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BOD higher than COD + toxicity?(2)
17658023
Posted: Tuesday, May 28, 2013 10:18 AM
Joined: 5/17/2013
Posts: 1


Hi everyone,

I'm a grad student, and have been trying to resolve multiple issues with the BOD test for a few months now.  Any advice or experience would be greatly appreciated.

 

The samples: effluents from constructed wetlands treating greywater.  Anaerobic conditions, high sulfides.

 

Procedure details- I've been using Polyseed and commercial GGA, DI with house-prepared nutrients for dilution water.  Both BOD and COD are run on the same day as sampling.

 

Issues- GGA always low, blanks periodically high, BOD always increases as sample volume decreases to the point that BOD > COD, BODs of split samples sent to a certified lab on two occasions were much lower than what I measured.

 

One example:

Dilution water blank = 0.3 mg/L

SCF = 0.97 mg/L

GGA = 120 and 135 mg/L

Sample 1:

3 mL: 2.41 mg/L depletion, BOD= 143.6 mg/L

5 mL: 3.00 mg/L depletion, BOD= 121.9 mg/L

10 mL: 4.49 mg/L depletion, BOD = 105.7 mg/L

20 mL- residual DO of 0.7 mg/L.

COD = 134 mg/L

Sample 2:

3 mL: 2.01 mg/L depletion, BOD = 104.2 mg/L

5 mL: 2.22 mg/L depletion, BOD = 75.1 mg/L

10 mL: 2.84 mg/L depletion, BOD = 56.0 mg/L

20 mL: 4.11 mg/L depletion, BOD = 47.1 mg/L

COD = 75.5 mg/L

 

Has anyone seen a situation like this before?  How would you interpret it?