Ceramic water-filtration membranes developed for industrial water reuse
By Paul Manison
It’s a considerable understatement to say the process industries use plenty of water. Given this inevitable propensity, sustainable filtration solutions that improve industrial water-reuse efficiency are needed.
The vast majority of current filtration applications use polymer or ceramic-tubular membranes.
However, it’s worth noting that high-quality porous ceramic membranes are being developed as part of innovative filtration systems. The aim is better filtration efficiency, cuts in maintenance downtime and less energy use. These are benefits that can reduce costs.
Polymer and ceramic-tubular membranes are used to clean water for its conservation and reuse in industrial and communal wastewater applications, including wastewater plants, residential and commercial buildings and even cruise ships.
Polymer membranes account for around 75% of the membrane filtration market, with ceramics taking up most the remaining 25%, along with several other materials, such as metallic membranes. Figure 1 illustrates the variety of filtration separation processes and their particular applications. Ceramic membranes are found generally in dairy production and widely used for fruit-juice clarification.
While polymer membrane use currently dominates filtration applications in the wastewater treatment sector, this is based primarily based on price considerations. However, while polymer membranes are less expensive than their ceramic equivalents, they require more frequent filter replacements.
Limited polymer uses
The low-resistance properties of polymer membranes, compared to their ceramic counterpart, limit their application. This is shown by the simple fact that, in general, the metal module housing of a ceramic membrane system sets the limits in processing aggressive media, rather than the filter itself. When polymer membranes are used in harsh environments, the filter material will wear first.
Moreover, flowrate is greater through ceramic membranes than through polymer membranes for a given pore diameter. A ceramic filtration unit requires a lower pressure — and less energy — to circulate fluid.
As water reuse becomes more imperative, some extremely demanding industrial applications will involve filtration of some truly challenging media. Due to ceramics’ superior chemical and abrasion resistance, for many important applications its use will be preferred over a polymer membrane.
While ceramics withstand pH values ranging from 0 to 14, polymer membranes have a much narrower pH range. Polymer membranes can be tailored to resist neutral, acidic or basic pHs, but generally not all three ranges with the same material.
Ceramic membranes are also better for high-temperature applications. They can be sterilized or steam-cleaned for specific applications, such as in the medical industry. This is not possible with polymers. Finally, ceramic has more strength and rigidity, giving it better dimensional stability under pressure than polymer.
Some demanding applications
In the filtration of water containing oils or fatty acids, it’s often the case that surfactant, emulsion-breaking chemical dispersants are required to limit the formation of an organic fouling layer on the surface of a polymer membrane. High-quality ceramic membranes are found to be more resistant to fouling, without the need for dispersant additives.
While fouling of both ceramic and polymer membranes is inevitable, ceramics allow a wider range of options when it comes to systems with a Clean-In-Place (CIP) process. In some applications, membranes have to be cleaned with harsh chemicals and withstand high-pressure spray from two directions. This is especially necessary during back-flushing, to prevent the formation of a fouling layer onto the membrane’s surface or in the pores, leading to a reduction in the membrane’s filtration capability.
For example, use of peroxide chemical cleaners or of high-temperature steam cleaning is acceptable for use with ceramic membranes, but can be an issue for polymers. In other words, a combusting high-temperature air-cleaning treatment can be used for ceramics, but would be likely to melt a polymer membrane. CIP operations can be performed automatically without interrupting the filtration process when ceramic membranes are used, but this is more difficult with polymeric membranes.
As industry adopts water reuse and conservation methods, it also must minimize the footprint of such activities within their facilities. Membrane selection and design plays a part in reducing footprint encroachment for companies and facilities using these systems. Newly developed ceramic membranes can achieve greater compactness — defined as increased surface area of membrane per volume unit — due to the versatility of their design and geometry. Compactness and improved design of ceramic filtration modules also contributes in increasing the energy efficiency of such systems.
The energy/water nexus
Energy and water are inextricably related. Energy is required for water extraction, treatment and transportation. Water is required to produce energy used in industrial processes, whether it is for hydropower, steam used to turn turbines or coolants. Having the energy needed to perform water-related processing is already a global issue.
It is almost inevitable that fossil-fuel and water shortages will increase due to population growth and climate-change effects. As a result of these and other pressures, the likelihood is great that manufacturing plants and industries using large amounts of water in their processes will be faced with legislation that charges them for both withdrawal and discharge of water.
In the future, businesses are likely to adapt by developing their own water filtration systems; this might even include the use of modular or local filtration systems rather than sending wastewater to a central system. And such filtration systems will need more sustainable membranes. What this means is that innovations to increase efficiency are needed, and will be scrutinized more closely in the near future.
According to Global Water Intelligence, scarcity is driving water reuse policies.
Population growth and economic factors, as well as the physical scarcity of water, have driven China’s water treatment and reuse efforts. China’s target is to increase water reuse from its current 14% percent to 25% by 2015. In China, agricultural use of water as a percentage of all uses has been reduced from 86% in 1980 to 65% in 2005 — it is targeted to be 50% in 2050.
Other examples include Saudi Arabia, whose goal is to increase water reuse from 11% to 65% by 2016, and Spain, which aims to increase water reuse from 11% to 40% by 2015.
Government intervention in water issues will become more prevalent as the need for clean drinking water increases pressure for industrial water reuse. For example, the state government in São Paulo, Brazil, has introduced initiatives to protect drinking water for the region’s inhabitants, including issuing regulations to restrict the industrial use of potable water. This is forcing businesses to look for ways to reuse their wastewater, or obtain recycled water from another source. Filtration facilities have already been proposed and built to meet São Paulo water needs.
In Japan, an early adopter of water reuse strategies, there are currently more than 90 ceramic membrane water filtration plants.
In the United States, treated water reuse is currently at about 11%. No official targets have been set, but the National Resources Defense Council (NRDC) states that 10 of the U.S.’s largest cities are in severe danger of water shortages in the relatively near future. The top three listed are Los Angeles, Calif., which imports water from Colorado; Houston, Texas, located in a high drought area; and San Antonio, Texas, identified by the NRDC as having a non-sustainable water supply. There is a great probability that regulations will eventually be imposed to respond to these shortages.
To address specific areas, several U.S. pilot plants are using ceramic membranes with ultrafiltration for industrial water reuse.
In a related example, a pilot ceramic membrane treatment system was developed in Parker, Colo., by the Parker Water and Sanitation District (PWSD) for one of its plants treating water from the local reservoir. Since 2011, the plant has helped address the area’s long-term water shortage problems; the water is used by residents for everyday water needs, to replenish the underground aquifer, and as a reserve for better water management during a drought. For this plant, to reduce the life-cycle cost of the installation, ceramic membranes were preferred to polymer ones. It is estimated that the ceramic membrane’s life will be up to 20 years.
Sustainable ceramic membranes developed
In response to these trends, ceramic water filtration and purification membranes are being developed for use in innovative filtration systems for industrial waste treatment processes. These systems will be more energy efficient during fluid circulation and cleaning than conventional systems with ceramic tubular membranes.
For example, Morgan Advanced Materials works closely with world leaders in ceramic membrane filtration solutions to develop durable and differentiated systems. Material scientists and product development engineers from Morgan Advanced Materials’ Innovation Hub, located in Great Britain, are working on new porous ceramic components with a filtration layer of around 25 µm, about a quarter of the diameter of a human hair. These materials are meant for use in micro- and ultrafiltration applications, including purifying industrial wastewater so it can be reused or safely discharged into the environment.
The reliability of the high-quality ceramic membranes being developed will enable businesses to reduce maintenance work and energy usage, leading to associated cost savings. Ceramics are robust and offer high performance in high-temperature environments, in the presence of harsh cleaning chemicals or where chemically aggressive or high-viscosity fluids need filtering. They can withstand the harsh environments found in wastewater treatment facilities and do not need replacement as frequently as many plastic alternatives.
Moreover, ceramic membranes can be manufactured to highly complex geometries and tight tolerances, facilitating more flexible, innovative designs that make filtration modules more energy efficient. This means that users can save energy and costs associated with pumping water through the filtration system.
Sustainability is increasingly important. The environmental benefits of water filtration are well recognized. New ceramic water filtration membranes aim to help businesses operate more efficiently, save money and conserve water.
Paul Manison is a business manager at Morgan Advanced Materials.
Morgan Advanced Materials manufactures components and sub-assemblies using an extensive range of materials, including structural and piezoelectric ceramics, dielectrics, braze alloys and specialist coatings. It works with manufacturers’ design and R&D teams at local, national and international level on
projects from concept and feasibility studies through prototype development to full production.