At LiqTech, we aim to reduce the impacts made from living an improved lifestyle. We minimize the impacts from some of the world’s most polluting industries, such as oil and gas, shipping, and power plants. We aim to ensure compliance, progress, and sustainability through unique filtration technologies.
Ceramic membranes are the core of these filtration technologies. Ceramic membranes comprise a durable and efficient filtration process for industrial liquids and wastewater, meeting many end-users stringent demands of today and the future by delivering constant compliance with strict environmental regulations and sustainable production. Not only may ceramic membranes hold one of the keys to protect scarce water resources and help achieve the SDGs, but they also contribute to low energy consumption, high capacity, and a small footprint. Let us dive into everything you need to know about ceramic membranes.
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1. What is a ceramic membrane?
A ceramic membrane is a coated membrane substrate made of inorganic materials. It enables thorough liquid filtration of even the most troublesome industrial liquids and wastewater. Thus, it is the most imperative part of an industrial water treatment unit.
Ceramic membranes can remove harmful elements such as suspended solids, oil droplets, oil emulsions, particles, heavy metals, pathogens, viruses, and bacteria.
Ceramic membranes can operate within various filtration ranges. They can remove targeted particles and filter liquids and wastewater at different levels, depending on the specific filtration requirements. This makes ceramic membranes extremely relevant for various complex filtration tasks within many industries, such as oil and gas filtration, flue gas condensate filtration, marine scrubber wash-water filtration, algae filtration, pool and spa filtration, and drinking water treatment. With more stringent environmental regulations, this is essential in order to ensure a compliant operation. Ceramic membranes, thereby, enable businesses to grow while meeting the sustainable standards of the future.
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2. Why use ceramic membranes?
With a global population on the rise, the demand for clean water rises as well. This has turned clean water into a scarce resource, and many countries and regions already experience severe water crises. And with this negative development, many strict environmental regulations have been implemented, which are essential to comply with. Therefore, we ought to manage our water resources efficiently to sustain human life, our continually improving lifestyles, and to run compliant business operations. And ceramic membranes are a vital solution in managing water compliantly, efficiently, and sustainably. As ceramic membranes remove harmful matter, it enables sustainable discharges of industrial liquids and wastewater as well as water recovery and recycling.
On top of ensuring a compliant and sustainable operation, ceramic membranes can deliver significant advantages to your business. Ceramic membranes are extremely durable and possess mechanical, thermal, and chemical strength, which deliver an extremely robust filtration solution and a long membrane lifetime. Some of the advantages for ceramic membranes are:
- Highest flux for any membrane material
- Chemically inert pH 0-14
- Thermally resistant up to 800 ˚C
- Low fouling tendency
- Low operational cost and low total cost of ownership
These strong membrane properties enable membranes to operate with aggressive fluids in even the harshest industries. This also makes ceramic membranes a more durable and viable alternative than polymeric membranes. As polymeric membranes are made of cheaper unsound material, they do not possess strong properties, which makes them inadequate for various aggressive fluids and harsh industries. Furthermore, they cannot withstand frequent cleaning or sterilization processes, which are heavily demanded by various industries such as food and beverage and pharmaceutical.
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3. How do ceramic membranes work?
In water filtration, there are four filtration ranges, which are:
- Microfiltration (MF)
- Ultrafiltration (UF)
- Nanofiltration (NF)
- Reverse osmosis (RO)
Ceramic membranes operate within microfiltration and ultrafiltration, also denoted low-pressure membrane filtration. Within these ranges, viruses, bacteria, oil emulsions, black carbon, and suspended solids are removed. It is the membranes’ pore size that determines which particles are retained. Within microfiltration, particles larger than 0.1 microns are retained and within ultrafiltration, particles between 0.1 and 0.01 microns are retained.
In a filtration process, feedwater enters the ceramic membranes, and a feed-pump initiates the filtering process by generating a pressure, which will cause the feedwater to permeate the membrane layer and substrate. Simultaneously, particles are retained by the membrane layer. This, ultimately, delivers a permeate stream and a concentrate stream, which can either be reused or safely and sustainably discharged. However, reusing the permeate or the concentrate can contribute to cost savings and process optimizations within other production areas.
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4. What is a ceramic membrane made of?
Ceramic membranes can be made of several inorganic materials, such as titania, alumina, zirconia, and silicon carbide. Based on more than 20 years of research within the filtration industry, LiqTech has developed a unique, patented silicon carbide (SiC) membrane. Being one of the hardest materials in the world, silicon carbide adds some unique features and advantages compared to other materials utilized for traditional ceramic membranes and polymeric membranes.
Silicon carbide is the second hardest element in the world, only beaten by diamonds, and the hardest man-made element ever produced. The material was discovered in an attempt to produce artificial diamonds. Silicon carbide is both ultra-hard and durable, contributing to an extremely long membrane lifetime. By utilizing silicon carbide in the membrane substrate and coating, the membrane is strong and solid. The membrane layer consists of small silicon carbide grains retaining the unwanted particles, while the grains in the membrane substrate are gradually larger for easier water permeation. Silicon carbide adds a lot of unique features, such as reduced fouling tendency, high flux, chemically inertness pH 0-14, thermally resistance up to 800 °C, and low power usage and low pressure.
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5. What does the membrane coating do?
The membrane coating is a dense layer evenly placed inside or outside a ceramic membrane, depending on its membrane support design. It is this extremely dense layer that facilitates the liquid filtration process, so without the membrane layer, the membrane would not work for liquid filtration. The coating is so dense that if the whole membrane was made of coating, it would be difficult for water to pass through it.
The membrane coating defines the membrane properties, such as porosity, flux, pore size, and mechanical strength. Membrane coating is made of an inorganic material such as zirconia, alumina, or silicon carbide. Based on more than 20 years of research and development, LiqTech’s membrane coating is made of extremely small silicon carbide grains due to the durable performance brought by this material.
It is of utmost importance to obtain an even coating layer as an uneven coating will lead to inhomogeneity. This will result in an uneven membrane roughness and porosity, which will make the membrane malfunction. Likewise, it is important to add the right amount of materials to obtain the correct rheological properties of the coating suspension.
The SiC support and the membrane layer can be seen in a scanning electron microscope (SEM). When a ceramic membrane has several membrane layers with a gradual decrease in pore size distribution from the support to the membrane layer, it is called asymmetric.
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6. How is a ceramic membrane cleaned?
As membranes operate, unwanted objects such as particles, oil, and biomass will accumulate on the membrane surfaces, causing membrane fouling. Fouling leads to decreased permeability, as permeate water will have to pass through fouling as well as the membrane. To stabilize the permeability, fouling objects are removed with a rinsing mechanism, which can happen either manually, semi-automatically, or automatically.
There are three cleaning processes. Process 1 is what happens most frequently, while process 3 is what happens most infrequently.
Process 1) Crossflow
Crossflow is a filtration method and a rinsing mechanism. In this process, the membrane surfaces are continually cleaned with feedwater. Due to the sheer force on the membrane surfaces, fouling is removed.
Process 2) Backwash
Backwash is a natural rinsing method. In this process, a backwash pump flushes water back through the membranes’ pores from the opposite direction. Particles, oil droplets, and biomasses are flushed back, enabling the membranes to continue operation without further cleaning.
Process 3) Cleaning-in-Place (CIP)
CIP is a chemical rinsing method. In this process, the ceramic membranes are fully cleaned by means of chemicals, heat, and water. By employing the right chemicals, targeted fouling is dissolved. A water filtration unit will typically have two CIP tanks. One CIP tank contains a chemical alkali to remove, e.g., grease, while the other tank contains a chemical acid to remove, e.g., minerals.
It is essential to find the perfect balance between operation and cleaning processes in order to maintain high permeability and low downtime. At LiqTech, we have more than 20 years of experience within membrane filtration and we can provide expert guidance in terms of preventive membrane cleaning.
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7. How is a ceramic membrane made?
Ceramic membranes can be produced using various production methods and equipment. At LiqTech, we have refined our membrane production to consist of four steps based on more than 20 years of experience.
Step 1 – Silicon carbide paste
In the first step, a silicon carbide paste containing raw materials such as silicon carbide powder, dispersant, and the solvent is made. This should be a homogeneous mass.
Step 2 – Membrane extrusion
In the second step, the paste is extruded to ceramic membrane support. This is essential to do when the paste is wet in order to form even the most complex geometries. At LiqTech, we both make standardized and customized solutions. Once the ceramic membrane support has the right geometry, it has to dry to become firm and stable.
Step 3 – Membrane coating
In the third step, a membrane layer containing silicon carbide is added inside the membrane support’s flow channels or outside the membrane support, depending on its design. This layer can be added utilizing three different methods, being 1) Spray coating, 2) dip coating, and 3) slip coating. Which method is the most suitable highly depends on the design of the support structure.
Step 4- Sintering
In the fourth step, the membrane is burned in a high-temperature furnace. The temperature is approximately 2100 ˚C, and the membrane is burned for 2-3 days. This process delivers the extreme physical and chemical properties of the membranes.
It is important to conduct quality tests during all the production phases in order to ensure solid and durable ceramic membranes with a long lifetime.
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8. What kinds of ceramic membrane support designs exist?
Several ceramic membrane support designs exist, bringing about their own unique advantages and facilitating different filtration principles. Based on more than 20 years in the filtration industry, LiqTech has thoroughly developed four types of ceramic membranes, being tubular membranes, flat sheet membranes, disc membranes, and Aquasolution® membranes.
Tubular membranes are the most common type. It is typically designed as a tube with 30 parallel flow channels to provide an extremely large surface area of impressive 0.33 m2 for one membrane. Still, it can also have one large flow channel. The tubular membrane is coated on the inside.
Hybrid technology membranes
Hybrid technology membranes are a form of tubular membranes. However, while the classic tubular membrane is made of 100 % SiC, the hybrid technology membrane combines zirconia oxide and silicon carbide to obtain the best from the two worlds. This provides ground-breaking membrane properties and an extremely efficient and thorough filtration process.
Flat sheet membranes
Flat sheet membranes are designed as rectangular membranes, which can be stacked in customized rack sizes and tower heights, providing a flexible solution. The flat sheet membrane is coated on the outside.
Disc membranes are designed as energy-efficient circular disc with internal permeation flow channels. The disc membrane is coated on the outside.
Aquasolution® membranes represent a patented LiqTech solution, specifically designed for liquids with a reduced number of bacteria and suspended solids, such as pool and spa areas. The Aquasolution® membranes are coated on the inside.
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9. What are the membrane filtration principles?
Ceramic membranes can employ one of two filtration principles, being inside-out filtration or outside-in filtration.
In inside-out filtration, the liquid is filtered inside the membrane, and permeate flows outside the membrane. To enable the filtration process, the ceramic membrane substrate is coated with a dense membrane layer of inorganic material inside the membrane substrate. Inside-out filtration covers two types of filtration processes, being crossflow filtration and dead-end filtration. Crossflow filtration is frequently utilized because of its strong operational reliability, high flux, and the fact that it also works as a rinsing mechanism that cleans the membrane surfaces.
In outside-in filtration, the liquid is filtered outside the membrane, and permeate flows inside the membrane. The ceramic membrane has several permeate outlets in order to lead the permeate stream to a permeate tank. To enable the filtration process, the ceramic membrane substrate is coated with a dense membrane layer of inorganic material outside the membrane substrate.
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