Biochemical oxygen demand or B.O.D is 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. The term also refers
to a chemical procedure for determining this amount. This is not a precise quantitative
test, although it is widely used as an indication of the organic quality of water. The BOD value
is most commonly expressed in milligrams of oxygen consumed per litre of sample during
5 days of incubation at 20 °C and is often used as a robust surrogate of the degree of
organic pollution of water. BOD can be used as a gauge of the effectiveness
of wastewater treatment plants. It is listed as a conventional pollutant in the U.S. Clean
Water Act. BOD is similar in function to chemical oxygen
demand, in that both measure the amount of organic compounds in water. However, COD is
less specific, since it measures everything that can be chemically oxidized, rather than
just levels of biologically active organic matter.
Background Most natural waters contain small quantities
of organic compounds. Aquatic microorganisms have evolved to use some of these compounds
as food. Microorganisms living in oxygenated waters use dissolved oxygen to oxidatively
degrade the organic compounds, releasing energy which is used for growth and reproduction.
Populations of these microorganisms tend to increase in proportion to the amount of food
available. This microbial metabolism creates an oxygen demand proportional to the amount
of organic compounds useful as food. Under some circumstances, microbial metabolism can
consume dissolved oxygen faster than atmospheric oxygen can dissolve into the water or the
autotrophic community can produce. Fish and aquatic insects may die when oxygen is depleted
by microbial metabolism. Biochemical oxygen demand is the amount of
oxygen required for microbial metabolism of organic compounds in water. This demand occurs
over some variable period of time depending on temperature, nutrient concentrations, and
the enzymes available to indigenous microbial populations. The amount of oxygen required
to completely oxidize the organic compounds to carbon dioxide and water through generations
of microbial growth, death, decay, and cannibalism is total biochemical oxygen demand. Total
BOD is of more significance to food webs than to water quality. Dissolved oxygen depletion
is most likely to become evident during the initial aquatic microbial population explosion
in response to a large amount of organic material. If the microbial population deoxygenates the
water, however, that lack of oxygen imposes a limit on population growth of aerobic aquatic
microbial organisms resulting in a longer term food surplus and oxygen deficit.
A standard temperature at which BOD testing should be carried out was first proposed by
the Royal Commission on Sewage Disposal in its eighth report in 1912:
" An effluent in order to comply with the general standard must not contain as discharged
more than 3 parts per 100,000 of suspended matter, and with its suspended matters included
must not take up at 65°F more than 2.0 parts per 100,000 of dissolved oxygen in 5 days.
This general standard should be prescribed either by Statute or by order of the Central
Authority, and should be subject to modifications by that Authority after an interval of not
less than ten years.
This was later standardised at 68°F and then 20°C. This temperature may be significantly
different from the temperature of the natural environment of the water being tested. Investigators
also decided to eliminate anaerobic conditions. Although the Royal Commission on Sewage Disposal
proposed 5 days as an adequate test period for rivers of the United Kingdom of Great
Britain and Ireland, longer periods were investigated for North American rivers. Incubation periods
of 1, 2, 5, 10 and 20 days were being used into the mid-20th century. Keeping dissolved
oxygen available at their chosen temperature, investigators found up to 99 percent of total
BOD was exerted within 20 days, 90 percent within 10 days, and approximately 68 percent
within 5 days. Variable microbial population shifts to nitrifying bacteria limit test reproducibility
for periods greater than 5 days. The 5-day test protocol with acceptably reproducible
results emphasizing carbonaceous BOD has been endorsed by the United States Environmental
Protection Agency. This 5-day BOD test result may be described as the amount of oxygen required
for aquatic microorganisms to stabilize decomposable organic matter under aerobic conditions. Stabilization,
in this context, may be perceived in general terms as the conversion of food to living
aquatic fauna. Although these fauna will continue to exert biochemical oxygen demand as they
die, that tends to occur within a more stable evolved ecosystem including higher trophic
levels. The BOD5 test
There are two commonly recognized methods for the measurement of BOD.
Dilution method This standard method is recognized by EPA,
which is labeled Method 5210B in the Standard Methods for the Examination of Water and Wastewater
In order to obtain BOD5, dissolved oxygen concentrations in a sample must be measured
before and after the incubation period, and appropriately adjusted by the sample corresponding
dilution factor. This analysis is performed using 300 ml incubation bottles in which buffered
dilution water is dosed with seed microorganisms and stored for 5 days in the dark room at
20°C to prevent DO production via photosynthesis. In addition to the various dilutions of BOD
samples, this procedure requires dilution water blanks, glucose glutamic acid controls,
and seed controls. The dilution water blank is used to confirm the quality of the dilution
water that is used to dilute the other samples. This is necessary because impurities in the
dilution water may cause significant alterations in the results. The GGA control is a standardized
solution to determine the quality of the seed, where its recommended BOD5 concentration is
198 mg/l ± 30.5 mg/l. For measurement of carbonaceous BOD, a nitrification inhibitor
is added after the dilution water has been added to the sample. The inhibitor hinders
the oxidation of ammonia nitrogen, which supplies the nitrogenous BOD. When performing the BOD5
test, it is conventional practice to measure only cBOD because nitrogenous demand does
not reflect the oxygen demand from organic matter. This is because nBOD is generated
by the breakdown of proteins, whereas cBOD is produced by the breakdown of organic molecules.
BOD5 is calculated by:
where: is the dissolved oxygen of the diluted solution
after preparation is the DO of the diluted solution after 5
day incubation is the decimal dilution factor
is the DO of diluted seed sample after preparation is the DO of diluted seed sample after 5 day
incubation is the ratio of seed volume in dilution solution
to seed volume in BOD test on seed 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 that absorbs carbon dioxide is added in 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 compared
to 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.
Dissolved oxygen probes: Membrane and luminescence Since the publication of a simple, accurate
and direct dissolved oxygen analytical procedure by Winkler, the analysis of dissolved oxygen
levels for water has been key to the determination of surface water purity and ecological wellness.
The Winkler method is still one of only two analytical techniques used to calibrate oxygen
electrode meters; the other procedure is based on oxygen solubility at saturation as per
Henry's law. Though many researchers have refined the Winkler analysis to dissolved
oxygen levels in the low PPB range, the method does not lend itself to automation.
The development of an analytical instrument that utilizes the reduction-oxidation chemistry
of oxygen in the presence of dissimilar metal electrodes was introduced during the 1950s.
This redox electrode utilized an oxygen-permeable membrane to allow the diffusion of the gas
into an electrochemical cell and its concentration determined by polarographic or galvanic electrodes.
This analytical method is sensitive and accurate to down to levels of ± 0.1 mg/l dissolved
oxygen. Calibration of the redox electrode of this membrane electrode still requires
the use of the Henry's law table or the Winkler test for dissolved oxygen.
During the last two decades, a new form of electrode was developed based on the luminescence
emission of a photo active chemical compound and the quenching of that emission by oxygen.
This quenching photophysics mechanism is described by the Stern–Volmer equation for dissolved
oxygen in a solution:
The determination of oxygen concentration by luminescence quenching has a linear response
over a broad range of oxygen concentrations and has excellent accuracy and reproducibility.
There are several recognized EPA methods for the measurement of dissolved oxygen for BOD,
including the following methods: Standard Methods for the Examination of Water
and Wastewater, Method 4500 O In-Situ Inc. Method 1003-8-2009 Biochemical
Oxygen Demand Measurement by Optical Probe. Test Limitations
The test method involves variables limiting reproducibility. Tests normally show observations
varying plus or minus ten to twenty percent around the mean.
Toxicity Some wastes contain chemicals capable of suppressing
microbiological growth or activity. Potential sources include industrial wastes, antibiotics
in pharmaceutical or medical wastes, sanitizers in food processing or commercial cleaning
facilities, chlorination disinfection used following conventional sewage treatment, and
odor-control formulations used in sanitary waste holding tanks in passenger vehicles
or portable toilets. Suppression of the microbial community oxidizing the waste will lower the
test result. Appropriate Microbial Population
The test relies upon a microbial ecosystem with enzymes capable of oxidizing the available
organic material. Some waste waters, such as those from biological secondary sewage
treatment, will already contain a large population of microorganisms acclimated to the water
being tested. An appreciable portion of the waste may be utilized during the holding period
prior to commencement of the test procedure. On the other hand, organic wastes from industrial
sources may require specialized enzymes. Microbial populations from standard seed sources may
take some time to produce those enzymes. A specialized seed culture may be appropriate
to reflect conditions of an evolved ecosystem in the receiving waters.
History of the use of BOD The Royal Commission on River Pollution, which
was established in 1865 and the formation of the Royal Commission on Sewage Disposal
in 1898 led to the selection in 1908 of BOD5 as the definitive test for organic pollution
of rivers. Five days was chosen as an appropriate test period because this is supposedly the
longest time that river water takes to travel from source to estuary in the U.K.. In its
sixth report the Royal Commission recommended that the standard set should be 15 parts by
weight per million of water. However in the Ninth report the commission had revised the
recommended standard :
" An effluent taking up 2-0 parts dissolved oxygen per 100,000 would be found by a simple
calculation to require dilution with at least 8 volumes of river water taking up 0.2 part
if the resulting mixture was not to take up more than 0.4 part. Our experience indicated
that in a large majority of cases the volume of river water would exceed 8 times the volume
of effluent, and that the figure of 2-0 parts dissolved oxygen per 100,000, which had been
shown to be practicable, would be a safe figure to adopt for the purposes of a general standard,
taken in conjunction with the condition that the effluent should not contain more than
3-0 parts per 100,000 of suspended solids.
This was the cornerstone 20:30 + full nitrification standard which was used as a yardstick in
the U.K. up to the 1970s for sewage works effluent quality.
The United States includes BOD effluent limitations in its secondary treatment regulations. Secondary
sewage treatment is generally expected to remove 85 percent of the BOD measured in sewage
and produce effluent BOD concentrations with a 30-day average of less than 30 mg/L and
a 7-day average of less than 45 mg/L. The regulations also describe "treatment equivalent
to secondary treatment" as removing 65 percent of the BOD and producing effluent BOD concentrations
with a 30-day average less than 45 mg/L and a 7-day average less than 65 mg/L.
Typical BOD values Most pristine rivers will have a 5-day carbonaceous
BOD below 1 mg/L. Moderately polluted rivers may have a BOD value in the range of 2 to
8 mg/L. Municipal sewage that is efficiently treated by a three-stage process would have
a value of about 20 mg/L or less. Untreated sewage varies, but averages around 600 mg/L
in Europe and as low as 200 mg/L in the U.S., or where there is severe groundwater or surface
water Infiltration/Inflow. BOD Biosensor
An alternative to measure BOD is the development of biosensors, which are devices for the detection
of an analyte that combines a biological component with a physicochemical detector component.
Biosensors can be used to indirectly measure BOD via a fast to be determined BOD substitute
and a corresponding calibration curve method. Consequently, biosensors are now commercially
available, but they do have several limitations such as their high maintenance costs, limited
run lengths due to the need for reactivation, and the inability to respond to changing quality
characteristics as would normally occur in wastewater treatment streams; e.g. diffusion
processes of the biodegradable organic matter into the membrane and different responses
by different microbial species which lead to problems with the reproducibility of result.
Another important limitation is the uncertainty associated with the calibration function for
translating the BOD substitute into the real BOD.
BOD Software sensor Rustum et al. proposed the use the KSOM to
develop intelligent models for making rapid inferences about BOD using other easy to measure
water quality parameters, which, unlike BOD, can be obtained directly and reliably using
on-line hardware sensors. This will make the use of BOD for on-line process monitoring
and control a more plausible proposition. In comparison to other data-driven modeling
paradigms such as multi-layer perceptrons artificial neural networks and classical multi-variate
regression analysis, the KSOM is not negatively affected by missing data. Moreover, time sequencing
of data is not a problem when compared to classical time series analysis.
See also Chemical oxygen demand
Theoretical oxygen demand Carbonaceous biochemical oxygen demand
Wastewater quality indicators discusses both BOD and COD as indicators of wastewater quality.
Further References
Rustum R., A. J. Adeloye, and M. Scholz Applying Kohonen Self-organizing Map as a Software
Sensor to Predict the Biochemical Oxygen Demand, Water Environment Research, 80, 32 – 40.
References
External links BOD Doctor - a troubleshooting wiki for this
problematic test