7/22/2026
| Dr. Markus Hoffmann
Why BSB is a key parameter in wastewater treatment
Biochemical Oxygen Demand (BSB) is one of the most important parameters in municipal wastewater treatment. In every wastewater treatment plant, it forms the basis for operational self-monitoring and the evaluation of biological treatment processes.
But what does the BSB value actually indicate? It shows how much dissolved oxygen microorganisms need to break down biodegradable organic carbon. This makes BSB a direct indicator of organic pollution load, the performance of biological treatment stages, and overall plant efficiency.
Compared to COD (Chemical Oxygen Demand), which represents all oxidizable substances, BSB specifically reflects biological degradability. In particular, the BSB/COD ratio supports sound process evaluation in practice.
What is BSB5 and how is it determined?
The classical reference method is defined in DIN EN ISO 5815-1. It describes the determination of BSB5, i.e., the biochemical oxygen demand over five days.
Procedure of the classical BSB5 measurement:
- Preparation of dilution series based on expected BSB following COD determination, including nitrification inhibitor
- Determination of oxygen concentration at the beginning
- Incubation at 20 °C for five days
- Second oxygen measurement
- Calculation of BSB in mg/L considering dilution factors
The result shows oxygen consumption by microorganisms. BSB5 is the amount of oxygen consumed over five days for the biological degradation of organic substances.
However, this method is time-consuming, labor-intensive, and not very flexible for routine plant operation.
Respirometric BSB measurement: the efficient alternative
For routine operation, respirometric BSB determination according to DIN EN 1899-2 has become established.
Functional principle:
- Microorganisms degrade organic carbon
- CO2 is produced
- CO2 is chemically absorbed (e.g., by sodium hydroxide)
- A pressure drop occurs in the closed system
- This drop is proportional to oxygen consumption
- Measuring range is adjusted via sample volume
Result: The BSB value is calculated directly in mg/L.
Advantages: no dilution required, easy handling, ideal for routine measurements, and high reproducibility.
Practical solution: BSB measurement with OxiTop®-i
The OxiTop®-i system is widely used in operational laboratories as a reliable solution for routine BSB measurement.
- Direct display of BSB values in mg/L without manual calculation
- Guided menu for easy parameter setup
- Individual ID assignment for clear sample identification
- Color coding (e.g., influent grey, effluent blue)
Flexible measurement strategy
- Measurement duration adjustable from 1 to 7 days
- Standard BSB5 measurement possible
- Suitable for short tests and long-term monitoring
5-day test procedure:
- Prepare the sample (including nitrification inhibitor)
- Determine measuring range and sample volume based on COD
- Transfer sample into the bottle from the overflow measuring flask
- Insert rubber sleeve with CO2 absorber into the OxiTop® bottle
- Set measuring range via sample volume and start measurement
- Incubate at 20 °C
- Automatic evaluation after five days
Result: BSB5 value displayed directly on the device.
Maximum efficiency in daily routine
The system is designed for practical operation:
- Intermediate values and curves available at any time
- No manual recording required
- Automatic data storage
- AutoTemp function for reliable results (15–21 °C)
- LED status indicators
- Integrated test function (OxiTop® PT)
- Compatible with existing lab equipment
What is BSB used for in practice?
- Control of biological processes
- Evaluation of treatment performance
- Continuous self-monitoring
The combination of BSB5, respirometric measurement, and modern systems enables reliable and efficient monitoring of biodegradability.
Conclusion: BSB measurement as the basis for efficient treatment
BSB remains the key parameter for assessing biological treatment performance. Systems like OxiTop®-i make measurement easier, more efficient, and more practical, supporting reliable plant operation.
Optimize your BSB measurement in wastewater treatment plants.
Discover the advantages of OxiTop®-i and take your BSB analysis to the next level.