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Pumps & Systems, October 2007

When the capabilities of membrane technologies are combined with the economies of activated sludge processes, there is the real potential for having "the best of both worlds" relative to wastewater treatment. 

Activated sludge processes have proven their effectiveness over many decades. These biological processes are relatively simple and are economical over a broad range of applications. However, activated sludge processes alone have their limits. These limits include an inability to reach extremely low effluent requirements for turbidity, suspended solids, and phosphorous, and the inability to remove certain pathogens.

Membrane technologies, by comparison, have the ability to remove virtually everything in the process stream and deliver effluent that is immediately ready for reuse. However, membranes-because of their ultrafiltration capabilities-are limited when it comes to handling high solids-laden flows. Such flows will quickly blind-over membrane units, and large amounts of energy are required to clear the membranes of debris and ensure adequate dissolved oxygen levels.

For some time, it has been "common practice" to combine the biological processes of the activated sludge process with the extraordinary capabilities of membrane technologies. Although this approach provides many of the benefits of the biologically-based activated sludge processes with the benefits of membrane ultrafiltration, it is at best imperfect.

Current approaches typically include immersion of the membrane in the biological process tankage, which submits the membrane to large amounts of contaminants. This results in increases in energy requirements and compromises membrane life. When thoroughly evaluated, energy costs and membrane replacement costs make this approach cost-prohibitive.

More recently, an approach has been developed that moves the membrane ultrafiltration function further down the process line. In this approach, the influent is first treated in one of several optimized versions of the activated sludge process and then-prior to reaching the membrane units-the process moves through a high-rate activated sludge clarification process. The addition of this intermediate step, placed ahead of the membrane unit process, removes much of the debris inherent in the activated sludge process.

In this approach, the flow reaching the membrane units is at the effluent contaminant levels of a highly effective treatment system. Then, and only then, the effluent is introduced to the membrane surfaces. Obviously, this dramatically increases the life of the membrane and reduces the energy requirements related to membrane fouling mitigation. Also, because resistance of flow through the membrane remains low-due to lack of contamination-pumping energy and associated wear-and-tear are significantly reduced.

Most importantly, this specific approach to combining biological and membrane process units has consistently demonstrated that it can deliver effluent to meet virtually any requirement, including California's Title 22 requirements for reuse.

This winning combination has proven itself in tough applications, with an installation at California's Eastern Municipal Water District in Perris, as well as other municipal wastewater treatment facilities in North America.

This process has been named IMASTM technology (which stands for Integrated Membrane Activated Sludge).

Operational experience has shown that this form of the "combined process" requires 50 percent to 75 percent less of the energy required by combinations where the membrane is exposed to the biological process.

The membrane is made of hydrophilic polyethersulfone, which naturally resists biofilm formation and fouling and is chemically resistant to oxidants as well as low and high pH levels. 

The membrane is integrally bonded to the support fabric to facilitate backflush capabilities. 

It is rare that a winning combination is found using proven components. It is even rarer when the combination leads to longer component life, consumes less energy than conventional approaches and delivers superior results. However, this is the promise and the reality of new technology. Everyone benefits from this environmental and technological breakthrough, but none more than the municipal budget managers faced with tighter regulations and greatly reduced funding sources.  

Ben Gould works at Ashbrook Simon-Hartley, 11600 East Hardy, Houston, TX 77093-1098, 800-362-9041, Fax: 281-449-1324, http://www.ashbrookcorp.com/.