At infinite dilution you will reach 40 ppt. At any finite dilution you will be above 40 ppt. You may have decided that (say) 40.1 ppt is close enough to 40 ppt, and then calculated your dilution volume.
Unless you have another stream in your factory such as boiler or cooling tower blowdown that can dilute the salinity down to below 4.0%. Generally seawater salinity is around 3% if I recall correctly.