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DO interference
James Royer
Posted: Tuesday, September 20, 2011 11:50 AM
Joined: 9/21/2009
Posts: 98

I would like to point out a potential problem for the BOD analysis and DO values if the wastewater plant is dechlorinating with sulfite. We recently had to start monitoring the effluent for DO. We chlorinate and dechlorinate with sodium bisulfite. A comparison of DO on the effluent indicates low values with the Winkler method as compared with the Hach LDO.


The Hach LDO reads right at saturation at temperature and with considerable mixing that should be the correct DO. The Winkler reads about 1.5 mg/L lower. There is a sulfite residual that would account for this. Standard Methods indicates that reducing agent will give low values with the iodimetric methods.


Thus if the Winkler method is used for BOD analysis then the initial DO would read low and the BOD of the sample would be calculated low. I know most analysts use the probe method so it would not be a problem. In the past if there was a question about DO results we deferred to the Winkler but maybe we should be selective about DO interferences and methods.