Reverse Osmosis Design Criteria and Fouling Control: A Feed-Type Design Checklist

1. Why Design Values Depend on Feed Type

Fouling of a reverse osmosis membrane is caused by particles and colloidal material in the feed that become concentrated at the membrane surface. The Silt Density Index (SDI) of the pretreated feed is the index of that fouling potential. Concentration of foulants at the membrane surface rises with increasing permeate flux, increasing element recovery and decreasing concentrate flow, so the stronger the expected fouling environment, the lower the average system flux must be set.

2. Design Limits by Feed Water Type

The following recommended values come from an industry RO design guideline. Average flux is in litres per square metre per hour; concentrate and feed flow in cubic metres per hour; pressure drop per pressure vessel in bar.

• Municipal supply: average flux 22 (19-25); lead element flux below 32; concentrate flow above 3.6; feed flow below 13; pressure drop below 2.0; element recovery below 15%; flux decline above 10% per year; salt passage increase above 10% per year.
• Brackish well water: average flux 29 (25-33); lead element flux below 43; concentrate flow above 3.0; feed flow below 16; pressure drop below 3.0; element recovery below 20%.
• Surface water with media filtration: average flux 21 (18-24); lead element flux below 31; concentrate flow above 3.6; feed flow below 13; pressure drop below 2.0; element recovery below 15%.
• Surface water with MF/UF: average flux 27 (23-29); lead element flux below 39; concentrate flow above 3.0; feed flow below 15; pressure drop below 3.0; element recovery below 17%.
• Secondary effluent with media filtration: average flux 11 (9-13); lead element flux below 19; concentrate flow above 3.6; feed flow below 12; pressure drop below 2.0; element recovery below 12%; flux decline above 15% per year; salt passage increase above 15% per year.
• Secondary effluent with MF/UF: average flux 17 (13-20); lead element flux below 25; concentrate flow above 3.6; feed flow below 13; pressure drop below 2.0; element recovery below 13%.
• Seawater intake with media filtration: average flux 14 (12-16); lead element flux below 28; concentrate flow above 3.6; feed flow below 14; pressure drop below 2.0; element recovery below 10%.
• Seawater intake with MF/UF: average flux 17 (15-19); lead element flux below 32; concentrate flow above 3.6; feed flow below 14; pressure drop below 3.0; element recovery below 12%.
• Seawater beach wells: average flux 17 (15-19); lead element flux below 35; concentrate flow above 3.6; feed flow below 15; pressure drop below 3.0; element recovery below 12%.
• RO permeate feeding a second pass: average flux 35 (30-39); lead element flux below 48; concentrate flow above 2.4; feed flow below 17; pressure drop below 3.0; element recovery below 30%.

Designing outside these bands is possible; it simply raises the probability of fouling.

3. Feed Water Gates

• SDI15: normally required below 5, preferably below 3. Above 5, rapid irreversible fouling occurs regardless of membrane quality.
• Free chlorine: polyamide thin-film composite membranes require below 0.1 mg/L and are practically operated at zero; breakthrough causes irreversible oxidation damage.
• Temperature: typically 5-45 degrees Celsius; the pH tolerance window for chemical cleaning is generally 2-11.
• Langelier Saturation Index: negative or near zero is preferred; a positive LSI calls for acid dosing or a change of antiscalant programme.

4. Four Coupled Design Variables

• Recovery: 50%-75% for single-stage systems, up to 85%-95% for multi-stage designs; seawater RO is normally limited to 40%-50% by osmotic pressure.
• Concentration factor approximates 1/(1 - recovery): about 2-fold at 50%, about 4-fold at 75% and about 10-fold at 90%. The higher the factor, the greater the risk of precipitating sparingly soluble salts such as calcium carbonate, calcium sulphate and silica.
• Operating pressure: roughly 10-20 bar for brackish water and reclaimed water, 55-80 bar for seawater, set by feed salinity, recovery and net driving pressure.
• Salt rejection: new thin-film composite elements typically reject 99.0%-99.8% of sodium chloride under standard test conditions (2000 mg/L NaCl, 25 degrees Celsius, 15% recovery, about 1.0 MPa). Industrial selection should be verified against the manufacturer certificate of analysis rather than data quoted for household point-of-use units.

The coupling matters: raising recovery raises concentrate osmotic pressure, which raises the feed pump pressure required and therefore specific energy. High recovery, high flux, high rejection and low energy cannot all be pushed to their limits at once; a designer must decide which two give way and confirm the choice with on-site piloting.

5. Specific Energy and Energy Recovery

• Brackish systems typically run at 0.5-2 kWh per cubic metre; seawater systems at 3-6 kWh per cubic metre.
• Energy recovery devices such as isobaric pressure exchangers typically cut seawater energy demand by 30%-60%.
• Specific energy outside those bands is a signal to re-audit the high pressure pump and energy recovery selection.

6. Diagnosing Four Recurring Failures

• Fouling (biological, organic, colloidal): permeate flow drops while feed pressure stays broadly constant. Cause is usually SDI above limit or inadequate pretreatment. Remedy: high-pH clean followed by low-pH clean, then upgrade the media filter.
• Scaling (calcium carbonate, calcium sulphate, silica): differential pressure across the last stage rises from a baseline near 0.5 bar to above 2 bar. Cause is usually recovery set too high or antiscalant under-dosed. Remedy: reduce recovery by 5-10 percentage points, verify antiscalant injection and adjust LSI.
• Oxidation damage: an abrupt 2-5 percentage point drop in rejection. Cause is usually chlorine breakthrough from exhausted activated carbon. The damage is irreversible; replace the carbon and re-audit the chlorine limit.
• Membrane compaction: gradual flux loss at constant pressure over 12-24 months after sustained operation above design pressure or temperature. Irreversible; elements must be replaced.

References

• Design Guidelines for the Design of Reverse Osmosis Membrane Systems (limits by feed water type)
• WHO, Guidelines for Drinking-water Quality (palatability reference for TDS)
• HJ 2010-2011, Technical Specifications for Membrane Biological Processes (basis for MBR as RO pretreatment)