RO unit with/without permeate pump

Recently was shopping for an RO unit and came across a model that had a 'non-electric permeate pump'. One of the alleged benefits is less waste water. For whatever reason, I expected the pump to be on the high side of the membrane (higher pressure means less waste water). But this one has the pump on the low side of the membrane. The pump has four ports; RO IN, RO OUT, Brine IN, and Brine OUT. Does it work because it relieves any pressure on the low side, therefore letting the membrane see the full water pressure delta across it at all times? Do typical 'no permeate pump' units get less and less efficient as the tank fills and builds pressure?

I'm just curious is all. I might swap it it for the less efficient non-pump unit because the permeate pump is obnoxiously loud. It also really 'shoots' the Brine Out water into the drain, enough that it actually sprays a few droplets up out of the drain on occasion. I installed the drain saddle as close to the p-trap as I could.

SpanishCastleAle said:

Recently was shopping for an RO unit and came across a model that had a 'non-electric permeate pump'. One of the alleged benefits is less waste water. For whatever reason, I expected the pump to be on the high side of the membrane (higher pressure means less waste water). But this one has the pump on the low side of the membrane. The pump has four ports; RO IN, RO OUT, Brine IN, and Brine OUT. Does it work because it relieves any pressure on the low side, therefore letting the membrane see the full water pressure delta across it at all times?

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Anything which increases the pressure across the membrane increases the flow. You can increase pressure across the membrane by increasing the pressure on the feed/brine side or decreasing the pressure on the permeate side. The latter is clearly what is involved here. The source water pressure operates the pump which sucks on the other side of the membrane. You are limited to 1 atmosphere increase i.e. if feed pressure is 30 psig (45 psia) the greatest pressure differential possible is 1 atm more than this i.e. 45 psia - 0 psia = 45 psia (3 atm) as opposed to a no pump system with the permeate going to atmosphere: (45 - 15 = 30 psia ~ 2 atmospheres.)

OTOH a pump on the feed side can give you greater flow and, thus, greater recovery. In my particular system, for example, the feed pump delivers 150 psig (11 atm absolute) and as the permeate goes to an atmospheric tank the pressure across the membranes is 10 atm. With this setting I can get pretty close to 50% recovery.

SpanishCastleAle said:

Do typical 'no permeate pump' units get less and less efficient as the tank fills and builds pressure?

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Yes, but that's what they are supposed to do. When bladder tank pressure plus osmotic pressure equals feed pressure flow stops (the tank is "full").

SpanishCastleAle said:

I'm just curious is all. I might swap it it for the less efficient non-pump unit because the permeate pump is obnoxiously loud. It also really 'shoots' the Brine Out water into the drain, enough that it actually sprays a few droplets up out of the drain on occasion. I installed the drain saddle as close to the p-trap as I could.

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High recovery is a trade. If your water contains temporary hardness setting a system for high recovery will result in early failure of the membrane (from clogging with calcium carbonate deposit). Lower recovery ameliorates this but results in more water "wasted". If you are hooked to a water meter and use the RO system for everything in the house then higher recovery (and pre treatment of the feed with a softener) is probably preferred. For occasional use as a source of brewing water this aspect shouldn't be a consideration.

Thanks. It is pretty much just for brewing. I just really wanted to try to reduce the waste water, not for money but just because I hate wasting water and brewing already uses a lot.

But I never water my lawn like everyone else it seems so I should get a break.

If you're going to pump into a bladder tank, then a permeate pump is a good idea. If you're going to pump into an open (non-pressurized) vessel, then you don't need the pump, but you'll need a float valve.

I agree that the pumps are annoyingly loud. I haven't figured out how to quiet it yet.

mabrungard said:

If you're going to pump into a bladder tank, then a permeate pump is a good idea. If you're going to pump into an open (non-pressurized) vessel, then you don't need the pump, but you'll need a float valve.

I agree that the pumps are annoyingly loud. I haven't figured out how to quiet it yet.

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This one has a 4 gal tank but I don't think it's a bladder tank. Their cheapest unit (5-stage, no pump) has what they call '3.2 Gallon NSF Certified Metal Storage Tank with Poly Liner' but they have an identical system with tank upgrade that has a '4 Gallon NSF Certified Composite One-Piece Poly Tank'. This unit with the permeate pump has the 'premium' tank.

We have pneumatic intensifiers at work so I figured this pump works similar to that (essentially uses different surface areas at each end of a piston and check valves) and it definitely has a similar sound.

Bladder tank is another type of pressure tank. Sounds like you've got it covered.

Those tanks are really small and you might consider adding an extra and larger tank to the system. I have my original 3 gal tank and an extra 20 gal tank. These are the total volume of the tanks and the amount of water that they will actually return is only about 50% of that volume. So you would have problems discharging a larger volume of RO at a single time for a larger batch. The larger tank was a recommendation from John Blichmann in his presentation at the National Homebrewers Conference in Minneapolis last summer. It is sage advice. Even 20 gal is not enough if I do a 10 gal batch.

There are a couple of ways to skin this cat. One is to simply store the RO water in your HLT and/or other brewing vessels and/or an atmospheric tank reserved for this purpose. You can just go with the system as shipped (i.e. with small bladder tank) and empty the small pressure tank into the storage tank several times over the course of the few days before a brew session or you can just open the valve and have the permeate dribble into the collection tank starting a few days before. This is obviously easier than emptying the small tank several times.

But a bigger tank is clearly the better way to go. The water is stored away from light and dust and is under pressure for easy dispense/measuring. The bigger the pressure tank the better from the point of view of having plenty available on brew day.

Another option is to have the RO cartridges drain directly into an atmospheric tank and then use a pump to fill a bladder tank from the atmospheric tank. This is really nice in that the pressure in the pressure tank can be set by you - not the feed water pressure and the bladder tank can be smaller. It is now the size of the atmospheric tank that determines how much water you have on hand. The RO water supply is now almost identical to your well's supply (if you are on a wll). The disadvantages here are that you now have 2 tanks (pressure and atmospheric) and that you must have pressure controls to turn the pump that fills the pressure tank on and off, float switches to make sure the pressure pump doesn't come on when the atmospheric tank is empty and shut off the RO unit should the atmospheric tank become full (and turn it back on when the level in the atmospheric tank drops), and overpressure protection to protect the pressure tank in case the rest of the controls fail to turn the pressure pump off.

Almost forgot a major, but unanticipated advantage: when my well pump failed on Christmas day (remember that was a Saturday last year) there was enough water in the RO system bladder and atmospheric tanks to get us through until Tuesday of the following week. No showers for anyone but at least we could cook, clean up and flush toilets.

Good info. I was going to fill a 5 gal water bottle during the week for now. It's nice that it all fits under the sink and still leaves decent storage space.