The biochemical oxygen demand (BOD5) is one of the most important parameters in water and wastewater technology. It indicates the amount of oxygen required by microorganisms to break down the biodegradable organic substances in a water sample within five days under standardized conditions. BOD5 is a key parameter for assessing the organic load of wastewater and serves as an indicator of the efficiency of biological purification processes.

Technical background

Basics of the BSB5

• Definition: BOD5 describes the oxygen consumption caused by the microbial decomposition of organic substances in a water sample within five days at a constant temperature of 20 °C.
• Unit: mg O₂/L (milligrams of oxygen per liter).
• Principle: Microorganisms oxidize the organic substances in the sample to carbon dioxide (CO₂) and water (H₂O), consuming oxygen in the process. This consumption is measured.

Significance of the BOD5

• Evaluation of the organic load: The BOD5 provides information on the quantity of biodegradable substances in the water.
• Monitoring wastewater quality: BOD5 is used to check compliance with legal limits before wastewater is discharged into bodies of water.
• Efficiency of biological treatment processes: The BOD5 is used to evaluate the performance of biological wastewater treatment plants such as activated sludge processes or biofiltration.

Measuring method

BOD5 is measured according to standardized procedures:

1. Trial preparation:
   • The water sample is diluted if necessary to keep the oxygen consumption within a measurable range.
2. Enrichment with microorganisms:
   • If necessary, the sample is inoculated with active sludge or other microorganisms.
3. Measurement of the oxygen content:
   • The oxygen content is measured at the beginning and after five days in an airtight container in the dark at a constant temperature.
4. Calculation of the BOD5:
   • The difference between the oxygen content at the beginning and at the end of the five days is the BOD5 value.

Areas of application

1. industrial wastewater treatment

• Application: The BOD5 is used in industrial wastewater treatment to assess the organic load, e.g. in the food, chemical or paper industry.
• Relevance:
  • Pre-treatment: Decide whether physical-chemical pre-treatment is required.
  • Biological treatment: Optimization of activated sludge processes or anaerobic processes.

2. monitoring of water quality

• Objective: To ensure that the discharge of wastewater does not lead to the eutrophication of water bodies.
• Anforderungen: Die BSB5-Werte müssen innerhalb der gesetzlichen Einleitgrenzwerte liegen (typisch: < 20 mg O₂/L).

3. management of wastewater facilities

• Process control: The BOD5 is used to monitor and control biological treatment stages in order to optimize the degradation performance of the microorganisms.
• Comparison of raw and treated wastewater: Determination of the purification performance (BOD5 degradation rate).