Factors Affecting Reverse Osmosis (RO) Desalination Rate and Optimization Directions

Factors Affecting Reverse Osmosis (RO) Desalination Rate and Optimization Directions
The reverse osmosis (RO) desalination rate serves as a core metric for evaluating a system's ability to remove salts (such as ions and organic matter) from water. Its influencing factors can be categorized into four major types: membrane characteristics, feedwater quality, operational process parameters, and system operation and maintenance. Detailed analysis follows:

I. Membrane Characteristics (Core Fundamental Factors)
The desalination performance of reverse osmosis membranes is determined by their inherent structure and material composition, constituting the fundamental factors influencing desalination rate:
Membrane Material and Structure
① Common membrane materials include polyamide (PA) composite membranes (high desalination rate, effective removal of organic matter and high-valent ions) and cellulose acetate (CA) membranes (strong chlorine resistance, but slightly lower desalination rate and susceptibility to hydrolysis).
② Pore size distribution, skin layer thickness, and crosslinking degree directly affect retention efficiency: Smaller pores, denser skin layers, and higher crosslinking yield higher salt rejection rates, but reduce permeate flux (requiring a balance between flux and salt rejection).
Membrane Molecular Weight Cut-off (MWCO) ① For RO membranes used in desalination, MWCO is typically <200 Da (Dalton). A smaller MWCO enhances retention of small-molecule salts (e.g., NaCl, CaCl₂).
Membrane fouling and aging
① Chemical degradation (e.g., oxidation, hydrolysis) or physical damage (e.g., scratches, compaction) during membrane operation causes pore size enlargement and irreversible decline in salt rejection rate.
② Residual production impurities (e.g., protective solutions, oil contamination) on the membrane surface that are not thoroughly cleaned will initially impair salt rejection efficiency.

II. Feedwater Quality Factors (Key External Conditions)
Feedwater quality directly determines membrane operating pressure and fouling risk, serving as a critical variable affecting salt rejection: Feedwater Salinity and Ion Types
① Higher feedwater salinity (e.g., seawater vs. freshwater) increases osmotic pressure across the membrane. Without corresponding pressure adjustment, salts may “penetrate” membrane pores, reducing salt rejection.
② Ion Valence: High-valent ions (Ca²⁺, Mg²⁺, SO₄²⁻) exhibit significantly higher desalination rates (typically >99.5%) than low-valent ions (Na⁺, Cl⁻, desalination rate approx. 99.0%-99.5%);
③ Ion radius: Smaller-radius ions (e.g., Li⁺) permeate membranes more readily than larger-radius ions (e.g., K⁺), resulting in slightly lower desalination rates.

Feedwater pH
① Polyamide (PA) membranes exhibit optimal pH performance between 6.5–7.5, where amide bond stability maximizes desalination efficiency;
② At pH < 4 or pH > 11, PA membranes are prone to hydrolysis degradation, increasing pore size and significantly reducing desalination efficiency;
③ Under extreme pH conditions, certain weak acid/base ions (e.g., HCO₃⁻, SiO₃²⁻) convert to molecular forms (e.g., H₂CO₃, H₂SiO₃). Membranes exhibit lower retention rates for molecular species than ionic species, leading to reduced desalination efficiency.
Feed Water Temperature
①Increased temperature accelerates the diffusion rates of water molecules and salt ions: Water molecule diffusion increases more significantly (flood increases), but the probability of salt ions penetrating the membrane also rises, causing a slight decrease in salt rejection rate (typically 0.1%-0.2% reduction per 1°C temperature increase).
② Excessively low temperatures (<10°C) cause a sharp drop in membrane flux. Increasing operating pressure to maintain flux may indirectly increase the risk of salt breakthrough.

Feedwater Contaminants
① Organic matter (e.g., humic acids, tannins, petroleum compounds): Adsorbs onto the membrane surface forming an “organic fouling layer,” which clogs pores and reduces membrane hydrophilicity. Simultaneously, some small-molecule organic compounds may permeate the membrane, leading to decreased salt rejection (especially TOC removal efficiency).
② Colloids and suspended solids (e.g., silt, iron/manganese oxides, microbial flocs): Form a “cake layer” that increases concentration polarization on the membrane surface, indirectly reducing salt rejection;
③ Oxidizing agents (e.g., Cl⁻, O₃): PA membranes are highly sensitive to free chlorine (tolerance < 0.1 mg/L). Chlorine oxidizes the membrane's amide bonds, causing irreversible structural damage and reduced salt rejection;
④ Heavy metal ions (e.g., Fe³⁺, Mn²⁺): Tend to precipitate on the membrane surface (e.g., Fe(OH)₃) or react with the membrane material, impairing retention efficiency.

III. Operational Process Parameters (Key Adjustable Factors)
Optimizing operational parameters maximizes salt rejection within membrane performance limits. Core parameters include:

Operating Pressure
① Operating pressure must exceed feed osmotic pressure: Higher pressure increases the driving force for water molecules to permeate the membrane, more effectively “squeezing” salt ions onto the feed side and boosting salt rejection;
② However, exceeding the membrane's rated pressure (e.g., 5.5–6.0 MPa for seawater RO membranes) causes irreversible membrane compaction and pore size reduction (potentially boosting short-term salt rejection but leading to rapid long-term flux decline and shortened membrane lifespan);
③ Insufficient pressure reduces water molecule driving force, allowing salt ions to remain or permeate, thereby decreasing salt rejection.

Recovery Rate
① Higher recovery rate (product water volume / feed water volume) increases salt concentration on the membrane concentrate side (exacerbating concentration polarization), amplifying the salt ion concentration gradient. This raises the probability of salt ion permeation through the membrane, thereby decreasing desalination efficiency.
② Excessively high recovery rates (e.g., >15% for a single membrane element) may also cause scaling on the concentrate side (e.g., CaCO₃ or CaSO₄ precipitation), further degrading membrane performance.
③ Conventional RO system recovery rates: Brackish water RO: 75%-85%; Seawater RO: 35%-45%. Adjust based on feedwater hardness and salinity.

Concentration Polarization and Flow Rate
① Concentration polarization: The phenomenon where salt concentration at the membrane surface exceeds that of the bulk solution increases salt permeation drive and reduces desalination rate.
② Higher feed-side flow rates (e.g., 2-3 m/s for brackish water RO membrane elements) effectively flush the concentrated salt layer from the membrane surface, mitigating concentration polarization and improving salt rejection. Conversely, excessively low flow rates exacerbate concentration polarization, leading to reduced salt rejection.

Product Water Backpressure
① Excessive product water backpressure (e.g., from clogged product water lines or overly tight valve closures) negates feed-side operating pressure, reduces water molecule permeation drive, and may cause concentrate backflow, resulting in a sharp drop in salt rejection rate;
② Standard requirement: Product water backpressure < 0.03 MPa.

IV. System Operation and Maintenance (Ensuring Long-Term Stable Desalination)
Pre-treatment Effectiveness
① Unremoved suspended solids/colloids cause membrane fouling; unremoved Cl⁻ leads to membrane oxidation; untreated hard water causes membrane scaling—all directly reduce desalination rate. Pre-treatment (e.g., quartz sand filtration, activated carbon filtration, softening, addition of reducing agents/scale inhibitors) is critical for maintaining membrane performance: (Example: Failure to add scale inhibitor during high feedwater hardness causes CaSO₄ crystallization on the membrane surface, scratching the membrane epidermis layer and leading to irreversible decline in salt rejection rate)

Cleaning and Maintenance
① Delayed cleaning: Accumulated membrane fouling causes continuous desalination rate decline (e.g., organic fouling reduces desalination by 5%-10%), with irreversible contamination.
② Improper cleaning: Using high-concentration acids/alkalis (exceeding membrane tolerance), excessively high cleaning temperatures, or mechanical cleaning that scratches the membrane surface can damage the membrane structure and reduce salt rejection rate.
③ Shutdown maintenance: Prolonged shutdown without soaking in a protective solution (e.g., 1% sodium bisulfite solution) causes membrane drying and oxidation, leading to reduced salt rejection rate.

System Design Rationality
① Membrane Element Arrangement: When multiple membranes are connected in series, failure to replace fouled/degraded front-end membranes promptly causes excessive load on downstream membranes, leading to overall desalination rate decline.
② Flow Distribution: Uneven feedwater distribution results in low flow rates and excessively high recovery rates in some membrane elements, causing localized desalination rate reduction. ③ Concentrate Drainage: Inadequate concentrate drainage leads to salt accumulation within the system, exacerbating concentration polarization.

Summary: Core Logic and Optimization Directions Affecting Desalination Rate Core Logic: Desalination rate fundamentally balances the “membrane's salt rejection capability” against the “driving force for salt permeation through the membrane.” The membrane's inherent rejection capability forms the foundation, while feedwater quality determines the “potential risk” of salt permeation. Operational parameters and maintenance determine whether this risk can be suppressed and the membrane's maximum performance realized.
Optimization Directions:
① Membrane Selection: Choose membranes suited to feedwater characteristics (salinity, pH, contaminant types) — e.g., seawater RO membranes for seawater, anti-fouling PA membranes for high-organic-content water.
② Pretreatment: Strictly control feedwater SDI (<5), turbidity (<0.1 NTU), and Cl⁻ (<0.1 mg/L); remove suspended solids, organics, and hardness.
③ Operating Parameters: Maintain optimal pressure (slightly above osmotic pressure + safety margin), recovery rate (matching membrane element rating), and flow rate (to mitigate concentration polarization);
④ Maintenance: Perform regular cleaning (physical + chemical), scheduled shutdown maintenance, and timely replacement of aged/contaminated membrane elements.