Reverse Osmosis · Water Treatment Notes

How an RO system moves
and balances water

Four figures, no electricity required. Follow the water from the city line, through the membrane, into the bladder tank, and out of your faucet — and see how the system shuts itself off when the tank is full.

Figure 01 — The whole loop

Where the water goes

Feed water is pushed through pre-filters into the membrane. There the stream splits in two: clean water (permeate) goes up to the tank and faucet; the leftover dirty water (reject) goes down to the drain.

Feed water Clean water (permeate) Reject water → drain — — — pressure sensing

Figure 02 — The tank with the rubber bag

How the bladder stores and pushes water

A rubber bladder holds the clean water. Around it is sealed air. As water fills the bladder, the air gets squeezed smaller, so its pressure climbs. That trapped air is the "spring" that pushes the water back out when you open the faucet.

EMPTY · air ≈ 7 psi

FILLING · air ≈ 20 psi

FULL · air ≈ 35 psi → shut off

Clean water (inside bladder) Sealed air (the spring)

Figure 03 — The split at the membrane

Why some water goes down the drain

The membrane only lets pure water pass through under pressure. The rejected contaminants must be flushed away with some water — that is the reject stream. A typical home system keeps about 1 part for every 3 parts sent to drain.

RECOVERY ≈ 25% — the pipe widths above are drawn to scale (1 : 3)

Why "waste" the water? The reject flow is what carries dissolved salts, PFAS, and other rejected matter off the membrane surface. Without it, the membrane would quickly clog. A flow restrictor on the drain line keeps just enough back-pressure so clean water keeps passing through instead of all rushing to drain.

Figure 04 — Interactive · the balance

Watch the system shut itself off

Press Produce and watch the tank fill: water rises, the trapped air is squeezed, and pressure climbs. When tank pressure reaches about two-thirds of the feed pressure, the ASO valve closes and production stops — no electricity, just pressure. Open the faucet to drop the pressure and it turns back on.

Tank fill: 0% · notice it stops well before 100%

7 psi

tank pressure · feed holds steady at 60 psi

● PRODUCING

The balance, in one line: feed pressure pushes water forward through the membrane; the filling tank pushes back. When back-pressure ≈ ⅔ of feed → the system rests. Draw water → pressure drops → it wakes up again. That self-regulating loop is why an RO system needs no pump or power to stay balanced.