Design Manual › Tertiary & Reuse › Reverse Osmosis
FREE ENGINEERING TOOL · REUSE
Reverse Osmosis for Water Reuse
Recovery is the number everyone wants to push, and the number two quiet constraints
will not let you push. As recovery rises the concentrate gets saltier, the osmotic pressure you must
beat rises with it, and the sparingly soluble salts arrive at their limit long before the membrane
does. This tool sizes the array from design flux, builds the pressure from osmotic pressure and
temperature-corrected permeability, and then runs the Langelier index and silica check on the
concentrate — the two that actually decide how far you can go.
1Duty & separation
Start with what the water is and what it has to become. Everything else — area,
pressure, energy — follows from those two numbers and the recovery you can actually achieve.
Results — permeate, area & pressure
Permeate
—m³/d
Membrane elements
——
Specific energy
—kWh/m³
| Item | Value | Basis |
|---|
2Membrane array
Area comes from flux; the element and vessel count comes from area. The array is then
staged 2:1 so the second stage keeps the crossflow up as the volume falls.
| Item | Value | Basis |
|---|
3Pressure & energy
Feed pressure has to beat the osmotic pressure of the concentrate, not of the feed —
and the temperature changes both the membrane's permeability and its salt passage.
| Item | Value | Basis |
|---|
4Scaling — the real recovery limit
This is where a reuse scheme gets decided. The concentrate, not the feed, is what
precipitates.
| Item | Feed | Concentrate |
|---|
5What passes, what does not
Rejection is not one number. Neutral species and small uncharged molecules cross RO
membranes freely, and that catches reuse schemes out.
| Species | Rejection | Consequence for reuse |
|---|
6Instrument schedule
RO is a differential-pressure and a two-conductivity process. Everything else is
there to protect the membrane.
| Point | Instrument | Why here |
|---|
Quote the RO instruments at factory price
Feed and permeate conductivity, differential pressure, flow, pH/ORP and level — all mappable to our catalogue and supplied at the manufacturer's ex-works price (our margin is the export rebate, not a markup on the device).
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Sources & parameter basis
| [1] | METCALF & EDDY / AECOM — membrane treatment: municipal reuse schemes typically treat secondary or tertiary effluent of 300–3000 mg/L TDS; single-pass brackish RO gives permeate well under 200 mg/L. |
| [2] | Element nominal salt rejection 96–99.5 % at standard test conditions (25 °C). Salt passage roughly doubles for every 5–6 °C rise, so a warm feed gives more water and dirtier water at the same time. |
| [3] | Recovery: 65–85 % for single-pass brackish RO on well-pretreated municipal effluent; 85–90 % only with concentrate recirculation or a second stage. Recovery is limited by concentration, not by the membrane. |
| [4] | Design flux 13–25 LMH for wastewater RO downstream of UF/MF; 8-inch element active area 30–40 m²; 4–7 elements per pressure vessel; arrays staged 2:1 or 3:2:1. |
| [5] | Permeability: flux = A·NDP, with A ≈ 1.5–4 LMH/bar at 25 °C, temperature-corrected by a factor of 1.03^(T−25). Osmotic pressure ≈ 0.00077 bar per mg/L TDS. Feed pressure = NDP + (π_avg − π_permeate) + ΔP/2. |
| [6] | High-pressure pump: kW = Q(m³/h)·P(bar)/(36·η). Brackish RO on municipal reuse typically lands at 0.4–0.8 kWh per m³ of permeate. |
| [7] | Langelier saturation index on the concentrate: pH_s = 9.3 + A + B − C − D with A = (log₁₀TDS − 1)/10, B = −13.12·log₁₀(T+273) + 34.55, C = log₁₀(Ca as CaCO₃) − 0.4, D = log₁₀(alkalinity as CaCO₃). Concentrate pH ≈ feed pH + log₁₀(CF). LSI above about +0.5 needs antiscalant and/or acid; above +1.8 reduce recovery. |
| [8] | Amorphous silica solubility is roughly 120 mg/L at 25 °C and rises with temperature and pH. Silica scaling is effectively irreversible — it is the constraint you cannot chemically fix after the event. |
Full derivation and worked example: ro-reuse.html. Push the
recovery from 75 % to 85 % and watch the concentrate LSI jump — the membrane would happily do it, the
calcium carbonate would not.
How do I calculate permeate and concentrate flow from RO recovery?
Recovery R = Qpermeate / Qfeed. So Qpermeate = R x Qfeed and Qconcentrate = Qfeed - Qpermeate. The calculator returns permeate and concentrate flow for any feed flow and target recovery you enter.
What actually limits RO recovery in water reuse - scaling, silica, or the Langelier index?
The recovery ceiling is set by concentrate scaling: the Langelier (LSI) index, silica saturation, and rising osmotic pressure. The calculator runs the Langelier and silica checks and shows the feed pressure needed, so you see the real recovery limit before the membranes foul.
How do I size RO membrane area and element count?
Membrane area = permeate flow / design flux, and the element count follows from area per element. The calculator converts your recovery and flux into membrane area, the number of elements, and specific energy.
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