By Joe Cohen
A new process offers a more efficient use of backwashable granular filtration media (silica sand,anthracite coal,pumice,granular activated carbon, ion exchange media). Offering improved efficiency via smaller, less-costly equipment providing more service flow and requiring less wash water, it could benefit applications from small point-of-entry (POE) systems through large commercial and industrial high rate filtration processes; even municipal water treatment plants may utilize this new process.
Inspiration for better filtration
Backwashable granular media filters are susceptible to many flow-related problems and inefficiencies:
- Malfunctions caused by channeling of the media
- Gas pockets within the media body
- Compromised flow rates due to low available media surface area and high resistance to flow
- Low dirt holding capacities resulting in relatively short filter cycles
- Inefficient backwash cycles that consume a lot of time and water
- Loss of media in excessive backwash flow
A resolution to design an improved process immune to all these problems led to the development of a new process called Radial Flow filtration (RFF). The issues with conventional gravity-reliant systems are all unquestionably undesirable and avoidable problems that can create consequential wasteful, costly and potentially dangerous results. Conventional water filtration equipment is big, expensive and inefficient, especially when compared to RFF. In light of acknowledged global water shortages, the water filtration industry should strive to develop new and more efficient water filtration techniques that provide better quality water at a faster rate. The new RFF techniques should have an associated lower equipment and operational cost, making the filtration technology affordable to the countries that need it most.
A working prototype was successfully constructed and tested by the developer in 1995. A test station was designed and built to test filtration performance comparatively between RF and conventional high rate filtration. To test the two filtration techniques on a level playing field, the test loop could select between the RF or the conventional filter, both in identical 25-inch diameter tanks and both with three cu.ft. granular media capacities.
As opposed to the conventional method of allowing gravity to compact the granular media inside a filter tank, the RF filter mechanically compacts its cache of granular media into a shape that substantially increased the available surface area: a rigid hollow orb (see media body illustration, Figure 1) suspended within the vessel. The incoming unfiltered water totally envelops this suspended media orb and flows radially inward at an accelerating velocity through the compacted media, to be filtered and collected at its hollow center.
With the same amount of media, this new geometry allows three times the available surface area of a conventional filter. The surface area of the media body (expressed in square feet) determines process speed and capacity for contaminant. The service flow and dirt holding capacity of a granular media filter are directly dependent on the available surface area of the media body. It was theorized that tripling the available surface area would triple both the service flow and the dirt holding capacity of the filter. Basically, the RF concept puts more of the media to work by locating all the media within seven inches of the exterior surface of the media body where it can actively filter the water. Many additional benefits of RF filtration became apparent during the development and testing stages.
The suspended orbicular media body of the RF filter provides a combination of increased available surface area plus reduced bed depth that generates less resistance to flow than a conventional gravitationally packed filter with less area and more bed depth.
In the comparative tests, the conventional filter, with a surface area of 3.4 ft2 (and with clean media) creates a pressure loss (DP) of 4.5 psi at its recommended maximum flow rate of 68 gpm. At the same DP (4.5 psi) the RF filter produced a flow of 130 gpm and it can be driven to 215 gpm at a DP of 10 psi with no disruption or movement of the media grains: basically, three times the flow with the same amount of media.
In a conventional filter, gravity exerts a compacting force on the media that becomes progressively stronger with depth, just like water pressure within a water column. The total available force of 3 cu ft of silica sand media is only equal to its submersed weight, about 200 lbs. Unfortunately, this force is weakest where its needed most: at the media body surface where channeling problems begin.
In the RF process, the granular media is captured and mechanically compacted between two spherical permeable barriers, the media container and its inner drain (which is the same spherical shape but smaller; see Figure 2.) The media container is composed of two elastomerically adjoined hemispheres that act dynamically on the media body like a spherical vice.
In the prototype used for testing, the compaction force was generated by the combination of a spring and the resistance to flow as the water passed inward through the media container. Other forces (electro-magnetic, hydraulic, or pneumatic) could also be used. At a service flow of 200 gpm our media container produced 1,160 lbs. of clamping force that is exerted equally, not progressively, throughout the media body, forcibly compacting it to minimum void and holding it rigid. Consequently, troublesome channeling and gas pocket problems are avoided.
During the backwash cycle, the media container hemispheres diverge, allowing the media body to fluidize while keeping the media grains captivated within the space between the media container and the inner drain. At the end of the backwash cycle, the media container instantly recompacts the media into the rigid orbicular shape for the next filtration cycle.
Backwashing is a blast
The RF filter backwash cycle offers major performance advantages, too. Since the granular media is captive in the space between the two spherical media barriers, that allows for backwashing with violently turbulent flows. The media cannot escape from the tank in the effluent flow – in fact, it is positioned in close proximity to the powerful backwash jets. Prototype testing with a backwash jet manifold of 16 high-velocity jets achieved a nozzle velocity of 16 ft/sec, approximately 390 times faster than the velocity within a conventional filter backwashing at a flux rate of 20 gpm/ft2.
These powerful jets blast the media clean in less than 45 seconds, as opposed to the three to five minutes required by conventional filters.
Also, unlike a conventional filter where up to 50 percent of the tank is freeboard, there is very little space inside an RF filter occupied by unfiltered water. Since the freeboard space within a conventional filter must be displaced with clean water at the end of the backwash cycle, time and water are wasted.
Further experimenting with the prototype proved, surprisingly, that the unit worked best with buoyant media. The use of a buoyant granular media within a regenerative backwash filter had never been accomplished prior to the invention of the RFF. Up to this point, the specific gravity (relative weight) of the media had always been a huge consideration for water filtration media, because conventional systems rely upon gravity to position the media grains. RFF does not rely upon gravity; it can use lightweight granular media – the lighter the better. A buoyant media fluidizes instantly in the backwash cycle, which improves efficiency and greatly reduces the overall weight of the system. This opens the door to the use of high performance synthetic granular filtration media.
When comparing production RF filters to conventional filters at similar flow rates, this weight savings (dry) calculates to 90 percent. Savings of floor space required calculate to 55 percent (see attachment). These space and weight savings have been of particular interest to mobile filtration companies. With conventional systems, they must truck the media separately to stay under the highway weight limits. With RF technology, they can ship ready-to-run filter plants.
In “Method and Apparatus for Filtering” (US Patent #3,557,955 issued in 1971) water filtration inventor Gene Hirs found polyethylene (PE) to be an exceptionally good granular water filtration media. Since then, a new super durable form of PE has emerged: Ultra High Molecular Weight Polyethylene (UHMW-PE). It seems an ideal material for water filtration media: it possesses a high resistance to abrasion, impact and wear. It is non-toxic, approved for food service applications by the Food and Drug Administration (FDA) and compliant with NSF/ANSI 61 for components that come in contact with potable water.
UHMW-PE is extremely chemically inert (it’s even used in automotive wet cell batteries). It has a low coefficient of friction which gives it a generally non-stick surface. It doesn’t adhere to the contaminants it traps so it makes an ideal media to regenerate by backwashing. This material fluidizes quickly and immediately separates from contaminants when backwashed. A buoyant material that obviously cannot be compacted by gravity, it can work within an RF Filter.
Tests were conducted using a special powdered UHMW-PE designed specifically for filtration. This powder has a material particle size range from 31 to 420 microns and is normally used to produce sintered (low pressure or pressure-less fusion molded) disposable filter cartridges that feature two to four mm absolute filtration performance (see side bar). Its popcorn-like grains provide a low resistance to water flow combined with a high dirt holding capacity. This material has a lot of surface area, (2,200 cm2/g) and it is very lightweight at 19 lbs/cu ft. Conventional filtration media weighs between 45 and 120 lbs per cu ft. and must be added to conventional filters AFTER installation, adding both cost and time required for an installation. They also require larger tanks with more media, further increasing the space, time and cost considerations of a water filtration system installation.
This media is designed to trap suspended solids with the geometry of the flowpaths it creates (just like kelp trapping a diver), not by adhesion to these solids. Although it was designed specifically for filtration, this product had never been used for filtration in its raw powdered form. If used in conjunction with the new RF process, it could very possibly provide an ideal backwashable media for the removal of cryptosporidium, the number one leading filterable contaminant to the worlds drinking water. UHMW media, being used successfully in a sintered form to remove cryptosporidium,could very possibly be used in the regenerative (reusable) technique of RFF to continually, efficiently and affordably remove the contaminant from public drinking water at high process rates.
Interestingly, during the process, this material forms a spongy cake that can be returned to its powdered form with backwash turbulence. At a flow rate of five gpm/ft2, the pressure loss (DP) is only 3.75 psi/inch of depth of mechanically compacted media. Tests were conducted of its ability to remove SAE Fine Test Dust from water as an indicator of its potential to remove Cryptosporidium. This test dust challenge contains particulate much smaller than Crytosporidium, made up of particulate ranging from .65 to 88 mm and, by volume, 20.4 percent is three mm or smaller. A single pass through a three-inch mechanically compacted bed of the UHMW-PE consistently removed 97.9 percent (by our initial measured tests) of the SAE Fine Test Dust. Based on these results, a 10-ft2 RF filter could be configured to provide 50 gpm absolute two to four mm backwashable filtration utilizing a three-inch bed of UWMW-PE. This filtration level could conceivably remove cryptosporidium from water at the acceptable 3.5 log rate without coagulants.
Replacable ‘media cells’ of the Future
This technology could be best commercialized in a singular size backwashable RF filter that provides 10 ft2 of external media body surface area. This size RF filter be a spherical 25″ diameter vessel featuring an easy-to-open flange at its equator. Each filter vessel would contain a quick-change RF ‘media cell’ (see cartoon). The media cell is the entire RF mechanism within the filter tank. It would include the media container, the inner drain and backwash jet manifold, and, of course, the media. The cell would quick-change with a bayonet lock fitting on the lower pipe flange. These filters would be installed in sequentially backwashed battery installations to accommodate a wide range of process flow rates and additional filters could be plugged in later to handle increased flow rates in the future. This modular approach allows system operators to adjust the number of filters online to the prevailing flow rate and to pull units offline individually for media cell changes without interruption of the service flow.
These multiple filter installations would all use the same backwash valve, a single three-port three-inch multiport valve on the influent line of each RF Filter. This method of valving is popular with today’s high rate filter manufacturers. It allows the sequential backwashing of filters with filtered water. This would standardize valving requirements for all RF systems regardless of the number of filters.
RF media cells could be produced with a variety of different media to address different user needs and could be promptly changed out to adjust to changes in the influent water quality.
In conclusion, RF filtration is a new, practical approach to automatable water filtration that offers promise of providing a much more affordable, efficient, and serviceable system. This new design filter, which fits through a standard doorway, could fill applications where traditionally, large high-rate filters are now used. Because gravity is not part of the equation, RF technology could also open the door to the use of backwashable granular media technology in zero gravity environments.
This filtration technology was presented to the US EPA/ETV steering committee in Cincinnati in May, 1997. Shortly thereafter the agency finalized a test protocol entitled, “NSF Equipment Testing Plan: Backwashable Depth Filtration For The Removal of Microbiological and Particulate Contaminants”. This new protocol will cover the RF process described in this article.
About the Author
Joe Cohen is Chief Operating Officer of Univalve LLC and FiltraSonics LLC in Denver, Colorado. Cohen has an extensive background with recreational water facilities as well as water filtration. He holds a number of patents relating to filtration and valving.