Let's start with the basics.  ...

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Let's start with the basics.  What is your TSS level?  Also look at the level of non-biodegradable oils in your effluent.  COD will reduce and measure lots of compounds that BOD bypasses because the BOD test is unreliable at low levels.  One of the things which can interfere with the COD test is chlorides.  Check your chloride levels and consult the Hach.com website for interference with the COD test.  TSS can also give you a high reading on COD but won't show up on BOD.