The need for fresh water increases every year, with demand driven by such factors as population growth, a rising global living standard, and an upswing in agricultural and industrial processes that require pure water. Suppliers of pumps, valves, piping, storage tanks, treatment chemicals, and testing and measurement instruments are well positioned to be enablers of growth in water markets going forward. And with revenues in the range of $400 billion per year with a strong year-over-year upsurge, the future looks bright in the water industry.
|Figure 1. Breakdown of Earth”s water resources.|
Earth contains approximately 335 million cubic miles of water. However, of the total water currently on the planet, approximately 97.5 percent is in the oceans and seas and not directly usable by humans. Only 2.5 percent is available as fresh water, with 70 percent of fresh water locked up in the ice caps, glaciers and mountain snow fields. That leaves just 30 percent (of fresh water) in ground water sources, with just 0.3 percent readily available in surface lakes and rivers. It has been estimated that if the entire world supply of fresh water were represented by a one-gallon jug of water, the fresh surface water readily available in lakes and streams for use by humans would be just one tablespoon. The breakdown of world water resources as estimated by the United Nations (www.un.org) is shown in Figure 1.
Consumption of water by human activity is shown in Figure 2. Irrigation is the largest consumer of water, followed by domestic use, industrial use, and use for watering of livestock. These are average worldwide values, and can vary substantially by geographic region and by the makeup of the local economy.
Water for domestic use and industrial use provides most of the opportunities for water treatment and filtration, despite being only 25 percent of usage by volume. The higher level water filtration processes for these applications (not including gravity separation and coarse straining) can be categorized in two major classes: granular media filtration and membrane filtration.
Granular media filtration removes suspended solids and macro-particles visible to the naked eye. Granular media can also remove some level of micro-particles, bacteria and parasites. This method of filtration is generally employed after gravity separation, and before other treatment processes. Particle removal occurs either on the surface of the filtration media (cake filtration), or throughout the depth of the media (depth filtration). Granular filtration can have applications as diverse as large-scale filtration in municipal water treatment plants to point-of-use filtration in domestic faucet filters.
|Figure 2. Fresh water consumption by usage.|
Membrane filtration, which is the primary focus of this article, provides a higher level of filtration — from the macro-molecular range to the ionic range. Membrane filtration can remove a wide spectrum of contaminants, including colloids (fine suspended solids), viruses, bacteria, and even dissolved salts to provide pure water suitable for human consumption and industrial processes. Membranes can also be applied to make wastewater suitable for re-introduction into natural water systems. Major national and international corporations are actively involved in researching advanced membrane filtration systems. The demand for membrane filtration is increased substantially by the increased price and decreased availability of pristine natural water supplies.
Membrane filtration is also a major technology for developing additional sources of water for human consumption. In 1961, President John F. Kennedy said in a speech, “If we could ever competitively, at a cheap rate, get fresh water from saltwater … [this] would be in the long-range interests of humanity, which could really dwarf any other scientific accomplishments.”
Today large-scale seawater desalination systems are found in areas of severe water shortage such as the Middle East, and also increasingly in many other parts of the world. According to the International Desalination Association (www.idadesal.org), there are more than 13,000 seawater desalination plants in operation, processing 12-billion gallons (44.4-million m3) of water per day.
Types Of Membrane Filtration
Membrane filtration can be broken down into four basic types:
• microfiltration (MF)
• ultrafiltration (UF)
• nanofiltration (NF)
• reverse osmosis (RO)
|Figure 3. Filtration processes and approximate range of filtration capabilities.|
All systems use pressure differential across the membrane to drive the basic filtration process. Efficiency factors include pressure, temperature, the quality of the source water, and the type of membrane. Figure 3 shows the range of filtration capabilities.
Reverse osmosis (RO) produces the highest level of filtration of the four listed membrane processes. Application of RO filtration is most often required to provide water that is virtually free of solids, salts, organics and colloids. This high removal efficiency is needed for applications such as desalination of seawater for human consumption, as well as where ultra-pure water is required for application in industrial processes, such as chip washing in the semiconductor industry, creation of water for injection (WFI) in the pharmaceutical industry, and water for super-critical boilers in the power generation industry, among others.
Reverse osmosis uses pressure on solutions with concentrations of salt to force fresh water to move through a semi-permeable membrane, leaving the salts behind. Desalination systems may operate at pressures ranging from 800 PSI to 1,200 PSI in order to achieve the desired efficiencies. The amount of desalinated water that can be obtained ranges between 30 percent and 85 percent of the volume of the input water, depending on the initial water quality, the quality of the product and the technology and membranes involved.
An RO system is made up of the following basic components: pretreatment, high-pressure pump, membrane assembly, and post-treatment. Pretreatment of feedwater is often necessary to remove contaminants and prevent fouling or microbial growth on the membranes, which reduces passage of feedwater. Pretreatment typically consists of filtration and either the addition of chemicals to inhibit precipitation or efficient filtering to remove solids. A high-pressure pump generates the pressure needed to enable the water to pass through the membrane.
The membrane assembly consists of a pressure vessel and a membrane that permits the feedwater to be pressurized against the semi-permeable membranes in a variety of configurations. The two most commercially successful membrane configurations are spiral-wound and hollow-fiber. Post-treatment prepares final product water for distribution, removes gases such as hydrogen sulfide, and adjusts pH.
The energy requirement for RO depends directly on the concentration of salts in the feedwater. Because neither heating nor phase change is necessary for this method, pressurizing the feedwater accounts for the major use of energy. As a result, RO facilities are most economical for desalinating brackish water and increase in cost as the salt content of the water increases.
|Figure 4. Desalination system flow diagram.
RO has become a relatively mature technology, and membrane systems are experiencing fast growth. Some of the largest new desalination plants under construction and in operation use RO membranes. Figure 5 provides approximate operating costs for a large municipal reverse osmosis seawater desalination plant and shows the major impact of electrical costs in operating high-pressure system pumps. Percentages will vary based on the size of the plant and the makeup of the source water. Desalination of brackish water typically requires much lower operating pressures (and hence lower electrical costs) than seawater desalination.
Some issues surrounding RO systems, particularly seawater desalination systems, include the substantial energy required to operate high-pressure pumps, and the re-introduction of concentrated brine back into the environment without adverse effects on the ecosystem. These issues are the focus of intensive research by suppliers of equipment to this industry, and solutions are currently available to at least partially mitigate the problems. Additional opportunities still exist in these areas for even greater improvements. Other opportunities include improved membranes that are more durable and increase the flux of pure water; new approaches to reduce biofouling in membranes; more effective energy recovery and use; and development of less expensive materials.
Microfiltration (MF) is a membrane filtration process that allows molecules the size of salts, sugars and proteins to pass through the membrane pores, while molecules the size of bacteria are rejected. Microfiltration has many applications in the field of biotechnology, food & beverage filtration and drinking water treatment. Microfiltration can also be used as pretreatment for reverse osmosis. Microfiltration is the least efficient membrane technology, but consumes the least amount of energy.
Ultrafiltration (UF) is another membrane filtration process for purifying liquids. Ultrafiltration typically rejects organics over 1,000 molecular weight (MW), while passing ions and small organics. Suspended solids and solutes of high molecular weight and of a size larger than the membrane pore size are retained on the feed side of the membrane. Ultrafiltration is best applied for the removal of particles in the approximate size range of 0.1um down to 0.01um.
Ultrafiltration will typically remove all bacteria, parasites, Giardia cysts, and viruses. In the pharmaceutical industry, ultrafiltration is used as a final filter in some water-purification systems. Ultrafiltration is used for a variety of other applications as well, including purifying and concentrating protein solutions. Growth in this sector has been associated with the food industry, and more recently in the purification of drinking water. In the spectrum of filtration technologies, ultrafiltration falls between microfiltration and nanofiltration.
Nanofiltration (NF) is a relatively recent development in membrane technology, with a high degree of filtration efficiency. It is efficient at removing particles in the size range of 0.01um down to 0.001um. It has applications in many industries, with particular application in the dairy segment of the food industry. Another fast growing application is in the filtration of water and wastewater, with annual revenues exceeding $1.6 billion. Its performance is close to that of RO, but with lower energy requirements, which makes it an attractive solution for many applications.
|Figure 5. Breakout of major operating costs for a municipal seawater desalination plant.
In general, membrane filtration technologies, including MF, UF, and NF, offer advantages relative to other purification techniques such as distillation. Advantages include operation at or near ambient temperatures, lower energy requirements, and reduced capital costs because no boilers or condensers are required.
Ultrapure Water (UPW)
The treatment and distribution of conventional ground and surface water for domestic use is relatively well known, as evidenced by the technology on display at treatment facilities, pumping stations, settling ponds and other infrastructure elements common to most developed communities.
Less well known, perhaps, are the applications of ultrapure water produced using one or more of the membrane technologies described above. Major industrial applications of UPW include power generation, chip cleaning in semiconductor manufacturing, flat panel display manufacturing, and pharmaceutical manufacturing.
The largest usage of UPW systems is in the semiconductor industry. In this industry, substantial amounts of ultrapure water are required for wafer cleaning and rinsing, chemical preparations, chemical-mechanical polishing, chemical vapor deposition, disc-drive manufacturing, immersion optical lithography, and other uses. Recent industry data indicates that approximately 500 gallons of ultrapure water are required in the preparation of a single 150-mm wafer. According to McIlvaine Company (www.mcilvainecompany.com), ultrapure water system sales in this sector are projected to exceed $2.5 billion per year in 2012. Much of this growth will come from Taiwan, Japan, South Korea and China.
The most rapidly growing segment for UPW is in flat-panel displays. Liquid crystal displays (LCDs) are the most common type of flat-panel display and have been widely used since the early 1970s. LCDs account for about 80 percent of the entire flat-panel market. A major use of ultrapure water in this segment is for critical substrate surface cleaning to remove all traces of foreign particles. Taiwan and South Korea are the leading suppliers of the display market segment and of UPW systems for that application.
The pharmaceutical industry uses ultrapure water in numerous ways, including preparation of water for injection (WFI) for human injectables, process water for chemical preparations, and in cleaning solutions, to name several. The pharmaceutical purchases of ultrapure water systems continue to grow at a steady rate, with projected worldwide sales of approximately $345 million in 2012. Within this sector, biotech is growing much faster than the total pharmaceutical UPW market, with strong growth especially in India.
The power industry is seeing significant activity in ultrapure water systems, particularly for supercritical steam boilers. Supercritical boilers require ultrapure water for operation, and are growing in numbers for new and updated power generation plants throughout the world, especially in China. Advantages of supercritical boilers include increased efficiency, lower greenhouse gas emissions, and lower fuel costs. Sales of UPW systems for coal-fired boilers are expected to be in the range of $975 million by 2012.
Ultrapure water systems are highly instrumented. Pump and valve purchases for ultrapure water systems are expected to reach approximately $350 million per year in 2012. Significant purchases will also be made for degasification, disinfection, ion exchange, storage and piping.
Continued Opportunities In Water Filtration & Infrastructure
As highlighted by United Nations and industry studies, the demand for pure water is on an upward sloping trendline that is expected to increase along with world population, increased industrialization, and increased living standards for a greater proportion of the world’s population. This increasing demand provides opportunities for large and small companies involved in the many aspects of water processing, treatment and distribution.
Domestically, the American Recovery and Reinvestment Act of 2009 (ARRA) of the federal stimulus program has earmarked approximately $7 billion in funding for water-related programs. This provides unprecedented opportunities for growth in domestic markets, augmented by powerful supply-and-demand market forces for growth in water markets throughout the rest of the world.
Tom Tschanz is a senior consultant with the McIlvaine Company. Mr. Tschanz has an extensive background in the electric utility, HVAC, and flow measurement categories. He earned his bachelor’s degree in Mechanical Engineering at Marquette University, has been trained in Six Sigma and Total Quality Management & Lean Manufacturing, and is a named inventor on two U.S. patents. In his current role for McIlvaine Company, Mr. Tschanz is responsible for analyzing technology segments, such as valves, pumps and filters. He can be reached at firstname.lastname@example.org.