Biochemical oxygen demand or BOD is a chemical procedure for determining the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period. It is not a precise quantitative test, although it is widely used as an indication of the organic quality of water. It is most commonly expressed in milligrams of oxygen consumed per liter of sample during 5 days (BOD5) of incubation at 20°C and is often used as a robust surrogate of the degree of organic pollution of water.
BOD directly affects the amount of dissolved oxygen in rivers and streams. The rate of oxygen consumption is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water.
The greater the BOD, the more rapidly oxygen is depleted in the stream. This means less oxygen is available to higher forms of aquatic life. The consequences of high BOD are the same as those for low dissolved oxygen: aquatic organisms become stressed, suffocate, and die.
Sources of BOD include topsoil, leaves and woody debris; animal manure; effluents from pulp and paper mills, wastewater treatment plants, feedlots, and food-processing plants; failing septic systems; and urban stormwater runoff.
BOD is affected by the same factors that affect dissolved oxygen. BOD measurement requires taking two measurements. One is measured immediately for dissolved oxygen (initial), and the second is incubated in the lab for 5 days and then tested for the amount of dissolved oxygen remaining (final). This represents the amount of oxygen consumed by microorganisms to break down the organic matter present in the sample during the incubation period.
BOD Measurement Problems
When encountering a problem with BOD values, most operators blame the instrument or probe first. After all, it’s what gives you the values! There can be many factors leading to the problem from dirty BOD bottles, DI water, bad seed, bubbles, non-linearity and so on.
Calibration: Regardless of technology, it is recommended to only calibrate the instrument once a day before readings are taken. Make sure there are no water droplets on the probe tip before calibrating and that the probe is in a 100% water-saturated air environment. A BOD bottle with a little water in the bottom is all that’s needed. Place the probe back in this bottle when not in use. Make sure this bottle is kept clean as shown in the bottle on the right of the image. The bottle on the left is not a clean environment and could affect the calibration.
Probe Care: Polarographic sensors must have fresh electrolyte and membranes along with a maintained anode and cathode for optimal performance. Optical sensors will need an occasional visual check of the paint layer on the sensor cap.
Warm-Up Time: Polarographic probes require sufficient warm-up time of 5-15 minutes before calibrating and using the probe. Improper warm-up time can easily lead to data drifting due to an inaccurate calibration. Optical probes do not require a warm-up period.
A variety of strategies are employed to deal with specific sample types. These include varying dilutions and diluent seeding. It is often desirable to distinguish between carbonaceous and nitrogenous demand, in which case a nitrification inhibitor is used. Toxic and chlorinated samples also need special handling. The operator must be familiar with standard methods and with the technical literautre on the subject.
When properly used, the BOD test provides a reliable characterization of wastewater. It can be expected to be a standard for regulatory agencies for many years even though its use as a control tool is limited by the 3 or 5 day wait required for the test (and sometimes 20 days!). Various methods (based on short-term monitoring and extrapolation) of quickly estimating the probable results of the BOD test on a sample have been devised and the interested reader is advised to consult appropriate literature but a 'true' BOD test requires time and incubation.
(complete Tech Note on BOD Measurements)
View our optical ProOBOD probe and learn more about this technology or read our tech note: Accurate BOD Measurements with Electrochemical and Optical Probes.
Essential BOD Posts on the Blog
BOD Testing Accuracy and Success - Are You Achieving This?
What is Affecting Your Dissolved Oxygen Measurements? Part 1 of 4
Dissolved Oxygen Measurements - Piece of Cake, Right?
Dissolved Oxygen Meters - Which Membrane Type Should I Use?