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Ultrafiltration (UF) is a low-pressure membrane process, capable of removing colloidal materials, fine suspensions, bacteria, virus, suspended solids and large molecular weight organic materials depending on the molecular weight cutoff of the membrane.
Ultrafiltration technology is one of the fastest-growing technologies in the water treatment industry. Membrane systems have been used for many years to clean water for reuse in hundreds of industrial applications, and can be used by carwashes as well.
This filtration provides a physical barrier to separate and concentrate suspended solids, oils, greases, waxes, and odor causing biological contaminants. The water produced from the system is so clean that it can be combined with reverse osmosis (RO) systems to produce drinking quality water. Ultrafiltration can provide water quality of .01 micron — small enough to remove almost all bacteria, viruses, and suspended solids. Ultrafiltration is a pressure-driven purification process in which water and low molecular weight substances permeate a membrane while particles, colloids, and macromolecules are filtered. The primary removal mechanism is size exclusion. The clean water produced from the system can be re-used or blended with other sources of water in industrial and commercial applications reducing water and sewer costs for many industries.
If you look at the chart (figure 1) above — provided by Koch Membrane Systems — you will see the different types of separation processes and note the specific micron/molecular weight cutoff of ultrafiltration technology. Depending on the water quality required/desired ultrafiltration provides the leap from waste or other process waters to high quality or high purity water.
These membranes come in many different shapes/sizes, and are constructed from different types of material. The most important factor in taking advantage of ultrafiltration technology is having a good water analysis to understand which type of membrane to use. Most installations even start with a small pilot system to verify the effectiveness of the equipment prior to full scale introduction. Most installations whether desalination or process water have a fairly consistent contaminant level, some process water like carwash reclaim can have a variety of contaminants or constituents introduced to the water that can have an impact on operation.
Fortunately, the technology has advanced over the years with the different types available. Membranes manufactured using PVDF (polyvinylidene fluoride) or ceramic material have a higher degree of tolerance to these upsets. Additional technological advancements have improved output and reduced capital costs. Ultrafiltration membranes function at lower pressures (5 – 150 psi) which minimizes construction cost and operating costs.
An example of one type of UF membrane is shown in Figure 2. This is the typical construction of a PVDF hollow fiber membrane. Hollow fiber membranes can operate two ways. "Outside in" which refers to the flow of the water from the outside of the fibers having the filtering process flow to the center of the fibers. "Inside out" refers to the water flowing inside the fibers and filtering out.
Ultrafiltration system design
Manufacturers understand most applications are custom designed to solve the many different types of problem water. As mentioned earlier the better the information available through a water analysis, the better the chance of an optimized solution for each project. A short pilot study placing a smaller version of the equipment on-site allows for a good design prior to design of a full scale system. Pilot studies can last a day, week, or more depending upon the circumstances. When you consider the typical return on investment, permitting, regulations, and environmental impact advantages of UF, a pilot study and good water analysis can eliminate cost and headaches. Optimized design, operation, and chemical costs (if required) can all be calculated with this data.
Figure 3 shows a typical design of an ultrafiltration system. It must be stressed that UF flow diagrams may differ between each project and this is only one example. Designs will vary depending on the requirements and problems to be treated. The plumbing in/out of a UF System usually consists of a raw water inlet, filtered water outlet and drain line. Nothing has a greater effect on the performance of any membrane-based filtration system than the feed water quality. For lasting performance it is important to supply the UF systems with the feed water quality shown below in Table A.
|Recommended feed water characteristics|
|pH||2 - 11|
|TDS||up to 10,000 mg/L|
|TSS||up to 500 mg/L|
|Temperature||15 to 35 C|
|Hardness||< 300 ppm as CaCO3|
|Turbidity||< 50 NTU|
|Iron||< 5 mg/L|
|Oils and Greases||< 0.1 mg/L|
|Solvents, phenols||< 0.1 mg/L|
Briefly outlined below are a few applications that benefit from ultrafiltration:
• Pretreatment of reverse osmosis and nanofiltration: Of all the pretreatment methods, the use of UF membrane is the most important step forward prior to reverse osmosis and nanofiltration. A good pretreatment will provide good quality feed water for subsequent RO and NF, and UF does just that through a consistent pore size membrane barrier that greatly surpasses the feed water quality requirement for RO and NF. The result is consistent and excellent feed water.
• Municipal water treatment: The world is experiencing fresh water scarcity like never before from population growth, rapid industrial expansion, global warming and weather change, and contamination. Large UF membrane modules can drive down the cost of municipal water treatment to treat more water. UF systems require minimal added chemicals and often reduce the need for chlorine and other biological disinfectants. Packaged UF systems can be low cost solutions for small residential area or communities trying to cope with new drinking water regulations for surface water supplies. UF membranes have been shown to be more practical and cost effective alternatives to conventional chemical and physical based water and wastewater treatment, involving coagulation, flocculation, clarification, and media filtration.
• Residential and commercial water treatment: Residential water quality fluctuates depending on the age of water pipes and type of water source. It can be affected by sediment, cloudiness, viruses, pathogens, cysts, rust and sand. Bacterias with typical length 0.4-14 micron to width of 0.2-2 micron can be safely removed using UF membrane in addition to viruses, pathogens, cysts and other micro-organisms. The result is clear water free of sediment and micro-organisms that cause people to get sick. There are also some commonly found water-borne parasites with tough walls that are chlorine resistant. The use of UV disinfection is not enough while UF membrane provides a safe barrier to keep out these harmful micro-organisms.
Desalination and ocean depth water filtration
During the process of making drinking water from brackish saline water or seawater, a combination of ultrafiltration followed by reverse osmosis can be very cost effective. UF membranes used as pretreatment can significantly extend the useful life of RO elements due to the removals of organisms and suspended matters in feed water. When treating brackish water with relatively low TDS, UF can be used as pretreatment to a reverse osmosis system, and also to produce permeate for blending with RO product. The result is water for distribution that not only tastes good but is less expensive to produce. The UF approach allows the designer to significantly reduce the size of the RO system and its overall costs.
Wastewater recovery and water reuse
With ever more stringent environmental laws today, recovering and reusing your process water as a renewable water source is becoming more important across many industries. UF can help you reuse water from process wastewater, rinsing operations, and low-solid effluents with water recoveries up to 95 percent of the volume of wastewater that would otherwise be lost through discharge. Wastewater is economically recycled using advanced PVDF ultrafiltration membrane systems in combination with RO systems. With advanced water recovery technologies, you can gain value from your process, reduce wastewater discharges, and improve your operations.
Stan Royal is vice president, sales, for Con-Serv Manufacturing, which is located in Lakeland, FL. He can be reached at email@example.com or Toll Free at (800) 868 9888. Website: www.con-servwater.com.