Well, this is a very loose talk. Even the plant suppliers also sound very loose when they easily claim that 50% of water is only usable, & the rest is thrown, waste. This has to be corrected properly, let's do it :-
It's not 50 % flat
Agreed, the RO system, in a way to filter out even the dissolved solids, reject certain salts, which go with water. However, this depends upon how much quantity of salts or solids are there in it first. There are many minerals, the collective count is called as the TDS. Roughly the count of the TDS will give you how many these salt particals are there in the Raw Water. It's measured in PPM (Particals per million) : How many salt particals are there in 10 Lakh total Water Particals.
There is a simple device called TDS Meter, which can easily tell you about the same. If the TDS is 200, meaning just 200 particals in total 10 Lakh (1 million) particals. Depending upon the level of impurity it is decided as to how much water can be bottled as pure water. It can be 90% also if the raw water is of very good quality.
It's not waste, It's Reject Water
It is allowable to used in own toilets/sanitary system or trees/plants/gardens to be used which ultimately will flow to the sewage system to get further filtered ultimately. Hence it is NOT waste water, it's reject water.
Can the Reject Water be re-used ?
Technically, yes. Practically : Not advisable for Water Bottling Plants. Why ? You can read a special post here on Zero Liquid Discharge Plants
