I. What is a Ceramic Membrane?
Simply put, a ceramic membrane is a precision filtration material made from inorganic ceramic materials (such as alumina, zirconia, titania, etc.) through special processes, possessing a highly regular microporous structure.
You can think of it as an extremely fine "sieve," but its separation accuracy is far superior to ordinary sieving. Its core working principle is "cross-flow filtration":
● The liquid to be treated flows parallel to the membrane surface at high velocity under pressure drive.
● Water and small molecular substances pass vertically through the micropores on the membrane wall under pressure, becoming the clear "permeate" or "product water."
● Pollutants (such as suspended solids, colloids, bacteria, macromolecular organics, etc.) are retained by the membrane and are discharged with the main liquid stream as it becomes concentrated, forming the "concentrate."
Based on pore size (or molecular weight cut-off), ceramic membranes can be divided into:
● Microfiltration (MF) membranes: Pore size 0.1-1.0 μm, used for removing suspended solids, bacteria, oil droplets, etc.
● Ultrafiltration (UF) membranes: Pore size 0.01-0.1 μm, used for removing colloids, proteins, macromolecular organics, viruses, etc.
In wastewater treatment, ultrafiltration-grade ceramic membranes are the most widely used.
II. How are Ceramic Membranes Processed and Manufactured?
Their manufacturing process can be described as a "combination of materials science and art," mainly including the following steps:
1.
Raw Material Preparation: High-purity ceramic powders (e.g., α-alumina, zirconia) are mixed with solvents, dispersants, binders, etc., in precise proportions to form a uniform suspension (slurry).
2.
Forming: The slurry is formed into a green body through extrusion molding (for tubular or multi-channel configurations) or tape casting (for flat sheet configurations). Multi-channel tubular membranes (e.g., 7-channel, 19-channel, 37-channel) are the mainstream structure, significantly increasing the membrane area per unit volume.
3.
Sintering: The formed green body is placed in a high-temperature sintering furnace (typically >1300°C) for sintering. This is the most critical step, where powder particles melt and recrystallize at high temperatures, forming a ceramic support with extremely high mechanical strength and a stable microporous structure.
4.
Coating and Secondary Sintering: To achieve smaller pore sizes (e.g., for UF membranes), a layer of finer ceramic material (e.g., zirconia, titania) slurry is coated onto the support body, followed by a second round of sintering at a lower temperature to form the final separation layer.
5.
End-Sealing and Assembly: The manufactured ceramic membrane tubes are sealed at the ends with special sealing materials (e.g., epoxy resin, glass glaze), then assembled into membrane elements (membrane cores) that can be placed into pressure vessels, and finally integrated into membrane stacks and membrane units.
III. How are Ceramic Membranes Practically Applied in Wastewater Treatment?
Ceramic membranes are typically not used as standalone treatment units in wastewater treatment but are embedded as the core solid-liquid separation unit within the entire process chain. Their main application modes are:
1. As an Advanced Treatment Unit (Replacing Secondary Sedimentation Tank + Tertiary Treatment)
● Process Flow: Traditional Activated Sludge Process → Ceramic Membrane Bioreactor (Ceramic MBR).
● Practical Application: Ceramic membrane modules are directly submerged in the biological reactor (submerged) or placed in an external circulation pipeline (sidestream), replacing the traditional secondary sedimentation tank.
● Microorganisms (activated sludge) are completely retained within the reactor by the ceramic membrane, achieving a complete separation of Hydraulic Retention Time (HRT) and Sludge Retention Time (SRT).
● The water filtered by the membrane is extremely clear and can directly meet reuse standards (e.g., for urban miscellaneous water, landscape environment water), eliminating the need for subsequent tertiary treatment facilities such as sedimentation, filtration, and disinfection.
● This is currently one of the mainstream advanced technologies for upgrading, expanding, and reusing municipal and industrial wastewater treatment plants.
2. As the Perfect Pretreatment Unit for RO Reverse Osmosis
● Process Flow: Conventional Pretreatment → Ceramic Ultrafiltration System → RO Reverse Osmosis → High-Quality Reuse Water.
● Practical Application: In industries requiring extremely high product water quality, such as electronics, power, and chemicals, RO systems need ultimate protection. Ceramic ultrafiltration can produce high-quality effluent with SDI <1 (far exceeding the RO feed water requirement of <3), greatly extending the service life of RO membranes (reducing cleaning frequency, minimizing scaling and fouling risks), and ensuring the stable operation of the subsequent RO system.
3. Treating High-Difficulty, Specialized Industrial Wastewater
Used for treating industrial wastewater with complex composition, strong acidity/alkalinity, containing oil, or high temperature, such as:
● Oily Wastewater: Metalworking emulsions, oilfield injection water. Ceramic membranes can efficiently break emulsions and separate oil and suspended solids.
● Chemical and Pesticide Wastewater: Their resistance to acids, alkalis, and organic solvents makes them irreplaceable.
● Textile and Dyeing Wastewater: Treating high-temperature dye wastewater and recovering dyes.
● Landfill Leachate: The durability advantages of ceramic membranes are significant due to the extreme water quality.
IV. Advantages and Disadvantages of Ceramic Membranes
Advantages:
1.
Extreme Physicochemical Stability: Resistant to strong acids, strong alkalis, and organic solvents; pH tolerance range of 0-14. This is unmatched by organic polymer membranes.
2.
Very Long Service Life: With normal use and cleaning, service life can reach 10-15 years or even longer, far exceeding the 3-5 years of organic membranes.
3.
Very High Mechanical Strength: Not prone to fiber breakage or damage; can withstand high-pressure backwashing and high-speed scouring; good recoverability.
4.
Excellent Cleaning Effectiveness: Can withstand frequent cleaning with strong acids, strong alkalis, and strong oxidants (e.g., sodium hypochlorite); flux recovery is thorough; capable of treating severely fouling wastewater.
5.
High Flux and Stability: Narrow pore size distribution, high and stable filtration accuracy, slow flux decline during long-term operation.
6.
High Temperature Resistance: Can treat wastewater from 0-90°C or even higher temperatures without cooling, saving energy.
Disadvantages:
1.
High Investment Cost: Complex raw materials and manufacturing processes lead to significantly higher initial investment compared to similar organic membranes (typically 3-8 times the cost of PVDF membranes). This is the main factor limiting its large-scale application.
2.
Brittleness: Ceramic material itself is brittle and fragile, susceptible to impact; requires careful handling during transportation and installation; must be protected from severe vibration and stress shocks.
3.
Low Module Packing Density: Compared to hollow-fiber organic membranes, the packing density of ceramic multi-channel membranes is lower, meaning slightly larger footprint for the same treatment capacity.
4.
High Weight: Places higher demands on membrane racking and civil structures.
V. Price Level
The pricing structure for ceramic membranes is complex and greatly influenced by brand (Kubota, Pall, Metawater, domestic Chinese brands, etc.), pore size, number of channels, length, and purchase volume.
● Unit: Usually priced per square meter (m²) of membrane area.
● Price Range: The current market price for ceramic membranes roughly falls within the range of 2000-6000 RMB/square meter.
● International brands (e.g., Japan's Kubota): High price, typically above 4000 RMB/m².
● Domestic brands (e.g., Jiangsu Jiuwu, Anhui Shilv, Beijing Meinaide, etc.): With technological advancements and localized production, prices are significantly competitive, currently dropping to the 2000-3000 RMB/m² range, offering increasing cost-effectiveness.
● Full System Investment: For a complete ceramic membrane water treatment project, the membrane cost itself accounts for about 30%-50% of the total system investment; the rest goes to pumps, valves, piping, skids, control systems, etc.
Although the initial investment is high, its ultra-long service life and very low replacement frequency can make it more economical over a full lifecycle cost analysis of 10+ years compared to organic membranes that require frequent replacement.
VI. Specific Application Case
Case Name: Upgrade and Reuse Project for a Large Industrial Park Wastewater Treatment Plant
● Background: The park's wastewater had complex composition and significant quality fluctuations. The existing process effluent could not stably meet the Class 1A standard. Furthermore, there was substantial internal demand for reuse water for landscaping and cooling tower makeup.
● Solution: Adoption of a "Modified A²O Process + Ceramic Membrane Bioreactor" as the core treatment process.
● Process Flow:
1.
Wastewater undergoes pretreatment through screens, grit chambers, and primary sedimentation tanks.
2.
Enters modified A²O biochemical tanks for nitrogen and phosphorus removal.
3.
The mixed liquor from the biochemical tank directly enters the submerged ceramic membrane tank (equipped with domestic 19-channel ultrafiltration ceramic membranes).
4.
Product water, after UV disinfection, is partially discharged, and partially enters the reuse water tank for plant area landscaping and cooling tower makeup.
5.
Concentrated sludge from the membrane tank is partially recycled to the biochemical tank, and excess sludge is discharged for treatment.
● Application Results:
● Effluent Quality: COD <30 mg/L, Suspended Solids (SS) undetectable, Turbidity <0.1 NTU, far exceeding the Class 1A standard and stably meeting reuse water standards.
● Stable Operation: Even with fluctuations in influent quality, the ceramic membrane ensured extremely stable effluent quality.
● Cleaning and Maintenance: Adopted a cleaning regimen of "Online Maintenance Cleaning (daily backwash with sodium hypochlorite + citric acid) + Offline Recovery Cleaning (monthly)". Flux recovery has been good, and the system has been operating stably for over 5 years.
● Economy: Although the project investment was about 15% higher than traditional processes + organic MBR, it saved the cost of subsequent tertiary treatment facilities. The membrane modules are expected not to require replacement for 10 years, making the long-term operational costs more advantageous.
