Mark,

Could you expand on your question. I am just not sure what you are trying to ask.

Different Mark

Standard Methods says you must dechlorinate chlorinated samples prior to analysis for NH3 or BOD. 

Dechlorinating agents such as sulfite can exhibit an oxygen demand so you don't want to "over" dechlorinate.  Small amounts of chlorine may dissipate while the sample awaits processing.

Would a sample that had 1.5 mg/L Cl2 residual be considered dechlorinated if you knocked it down to 0.1 mg/L?  Would that level inhibit a BOD test?  Would that level interfere with an ammonia test by ISE?

Mark, if you are looking for a chlorine concentration below which the BOD is not affected, I don't think you are going to find one (someone else will have to address NH3).  I have never seen such a concentration specified anywhere. 

The result you get for BOD is determined by the method...follow the method, and the result you get IS the desired result (if all other method requirements are met).  If you dechlorinate with sodium sulfite by titration, and add the number of drops determined as called for in the method, you have "dechlorinated" as required by the method.  Is there still some chlorine in the sample!  Probably so, most prominently as combined chlorine which releases slowly.  Does that chlorine "interfere" with the test?  Depends on what you mean by "interfere".  It will kill some of the seed bacteria, but written between the lines in the method, that is acceptable because you have followed the method.

I would imagine the reason you are asking the question is that you might want to do a quantitative test for chlorine (e.g., maybe total chlorine by DPD) and if the result is less than a certain number, you wouldn't have to dechlorinate.  Such reasoning would be valid except for one thing...it doesn't follow the method.

Your concert over excess sodium sulfite having an oxygen demand is valid.  That's why when you reach the end point on the titration when determining how many drops are needed to dechlorinate each bottle, you don't wait for a while to see if more titrant is needed...that could go on all day because of the slow release of chlorine from combined forms.  Get to the first end point, record the number of drops, and go for it!

Our permit requires that we have a total chlorine residual of less than 0.05 mg/L in the effluent. Normally there is a sulfite residual in the sample as we titrate it with a Hach drop count method. If there was a 0.04 chlorine residual we would add a drop of 1 % sodium sulfite and check to make sure that we get a 0.00 chlorine residual or a sulfite residual.

As for doing BOD's for another WWTP I would ask them to dechlor the sample prior to delivering the sample to your lab. They know the chlorine residual and how much sulfite to add to dechlor it better than anyone else. you would just have to check for chlorine or sulfite residual to ensure no chlorine residual.

You do have to be aware that there are some interferences with the DPD residual chlorine test. Oxidized manganese is the most common and nitrite if present can interfer. We use the ampherometric method of chlorine analysis to avoid interference.

Excess sulfite from effluent dechlorination is part of the BOD result but excess sulfite from lab dechlorination is a positive bias.