Demystifying the Permeate Pump
What is a permeate pump:
The permeate pump is one of my favorite RO/DI accessories. It is used to reduce the waste water of an RO/DI drinking water unit. Its a really unique device and its one of the few things in the world where you really seem to get something for nothing. The permeate pump is a hydraulically powered pump that is used to force ro product water (also called permeate) into the pressured storage tank. The unique thing about the pump though is that it does this without using electricity. The pump is hydraulically powered using the energy from the waste water (also called brine water) that is going down the drain. The higher pressure on the waste water line is basically using the mechanisms in the pump (described below) to push the water into the tank. By pushing the water in and isolating the backpressure of the pressurized storage tank from the RO membrane it makes the unit much more efficient (less waste, higher output, etc).
Who does it apply too:
Its important to note that the permeate pump really only applies to applications with a significant amount of backpressure on the membrane. The most common installation where your going to find this is on a system with a pressurized storage tank (usually a system used for drinking water). This doesn’t apply to people who just have an “atmospheric” storage tank. Atomospheric storage tanks are things like barrels, stock tanks, aquariums, etc where the tank itself is open to the air. In these situations there isn’t actually any backpressure (save for a bit of gravity) so there isn’t any backpressure for the permeate pump to isolate the membrane from. In other words, while it won’t hurt anything, it won’t do much.
So why is backpressure important? Because it fights against the incoming pressure that your membrane so desperately needs. We all know that low water pressure decreases the performance of an RO membrane. When the pressure is low you end up with decreased water production, increased waste water, lower rejection rates/higher TDS, and reduced membrane life. So lets say you have 50psi (right on spec for a Dow Filmtec 75gpd membrane) on your drinking water system. You are ready to go right? Well not so fast. Sure your unit is going to work. When it first turns on you will even have great performance, assuming other conditions are perfect you are likely making 75gpd, (ie. ~3gph) and a waste to product water ratio of about 3:1 and everything is just rocking. The problem is that if you come back to that unit in an hour and checked it you would likely see that its running at something like 10 gallons per day at a 10:1 waste water ratio. Why you ask? Yeah you guessed it, backpressure!
As your RO membrane produces water it starts to fill the pressurized storage tank. The tank is sealed and a fixed volume so as the water fills the tank, the pressure in the tank begins to increase (this is why when you go to dispense it, it forces its way out and at a fast speed, because its being pushed out under that pressure). This pressure that builds up is the backpressure, the more pressure that builds up the more backpressure there is pushing back on the membrane. The backpressure basically has a cancelling effect on the incoming pressure, so if you have 50PSI coming in and the storage tanks pressure is slowly increasing to 20psi of backpressure its like your membranes operating pressure is slowly declining from 50psi down to 30psi. So as that backpressure builds up your creating less product water, more waste water, and lower rejection rates. The first gallon of water into the tank might take 20 minutes, the last gallon of water in the tank might take an hour. The actual numbers are going to vary depending on water temperature and other factors but these numbers are pretty typical. A permeate pump isolates the pressurized storage tank from the membrane so that it can run at its full potential from empty tank to full tank (technically you do see a little bit of performance loss right at the end), all just by utilizing the hydraulic pressure of the waste water (which you weren’t going to use anyway). Pretty sweet!
Installing a permeate pump is also highly encouraged if you want to connect your drinking water system to an icemaker, coffee maker, or other device. Most appliances require higher water pressure then you would get out of a typical drinking water setup without a permeate pump. The main reason is that a permeate pump has the added benefit of also working as an auto shut off valve, except its an auto shut off valve that turns the unit off when the product water line has reached line pressure. This is in comparison to a normal auto shut off valve that will turn your unit off when pressure reaches roughly 60% of line pressure.
Folks often confuse permeate and booster pumps so its worth pointing out the distinctions between the two. A booster pump is an electronically powered pump that installs before an RO unit and is used to increase the incoming pressure to the RO/DI unit. Most membranes require 50-65psi of water pressure in order to operate at spec, if your pressure is lower then that you will get decreased performance (low enough and it may not work at all). If your pressure is low, a booster pump is designed to fix that. A permeate pump on the other hand is designed to reduce backpressure, but it does NOT increase your incoming pressure. If your incoming pressure is low, the permeate pump will not increase it and a booster pump will still be necessary.
How does it work:
How the permeate pump accomplishes this is actually pretty amazing and while it seems like a lot to grasp at first, its actually pretty simple. Inside the permeate pump is a bladder that is divided in two, one side holds product water and the other side waste water. The in/out ports have a series of check valves and flow valves controlled by the mechanics of the pump and the pump essentially operates on two strokes.
On the first stroke of the pump permeate water is allowed to enter one side of the bladder. As the bladder fills the divider in the bladder pushes towards the waste water side of the pump until it is full. During this time a valve is closed that blocks waste water from exerting pressure on the bladder. The exit port for the waste water to the drain is open and it freely enters and leaves the pump. Once the bladder is full the pump moves on to its power stroke.
On the pumps power stroke the port allowing the waste water to exit is closed and the port to the waste water side of the bladder is opened. This allows the waste water to push against the bladder. This force pushes the product water that has been saved up in the bladder, out of the bladder and into the storage tank. That way you are using the force of the waste water to push the water into the tank instead of the force of the product water line (which would result in a lot of backpressure on the membrane).
Installation of the permeate pump is a breeze and the instructions are printed right on the pump, you just have to get used to some terms you may not use every day. The pump itself has four 1/4″ push connect ports on it. There is a Permeate In, Permeate Out, Brine In, and a Brine Out. While these terms are pretty common in the RO industry there not as common in the hobby so I will explain. Permeate is just the product water and Brine is the waste water. So basically, product water in/out, waste water in/out. Not too complicated! There is one catch, you do need to follow the big arrow that says “this side up”. This allows the air bubbles to escape the pump.
To install the permeatepump take the product water line out of the membrane and route it to the “Permeate In” port on the pump. The “Permeate Out” line then connects to the rest of your drinking water setup (generally the line that goes to your faucet/storage tank). To install the waste water line route the waste water tube (after the flow restrictor) into the “Brine In” and connect the “Brine Out” to the line that goes to your drain. Be sure to mount the pump with the outlets facing up and you are ready to go.
Once the permeate pump is installed you can go ahead and remove the auto shut off valve. This step isn’t necessary but is highly recommended if you plan to use the unit to supply water to an icemaker, coffee maker, etc because it will allow the tank to pressurize up to line pressure. Just removing the valve is all you need to do. The auto shut off function of the permeate pump will work automatically.
I hope that clears things up for folks. How many folks have a drinking water system, do you have a permeate pump too? Any more information you think should be added? Let me know in the comments below!