How can you tell the efficiency (# of gallons of waste/gallon of purified water) of a particular RO unit? I'd eventually like to get as efficient a unit as I can afford, but I can't find this data for most units. Usually all I find is the number of gallons/day the unit can produce.
All RO and RO/DI units on the market, with only a couple exceptions, are 4:1 waste ratio. Thats 4 gallons of brine or waste for every 1 gallon of permeate or treated water. This is not "efficiency" in the RO world, efficiency is considered the rejection rate or how much TDS the membrane removes, typically 90 to 98%.
Membranes must be flushed so they do not plug or foul and become useless. The best way to accomplish this is by that 4:1 waste ratio which continously flushes the membrane while it is in use. There are those who offer a "flush kit" which is basically a ball valve that bypasses the flow restrictor which you open to manually flush TDS away. So far I have not seen any data to back up these flush valves so I would say they have limited value if any at all. To be of any benefit they would have to be used each and every time before you shut the RO system off to flush any accumulated TDS away from the membranes surface. The trouble is we all automate our water making with float valves or float switches and solenioids so we never know when the RO is shutting off and would miss the flush. Once you have missed flushing it a few times the solids begin to solidify or cake on the membran and can never be flushed away so its best to stick with the 4:1 waste ratio.
There are a few vendors promoting low waste systems but this is something major manufacturers and vendors have tried for 25 years and dismissed since it does not work well long term. There is one exception to this rule but it is also a $800 system which is microprocessor controlled and uses DI water to flush the system not tap water so is proven to work well. Not cheap but worth it in the long run if you plan to keep the system for long and make much water.
In reality RO and RO/DI systems don't waste a significant amount of water. You probably flush 4 or 5 times that amount down the drain each day with the toilet and shower. I would pay more attention to how efficient, the rejection rate, the system is as this leads to cost savings in membrane and DI replacements and higher water quality.
I too think that 4 (or 5) to 1 is typical. There are some that are listed as better, but I never looked at price (I have had mine for some 15 years). They may be a bit more efficient at higher temperatures (they will certainly produce more at higher temperatures) and a permeate pump might help. As the membrane ages it will get worse.
Membranes are less efficient at higher temperatures. Colder water actually produces higher rejection rate of removal efficiency.
The standard waste ratio is 4:1, not 5:1.
Permeate pumps ONLY work on drinking water type systems with a pressure tank, they use the pressure in the tank to drive the pump. They will not work on a reef type system which goes to a ATO storage reservoir or bucket.
A little research proves that salt rejection is better at cooler temperatures, so my comment was way out in left field.
4:1 is a claim all manufacturers make, but that assumes no back pressure - which would be true if you were not filling a tank. Do many folks really just dump it to a bucket? I don't...
My only experience with permeate pumps is with Aquatec. They claim to improve a host of things...I only know that they have worked well for me for some 15 years.
Permeate pumps ONLY work with a pressure tank period. Without the backpressure they do not function at all.
4:1 is not a claim, its what they are supposed to be set at with or without backpressure with a properly designed system. Many vendors use fixed type restrictors, most better vendors use capillary tubes which you take the time to trimn for an exact waste ratio. I wouldn'y own a fixed restrictor, RO is not really a plug and play or one size fits all. A few minutes fine tuning can extend the life of a membrane by years and reduce unnecessary waste significantly.
All of your comments need further research. I can provide more documentation if it would help.
I know its pretty technical and hard to follow but basically what is says is colder water has a higher viscosity so is more resistant to flow through a semipermeable RO membrane, thus lower TDS in the final product or permeate water but with higher waste.
Here is another article from Applied Membranes which has some easy to follow Figures or Graphs showing the effects of pressure, temperature and solute concentrations on both rejection rate and recovery rate. http://www.watertreatmentguide.com/f...erformance.htm
Here is an article from Spectrapure that gives you an overview of how a permeate pump works in conjunction with a pressure tank to keep that waste ratio at the recommended 4:1. Again permeate pumps will only work with pressure tanks so are generally not suitable for a Reef quality RO/DI system since pressure tanks tend to concentrate TDS due to TDS creep. http://www.spectrapure.com/faq_p0.htm#permeatePump
Here is the Aquatec Permeate Pump page. I swear by these for drinking water and pressurized systems, they really do work in that situation. http://www.aquatec.com/permeate.htm
Most of us with reef systems do really just dump it in a bucket, jug or Brute trashcan. The benefits are many compared to a pressurized system.
RO unit efficiency
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