Hard to say sensible things ...
Published by Martin P. Pelle, Water Technology Advisor, Engineering & Built Project Utility Team EMEA at Cargill
Hard to say sensible things without having wider range of analyses. I assume 'RO first round' means first pass RO permeate? 'RO second round' = second pass RO? If so, why is the second pass RO water dirtier than your first? Strange configuration? You say tube well, suggesting it may be from deeper? If from deep underground you would expect anaerobic water.....which would mean nitrate shouldn't even be there? SO: or you are aerating your well water (maybe to remove iron or manganese or methane?...but I do not see the full range analyses) or it is from a shallow well, or there is pollution entering the watertable of your well. If so: try/drill a deeper well?
Another suggestion that may help: continuous sandfiltration with continuous backwash via air-lift pump, that simultanuously makes the filter bed circulate slowly. Controlled methane dosing into the bed allows limited amount of biomass to remove the NO3- to N2. DynaSand is one tradename (ao Nordic Water), also Paques offers it. Good luck.