Membrane use for water treatment is on the rise. (Cover Article).
In 1995, a three-mgd membrane filtration plant designed by Black & Veatch in Fort Lupton, Colorado, was considered large. Now the larger facilities are more than 20 times that size. The engineering challenge is how to integrate membranes with other processes to achieve multiple treatment goals.
One example is the Minneapolis Water Works (MWW) program of replacing granular media filters with membranes downstream from lime softening and coagulation-sedimentation processes to treat 160 mgd of drinking water and yield a combination of softening, control of dissolved organic materials, filtration, and disinfection.
Scott Freeman and Srinivas Veerapaneni, Ph.D.
WHY MEMBRANE FILTRATION?
What are MF/UF membranes and why have they become popular so quickly? These membranes are formed into hollow fibers, just slightly larger than a human hair. Submicron-sized pores in the fiber wall filter impurities from the water.
Membrane filters are made of plastics with complex names such as polyvinylidenefluoride. Although membrane chemistry may sound complicated, the technology makes it easier for a water plant operator to ensure removal of small particles and pathogens--and therefore makes it easier to protect public health.
"Membranes provide an excellent barrier to microbial contaminants. Generally, physical removal is better than chemical inactivation as it avoids possible future concerns over newly discovered byproducts," says MWW Director Adam Kramer.
In the 1990s, the water industry became increasingly concerned about the presence of certain pathogens in source water, including Cryptosporidium oocysts. Because these pathogens are resistant to traditional disinfectants, it is advantageous to remove them ahead of chemical disinfection. If they are not completely removed, consumers may suffer gastrointestinal illnesses that are painful and can sometimes prove fatal for individuals whose immune systems have been weakened by chemotherapy or illnesses such as AIDS.
Conventional granular media filters can successfully remove the pathogens, but the effectiveness of removal depends on proper operation of the coagulation-flocculation process upstream. Coagulant chemicals may join with small particles like occysts to form masses large enough to be settled or adsorbed onto the media, preventing their passage into the filtered water.
If coagulation, flocculation, sedimentation, and granular media filtration processes are correctly combined and applied, protozoan removal credits of 2 to 2.5 log (which means 99 to 99.68 percent removal) can be achieved. But to attain this level of performance with granular media filters, plant operators must periodically adjust the coagulant close and verify that the filtered water turbidity is less than 0.3 nephelometric turbidity units (Ntu). To be on the safe side, many utilities target a turbidity goal of 0.11 Ntu.
The operating mechanism of membranes is completely different than that of granular filter media. The pores in the membrane fiber wall are much smaller than the protozoan cysts. (Giardia cysts are nominally five to 14 micron and Cryptosporidium oocysts two to seven micron.) Thus the microbes are easily removed by simple size exclusion, independently of pretreatment coagulation conditions. As Jack Schulze, team leader of Surface Plant Evaluation for the Texas Natural Resources Conservation Commission, humorously explains, "You can't shove a cow through a barbed wire fence."
The automatic result is a high level of removal of small particles, resulting in finished water turbidities of less than 0.08 Ntu--frequently much less. Based on the results of an extensive microbial challenge test program, the California Department of Health Services has approved MF/UF systems with fourlog (99.99 percent) removal credit for Giardia and Cryptosporidium. Significantly higher values--up to eight-log--are routinely observed during tests.
Membranes offer a new way to verify the level of pathogen removal at full-scale facilities. Water plants that use granular media filters monitor data from turbidimeters and sensitive particle counters to document the removal of cyst-size material. Membrane plants apply these same methods as well as automated integrity verification systems to check for leaks. This type of system, which is similar to filling the inner tube of a bicycle tire with air while submerged in a bucket of water, helps an operator prove that there are no significant defects and helps locate any leaks that may occur.
Utilities report fewer than one broken fiber a month per 10 mgd of capacity, well under any significant level of concern. Even with a few breaks, large facilities maintain a high log removal.
BACKWASHING AND CLEANING
During the filtration cycle, a layer of material collects on the dirty side of the membrane surface. This filter cake is removed by a periodic automated backwash, similar to the operation of a granular media filter. Some systems apply an air scour during the backwash to improve the effectiveness. In many cases, operations are simplified by periodically applying a chemically enhanced backwash with a low concentration of citric acid or hypochlorite solution to further improve the efficiency. At less frequent intervals, after weeks or months of operation, a small portion of the filter cake accumulates, increasing pressure drop across the membrane. This is removed by chemically cleaning the membranes while in place with acid, caustic, and sometimes hypochlorite solutions. Typical cleaning agents include citric acid and sodium hydroxide.
In the early applications of MF/UF for surface water treatment--during the mid-1990s--membranes were used primarily in a simple filtration mode. Without any additional process steps, the small pores in the membrane wall produce well-filtered finished water. Simple filtration suffices if removal of particulate material is the only treatment goal.
The three-mgd membrane water treatment plant in Fort Lupton, Colorado, is an example of this, producing clear and clean filtered water. However, many situations require removing dissolved contaminants such as the compounds that cause taste and odor problems, as well as the naturally occurring organic material that reacts with chlorine to form disinfection byproducts (DBPs), trihalomethanes, and haloacetic acids. Other processes can be integrated with membranes to achieve multiple treatment goals, as illustrated in the examples below.
Minneapolis, Many treatment plants use lime softening to produce softer, aesthetically pleasing water. Minneapolis is applying a new approach, integrating lime softening with membrane filtration.
"Minneapolis is a leader among the increasing number of communities that are updating their aging water systems. Portions of the Columbia Heights plant have been in operation for nearly 90 years," says Black & Veatch Project Manager Chad Hill, who is based in the company's Minneapolis office.
Minneapolis was confronted with the need to upgrade aging filtration equipment at its 70-mgd Columbia Heights and 90-mgd Fridley plants, which treat water from the upper Mississippi River. Following an extensive evaluation process, the city selected our firm to design the replacement of granular media filters with membranes at Columbia Heights. Upon completion in 2004, the Columbia Heights Membrane Filtration Plant will be the world's largest lime softening-membrane facility and the largest UF plant in North America.
The Fridley design is scheduled to follow two years later. Pilot systems for two membrane systems were conducted onsite to verify operating conditions and microbial removal. One system was chosen in a competitive bidding process that took into account the initial capital cost as well as the present worth of major operating expenses. Unlike the purchase prices for most types of equipment, membrane costs decrease each year. Minneapolis proved no exception, with a cost of $16 million yielding a unit cost of less than $ 0.23/gpd for the membrane system.
Scottsdale Chaparral WTP. Many traditional water treatment plants supply coagulated-flocculated water directly to granular media filters in direct filtration process. Scottsdale's new 30-mgd Chaparral water treatment plant will apply direct filtration to membranes. Raw water from the Arizona Canal will be dosed with approximately 15 mg/L of ferric sulfate coagulant, which adsorbs dissolved contaminants into particles and allows membranes to filter them from the water.
A unique aspect of this plant is the use of coagulation-flocculation-MF/UF to remove dissolved arsenic, a pollutant of growing concern for many water supplies. Scottsdale chose membranes for various reasons, including space limitations. Four types of membranes were compared in onsite pilot studies, and the full-scale equipment will be selected in an evaluated competitive bidding process. A post-membrane granular activated carbon step will remove traces of taste-and odorcausing compounds and DBP precursors, so this plant will serve as an example of combining direct filtration and post-treatment methods.
Elsewhere in the city, the award-winning Black & Veatch-designed Scottsdale Water Campus incorporates MF as well as reverse osmosis to treat was tewater to potable quality for reclamation through aquifer recharge.
Bakersfield, California. In some cases, a sedimentation step between coagulation-flocculation and filtration improves the overall cost-effectiveness of treatment. Our firm has taken this approach in the design build delivery of the 20-mgd California Water Service Company plant in Bakersfield. The Kern River, the source of raw water for the new plant, is typically not a "flashy" source; however, turbidity can increase from 15 to more than 1,000 Ntu during spring rains. Sedimentation will dampen these peaks and augment removal of dissolved color and DBP precursors.
Before proceeding with detailed design of the facility, the project team pre-selected the membrane equipment through a competitive bidding process based on life-cycle cost analysis. The cost of the entire project, including membranes, is $30 million, and it is scheduled to begin operation in mid-2003. Black & Veatch is designing a similar 36mgd facility for the South San Joaquin Irrigation District in Stanislaus County, California.
PUMPS OR A NATURAL SIPHON?
In the past, each MF/UF membrane filtration train has been served by an individual pump. To simplify operation and beneficially use the hydraulic profile available at most water treatment plants, our firm has developed an innovative naturalsiphon approach for the 96-mgd Choa Chu Kang Waterworks in Singapore, which will be the largest membrane water plant in the world. As part of a major plant upgrade, submerged membranes will be installed in the existing filter boxes to improve the already high quality of the finished water. After a siphon has been established, a 25-ft elevation change between the filter boxes and the filtrate storage tank will draw water through the membrane. Onsite pilot tests verified the operating conditions and the membrane pretreatment of alum-based coagulation-flocculation-sedimentation.
The use of membrane filters for treatment of surface water has expanded rapidly, in terms of number of installations as well as geographic range. MF/UF membrane plants are now operating in more than half of the states in the United States and in eight other countries.
MF/UF yields high-log removals of Giardia cysts and Cryptosporidium oocysts and produces low filtered water turbidity, independent of upsets in pretreatment coagulation. The physical barrier provided by membranes, coupled with integrity verification, makes it relatively easy for utilities to produce well-filtered and safe potable water.
New approaches are being developed to integrate membranes with other treatment steps to achieve multiple treatment goals including softening, removal of arsenic, and elimination of compounds that cause taste and odor problems or produce DBPs. MF/UF has matured into one of the main filtration methods, and membrane facilities are sprouting up all over. The performance of membrane filtration is now well demonstrated, so membrane options should be considered for any new surface water plant or major refurbishment project.
Mr. Freeman is a Senior Membrane Process Engineer and Dr. Veerapaneni is a Process Engineer with Black & Veatch, Kansas City, Missouri. Both are members of the company's global team of water and wastewater treatment process specialists.
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|Author:||Freeman, Scott; Veerapaneni, Srinivas|
|Date:||Jun 1, 2002|
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