TSB107

Summary

Product identification

This technical bulletin provides general information on foulants and cleaning procedures for Hydranautics' Composite Polyamide Reverse Osmosis (RO) membrane elements. This includes product lines such as ESPA, ESNA, CPA, LFC, NANO, and SWC. The information applies to 4-inch, 6-inch, 8-inch, 8.5-inch, and 16-inch diameter RO membrane elements.

Operating limits

Critical Membrane Protection Warnings:

• Composite Polyamide type RO membrane elements must not be exposed to chlorinated water under any circumstances, as this will cause irreparable damage.
  • Ensure no trace of chlorine is present in feedwater, cleaning, or storage solutions.
  • Neutralize chlorine residual with sodium bisulfite solution (1.8 to 3.0 ppm sodium bisulfite per 1.0 ppm free chlorine).
• Avoid the use of cationic surfactants in cleaning solutions, as irreversible fouling may occur.
• Membrane elements shall not be exposed to feedwater containing oil, grease, or other foreign matter that can chemically or physically damage the membrane's integrity.
• During cleaning, the permeate line and any permeate valves must always be open to atmospheric pressure to prevent damage from excessive permeate back-pressure.

Cleaning Triggers and Performance Degradation: Cleaning should occur when the RO shows evidence of fouling, just prior to a long-term shutdown, or as part of scheduled routine maintenance. Membranes should be maintained in a clean or "nearly clean" condition to prevent excessive fouling. Cleaning is recommended before the following limits are exceeded:

• Normalized permeate flow decrease is less than 10%.
• Normalized permeate quality decrease is less than 10%.
• Normalized pressure drop (feed to concentrate headers) increase is less than 15%.

If normalized membrane performance drops 30% to 50%, it may be impossible to fully restore performance to baseline conditions. A rough rule of thumb for acceptable cleaning frequency is once every 3 to 12 months.

Foulants and Cleaning Procedures

Common Foulants: RO membrane elements are subject to fouling by suspended or sparingly soluble materials present in the feedwater. Common foulants include:

• Calcium carbonate scale
• Sulfate scale of calcium, barium, or strontium
• Metal oxides (iron, manganese, copper, nickel, aluminum, etc.)
• Polymerized silica scale
• Inorganic colloidal deposits
• Mixed inorganic/organic colloidal deposits
• NOM organic material (Natural Organic Matter)
• Man-made organic material (e.g., antiscalant/dispersants, cationic polyelectrolytes)
• Biological (bacterial bioslime, algae, mold, or fungi)

RO Troubleshooting Matrix: Table 1 "RO Troubleshooting Matrix" summarizes the expected effects that common foulants have on performance regarding pressure drop, feed pressure, and salt passage, often indicating a specific location (e.g., first stage, last stage). See the official PDF for exact tabulated values.

Feed water

Pre-treatment Considerations:

• Ensure excessive amounts of coagulant are not fed to the RO system, as it can lead to fouling. Polyelectrolyte fouling can be very difficult to remove.
• Excessive inorganic coagulant can be measured using SDI filter equipment; iron on the SDI filter pad should typically be 3 µg/pad and never above 5 µg/pad.
• Particle counters are effective for measuring feedwater suitability: particles greater than 2 microns in size should be < 100 particles per millilitre.
• Contact Hydranautics technical department if phosphate levels in the feed are 5 ppm or higher.

Cleaning Chemistry

It is recommended that all RO membrane cleaning operations should be closely coordinated with Hydranautics during the RO membrane element warranty period.

Recommended Cleaning Sequence (for complex fouling):

1. Flushing with permeate with addition of non-oxidizing biocide (DBNPA or similar type) at the end of the flushing.
2. High pH CIP – Temperature versus pH as per recommendations.
3. Flushing with permeate until pH on the brine side is below pH 8.5.
4. Low pH CIP.
5. Acid flushing with permeate and non-oxidizing biocide (DBNPA or similar type).

General Precautions for Chemical Selection and Usage:

• Verify proprietary chemicals are qualified for use with Hydranautics membranes; instructions should not conflict with Hydranautics recommendations.
• Use the least harsh cleaning regimen (pH, temperature, contact time) to optimize membrane life.
• Clean at recommended target temperatures for efficiency and membrane life.
• Use minimal chemical contact time.
• Adjust pH prudently; a "gentle" pH range is 4 to 10, while the harshest is 2 to 12.
• Oil and biologically-fouled membranes should not use a low pH clean-up first, as oil and biological matter will congeal.
• Cleaning and flushing flows should be in the same direction as normal feed flow to avoid telescoping and element damage.
• For multi-stage RO, clean one stage at a time to optimize flow velocities.
• Flushing out detergents with higher pH permeate can reduce foaming.
• Verify proper disposal requirements for cleaning solutions.
• If biologically fouled, consider a sanitizing biocide chemical before and after cleaning.
• Always wear safety glasses and protective gear when working with chemicals.
• Do not mix acids with caustics; thoroughly rinse the first cleaning solution before introducing the next.

Hydranautics Recommended Chemical Cleaning Solutions (Based on Foulant):

Description of Cleaning Solutions:

• Solution 1: Low pH cleaning solution of 2.0% (w) citric acid (C₆H₈O₇). Useful for inorganic scale (CaCO₃, sulfates), metal oxides/hydroxides (Fe, Mn, Ni, Cu, Zn), and inorganic-based colloidal material. Citric acid is a powder.
• Solution 2: High pH cleaning solution (target pH 10.0) of 2.0% (w) STPP (sodium tripolyphosphate, Na₅P₃O₁₀) and 0.8% (w) Na-EDTA (sodium salt of ethylaminediaminetetraacetic acid). Recommended for calcium sulfate scale and light to moderate natural organic foulants. STPP is an inorganic chelating agent/detergent. Na-EDTA is an organic chelating agent. Both are powders.
• Solution 3: High pH cleaning solution (target pH 10.0) of 2.0% (w) STPP (Na₅P₃O₁₀) and 0.025% (w) Na-DDBS (sodium salt of dodecylbenzene sulfonate, C₆H₅(CH₂)₁₂-SO₃Na). Recommended for heavier levels of natural organic foulants. Na-DDBS is an anionic detergent.
• Solution 4: Low pH cleaning solution (target pH 2.5) of 0.5% (w) HCl (hydrochloric) acid. Harsher than Solution 1, useful for inorganic scale and metal oxides/hydroxides. HCl is available in concentrations like 18⁰ Baume (27.9%), 20⁰ Baume (31.4%), 22⁰ Baume (36.0%).
• Solution 5: Lower pH cleaning solution (natural pH 4-6) of 1.0% (w) Na₂S₂O₄ (sodium hydrosulfite). Useful for metal oxides/hydroxides (especially iron fouling), and to a lesser extent, sulfates. Strong reducing agent, requires proper ventilation. Powder form.
• Solution 6: High pH cleaning solution (target pH 11.5) of 0.1% (w) NaOH (sodium hydroxide) and 0.03% (w) SDS (sodium dodecylsulfate). Useful for natural organic foulants, mixed organic/inorganic colloidal foulants, and biological material. SDS is an anionic surfactant causing foaming. Considered a harsh regimen.
• Solution 7: High pH cleaning solution (target pH 11.5) of 0.1% (w) NaOH (sodium hydroxide). Useful for polymerized silica. Considered a harsh regimen.

Recipes for Cleaning Solutions (per 100 U.S. gallons / 379 liters of make-up water):

Make-up water quality should be RO permeate or deionized (DI) quality, free of chlorine and hardness.

pH and Temperature Limits for Cleaning: Table 4 provides Hydranautics pH and temperature limits for cleaning for various membrane types (e.g., NANO-SW, ESNA1-LF, ESPA1, CPA3, SWC4+). These limits define continuous operation pH ranges and maximum cleaning temperatures, which vary depending on the duration and intensity of the cleaning. Cleaning at the extremes may be more effective but can shorten membrane life due to hydrolysis. Cleaning exposure at pH limits is limited to 60 minutes below 40 °C and 30 minutes above 40 °C. See the official PDF for specific values for your membrane type.

Mechanical / hydraulic

Cleaning and Flushing Flow Rates per RO Pressure Tube: Pressures are not to exceed 60 psi (4 bar) at the inlet to tubes.

Cleaning Solution Volume Requirement per RO Element: These volumes do not include volumes required for piping, filters, etc., nor the initial 20% of volume dumped to drain.

RO Cleaning Skid Components: A well-designed RO cleaning skid is required for on-site cleaning.

• RO Cleaning Tank: Sized to accommodate water displacement, allowing 100% drainage, easy chemical introduction, recirculation, proper venting, overflow, and a return line near the bottom to minimize foam.
• RO Cleaning Pump: Sized for proper cross-flow velocity. Maximum recommended pressure is 60 psi (4 bar) at the inlet to pressure vessels.
• RO Cleaning Cartridge Filter: Normally 5 to 10-micron, designed to remove dislodged foulants.
• RO Tank Heater or Cooler: Maximum design temperature for cleaning is 113 °F (45 °C). Note that the cleaning pump can impart heat.
• RO Tank Mixer: Recommended for optimal chemical mixing.
• Instrumentation: To monitor flow, temperature, pressure, and tank level.
• Sample Points: For pH and TDS measurements off the pump discharge and concentrate return line.
• Permeate Return Line: Required for returning a small amount of cleaning solution that permeates through membranes back to the cleaning tank.

Cleaning Procedures

The time required to clean a stage can range from 4 to 8 hours.

General Procedure for Cleaning RO Membrane Elements:

1. Low Pressure Flush: Flush pressure tubes with clean water (RO permeate or DI quality, free of hardness, transition metals, and chlorine) at 60 psi (4 bar) or less for several minutes, sending flush water to drain.
2. Mix Cleaning Solution: Prepare a fresh batch of selected cleaning solution in the tank, using clean RO permeate or DI water. Adjust temperature and pH to target levels.
3. Circulate Cleaning Solution: Circulate cleaning solution through pressure tubes for approximately one hour or the desired period.
   • Initially, send displaced water to drain.
   • Divert up to 20% of the most highly fouled cleaning solution to drain before returning to the tank.
   • Slowly throttle flow rate: 1/3 max design flow for first 5 minutes, 2/3 max design flow for second 5 minutes, then full max design flow.
   • Readjust pH if it changes more than 0.5 pH units.
4. Optional Soak: An optional soak and recirculation sequence can be used, typically 1 to 8 hours, maintaining proper temperature and pH. Do not exceed maximum pH and temperature limits for specific elements.
5. Cleaning Rinse: After chemical cleaning, perform a low-pressure rinse with clean water (RO permeate or DI quality) to remove chemical traces. Drain and refill the cleaning tank with clean water. Rinse pressure tubes by pumping rinse water through them to drain.
6. Final Low Pressure Clean-up Flush: Once system is rinsed of chemicals, perform a final flush using pretreated feedwater at less than 60 psi (4 bar). The permeate line should remain open to drain. Continue until flush water is clean, free of foam or residues (15-60 minutes). Use conductivity meter (within 10-20% of feedwater) or pH meter to compare flush water to feed water.
7. Restart RO: After all stages of a train are cleaned and flushed, restart the RO. Divert permeate to drain until it meets quality requirements (conductivity, pH). Stabilization can take hours to days, especially after high pH cleanings.

Alternative Cleaning Procedures

Hydranautics notes awareness of several alternative methods for recovering membrane performance, though it does not guarantee effectiveness or accept responsibility for adverse effects.

• Electro Magnetic Fields (EMF): Claims suggest continuous EMF during operation reduces fouling and scaling tendency.
• Direct Osmosis at High Salinities (DO-HS): Involves a daily pulse of ~7% NaCl into the high-pressure pump suction for 6-12 seconds, momentarily changing RO flow to direct osmosis. This draws permeate back through the membrane, potentially lifting foulants, dehydrating bacteria, and sweeping debris.
• Proprietary Cleaning Chemicals: Various suppliers offer proprietary formulations, often enhanced versions of generic chemicals.
• Air Scouring: Using two-phase (air bubbles and water) flow can increase shear forces and improve foulant removal, primarily used for individual elements.

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Disclaimer: This summary is based on extracted text from a technical datasheet. It may not include all figures, footnotes, or the latest revisions. For contractual data and complete information, always refer to the official OEM PDF document revision used on the project.

Official datasheet (PDF)

PDF datasheet