Abstract:
Locally backwashing portions of filter media allows a simple and effective design of intake and pretreatment units, as well as their integration. An enclosure is used to limit portions of filter media and backwash them locally by suction, utilizing filtered water from adjacent filter media as the back wash water. Wastewater is produced at small amounts that allows efficient sludge treatment. This design enables water pretreatment at the intake unit, simplifying overall plant design and preventing damage to organisms living outside the intake unit.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of water treatment, and more particularly, to a filtering system. 
       BACKGROUND 
       [0002]    Water treatment systems such as seawater desalination facilities and osmotic power plants (facilities that generate energy from a difference in osmotic pressures of e.g. seawater versus river water) include an intake unit for delivering water from its source (e.g. sea or river) to the system, and a pretreatment unit for removing floating and dissolved material from the delivered water, in order to prepare the water for the main membrane process. 
         [0003]    Two types of intake units are open intakes and infiltration intakes (or infiltration galleries). Open intakes draw water via piping directly from the source. Open intakes typically employ screen meshes to filter out large debris and prevent fish or other marine life from being drawn into the pumps. However, millions of fish and other small marine organisms, with a width of under 2 cm are sucked into the piping, leading to considerable damage, both to the environment and to facilities. Damage is inflicted on both large aquatic organisms such as fish or crabs that are trapped against the intake screens and drown or suffocate, and on small marine organisms such as fish, fish eggs, larvae or plankton that is drawn into the intake system and is killed by the plant equipment. 
         [0004]    Infiltration intakes, or galleries, are built in the seabed by the installation of horizontal drain systems. The drain system is placed in the natural filtration media sand, and the seawater is slowly filtered by the sand. This media is naturally cleaned by waves and storms. Horizontal drain systems deliver water to the pumping station located on the seashore. Infiltration galleries, while protecting the marine environment, can only be installed in areas with naturally occurring sands. Another major limitation is that these systems clog over time and it is highly difficult, or in some cases impossible, to clean them. Clogged media reduces the throughput through the system by two orders of magnitude (e.g. from 10 to 0.1 m 3 /hr). 
         [0005]    Pretreatment units employ a layer of filter media supported by a drainage layer. Water is introduced above the filter media, and is pretreated by flowing through the filter media which removes floating and dissolved material therefrom. The filter media is gradually clogged by the removed material, and periodical global backwashing is used to clean the filter media. Global back washing produces huge amounts of wastewater which leads to environmental and technical problems. The backwashing process also involves interrupting the operation of the filter, and this is a major drawback too. 
         [0006]    Such a global backwashing system is illustrated in  FIG. 1A .  FIG. 1A  illustrates a prior art filter cleaning method for back washing a filter  90  that is used to filter water  91  through filter media  92  into a drainage layer  96  (under-drain) that supports filter media  92 . The filter cleaning method uses an external source of backwash water that is pumped throughout the whole filter  90  to backwash the filter globally (see arrows). The backwash water is then removed gravitationally through a discharge channel  132 . The large volume of backwash water requires operation of the filter with a high level of water  91  above filter media  92  (denoted in  FIG. 1A  by H) to allow for expansion of the filter media, during which sludge is released from the filter media particles. The necessarily large water head has severe constructional implications, as the substrate must support the large pressures. Hence, prior art backwashing systems suffer from a severe limitation. Clearly, this method of backwashing is not applicable to infiltration intakes as they are open to the water source (such as a sea or a river) and contamination of the source with the backwash water is hardly acceptable. 
       BRIEF SUMMARY 
       [0007]    One aspect of the present invention provides a method of backwashing a filtering system having a layer of filter media supported by a drainage layer. The method comprises: sequentially backwashing partial volumes of the filter media by (i) sinking the enclosure into the filter media until the enclosure is supported on the drainage layer by reducing the air pressure in an upper part of the enclosure once the open lower end of the enclosure is immersed in filter media, to enclose the partial volume of filter media within the enclosure, (ii) generating local backwashing of the partial volume of the filter media enclosed in the enclosure, during continued operation of global filtering through filter media outside the enclosure in the filtering system, by generating suction in the upper part of the enclosure that initiates a water flow from the upper part of the enclosure through the pipe system to the discharge structure, wherein the water flow introduces into the lower end of the enclosed partial volume of filter media filtered water from the drainage layer which expands the enclosed filter media and releases sludge therefrom to the water flow, and (iii) raising the enclosure above the surface of the filter media to release the backwashed enclosed volume of filter media, by injecting air and increasing air pressure in the upper part of the enclosure to float the enclosure above the filter media. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    For a better understanding of embodiments of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout. 
           [0009]    In the accompanying drawings:  FIG. 1B  illustrates schematically the idea behind the proposed local backwashing apparatus and method in respect to the prior art, according to some embodiments of the invention;  FIG. 2  is a schematic block diagram illustrating an integrated intake and pretreatment unit for a water treatment plant, according to some embodiments of the invention;  FIGS. 3A-3E  schematically illustrate the operation of the local backwashing apparatus within a filtering system , according to some embodiments of the invention;  FIGS. 4A-4C  schematically illustrate integrated intake and pretreatment units for a water treatment plant with various configurations of the supporting structure for the local backwashing apparatus, according to some embodiments of the invention; and 
           [0010]      FIG. 5  is a schematic flowchart illustrating a method of supplying a water treatment plant with water and backwashing of the filtering system, according to some embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
         [0012]    Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways and is limited only by the appended claims. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0013]    The following illustrates a filtering system and an apparatus for performing local or spot backwashing of a water treatment unit (including also water intake units and water pretreatment units) that filters water using a layer of filter media (e.g. sand filter using sand). The apparatus is moved above the surface of the filter media and is inserted at different parts of it consecutively, to eventually backwash the whole volume of the filter media. 
         [0014]    The apparatus comprises an enclosure (that may be supported on a bridge, a tower, a crane or a floating platform) that is sunk into the filter media and encloses a portion of it. The enclosure is supported on the draining structure at the base of the filter media. Sinking the enclosure is carried out by lowering the air pressure in its upper part. 
         [0015]    Once sunk into the filter media, the lowered air pressure in the upper part of the enclosure is used to initiate suction in the enclosure that expands the enclosed portion of filter media and removes water with sludge from it. The water is supplied into the enclosure from filtered water entering the enclosure from the surrounding filter media outside the enclosure, as the filtration continues globally in the water treatment unit (through filter media outside the enclosure in the filtering system). The enclosure allows a local rise in the water level that does not produce a large head when viewed on the scale of the whole system, and hence does not require massive structural adaptations. Indeed, using local backwashing allows the water level above the filter media to be low, simplifying the construction of the whole system. 
         [0016]    After water with sludge is removed, the enclosed portion of filter media is allowed to settle, and the enclosure is raised out of the filter media by increasing the air pressure in its upper part. Upon this increase, the enclosure floats and is moved to another area of the filter. 
         [0017]    As a result, several benefits are achieved. Firstly, the design of the water treatment plant is simplified by using a single filter system which integrates the intake and pretreatment functions instead of having two separate units. Secondly, the filtering unit has a better design than prior art pretreatment units due to the lower water head that is made possible by the local backwashing apparatus and method. Thirdly, the backwashing process is more efficient, using a much smaller quantity of water, avoiding the use of large external reservoirs of both backwash water and sludge holding wastewater, and allowing the continued operation of the filtering system during the local backwashing. Fourthly, intake of water from the source is made possible without damage to the organisms living in the source, as the filtering system is gentle (no powerful suction but gentle flow through the filter media), and yet in an efficient manner that prevents clogging. 
         [0018]    The following drawings illustrate the system and method graphically. 
         [0019]      FIG. 1B  illustrates schematically the idea behind the proposed local backwashing apparatus  110  and method  200  ( FIG. 5 ) in respect to the prior art ( FIG. 1A ), according to some embodiments of the invention, and  FIG. 2  is a schematic block diagram illustrating an integrated intake and pretreatment unit  100  for a water treatment plant  60 , according to some embodiments of the invention. 
         [0020]    Referring to  FIG. 2 , integrated intake and pretreatment unit  100  comprises a filtering system  99  with a local backwashing apparatus  110  configured to supply water to water treatment plant  60  from a water source  70 . For example, water treatment plant  60  may be a desalination plant and water source  70  may be the sea, or water treatment plant  60  may be an osmotic power production plant, receiving seawater via a seawater integrated intake and pretreatment unit  100  and river water via a river integrated intake and pretreatment unit  100 , with water source  70  being the sea (or the ocean) and a river, respectively. Water treatment plant  60  may be any other type of facility, for example, drinking ware facilities, irrigation facilities or any other system that removes water from a natural body of water. Water treatment plant  60  operates with water received from integrated intake and pretreatment unit  100  without any further need for pretreatment. 
         [0021]    Local backwashing apparatus  110  for filtering system  99  comprises ( FIG. 2 ) an enclosure  119  for carrying out the local backwashing (see  FIGS. 3A-3E  and  5 ), a supporting structure  105  movably supporting enclosure  119 , the supporting structure  105  (see  FIGS. 4A-4C ) configured to position enclosure  119  at a specified spot on the surface, a pneumatic system  140  in fluid communication with an upper part of the enclosure  119  and configured to determine an air pressure in the upper part, a pipe system  133  connecting the upper part of enclosure  119  with pneumatic system  140  and with a discharge structure  132  for receiving wastewater with sludge generated in the local backwashing process, and a control unit  150  connected to supporting structure  105  and pneumatic system  140  and configured to conduct local backwashing of consecutive partial volumes of filter media  92 . 
         [0022]    As illustrated in  FIG. 1B  and in contrast to the prior art ( FIG. 1A ), system  100  and method  200  backwash portions of filter media  92  locally, using much smaller quantities of water than the prior art systems. As a consequence, the high level H of water  91  above filter media  92  is not necessary and can be replaced by a much lower level h of water  91  above filter media  92 , a fact which significantly simplifies the construction of filter  90 . For example, prior art filters are built to allow H=3 meters of water  91  above a 1.5 meter filter media layer  92 , while the proposed filter may operate using less than a h=1 meter water layer. As the water level is reduced, pumps  143  are added to move the filtered water through drainage layer  96  to their destination. 
         [0023]    Using smaller amounts of backwash water also allows using filtered water instead of water from an external source, and most significantly back wash portions of filter media  92  during the actual filtering process, without interruption to the global process. Using smaller amounts of backwash water also does not require building an external reservoir for receiving the backwash water with sludge for treatment. In principal, discharge channel  132  may be sufficient to handle the backwash water, or sludge may be separated from the water flow and treated separately by a sludge treatment unit  55 . Moreover, as backwashing is carried out locally, it does not limit the overall filter size, as global backwashing does. While prior art filters are limited to around 100 m 2 , the proposed filtering system  99  may be built to much larger sizes, for example 500-600 m 2 . 
         [0024]    The actual local backwashing is carried out within a limited portion of filter media  92 , e.g. enclosed in enclosure  119 , that is expanded to release sludge from the filter media  92 . Water with the sludge in is then moved via pipe system  133  to discharge structure  132 . The flow of water with sludge may be initiated pneumatically by pneumatic system  140  over a three way valve  135  that prevents the water flow from reaching an air pump (not shown) of pneumatic system  140 . The pipe system  133  comprises valves  134  for regulating air and water flow therethrough, as explained below. 
         [0025]      FIGS. 3A-3E  schematically illustrate the operation of local backwashing apparatus  110  within a filtering system, according to some embodiments of the invention, 
         [0026]    Local backwashing apparatus  110  for filtering system  99  comprises ( FIG. 2 ) an enclosure  119  having an open lower end and positioned above a surface of filter media of the filtering system, the filter media  92  being supported by a drainage layer. 
         [0027]    Enclosure  119  may have two main positions—an inactive position  117  and an inserted position  118 . In the inactive position  117 , an open lower end  128  of enclosure  119  is plunged in water  91  above filter media  92 , e.g. on supporting structure  105  such as a bridge or a crane. In inserted position  118 , open lower end  128  of enclosure  119  is inserted into filter media  92 , enclosing a portion of filter media  92  for backwashing. In inserted position  118  enclosure  119  is supported upon drainage layer  96  and isolates the portion of filter media  92  from the surrounding filter media  92 . Enclosure  119  is arranged to backwash this portion during the continuing filtration of water  91  in filter  90 , as explained below. Pneumatic system  140  is connected to an upper closed end  127  of enclosure  119  and is arranged to determine a pressure in upper closed end  127 . 
         [0028]    Pneumatic system  140  is connected to upper end  127  and discharge structure  132  via valves  134 ,  135  that regulate air and water flow to and from enclosure  119 . Once water flow is established (priming), it continues due to the hydrostatic pressure difference that results from the difference in water level h between enclosure  119  and discharge structure  132 . The regulation of air and water flow is carried out by controlling valves  134 ,  135  controlled by controlling unit  150 . Pneumatic system  140  is further arranged to interrupt the water flow and increase air pressure in upper end  127 . 
         [0029]    Control system  150  conducts local backwashing of consecutive partial volumes of filter media  92  by the following stages. These stages are also illustrated in  FIG. 5 , being a schematic flowchart illustrating method  200  of supplying water treatment plant  60  with water and backwashing of filtering system  99 , according to some embodiments of the invention. 
         [0030]    First, enclosure  119  is positioned at a specified spot on the surface that corresponds to the partial volume of filter media, i.e. over the spot that is to be backwashed (stage  225 ). The positioning may be carried out by lifting enclosure  119  above the surface and then lowering it into the filter media, by dragging or rolling enclosure  119  on the surface, or by any other positioning method. Then ( FIG. 3A ), enclosure  119  is sunk into filter media  92  (stage  230 ) until enclosure  119  is supported on drainage layer  96  ( FIG. 3B ) by reducing the air pressure in the upper part  127  of enclosure  119  (stage  232 ) through pneumatic system  140  once open lower end  128  of enclosure  119  is immersed in filter media  92  ( 129 E in  FIG. 3E , representing the end of a former local backwashing process), to enclose the partial volume of filter media  92  within enclosure  119  (stage  234 ). At the beginning of the sinking process, lower end  128  is in the filter media and partially filled with water  129 A, at the end of the sinking process, lower end  128  is filled with a local portion of the filtering media  129 B. To initiate sinking stage  230 , it is favorable that edge  109  is within filter media  92 , to prevent excessive water flow that may interfere with the process. 
         [0031]    Local backwashing of the partial volume of filter media  92  enclosed in enclosure  119  is then generated during continued operation of global filtering in the filtering system (through filter media outside the enclosure in the filtering system) (stage  240 ), by generating suction through pneumatic system  140  (stage  242 ) that initiates a water flow from upper part  127  of enclosure  119  through pipe system  133  to discharge structure  132  (stage  250 ). 
         [0032]    The water flow introduces into the enclosed partial volume of filter media filtered water from drainage layer  96  that is filtered by the rest of the filter media, surrounding enclosure  119  (stage  244 ) which expands the enclosed filter media and releases sludge therefrom (stage  246 ) to the water flow that flows to discharge structure  132  (stage  250 ). Water flow is maintained by utilizing the height difference h between the water level in enclosure  119  (that may locally be higher than the water level in filter  90 ) (stage  252 ). An edge  109  ( FIG. 3A ) of open lower end  128  of enclosure  119  may be shaped to prevent filter media flow into enclosed partial volume  129 C ( FIG. 3C ). As edge  109  is supported on an upper layer  96 A of drainage layer  96  (upper layer  96 A of drainage layer  96  supports filter media  92  and allows water move through, a lower layer  96 B collects the water), good contact may be achieved e.g. by widening edge  109 . Furthermore, the form of edge  109  may be designed to transmit forces (weight and contacting impact) from enclosure  119  to upper layer  96 A in a non damaging manner. Local backwashing is carried out by water from drainage layer  96  moving into the enclosed filter media, expanding it and removing sludge from the filter media particles. Hence, filtered water that is filtered during the local backwashing by other parts of the filter media outside the enclosure, is used for backwashing the enclosed filter media, without need for an external water source. 
         [0033]    Expanded filter media  129 C fills most of the volume of enclosure  119 , and is agitated due to the suction, water flow into the enclosure and water flow out of the enclosure  119 . The agitation separates the sludge particles gravitationally from the filter media, as sludge particles float in the water and filter media sinks. The portion of backwashed filter media may be allowed to settle before enclosure  119  is removed, in order to prevent horizontal mixing of filter media  92  which may decrease the efficiency of the local backwashing. 
         [0034]    Finally, enclosure  119  is raised above the surface of filter media  92  (stage  260 ) to release the backwashed enclosed volume of filter media, by injecting air and increasing air pressure in the upper part of enclosure  119  (stage  262 ) through the pneumatic system to float enclosure  119  above filter media  92  (stage  264 ). Settled backwashed filter media  129 D starts filtering water  91  at high efficiency once enclosure  119  is removed. 
         [0035]    The backwashing of partial volumes may be carried out sequentially (stage  272 ) to backwash a whole volume of filter media  92  at a frequency between five times a day and once in three months, depending on the sizes of filter  90  and enclosure  119 , the clogging rate of the filter media, water throughput, technical parameters of operation, etc. 
         [0036]    Method  200  may further include supplying water treatment plant  60  with water by constructing an intake unit as filtering system  99  (stage  210 ) with filter media  92  and applying local backwashing (stage  220 ) to maintain filtering system  99  operative. 
         [0037]      FIGS. 4A-4C  schematically illustrate integrated intake and pretreatment units  100  for water treatment plant  60  with various configurations of supporting structure  105  for local backwashing apparatus  110 , according to some embodiments of the invention. 
         [0038]      FIG. 4A  illustrates a concrete filter  90  as filtering system  99  with a fixed bridge as supporting structure  105  (pipe system connecting enclosure  119  to discharge structure is not shown). 
         [0039]      FIG. 4B  illustrates a constructed filtering system  99  with a floating bridge as supporting structure  105  (pipe system connecting enclosure  119  to discharge structure is not shown). The construction may be carried out e.g. by digging and lining a volume for filter  90  and then bringing filter  90  into contact with water source  70 . 
         [0040]      FIG. 4C  illustrates a constructed filtering system  99  with rotating bridges as supporting structure  105  (pipe system connecting enclosure  119  to discharge structure is not shown). In the illustrated example, enclosures are mounted pairwise on the rotating bridges to achieve a high rate of local backwashing. Multiple enclosures  119  may be used with any configuration of supporting structure  105  and control unit  150  may be adapted to control and manage any number of simultaneously operating enclosures  119 . In a similar manner, enclosure  119  may be supported by a crane as supporting structure  105 . 
         [0041]    In embodiments, filtering system  99  may be elongated and enclosure  119  may span a width of filtering system  99 , e.g. have the width of bridges  105  in  FIGS. 4A  and  4 B. In some embodiments, open lower end  128  of enclosure  119  may be small, e.g. have an area between 0.1 m 2  and 10 m 2  to simplify pipe system  133 , pneumatic system  140 , supporting structure  105  and their control. 
         [0042]    Integrated intake and pretreatment unit  100  prevents damage to the natural fauna in the body of water from where water is taken. As the intake is carried out through the filter media, there are no open pipes or intake screens that damage organisms such as fish, and no open fast flowing water bodies that may remove and kill organisms. 
         [0043]    In the above description, an embodiment is an example or implementation of the invention. The various appearances of “one embodiment”, “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. 
         [0044]    Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. 
         [0045]    Embodiments of the invention may include features from different embodiments disclosed above, and embodiments may incorporate elements from other embodiments disclosed above. The disclosure of elements of the invention in the context of a specific embodiment is not to be taken as limiting their used in the specific embodiment alone. 
         [0046]    Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above. 
         [0047]    The invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described. 
         [0048]    Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined. 
         [0049]    While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention, as limited by the claims.