Whole house reverse osmosis filtration pushes the entire household supply against a semipermeable membrane, splitting it into treated permeate and a concentrate stream that goes to drain. Understanding the process explains everything else about these systems: why they need pretreatment, why they store water, and why the finished water has to be corrected before it enters your pipes.
Crossflow, not dead-end filtration
A cartridge filter is dead-end: everything goes in one side and the captured material stays in the media until you change it. A membrane works differently. Feed water flows across the membrane surface while a portion passes through it, and the remainder sweeps the rejected salts and contaminants along and out to drain. That crossflow is what keeps the membrane from immediately caking over. It also explains the concentrate stream that people mistake for waste in a broken system: it is continuous and it is doing the job of keeping the surface clean while the system produces.
Pretreatment keeps the membranes alive
Three things kill residential membranes. Chlorine and chloramine attack the membrane material chemically, so a carbon stage upstream is mandatory on a municipal supply. Hardness scales the surface as water is concentrated against it, so a softener goes ahead of the membranes on hard water. Particulate and oxidized iron blind the surface mechanically, so sediment removal and, on a well, iron and manganese removal come first. Systems also monitor pressure across the array: a rising differential means fouling, and there is a cleaning regime with acidic and alkaline solutions that recovers performance before replacement becomes necessary.
Storage and repressurization
Membranes produce steadily and slowly, while a house draws in short heavy bursts. So permeate goes into an atmospheric storage tank, and a pump takes water from that tank back up to household pressure. A level control shuts production down when the tank is full and restarts it as the level falls. This is the architectural difference from a point-of-use system, and it drives most of the cost, the floor space and the electrical requirement. It also means the tank has to be sized against the household peak, kept covered and vented against contamination, and given an overflow to a drain.
Permeate is aggressive until you correct it
Reverse osmosis removes the calcium, magnesium and bicarbonate that buffer water and keep it chemically stable. What comes out is low in dissolved solids, slightly acidic and hungry for minerals, which makes it corrosive to copper and to the brass in valves and fittings. Every properly designed whole house system therefore finishes with pH correction, a calcite bed, or a controlled blend of pretreated water back into the permeate. Skipping that stage does not show up on day one; it shows up as blue-green staining and pinhole leaks later. Our whole house reverse osmosis system page covers how vendors specify that stage.
Questions people ask about whole house reverse osmosis filtration
Does whole house reverse osmosis filtration remove everything?
It removes most dissolved contaminants, but it is not a disinfection barrier and some small uncharged molecules pass. Pretreatment and post-treatment stages cover the gaps.
Why is there always water running to the drain?
That is the concentrate stream that sweeps rejected contaminants off the membrane surface. It runs whenever the system is producing and is a normal part of the process.
Can membranes be cleaned instead of replaced?
Yes. A rising pressure differential across the array indicates fouling, and a chemical clean often restores performance. Replacement follows when rejection no longer recovers.
Why does the treated water need minerals added back?
Permeate is low in dissolved minerals and slightly acidic, which makes it corrosive to copper plumbing and flat tasting. A calcite bed or blending stage corrects both.