Water Treatment Technologies | Differences Between RO Reverse Osmosis Membranes, Ultrafiltration Membranes, and Nanofiltration Membranes

I. Comparison of Reverse Osmosis Membranes, Ultrafiltration Membranes, and Nanofiltration Membranes

1. Reverse Osmosis Membrane: The most refined membrane separation product, effectively retaining all dissolved salts and organic molecules larger than 100 molecular weight while allowing water molecules to pass through. RO membranes are widely used in seawater and brackish water desalination, boiler feedwater, industrial pure water and electronic-grade high-purity water production, drinking purified water manufacturing, wastewater treatment, and specialized separation processes.

2. Ultrafiltration Membrane: Retains macromolecules and proteins ranging from 0.002 to 0.1 microns. Ultrafiltration membranes permit passage of small molecules and dissolved solids (inorganic salts) while retaining colloids, proteins, microorganisms, and macromolecular organic compounds. The molecular weight cut-off range indicating ultrafiltration membrane pore size typically spans 1,000 to 500,000. Ultrafiltration membranes typically operate at pressures of 1-7 bar.

3. Nanofiltration Membranes: Retain substances at the nanometer scale (0.001 micrometers). Operating between ultrafiltration and reverse osmosis, nanofiltration membranes retain organic molecules with molecular weights of approximately 200-800 MW. with dissolved salt retention ranging from 20% to 98%. It removes soluble monovalent ions less effectively than multivalent ions. NF is commonly used to remove organic matter and colorants from surface water, hardness and radium from groundwater, and partially remove dissolved salts. It is also employed in food and pharmaceutical production for extracting and concentrating valuable substances. NF membranes typically operate at pressures of 3.5 to 30 bar.

II. Comparative Advantages of Reverse Osmosis and Ultrafiltration Membranes

Reverse osmosis membranes possess pore sizes one-hundredth the size of ultrafiltration membranes. Consequently, reverse osmosis water treatment systems effectively remove chemical contaminants such as heavy metals, pesticides, and trichloromethane from water sources—contaminants that ultrafiltration systems cannot eliminate. Conversely, particulate contaminants and bacteria removed by ultrafiltration systems are completely eliminated by reverse osmosis.

(1) Both reverse osmosis and ultrafiltration rely on membrane elements as their core components. The primary differences are twofold:

1. The treated water quality differs from the standards set by health authorities. For example, regarding bacterial indicators in the treated water, ultrafiltration is classified as a “general water treatment device” with a total colony count of 100 per milliliter. Reverse osmosis water treatment equipment, however, has a stricter requirement of 20 per milliliter. Naturally, the treated water quality from reverse osmosis equipment is significantly better than that from ultrafiltration. 2. Reverse osmosis systems provide dual-purpose water: pure water for drinking and concentrate for washing. Ultrafiltration is typically used solely for washing water. However, when municipal water quality is high, ultrafiltration can also serve as drinking water.

(II) Advantages of Ultrafiltration: Typically requires no pumps, consumes no electricity, and poses no electrical safety concerns; features fewer connections and lower water pressure, resulting in relatively lower failure rates and leakage probabilities; boasts simple structure and lower cost. Disadvantages include: poor removal of chemical contaminants; limited effectiveness during water supply emergencies; slightly inferior taste of treated water; inability to reduce water hardness—highly hard tap water may still cause scaling in cooking vessels. Ultrafiltration membranes remove macromolecules, colloids, proteins, and particulates from solutions, featuring low operating pressure, high water production rates, and ease of operation. Testing the deep purification of raw water for liquor production using hollow fiber ultrafiltration membrane systems demonstrates that ultrafiltration membrane water purification effectively eliminates secondary contamination in water distribution networks, further enhancing water quality.

(3) Advantages of Reverse Osmosis Water Treatment Equipment: Ensures water safety by effectively removing various harmful impurities; performs well during water supply emergencies; produces water with a pleasant taste; significantly reduces water hardness, preventing scale buildup in cooking vessels. Disadvantages include: requires pumps and consumes electricity, posing electrical safety concerns; features multiple connections and operates at high pressure, resulting in relatively higher failure rates and leakage probabilities; has a complex structure and is comparatively expensive.
 

III. Differences Between Ultrafiltration Membranes, Nanofiltration, and Reverse Osmosis

(A) Ultrafiltration Membranes

Ultrafiltration membranes employ a pressure-driven membrane separation technology. Under specific pressure conditions, small-molecule solutes and solvents pass through a specially designed membrane with defined pore sizes, while large-molecule solutes are retained on one side of the membrane. This process achieves partial purification of macromolecular substances. The advantages of ultrafiltration technology include simple operation, low cost, and no requirement for chemical additives. Crucially, its mild processing conditions avoid phase changes compared to evaporation or freeze-drying, while preventing alterations in temperature or pH. This prevents denaturation, inactivation, and autolysis of biomolecules. In biomolecular preparation, ultrafiltration is primarily used for desalting, dehydration, and concentration. Ultrafiltration also has limitations; it cannot directly produce dry powder formulations. For protein solutions, it typically achieves only 10–50% concentration. It is suitable for both household and industrial applications. The key to ultrafiltration technology is the membrane. Membranes come in various types and specifications, selectable according to operational requirements.

(II) Nanofiltration

Nanofiltration operates between ultrafiltration and reverse osmosis. It is currently primarily used for desalination in water treatment plants or industrial applications, achieving desalination rates exceeding 90%. While reverse osmosis achieves desalination rates above 99%, nanofiltration offers significant cost savings when water quality requirements are not exceptionally stringent.
 

(III) Reverse Osmosis

Reverse osmosis is a membrane separation filtration technology driven by pressure differentials. Originating from 1960s aerospace research in the United States, it was later adapted for civilian use. It is now widely applied in scientific research, pharmaceuticals, food processing, beverages, and seawater desalination. Applications include: - Production of space water, purified water, and distilled water - Alcohol manufacturing and dilution water - Preliminary water preparation for pharmaceuticals and electronics - Concentration, separation, purification, and water formulation in chemical processes - Boiler feedwater desalination and softening - Desalination of seawater and brackish water - Water supply and wastewater treatment for papermaking, electroplating, and dyeing industries

IV. Application of Different Membranes in Water Treatment: Forward Osmosis, Reverse Osmosis, Ultrafiltration, Nanofiltration
(1) Principle of Forward Osmosis (FO)

A semipermeable membrane that allows solvent molecules but not solute molecules separates the solvent from the solution. Under osmotic pressure, solvent molecules spontaneously pass through the membrane from the solvent side to the solution side. This phenomenon is known as osmosis, or “forward osmosis.”
 

1. Applications of Forward Osmosis Membranes in Water Treatment

(1) Seawater Desalination FO for seawater desalination is one of its most extensively researched fields. Early application studies were primarily documented in patents, but most of these were immature and lacked high feasibility.

(2) Industrial Wastewater Treatment Early studies reported using FO membranes for treating low-concentration heavy metal wastewater. However, due to severe fouling and rapid flux decline of the RO (reverse osmosis) membranes employed, this approach was not pursued further.

(3) Landfill Leachate Treatment The Coffin Butte Landfill in Corvallis, Oregon, USA, generates (2–4) × 10⁴ m³ of landfill leachate annually. To meet water quality standards for land application, the effluent TDS must be reduced below 100 mg/L.

(II) Reverse Osmosis Membrane Technology

1. Principle of Reverse Osmosis (RO)

Reverse osmosis is a pressure-driven membrane separation process. To generate reverse osmotic pressure, a pump applies pressure to a saline solution or wastewater, overcoming natural osmotic pressure and membrane resistance. This forces water through the reverse osmosis membrane while retaining dissolved salts and contaminants on the opposite side.

2. Applications of Reverse Osmosis Membranes in Water Treatment

(1) Conventional Applications of RO Membranes in Water Treatment Water is an indispensable resource for human survival and production activities. With freshwater resources becoming increasingly scarce, the global capacity of reverse osmosis water treatment plants has reached millions of tons per day.

(2) Applications of RO Membranes in Municipal Wastewater Treatment Currently, the use of RO membranes in advanced municipal wastewater treatment—particularly for secondary effluent reuse and reclaimed water applications—has gained significant attention.

(3) Application of Reverse Osmosis Membranes in Heavy Metal Wastewater Treatment Conventional treatment methods for wastewater containing heavy metal ions merely transfer pollution by converting dissolved heavy metals into precipitates or more manageable forms. Final disposal often involves landfilling, yet the risk of secondary pollution to groundwater and surface water persists long-term.

(4) Application of Reverse Osmosis Membranes in Oily Wastewater Treatment Oily wastewater represents a widespread and voluminous industrial effluent. Direct discharge into water bodies forms surface oil films that inhibit oxygen dissolution, leading to anoxic conditions, biological die-off, foul odors, and severe ecological contamination. Treatment of oilfield produced water containing 3.5 mg/L oil and (16–23) mg/L total organic carbon (TOC) to boiler feedwater quality, with the treated water reused as power plant boiler feedwater.

(III) Nanofiltration Membrane Technology

1. Principles of Nanofiltration (NF)

Nanofiltration (NF) is a novel molecular-level membrane separation technology and a current research hotspot in the global membrane separation field. NF membranes feature pore sizes above 1 nm, typically ranging from 1–2 nm. Their solute retention capacity lies between RO and UF membranes. While RO membranes exhibit high removal rates for nearly all solutes, NF membranes achieve high removal rates only for specific solutes. NF membranes can remove divalent and trivalent ions, organic compounds with Mn ≥ 200, as well as microorganisms, colloids, pyrogens, and viruses. A key characteristic of NF membranes is their inherent membrane charge. This charge enables high desalination performance even at very low pressures (as low as 0.5 MPa) and allows membranes with molecular weight cut-offs in the hundreds to remove inorganic salts. It is also the primary reason for NF's relatively low operating costs. NF is suitable for various saline water sources, typically achieving water recovery rates of 75%–85%. For seawater desalination, recovery rates range from 30%–50%, with no acidic or alkaline wastewater discharge.

2. Applications of Nanofiltration Membranes in Water Treatment

(1) Applications in Drinking Water

Nanofiltration operates at low pressures, making it the preferred process for drinking water preparation and advanced purification. Nanofiltration technology can remove the vast majority of Ca²⁺ and Mg²⁺ ions, making desalination the most common application area for NF. Membrane water treatment technology is comparable to conventional lime softening and ion exchange processes in terms of investment, operation, maintenance, and cost. However, it offers advantages such as no sludge production, no regeneration requirements, complete removal of suspended solids and organic matter, simplified operation, and reduced footprint, resulting in numerous practical applications. Nanofiltration can be directly applied for softening groundwater, surface water, and wastewater, and also serves as pretreatment for reverse osmosis (RO), photovoltaic-powered desalination systems, and other processes.

(2) Application of Nanofiltration Membranes in Seawater Desalination Seawater desalination refers to the process of reducing the salinity of seawater (typically 35,000 mg/L) to drinking water quality below 500 mg/L.

(3) Application of Nanofiltration Membranes in Wastewater Treatment A. Domestic Sewage B. Textile and Dyeing Wastewater C. Leather Processing Wastewater D. Electroplating Wastewater E. Pulp and Paper Mill Wastewater.