7/17/2019 Expeiment 8

http://slidepdf.com/reader/full/expeiment-8 1/1

Questions:

1) Define BOD?Ans: BOD stands for Biochemical oxygen demand. Biochemical oxygen demand  or B.O.D.  is the

amount of dissolved oxygen needed by aerobic biological organisms in a body of water to brea down

organic material !resent in a given water sam!le at certain tem!erature over a s!ecific time !eriod. "he

 biochemical oxygen demand #BOD) is an em!irical test$ in which standardi%ed laboratory !rocedures are

used to estimate the relative oxygen re&uirements of wastewaters$ effluents and !olluted waters. 't is the

measure of re&uired oxygen demand for the degradation of organic material for both carbon and nitrogen

demand. "he BOD value is most commonly ex!ressed in milligrams of oxygen consumed !er liter of 

sam!le during ( days of incubation at * +, and is often used as a robust surrogate of the degree of 

organic !ollution of water.

) -hy natural water and ta! water cannot be used for !re!aring dilution media in the BOD test?

Ans: ,hlorine is often !resent in ta! water in order to control microbial contamination. As chlorine

would interfere with the microorganisms used in the BOD test$ it should be removed from the water used

for the dilutions and the blans. "he same is true of other commonly used ta! water disinfectants#chloramines$ etc.)

 

ost natural waters contain small &uantities of organic com!ounds. A&uatic  microorganisms have

evolved to use some of these com!ounds as food. icroorganisms living in oxygenated waters use

dissolved oxygen to convert the organic com!ounds into energy for  growth and re!roduction. /o!ulationsof these microorganisms tend to increase in !ro!ortion to the amount of food available. "his microbial

metabolism creates an oxygen demand !ro!ortional to the amount of organic com!ounds useful as food.

0nder some circumstances$ microbial metabolism can consume dissolved oxygen faster than atmos!heric

oxygen can dissolve into the water. o that2s why natural water and ta! water cannot be used for 

 !re!aring dilution media in the BOD test.

3) -hat is the role of nitrifying bacteria in BOD test?

Ans: 4itrifying bacteria is the bacteria which oxidi%e !roteinous matter for energy. "he nitrifying bacteria

are usually !resent in relatively small numbers in untreated domestic wastewater. "he re!roductive rate at*, is such that their !o!ulations do not become sufficiently large to exert an a!!reciable demand for 

oxygen until about 5 to 1* days. Once the organisms become established$ they oxidi%e nitrogen in the

form of ammonia to nitrates and nitric acids in amounts that induce serious error in BOD estimation.

6) -hat is the history of BOD test?

Ans: "he 7oyal ,ommission on 7iver /ollution$ which was established in 158( and the formation of the

7oyal ,ommission on ewage Dis!osal in 1595 led to the selection in 19*5 of BOD( as the definitive test

for organic !ollution of rivers.

() -hat is seeding?Ans: /rovides microorganisms to oxidi%e organic matter. "o ensure that all other conditions are e&ual$ a

very small amount of microorganism seed is added to each sam!le being tested. "his seed is ty!ically

generated diluting organisms with buffered dilution water. "his seed source will be the source of bacteria

added to the sam!le. "he bacteria are added to oxidi%e the biodegradable organic matter in the sam!le.

"his is the bacbone of the ex!eriment. 'f your bacteria fail to oxidi%e the organic matter$ then the test

will not wor. 't is also im!ortant to run a seed control to account for any BOD in the seed material.