Water Quality Monitoring: Biochemical Oxygen Demand

Biochemical Oxygen Demand

Biochemical oxygen demand is a measure of the quantity of oxygen used by microorganisms (e.g., aerobic bacteria) in the oxidation of organic matter.

Natural sources of organic matter include plant decay and leaf fall. However, plant growth and decay may be unnaturally accelerated when nutrients and sunlight are overly abundant due to human influence.

Urban runoff carries pet wastes from streets and sidewalks; nutrients from lawn fertilizers; leaves, grass clippings, and paper from residential areas, which increase oxygen demand.

Oxygen consumed in the decomposition process robs other aquatic organisms of the oxygen they need to live. Organisms that are more tolerant of lower dissolved oxygen levels may replace a diversity of more sensitive organisms.

Test Equipment

The same equipment that was used for the dissolved oxygen test

Additional Data

In this test, the dissolved oxygen level of a water sample is measured five days after it was collected. On the day of collection, the DO level is measured in an initial sample. The biochemical oxygen demand is the difference between DO levels in the two samples, so the level of the original sample must be known.

Procedure

Follow directions through step four of the DO measurement. The sample bottle which was stored in the paper bag and should be placed in the dark and incubated for five days at 20 degrees Celsius, which is approximately room temperature. If no incubator is available, place the sample bottle and bag into a "light-tight" drawer or cabinet.
After five days, determine the DO level of the sample by repeating steps five and six of the treatment procedure and all steps of titration and cleanup.

Clean-up

Follow instructions for the dissolved oxygen test.

Water Quality Index

Note: If biochemical oxygen demand is greater than 30 ppm,
the quality index equals 2.

Calculations

Subtract the DO level of the five-day-old sample from the DO level found in the original sample to derive biochemical oxygen demand (ppm).

Convert biochemical oxygen demand (ppm) to water quality index.

Biochemical oxygen demand:  (ppm)

                           

      Water quality index: