What types of membrane modules are used in MBR membrane processes?
The membrane module in an MBR membrane process is the core unit for solid-liquid separation. Its types are primarily categorized by its structural form and installation method. Different types have distinct characteristics in terms of fouling resistance, treatment efficiency, and applicable scenarios. This article will introduce several common membrane module types:
I. Classification by Membrane Structure
1. Hollow Fiber Membrane Module (Most Widely Used)
Structural Features: The membrane is fibrous, typically 0.1-2mm in diameter. Each membrane filament is a hollow tube. There are two types: “internal pressure” (raw water enters from the inside of the membrane filament and purified water seeps out from the outside) and “external pressure” (raw water enters from the outside of the membrane filament and purified water seeps out from the inside). The ends of the membrane filaments are fixed to end caps to form a bundled module, which is then placed in a membrane tank or pressure vessel.
Advantages: Large specific surface area, high membrane area per unit volume, high treatment efficiency; compact structure, small footprint; suitable for large-scale wastewater treatment, such as municipal and industrial wastewater.
Disadvantages: The membrane filaments are thin and easily clogged by impurities such as hair and fibers; if the external pressure membrane filaments break, sludge may leak.
Applications: Municipal sewage reuse, large-scale industrial wastewater treatment, such as in printing and dyeing, and food processing.
2. Flat-Plate Membrane Module
Structural Features: The membrane sheets are flat (mostly made of PVDF or PES), stacked together on a support frame to form a module. Water flow channels are left between the membrane sheets, allowing raw water to flow over the membrane surface and purified water to enter the water collection chamber through the membrane pores.
Advantages: The membrane surface is flat, highly resistant to contamination, and resistant to impurities. Cleaning is easy; the module can be directly disassembled for wiping or soaking. The membrane has high mechanical strength and a service life of over 5 years.
Disadvantages: Small specific surface area, resulting in lower treatment efficiency per unit volume than hollow fiber membranes. The module is heavy, making installation and maintenance relatively cumbersome.
Applications: Wastewater containing high levels of suspended solids, such as slaughterhouse wastewater, decentralized rural wastewater, and applications requiring high membrane module stability.
3. Tubular Membrane Modules
Structural Features: The membrane is tubular, with a large diameter of 10-20mm, similar to a pipe. Multiple membrane tubes are installed in parallel in a pressure vessel. Raw water flows under high pressure within the tubes, and purified water seeps through the tube walls.
Advantages: The large inner diameter of the membrane tubes provides strong pollution resistance and can treat wastewater containing high concentrations of suspended solids, grease, or particulate matter, such as oilfield wastewater and papermaking wastewater. It is resistant to clogging and suitable for high-viscosity fluids.
Disadvantages: Small specific surface area, low treatment efficiency; high equipment cost, high energy consumption, and the need for high-pressure operation.
Applicable Applications: High-difficulty industrial wastewater, wastewater containing oil or residue, and wastewater with low pretreatment requirements.
II. Classification by Installation Method
1. Submerged Membrane Modules
Installation Method: The membrane module is directly immersed in the mixed liquid of the biological reactor. Air bubbles generated at the bottom of the tank flush the membrane surface, creating aeration and negative pressure extraction of produced water.
Advantages: Low energy consumption, no need for a high-pressure pump; simple structure, easy maintenance; suitable for small and medium-sized sewage treatment systems.
Applicable Applications: Reclaimed water reuse in urban communities, decentralized sewage treatment in rural areas.
2. External (Pressure-Type) Membrane Modules
Installation Method: The membrane module is independent of the biological reactor and placed in a pressure vessel. Raw water is pressurized by a high-pressure pump and enters the membrane module, where it is driven by pressure to produce purified water through the membrane.
Advantages: The membrane module is separated from the biological reactor, allowing for independent control of operating parameters such as pressure and flow. This prevents direct contamination of the membrane by sludge in the biological reactor, making it suitable for wastewater with high pollution loads.
Disadvantages: Reliance on high-pressure pumps leads to high energy consumption; high equipment investment costs.
Applicable Applications: High-concentration industrial wastewater, such as chemical and pharmaceutical wastewater, where strict control of operating parameters is required.
III. Other Special Types
Spiral-wound Membrane Modules: Similar to reverse osmosis membrane structures, the membrane and flow screen are wound around a central tube. Raw water flows along the flow screen, and purified water passes through the membrane into the central tube. However, due to its poor pollution resistance, it is rarely used in MBR systems and is primarily used for treating low-suspended-solids wastewater.
Ceramic membrane modules: Made of ceramic (alumina, zirconia), they are resistant to high temperatures, strong acids and alkalis, and offer excellent pollution resistance. However, they are expensive and are therefore only suitable for special industrial wastewaters such as high-temperature, highly corrosive wastewater.
IV. Key Selection Factors
Water Quality Characteristics: Flat sheet membranes are preferred for wastewaters containing high suspended solids and fibers; external hollow fiber membranes or ceramic membranes can be used for high-concentration, difficult-to-degrade wastewater.
Treatment Scale: Hollow fiber membranes are commonly used for large-scale projects with a capacity of 10,000 tons; flat sheet membranes can be used for smaller projects with a capacity of less than 1,000 tons.
Operation and Maintenance: Flat sheet membranes are preferred for low maintenance costs; hollow fiber membranes are preferred for high efficiency and compactness.
Different membrane modules have different advantages. In practical applications, a comprehensive selection process based on water quality, cost, and O&M requirements should be considered to maximize the stability and cost-effectiveness of the MBR system.