Why Salt Passage Rises Sharply? Root Causes & Replacement Solutions

Why Salt Passage Rises Sharply? Root Causes & Replacement Solutions

The sharp increase of salt passage of imported RO membranes originates from three irreversible structural damages: active layer oxidation, pressure compaction and spacer blockage. This article establishes quantitative judgment standards via operating parameter monitoring and membrane autopsy, analyzes inherent material and structural defects of mainstream imported membrane models, and proposes systematic replacement and long-term operation optimization schemes relying on self-developed cross-linked polyamide substrate RO membranes featured with superior anti-oxidation, anti-compaction and wide-spacer performance. Targeted chemical cleaning standards and technology-only FAQ are supplemented to solve the permanent decay of desalination capacity fundamentally.

Overview of Industrial Pain Points

Sustained sharp growth of salt passage is the most costly irreversible failure of spiral-wound RO elements for industrial pure water, brackish water desalination and reclaimed water reuse. For mainstream imported polyamide RO membranes including CPA, LFC, and SWC series, operators mostly adopt temporary chemical cleaning and antiscalant dosing to mitigate TDS over-limit, yet fail to solve the root irreversible structural damage.

Three core irreversible failure modes fundamentally trigger runaway salt passage: oxidative degradation of polyamide active layer, permanent compaction of separation layer under long-term high pressure, and spacer channel blockage induced by mixed foulants. All three damage types cannot be fully restored via CIP cleaning, and repeated mainteInherent Defects of Mainstream Imported RO Series Triggering Three Irreversible Failuresnance will only accelerate performance recession and raise overall operation costs.

This guide systematically sorts out formation mechanism, judgment standards and hidden structural defects of imported membranes corresponding to each failure, and launches targeted replacement optimization schemes based on our self-developed cross-linked polyamide substrate RO membrane with outstanding anti-oxidation, anti-compaction and anti-fouling performance, helping water treatment plant technicians eliminate salt passage surge from the source and extend membrane service life.

Three Core Irreversible Failures & Salt Passage Deterioration Mechanism

Conventional imported aromatic polyamide active layers contain a large number of fragile amide molecular cross-linking bonds. Trace residual chlorine, hydrogen peroxide, chlorine dioxide in feed water, coupled with catalytic heavy metal ions (Fe/Mn/Cu), will break molecular cross-linking chains and form irreversible micro-pores on the separation surface. The nano interception barrier of salt ions is permanently destroyed, directly leading to continuous salt passage rise.

The damage is hidden and progressive: slight oxidation only reduces salt rejection by 2%–5%; severe oxidation will lift salt passage by more than 100%, with abnormal permeate flux surge. Conventional imported membranes have low oxidation tolerance, even 0.05mg/L long-term residual chlorine will trigger irreversible degradation within 6–12 months.

Imported standard polyamide active layer adopts thin single-layer forming process, with loose internal molecular gaps. Long-term overpressure operation, excessive single-stage recovery rate and frequent high-low pressure impact will continuously squeeze the active layer, resulting in permanent shrinkage of molecular gaps. The ion transmission channel widens, and salt interception efficiency drops permanently.

Typical symptom of compaction failure: normalized flux declines year by year, salt passage rises synchronously, and differential pressure has no obvious fluctuation, which is easily misjudged as mild organic fouling by field staff.

Most early imported membranes adopt thin 28mil narrow feed spacer. Inorganic scaling, biological slime and organic colloid easily adhere to the spacer surface to form blockages, creating local low-flow dead zones inside the membrane channel.

Severe concentration polarization accumulates high-concentration salt ions on the membrane surface, which indirectly accelerates oxidation and compaction damage of the active layer, forming a vicious cycle of salt passage continuous deterioration. Blockage failure is accompanied by obvious monthly ΔP sharp increase, and salt passage rebound rapidly after each chemical cleaning.

Our self-developed cross-linked polyamide substrate optimizes molecular cross-link density in film forming, embeds anti-oxidation functional groups into molecular chains, thickens and reinforces the active layer to improve mechanical compression resistance, and equips upgraded wide 31mil trapezoidal spacer to enhance cross-flow scouring force, simultaneously resisting the three irreversible damage factors from material and structural design.

Step-by-Step Parameter Quantitative Diagnosis Process

Before disassembling membrane elements for autopsy, technicians can complete preliminary classification of three irreversible failure modes through four groups of normalized operating parameters, without system shutdown loss. This quantitative diagnosis flow applies to all imported replacement models and our full cross-linked polyamide RO series, avoiding blind disassembly and unnecessary downtime.

Normal stable annual salt passage fluctuation shall not exceed 15% of initial value. Salt passage rises continuously, ΔP basically unchanged, flux slightly increased: typical oxidative damage of active layer.

Salt passage rises year by year, normalized flux decreases significantly, ΔP no abnormal growth: permanent active layer compaction.

Salt passage surges synchronously with monthly ΔP growth over 15%, flux fluctuates repeatedly after cleaning: spacer blockage + severe concentration polarization.

Monthly ΔP growth less than 10% belongs to normal operation range; narrow-spacer imported BW30HR LE, CPA series elements generate 60% faster ΔP rising speed than our wide spacer cross-linked polyamide membrane under identical water quality. Persistent ΔP surge is the core marker of spacer channel blockage.

Eliminate temperature interference and calculate standardized flux: 

Flux abnormal rise + salt passage jump = oxidation perforation of active layer; 

Flux steady decline + slow salt passage growth = long-term high-pressure compaction; 

Flux fluctuates up and down after each CIP = repeated blockage of feed spacer.

Detect online residual chlorine, ORP value to judge oxidation risk; SDI stable above 5 accelerates spacer fouling; recovery rate exceeding design upper limit aggravates concentration polarization and compaction damage. Our cross-linked polyamide membrane tolerates short-term oxidant fluctuation and high SDI impact better than conventional imported polyamide elements.

Deep Diagnosis: Membrane Element Autopsy & Failure Visual Identification

When parameter diagnosis cannot confirm the dominant irreversible failure mode, offline membrane autopsy is the most accurate detection method, with three intuitive identification operations suitable for field maintenance teams without professional laboratory equipment.

Oxidation damage: Membrane sheet surface loses gloss, partial areas appear matte white, tiny pinholes visible under light observation, no obvious foulant adhesion on spacer.

Active layer compaction: Membrane surface tight and shriveled, uniform thinning of separation layer, spacer clean without sediment accumulation.

Spacer blockage: Gray-black slippery bio-slime or white hard crystal deposits fully attached to trapezoidal spacer, flow channel partially blocked, uneven membrane surface scaling.

Flush contaminated membrane surface with clean permeate: Oxidized membrane leads to sharp conductivity rise; Compacted membrane has obviously lower flushing flux; Blocked spacer membrane obstructs flow and sheds massive sediment.

Dry weight comparison test between failed element & new element: Oxidation failure weight nearly unchanged; Compaction brings slight weight gain; Spacer blockage leads to over 8% weight increase due to foulants.

Inherent Defects of Mainstream Imported RO Series Triggering Three Irreversible Failures

The three salt-passage-related irreversible failures do not occur randomly; they are closely bound up with the material formula and structural design defects of different imported membrane series, which is the core comparison advantage of our cross-linked polyamide substrate RO membrane.

BW30HR LE / Early BW30 Series adopt 28mil narrow thin spacer + low cross-link polyamide layer. Low molecular cross-link density leads to weak anti-oxidation capacity, narrow flow channel causes frequent spacer blockage, thin active layer suffers easy compaction under high pressure. Replace with our equivalent cross-linked polyamide 8040 brackish water membrane with 31mil wide spacer and high cross-link anti-oxidation substrate.

CPA Series conventional brackish water membrane uses single thin polyamide active layer. Single-layer separation film has low mechanical strength and oxidation resistance, negative zeta potential accelerates organic adhesion to block spacer. Our neutral hydrophilic cross-linked polyamide membrane thickens separation layer and adds anti-oxidation groups to solve three defects simultaneously.

ESPA low-pressure series adopts ultra-thin active layer design to cut operating pressure, sacrificing cross-link density and structural durability, bringing high risk of oxidation perforation and pressure compaction. Our low-energy cross-linked polyamide membrane balances low pressure operation, high cross-link density and wide spacer structure.

LFC anti-fouling series only supports narrow chemical cleaning pH window (2–11). Frequent acid & alkali cleaning will gradually erode the thin polyamide layer, aggravating oxidation and compaction damage year by year. Our anti-fouling cross-linked polyamide membrane supports full 1–13 pH cleaning range for deep pollutant stripping without damaging active layer structure.

SWC ordinary seawater membrane uses low cross-link polyamide without anti-oxidation modification. Seawater pretreatment dechlorination is unstable, thin spacer accelerates boron salt deposition blockage. Our high boron rejection seawater cross-linked polyamide membrane enhances oxidation resistance and equips wide spacer to suppress fouling.

Targeted Replacement & Long-Term Optimization System Based on Cross-Linked Polyamide Membrane

Short-term chemical cleaning only removes surface foulants, and cannot repair oxidized/compacted active layer; fundamental control of salt passage surge relies on three-dimensional optimization system including membrane hardware replacement matching, standardized operation parameter adjustment and scientific pretreatment & chemical dosing.

System dominated by oxidative damage (unstable dechlorination, residual chlorine fluctuation): Select full anti-oxidation cross-linked polyamide brackish/seawater RO membrane, high molecular cross-link density with built-in oxidation-resistant functional groups, improve tolerance to trace chlorine, hydrogen peroxide and heavy metal catalysis.

System dominated by active layer compaction (long-term high pressure, over-high recovery): Adopt thickened high compression resistance cross-linked polyamide membrane, reinforce separation layer mechanical structure to avoid permanent shrinkage under pressure.

System dominated by spacer blockage (high hardness, high TOC, surface water/recycled water): Deploy 31mil wide trapezoidal spacer cross-linked polyamide anti-fouling membrane, strengthen cross-flow turbulence scouring, reduce organic/inorganic sediment adhesion.

Control brackish water system recovery rate ≤75%, seawater system ≤48% to avoid extreme concentration polarization accelerating compaction & oxidation; implement automatic low-flow flushing for shutdown over 2 hours to eliminate stagnant high-concentration brine.

Stabilize operating pressure within design rated range to prevent overpressure compaction; install online residual chlorine/ORP interlock alarm to cut off oxidant inflow in real time.

Configure multimedia filter + 5μm precision security filter to control feed SDI stably below 5; equip activated carbon filter + sodium bisulfite dosing double dechlorination protection to eliminate oxidation hidden danger.

Customize matched antiscalant according to water hardness, silica, boron content to reduce spacer scaling risk; periodic low-dose compatible biocide dosing to inhibit biofilm without damaging cross-linked polyamide active layer.

Targeted CIP Cleaning Protocol For Three Irreversible Failure Auxiliary Maintenance

Cleaning can only alleviate spacer blockage symptoms, cannot reverse oxidation and compaction damage; improper cleaning formula will further erode imported thin polyamide layer, while our cross-linked polyamide membrane supports wide pH cleaning window for thorough maintenance.

Prepare citric acid cleaning solution pH 2.5–3.5, circulate at 25–30℃ for 60–90 mins to dissolve calcium, silica crystal deposits on spacer, neutral rinse until effluent pH returns to neutral range.

NaOH + compatible surfactant mixed solution pH 10–11 circulate 90–120 mins to strip organic attachments; add membrane-compatible biological enzyme for severe biofouling, extend circulation to 120–180 mins to decompose polysaccharide biofilm matrix.

Adopt mild neutral cleaning agent without strong acid/alkali long soaking, avoid further thinning of already damaged active layer; strictly forbid oxidizing cleaning agents to prevent secondary oxidative corrosion.

FAQs

Q1: After replacing failure-prone BW30 narrow-spacer imported membrane with our high cross-link anti-oxidation cross-linked polyamide element, can salt passage rising trend be effectively suppressed?

A1: Yes. Our product adopts high-density cross-linked polyamide substrate with built-in anti-oxidation functional groups to resist trace oxidant corrosion; matched upgraded 31mil wide trapezoidal spacer reduces concentration polarization and spacer blockage probability; thickened active layer strengthens compression resistance. Under identical feed water and operation parameters, the annual salt passage growth rate can be reduced by over 70%, effectively restraining long-term desalination performance recession.

Q2: Long-term system overpressure leads to irreversible active layer compaction and salt passage surge, can chemical cleaning restore original rejection performance?

A2: Impossible. Compaction belongs to permanent physical shrinkage of polyamide molecular gaps, and cleaning agents cannot reconstruct the loose molecular structure of the active layer. The only fundamental solution is to replace with our high compression resistance cross-linked polyamide membrane, and adjust operating pressure & recovery rate to standard design values synchronously.

Q3: The pretreatment dechlorination system frequently fails, imported membrane suffers progressive oxidative damage, will switching to our cross-linked polyamide membrane completely eliminate oxidation risk?

A3: It can greatly improve oxidation tolerance, but cannot replace standard dechlorination pretreatment protection. Our substrate can withstand short-term trace oxidant impact without immediate salt passage jump, yet long-term continuous chlorine inflow will still cause slow degradation. The complete solution is to upgrade to anti-oxidation cross-linked polyamide membrane + double dechlorination interlock protection.

Q4: Mixed installation of original CPA imported membrane and our cross-linked polyamide element in one pressure vessel, will it accelerate salt passage uneven rise?

A4: Mixed assembly is not recommended. Two types of membranes differ in molecular cross-link density, spacer thickness, surface charge and anti-fouling modification, resulting in inconsistent flow distribution, fouling accumulation and oxidation resistance inside the membrane shell, partial elements will face faster salt passage growth. All elements inside one pressure vessel shall adopt identical cross-linked polyamide replacement model to achieve unified long-term stable performance.

Q5: Surface water project with high organic load, original LFC membrane spacer blocks repeatedly, salt passage rebounds sharply after each CIP, what targeted replacement scheme to choose?

A5: Select our neutral hydrophilic wide-spacer anti-fouling cross-linked polyamide RO membrane. Neutral surface reduces organic macromolecule adhesion; wide spacer enhances cross-flow scouring force to delay blockage cycle; full 1–13 pH cleaning window realizes thorough pollutant stripping each cleaning, fundamentally cut salt passage rebound amplitude after maintenance.

Related Product Links

BW30HR LE-440i

BW30HR LE-400/34i

BW30HR LE Series

BW30-400/34

BW30-400/34i