I get a lot of calls from skippers who want help operating their reverse osmosis (RO) water makers. This guide explains the basics—how marine RO systems work, the main types you’ll see aboard, and step-by-step tips for safe operation, flushing, and storage.

What Is Reverse Osmosis?

Reverse osmosis forces seawater across a semi-permeable membrane under high pressure. Because water molecules are smaller than dissolved salts and contaminants, clean water (often called product or permeate) passes through the membrane while concentrated seawater (brine) goes overboard. Only a portion of the feed water becomes product; the rest is discharged as brine to carry away salts and debris.

Key Points

  • RO requires high pressure and a steady flow of seawater.

  • The membrane is not a filter; it’s a separation process that needs flow to keep the membrane surface clean.

  • Product water quality is typically monitored in parts per million (ppm) of total dissolved solids (TDS).

Common Marine RO System Types

1) High-Pressure Pump Systems

These use a high-volume feed pump and a piston-style high-pressure pump (similar to a pressure washer) to achieve operating pressure.

  • Pros: High output—makes a lot of fresh water quickly.

  • Cons: Higher power draw; usually requires a robust AC generator.

2) Energy-Recovery Systems

Energy-recovery designs (e.g., Spectra’s Clark Pump) reuse the energy in the brine discharge to reduce load.

  • Pros: Lower power consumption; often run on a vessel’s DC system.

  • Cons: Typically lower product flow per hour than high-pressure AC systems—great for sailboats or boats without large generators.

Controls: From Simple to Fully Automated

Marine water makers range from analog units with panel-mounted flow meters and a pressure gauge to fully automated systems with a remote control/display. Advanced panels can show:

  • PPM/TDS (water quality)

  • Product rate

  • Filter status

  • Faults/alerts

  • Programmable fresh-water flush intervals

Membranes: Sizes, Limits & Care

Recreational systems commonly use 2.5” × 21” or 2.5” × 40” membranes; larger or commercial units often use 4” × 40” or bigger. Always follow the manufacturer’s maximum pressure rating—exceeding it can damage the membrane.

Biofouling (biological growth) is the #1 membrane killer. Because seawater is loaded with organic life, most systems fresh-water flush the membrane after each run. Automated models typically flush every few days (e.g., about every 5 days); make sure the domestic fresh-water pump remains on so the auto-flush can function.

Chlorine caution: Chlorinated water can damage membranes. Systems include an activated carbon (charcoal) filter on the flush line—replace it about every 6 months or per the manufacturer.

Oil/diesel caution: Oil will foul a membrane. Before running your system in a marina, look for any fuel sheen; if present, wait until the water is clear. Install the seawater intake as low as practical to reduce the chance of drawing in surface contaminants.

Filtration: Strainers & Cartridges

Keep the sea strainer clean and change pre-filters regularly:

  • Typical sequence: 20-micron followed by 5-micron.

  • Store spares aboard (three full sets is a good baseline).

  • Mount filters where they’re easy to service and can drain cleanly to the bilge.

Understanding Water Quality (PPM/TDS)

Many systems divert product overboard until TDS falls below a set threshold (often around 750 ppm). Healthy systems commonly produce ~100–250 ppm (lower is better), and a rising ppm can signal membrane or pre-filter issues.

First Commissioning or Restart: A Practical Walkthrough

Always consult your specific manual first. If you don’t have one, download it from the manufacturer and keep a printed copy aboard.

Before you begin

  • Gather supplies: new filter cartridges, carbon flush filter, service hoses, bucket (5 gal), filter wrench, and storage/pickling chemicals.

  • Ensure the pressure relief/regulator is backed off (open).

Step-by-step

  1. Close the seacock. Remove, clean, and reinstall the sea strainer.

  2. Replace pre-filters (20 µm & 5 µm). Use a proper filter wrench—avoid pipe wrenches or large pliers.

  3. Open the seacock and check for leaks.

  4. Start the feed pump (if separately switchable) to purge air. If not, start the machine and let it purge. Expect noise—get familiar with normal sounds.

  5. Confirm a strong brine discharge overboard; verify there’s no fuel sheen on the water.

  6. Let the system run 30–60 minutes to flush chemicals/air and check for leaks. Tighten filter housings if needed and replace O-rings if leaks persist.

  7. Start the high-pressure pump. Gradually increase pressure with the regulator (or, for a Clark Pump system, close the black knob as specified).

  8. Do not exceed the rated pressure for your membrane.

  9. Watch the product flow meter climb and check TDS/ppm with your meter (panels can foul; taste and meter readings help confirm quality).

  10. If flow is low or ppm stays high, consider membrane cleaning chemicals (noting they can shorten membrane life).

Operating Tips & Troubleshooting

  • Default to low pressure when something seems off; flushing at reduced pressure can help clear contamination.

  • Learn your pressure relief valve—when to open (pickling, leak checks, air purge) and how to ramp up safely.

  • If product ppm is high or flow is low, check: strainer, pre-filters, carbon filter (for flush), air leaks, pump performance, and finally membrane condition.

Pickling & Winter Storage (Step-by-Step)

If you’re laying up the boat, pickle the system to prevent biological growth.

You’ll need: manual, bucket with lid, service hoses, non-chlorinated water (RO product or distilled), and the manufacturer-approved storage chemical (or propylene glycol in freezing climates).

  1. Back off the pressure regulator/relief.

  2. Run two fresh-water flush cycles (confirm the carbon filter is fresh to remove chlorine).

  3. Mix storage chemical per instructions in the bucket (using RO or distilled water).

  4. Move diversion/service valves as specified, connect hoses, and recirculate through the bucket until the system is fully treated.

  5. Return valves to run position and shut down.

  6. Remove and discard pre-filters and the carbon flush cartridge.

  7. Install new cartridges but leave housings empty for lay-up (follow your manufacturer’s guidance).

  8. Close the seacock.

  9. Label the system with date and storage status. You’re done until spring.

Words to Know (Quick Glossary)

  • Reverse Osmosis (RO): Pressure-driven separation that removes water from seawater through a membrane.

  • Product (Permeate): The drinkable fresh water produced.

  • Pickle: Store the membrane with preservative chemistry for lay-up.

  • Brine: Concentrated seawater discharge.

  • Pre-Filter: Cartridge filters (e.g., 20 µm & 5 µm) that remove sediment/algae before the pump/membrane.

  • Sea Strainer: Intake strainer that removes large debris/grass.

  • Feed Pump: Supplies seawater at volume to the high-pressure or energy-recovery stage.

  • High-Pressure Pump: Piston pump that generates RO operating pressure (often >600 psi).

  • Energy-Recovery/Regenerative Pump: Recycles brine energy to reduce power consumption (e.g., Clark Pump).

  • Membranes: RO elements (e.g., 2.5”×21”, 2.5”×40”, 4”×40”); quantity/size determines capacity.

  • PPM/TDS: Parts per million of dissolved solids; lower indicates purer water.

Need Hands-On Help?

If you’d like on-board training—from first start to flushing, troubleshooting, and pickling—I can meet you at your slip and walk you through your exact system.

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