (68 °F)
in a controlled environment. The test period can be 5, 7 or 10 days, contingent on
the analysis or protocol. The BOD test measures the quantity of oxygen consumed
by bacteria that oxidize organic matter in a water sample. The test is used to
measure waste loadings at wastewater treatment plants and to examine the
efficiency of wastewater treatment.
BOD test results help find general oxygen uptake patterns. This lets operators
estimate plant operating efficiency and find correct treatment procedures.
Advantages to the BODTrak™ II as an alternative to the dilution method are:
• Minimal time to prepare a sample.
• Decreased total test time.
• The BODTrak II method gives results comparable to the diluton method (BOD5)
in 2 to 3 days.
• Calibration and dissolved oxygen measurement are not necessary.
• The BODTrak II test is easy to monitor.
• The sample is stirred constantly and kept in natural conditions. This makes the
BODTrak II results similar to occurrences found in a natural environment. The
dilution method supplies no additional oxygen to the sample. This causes a
higher percentage of oxygen depletion and possible retardation of biochemical
reactions.
• The BOD can be monitored at any time because the instrument continuously
shows the BOD result. Pressure changes in the closed BODTrak II system are
shown graphically in milligrams per liter (mg/L) on an LCD. The system supplies
360 uniform data points over the selected time period.
• The BODTrak II system continuously removes carbon dioxide from the system
so that the pressure difference monitored is proportional to the quantity of
oxygen used.
• Degassing can cause negative errors when heat is applied to a sample to
achieve experimental temperature. The BODTrak II adjusts for this occurrence.
The BODTrak II does not start the test until the temperature gets to equilibrium.
2.2.1

Oxygen transfer to sample

Bacteria in the sample use oxygen while consuming organic matter in the sample
bottles. The air in the bottle above the sample contains 21% oxygen and
replenishes the dissolved oxygen used by the bacteria. During the test period, stir
bars continually mix the sample in each bottle. This moves oxygen from the air to
the sample and helps simulate natural conditions.
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