Membrane filtration is nothing new in the area of water treatment. But while the large majority of municipal water and wastewater treatment plants still use conventional filtration methods, membranes continue to be recognized as the modern filtration technology of choice.
Various aspects of water treatment have advanced technologically in recent years, and the innovation in water filtration presents a unique aspect of that growth. In response to water quality issues, utilities aim to implement proactive measures to ensure the safety of their water. As an example, we needn?t look further than current events. Take the recent Elk River chemical spill in West Virginia that affected the water supply of more than 300,000 residents in nine West Virginia counties. While no deaths resulted, the incident was the subject of national attention. Legislation has already been proposed to help ensure the prevention of similar incidents as U.S. Senators in February drafted the Chemical Safety and Drinking Water Protection Act, aimed at providing further oversight of chemical facilities and mitigating threats to clean water.
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Needed projects in this area may be more prevalent than ever, as drinking water continues to become a concern all across North America. Recent U.S. EPA projections show that $682 billion in infrastructure improvements are needed over the next 20 years to meet safe drinking water and sanitation standards in the United States alone. With this in mind, examining trends across the area of water treatment warrants a closer look. One of those notable trends is in filtration.?? ?
Conventional treatment of potable water generally consists of processes such as coagulation, flocculation, sedimentation, filtration and disinfection. Not every water utility has a process that encompasses each of these steps, but many have a process that does utilize some form or combination of these different treatment stages. For the filtration process specifically, using membranes is practical for several reasons.
?In my opinion, I think it is a treatment process of choice,? said Marcus Firman, chief operating officer of Collingwood Public Utilities in Collingwood, Ontario. ?The one good thing about membrane technology is you don?t produce poor quality water even if you don?t maintain the membranes. You just produce less. But the quality of the finished product remains pristine.?? ?
Town of Collingwood, Ontario
The Town of Collingwood is located about 90 miles northwest of Ontario on the Georgian Bay. The town services a total population of about 24,000 with a pumping capacity of about 31.1 megaliters of water per day (approximately 8.2 mgd). Collingwood also supplies water to two other municipalities ? the neighboring Town of Blue Mountains and the Town of New Tecumseth, which is about 40 miles to the southeast. The utility uses surface water from Nottawasaga Bay, Georgian Bay and Lake Huron as its source of raw water treated at the plant.
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The treatment plan itself is rather simple, comprising the processes of filtration and disinfection. The direct filtration process is completed without any coagulants, or chemicals, to aid in the treatment, followed by chlorine disinfection to kill any remaining microorganisms. In contrast to a conventional filtration plant that may use gravity sand or gravel filters, Collingwood?s filtration plant uses membranes.
According to Firman, Collingwood?s membrane filtration plant was the first municipal surface water treatment plant in North America to use submersible membranes. The plant was commissioned in the late 1990s after an outbreak of cryptosporidium in the area a couple years earlier prompted the municipality to take action. ?
?Although [the outbreak] was never conclusively linked to the drinking water supply, it also couldn?t be refuted because there was no filtration process in place,? said Firman. ?Because of that, the Medical Officer of Health required that an absolute barrier be put in place.?
The absolute barrier the utility decided on was a new treatment plant that would encompass the filtration process using hollow fiber membranes and disinfection.
Firman, who was not yet working for Collingwood at the time and was involved in the facility design and construction as project manager while working for an engineering consultant, said membranes were the clear choice for the plant because they have the ability to filter out bacteria and other harmful microorganisms that sand filters alone cannot.
?[Membranes] are the only thing I believed would cover the criteria of what the Medical Offer of Health was requiring because sand filters alone aren?t considered an absolute barrier,? he said.
?When talking about cryptosporidium, for example, the beauty of membranes is that the pore sizes [of the membranes] are 0.01 um (micrometers) and cryptosporidium is typically 2.0 um in size, so it doesn?t fit in the hole. With sand filters, statistically (2 log removal or 99 percent compared to a 6 log or 99.9999 percent challenge passed by membranes) some bacteria may pass through.?????? ?
Collingwood?s Membranes
According to an overview provided by Koch Membrane Systems, membranes are manufactured in a variety of configurations including hollow fiber, spiral and tubular shapes. These configurations include: ultrafiltration; reverse osmosis, which are most often used in desalination or reuse applications; nanofiltration; and microfiltration.
Collingwood uses ZeeWeed membranes, a technology that was originally developed by Canadian company, ZENON Membrane Solutions, but has since been purchased by GE. ZeeWeed membranes are available in a variety of spiral wound and hollow fiber types for use in different applications ranging from drinking water filtration to wastewater treatment, industrial process water and seawater desalination. ?
The concept is that raw water first flows by gravity into the membrane basins. The membranes are then submersed in the raw water while suction is applied. Air is blown into the membrane basins to keep the particles in suspension and assist in removing material from the membrane. The water passes through the membrane leaving contaminants on the outside.
As Firman described, the hollow fiber ZeeWeed membranes Collingwood uses essentially look like long strands of spaghetti, or straws.
?Water is sucked through the straw from the outside into the center of the straw,? he said. ?Again, the membrane has a pore size of about 0.01 um and you suck it through the straw by applying a low pressure suction. So the water passes through the membrane leaving contaminants on the outside.?
Pumps are used to apply suction to the membrane while other pumping systems reject concentrated lake water. The result is high quality potable water free from bacteria that might otherwise make its way through conventional filtration.
Firman also said one of the advantages of using membrane filtration versus conventional is that membrane filtration is compact. Because off this, installation costs are generally lower because membrane systems don?t require large buildings. ?If we were to use sand filters at Collingwood, the land area required to provide enough filtration surface area would be at least three times the land area that we?re using for membrane filtration,? he said. ?So this is a very compact technology.?
Additional Membrane Applications
In addition to potable water applications, membrane technology is also used to treat wastewater, and in many cases, applying membrane bioreactor technology in reuse applications. This technology has also progressed in recent years due to stricter wastewater discharge regulations.
Membrane bioreactor systems generally incorporate hollow fiber membranes and can be used to replace conventional treatment and combine clarification, aeration and filtration into a simple and cost-effective process to reduce operating costs. The result is a consistent, high quality effluent suitable for discharge or reuse applications.
Recently, the Irvine Ranch Water District (IRWD) in Southern California began producing permeate from its newly installed 10 mgd membrane bioreactor (MBR) unit. The project is coming online just in time to distribute much needed recycled?water for a variety of non-potable uses in drought stricken Southern California. The project is part of the Michelson Water Recycling Plant Phase 2 Expansion, which was awarded Water Treatment Project of the Year by the American Society of Civil Engineers? Orange County Branch in 2013. According to the IRWD, construction and capacity expansion of the membrane bioreactor is expected to be fully completed by summer 2014.
Reverse osmosis using membranes can also be used for seawater desalination. For municipal applications, utilities have used this process to reduce or eliminate saltwater intrusion of drinking water supplies. In 2007, the drinking water utility in Tequesta, Fla., discovered salt water intrusion in the superficial aquifer for the town?s water supply. Initially the utility wanted to draw water from a deeper aquifer but realized that water was far more brackish with a much higher salt content and would require desalination before it was suitable for consumer use.
Arcadis Engineering worked with the utility to design a water treatment system capable of providing the level of desalination needed. ?We were confident that a membrane process, specifically reverse osmosis, would be the optimal approach,? said William Reese, vice president with Arcadis, in a release about the project.
Once the system was up and running, the filtered water from the RO membranes was pumped to a finished water tank to be mixed with water still being drawn from the upper aquifer. The entire mix is then treated via the older filtration process and goes into a clear well. The RO permeate now being generated is a high-quality water to complement the old water source.
?Combining the two water sources produces a perfect blend,? said Reese. ?By blending it (the filtered aquifer water) with the newer source, we still have enough calcium for taste concerns but overall it?s a purer end-product. And with over 5.1 mgd of total capacity, we?re more than satisfying the needs of customers, even in times of peak demand.?
While they do demonstrate the wide-ranging uses of the technology, these projects are but a few examples among the many projects completed using membrane filtration for water, wastewater, reuse and desalination applications.
Andrew Farr is associate editor of UIM.
Cryptosporidium is a type of parasite capable of infecting humans and animals and can be washed into rivers and lakes by heavy rains and surface runoff. Drinking water contaminated with the parasite is the most common way cryptosporidium can spread, and it can cause intestinal sickness in humans that can last up to a month. In the United States specifically, cryptosporidium?is one of the most frequent causes of waterborne disease among humans, according to the Centers for Disease Control and Prevention. No deaths resulted from the outbreak in Collingwood, but at the time, the only treatment process the town had in place was disinfection using chlorine.
