What is Biological Oxygen Demand or BOD?
BOD, or Biochemical Oxygen Demand, is a parameter which indicates the amount of
biodegradable organic matter present in water. A high amount of biodegradable matter in water
would result in high oxygen demand thereby consuming more oxygen from the water.
(BOD) is a critical parameter for adherence to environmental norms. Low BOD is an indicator of
good quality water, while high BOD indicates polluted water. Monitoring BOD gives an
understanding of the efficiency of plant operations, and also early warnings of any unrest in the
main process.
Why is BOD monitoring so important?
Apart from swage, effluents of industries like dairy, food processing, beverage, paper and
distilleries exert a high level of BOD load on natural sources of water. These effluents, when
mixed with a common source, put a lot of stress on the dissolved oxygen (DO) levels. When the
DO drops below a certain level, aquatic life in that water cannot sustain. A drop in DO levels
also encourages growth of aquatic weeds, which impact the co-environment and reduce water
availability. It is therefore critical to monitor BOD load from industrial effluents.
What are the limitations in BOD measurement?
In the conventional methods of BOD measurement, the results are obtained after 3- 5 days.
This means, the BOD level in the effluent is understood only after 3-5 days of the sample being
taken.
BOD levels are dependent on the actions of the microorganisms found in wastewater and, as
such, are subject to a number of variations. These variations can be caused by various factors,
including changes in temperature, weather, composition of incoming sewage, in plant
operations and sampling points. Results can vary widely from day to day, or even hour to hour.
Since the BOD test is dependent on biological activity, the major interferences will be those
substances which inhibit the growth of the microorganisms. These will include chlorine, caustic
alkalinity or acidity, mineral acids, and heavy metals (such as copper, zinc, chromium, and
lead).
Excessive nitrites can interfere with the BOD determination. Growth of algae in the presence of
light can cause problems by actually increasing the DO of the sample before testing, which must
be removed by deaeration.
In addition, the rate and degree that organic matter in wastewater is decomposed (or oxidized)
by the normal bacteria present in a sample is largely dependent on the characteristics of the
organic matter. For example, some organic matter (like sugars or starches) is oxidised very
easily and rapidly, and will almost always result in measurable BOD. Other organic matter,
however, is sometimes resistant to biological oxidation, and may require special acclimated
bacteria to oxidise the material and to show a BOD. Although this is what actually happens in
nature, it causes significant variation in BOD results from sample to sample.
During this time, it is essential that the sample be kept at constant temperature in order to
accurately record the oxygen requirement for organic matter to decompose.
Commonly used BOD measurement techniques
Considering the drawbacks of conventional BOD measurement methods, industries today
depend on indirect indicative parameters like COD (chemical oxygen demand). COD
measurement involves addition of chemicals to the sample, which then needs treatment before
disposal into the environment.
There are two commonly recognised methods for the measurement of BOD
Dilution method: To ensure that all other conditions are equal, a very small amount of micro-
organism seed, typically generated by diluting organisms with buffered dilution water, is added
to each sample. The BOD test is carried out by diluting the sample with oxygen saturated
dilution water, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen (DO)
and then sealing the sample to prevent further oxygen dissolving in.
The sample is kept at 20°C, away from light, to prevent photosynthesis. After five days, the
dissolved oxygen is measured again. BOD is arrived at by calculating the difference between
the final DO and initial DO.
Manometric method: This method is limited to the measurement of the oxygen consumption
due only to carbonaceous oxidation. Ammonia oxidation is inhibited. The sample is kept in a
sealed container fitted with a pressure sensor. A substance, typically lithium hydroxide, that
absorbs carbon dioxide is added into the container above the sample level. The sample is
stored in conditions identical to the dilution method. Oxygen is consumed and, as ammonia
oxidation is inhibited, carbon dioxide is released.
The total amount of gas, and thus the pressure, decreases because carbon dioxide is absorbed.
From the drop of pressure, the sensor electronics computes and displays the consumed
quantity of oxygen.
The main advantages of this method over the dilution method are:
• Simplicity: no dilution of sample required, no seeding, no blank sample
• Direct reading of BOD value
• Continuous display of BOD value at the current incubation time
A major limitation of the manometric method is that, this too, takes 3-5 days for values to be
monitored, hence it may not be possible to take timely corrective action to control BOD values.
The Forbes Marshall Solution
The BioSens™ from Forbes Marshall, developed in technical collaboration with IGIB/CSIR, is a
unique biological oxygen demand analyser that provides measurement of BOD levels in about
40 minutes.
The technique deploys a special immobilised microbial membrane and a standard DO probe,
together forming a Biosensor. It does not call for incubation, titration, dilution, filtration or
conditioning of sample. It is a direct measuring technique, so there is no need for further
cumbersome calculations of BOD value.
The biosensor is immersed in buffer solution to be stabilized. Dissolved oxygen in the buffer
diffuses through the outer nylon cloth and then the immobilized membrane. At this interface,
part of the oxygen is consumed by the immobilized microbes. The remainder diffuses further
through the permeable sheet of the DO probe cap and is detected by its electrodes. The signal
reaches a stabilized level indicated as Ii. This represents the endogenous respiration rate of the
microbes.
When an effluent sample is added into the buffer solution and stirred, the organic content in the
sample also diffuses through the outer nylon cloth and then the immobilized membrane. At this
interface, the immobilized microbes assimilate the organic content that results in an increase in
their respiration rate and consequently their oxygen consumption. This rapidly reduces the
amount of dissolved oxygen that diffuses across the permeable sheet of the DO probe cap and
is detected by its electrodes as a falling DO level.
This is shown by the drop in the signal level until it reaches a new steady state indicated by If
within 20 minutes.
As the above behavior is caused purely by the organic content, the biosensor signal must be
proportional to the concentration of the organic content.
The difference in the signal levels [∆I= If - Ii] is used by BioSens™ to internally compute the BOD
value of the sample in terms of mg/l and display it directly on the screen.
Advantages of the Forbes Marshall BioSens™
- Result in 40 minutes, without any incubation at specific temperature
- No use of hazardous chemicals and strong acids
- Sample dilution directed by the instrument itself
- Batch variation change in BOD can be easily monitored
- Backup of reports and printing of reports over dedicated USB ports
- Automatic storage of calculations and related data