Hardcore Analysis丨Water Treatment Membrane Technologies (UF, NF, RO): Deep Dive into Pros & Cons

Nanofiltration (NF) Membrane:

• Can intercept substances at the nanoscale (0.001 microns).

• Operates in a range between Ultrafiltration (UF) and Reverse Osmosis (RO).

• Rejects organic molecules with a molecular weight of approximately 200-800 Daltons.

• Capable of rejecting 20%-98% of dissolved salts, with lower removal rates for monovalent ions compared to multivalent ions.

• Primarily used to remove organics and color from surface water, hardness and radium from groundwater, and partially remove dissolved salts.

• Used for extraction and concentration of valuable substances in food and pharmaceutical production.

• Operating pressure typically ranges from 3.5 to 30 bar.

Reverse Osmosis (RO) Membrane:

• The finest membrane separation product; effectively rejects all dissolved salts and organics with a molecular weight > 100 Daltons, while allowing water molecules to pass.

• Widely applied in seawater and brackish water desalination, boiler feed water makeup, industrial pure water and electronic-grade ultrapure water production, drinking water purification, wastewater treatment, and specialized separation processes.

Ultrafiltration (UF) Membrane:

• Can intercept macromolecules and proteins between 1-20 nanometers in size.

• Allows small molecules and dissolved solids (inorganic salts) to pass while rejecting colloids, proteins, microorganisms, and large organic molecules.

• Operating pressure typically ranges from 1 to 5 bar.

Differences Between UF, NF, and RO Membranes:

• Ultrafiltration (UF): A pressurized membrane separation technology. Under pressure, small solute molecules and solvent pass through a membrane with specific pore sizes, while larger solute molecules are retained, partially purifying the solution.

• Nanofiltration (NF): Operates between UF and RO. Primarily used for municipal or industrial desalination. Achieves >90% salt rejection. While RO achieves >99% rejection, NF offers significant cost savings if the highest water purity isn't strictly required.

• Reverse Osmosis (RO): A membrane separation filtration technology using pressure difference as the driving force. Extensively used in research, medicine, food & beverage, seawater desalination, etc. Applications include: production of space water, purified water, distilled water; water for beverage dilution in alcohol production; pretreatment for water in pharmaceutical and electronics industries; concentration, separation, purification, and solution makeup in chemical processes; desalination/softening for boiler feed water; seawater/brackish water desalination; water supply and wastewater treatment for papermaking, electroplating, printing, and dyeing industries.

Comparison of RO vs. UF Membranes:

• RO membrane pores are approximately 1/100th the size of UF membrane pores. Consequently, RO equipment can effectively remove chemical contaminants like heavy metals, pesticides, and chloroform from water, which UF purifiers cannot remove. UF can remove particulate pollutants and bacteria, all of which RO also removes effectively.

• Both RO and UF systems use membrane elements as their core component. Key differences:

  1. Water Quality & Regulatory Standards: For example, the bacterial count standard for effluent: UF systems under "General Water Purifiers" allow up to 100 CFU/mL; RO systems are stricter, allowing only 20 CFU/mL. RO effluent quality is significantly superior.

  2. Water Use Philosophy: RO systems provide split-stream output: purified water for drinking/concentrate for washing. UF output is generally used for washing, though it can be used for drinking if the feed water quality is excellent. (Note: This depends heavily on source water quality and specific UF system capability).

Advantages of Ultrafiltration (UF):

• Generally does not require a pump, consumes no electricity, eliminating electrical safety concerns.

• Fewer connections, lower water pressure requirements, resulting in lower failure and leakage rates.

• Simple structure, lower cost.

• Easy operation, low operating costs, requires no chemical additives.

• Operates under mild conditions (no phase change, minimal temperature/pH change), preserving the activity of biological macromolecules.

• Primarily used for desalting, dehydration, and concentration of biological macromolecules.

Advantages of Reverse Osmosis (RO) Systems:

• High water safety; effectively removes various harmful impurities.

• Performs well during water supply emergencies (presuming pretreatment is adequate).

• Improved water taste.

• Effectively reduces water hardness, minimizing scale formation in water heating vessels.

Applications of Different Membranes in Water Treatment:

• Nanofiltration (NF): Drinking water preparation and advanced purification; wastewater treatment (municipal sewage, textile/dyeing effluent, tannery effluent, electroplating effluent, papermaking effluent).

• Forward Osmosis (FO): Seawater desalination, industrial wastewater treatment, landfill leachate treatment.

• Reverse Osmosis (RO): Conventional water purification, municipal wastewater treatment, heavy metal wastewater treatment, oily wastewater treatment, etc.

Water Treatment Membrane Technology: Principles and Process Flows

Utilizes membrane technology as the core filtration method, supplemented by pretreatment and other equipment to achieve purification goals.

Two-Stage RO Purified Water Process:
Raw Water → Quartz Sand Filter → Activated Carbon Filter → Safety Filter → Multi-stage Pump → Primary RO Membrane (Concentrate Recovery) → Multi-stage Pump → Secondary RO Membrane (Concentrate Recovery) → Purified Water

Single-Stage RO Purified Water Process:
Raw Water → Quartz Sand Filter → Activated Carbon Filter → Safety Filter → Multi-stage Pump → Primary RO Membrane (Concentrate Recovery) → Purified Water

Mineral Water / Mountain Spring Water / Purified Water Process:
Raw Water → Quartz Sand Filter → Activated Carbon Filter → Safety Filter → Ultrafiltration (UF) Membrane → Mineral Water / Mountain Spring Water / Purified Water

Ultrapure Water / Deionized Water Process:
Raw Water → Quartz Sand Filter → Activated Carbon Filter → Safety Filter → Multi-stage Pump → Primary RO Membrane (Concentrate Recovery) (or use Two-Stage RO) → Primary Mixed Bed (MB) Unit → Secondary Mixed Bed (MB) Unit → Ultrapure Water