BIOCHEMICAL OXYGEN DEMANDNAME = VIPUL KUMARCLASS = 11th RAMANROLL NO = 24

SUBJECT = CHEMISTRY TEACHER = AMAN PARAKH

BOD is the amount of dissolved oxygen needed by aerobic biological organism in a body of water to breakdown organic material present in a given water sample at certain temperature over a specific time period (Numerous 1974, Tchbanglous and Schroeder ,1985)

CH2O + O2 CO2 + H2O

Most of Bacteria in the aquatic columns are aerobic. Escherichia coli, Bacillus subtilis, Vibrio cholera

The Relationship Between Temperature and Oxygen Solubility

Temperature(degrees C)

Oxygen Solubility(mg/L)

0 14.65 12.8

10 9.015 10.220 9.225 8.6

100 boiling 0

Higher the temperature of water higher will be the rate of respiration. So, concentration of oxygen decreases.Many Animal species can grow and reproduce normally when dissolved oxygen level is ~ 5.0 mg/L.

Atmosphere contains 21% oxygen (210000 mg/dm3)

HYPOXIA: When dissolve oxygen content below 3.0 mg/L. Many Species move elsewhere and immobile species may die

ANOXIA: When dissolve oxygen content below 0.5 mg/L. All aerobic species will die

Sources of organic matter

Fertilizer contains Nitrate contributes to high BOD

Phosphate present in Soap and detergent that enhances the growth of algal blooms. As a result depletion of oxygen occur.

High BOD and Low BOD:

In a body of water with large amount of decaying organic material , the dissolved oxygen level may drop by 90 %, this would represent High BOD

In a body of water with small amount of decaying organic material , the dissolved oxygen level may drop by 10 %, this would represent Low BOD

ANALYSIS OF BOD OF WATERUse glass bottles having 60 mL or greater capacity. Take

samples of water.

Turn on the constant temperature chamber to allow thecontrolled temperature to stabilize at 20°C ±1°C.

Record the DO level (ppm) of one immediately.

Place water sample in an incubator in complete darkness at 20 C for 5 days. Exclude all light to prevent possibility of photosynthetic production of DO

If don't have an incubator, wrap the water sample bottle in aluminum foil or black electrical tape and store in a dark place at room temperature (20o C or 68 °F).

DILUTION OF SAMPLE Most relatively unpolluted streams have a BOD5 that ranges from 1

to 8 mg/L Dilution is necessary when the amount of DO consumed by

microorganisms is greater than the amount of DO available in the air-saturated.

If the BOD5 value of a sample is less than 7 mg/L, sample dilution is not needed.

The DO concentration after 5 days must be at least 1 mg/L and at least 2 mg/L lower in concentration than the initial DO

(American Public Health Association and others, 1995).

BOD RANGE Millilitre of Sample Millilitre of Water

0-7 mg/L 300 0

6-21 mg/L 100 200

12-42 mg/L 50 250

30-105 mg/L 20 280

60-210 mg/L 10 290

[Adapted from Sawyer and McCarty, 1978. BOD5, 5-day biochemical oxygen demand]

Recommended sample volumes

BOD of the dilution water is less than 0.2 mg/L.

Discard dilution water if there is any sign of biological growth.

pH of the dilution water needs to be maintained in a range suitable for bacterial growth

Bacterial growth is very good between 6.5 to 7.5

Sulfuric acid or sodium hydroxide may need to be added to the dilution water to lower or raise the pH, respectively.

CALCULATION:The general equation for the determination of a BOD5 value is:BOD = D1-D2/PWhere D1 = initial DO of the sample,D2 = final DO of the sample after 5 days, andP = decimal volumetric fraction of sample used.

If 100 mL of sample are diluted to 300 mL, then P = 0.33. Notice that if no dilution was necessary, P = 1.0 and the BOD5 is determined by D1 - D2.

DO depletion of at least 2 mg/L, and there is no evidence of toxicity

Nature BOD Reaction

* -dC/dt = kC, where C = concentration of oxidizable organic matter at start of the time interval t, and k is the rate constant for the reaction

First Order Reaction: Rate is proportional to the amount of oxidizable organic matter remaining at any time

CHEMISTRY BEHIND ELECTRODE

INTERPRETATION OF BOD LEVEL

BOD LEVEL STATUS1-2 mg/L CLEAN WATER 3-5 mg/L MODERATELY CLEAN6-9 mg/L POLLUTED WATER> 10 mg/L VERY POOR QUALITY

Generally, when BOD levels are high, there is a decline in DO levels. This is because the demand for oxygen by the bacteria is high and they are taking that oxygen from the oxygen dissolved in the water

The standard oxidation (or incubation) test period for BOD is 5 days at 20 degrees Celsius (°C) (BOD5).

Twenty days is considered, by convention, adequate time for a complete biochemical oxidation of organic matter in a water sample, BOD results represent approximate stream oxygen demands because the laboratory environment does not reproduce ambient stream conditions such as temperature, sunlight, biological populations, and water movement

The samples must be seeded with a population of microorganisms to produce an oxygen demand

Interferences present in a water sample can inhibit biochemical oxidation and interfere in BOD process.

Copper, lead, chromium, mercury, and arsenic are examples of Interferences

In the USA, it has been estimated that each year 7.1 million suffer from a mild to moderate infections, from severe waterborne diseases, in estimated 12,000 deaths a year