BIOCHEMICAL OXYGEN DEMAND

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 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

Dilution method

To ensure that all other conditions are equal, a very small amount of micro-

organism seed is added to each sample being tested. This seed is typically

generated by diluting activated sludge with de-ionized water. The BOD test is

carried out by diluting the sample with oxygen saturated de-ionized 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 in the dark to prevent photosynthesis (and thereby the addition of

oxygen) for five days, and the dissolved oxygen is measured again. The difference

between the final DO and initial DO is the BOD.

The loss of dissolved oxygen in the sample, once corrections have been made for

the degree of dilution, is called the BOD5. For measurement of carbonaceous

BOD (cBOD), a nitrification inhibitor is added after the dilution water has been

added to the sample. The inhibitor hinders the oxidation of nitrogen.

BOD can be calculated by:

 Undiluted: Initial DO - Final DO = BOD

 Diluted: ((Initial DO - Final DO)- BOD of Seed) x Dilution Factor

BOD is similar in function to chemical oxygen demand (COD), 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.

Manometric method

This method is limited to the measurement of the oxygen consumption due only to

carbonaceous oxidation. Ammonia oxidation is inhibited.

that absorbs carbon dioxide (typically lithium hydroxide) 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

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

lower the test result.[2]:85

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

which was established in 1865 and the formation of the Royal Commission on

Sewage Disposal in 1898 led to the selection in 1908 of BOD 5 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 1912, the commission also set a standard of 20

ppm BOD5 as the maximum concentration permitted in sewage works discharging

to rivers, provided that there was at least an 8:1 dilution available at dry weather

flow. This was contained in the famous 20:30 (BOD:Suspended Solids) + 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.[3]

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. (The generally lower values in

the U.S. derive from the much greater water use per capita than in other parts of

the world.)[1]

BOD Biosensor

An alternative to measure BOD is the development of biosensors, which are

devices for the detection of an analytic that combines a biological component with

a physicochemical detector component. Biosensors can be used to indirectly

measure BOD via a fast (usually <30 min) to be determined BOD substitute and a

corresponding calibration curve method (pioneered by Karube et al., 1977).

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 results (Praet et al., 1995). Another important limitation is the

uncertainty associated with the calibration function for translating the BOD

substitute into the real BOD (Rustum et al, 2008).

Rustum et al. (2008) 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 perceptions artificial neural networks

(MLP ANN) 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.

CHEMICAL OXYGEN DEMAND

environmental chemistry, the chemical oxygen demand (COD) test is

commonly used to indirectly measure the amount of organic compounds in water.

Most applications of COD determine the amount of organic pollutants found in

surface water (e.g. lakes and rivers), making COD a useful measure of water

quality. It is expressed in milligrams per liter (mg/L), which indicates the mass of

oxygen consumed per liter of solution. Older references may express the units as

parts per million (ppm).

oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions.

The amount of oxygen required to oxidize an organic compound to carbon dioxide,

ammonia, and water is given by:

This expression does not include the oxygen demand caused by the oxidation of

ammonia into nitrate. The process of ammonia being converted into nitrate is

referred to as nitrification. The following is the correct equation for the oxidation

of ammonia into nitrate.

It is applied after the oxidation due to nitrification if the oxygen demand from

nitrification must be known. Dichromate does not oxidize ammonia into nitrate, so

this nitrification can be safely ignored in the standard chemical oxygen demand

test.
The International Organization for Standardization describes a standard method for

measuring chemical oxygen demand in ISO 6060

Blanks

Because COD measures the oxygen demand of organic compounds in a sample of

water, it is important that no outside organic material be accidentally added to the

sample to be measured. To control for this, a so-called blank sample is required in

the determination of COD (and BOD -biochemical oxygen demand - for that

matter). A blank sample is created by adding all reagents (e.g. acid and oxidizing

agent) to a volume of distilled water. COD is measured for both the water and

blank samples, and the two are compared. The oxygen demand in the blank sample

is subtracted from the COD for the original sample to ensure a true measurement

of organic matter.

Measurement of excess

For all organic matter to be completely oxidized, an excess amount of potassium

dichromate (or any oxidizing agent) must be present. Once oxidation is complete,

the amount of excess potassium dichromate must be measured to ensure that the

amount of Cr3+ can be determined with accuracy. To do so, the excess potassium

dichromate is titrated with ferrous ammonium sulfate (FAS) until all of the excess
oxidizing agent has been reduced to Cr 3+. Typically, the oxidation-reduction

indicator Ferroin is added during this titration step as well. Once all the excess

dichromate has been reduced, the Ferroin indicator changes from blue-green to

reddish-brown. The amount of ferrous ammonium sulfate added is equivalent to

the amount of excess potassium dichromate added to the original sample. and also

we can determine COD by boiling the water sample and we can determine CO2

ratio by the infra-red analyzer

Preparation Ferroin Indicator reagent

A solution of 1.485 g 1,10-phenanthroline monohydrate is added to a solution of

695 mg FeSO4·7H2O in water, and the resulting red solution is diluted to 100 mL.

Calculations

The following formula is used to calculate COD:

where b is the volume of FAS used in the blank sample, s is the volume of FAS in

the original sample, and n is the normality of FAS. If milliliters are used

consistently for volume measurements, the result of the COD calculation is given

in mg/L.
The COD can also be estimated from the concentration of oxidizable compound in

the sample, based on its stoichiometric reaction with oxygen to yield CO 2 (assume

all C goes to CO2), H2O (assume all H goes to H2O), and NH3 (assume all N goes

to NH3), using the following formula:

COD = (C/FW)(RMO)(32)

Where C = Concentration of oxidizable compound in the sample,

FW = Formula weight of the oxidizable compound in the sample,

RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable

compound in their reaction to CO2, water, and ammonia

For example, if a sample has 500 wppm of phenol:

C6H5OH + 7O2 → 6CO2 + 3H2O

COD = (500/94)(7)(32) = 1191 wppm

Inorganic interference

Some samples of water contain high levels of oxidizable inorganic materials which

may interfere with the determination of COD. Because of its high concentration in
most wastewater, chloride is often the most serious source of interference. Its

reaction with potassium dichromate follows the equation:

Prior to the addition of other reagents, mercuric sulfate can be added to the sample

to eliminate chloride interference.

The following table lists a number of other inorganic substances that may cause

interference. The table also lists chemicals that may be used to eliminate such

interference, and the compounds formed when the inorganic molecule is

eliminated.