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Ultra Filtration Systems
Replacing Conventional Water Pretreatment Processes with Membrane Filtration
The membrane filtration processes of Ultrafiltration (UF) is rapidly becoming the preferred choice for water pretreatment in industrial and municipal environments. Recent developments in membrane types and cartridge configurations have enhanced the economic viability and technical reliability in large volume applications such as surface water pretreatment and industrial waste water recycle.
Membrane systems are now being used for boiler feed, cooling towers and process water in the chemical, petrochemical, power generation, semiconductor, and beverage industries. Systems may be designed for treating raw water directly from a river, lake or other surface source. The UF system is designed as a single step unit processes that replaces multiple step processes such as solids contact clarification with coagulant feeds and one or more steps of multimedia type depth filtration.
For zero discharge wastewater treatment systems, water is treated directly from biological treatment and secondary clarifiers. The water is then recycled to the front end of the plantâs feed water treatment process.
UF system is usually designed as a continuous process, with operation 24 hours per day. Typically units are back-flushed every hour for about 30-60 seconds and periodically cleaned with chemicals restore water flux. Systems are designed with recovery rates up to 99.5%, allowing for maximum efficiency of the water treatment system. The concentrated retentate from the ultrafiltration system may be further dried and removed as a solid.
The Membrane as a Physical Barrier
The UF system provides a physical barrier between feed and product water. Membrane processes do not rely on chemical precipitation, depth filtration or biological activity as with conventional water treatment. The quality of the filtrate from a membrane filter is dependent only on the pore size of the membrane.
Since the separation process is based on particle size, wide variations in feed water such as high turbidity found during flood stages of surface waters, result in no change of product water quality. Consistent, high quality water is always produced by the membrane filter.
For example, a river water system may have a feed water turbidity of 5-50 NTU during normal operation, but flood stages may result in a turbidity of 150+ NTU water due to silt and runoff. When using an ultrafilter, product water will not vary in turbidity quality, with an expected turbidity of less than 0.1 NTU in all cases. Product water quality is independent of feed water composition.
Improving Reverse Osmosis System Performance
When used as a pretreatment step to reverse osmosis, the ultrafilter will produce a feed water quality that significantly enhances the performance of the RO system. Reverse osmosis systems require a feed with a low silt density index (SDI).
Ultrafiltration produces a water with an SDI of less than 1.0 by eliminating suspended solids, colloids, bacteria and particulate matter. Cleaner feed water has the effect of decreasing the RO cleaning frequency and enhancing membrane life.
As an example, an RO system experiencing biological fouling with a once/week cleaning, may now be cleaned once every several months with a change in RO membrane life from 1-2 years to 5 years or more.
With a cleaner water feed to the RO, the design of the RO system may be significantly smaller than conventional treatment designs. Flux increases of 30-100% are found when using UF as a pretreatment step resulting in reduction in the size of the RO system for a given flow rate and therefore the cost of the RO system. Alternatively, in plants where energy consumption is the primary issue, the operating pressure of a system may be decreased, resulting in a reduction of energy costs.
Cleaner feed water to the RO also has the potential of increasing the recovery of the RO system. By removal of fouling components before the RO system, a 10-15% recovery increase is predicted assuming that the fouling species is colloidal or particulate in nature.
Chemical Free Water Treatment
Most membrane processes do not require chemical addition steps to precipitate or flocculate water contaminants, making membrane processes both economically and environmentally attractive. The concentrate from the water treatment plant will be free from added components that may cause environmental discharge problems relating to increased organics, or increased disposal mass. The concentrated streams from the membrane processes are concentrated native components of the original water stream. Except for small volumes of chemicals used to clean or sanitize the membrane systems, water treatment with membranes is a chemical free process.
Lower Capital and Installation Costs
Ultrafiltration systems are competitive and often less expensive than conventional treatment using clarifiers and multimedia filters. Membrane filtration systems are modular in design and require minimal on site installation investment. Compared to conventional systems which require more extensive civil work, foundations, concrete work, and tankage, membrane systems are factory fabricated for quick and easy on-site installation. A typical UF/RO system requires 30-50% of the footprint space of a conventional treatment plant, and is significantly lighter in weight.
High Removal Efficiencies
Both ultrafiltration and microfiltration membranes are designed to remove a wide range of contaminants from surface water and industrial waste waters, but because of its improved rejection properties, ultrafiltration is typically selected over microfiltration. Ultrafiltration membranes have pore sizes of approximately 0.02-0.03 microns and are used because of their ability to almost completely retain both suspended and colloidal particles. Essentially all silt, clay, particulate matter, colloidal silicates, insoluble iron, and microorganisms are removed by the ultrafiltration process.
The high level of removal for colloidal silicates and colloids of iron, manganese, aluminum and other metals make ultrafiltration an ideal pretreatment to reverse osmosis, nanofiltration, ion exchange, and other water treatment processes where colloids may present a problem with downstream unit operations.
Membrane filtration also allows for almost complete removal of microorganisms such as bacteria, algae, fungus and viruses. Ultrafiltration produces the highest level of microorganism removal and is the membrane of choice for most potable and industrial processes with high levels of microorganism contamination. For industrial RO pretreatment processes, the high microorganism removal of a UF membrane minimizes biofouling at the RO system.