WEF Discussions — Now on LinkedIn!

WEF is pleased to announce the creation of free LinkedIn groups associated with its technical discussion forum topics. Through these LinkedIn groups, members will be able to view and participate in discussions, as well as communicate with one another via direct messages. Share your experiences and knowledge, ask questions and respond to other discussions as frequently as you like!

You must have a LinkedIn profile to join a LinkedIn group; you can create your profile here. If you already have a LinkedIn profile, all you need to do is submit a request to join one or all of the following groups:

Water Environment Federation (Main) | Biosolids | Collection Systems | Nutrients | Stormwater 
Utility Management | Water Reuse | Water for Jobs | Watershed Management | Laboratory Practices 

RSS Feed Print
BOD higher than COD + toxicity?(2)
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?

Luis Manriquez_1
Posted: Thursday, February 19, 2015 10:50 AM
Joined: 1/6/2010
Posts: 22

At first glance it does look like toxicity. In this case I would only use the result from the smallest volume. However, SM now requires reporting of the average of all replicates, albeith with a qualifier.


Posted: Tuesday, April 26, 2016 9:30 PM
Joined: 4/26/2016
Posts: 1


Have you tested your effluents for ammonia? The COD test doesn't oxidize ammonia. If your BOD sample does not include a nitrification inhibitor and the effluent has nitrifiers as part of its microfauna community, the BOD sample may be oxidizing the ammonia. It doesn't take a lot of ammonia to exert a high oxygen demand. The rule-of-thumb that nitrification does not occur until 5 days into a test is just that a rule-of-thumb; it may not apply to your real-world situation.