Classification and basis of membrane classification

1. Membrane Material

(1) High-molecular organic membrane materials: Polyolefins, polyethylenes, polyacrylonitrile, polysulfones, aromatic polyamides, fluoropolymers, etc.

Organic membranes are relatively low in cost, inexpensive to manufacture, have mature manufacturing processes, and have a variety of pore sizes and forms, making them widely used. However, they are prone to fouling during operation, have low strength, and short service life.

(2) Inorganic membranes: These are a type of solid-state membrane made of inorganic materials such as metals, metal oxides, ceramics, porous glass, zeolites, and inorganic polymeric materials.

Inorganic membranes used in MBRs are mostly ceramic membranes. Advantages: They can be used in environments with pH=0~14, pressure P<10MPa, and temperature <350℃. They have high flux, relatively low energy consumption, and are highly competitive in the treatment of high-concentration industrial wastewater. Disadvantages: High cost, alkali-resistant, low elasticity, and difficult membrane processing and preparation.

2. Membrane Pore Size

Membranes used in MBR processes are generally microfiltration membranes (MF) and ultrafiltration membranes (UF), mostly using 0.1～0.4μm pore sizes, which is sufficient for solid-liquid separation membrane reactors.

Commonly used polymer materials for microfiltration membranes include: polycarbonate, cellulose ester, polyvinylidene fluoride, polysulfone, polytetrafluoroethylene, polyvinyl chloride, polyetherimide, polypropylene, polyetheretherketone, polyamide, etc.

Commonly used polymer materials for ultrafiltration include: polysulfone, polyethersulfone (PES), polyamide, polyacrylonitrile (PAN), polyvinylidene fluoride, cellulose ester, polyetheretherketone, polyimide, polyetheramide, etc.

3. Membrane Modules

To facilitate industrial production and installation, improve membrane efficiency, and achieve the maximum membrane area per unit volume, membranes are usually assembled in a basic unit device in a certain form. Under a certain driving force, the separation of various components in the mixed liquid is completed. This device is called a membrane module.

There are five commonly used membrane module types in industry:

Plate and frame module, spiral wound module, tubular module, hollow fiber module, and capillary module. The first two use flat sheet membranes, while the last three use tubular membranes. Tubular membranes have a diameter >10mm; capillary membranes 0.5~10.0mm; hollow fiber membranes <0.5mm.

4. Characteristics of Various Membrane Modules

(1) Plate and Frame Type

One of the earliest membrane module types used in MBR processes, similar in appearance to ordinary plate and frame filter presses.

Advantages: Simple manufacturing and assembly, easy operation, easy maintenance, cleaning, and replacement.

Disadvantages: Complex sealing, large pressure drop, low packing density.

(2) Tubular Type

Consists of a membrane and its support. There are two operating modes: internal pressure and external pressure. Internal pressure is mostly used in practice, where the feed water flows from inside the tube and the permeate flows from outside the tube. The membrane diameter is between 6~24mm.

Advantages: Liquid can be controlled for turbulent flow, less prone to clogging, easy to clean, low pressure drop.

Disadvantages: Low packing density.

(3) Hollow Fiber Type

The outer diameter is generally 40～250μm, and the inner diameter is 25～42μm. In MBR, the module is often placed directly into the reactor without a pressure-resistant container, forming an immersed membrane-bioreactor. It is generally an external pressure membrane module.

Advantages: High pressure resistance, not easily deformed, no support material required; high packing density; relatively low cost; long lifespan; nylon hollow fiber membranes with stable physical and chemical properties and low water permeability can be used.

Disadvantages: Sensitive to clogging, fouling and concentration polarization have a great impact on membrane separation performance.

5. General Requirements for MBR Membrane Module Design

(1) Provide sufficient mechanical support for the membrane, smooth flow channels, no flow dead zones or stagnant zones;

(2) Low energy consumption, minimizing concentration polarization, improving separation efficiency, and reducing membrane fouling;

(3) As high a packing density as possible, easy installation, cleaning, and replacement;

(4) Sufficient mechanical strength, chemical and thermal stability.

The selection of membrane modules should comprehensively consider their cost, packing density, application scenarios, system process, membrane fouling and cleaning, and service life.