Biochemical oxygen demand

By Dr Utpal SharmaAssist. Professor

Department of Community Medicine, SMIMS

For the beginning….. Water is “polluted” by many organic matter in its course of flow

When organic matter is present in a water supply, the bacteria present in water will begin the process of breaking down this waste.

With this much of the available dissolved oxygen is consumed by aerobic bacteria, robbing other aquatic organisms of the oxygen they need to live.

Biological Oxygen Demand (BOD) is a measure of the oxygen used by microorganisms to decompose this waste.

A large quantity of organic waste in the water supply, guarantees a large number bacteria present to decompose this waste.

In this case, the demand for oxygen will be high (due to all the bacteria) so the BOD level will be high.

As the waste is consumed or dispersed through the water, BOD levels will begin to decline.

Cont…Definition…. The amount of oxygen absorbed by a sample of sewage during a

specific period, generally 5 days at a specific temperature, generally 20° C for the aerobic destruction of the organic matter by living organisms.

Total BOD is of more significance to food webs than to water quality.

Cont….Nitrates and phosphates in a body of water can

contribute to high BOD levels.

Nitrates and phosphates are plant nutrients and can cause plant life and algae to grow quickly.

When plants grow quickly, they also die quickly.

This contributes to the organic waste in the water, which is then decomposed by bacteria resulting in a high BOD level.

Is Biochemical oxygen demand different from Biological oxygen

demand….???

Biochemical oxygen demand measures the molecular oxygen utilized for the biochemical degradation of organic material (carbonaceous demand) and…..

……the oxygen used to oxidize inorganic material such as sulfides and ferrous ion.

It also may measure the amount of oxygen used to oxidize reduced forms of nitrogen (nitrogenous demand).

Biological oxygen demand only addresses the oxygen used up by the bacteria to degrade organic substances.

Difference between Aerobic and Anaerobic Decomposition

Element Aerobic Decay Product

Anaerobic decay product

C CO2 CH4

N NO3 NH3

H H2O CH4, NH3, H2S, H2O

S SO4²- H2S

P PO4³- PH3

Biochemical oxygen demand takes into account of oxygen used up for degradation of both carbonaceous and

inorganic materials

Environmental significance Principle test for biodegradability of any sample and strength of the

waste so measures the amount of pollution.

Important parameter to assess the pollution of surface and ground waters where contamination occurred due to disposal of domestic and industrial effluents.

Allows calculation of the effect of the discharges on the oxygen resources of the receiving water.

Measurement of BOD in raw (influent) and treated (effluent) wastewaters is a standard practice to evaluate treatment facility performance.

Data from BOD tests used for the development of engineering criteria for the design of wastewater treatment plants.

Cont….

Used in studies to measure the self-purification capacity of streams

Serves regulatory authorities as a means of checking on the quality of effluents discharged to stream waters.

One of the most important method in sanitary analysis to determine the polluting power, or strength of sewage, industrial wastes or polluted water.

It serves as a measure of the amount of clean diluting water required for the successful disposal of sewage by dilution.

And we expect…..

Any effluent to be discharged into natural bodies of water should have BOD less than 30 mg/L.

Drinking water usually has a BOD of less than 1 mg/L.

But, when BOD value reaches 5 mg/L, the water is doubtful in purity.

Items BOD standards(mg/L)

Most pristine rivers <1

Moderately polluted rivers 2-8

Ordinary domestic sewage 150- 200

Municipal sewage efficiently treated <20 

This is it …

Factors affecting BOD…Temperature Elevated temperature decreases the level of DO of water harming

aquatic organisms like fish, amphibians and others

Increases the metabolic rate of aquatic animals and enzyme activity, resulting in consumption of more food in a shorter time

High temperature limits oxygen dispersion into deeper waters, contributing to anaerobic conditions.

This can lead to increased bacterial levels when there is ample food supply.

Higher water temperature increases plant growth rates

This results in a shorter lifespan and species overpopulation causing an ”algae bloom” which reduces oxygen levels.

Eutrophication

Cont…

Eutrophication occurs due to oversupply of nutrients, which causes explosive growth of plants and algae.

when such organisms die, consume the oxygen in the body of water, thereby creating the state of hypoxia.

The primary limiting factor is phosphorus which promotes excessive plant growth and decay, favouring simple algae and plankton, and causes a severe reduction in water quality.

Phosphate adheres tightly to soil, so it is mainly transported by erosion.

Once into the lakes, phosphate is extracted into water is slow, hence the difficulty of reversing the effects of eutrophication

The source of this excess phosphate are detergents, industrial/domestic run-off, and fertilizers.

At higher levels of BOD…. At high BOD levels, organisms such as macro invertebrates

that are more tolerant of lower dissolved oxygen (i.e. leeches and sludge worms) may appear and become numerous.

Organisms needing higher oxygen levels (i.e. caddisfly larvae and mayfly nymphs) will NOT survive.

The basics of BOD test.. The first step is to obtain equal volumes of water from the area to be tested Dilute each specimen with a known volume of distilled water which has been

thoroughly shaken to insure oxygen saturation.

Oxygen meter is used to determine the concentration of oxygen within one of the vials.

The remaining vial is than sealed and placed in darkness and tested for oxygen content five days later.

BOD is then determined by subtracting the second meter reading from the first.

Possibilities…Water from an exceptionally clear lake might show a BOD of less than 2 ml/L of water. Raw sewage may give readings in the hundreds and food processing wastes may be in the thousands.

Apparatus required

Reagents required

Step 1

Sample collection Samples for BOD test can be either grab or composite.

Composite sample more preferred as it is more representative of the wastestream over a period of time.

Samples should be taken at a point of well-mixed and proportional to the amount of the flow.

 Preservation of sample is not practical as biological activity will continue after a sample has been taken

Testing should be started as quickly as possible, preferrrably <6hrs.

If not possible samples should be kept ≤ 4°C.

Do not allow samples to freeze.

Samples may be kept for no more than 48 hours before beginning the BOD test.

Step 2

Dilution The BOD concentration in waste water is more than sample due to

biological activity.

So, it is necessary to dilute the sample before incubation to bring the oxygen demand and supply into appropriate balance

If not diluted, microorganisms will use up the DO in the BOD bottle before the five day incubation time is up.

There is no way of knowing at what point during the five days the DO reached zero.

Dilution water It is high quality organic free water . The required volume of water is aerated with a supply of clean compressed air

for at least 12 hours. It is stabilized by incubating it at 20ºC for at least 4 hours.

Step 3Pretreatment Samples with extreme pH values (>8.5 or <6.0) must be neutralized to pH

7.0 prior to testing.

This is done by adding either or H2SO4 or NaOH

Any samples containing residual chlorine must be pretreated to remove chlorine before the test is run.

This is done by adding sodium sulfite to the sample

Step 4 Seeding The process of adding live bacteria to a sample.

Samples form sources like high temperature, extreme pH or having heavy metals could kill or injure the microorganisms

The condition must be corrected and healthy active organisms added

Laboratory procedure Completely fill two BOD bottles with dilution water.

Into additional BOD bottles, partially filled with dilution water, carefully mix the proper volume of sample.

Add dilution water until the bottles are completely filled.

Stopper each bottle taking care to avoid trapping air bubbles inside the bottles as the bottle stoppers are inserted.

Fill the top of each bottle neck around the stopper with dilution water.

Determine the initial DO content of each set of duplicate bottles, including the dilution water blank

Place the remaining bottles in the incubator at 20°C and incubate for five days.

Cont…. At the end of exactly five days (+/‑3 hours), test the DO content of

the incubated bottles.

 Calculate the BOD for each dilution.

The dilution water blanks are used only to check the quality of the dilution water.

If the quality of the water is good and free from impurities, the depletion of DO should be less than 0.2 mg/L.

In any event, do not use the depletion obtained as a blank correction.

Samples treated for chlorine should always be seeded

Conclusions CALCULATIONS

BOD, mg/L = (Initial DO ‑ Final DO) x bottle volume)

Major interferences of BOD test are substances that inhibit the growth of the microorganisms viz. chlorine, mineral acids, and heavy metals

Excessive nitrites can interfere with the BOD determination.

Growth of algae in the presence of light can increase the DO

Artifacts in BOD testing results from residues building up in the BOD and dilution water bottles.

Variations in BOD test is subject to a number of factors temperature, weather etc. results can vary widely from day to day, or even hour to hour.

Sample volume

Thank you folks…for

your patience