Correct Me if I'm Wrong - RO Question

[WibblyPig]

The water comes in, goes through 3 filters and then hits the reverse osmosis filter before going onto the DI cartridge.

 

The RO filter rejects 4 or 5 gallons of water for every 1 it makes. That rejected water is still cleaner than tap water as it has gone through 3 steps of filtration.

 

Any reason I shouldn't collect it in a Brute and use it for water changes on my African Cichlid tank?

[Neebles]

I wouldn't. The waste water is going to contain more concentrated levels of the nasties you are trying to filter out. You'd be better off with tap water.

[Tyron]

I just asked brs a similar question (about drinking water) and they told me that the tds of the rejected (waste) water would be higher than your source water.

[AZDesertRat]

No.
The tap water goes through a sediment filter to remove large floaty particles then through one or two stages of carbon to remove chlorine and maybe a little bit of organics then through the RO membrane which does 90-98% of the actual treatment then gets polished off by the DI resin.
The waste or more accurately brine stream has been though two "stages", sediment and carbon but these are really more asthetics than actual treatment since it has done very little physical treatment other than to remove the residual chlorine put in the water to protect us from viruses and bacteria and which have not been removed from that brine. The 3 or 4 gallons of brine also contain the now concentrated waste or contaminants removed from that 1 treated gallon so the TDS is now 20 to 25% higher than the original tap water was to begin with.

The sediment and carbon are really only there to protect the RO membrane which in turn acts as pretreatment for the DI resin.

Sediment and carbon filters have micron ratings of like 0.5, 1, 5 and 10 microns, pretty big really. The RO membrane removes down to 0.0001 microns, that is thousands of times smaller than the first two are capable of. You can see 40 microns with the unaided human eye so 5 or 10 microns is pretty darn big where 0.0001 microns is basically down to the atom or molecule level.

Take a look at this chart which shows the physical size of contaminants and what gets removed by what.

 

http://www.filters-cartridges.com/Filtration-Separation-Spectrum.pdf