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Some BOD problems

Practice, Practice, Practice

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Practice Problem #1

200 mL of Genesee river water wascollected from jst below t!e brewer" 2mL of river water dilted to 1 L, aerated

and seeded $!e dissolved o%"&encontent was '( m&)L initiall" *fter +da"s, t!e dissolved o%"&en content !addropped to + m&)L *fter 20 da"s, t!edissolved o%"&en content !ad droppedto +- m&)L .!at is t!e ltimate BOD/

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Soltion

.e !ave mltiple data points so we dont

need to assme t!e rate constant, , to

be 02- da"s31

4ow wold "o se t!e data to calclate

/

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$!e e5ation

BOD6 7 BOD 813e3t9

$!e problem is, we !ave : nnowns

So, even if we now 2 of t!em 8for e%ample, t!eBOD6 at a &iven time9, we still !ave 2 left

2 nnowns re5ire 2 e5ations to determinet!em

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$!e e5ation

BOD6 7 BOD 813e3t9

is a constant

BOD is a constant

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$!e e5ation

BOD+ 7 BOD 813e38+ da"s99

BOD20 7 BOD 813e3820 da"s99

;f we compare t!e ratio, t!e BODcancels

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$!e e5ation

BOD+ 7 BOD 813e38+ da"s99

BOD20  BOD 813e3820 da"s99

BOD+ 7 813e38+ da"s99

BOD20  813e3820 da"s99

 *nd we now BOD+)BOD20 ;ts jst anmber, call it <

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$!e e5ation

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$!e e5ation

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>or or particlar problem?

200 mL of Genesee river water wascollected from jst below t!e brewer" 2mL of river water dilted to 1 L, aerated

and seeded $!e dissolved o%"&encontent was '( m&)L initiall" *fter +da"s, t!e dissolved o%"&en content !addropped to + m&)L *fter 20 da"s, t!edissolved o%"&en content !ad droppedto +- m&)L .!at is t!e ltimate BOD/

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Soltion

BOD+ 7 '( m&)L + m&)L 7 +0 m&)L

  2 mL)1000 mL

BOD20 7 '( m&)L +- m&)L 7 12+0 m&)L

  2 mL)1000 mL

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BOD+ 7 813e38+ da"s99

BOD20  813e3820 da"s99

+0 7 813e38+ da"s9912+0 813e3820 da"s99

0'@ 7 813e38+ da"s99

  813e3820 da"s99

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0'@ 7 813e38+ da"s99

  813e3820 da"s99

0'@ 0'@ e38209 7 1 e38+9

e38+9  0'@ e38209 7 130'@ 7 02:

.e jst &rap! t!e left side as a fnction of andloo to see w!ere it e5als 02: 8or "o can se

solver on "or calclator9

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eA3+ 3 0'@eA20

0 02:

002+ 0:21+-:

00+ 0:212

00'+ 0+1''1

01 0+0-@'@012+ 0:'2(''

01+ 0:-:+2(

01'+ 0--12

02 0-+-@

022+ 0-1@21

02+ 02(1-(:

02'+ 02:'-:

0- 02212:@

0-2+ 01+'@

0-+ 01'-0(1

0-'+ 01+2-+

0: 01-+0(

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$!e vale is=

02( da"31

o can t!en se t!is and eit!er of t!eBOD6 to calclate ltimate BOD

BOD+ 7 BOD 813e38+ da"s99

+0 m&)L 7 BOD 81 e380#2(98+99

BOD 7 12@1 m&)L

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Comparison to $!eoretical

;f we !ad simpl" assmed 702- da"s31

BOD+ 7 BOD 813e38+ da"s99

+0 m&)L 7 BOD 81 e380#2-98+99

BOD 7 1-0 m&)L

 *nd, if we calclated t!e BOD from t!e 20da" data=

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G4E

BOD20 7 BOD 813e3820 da"s99

12+0 m&)L 7 BOD 81 e380#2-982099

BOD 7 12@2 m&)L

$!e ltimate BOD will not a&ree since t!e is Fwron&

.!ic! wold "o se/

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20 da" is alwa"s better 

20 da" s!old alwa"s be more accrate

o are avera&in& more da"s *HD t!e

reaction s!old be 0IJ complete b"

t!en 8actall" J if t!e assmed iseven close to correct9

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Practice Problem #2

200 mL of Genesee river water was collectedfrom jst below t!e brewer" 2 mL of riverwater dilted to 2+0 mL, aerated and seeded$!e dissolved o%"&en content was '@ m&)L

initiall" *fter + da"s, t!e dissolved o%"&encontent !ad dropped to +' m&)L * secondsample was obtained @0 da"s later andretested in identical fas!ion $!e intialdissolved o%"&en was '+ m&)L and, after +da"s, dropped to +- m&)L .!at is t!eltimate BOD for eac! of t!e samples/ .!ic!water sample was cleaner/

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Soltion

BOD+,1 7 '@ m&)L +' m&)L 7 2-( m&)L

  2 mL)2+0 mL

BOD+,2 7 '+ m&)L +- m&)L 7 2'+ m&)L

  2 mL)1000 mL

Can "o alread" tell w!ic! is dirtier/

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Soltion

Can "o alread" tell w!ic! is dirtier/

Since is constant, t!e BOD+ is as &ood ameasre as t!e ltimate BOD $!e 2nd 

test sample is dirtier t!an t!e first

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ltimate BOD calclation

Sample #1

BOD+ 7 BOD 813e38+ da"s99

2-( m&)L 7 BOD 81 e3802-98+9

92-( m&)L 7 BOD 80@(--9

BOD 7 -:( m&)L

Sample #22'+ m&)L 7 BOD 80@(--9

BOD 7 :02 m&)L

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B$ B$ B$

 *lwa"s eep in mind t!e limitations of an" test?

BOD is not foolproof? t!e bi&&est falt bein& t!at

it will miss !ms 8non3biode&radable or&aniccomponds9

Generall", if it is wron&, it is too low *lt!o&! it

can also erroneosl" detect c!emical o%idationof inor&anic componds 8metals9 bt t!is issmaller t!an t!e !ms problem

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