Started Date :13.07.16- 18.07.16 ( test A)
Initial DO =2.22 mg/L. Blank DO= 8.72( after long time aeration ). Sample was diluted 3( 10ml- 20 ml-50 ml to 300 ml).
calculation : Test -1 ( for 10 ml )= (8.72-6.44 )*300/ 10 =68.4 mg/l
Test -2 ( for 10 ml )=(8.72-6.60 )*300/ 10 =63.6 mg/l
Test -3 ( for 10 ml )=(8.72-6.80 )*300/ 10 =57.6 mg/l
average=63.2 mg/l
Test -4( for 20 ml )=(8.72-1.63 )*300/ 20 =106.35 mg/l
Test -5( for 20 ml )=(8.72-1.84 )*300/ 20 =103.2mg/l
Test -6( for 20 ml )=(8.72-1.39 )*300/ 20 =109.95 mg/l
average=106.5 mg/l
Test -7( for 50 ml )=(8.72-0.29 )*300/ 50 =50.58 mg/l
Test -8( for 50 ml )=(8.72-0.48 )*300/ 50 =49.44 mg/l
Test -9( for 50 ml )=(8.72-0.32 )*300/ 50 =50.4 mg/l
average=50.14 mg/l
Test(B)
i started again taking new sample from same source.testing date 21.07.16- 26.07.16. everything is like previous.
average result of 10 ml= 11.6 mg /l (17.7+8.7 +8.4)
average result of 20 ml= 20.95 mg /l (22.95 +17.25+22.65)
average result of 50 ml= 47.22 mg /l (46.86+47.58).
i am very confused .what is my mistake.which is correct ?.
Hi Mijan,
although not being an expert in this field, I suspect the answer is in the "B" in BOD, Biological Oxygen Demand. The oxygen uptake is determined by the micro-ororganisms in you sample and the (albeit small) variations in the conditions under which your sample is stored. BOD is not to be compared with normal Chemical analysis.
kind regards
Francois
BOD is a dynamic system so slight variation is expected. Time and temperature can slightly influence your results. This test can not be equated with other chemical analysis.
Did you control the quantity of your seed sludge? How did you measure the DO?
what test did you use; did you use any particular seed and mixing in your BOD tests?
There is the oxytop equipment that makes easier to carry BOD; and finally what is your BOD/COD ratio?
I did not use seed sludge. test was carried out by DO probe ( HACH). Observe the result . no synchronized with each other.
https://water.usgs.gov/owq/FieldManual/Chapter7/NFMChap7_2_BOD.pdf
No seed sludge, which may affect the results. Also look at the DO guidelines in the US Army method. Seed sludge may be discussed in 'Standard Methods', but I don't have access to this at home.
I think you need to carry out COD test. It may help since BOD is highly dependent on microbial activity.
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