I. Structural Characteristics and Separation Mechanisms
Ultrafiltration (UF) is a pressure-driven membrane separation technology, with a molecular weight cut-off typically ranging from 10,000 to 100,000 daltons, and a physical pore size of approximately 0.01–0.1 micrometres.
Hollow-fibre structure: The membrane fibres appear as fine tubes, with an outer diameter typically ranging from 0.8 to 2 mm. This structure provides a vast effective filtration area per unit volume, significantly enhancing the packing density of the module.
Filtration Modes:
Outside-In: Feed water permeates into the lumen from the outer side of the membrane fibre, with contaminants deposited on the outer wall. This facilitates air scouring and physical flushing, making it suitable for water with high turbidity and high levels of suspended solids.
Inside-Out: Raw water flows into the membrane fibres from the inside, and the permeate flows outwards. With fixed flow paths and favourable hydraulic conditions, this configuration is commonly used for fine filtration of low-turbidity feedwater
II. Key Advantages of PVDF Material
Compared with traditional membrane materials such as polysulphone (PSu), polyethersulphone (PES) or polypropylene (PP), PVDF demonstrates unique competitive advantages in complex industrial water systems:
1. Extremely high tensile and mechanical strength: PVDF membrane fibres prepared using the thermally induced phase separation (TIPS) or non-solvent-induced phase separation (NIPS) methods possess excellent toughness, an extremely low fibre breakage rate, and the ability to withstand frequent high-pressure backwashing and air scrubbing.
2. Outstanding resistance to oxidation and chemicals: They can withstand long-term cleaning with high-concentration chlorine and strong acids and alkalis. This characteristic enables them to fully restore flux through intensive chemical cleaning (CIP) when faced with severe organic or microbial contamination.
3. Ageing resistance and long service life: They possess inherent resistance to UV radiation and ageing, with a typical industrial design service life of 3–5 years or more.
III. Key Industrial Applications
PVDF hollow-fibre ultrafiltration membranes can be used in the water treatment process chain both as a final purification stage and as a pre-treatment barrier for advanced treatment:
Reverse osmosis (RO) and seawater desalination pre-treatment: As pre-treatment for the feed water to RO systems, they can effectively reduce the risk of physical scaling and biological fouling of the downstream RO membranes.
MBR (Membrane Bioreactor): Submerged PVDF MBR membrane modules are placed directly in the aeration tank, replacing traditional secondary sedimentation tanks to achieve solids-liquid separation; the effluent quality can directly meet the standards for reclaimed water reuse.
Advanced Treatment of Industrial Wastewater and Zero Liquid Discharge (ZLD): In wastewater from textile printing and dyeing, electroplating, petrochemical and coal chemical industries, it efficiently retains suspended solids and macromolecular organic matter, providing high-quality feedwater for subsequent concentration and crystallisation units.
Upgrading of Municipal Drinking Water Systems: Effectively retains pathogenic microorganisms (such as Cryptosporidium and Giardia), algae and colloids in surface water, ensuring the biological safety of drinking water.