Reverse osmosis permeate discharge pressure

[giuman]

Dear All,
I work in the water treatment field, especially in the design and construction of reverse osmosis skids.
I would like to know why the discharge pressure of permeate water from Reverse Osmosis is always 1 bar, or tipically near atmospheric pressure.
Also from literature is stated this condition, but I cannot find the reason why.

Thanks in advance
GM

 

[itsmoked]

My guess is this: A lot energy is required for the high pressures involved in RO. Any pressure in the permeate adds directly to the energy needed to create it. It's been found that 1 bar "is enough". Makers want their energy demands to be the lowest against competitors, any higher than the next company's and they're in a bad marketing position since energy is the biggest cost of running RO.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

RO essentially works by increasing pressure one side and then the water leaks out of the membrane

Any pressure on the pure water side will impact on the operation.

Most of the time this water just goes into a tank - why do you need pressure?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ashtree]

Actually it can be any pressure you like, and many plants that are multi-stage use bigger feed pumps and permeate throttling on the first stage and no interstage boost pump. But any permeate pressure above atmospheric will require the same pressure added to the feed. It is not unusual to supply enough pressure to lift the permeate into a tank. But that extra lift means increasing the feed by the same extra head.

Two traps however.

If the RO plant is achieveing 75% recovery, every Gallon of permetae you produce and lift into that tank, will require you to lift the pressure on 1.33 galloons of feedwater.

RO membranes also have limits on back pressure when shut off, usually 30-50kpa. So if you are lifting the permeate 6metres into a tank , you have to make sure that you protect the membranes against the excessive back pressure. You might do it with shut off valves but many systems have a drain down valve to dump the head pressure on shut down.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[nondimensional]

Instead of asking yourself why typical systems don't have more than 1 barg, ask why do I need anymore pressure. Agree with LittleInch here:

Most of the time this water just goes into a tank - why do you need pressure?

Also, agree with ashtree on higher energy consumption for feed water pump, although with a good energy recovery device you can recover most of it.

 

[ashtree]

You cannot recover the energy it takes to lift the water into the tank, otherwise you are getting something for nothing.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[nondimensional]

Ashtree,
I will have to respectfully disagree.

1x water goes to tank. You indicated that 1.33X water will have to be pressurized. The .33X is brine reject. Whatever additional head you put into the additional 0.33X water is still retained by that water, and CAN be recovered. If the tank is 200 ft higher (for whatever reason), then yes the 1.33x gets additional energy, but that is required to push the water higher. If you would rather have the RO Feed pumps not do this additional head, you will need a separate booster pump anyway. So, besides the inefficiencies of ERD system, energy can be recovered.

 

[ashtree]

I agree with your maths and totally understand. The ERD can not recover the energy required to lift the 1.0 into the tank. Such energy is stored as potential energy and is recovered when the water runs out of the tank by gravity.
The OP does not state what size systems he is dealing with and ERDs are normally only economical on what many wouuld call larger scale or on seawwater projects

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Simply inlet pressure to RO less the resistance across the RO membrane.