In less than 20 years, low-pressure membrane filtration has successfully gained acceptance in the municipal marketplace. In 1989, Memcor membrane technology was implemented for a 1-mgd municipaldrinking water system—a large system
for that time. Within five years, more than 20 municipalities were using these membranes. Today, the technology has been implemented in thousands of communities worldwide, producing up to 120 mgd.
Once thought to have limited use, low-pressure membranes are now utilized in water treatment, wastewater treatment, reclamation, and pretreatment to RO for desalination applications. The once “alternative technology” has become a technology of choice.
This acceptance rate is a result of ongoing innovation, advancing performance and decreasing costs. This article summarizes 20 years of innovation and the evolution of low-pressure membranes.
Fiber advancements
Polypropylene hollow fiber membranes were once the most common membranes in the municipal market. These fibers, having a nominal pore size of 0.2 microns, achieved greater than 4-log removal of Giardia and Cryptosporidium. Made of a single polymer, these homogenous fibers could withstand backpressures up to 100 psi and set the standards for direct integrity testing.
Market demand called for the development of oxidant-resistant membranes. Although numerous versions were commercially available, few could withstand the integrity testing used with the polypropylene fibers.
Unlike horizontally aligned membranes or thin-skinned PVDF membranes that have a potential for breakage or delamination, Memcor PVDF membranes are made of a robust single polymer, designed to withstand high pressure differentials and conservative integrity-testing standards, assuring long-term quality.
Because of their strength and integrity, the membrane modules can be configured in both pressure and submerged systems, depending on the requirements of the particular application. With a pore size of 0.04 microns, they achieve higher virus reduction than polypropylene membranes.
System configurations
When membranes were first introduced to the municipal water industry, most systems were vertically configured, pressure-driven systems with modules placed in pressure vessels and arranged on skids. Today, both pressure-driven and vacuum-driven (submerged) systems are commercially available, each having benefits and advantages for certain applications.
Pressure-driven membranes have inherent advantages in applications, such as cold water, where water viscosity can quickly limit the trans-membrane pressure of submerged membrane systems. Memcor pressure systems are widely used in pretreatment to RO applications, drinking water and reclamation plants.
Submerged membrane systems
have advantages, such as their ability to retrofit into existing conventional systems to increase capacity. Existing plants built in the late 1970s often are structurally sound but face challenges meeting enhanced regulatory requirements or increased capacity demands. Submerged membranes are installed into these existing bays, often with minimal piping changes.
In 1998, Memcor submerged membranes became available for municipal use and are used today in reuse, drinking water and wastewater applications. Kennewick, Wash., retrofitted a 7.5-mgd conventional plant to membranes, doubling the capacity to 15 mgd without adding additional filter area.
Growing application base
Many membrane bioreactors and seawater desalination applications now use low-pressure membranes rather than conventional processes. Seawater desalination processes often use RO to remove salts and dissolved matter. To ensure optimal performance, RO membranes require consistent, high quality feedwater. Although a challenge for conventional filtration, low-pressure membranes produce consistent water quality with SDI less than 3, typically less than 2. The result is enhanced performance and prolonged RO life.
In addition to increasing performance, low-pressure membranes have significant cost advantages over conventional pretreatment. Chemical pretreatment is typically eliminated; process steps are removed; and overall footprint is reduced. The result is a more cost-effective solution, adding additional value to desalination applications.
Membrane bioreactors are also widely accepted. Unlike thin-skinned membranes that rely on a nylon substrate for support, Memcor membrane fibers are made of a single polymer structure. This monolithic design eliminates the potential for delamination and failure due to surface abrasion.
As regulatory requirements tighten, membrane technology advantages outweigh conventional technologies in terms of water quality and cost per gallon treated. Memcor membranes, with a pore size of 0.04 microns, provide physical barrier, removing pathogens and colloidal matter to provide greater than 4-log rejection, thus reducing the amount of disinfection required. Reducing chemical disinfection helps to reduce the formation of disinfection byproducts, a major benefit in meeting the Long Term 2 Enhanced Surface Water Treatment Rule.
In addition, costs are reduced with Memcor membranes. Chemical consumption for pretreatment is typically reduced, lowering overall operating costs. The compact nature of membrane technology often eliminates process trains and reduces filter areas, thus reducing the overall footprint and capital cost.
Evolution continues
Low-pressure membrane technology continues to evolve as fiber research and system designs advance, and market acceptance increases. The global market and numerous applications for membranes will continue to grow as costs and performance continue to outweigh those of conventional technologies.
These affordable, reliable systems are available for customers needing 20 gpm to more than 100 mgd, from drinking water to wastewater reuse.
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