Patent Publication Number: US-2007102337-A1

Title: Deep bed filter

Description:
FIELD OF THE INVENTION  
      This instant invention is generally related to deep bed filtration systems as described in U.S. Pat. Nos. 3,814,247, 3,900,395, 4,048,068, 4,197,205 and 4,197,208, all to Hirs.  
     BACKGROUND OF THE INVENTION  
      The instant invention relates generally to an improved deep-bed filter used for filtering water, sewage or other aqueous liquids, and specifically to an improved apparatus and method for liquid filtration utilizing a polymeric filtration media layer for enhanced contaminant filtration with minimum pressure drop through the filter.  
      Typical deep bed filters often have difficulty handling peak or emergency loads without over-design of the filter or the addition of an auxiliary filter to handle the extra contaminant load. Simply adding layers of filtration media to existing deep bed filters to solve the aforementioned problems is ineffective. Furthermore, backwashing many prior art filters having media of varying particle sizes but identical specific gravities results in a reversal of the media grading order, i.e., from small to large.  
      This reverse gradation problem has been solved to some degree by using media materials having differing specific gravities. However, even when using materials of such different specific gravities as anthracite and sand, if the granules of coal are large enough they will stratify at lower levels within the filter bed. These aforementioned dual media filters are generally used to handle increased turbidity loads and will provide longer periods of filter operation between backwashing. However, when turbidity gets very high and coagulants must be used these filters are still subject to surface binding, thereby requiring frequent backwashing.  
      The primary problem with known in the art dual media deep bed filters is that large coagulated particles and floc that are larger than the voids in the top layers of media are captured at the surface instead of passing into the depth of the media. This buildup of surface contaminants on the filter causes pressure buildup on the filter surface, thereby restricting flow of high turbidity liquids and preventing effective use of adequate chemical flocculating agents. Furthermore, during filter backwashing media classification takes place and the coal fines settle at the surface of the filter, thereby closing off any voids and further restricting fluid flow through the filter. Increased relative turbidity is often the result.  
     SUMMARY OF THE INVENTION  
      The instant invention utilizes an additional filtration media layer that is both larger and lighter than sand disposed below. This upper media layer mixes with the sand to enhance and maintain interstitial sites at the filter surface and throughout the mixed strata. Furthermore, this media is comprised of a plurality of cylindrical polymeric particles, each cylinder having a carefully selected diameter to maximize mixing with the finer sand particles subsequent to filter backwashing. This uppermost media strata prevents excessive buildup of contaminants on the surface of the deep bed filter, thereby enhancing fluid flow therethrough, even during periods of high contaminant loading. Other uses and advantages of the instant invention will become apparent from the detailed description of the preferred embodiments below in conjunction with the accompanying drawing Figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic view of a prior art deep bed filter and a deep bed filter in accordance with the instant invention.  
       FIG. 2  is a graph of deep bed filter performance curves, in accordance with the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring now to  FIG. 1  and in accordance with a preferred embodiment of the instant invention an improved deep bed filter  10  comprises a vessel  20  having a fluid inlet  30  and a fluid outlet  40 . The vessel  20  may be any known in the art filter container or containment area capable of accepting a contaminant to be filtered. The inlet  30  supplies contaminated fluid to the filter  10 , and is in fluid communication with an inlet valve  32  and an outlet backwash valve  34 . Similarly, the outlet  40  is in fluid communication with both an outlet valve  42  and an inlet backwash valve  44 . When filter  10  backwashing is desired, both the inlet and outlet valves  32 , and  42  respectively, are closed, and both backwash valves  34  and  44  are opened, to allow fluid to be pumped from the bottom of the filter  10  to the top thereof.  
      The vessel  20  contains a plurality of layers of filter media including a layer of sand  50  over which is placed a layer of polymeric particles  70 . The filter media typically contains large granular particles for example, below the filter sand  50 . The polymeric particles  70  are cylindrical in shape, having a diameter of about 1.5 mm and a length of about 6 to 12 mm. Furthermore, in one embodiment, the polymeric particles  70  may have a specific gravity of approximately 1.15. The cylindrical shape and specific gravity of the particles  70  are selected to allow settling thereof at a rate which permits mixing with the finer sand particles  50  as the filter media layers settle subsequent to backwashing.  
      The cylindrical polymeric particles  70  mix well with the lower layer of sand particles  50  to form an intermediate layer  60  containing polymeric media and sand particles. This feature of the instant invention provides a significant improvement over prior art deep bed filtration systems.  
      Since the upper portion of the sand particles  50  tend to be relatively small, filter  10  surface loading often occurs around this area of the filter when high concentrations of organic contaminants or chemical flocs are present in the fluid to be filtered. The mixing action of the cylindrical polymeric particles  70  with the finer sand particles  50  creates a plurality of voids or interstices forming the intermediate layer  60 . These voids permit contaminant penetration into the filter  10 , thereby greatly enhancing filter efficiency due to significantly decreased surface loading.  
      The selection of size, shape, and specific gravity of the polymeric particles  70  is crucial to proper filter operation. The polymeric particle  70  characteristics are selected to provide a desired amount of mixing with the sand  50  to facilitate filter penetration. For example, prior art spherical polymeric particles are less desirable for use in deep bed filters because the rate of settling of a sphere is too great, thereby causing the particles to settle to the filter bottom. In one embodiment, the layer  70  ranges from 6 to 12 inches in the filter.  
      Since a cylinder has a higher area to volume ratio than a corresponding spherical particle, and therefore settles at a much slower rate, cylindrical particles are superior for use as a top media layer. Furthermore, polymeric particles having a specific gravity of 1.15 have an ideal settling rate to facilitate mixing with the sand particles  50 , wherein exemplary sand particles range from 45-55 mm. Specifically, nylon or polypropylene composite rods having a specific gravity of 1.15 are commercially available and are readily cut into cylindrical particles  70  of appropriate size.  
      In one embodiment, a plurality of sand layers may be formed within the filter  10 . As shown in  FIG. 1 , the first filtering sand layer  50  may be sized at 45-55 mm particles. A second supporting sand layer  52  may be formed beneath first sand layer  50 , wherein sand layer  52  is formed with medium sand particles larger in size than first sand layer  50 . A third supporting sand layer  54  may be formed beneath the second supporting sand layer  52 , wherein sand layer  54  has relatively larger or coarser sand particles than sand layer  52 . A layer  56  of supporting gravel may then if desired be layered beneath the third sand layer  54  for final passage of the liquid prior to exiting the filter  10 .  
      As shown in the table given below, an experimental filter formed in accordance with the present invention was compared to a plant filter formed as known in the art. The experimental filter indicated in  FIG. 2  was sized to have a diameter of 18 inches and a height of 60 inches. The polymeric layer  70  was about three inches in depth. The layer of filter sand  50  had particles sized at about 45-55 mm and was about 18 inches in depth. The flow rate through the experimental filter was about 7.5 gallons/square foot/minute on average. The total hours of operation were 1200 hours. The cycle time of filtration was about 82 hours. 60% of the water volume indicated a clarity better than 0.02 NTU, or national turbidity units.  
                           TABLE 1                                   Experimental Filter   Plant Filter                                                Size   18″ Diameter ×   8′ Diameter × 20′ Tank           60″ height       Top Layer   3″ Polymer   24″ Anthracite       Filter Sand   18″ of .45-.55 mm 2     15-21″ of .45-.55 mm 2         Hours of Run   about 1200 hours   about 1200 hours       Time       Flow Rate   about 7.5 gal/(ft 2  · min)   about 2.5 gal/(ft 2  · min)       Cycle Time   82 hours   100 hours       Clarity   60% less than 0.02 NTU   25% less than 0.02 NTU       Total Volume   950,000 gallons   28,000,000 gallons       Filtered                  
 
      As shown in the table given above, an experimental filter formed in accordance with the present invention was compared to a plant filter formed as known in the art. The experimental filter indicated in  FIG. 2  was sized to have a diameter of about 18 inches and a height of about 60 inches. The polymeric layer  70  was about three inches in depth. The layer of filter sand  50  had particles sized at about 45-55 mm and was about 18 inches in depth. The flow rate through the experimental filter was about 7.5 gallons/square foot on average. The total hours of operation were 1200 hours. The cycle time of filtration was about 82 hours. The clarity was about 60% of the volume was less than or better than 0.02 NTU, or national turbidity units.  
      With regard to clarity as shown in the table, “60% less than 0.02 NTU” or “25% less than 0.02 NTU” is defined as meaning that 60% of the total volume of water has clarity better than 0.02 NTU, or, that 25% of the total volume of water had clarity better than 0.02 NTU, respectively. The results  
      The plant filter indicated in Table 1 was sized at about eight feet by about 20 feet. The layer of anthracite was about 24 inches in depth. The filter sand was about 21 inches in depth and sized at about 45-55 mm. The flow rate through the plant filter was about 2.5 gallons per square foot. The cycle time of filtration was about 100 hours. The clarity was about 25% less than or better than 0.02 NTU.  
      The experimental filter was operated parallel to the plant filter and samples taken simultaneously. Water was supplied from ground wells having a relatively high hardness content of 300 ppm. Lime treatment was used to reduce the hardness. The city utilizing the plant filter had approximately 100,000 water users. For comparative purposes, an extensive run time of 1200 hours was employed. The average experimental filter run or cycle time was 82 hours before backwashing. The plant filter ran for 100 hours before backwashing. The object of the experiment was to determine if the effluent quality could be improved while concurrently raising the flow rate. It will be appreciated that in general, effluent quality is compromised as flow rates are increased unless more sophisticated filtration is used.  
      The plant flow rate was maintained at about 2.5 gallons per minute per square foot while the experimental filter was maintained at about 7.5 gallons per minute per square foot. The total amount filtered by the experimental filter was about 950,000 gallons. As shown in  FIG. 2 , the experimental filter ran 640 hours (out of 1200 total hours) with turbidity better than 0.02 N.T.U. (national turbidity units). In contrast, the plant filter only ran about 260 hours (out of 1200 total hours) with turbidity better than 0.02 N.T.U. Overall, the experimental filter provided water significantly less turbid than the plant filter over the 1200 hour run time. Stated another way, the experimental filter only resulted in about 100 hours of water quality indicating 0.10 N.T.U. On the other hand, the plant filter resulted in about 480 hours of water quality indicating 0.10 N.T.U., notwithstanding substantially lower relative flow rates through the plant filter. It should be appreciated that optimization of coagulating and/or flocculating treatment would likely improve the experimental filter performance indicated by the data presented. For the sake of comparative purposes, the water entering the experimental filter and the plant filter were chemically treated in the same manner.  
      It can therefore be concluded that as indicated in the present test and as compared to the plant filter, the present or experimental filter provided about 250% or more improvement in the water quality as measured by turbidity. It should further be emphasized that the substantial improvement in turbidity was realized while increasing the experimental filter flow rate to about 300% of the plant filter flow rate. This is certainly unexpected, and counterintuitive to what one of ordinary skill would expect. Typically, improved turbidity is most often achieved by reducing flow rates, increasing chemical treatment, and/or providing additional filtration subsequent to the deep bed filter. All of these options, however, translate into greater operating costs. The present system provides improved water quality at higher flow rates and at relatively lower costs.  
      While the preferred embodiments of the invention have been described in detail, it will be appreciated by one of ordinary skill in the art that the instant invention is susceptible of various modifications without departing from the scope of the following claims.