Biochemical oxygen demand (BOD) 
and degradation tests

Why is BOD important?

• Environmental assessment
  
  • indicates the degree of water pollution (water quality)
  • Low oxygen levels threaten aquatic ecosystems

• Wastewater analysis
  
  • Monitoring treatment performance in wastewater treatment plants
  • Assessment of discharge limits

• Process optimization
  
  • Control of biological treatment processes
    Monitoring of industrial wastewater

How is BOD measured?

Many microorganisms obtain energy by taking in oxygen. It is precisely this biochemical oxygen demand that the measurement is intended to determine.
Bacteria take in oxygen and release carbon dioxide - carbon combines with oxygen to form carbon dioxide. Since one molecule of oxygen is converted into one molecule of carbon dioxide, BOD can generally be determined in two ways: directly via oxygen consumption or indirectly via carbon dioxide production. Respirometric methods use the second approach and measure the resulting change in pressure.

But where does this pressure change come from? One mole of oxygen—that is, 6.022 × 10²³ molecules—has a volume of 22.4 liters at 25 °C and 1013 hPa. One mole of carbon dioxide has exactly the same volume. So when oxygen is exhaled as carbon dioxide, there is initially no change in pressure.
This is where sodium hydroxide, located in the bottle’s neck, comes into play. Sodium hydroxide chemically reacts with the resulting carbon dioxide to form sodium carbonate, thereby binding it. The volume of oxygen consumed is not replaced - the pressure in the bottle drops. This removes the carbon dioxide formed from the gas phase, and the absorption creates a measurable vacuum.

Pressure drop (Δp) in the closed bottle = O₂ consumed = BOD value

What is the difference between BOD and COD?

The OxiTop® System from WTW:

OxiTop® is a product family available in two versions:

• The OxiTop®-i is specifically designed for measuring municipal wastewater or lightly polluted industrial wastewater. The measuring range is between 0 and a maximum of 4000 mg/l BOD.
• The OxiTop®-IDS offers various operating modes that enable standard BOD measurement, the preparation of defined dilution series to extend the measurement range up to 400,000 mg/L, as well as other specialized degradability tests:
  
  • Standard BOD measurement for self-monitoring according to DIN EN 1899-2 (H55)
  • Soil respiration according to DIN EN ISO 16072:2011-09 and the respiration activity of biomechanically treated waste (AT4)
  • Aerobic degradability according to OECD 301F/DIN EN ISO 9408
  • Anaerobic degradability according to VDI 4630