I think even though your TDS ...

I think even though your TDS is very high (and there are ways to drop it).... What is your BOD level as that will override most, if not regulations and therefore determine what treatment I would recommend. Quite honestly, the fastest way to drop TDS especially highly organic TDS is to discharge the permeate from the sand filter to a settling tank for at least 90-120 minute and then through carbon filters.  The settling tank should remove at least half the TDS and if not, a coagulant and possibly a polymer may need to be added and mixed into the sand permeate  (in a separate tank) prior to entering the settling tank.  This would be the cheapest option by far but as with anything I do I would always run bench scale tests to make sure the performance fits your needs.  If this initial option fails then we need to jump to a whole other level of treatment which I won't even start talking about here as it will take pages to write.  I always start with basics before going further.  And no the polymer will not wreck your GAC filters just in case you are wondering... We do it all the time. 

Kurt