Worldwide water scarcity, combined with an increase in population growth, is leading to the realization that water production and reuse must become more efficient. Industries that use large amounts of water, including the food and energy industries, are increasingly on the lookout for sustainable filtration solutions that improve industrial water reuse efficiencies.
While the vast majority of applications use either polymer or ceramic tubular membranes, new high-quality porous ceramic membranes are being developed as part of an innovative filtration system with the aim of improving filtration efficiency, reducing the amount of maintenance downtime and decreasing energy usage. These operational benefits can also result in reduced operating costs.
Filtration System Basics
Polymer or ceramic tubular membranes are currently used to clean, conserve and reuse water in a variety of industrial and communal wastewater applications, including wastewater plants, buildings, and even cruise ships. Currently, polymer membranes account for around 75% of the membrane filtration market, with ceramics accounting for the majority of the remaining 25% (along with a small number of other materials, such as metallic membranes). Figure 1 illustrates the variety of filtration separation processes available and provides an overview of their use in particular applications.
The use of ceramic membranes is common in dairy production and fruit juice clarification, whereas polymers currently dominate the wastewater treatment sector. Due to their lower purchase price, systems using polymer membranes are widespread in the wastewater filtration market. However, while polymer membranes are cheaper to manufacture than their ceramic equivalents, polymer systems require more frequent filter replacements.
Compared to ceramic membranes, the low resistance properties of polymer membranes limit the number of applications in which they can be used. In general, the ceramic membrane filtration module housing, which is often metallic, sets the limitations for processing aggressive media, rather than the filter itself. Inversely, when polymer membranes are used in harsh environments, the filter material will wear first. In addition, the flow rate through ceramic membranes can be greater than through polymer membranes for a given pore diameter. A ceramic filtration unit also requires a lower pressure (and less energy) to circulate fluid.
As water reuse becomes a more important issue, some extremely demanding industrial applications are faced with the need to filtrate increasingly challenging media. Due to the ceramic materials’ superior chemical and abrasion resistance, their use is preferred over polymer membranes for some applications. One example is the separation of oil, water and sand in the oil industry—a process that is both chemically aggressive and abrasive.
While ceramics can withstand pH values ranging from 0-14, polymer membranes can only withstand a much narrower pH range; they can be tailored to resist neutral, acid 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, which is not possible with polymers. Moreover, ceramics have more strength and rigidity, giving them better dimensional stability under pressure than polymer materials.
In the filtration of water containing oils or fatty acids (e.g., surfactants), emulsion-breaking chemical dispersants are often required to limit the formation of an organic fouling layer on the surface of polymer membranes. High-quality ceramic membranes have been found to be more resistant to fouling without the need for dispersant additives.
While the fouling of both ceramics and polymers is inevitable, ceramics can enable 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 be able to withstand high pressure from two directions. This is especially necessary during back-flushing to prevent the formation of a fouling layer to the membrane’s surface or in the pores, leading to a reduction in the membrane’s filtration capability.
For example, peroxide chemical cleaners or high-temperature steam cleaning is acceptable for ceramics, but can be an issue for polymers. A combusting, high-temperature air cleaning treatment can be used for ceramics but would likely 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.
At the same time as industries are adopting water reuse and conservation methods, they are trying to reduce the footprint of such activities on their facilities. Membrane selection and design plays a part in reducing facility footprint in the businesses or communal buildings using these systems. Newly developed ceramic membranes can achieve a great compactness (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 to increasing the energy efficiency of such systems.
A Key Global Issue
Energy and water are inextricably related. Energy is required for producing water (for extraction, treatment and transportation), and water is required for producing energy (hydropower, steam to turn turbines, or as coolants in industrial processes). Producing the energy needed to produce water and vice versa is already becoming a key issue worldwide. Fossil fuel shortages and water demand will also increase due to population increases, energy demand, and the effects of climate change.
As a result of pressure from increased water demand, there is now a greater likelihood that manufacturing plants and industries that use 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 may even include the use of modular or local filtration systems, rather than sending wastewater to a central system. 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. In China, agricultural use of water has been reduced from 86% in 1980 to 65% in 2005; it is targeted to be 50% in 2050. 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% to 25% by 2015. Other examples include Saudi Arabia, where the 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 will become more prevalent as the need for drinking water increases pressures 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, 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 U.S., 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, which imports water from Colorado; Houston, located in a high-drought area; and San Antonio, identified by the NRDC as having a non-sustainable water supply. Regulations will likely be imposed to respond to these shortages.
To address specific areas, several U.S. pilot plants are using ceramic membranes with ultra-filtration for industrial water reuse. A pilot ceramic membrane treatment system was developed by the Parker Water and Sanitation District (PWSD) for one of its water treatment plants, which treats 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, ceramic membranes were preferred over polymer to reduce the lifecycle cost of the installation. It is estimated that the ceramic membrane’s life will be up to 20 years.
Porous Ceramic Components
In response to these trends, new ceramic water filtration and purification membranes are being developed for use in an innovative filtration system for industrial waste treatment processes. These systems will be more energy efficient to clean and circulate fluid than conventional systems with ceramic tubular membranes.
Material scientists and product development engineers 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 systems are designed for use in micro- and ultra-filtration applications, including those that purify industrial wastewater so it can be reused or safely discharged into the environment.
The reliability of the high-quality ceramic membranes under development will enable businesses to reduce maintenance and energy usage and achieve the associated cost savings. The ceramic membranes 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 to be replaced as often as many plastic alternatives.
In addition, ceramic membranes can be manufactured to highly complex geometries and tight tolerances, facilitating more flexible and innovative designs that make filtration modules more energy efficient. This means that businesses can save energy and costs associated with pumping water through the filtration system.
Sustainability is becoming increasingly important in businesses, and 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.
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