When a reverse osmosis system underperforms on low pressure, there are two devices commonly recommended and they are not alternatives. A booster pump raises the feed pressure into the membrane. A permeate pump uses the energy in the reject stream to help fill the tank. They fix different problems, and buying the wrong one leaves the original symptom in place.
What low feed pressure does
Reverse osmosis is driven by pressure across the membrane, so when line pressure is low the system produces slowly, rejects fewer dissolved solids and sends a greater proportion of water to drain. The tank takes far longer to fill and the faucet delivers a weak flow. Low pressure also gets worse as the tank fills, because the back pressure from the storage tank opposes the feed pressure, which is why production tails off badly towards the end of a fill on a marginal supply.
When a booster pump is the answer
When the incoming line pressure is genuinely below what the system needs. A booster pump sits before the membrane, raises the feed pressure to a proper working figure, and improves production rate, rejection quality and the waste ratio all at once. It needs a power supply under the sink and adds a pump that can be audible, plus usually a pressure switch and a solenoid to stop it running when the tank is full. It is the correct fix for a low pressure supply.
When a permeate pump is the answer
When line pressure is adequate but the system still wastes a lot of water and slows as the tank fills. A permeate pump uses the hydraulic energy in the reject stream, which would otherwise go to drain, to push permeate into the tank, isolating the membrane from the tank's back pressure. It needs no electricity, reduces water sent to drain and lets the tank fill fuller. It does not raise feed pressure, so it will not rescue a genuinely low pressure supply.
Diagnosing before buying either
Measure the line pressure with a gauge on the cold feed rather than assuming, then check the storage tank's air charge with the tank empty, since a lost or wrong charge produces exactly the symptoms both pumps are sold to fix and costs nothing to correct. Check the flow restrictor and the automatic shutoff valve too. Fixing a tank charge is a ten minute job and is the single most common answer to a reverse osmosis system that has gone weak. Our RO installation guide covers the sequence.
Questions people ask about ro pump booster
Booster pump or permeate pump?
A booster pump when feed pressure is genuinely low. A permeate pump when pressure is adequate but the system wastes water and fills slowly.
Does a permeate pump need power?
No. It uses the hydraulic energy in the reject stream that would otherwise go to drain, so it needs no electrical supply.
Why does production slow as the tank fills?
The tank's back pressure opposes the feed pressure. A permeate pump isolates the membrane from that back pressure.
What should I check before buying a pump?
Line pressure with a gauge, and the storage tank air charge with the tank empty. A lost charge causes the same symptoms and is free to fix.