SWRO System Design Guide

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This guide walks through the engineering decisions required to design a seawater reverse osmosis (SWRO) plant: from initial site/water assessment through pretreatment, membrane and pump selection, energy recovery, controls, and post-treatment. It assumes familiarity with the fundamentals in our Reverse Osmosis Basics guide.

1. Site Assessment & Water Analysis

The single most important input to SWRO design is a complete, recent feed-water analysis. At minimum, obtain:

• TDS (35,000 mg/L typical open ocean; 38,000–42,000 in the Arabian Gulf and Red Sea).
• Temperature profile — min, max, design. Membrane flux drops ~3% per °C below 25 °C; pump and ERD sizing must accommodate winter cold.
• SDI₁₅ (target < 3 after pretreatment per ASTM D4189).
• Full ionic composition — Na, K, Ca, Mg, Cl, SO₄, HCO₃, Br, F, NO₃, Si, B.
• Boron — 4–5 ppm typical seawater; WHO guideline 2.4 mg/L; irrigation limit often < 0.5 mg/L. May require second-pass RO or boron-selective ion exchange.
• Organics (TOC), turbidity, oil & grease, algae counts — biofouling indicators.
• Free chlorine, ORP, dissolved oxygen, H₂S.

See our Water Quality Parameters guide for interpretation. Open intakes require more aggressive pretreatment than beach wells; algae blooms drive UF selection.

2. Capacity Sizing

Define:

• Net product capacity (m³/day or GPD) — what the user actually receives.
• Plant availability — typically 90–95% accounting for CIP, cartridge changes, intake outages.
• Peak vs. average demand — if peak/avg > 1.3, consider product storage instead of oversizing.
• Redundancy — N+1 trains for critical service. Two 50% trains and a 50% spare is common for municipal < 10 MGD.
• Future expansion — size building, intake, and outfall for the 10-year forecast even if Phase 1 stops short.

Design capacity = (Net capacity) ÷ (Availability) × (1 + Peak factor margin).

3. Recovery & Concentration Polarization

SWRO single-pass recovery is typically 35–50%. Higher recovery means smaller intake/outfall, less feed pumping, but higher brine TDS and osmotic pressure. Brine TDS scales roughly as:

C_brine ≈ C_feed × (1 − Y · R) / (1 − Y)

At Y = 45% on 38,000 mg/L feed (R = 99.7%), brine reaches ~69,000 mg/L with π ≈ 55 bar. Concentration polarization factor β (1.1–1.2 design target) raises the effective wall concentration further. Check the projection software (DuPont WAVE, Hydranautics IMSDesign, Toray DS2) for maximum-element-recovery flags and scaling indices (LSI, S&DSI, CaSO₄, BaSO₄, SrSO₄, SiO₂).

4. Pretreatment Selection

Pretreatment must deliver SDI₁₅ < 3, turbidity < 0.2 NTU, free chlorine < 0.1 ppm, and adequate antiscalant dose to RO feed.

Step	Equipment	Notes
Coarse/fine screening	Drum or band screens, 1–3 mm	Open intake only
Coagulation	FeCl₃ or polyaluminum dosing	For algal/organic feeds
Clarification / DAF	Dissolved-air flotation	Algae blooms, high TOC
Media filtration	Dual-media (anthracite/sand), pressure or gravity	5–10 gpm/ft²
Ultrafiltration (UF)	Hollow-fiber UF (Inge, Pentair, Toray)	Preferred for open intakes; SDI<2
Cartridge filter	5 µm nominal pleated PP	RO guard; 3–5 gpm per 10″ element
Antiscalant	King Lee Pretreat Plus 0100, Genesys LF, Avista Vitec	2–5 ppm typical
Dechlorination	Sodium metabisulfite (SMBS) or activated carbon	3 ppm SMBS per ppm Cl₂

5. Membrane Selection

• DuPont FilmTec SW30HRLE-440i / SW30XLE — benchmark high-rejection low-energy SWRO element; 99.8% nominal rejection; 7,500 gpd nominal flow at 800 psi test.
• LG Chem NanoH2O SW440 ES — thin-film nanocomposite (TFN); high flux at lower pressure, good boron rejection.
• Toray TM820V-440 — very high rejection and chemical durability.
• Hydranautics SWC6 MAX — balanced flow/rejection, robust for variable feeds.

Trade-off: high-rejection (HR) elements give better product TDS and boron rejection at higher feed pressure; low-energy (LE) elements cut SEC but pass slightly more boron and TDS. For potable, two-pass with partial second-pass on the front-end permeate is common when boron must reach < 0.5 mg/L.

Design flux 12–15 LMH (7–9 GFD) for open intakes; 14–17 LMH for beach-well feeds. Vessels of 7 elements each, 6–8 elements is standard.

6. High-Pressure Pump Selection

Pump Type	Strengths	Typical Range
Danfoss APP axial piston	High efficiency (88%+), compact, oil-free, ideal for small-medium SWRO and solar-driven systems	0.4–88 m³/h
CAT triplex plunger	Robust, serviceable, good for variable-pressure containerized units	0.5–25 m³/h
Grundfos CR / CRN multistage	Stainless multistage centrifugal; widely available; lower efficiency at SWRO pressure	1–180 m³/h
FEDCO MSD / MSS	Investment-cast Super Duplex, water-bearing technology, 87% hydraulic efficiency	7.5–1,080 m³/h

7. Energy Recovery Choice

For any SWRO above ~30 m³/day, an energy recovery device (ERD) pays back quickly. See our Energy Recovery guide for the math.

Criterion	FEDCO HPB Turbocharger	ERI PX Pressure Exchanger
Transfer efficiency	80–83%	95–97%
Mixing (brine into feed)	None (separate streams)	1–3% (ceramic rotor)
Booster pump needed?	No (built-in boost)	Yes (small circulation pump)
Footprint	Compact, single device	Multiple PX units in parallel
Maintenance	No external lubrication, single rotor	Ceramic rotor — 15+ yr life
Best fit	Single-train 50–5,000 m³/day, simpler hydraulics	Large municipal plants where SEC dominates

8. Pressure Vessel Sizing

Codeline 80S100 (1,000 psi) and 80S125 (1,250 psi) are the workhorse FRP pressure vessels for 8″ SWRO elements. Element count per vessel is typically 6 or 7. The membrane projection software determines:

• Number of vessels per stage
• Elements per vessel
• Lead-element flux (avoid exceeding manufacturer limits)
• Last-element recovery (avoid scaling)

9. Controls & Instrumentation

Modern SWRO trains run on Allen-Bradley CompactLogix or Siemens S7-1200/1500 PLCs with HMI (FactoryTalk View, WinCC) and optional SCADA integration. Required loops:

• Feed flow, permeate flow, brine flow (electromagnetic)
• Feed pressure, membrane inlet pressure, ΔP each stage, ERD pressures
• Feed conductivity, permeate conductivity per train and per vessel (for probe-down element ID)
• pH, ORP (post-dechlor), free chlorine (post-dosing if used)
• Tank levels, antiscalant/SMBS day-tank levels
• Pump vibration and motor current
• VFDs on feed pump and ERD booster for soft start and capacity trim

10. Post-Treatment

Permeate at 200–400 mg/L TDS is corrosive and lacks alkalinity. For potable service:

• Remineralization — calcite contactor (limestone bed) with CO₂ injection, or lime + CO₂ dosing, to add 40–80 mg/L Ca²⁺ and 60–100 mg/L alkalinity. Target LSI > 0.
• UV disinfection — 40 mJ/cm² minimum.
• Chlorination — sodium hypochlorite for residual.
• Final pH adjustment — NaOH if needed.

Industrial uses (boiler feed, semiconductor, pharma) often require additional polishing (EDI, mixed-bed, degas) instead of remineralization.

11. Specific Energy Targets

With a properly designed ERD train, modern SWRO can achieve 2.5–4.0 kWh/m³ total plant SEC (including intake pumping, pretreatment, HP feed, post-treatment, product pumping). The HP-feed-only contribution can be as low as 1.8–2.2 kWh/m³. Without ERD, expect 5.5–8 kWh/m³ at HP feed alone.

12. Sample Single-Train P&ID Description

A representative 200 m³/day (53,000 GPD) containerized SWRO train using FEDCO HPB-60 energy recovery:

1. Seawater intake pump feeds a 25 m³ equalization tank.
2. Multimedia filter (dual-media, 8 gpm/ft²) followed by 5 µm cartridge filter.
3. Antiscalant dosing (3 ppm) and SMBS dosing (3 ppm).
4. High-pressure pump (Danfoss APP 21, ~21 m³/h, 70 bar) delivers feed to HPB-60.
5. HPB-60 boosts brine-side feed by ~24 bar.
6. Combined feed enters two 8″ pressure vessels in parallel, each loaded with 7 FilmTec SW30HRLE-440i elements.
7. Permeate to product storage; brine through HPB-60 brine port to outfall.
8. Permeate post-treatment: calcite contactor + UV.
9. CIP skid with heated tank, dedicated CIP pump, and chemical totes.
10. Allen-Bradley CompactLogix PLC, PanelView 7 HMI, conductivity / flow / pressure instrumentation throughout.