Biochemical oxygen demand (BOD): how can the biodegradable organic load in water be quantified?

Biochemical Oxygen Demand (BOD) is the benchmark parameter for quantifying the biodegradable organic load of water. For comparable, regulator-accepted results, measurements must be carried out over 5 days at 20°C, follow standardised procedures (ISO 5815, EN 1899, APHA 5210), and maintain tight control of temperature and dissolved oxygen. Dutscher supplies respirometric BOD systems, dilution and O?-probe solutions, and 20°C thermostatic incubators with accessories, to combine accuracy, traceability and simplicity. From routine analysis at wastewater treatment works to pilot trials in R&D, our equipment safeguards preparation, incubation, readout and metrological validation.

Which instruments are used to measure BOD, and how do they work?

Two approaches dominate the measurement of BOD. Respirometric (manometric) systems use hermetically sealed bottles containing a CO? absorbent (often NaOH): at 20 °C, the oxygen consumed by biodegradation causes a pressure drop measured by a sensor. The instrument converts this kinetics into mg/L O?, with logging and alarms. Strengths: automation, full traceability, minimal handling. Watchpoints: bottle tightness, seeding quality and thermal stability.

The dilution and dissolved oxygen probe method (APHA 5210 B, ISO 5815) measures the initial and final O? of a 300 mL BOD bottle after 5 days in a 20 ±1 °C incubator, in the dark. Optical (LDO) sensors offer low maintenance and minimal drift, while polarographic/galvanic probes remain economical but require membranes and electrolyte. Nutrients, seed and, if needed, ATU are added to determine CBOD by inhibiting nitrification.

BOD thermostatic incubators ensure the target temperature and darkness; gentle shaking platforms improve homogeneity. Borosilicate glass bottles for BOD with ground-glass stoppers and overflow necks ensure a constant volume and minimise bubbles. Standards such as GGA (glucose–glutamic acid) enable quality control.

Selection criteria: matrix (wastewater, industrial effluents, surface water), throughput (number of positions), compliance with standards, intended measurement range, ergonomics (interfaces, data export), maintenance and total cost over the life cycle. Optical solutions reduce consumables and waste; respirometric systems maximise repeatability over long runs.

Type	Principle	Applications
Respirometric	Pressure ? via CO? absorption; conversion to mg/L O?	Multi-sample routines, kinetic monitoring, traceability
Dilution + O? probe	Initial/final DO with LDO or polarographic	Regulatory BOD5 control, GGA checks, CBOD
BOD incubator	Stable 20 °C, dark	BOD5 compliant with ISO 5815/EN 1899, teaching

In which sectors is BOD measurement used?

BOD is fundamental in the environment sector for tracking wastewater treatment plants (influent/effluent) and monitoring receiving waters. In food and drink (dairies, abattoirs, sugar factories), it steers the pre-treatment of discharges. In chemicals, cosmetics and pharmaceuticals, it checks effluent compliance and optimises biological processes. Testing laboratories, engineering consultancies and centres for pilot trials use it to size and validate treatment trains (activated sludge, MBBR, SBR). In R&D and education, it supports the assessment of biodegradability and the comparison of treatment options. Shared goal: quantify the biodegradable organic load to protect receiving environments and ensure regulatory compliance.

Available BOD instruments and accessories

Frequently asked questions about BOD measurement (Biochemical Oxygen Demand)

What’s the difference between BOD and COD?
BOD measures biologically consumed oxygen; COD the total chemical oxidation. COD is faster to run, BOD reflects biodegradability.

Do I need a dedicated incubator at 20 °C?
Yes. BOD5 requires 20 ±1 °C and darkness. A stable BOD incubator ensures repeatability and compliance with ISO 5815/EN 1899.

Respirometric or dilution method: how to choose?
Respirometric for batches and automated traceability; dilution + LDO for flexibility and low per-test cost. Decide based on matrix, throughput and QA/QC needs.

Why use ATU and talk about CBOD?
ATU inhibits nitrification. CBOD isolates the carbonaceous BOD, useful when ammoniacal nitrogen biases results via nitrifier oxidation.

How do I ensure result quality?
Check blanks, the GGA standard, seeded controls, bottle seal integrity, probe calibration, and keep 20 °C constant over 5 days.

Do optical probes need maintenance?
LDO minimises membranes and electrolytes to replace. Clean the window, replace the optical cap as recommended, and calibrate periodically.

What volumes and bottles should I use?
BOD 300 mL bottles with an overflow neck ensure constant volume, limit bubbles and deliver reliable oxygen readings.