Is mixing of treated and potable water required before releasing into domestic flush systems?

We are from a society with 840 flats in 4 blocks of 21 floors each. Our builder and the STP person says that it is recommended to have a 50/50 or 60/40 ratio of sewage treated water and potable water before it is released to our domestic flush tanks. Is this ratio justified? We have seen earlier that when 100% STP treated water is used in flushes they stink and has a slightly yellowish colour but when mixed with 50% potable water they seem good. Pls advise if this is the norm.