The influence of reverse osmosis membrane on the treatment effect of different water qualities

In reverse osmosis (RO) applications, feed water quality exerts a primary influence on membrane performance, dictating flux, salt rejection, fouling propensity, energy consumption, and cleaning frequency. Surface waters rich in particulate and natural organic matter (NOM) promote organic and biofouling, whereas groundwater or brackish sources characterized by hardness, silica, and metal ions pose inorganic fouling and scaling challenges. Seawater’s high salinity requires elevated operating pressures that exacerbate scaling and biofilm formation . Wastewater effluents, laden with dissolved organics and micropollutants, can rapidly foul membranes, demanding rigorous pretreatment and robust cleaning protocols. Across these contexts, tailored pretreatment, judicious membrane selection, and optimized operational control are essential to sustain RO efficiency and longevity.

Introduction
Reverse osmosis (RO) separates water from dissolved solutes by applying hydraulic pressure across a semipermeable membrane. RO now accounts for over 65 % of global desalination capacity, underscoring its central role in water treatment and reuse. However, membrane performance is highly sensitive to feed water characteristics—organic load, ionic composition, particulate matter, and biological activity—all of which can reduce permeate flux, lower salt rejection, elevate energy consumption, and shorten membrane service life.

Types of Feed Water and Their Impacts
Surface Water
Surface sources (rivers, lakes) often contain high levels of suspended solids, turbidity, and NOM, leading to rapid membrane fouling and spacer clogging if not adequately pretreated . Minimal pretreatment of surface water fails to stabilize RO operation, as particulate and organic matter accumulate on membrane surfaces, triggering concentration polarization and fouling.

Groundwater / Brackish Water
Brackish groundwater features moderate salinity and hardness, reducing osmotic pressure compared to seawater but raising risks of calcium carbonate and silica scaling . Silica scaling, in particular, is difficult to inhibit with conventional antiscalants, requiring specialized control strategies to preserve flux and avoid irreversible fouling.

Seawater
Seawater RO (SWRO) operates at high pressures (typically 55–80 bar) to overcome osmotic pressures of ~25–30 bar, which increases energy demand and accentuates membrane compaction and fouling. Inorganic scaling (calcium sulfate, carbonate) and biofouling are prevalent and necessitate rigorous antiscalant dosing, biofilm control, and frequent cleanings.

Wastewater Effluent
RO treatment of municipal or industrial wastewater effluents tackles high loads of dissolved organics, micropollutants, and residual disinfectants, leading to severe organic and biofouling . Hospital WWTP effluents treated with polyamide RO membranes show significant reductions in TDS and COD but suffer rapid flux decline without preRO filtration and controlled operating conditions.

Effects on Membrane Performance
Fouling Mechanisms
Organic fouling arises from adsorption of lowmolecularweight organic compounds (LMWOC) onto membrane surfaces and within pores, hindering water permeability . Inorganic fouling involves precipitation of sparingly soluble salts (e.g., calcium carbonate, sulfate) on the membrane face, creating rigid scales that block pores and damage the active layer. Biofouling, driven by microbial attachment and biofilm growth, further reduces flux and encourages localized scaling.

Scaling
Calcium sulfate and carbonate scaling typically occur when hardness ions exceed solubility limits at the membrane surface, accelerated by concentration polarization . Silica, present as colloidal or dissolved species, forms hard, tenacious deposits that resist traditional antiscalants, especially in brackish water RO systems.

Scaling on the spacers of a Reverse Osmosis Membrane

Permeate Flux and Salt Rejection
Permeate flux increases with transmembrane pressure but plateaus due to concentration polarization and compaction effects. Higher pressures marginally improve salt rejection, yet beyond optimal ranges can reduce rejection due to increased convective forcing of solutes through defects and fouling layers.

Energy Consumption and Operating Pressure
Fouling and scaling raise hydraulic resistance, necessitating higher feed pressures to maintain flux, thus escalating specific energy consumption (SEC) and operational costs . Cleaning cycles become more frequent, further adding chemical use and downtime.

Mitigation Strategies
Pretreatment
Effective pretreatment—coagulation, media filtration, ultrafiltration—removes particulates and NOM, curbing fouling potential and stabilizing RO performance. For surface water, dual‐media filters and activated carbon adsorption reduce organic loading and protect downstream RO modules.

Cleaning and Antiscalants
Physical (backwashing, forward flushing) and chemical (acids, alkalis, oxidants) cleaning protocols target specific foulant types—organics, inorganics, biofilms—to restore flux and rejection. Antiscalant dosing, pH adjustment, and antifoaming agents help prevent scale nucleation and biofilm formation.

Membrane Surface Modification
Recent advances in surface grafting (e.g., polyacrylic acid, grapheneoxide coatings) have yielded membranes with smoother, more hydrophilic, and negatively charged surfaces, enhancing fouling resistance and flux retention.

Case Studies

• Brackish Water Desalination in Tunisia: A fullscale plant exhibited stable longterm operation with periodic cleaning; fouling characterization revealed mixed organic–inorganic layers requiring tailored cleaning regimes.
• Hospital WWTP Effluent Recycling: Polyamide RO membranes reduced TDS from ~1 500 mg/L to <20 mg/L and COD by >90 %, but permeate flux declined by 25 % over 30 days without advanced pretreatment and optimized pressure settings.

Conclusions
Feed water quality is the paramount factor governing RO membrane performance. Surface waters demand robust particulate and NOM removal; brackish sources require silica and hardness control; seawater treatment must balance high‐pressure operation with scaling management; wastewater effluents call for rigorous pretreatment and tailored cleaning. Innovations in membrane materials, pretreatment techniques, and real‐time fouling monitoring continue to enhance RO resilience and efficiency across diverse water qualities. Consistent application of these strategies ensures sustainable RO operations, prolonged membrane lifespans, and reliable production of highquality permeate.