Biochemical Oxygen Demand (BOD)

What is BOD?

Biochemical Oxygen Demand or Biological Oxygen Demand, is a measurement of the amount of dissolved oxygen (DO) that is used by aerobic microorganisms when decomposing organic matter in water.

How is BOD measured?

The most common approved method for determination of Biochemical Oxygen Demand is Standard Methods 5210B. A sample is first analyzed and conditioned to ensure favourable growth conditions for bacteria, which may include adjustment for pH, neutralization of residual chlorine, and/or reduction of DO in supersaturated samples. The sample is then diluted and the appropriate amount of seed bacteria added. The initial dissolved oxygen content is recorded and the sample is then incubated for 5 days at 20°C. After the 5 day period, the sample is removed from the incubator and the final dissolved oxygen reading is taken. BOD is calculated from the DO depletion and volume of sample used following the formula below:

BOD5 = BOD mg/L = [(IDO -DO5) – seed correction] x dilution factor

Why is BOD important?

Biochemical Oxygen Demand is an important water quality parameter because it provides an index to assess the effect discharged wastewater will have on the receiving environment. The higher the BOD value, the greater the amount of organic matter or “food” available for oxygen consuming bacteria. If the rate of DO consumption by bacteria exceeds the supply of DO from aquatic plants, algae photosynthesis or diffusing from air, unfavourable conditions occur. Depletion of DO causes stress on aquatic organisms, making the environment unsuitable for life. Further, dramatic depletion can lead to hypoxia or anoxic environments. BOD is also used extensively for wastewater treatment, as decomposition of organic waste by microorganisms is commonly used for treatment.

Regulations for BOD will vary by country and region. In general, maximum allowable concentration for direct environmental wastewater discharge fall around 10 mg/L BOD and maximum allowable concentrations for discharge to sewer systems around 300 mg/L BOD.

Measuring BOD with UV-VIS Absorbance technology

While the Biochemical Oxygen Demand test provides the most relevant information for environmental impact and wastewater treatment, it provides little to no value from an operational perspective due to the length of the test.

Using UV-VIS absorbance technology, real-time continuous results of BOD can be achieved. Organic matter naturally absorbs light in the 200-400 nm range. Using multiple wavelengths for organics detection, along with compensation wavelengths in the 500-600 nm range, excellent calibrations can be made for BOD.

Where is BOD measured?

  • Influent wastewater streams for process control
  • Effluent wastewater streams to sewer or environment for regulatory compliance