Consider treating your CT ...

Consider treating your CT blowdown to remove hardness, silica, and TSS via lime softening.  Then filter with media filter or better yet UF prior to feeding an RO.  The RO reject is discharged as a high TDS stream and the high quality permeate is returned to the tower.  This effectively increases the cycles of concentration of your tower and reduces water consumption.  This scheme can be costly, so you have to look at how much you'll save on water vs. the capital cost to see of the ROI is acceptable.  This approach is normally used is arid areas where water is in short supply (such as the US Southwest).