A permeate pump is a small non-electric device fitted to a reverse osmosis system that uses the energy in the waste stream to push treated water into the storage tank. It solves a specific problem: as the tank fills, back pressure builds against the membrane, production slows and the waste ratio gets worse. The pump takes that back pressure out of the equation.
The problem it solves
A reverse osmosis membrane produces at a rate set by the difference between feed pressure on one side and the pressure it has to push against on the other. In a tank system the treated water is pushed into a bladder tank, and as that tank fills the bladder pushes back harder. By the time the tank is two thirds full the membrane may be working against most of the incoming pressure, so production crawls and the ratio of water sent to drain against water produced gets steadily worse. The system still works, it just works badly at exactly the time it is doing most of its filling.
How the pump gets around it
A permeate pump sits between the membrane and the tank and is driven entirely by the waste stream, which would otherwise go straight to drain carrying its pressure with it. That flow drives a hydraulic piston that pushes treated water into the tank in small pulses, so the membrane never sees the tank's back pressure directly. It is not electric, needs no outlet under the sink, and makes a distinct clicking or knocking noise as it cycles, which is normal operation rather than a fault. Waste water still goes to drain, but its pressure does useful work first.
What you actually get
Three things improve together. Production rate rises, because the membrane spends the fill cycle at closer to full differential pressure. Waste ratio improves, because less water goes to drain per unit of treated water produced. Water quality improves slightly, since membranes reject dissolved solids better at higher effective pressure, which is why the last glass out of a slowly filled tank often tastes worse than the first. In homes with low incoming pressure, all three effects are larger, which is why low-pressure households are the strongest case for fitting one.
When to fit one, and when not to
Fit one if you have a tank-style under-sink system, incoming pressure at the low end of what your unit specifies, and you are irritated by slow refill or by how much water goes to drain. Do not fit one to a tankless reverse osmosis system, because there is no storage tank creating back pressure and nothing for the pump to solve. Do not fit one alongside an electric booster pump unless the manufacturer says the pair is supported. And do not expect it to fix a fouled membrane; if production dropped suddenly, change the pre-filters first and ask us for a quote if the diagnosis is unclear.
Questions people ask about permeate pump
Does a permeate pump need electricity?
No. It is driven entirely by the pressure in the reverse osmosis waste stream, which is why it can be fitted in a cabinet with no outlet available.
Why does my permeate pump click?
The clicking is the hydraulic piston cycling as it pushes treated water into the tank. Regular clicking during production is normal; continuous clicking when nobody is drawing water suggests a leak or a failed check valve.
Will a permeate pump work with a tankless RO system?
No. Tankless systems have no storage tank building back pressure against the membrane, so there is nothing for a permeate pump to improve.
Does a permeate pump replace a booster pump?
They do different jobs. A booster pump raises feed pressure into the membrane; a permeate pump removes back pressure from the tank side. Very low pressure homes sometimes need the booster instead.