If the RO plants are inland ...

If the RO plants are inland brackish and are very small , then  lined evaporation ponds  with  fine jet spray (eg Turbomister)  will often be the lowest solution.  However the land requirement is high,  double liners can be demanded by environment bodies , and the final salt residue will need disposal, and there are issues of wild life protection  .     Thermal Zero Liquid Discharge plants are insanely expensive  in capex and opex .   Shallow well disposal has no sustainable future.       If a country  has many such  RO plants then it could consider a strategic  pipe network for brine  taking the brine to the sea.   If there is an existing solar salt farm then that could be used but it environmentally problematic to build a new solar salt farm  (bitterns issue).     If the RO plant is already near the coast, then  disposal to the sea  with jet diffusers is always the best solution.