L-16 and 17Biochemical Oxygen Demand

(BOD)

Environmental Engineering-II

Definition• Biochemical oxygen demand or

B.O.D. is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water or organisms in a body of water or wastewater sample to break down organic material present in a given water or wastewater sample at certain temperature (200C) over a specific time period (5 days).

• Used to assess the relative strength of a waste

• The amount of oxygen required to stabilize a waste if discharged to a surface water.surface water.

• 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 consumed per litre of sample during 5 days of incubation at 20 °C and is often used as a indicator of the degree of organic pollution of water.

Background• Most natural waters and wastewaters

contain small quantities of organic compounds.

• Aquatic microorganisms have evolved to use some of these compounds as to use some of these compounds as food.

• Microorganisms living in oxygenated waters use dissolved oxygen to convert the organic compounds into energy 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. the water.

• Fish and aquatic animals and plants may die when oxygen is depleted by microbial metabolism

Types of BOD• There are two stages of

decomposition in the BOD test:

i. a carbonaceous stage and

ii. a nitrogenous stage.ii. a nitrogenous stage.

• The carbonaceous stage, or first stage, represents that portion of oxygen demand involved in the conversion of organic carbon to carbon dioxide.

• The nitrogenous stage, or second stage, • The nitrogenous stage, or second stage, represents a combined carbonaceous plus nitrogenous demand, when organic nitrogen, ammonia, and nitrite are converted to nitrate. Nitrogenous oxygen demand generally begins after about 6 days.

TEST• The test for biochemical oxygen

demand (BOD) is a bioassay procedure that measures the oxygen consumed by bacteria from the decomposition of organic matter.decomposition of organic matter.

• The change in DO concentration is measured over a given period of time in water samples at a specified temperature.

• In a nutshell, BOD gives a measure on the impact of a waste(water) on the oxygen content of a receiving System(stream/river/lake).

• Wastes are broken down by • Wastes are broken down by microbial organisms (frequently referred to as “bugs” or “microorganisms”), and the bugs, in turn, require oxygen for this monumental effort.

• Thus, in order for this test to "work", you need

(1) a food source (Organic matter),

(2) a nice population of bugs,(2) a nice population of bugs,

(3) available oxygen (DO) to drive the bugs, and

(4) a system which provides a hospitable environment for the bugs (incubation temp.).

Procedure A. Dilution

• Dilution Factor . The dilution factor, DF, is the ratio of the final volume (e.g., for the bottle method, the volume of the BOD bottle, usually 300 mL; for the graduated cylinder method, the volume of the cylinder method, the volume of the cylinder, usually 1,000 mL) to the volume of sample therein. DF for the bottle method = Volume of Diluted Mixture/Volume of Sample in Mixture.

B. DO fixation • 1. Slowly siphon three portions of aerated

dilution water into three separate BOD bottles. Avoid adding atmospheric O2 to dilution water.

2. To two of the three BOD bottles, add 1 2. To two of the three BOD bottles, add 1 ml MnS04 solution, followed by 1 ml alkali-iodide-azide reagent. Submerge pipette tips in sample when adding reagents. Rinse tips well between uses.

3. Stopper carefully to exclude air bubbles; mix by inverting bottle several times.

4. When precipitate has settled to about half the bottle volume, carefully half the bottle volume, carefully remove the stopper and add 1.0 ml conc. sulfuric acid. Re-stopper and mix by gentle inversion until the iodine is uniformly distributed throughout the bottle.

C. DO Measurement 5. Transfer 203 ml of sample into conical

flask and titrate with 0.0250N sodium thiosulfate to a pale straw color.

6. Add 1-2 ml of starch solution and continue to titrate to first disappearance continue to titrate to first disappearance of the blue color. (200 ml of original dilution water is equal to 203 ml of dilution water plus reagents.)

•

• Determine the initial concentration of dissolved oxygen in one bottle of the mixture of sample and dilution water (DO ), and in one of the water (DOinitial), and in one of the bottles containing only dilution water.

• Place the other bottles in the incubator

• Incubate the blank dilution water and the diluted samples for 5 days in the dark at 20 °C.

• After 5 days, remove the bottles, fix the DO and measure the DO (DOfinal)DO and measure the DO (DOfinal)

• Calculate BOD5• BOD5 (mg/L) = {DOinitial (mg/L) of first

bottle - DOfinal (mg/L) of second bottle} x dilution factor

BOD incubator and BOD

bottle

BOD monitors

BOD monitors

Chemical Oxygen Demand (COD)

• Like BOD, COD provides a measure of the amount of organic compounds in water. The difference is that COD is less specific since it measures is less specific since it measures everything that can be chemically oxidized rather than just levels of bio-degradable organic matter.

• COD also is different in that it reflects the oxidation based on a specific chemical oxidant (dichromate).

• COD value is always greater than BOD for every wastewater sample.BOD for every wastewater sample.

Objective Questions1. COD value is always _____________than

BOD value.

2. First stage BOD is also known as ___________________________.

3. Second stage BOD is also known as 3. Second stage BOD is also known as _______________________.

4. For BOD test sample is incubated at ____0C.

5. ______ is indicator of the degree of organic pollution of water.

6. ______test is basis for BOD test.

Theory QuestionsQ1. Define the following

i. BOD

ii. Dilution factor

iii. COD

Q2. Explain ‘BOD test’.