Theory & Overview of Reverse Osmose (RO)

Summary

What is Reverse Osmosis?

Reverse Osmosis (RO) is a process technology used to purify water for diverse applications such as semiconductors, food processing, biotechnology, pharmaceuticals, power generation, seawater desalting, and municipal drinking water. The RO industry, which began with initial experiments producing small quantities of water, now has a combined worldwide production exceeding 1.7 billion gallons per day.

Historical Background

Research into Reverse Osmosis started in the 1950s at the University of Florida with Reid and Breton demonstrating desalination properties of cellulose acetate membranes. Loeb and Sourirajan further developed the technology by creating the first asymmetric cellulose acetate membrane.

This early research led to new and improved RO element configurations, predominantly spiral wound elements, and in some cases, hollow fiber elements. In the early 1980s, US Government Labs developed the first Composite Polyamide membrane, which offered significantly higher permeate flow and salt rejection than cellulosic membranes. With the introduction of products like the ESPA3 by Hydranautics, the industry has achieved a 20-times increase in flow per pressure over original cellulosic membranes, coupled with an order of magnitude decrease in salt passage.

Key Concepts in Reverse Osmosis

• Semi-permeable: Refers to a membrane that selectively allows certain species to pass through while retaining others, with water passing at a much faster rate than dissolved solids (salts).
• Osmosis: A natural process where fluid flows across a semi-permeable membrane from a region of higher chemical potential (e.g., pure water) to a region of lower chemical potential (e.g., a salt solution) until equilibrium is restored, or the hydrostatic pressure differential equals the osmotic pressure. Osmotic pressure is a solution property proportional to salt concentration.
• Reverse Osmosis: Occurs when an external pressure, greater than the osmotic pressure, is applied to the salt solution side of a semi-permeable membrane, causing solvent (water) to flow to the pure water side. Applied pressure is the driving force, and more energy is required for solutions with higher salt concentrations.

Osmotic Pressure

The osmotic pressure (P_osm) of a solution can be determined by measuring the concentration of dissolved salts: P_osm = 1.19 (T + 273) * Σ(mᵢ) where P_osm is in psi, T is temperature in °C, and Σ(mᵢ) is the sum of molal concentrations of all constituents. An approximation suggests 1000 ppm of Total Dissolved Solids (TDS) equals about 11 psi (0.76 bar) of osmotic pressure.

Water Transport

The rate of water flow (Q_w) through a semi-permeable membrane is defined by: Q_w = (∆P - ∆P_osm) * K_w * S/d This is often simplified to: Q_w = A * (NDP) where A is a unique constant for each membrane material type, and NDP is the net driving pressure or net driving force. The rate of water flow is proportional to the net driving pressure differential.

Salt Transport

The rate of salt flow (Q_s) through the membrane is defined by: Q_s = ∆C * K_s * S/d This is often simplified to: Q_s = B * (∆C) where B is a unique constant for each membrane type, and ∆C is the salt concentration differential across the membrane. The rate of salt flow is proportional to the concentration differential and is independent of applied pressure.

Salt Passage and Rejection

• Permeate Salinity (C_p): Depends on the relative rates of water and salt transport: C_p = Q_s / Q_w.
• Salt Passage (SP): The ratio of salt concentration in the permeate (C_p) to the mean feed concentration (C_fm): SP = 100% * (C_p / C_fm). Salt passage is an inverse function of pressure; it increases as applied pressure decreases.
• Salt Rejection (SR): The opposite of salt passage: SR = 100% - SP.

Permeate Recovery Rate (Conversion)

Recovery (R) is the rate of feed water converted to product (permeate): R = 100% * (Q_p / Q_f) where Q_p is product water flow rate and Q_f is feed water flow rate. Higher recovery rates increase salt concentration on the feed-brine side, increasing salt flow across the membrane and osmotic pressure, thereby reducing the net driving pressure and product water flow.

Concentration Polarization

As water passes through the membrane and salts are rejected, a boundary layer with higher salt concentration than the bulk solution forms near the membrane surface. This is called concentration polarization, and it:

1. Increases osmotic pressure (∆P_osm) at the membrane surface, reducing the net driving pressure differential.
2. Reduces water flow (Q_w) across the membrane.
3. Increases salt flow (Q_s) across the membrane.
4. Increases the probability of sparingly soluble salts precipitating and causing membrane scaling.

The Concentration Polarization Factor (CPF) is the ratio of salt concentration at the membrane surface (C_s) to bulk concentration (C_b): CPF = C_s / C_b CPF is directly proportional to permeate flow (Q_p) and inversely proportional to average feed flow (Q_favg). Hydranautics recommends a CPF limit of 1.20, which corresponds to 18% permeate recovery for a 40-inch long membrane element.

Commercial RO Membrane Technology

Commercial RO membranes consist of a thin polymeric film several thousand Angstroms thick cast on a fabric support. Key properties include high water permeability, high semipermeability (water transport much higher than ion transport), stability across a wide pH and temperature range, and good mechanical integrity. Commercially useful membrane life is typically 3 to 5 years. Two main polymeric materials are used: Cellulose Acetate (CA) and Composite Polyamide (CPA).

Cellulose Acetate (CA) Membranes

Original CA membranes were made from cellulose diacetate polymer; current versions use a blend of cellulose diacetate and triacetate. They are formed by casting an acetone-based solution onto a non-woven polyester fabric, followed by a cold bath and high-temperature annealing (60 - 90 °C). The annealing step improves semipermeability, decreasing water transport and significantly reducing salt passage. CA membranes have an asymmetric structure with a dense surface layer of about 1000 - 2000 Å (0.1 - 0.2 micron) responsible for salt rejection, and a spongy, porous film providing high water permeability. CA membranes tolerate limited levels of free chlorine and have a smooth surface with little surface charge, making them suitable for feed water with high fouling potential like municipal effluent and surface water.

Composite Polyamide (CPA) Membranes

CPA membranes are manufactured in two steps:

1. A porous polysulfone support layer is cast onto a non-woven polyester fabric. This layer is not semi-permeable.
2. A semi-permeable membrane skin is formed on the polysulfone substrate via interfacial polymerization of monomers. This allows independent optimization of the support and salt-rejecting skin properties. CPA membranes generally offer higher specific water flux and lower salt passage than CA membranes, and they are stable over a wider pH range. However, they are susceptible to oxidative degradation by free chlorine.

Membrane Module Configurations

The two primary module configurations for RO are hollow fiber and spiral wound. Tubular and plate and frame configurations are used in specific industries but are less common for general RO.

Hollow Fiber (HFF) Membranes

HFF membranes are hollow fibers extruded from cellulosic or non-cellulosic material, asymmetric in structure, and typically about 42 micron (0.0016 inch) I.D. and 85 micron (0.0033 inch) O.D. Millions of these fibers are bundled, folded, and sealed with epoxy into a module (permeator), usually 10 to 20 cm (4 to 8 inches) in diameter and about 137 cm (54 inches) long. Pressurized feed water flows radially around the fiber bundle, permeates through the fiber walls into the hollow core, and exits as product.

Hollow fiber units operate in a non-turbulent or laminar flow regime, with low permeate water flow per unit area, resulting in lower concentration polarization. They require a minimum reject flow to minimize concentration polarization and maintain even flow distribution. A single hollow fiber permeator can operate at up to 50-percent recovery. They offer a large membrane area per unit volume, leading to compact systems. HFF modules require feed water of better quality (lower suspended solids) than spiral wound modules due to close-packed fibers and tortuous feed flow.

Spiral Wound Membranes

Spiral wound elements consist of two flat sheets of membrane separated by a permeate collector channel, sealed on three sides to form a leaf. A feed/brine spacer sheet is added, and multiple leaves are wound around a central perforated plastic permeate tube. Typical industrial spiral wound elements are approximately 100 or 150 cm (40 or 60 inches) long and 10 or 20 cm (4 or 8 inches) in diameter.

Feed/brine flows axially from the feed end to the brine end, parallel to the membrane surface. The feed channel spacer induces turbulence and reduces concentration polarization. Manufacturers specify brine flow requirements to control concentration polarization, limiting recovery per element to 10 - 20 percent. To achieve higher overall recovery, systems are typically staged with three to six elements connected in series within a pressure tube. A single pressure vessel with four to six elements can operate at up to 50-percent recovery. Spiral wound elements are commonly made with cellulose diacetate and triacetate (CA) blends or thin film composites (e.g., polyamide, polysulfone, polyurea). Composite membranes generally offer higher rejection at lower operating pressures than CA blends.

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Disclaimer: This summary is based on extracted text and may not include all figures, footnotes, or the latest revisions from the original OEM PDF. For contractual data, always refer to the specific OEM PDF revision used on the project.

Official datasheet (PDF)

PDF datasheet