How to Start Up a Marine SWRO Watermaker: Pre-Commissioning, Pretreatment, and Energy Recovery
Posted by ForeverPure Engineering Team on May 4th 2026
Onboard SWRO doesn't fail at sea because of pump curves — it fails because of biofilm in the pretreatment, salt creep into the high-pressure pump, or an energy-recovery device primed against the wrong direction. Marine and offshore commissioning is the same chemistry as land-based SWRO with three extra problems: vessel motion, intermittent operation, and inaccessible spares. This guide covers the start-up sequence used on offshore platforms and large yachts.
1. Verify Sea-Chest Intake and Strainer
Sea-chest (intake) screen at 2-3 mm; downstream basket strainer at 200-400 µm. Inspect for marine growth before commissioning — barnacles and mussels colonize within weeks. Specify silver- or copper-impregnated paint inside the sea-chest. Locate intake away from the engine sea-water cooling discharge to avoid hot-water plumes.
2. Pressure-Test the High-Pressure Loop at 1.5× Operating Pressure
SWRO operates 800-1200 psi. Hydrotest at 1500-1800 psi for 30 minutes. Marine systems use Duplex 2205 or super-duplex stainless plumbing because 316L pits in seawater. Inspect every flange, elbow, and instrument fitting. Sealing surface flatness within 0.005" matters — corrosion on a flange face will leak permanently.
3. Commission the Pretreatment Train
Standard sequence: intake → strainer → media filter (multimedia or sand) → cartridge filter (5 µm) → SBS/carbon dechlorination if intake chlorinated → high-pressure pump. Some platforms add ultrafiltration before cartridge for high-TOC offshore intakes. SDI15 after pretreatment must be < 4 (warranty) or ideally < 3.
4. Prime the Energy Recovery Device
Pressure exchangers (ERI PX) and Danfoss APP/iSave systems require careful priming — they spin at high RPM and dry-running destroys the rotor. Manufacturer procedure: fill the device with feed water, vent all air via top-mounted bleeders, prime the booster pump downstream, slowly bring the high-pressure pump online. Skip a step and the rotor flashes to vapor on first start.
5. Load Membranes With Anti-Bacterial Glycerin
SWRO elements ship with sodium-metabisulfite preservative. For marine vessels stored intermittently, reload elements with USP-grade glycerin + 1% propylene glycol after every long port stay (> 30 days). Loading direction matters: feed-end seal first, then concentrate-end. Lubricate o-rings with food-grade glycerin only.
6. Slow-Ramp High-Pressure Pump and Verify NDP
Increase pressure no faster than 50 psi/min. Watch first-element NDP — too low means too little flux, too high means crushed elements. Net Driving Pressure typical 500-700 psi at 30,000-35,000 ppm seawater. Permeate flow target on a 6-element vessel: 250-400 GPD per element. Salt rejection ≥ 99.5% on day one.
7. Set Antiscalant and Biocide Dosing Cadence
Antiscalant 4-6 ppm continuous (saturation indices for Ba/Sr/SO4 in seawater are easier than brackish but not zero). Shock biocide weekly: 1-2 mg/L DBNPA contact for 30 min, then full freshwater flush (the membranes can't hold biocide between operations). Document every dose.
8. Establish Freshwater-Flush Cycle for Idle Periods
Whenever the SWRO will be idle > 24 hours: full system flush with permeate or low-TDS make-up water for 5-10 minutes to displace seawater. Idle seawater grows biofilm rapidly. Long-term lay-up (> 2 weeks): glycerin reload or wet-store with 1% sodium metabisulfite.
9. Validate USPHS / Class Society Compliance and Document
USCG Vessel Sanitation Program (cruise), DNV-GL or ABS for offshore platforms — every flag/class society has a potable-water compliance regime. Sample log: feed/permeate TDS, free chlorine, total chlorine, HPC, total coliform, Legionella periodically. Submit pre-departure or pre-platform-handover packet. Marine and offshore overview.
Frequently Asked Questions
What permeate quality target for ship's potable water?
USPHS Vessel Sanitation Program: < 500 mg/L TDS, < 0.5 NTU turbidity, free chlorine 0.2-5.0 mg/L. WHO drinking-water standards apply for international voyages. Post-treatment chlorination is mandatory before entering the storage tank.
Why is biofouling worse offshore?
Open-ocean intake water carries higher TOC and biofilm-forming bacteria than terrestrial brackish wells. Intermittent operation (start-stop cycles between port stays or shift schedules) gives biofilm time to mature inside the pretreatment train. Continuous operation with low-dose chlorination at the intake (followed by SBS or carbon dechlorination) is the standard control.
Is an Energy Recovery Device necessary?
For systems > 5 m³/h (≈ 22 GPM), yes — energy recovery cuts SEC from 6-8 kWh/m³ down to 2-3 kWh/m³. Danfoss APP/PAH with iSave or APP-PSO, or Energy Recovery PX rotary devices, dominate the market. Smaller yacht systems use clutched pressure exchangers or just accept the higher SEC.
What about deck movement?
Membrane elements tolerate vessel motion fine. The risk is at the pretreatment cartridge filters and the chemical-feed metering pumps — splash and tilt cause priming loss. Specify gimballed metering pumps and pressurized cartridge housings with bleeders that hold water on tilt.
Talk to a ForeverPure Engineer
Industry-specific water treatment requires industry-specific equipment selection. Our application engineers ship and commission systems to marine and offshore sites worldwide and will scope the right equipment for your operating environment, regulatory regime, and uptime requirements.
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