22 - Some BOD Problems

Some BOD problems Practice, Practice, Practice

Practice Problem #1 200 mL of Genesee river water was collected from jst below t!e brewer" 2 mL of river river water water dilted dilted to to 1 L, aerated and seeded $!e dissolved o%"&en content was '( m&)L initiall" *fter + da"s, t!e dissolved o%"&en content !ad dropped to + m&)L *fter 20 da"s, t!e dissolved o%"&en content !ad dropped to +- m&)L .!at is t!e ltimate BOD/

Soltion .e !ave mltiple data points  so we dont need to assme t!e rate constant, , to be 02- da"s 31 4ow wold "o se t!e data to calclate /

$!e e5ation BOD6 7 BOD 813e3t9 $!e problem is, we !ave : nnowns So, even if we now 2 of t!em 8for e%ample, t!e BOD6 at a &iven time9, we still !ave 2 left 2 nnowns re5ire 2 e5ations to determine t!em

$!e e5ation BOD6 7 BOD 813e 3t9  is a constant BOD is a constant

$!e e5ation BOD+ 7 BOD 813e 38+ da"s99 BOD20 7 BOD 813e3820 da"s99 ;f we compare t!e ratio, t!e BOD cancels

$!e e5ation BOD+ 7 BOD 813e38+ da"s99 BOD20 BOD 813e3820 da"s99 BOD+ 7 813e38+ da"s99 BOD20  813e3820 da"s99  *nd we now BOD+)BOD20 ;ts jst a nmber, call it <

$!e e5ation < 7 813e38+ da"s99   813e3820 da"s99  *nd we jst solve for = 4ow wold "o do t!at/

$!e e5ation or or particlar problem? 200 mL of Genesee river water was collected from jst below t!e brewer" 2 mL of river water dilted to 1 L, aerated and seeded $!e dissolved o%"&en content was '( m&)L initiall" *fter + da"s, t!e dissolved o%"&en content !ad dropped to + m&)L *fter 20 da"s, t!e dissolved o%"&en content !ad dropped to +- m&)L .!at is t!e ltimate BOD/

Soltion 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

BOD+ 7 813e38+ da"s99 BOD20  813e3820 da"s99 +0 7 813e38+ da"s99 12+0 813e3820 da"s99 0'@ 7 813e38+ da"s99   813e3820 da"s99

0'@ 7 813e38+ da"s99  

813e3820 da"s99

0'@  0'@ e 38209 7 1  e38+9 e38+9  0'@ e38209 7 130'@ 7 02: .e jst &rap! t!e left side as a fnction of  and loo to see w!ere it e5als 02: 8or "o can se solver on "or calclator9



eA3+ 3 0'@eA20

0

02:

002+

0:21+-:

00+

0:212

00'+

0+1''1

01

0+0-@'@

012+

0:'2(''

01+

0:-:+2(

01'+

0--12

02

0-+-@

022+

0-1@21

02+

02(1-(:

02'+

02:'-:

0-

02212:@

0-2+

01+'@

0-+

01'-0(1

0-'+

01+2-+

0:

01-+0(

$!e  vale is= 02( da"31 o can t!en se t!is and eit!er of t!e BOD6 to calclate ltimate BOD

BOD+ 7 BOD 813e 38+ da"s99 +0 m&)L 7 BOD 81  e 380#2(98+99 BOD 7 12@1 m&)L

Comparison to $!eoretical ;f we !ad simpl" assmed 702- da"s 31

BOD+ 7 BOD 813e 38+ da"s99 +0 m&)L 7 BOD 81  e 380#2-98+99 BOD 7 1-0 m&)L  *nd, if we calclated t!e BOD from t!e 20 da" data=

G4E BOD20 7 BOD 813e3820 da"s99 12+0 m&)L 7 BOD 81  e 380#2-982099 BOD 7 12@2 m&)L $!e ltimate BOD will not a&ree since t!e  is Fwron& .!ic! wold "o se/

20 da" is alwa"s better  20 da" s!old alwa"s be more accrate o are avera&in& more da"s *HD t!e reaction s!old be 0IJ complete b" t!en 8actall" J if t!e assmed  is even close to correct9

Practice Problem #2 200 mL of Genesee river water was collected from jst below t!e brewer" 2 mL of river water dilted to 2+0 mL, aerated and seeded $!e dissolved o%"&en content was '@ m&)L initiall" *fter + da"s, t!e dissolved o%"&en content !ad dropped to +' m&)L * second sample was obtained @0 da"s later and retested in identical fas!ion $!e intial dissolved o%"&en was '+ m&)L and, after + da"s, dropped to +- m&)L .!at is t!e ltimate BOD for eac! of t!e samples/ .!ic! water sample was cleaner/

Soltion 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/

Soltion Can "o alread" tell w!ic! is dirtier/ Since  is constant, t!e BOD + is as &ood a measre as t!e ltimate BOD $!e 2 nd test sample is dirtier t!an t!e first

ltimate BOD calclation Sample #1

BOD+ 7 BOD 813e38+ da"s99 2-( m&)L 7 BOD 81  e 3802-98+99 2-( m&)L 7 BOD 80@(--9 BOD 7 -:( m&)L Sample #2 2'+ m&)L 7 BOD 80@(--9 BOD 7 :02 m&)L

B$ B$ B$  *lwa"s eep in mind t!e limitations of an" test? BOD is not foolproof? t!e bi&&est falt bein& t!at it will miss !ms 8non3biode&radable or&anic componds9 Generall", if it is wron&, it is too low *lt!o&! it can also erroneosl" detect c!emical o%idation of inor&anic componds 8metals9  bt t!is is smaller t!an t!e !ms problem

Konroe Cont" .ater *t!orit" 

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