another RO waste water ques.

Yes and that is exactly how Spectrapure plumbs their 180 GPD dual membrane RO/DI as do others. They take the waste from the first membrane and feed it into the second membrane so you get double the production but they recommend you still keep the waste ratio up to ensure the membrane stays flushed. There are some who promote this as a water saving device but long term it does not work for most people as the second membrane fouls prematurely due to the concentrated TDS from the first membrane.

My personal system is the MaxCap UHE 100 GPD ultra low waste which uses a booster pump to flush the membrane with stored DI water and pickles the membrane in DI when its not in use. I run it at slightly less than 1:1 waste ratio, my tap TDS averages 600-800 and the unit has been in operation about 18 months with only one MaxCap DI filter change.

Boosting the pressure to a RO membrane does increase the removal efficiency or rejection rate but it does not reduce the waste ratio. The need to flush the solids away is still there.

 

+1 I was just reading about this today. There are some systems that feed the waste water back into the RO system so that it is reused.

 

There are not any where no water is wasted, thats not possible. There are some which reduce the waste such as the UHE, others that claim to reduce the waste by reducing the waste ratio but shorten the membrane land DI life in the process and yet another like the watts Premier Zero waste which dumps the waste into your hot water heater again which is not a good idea. All membranes must be flushed, there is no way around it, its all in how you deal with that waste.