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sludgejudgeH

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COD testing/ chloride
Mar 06^th, 2013, 12:31pm

A plant has chlorides, but not high (100-150 mg/L average in effluent)- currently using mercury free COD vials. In house data is reliable, just not sure how accurate. Going to look at running some split samples with the mercury vials to find out.

I was wondering if anyone has run into a similiar situation and what they found- were the mercury free ones accurate, high, low etc. I know the answer is to get the mercury vials and compare, but just wondering if anyone has anything to share on or what to expect.

thanks in advance,

Ryan Hennessy

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joeb YaBB Newbies Offline I love Water and Wastewater.com Posts: 12 Mt. Pleasant, SC Gender: Company or Organization: Environmental Express	Re: COD testing/ chloride Reply #1 - Mar 6^th, 2013, 2:49pm Chlorides will typically present a positive interferent as they are oxidized by the dichromate in the COD tubes. You want the mercuric sulfate in the COD tube to be at a 10:1 ratio of merucric sulfate to chlorides to work most effectively. Joe
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DrHenry God Member Offline Ride, boldly ride, The shade replied... Posts: 788 Raleigh, NC Company or Organization: CESTAB (see bio)	Re: COD testing/ chloride Reply #2 - Mar 6^th, 2013, 5:35pm Ryan, we have never had trouble up to 1000 mg/L chloride using the high or low range tubes. What you have to watch is that with the mercury tubes more COD may be detected, even if the chlorides were not present. You also need to run some dilutions to make sure you get the same answers.
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H2O_Tech Junior Member Offline Water and Wastewater.com is the best! Posts: 86	Re: COD testing/ chloride Reply #3 - Mar 7^th, 2013, 12:09pm I've got a copy of a guide that we no longer have on our web site that has some good information on mercury-free COD. Last revised in 2002. Has a chart that was developed to "estimate" differences based on the chloride and ammonia levels. Drop me an email if you want a copy. rsedlace@hach.com
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sludgejudgeH Full Member Offline I love Water and Wastewater.com Posts: 125 Wisconsin Gender: Company or Organization: Woodard and Curran	Re: COD testing/ chloride Reply #4 - Mar 11^th, 2013, 6:32am thanks everybody!
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Terry Powers

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Re: COD testing/ chloride
Reply #5 - Mar 13^th, 2013, 7:40pm

We have a COD monthly average limit of 60 mg/L. Our Influent chloride level is ~35 mg/L and Final Effluent is ~55 mg/L. We always use COD vials with mercuric sulfate.

There was a month several years ago when our Final effluent COD jumped from 50 mg/L to 80 mg/L in several samples. The operators were going crazy trying to find out what was wrong. With 2/3 of the month completed we had a COD average of 67 mg/L

I started looking at our lab worksheets and procedures and noticed we'd started using a new box of COD vials, ones without mercuric sulfate (we always record the lot# of the vials used in the test). I'd ordered a new batch of vials using the wrong catalog number. That was the cause of the COD increase.

It makes a difference.

Terry Powers

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