In order to understand how the Vortisand® Cross-Flow Microsand Filter (CMF) can overcome media loss during standard operation, we first need to understand how media loss can occur within a conventional multi-media or sand filter.
Conventional Filtration & Media Loss
The efficiency of a sand filter relies on both its design and the application in which it is being used for. In wastewater and/or surface water filtration applications, the quality of the water can vary depending on the performance of the treatment system that is currently in place prior to the filter. As it pertains to surface water it is important to note that the environment and season may also play a crucial role in water quality, altering the levels of TSS from one season to the next. Here is a list of the most common challenges that lead to media loss within a conventional media filter.
|Development of cracks and contraction of filter bed||If the filter bed is not cleaned properly, the grains of the filter-bed (filtering medium) become coated. As the filter compresses, cracks develop, especially at the sidewalls of the filter. Ultimately, mudballs may develop. This problem can be controlled by adequately backwashing and scouring the system.|
|Loss of filter medium or media (mechanical)||With time, some of the filter media may be lost during backwash and through the underdrain system (where the gravel support has been upset or where the underdrain system has been installed incorrectly). The loss of the media can be minimized through the proper placement of washwater troughs and underdrain systems. Special baffles have also proven to be effective.|
|Loss of filter medium or media (operational)||Depending on the characteristics of the biological floc, grains of the filtering media can become attached to it, forming aggregates light enough to be floated away during the backwashing. The problem can be minimized by the addition of an auxiliary air and/or water scouring system.|
Extracted from: Wastewater Engineering, Treatment and Reuse, Metcalf & Eddy, Fourth Edition, McGraw-Hill, 2003, 1819 pages
High Efficiency Cross-Flow Filtration & Media Loss
The design of the Vortisand® system is optimized to maximize filtration performance while minimizing media loss commonly experienced through the backwash waste outlet, and to virtually eliminate any media loss during filtration.
Media Leaks Within the Filter
The Vortisand system utilizes fine sand (150 microns) to create its media bed, as a result special design considerations are made to prevent such fine media from leaking out from the bottom of the vessel.
A. Cross-Flow Injectors
Vortisand filtration systems use exclusive VortiJet™ injectors to generate cross-flow patterns that sweep the surface of the media. This sweeping motion causes a portion of the water to flow parallel to the top section of the media bed, allowing for high filtration rates while preventing fouling, surface blinding and channeling.
The cross-flow injectors are designed to evenly distribute the inlet water across the surface of the media bed. Therefore, the inlet flow is not concentrated in one area of the media bed. Ultimately, this technology drastically reduces the possibility of erosion and channeling from occurring. The graph below showcases the generated cross-flow action that creates a surface sweeping motion, keeping larger particles in suspension to later be removed through the backwash process.
When comparing the above graph to the flow patterns created by other cross-flow technologies and older generations of the “vortexing” injector system, we can see various concerns that can lead to channeling. Such outdated technology creates an uneven flow of distribution and increases the likelihood of channeling and media loss. This is important to understand as not all filtration systems utilizing similar technology are created equal. This is visible within the diagram displayed on the right, a media surface rendering of a single injector filter.
B. Support Media Layers and Wedge Wire Collectors
The microsand media performs all the filtration for the Vortisand system and takes up the first 8 inches of the media bed. Beneath the microsand there are additional layers of progressively coarser media that act as support for the microsand. The design is made in such a manner that the microsand cannot leak out from the vessel. Finally, a wedge-wire screen is evenly collecting the filtered water at the bottom of the vessel, further protecting the system from media loss.
To verify the filter’s resistance to channeling and media loss, the Vortisand CMF technology was tested until the differential pressure (dP) reached 28 psi (backwash is normally triggered at 17 psi). Yet, the media bed remained stable throughout this process without any media loss, while demonstrating a great resistance to channeling or surface blinding.
Media Loss Through Backwash
A. Rinsing the Media
In order to cleanse the media, the backwash flow needs to be high enough to fluidize the media, to remove and expulse the suspended solids (TSS) that were trapped during filtration. The flow also has to be low enough to ensure the media is not dragged out to the drain. To control this, a flow controller is installed and set to the exact flow required to backwash the filter. The flow controller will keep the exact backwash flow as long as the inlet pressure is between 30 to 100 psig.
A special sequence that has been developed for the Vortisand CMF is the MiniBW™ cycle. For example, if small air bubbles enter the filter and are being trapped within the media, this phenomenon is called “air entrapment”, the pressure drop across the filter would increase without having reached it with TSS removal. To prevent this, the Vortisand can be programmed to perform a MiniBW™ for less than one minute to simply release these bubbles but not allow the media to flow out of the vessel, and to return to the initial clean filter differential pressure drop.