Around 75% of the Water fed ...

Around 75% of the Water fed into a RO filter, is wasted. Only 25% gets extracted as drinking Water.

 I think that given the high conductivity of the RO reject, electro dialysis could also be considered.The answer depends on the cost you are waling to pay for concentrate disposal and the volume of concentrate that has to be disposed of per day.  For small volumes (less than 1,000 m3/day) the lowest cost disposal is the discharge to the closest sewer system.  With the increasing cost of water and waste discharge, more companies are looking to recover and reuse RO reject water. While it is possible to reuse the reject water from an RO system by feeding it directly into a 2nd RO unit, the likelihood of scaling or fouling the 2nd RO without pre-treatment is rather high.

ENCON Mechanical Vapour Compression (MVC) Evaporators and ENCON Thermal Evaporators have proven to be effective technologies for dewatering RO reject / RO concentrate waste streams. In brief, evaporation is a time-tested methodology for reducing the water portion of water-based waste. The evaporator converts the water portion of water-based waste to water vapour, while leaving the higher boiling contaminants behind. This greatly minimizes the amount of waste that needs to be hauled off-site

Evaporation technology has always been more “hands off” than other wastewater treatment methodologies resulting in a dramatically lower labour cost. Evaporation technology can handle a much wider range of waste streams compared to membranes and traditional physical / chemical treatment methodologies. Finally, evaporation does a much better job of concentrating waste streams compared to other methods, thereby yielding a lower disposal volume and cost.

The various methods of disposal reject is as follows.
1. Watering plants.
2. Send to sewage disposal.
3. Reuse by passing through softener bed.
4. You can also evaporate , condensate can be reused, but costly, will lead to scaling/fouling.
5. You can also use this High TDS water for toilet flushing.

6. Waste water reusing method for washing clothes.