Using Your Membrane
Immersed ultrafiltration membranes as an alternative to traditional water processes
The most commonly used methods for water treatment have been around for more than a century and rely on large basins to accommodate the processes of sedimentation, sand filtration and the addition of chemicals. These methods often do not address the current pressures placed on water resources, falling short of stringent new drinking water regulations.
Today, the emphasis in water treatment is shifting from inactivation of contaminants to their removal. This transition is being driven by the discovery of parasites such as Giardia cysts and Cryptosporidium oocysts, for which chlorination has limited efficiency. In addition, the use of chemicals may result in the production of harmful by-products and the generation of large quantities of treatment residuals (sludge).
The most common membrane water treatment process is reverse osmosis (RO), which is used for brackish or seawater desalination. However, other membrane processes, including microfiltration (MF) and ultrafiltration (UF), are rapidly developing due to increasingly stringent microbial and turbidity regulations; increased consumer awareness and expectations of drinking water quality; the reduction in the use of chemicals; and technological advances that have reduced the capital and operating cost of MF/UF systems.
A common configuration for MF/UF technologies is the hollow-fiber membrane, a thin tube ranging in diameter from 0.5 to 2.0 millimeters (mm) with a porous wall. Under a pressure gradient, the feed water flows through the membrane wall, either in an outside-in or inside-out flow path. Thousands of hollow-fiber membranes are assembled into a module that constitutes the building block for constructing a membrane system. The filtration modules are supported by the required ancillary equipment, including interconnecting piping, feed and backpulse pumps, blowers, valves, instrumentation, controls, etc.
Immersed membranes differ from conventional membranes in that pressure vessels are not required. Instead, shell-less hollow fibers are immersed directly into an open tank, and a gentle suction is applied to draw clean water through the fibers.
The suction approach has many advantages, including lower energy requirements, small plant footprints, low infrastructure development and upgrade costs and the ability to treat high-suspended solids with minimal fouling. It is particularly suited to large capacity applications where economies of scale enable the immersed membrane process to compete effectively with traditional water treatment processes.
Immersed membranes represent a paradigm shift from conventional membrane filtration technologies. Historically, conventional membrane systems have focused on achieving higher fluxes (flow per unit surface area) in order to reduce membrane surface area and thus reduce cost. These efforts were primarily directed at reducing the capital cost of pressurized membrane systems, where the modules have a low packing density and require significant ancillary equipment. Conversely, immersed membrane systems have a high packing density and are designed to operate at low and stable fluxes when compared to other membrane technologies. This provides two key benefits to the end user:
- At low fluxes, factors such as tangential flow, air scrubbing, back pulsing and chemical cleaning become less critical. This means that an immersed membrane system offers comparable operations simplicity to conventional water treatment processes, such as sand filtration.
- When the cost of implementing membrane technology is examined with a life cycle analysis approach, it becomes evident that energy and membrane replacement costs are dominant factors; both of these costs are reduced with immersed membranes.
As membranes have continued to advance over the past few years, they have emerged as a treatment alternative for:
- Direct filtration;
- Iron and manganese removal (achieved by coupling membrane filtration with an oxidation step);
- Color and total organic carbon (TOC) removal (achieved by coupling membrane filtration with coagulation);
- Conventional water treatment plant residual concentration;
- Pre-treatment for RO and nanofiltration; and
- Wastewater treatment and reuse for municipal and industrial applications (achieved by coupling membranes with biological treatment).
Treating Low TOC Water
Filtration of low TOC source water using immersed membranes does not require a pre-treatment step, even if the feed water contains fine suspended particles. Membrane filtration effectively replaces the coagulation, flocculation, clarification and sand filtration steps of conventional plants in a single step. Immersed directly in the process tanks, the membranes do not require a pressure vessel and are immune to the presence of high solids in the tank. For surface water plants, the performance of the membranes is unaffected by the feed water?s seasonal high turbidity peaks.
In addition to being a positive barrier to parasites, immersed membranes are being increasingly selected for small and large treatment plants because they are cost-effective and ideally suited for future capacity expansion. Unlike conventional pressurized membranes, immersed membranes can be installed directly in new or existing tanks, utilizing flexible combinations of the basic membrane modules to accommodate various treatment capacities and spatial requirements. This allows for the convenient retrofit of existing sedimentation basins, filters or other tanks with MF/UF membranes.
The use of large process trains (>5 million gallons per day (mgd)) with the immersed membrane system also reduces requirements for pumps, blowers and valves. These reductions in equipment translate into a smaller building footprint, reduced system complexity and lower capital costs. The huge space-savings resulting from the replacement of the concrete sedimentation basins and filters further reduces capital costs, which off-set the higher operational costs of immersed membrane filtration.
Treating High TOC Waters
The application of immersed MF/UF membranes using enhanced coagulation has been successful in applications where color and TOC removal are needed for producing drinking water. Color removal rates of up to 95 percent and TOC removal rates of up to 75 percent have been achieved utilizing this process.
Immersed membrane filtration enables a single tank coagulation-flocculation-UF process to replace the coagulation-flocculation-sedimentation-filtration stages of a conventional plant. The flocculation hydraulic detention times (HDT) for immersed UF membranes are a fraction of those used for sedimentation because pin-head size floc is all that is needed for effective removal. A high solids concentration is maintained in the membrane tank to promote the attachment of organics onto the small metal hydroxide flocs. The barrier characteristics of the ultrafilter are used to separate flocculated particles and precipitates. Overall recoveries range from 92 to 98 percent, depending on the treatment process used.
Powdered activated carbon can also be used in combination with enhanced coagulation to reduce taste and odors.
The use of immersed membranes for drinking water treatment has become the preferred alternative to conventional filtration technologies. Immersed membrane advancements, lower costs and increased user familiarity with membrane technology continue to lead the paradigm change in water treatment technologies. They are now being used around the world for water treatment and supply.
This article originally appeared in the 05/01/2002 issue of Environmental Protection.