You are overdosing the sodium sulfite in your water. It will exhibit an oxygen demand of its own. That amount might be good for wastewater but, as you have found out, it will ruin your BOD results. Instead of just adding a bulk amount indiscriminate of the true TRC level I would recommend one of the following options.

• First off you can try to avoid using the sodium sulfite entirely. Either get a better source of water that is chlorine free or
• Allow it to sit for a while before using it so the chlorine can dissipate.
• Do the dechlorination exactly like you would for a sample. Get 50 mL of the dilution water, treat it with acid, KI, and starch indicator then titrate it with your sulfite solution. Add a proportional amount to the dilution water.

These are in order of desirability for strictest method compliance. Because SM does not address the removal of TRC from the source water, you will probably be out of compliance with any moderately strict auditor that comes by if you try treating it.