AWP Labs · Water
RO system sizer
Enter the water you need and the water you have. Get feed and concentrate flows, permeate quality, osmotic pressure and the number of membrane elements and pressure vessels, arranged in stages with hydraulic checks.
Your inputs
Water source
Share of the feed that becomes product water.
Typical: seawater 7–10, wastewater 8–12, surface water 10–14, well water 14–18 gfd.
Results
Membrane elements required
46in 8 pressure vessels · Array 5 : 3
48 installed at 13.0 gfd average operating flux
- Feed flow
- 231.5gpm
- 1,262 m³/day
- Concentrate flow
- 57.9gpm
- 315 m³/day
- Concentration factor
- 3.97×
- Permeate TDS
- 18.5mg/L
- Concentrate TDS
- 7,945mg/L
- Osmotic pressure
- 91psi
- concentrate · feed 23 psi
- Feed per first-stage vessel
- 46.3gpm
- maximum 75 gpm
- Concentrate per last-stage vessel
- 19.3gpm
- minimum 16 gpm
Vessel array
Stage 1 · 5 vessels
Stage 2 · 3 vessels
How this is calculated
Flows. Feed = Permeate ÷ Recovery and Concentrate = Feed − Permeate.
Salt balance. Salts stay on the feed side, so the water gets saltier as it moves down the vessel. The average feed-side concentration is Feed TDS × ln(1 ÷ (1 − R)) ÷ R. Permeate TDS is that average times salt passage (1 − rejection), and concentrate TDS follows from the mass balance (Feed TDS − R × Permeate TDS) ÷ (1 − R).
Membranes. Membranes lose output as they age, so the design uses 85% of the clean flux: Elements = Permeate (gpd) ÷ (Flux (gfd) × 0.85 × Element area (ft²)), rounded up, then grouped into pressure vessels.
Stages. One vessel can only recover about 50–55%, so higher recoveries are split into stages with fewer vessels toward the end. Up to 55% recovery is 1 stage. From 56% to 80% it is 2 stages, where stage 2 holds the smaller of 40% of the vessels and concentrate gpm ÷ 16 (rounded down, at least 1). Above 80% it is 3 stages, at about 50%, 30% and 20% of the vessels.
Hydraulic checks. Concentrate per last-stage vessel must stay above 16 gpm for 8-inch elements (4 gpm for 4-inch), or the membranes foul and scale. Feed per first-stage vessel must stay below 75 gpm for 8-inch elements (16 gpm for 4-inch).
Assumptions
- Single-pass system with one average flux across all elements.
- Element count includes a 15% allowance for flux decline over the membrane life.
- Osmotic pressure uses about 0.0115 psi per mg/L, which fits sodium chloride waters.
- Temperature and feed pressure are not modelled.
- A preliminary sizing only. AWP runs a full membrane projection for every quote.
A preliminary estimate for planning. AWP confirms every figure against your water, site and permit before quoting.
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