Permeate pumps for RO systems?

[MateoinLosAngeles]

I've heard that buying a permeate pump, which costs about $50, and adding it to a classic under sink 5 stage RO system can help both increase water pressure and reduce waste water.

Is there any truth to that? Has anyone installed an additional permeate pump or bought a RO system with one?

[Ray]

That is NOT the purpose of a permeate pump.

A few RO basics to start:

(1) The throughput of the membrane is greater when the pressure difference across the membrane between raw incoming-, and pure outgoing (permeate) streams is greater.

(2) The flush water flow is fixed by the flow restrictor.

As the pressure tank fills, the backpressure to the RO system increases. That decreases the pressure differential across the membrane, slowing pure water output. So a system has its maximum permeate output when the tank is empty, but that gets less and less as the tank fills. (The flush water flow remains unchanged). When that backpressure reaches 2/3 of the incoming pressure, the hydraulic cutoff stops the flow of incoming water, shutting off both outgoing streams.

A permeate pump connects to both the pure water and flush water lines, and uses the flow of the flush water to drive an impeller that "pulls" on the pure water line from the system, reducing the apparent backpressure from the tank. So, the flow into the tank becomes more even, empty to full, meaning an average output higher than without the pump, hence, more efficient - the waste water flow is the same, but as it takes less time for the tank to fill, its overall volume is decreased.

Another RO fact:
(3) Membrane capacities are rated with water in the membrane housing being 77°F and 65 psi. Lower decreases output; higher increases it.

There are "booster" pumps to be placed before the system to increase the incoming pressure, which is a lot better for increasing production and decreasing the total water consumption.

If you have a 100 gpd system operating at test conditions, with a permeate pump, the membrane still can only produce 100 gallons of pure water a day. A booster pump on the input side, however, can increase the incoming pressure. At about 50 psi, the output is increased about 70%, making it a 170 gpd system.

[MateoinLosAngeles]

Thanks so much Ray, this is really helpful.

So basically there's no way to increase the output of RO water from the tank using either of those methods.

- A permeate pump could technically reduce the "amount" of flush water by evening the flow of RO water into the tank, making the process take less time, but it won't increase output from the tank or increase GPD.

- A booster pump placed before the system, can increase the GPD of the system, and perhaps reduce the overall water consumption by also reducing the time that the tank will take to fill up. It won't increase output from tank either.

Did I get that right? Is there a pump that can be installed from the tank to speed up the flow from tank to faucet?

I would imagine having the booster pump before the system will speed up the time that the tank takes to be filled. If a pump is installed to empty the tank faster (increasing the flow from the tank), having both systems could "even out" the flow?

[Ray]

Originally Posted by MateoinLosAngeles

Thanks so much Ray, this is really helpful.

So basically there's no way to increase the output of RO water from the tank using either of those methods.

- A permeate pump could technically reduce the "amount" of flush water by evening the flow of RO water into the tank, making the process take less time, but it won't increase output from the tank or increase GPD.

Correct

- A booster pump placed before the system, can increase the GPD of the system, and perhaps reduce the overall water consumption by also reducing the time that the tank will take to fill up. It won't increase output from tank either.

Yes, a booster increases the generation of pure water, but the flow restrictor keep the flush water flow unchanged, thereby increasing the efficiency.

Did I get that right? Is there a pump that can be installed from the tank to speed up the flow from tank to faucet?

I would imagine having the booster pump before the system will speed up the time that the tank takes to be filled. If a pump is installed to empty the tank faster (increasing the flow from the tank), having both systems could "even out" the flow?

The water output of a residential system is propelled by the air pressure below a rubber membrane in the tank, which is limited. Add to that the fact that delivery from the tank is typically 1/4" or 3/8" tubing, and you can see why it's limited, but for the intended purpose, cooking and drinking, it's not an issue.

Folks using RO for irrigation typically have the system deliver water through a float valve into a vented tank. When the tank is full, the float valve closes, increasing the backpressure to the system and the hydraulic cutoff stops all flow. When water is used, it opens again and reills the tank.

Delivery from the tank for irrigation is then done with a separate pump and (often) garden hose.