Abstract:
A method and apparatus for retrofitting existing waste water treatment facilities having at least one existing basin including installing generally vertical partitions at spaced locations in the at least one existing basin in order to divide the at least one existing basin into a plurality of treatment stage regions, installing at least one air lift in each of the plurality of treatment stage regions, loading each treatment stage regions with a quantity of floatable porous particles, supplying waste water to at least one of the plurality of treatment stage regions and allowing the waste water, but generally not the particles, to flow from at least one of the plurality of treatment stage regions to at least another of the plurality of treatment stage regions and operating the at least one air lift in each of the plurality of treatment stage regions to provide aerobic waste water flow therein in operative engagement with the floatable porous particles.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    The following patents and publications are believed to represent the current state of the art:  
         [0002]    U.S. Pat. Nos. 3,133,017; 4,045,344; 4,137,171; 4,231,863; 4,256,573; 4,374,730; 4,394,268; 4,521,311; 4,454,038; 4,521,311; 4,566,971; 4,599,174; 4,810,377; 4,820,415; 4,839,053; 5,030,353; 5,200,081; 5,202,027; 5,554,289; 5,698,094; 6,036,863.  
         [0003]    French Patent FR 2,707,183. 
     
    
     
       FIELD OF THE INVENTION  
         [0004]    The present invention relates to water treatment generally and more particularly to systems and methodologies for biological water treatment.  
           [0005]    A NEW PROCESS FOR ENRICHING NITRIFIERS IN ACTIVATED SLUDGE THROUGH SEPARATE HETEROTROPHIC WASTING FROM BIOFILM CARRIERS by Denny S. Parker, Bjorn Rusten, Asgeir Wien and Jon G. Siljudalen, Brown and Caldwell, P.O. Box 8045 Walnut Creek, Calif. 94596-1220, WEFTEC 2000, Copyright 2000 Water Environment Federation;  
           [0006]    PILOT STUDY TO FULL SCALE TREATMENT-THE MOVING BED BIOFILM REACTOR EXPERIENCE AT THE PHILLIPS 66 BORGER REFINERY by Chandler H. Johnson and Michael W. Page, WEFTEC 2000, Copyright 2000 Water Environment Federation;  
           [0007]    UPGRADING TO NITROGEN REMOVAL WITH THE KMT MOVING BED BIOFILM PROCESS by Bjorn Rusten, Jon G. Siljudalen and Bjomar Nordeidet, Wat. Sci. Tech. Vol 29, No. 12, pp 185-195, 1994;  
           [0008]    THE TWO STAGE MOVING BED/ACTIVATED SLUDGE PROCESS, AN EFFECTIVE SOLUTION FOR HIGH STRENGTH WASTES by Narinder Sunner, Chris Evans, Graig Siviter and Tom Bower, Water and Environmental Management, Volume 13, Number 5, October, 1999;  
           [0009]    UPGRADING WASTEWATER TREATMENT PLANTS BY THE USE OF BIOFILM CARRIERS, OXYGEN ADDITION AND PRE-TREATMENT IN THE SEWER NETWORK by Anette Aesoy, Hallvard Odegaard, Marius Haegh, Frode Risla and Greta Bentzen, Water Science &amp; Technology, Vol 37, Number 9, 1998.  
           [0010]    APPLICATION OF INVERSE FLUIDIZATION IN WASTEWATER TREATMENT: FROM LABORATORY TO FULL-SCALE BIOREACTORS, by D. G. Karamanev and L. N. Nikolov, Environmental Progress, Vol. 15, No. 3, pp 194-196, Fall 1996.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention seeks to provide improved systems and methodologies for biological water treatment.  
           [0012]    There is thus provided in accordance with a preferred embodiment of the present invention a method for retrofitting existing waste water treatment facilities having at least one existing basin. The method includes installing generally vertical partitions at spaced locations in at least one existing basin in order to divide the existing basin into a plurality of treatment stage regions, installing at least one air lift in each of the plurality of treatment stage regions, loading each treatment stage regions with a quantity of floatable porous particles, supplying waste water to at least one of the plurality of treatment stage regions and allowing the waste water, but generally not the particles, to flow from at least one of the plurality of treatment stage regions to at least another of the plurality of treatment stage regions and operating the air lift in each of the plurality of treatment stage regions to provide aerobic waste water flow therein in operative engagement with the floatable porous particles.  
           [0013]    There is also provided in accordance with a preferred embodiment of the present invention a method for waste water treatment employing at least one basin. The method includes installing generally vertical partitions at spaced locations in at least one basin in order to divide the basin into a plurality of treatment stage regions, installing at least one air lift in each of the plurality of treatment stage regions, loading each treatment stage regions with a quantity of floatable porous particles, supplying waste water to at least one of the plurality of treatment stage regions and allowing the waste water, but generally not the particles, to flow from at least one of the plurality of treatment stage regions to at least another of the plurality of treatment stage regions and operating the air lift in each of the plurality of treatment stage regions to provide aerobic waste water flow therein in operative engagement with the floatable porous particles.  
           [0014]    There is further provided in accordance with another preferred embodiment of the present invention a retrofitted waste water treatment apparatus. The apparatus includes at least one existing basin, generally vertical partitions located at spaced locations in the existing basin in order to divide the existing basin into a plurality of treatment stage regions, at least one air lift located in each of the plurality of treatment stage regions and a quantity of floatable porous particles loaded into each of the plurality of treatment stage regions, whereby supplying waste water to at least one of the plurality of treatment stage regions and allowing the waste water, but generally not the particles, to flow from at least one of the plurality of treatment stage regions to at least another of the plurality of treatment stage regions and operating the air lift in each of the plurality of treatment stage regions provides aerobic waste water flow therein in operative engagement with the floatable porous particles.  
           [0015]    There is further provided in accordance with yet another preferred embodiment of the present invention a waste water treatment apparatus. The apparatus includes at least one basin, generally vertical partitions located at spaced locations in the basin in order to divide the basin into a plurality of treatment stage regions, at least one air lift located in each of the plurality of treatment stage regions and a quantity of floatable porous particles loaded into each of the plurality of treatment stage regions, whereby supplying waste water to at least one of the plurality of treatment stage regions and allowing the waste water, but generally not the particles, to flow from at least one of the plurality of treatment stage regions to at least another of the plurality of treatment stage regions and operating the air lift in each of the plurality of treatment stage regions provides aerobic waste water flow therein in operative engagement with the floatable porous particles.  
           [0016]    Further in accordance with a preferred embodiment of the present invention at least some of the vertical partitions are spaced from a bottom of the basin in order to allow the waste water to flow thereunder between adjacent ones of the plurality of treatment stage regions.  
           [0017]    Still further in accordance with a preferred embodiment of the present invention the air lift includes the air diffuser disposed underlying a peripheral enclosure which defines a column of water and is lifted by air diffusing upwardly from the air diffuser therethrough.  
           [0018]    Additionally in accordance with a preferred embodiment of the present invention the peripheral enclosure includes a cylindrical enclosure. Alternatively, the peripheral enclosure includes a plurality of spaced generally vertical walls which extend between walls of the basin and are separated from the bottom of the basin.  
           [0019]    Further in accordance with a preferred embodiment of the present invention the floatable particles include porous plastic particles having a density lower than that of pure water. Preferably, the particles have a specific gravity between 0.65 and 0.95 and have an irregular shape, whose largest dimension is generally between 4-10 mm.  
           [0020]    Additionally in accordance with a preferred embodiment of the present invention, the particles have a total porosity exceeding 50 % and have a mean pore diameter of pores, whose diameter exceeds 10 microns, of about 20 microns.  
           [0021]    Further in accordance with a preferred embodiment of the present invention the generally vertical partitions divide the basin into between 4 and 12 process stages.  
           [0022]    Still further in accordance with a preferred embodiment of the present invention the air lift includes a series of air lifts arranged in the multiple process stages. Preferably, the series of air lifts includes at each process stage an initial air lift assembly and at least one intermediate air lift assembly. The initial air lift assembly typically includes a upstream partition which extends downwardly from a top location above a water level in the basin to a bottom location spaced from the bottom of the basin.  
           [0023]    Further in accordance with a preferred embodiment of the present invention the upstream partition extends fully from side to side of the basin.  
           [0024]    Additionally or alternatively the upstream partition is attached to a deflector which extends in a downstream direction from the upstream partition at the water level.  
           [0025]    Still further in accordance with a preferred embodiment of the present invention the initial air lift assembly also includes a downstream partition which extends fully from side to side of the basin but does not extend up to the water level.  
           [0026]    Moreover in accordance with a preferred embodiment of the present invention the intermediate air lift assembly includes an upstream partition which extends downwardly from a top location below the water level in basin to a bottom location spaced from the bottom of the basin.  
           [0027]    Further in accordance with a preferred embodiment of the present invention the vertical partitions each extend fully from side to side of the basin.  
           [0028]    Additionally in accordance with a preferred embodiment of the present invention the intermediate air lift assembly includes an upstream partition separated from a deflector plate which extends in a downstream direction from the upstream partition at the water level. Preferably, the intermediate air lift assembly also includes a downstream partition which does not extend up to the water level or as close to the bottom of the basin as does the upstream partition.  
           [0029]    Still further in accordance with a preferred embodiment of the present invention the step of installing also includes installing a final air lift assembly including an upstream partition which extends downwardly from a top location below the water level in the basin to a bottom location spaced from the bottom of the basin and extends fully from side to side of the basin. Preferably, the final air lift assembly also includes a downstream partition which also extends fully from side to side of the basin and extends to a top location above the water level and closer to the bottom than does the upstream partition. Additionally or alternatively, the downstream partition is attached to a deflector plate which extends in an upstream direction from downstream partition at a location at the water level.  
           [0030]    Further in accordance with a preferred embodiment of the present invention the air lift includes a plurality of air lift assemblies each including upstream and downstream partitions: a first plurality of air diffusers are disposed at the bottom of the basin intermediate upstream and downstream partitions of the plurality of air lift assemblies and a second plurality of air diffusers, lesser in number than the first plurality of air diffusers, are disposed at the bottom of the basin intermediate the plurality of air lift assemblies.  
           [0031]    Preferably, the first plurality of air diffusers intermediate the upstream and downstream partitions of each air lift assembly causes water to flow upward between the upstream and downstream partitions of each air lift assembly. Additionally, the second plurality of air diffusers intermediate the plurality of air lift assemblies allows water to flow downward.  
           [0032]    Still further in accordance with a preferred embodiment of the present invention the step of loading includes loading 10-40 percent of the volume of the basin with particles in absence of water flow.  
           [0033]    Additionally in accordance with a preferred embodiment of the present invention the step of supplying includes providing a continuous flow of water from the upstream side of the basin from the waste water inlet to the treated water outlet. Typically, the flow is an undulating flow and includes passage under upstream partitions which is of relatively low volume and generally does not carry floating particles into the air lift, thereby constraining the particles to reside outside of and between the air lift.  
           [0034]    Further in accordance with a preferred embodiment of the present invention, the method also includes controlling the flow velocity of water by controlling operation of the first and second pluralities of air diffusers.  
           [0035]    Further in accordance with a preferred embodiment of the present invention the air lift includes an adjustable angle deflector.  
           [0036]    Still further in accordance with a preferred embodiment of the present invention the air lift includes an integral curved downstream partition and deflector.  
           [0037]    Further in accordance with a preferred embodiment of the present invention the method also includes installing a denitrification unit in at least one of the plurality of treatment stage regions. Preferably, the denitrification unit includes a plurality of axial pumps which provide lift generally without an air flow, thereby to provide an anoxic de-nitrification process.  
           [0038]    Further in accordance with a preferred embodiment of the present invention the air lift includes an array of air lifts and wherein the array of air lifts includes a multiplicity of cylindrical air lifts arranged in the plurality of treatment stage regions and separated by the vertical partitions which extend from a bottom location and is spaced from a bottom of the basin by a first vertical separation.  
           [0039]    Preferably, the cylindrical air lifts each include: a hollow shaft which extends from a bottom location spaced from a bottom of the basin by a second vertical separation which exceeds the first separation, a deflector which is disposed in spaced relationship over each hollow shaft and is disposed at the water level and at least one air diffuser which is disposed underlying each hollow shaft to provide an air lift therethrough, thereby causing water to flow into the hollow shafts and upwardly through the hollow shafts, the deflectors causing the water exiting the tops of the hollow shafts to move sideways and downwardly.  
           [0040]    Additionally in accordance with a preferred embodiment of the present invention the cylindrical air lifts also includes a plurality of air diffusers disposed immediately upstream of each the vertical partition for providing control of particle movement and prevention of particle migration.  
           [0041]    Further in accordance with a preferred embodiment of the present invention the step of operating produces fluidization of the particles. Preferably, the operating step is operative, when the particles become heavily coated with biomass to cause the particles sometimes to enter the air lift and to be sloughed of some of the biomass as they are propelled upwards by the action of the air lift.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0042]    The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:  
         [0043]    [0043]FIGS. 1A and 1B are simplified illustrations of two types of prior art waste water treatment systems, which respectively employ surface aerators and diffused air aeration;  
         [0044]    [0044]FIG. 2 is a simplified illustration of a retrofit of a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with a preferred embodiment of the present invention;  
         [0045]    [0045]FIG. 3 is a sectional illustration taken along lines III-III in FIG. 2;  
         [0046]    [0046]FIG. 4 is a simplified illustration of the retrofit of FIGS. 2 &amp; 3 showing water flows;  
         [0047]    [0047]FIG. 5 is a sectional illustration taken along lines V-V in FIG. 4, showing water flows;  
         [0048]    [0048]FIG. 6 is a sectional illustration corresponding to FIG. 3 and showing particles located in the retrofit of FIG. 2 in the absence of fluid flow;  
         [0049]    [0049]FIG. 7 is a sectional illustration corresponding to FIG. 6 and showing water flows and fluidization of particles thereby;  
         [0050]    [0050]FIGS. 8A, 8B,  8 C &amp;  8 D are simplified illustrations of four embodiments of a unidirectional rectangular airlift used in the embodiment of FIGS.  2 - 7 ;  
         [0051]    [0051]FIGS. 9A, 9B,  9 C &amp;  9 D are simplified illustrations of four embodiments of a bidirectional rectangular airlift used in the embodiment of FIGS.  2 - 7 ;  
         [0052]    [0052]FIG. 10 is a simplified illustration of a denitrification unit useful in the embodiment of FIGS.  2 - 7 ;  
         [0053]    [0053]FIG. 11 is a simplified illustration of a retrofit of a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with another embodiment of the present invention;  
         [0054]    [0054]FIG. 12 is a sectional illustration taken along lines XII-XII in FIG. 11;  
         [0055]    [0055]FIG. 13 is a sectional illustration corresponding to FIG. 12 and showing water flows;  
         [0056]    [0056]FIG. 14 is a sectional illustration corresponding to FIG. 12 and showing particles located in the retrofit of FIG. 11 in the absence of fluid flow;  
         [0057]    [0057]FIG. 15 is a sectional illustration corresponding to FIG. 14, showing water flows and fluidization of particles thereby;  
         [0058]    [0058]FIG. 16 is a simplified illustration of a denitrification unit useful in the embodiment of FIGS.  11 - 15 ; and  
         [0059]    [0059]FIGS. 17A, 17B,  17 C,  17 D and  17 E are simplified illustrations of various deflectors useful in the embodiment of FIGS.  11 - 15 .  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0060]    Reference is now made to FIGS. 1A and 1B, which are simplified illustrations of two types of prior art waste water treatment systems, which respectively employ surface aerators and diffused air aeration.  
         [0061]    As seen in FIG. 1A, one conventional type of prior art waste water treatment system comprises a basin  10  having a waste water inlet  12  and a treated water outlet  14 . A plurality of surface aerators  16  are disposed at the water level of water in basin  10  and are operative to aerate the water therein, thus promoting biological activity and biological decomposition of organic material therein.  
         [0062]    Another conventional type of prior art waste water treatment system is shown in FIG. 1B and comprises a basin  20  which may be identical to basin  10  (FIG. 1), having a waste water inlet  22  and a treated water outlet  24 . A plurality of air diffusers  26  are disposed at the bottom of basin  20  and are coupled by air conduits  28  to an air blower  30 . Operation of blower  30  causes air to bubble upwardly through waste water in basin  20 , thus promoting biological activity and biological decomposition of organic material therein.  
         [0063]    Reference is now made to FIGS. 2 &amp; 3, which are simplified illustrations of a retrofit to a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with a preferred embodiment of the present invention. As shown in FIGS. 2 and 3, it is a particular feature of the present invention that a series of air lifts are retrofitted into a conventional waste water treatment system including a basin  40  having a waste water inlet  42  and a treated water outlet  44 .  
         [0064]    In accordance with a preferred embodiment of the invention, a series of air lifts  50  is arranged in multiple process stages, typically 4-12 in number. Each process stage includes an initial air lift assembly, here designated by reference numeral  52  and at least one intermediate air lift assembly, here designated by reference numeral  54 . A final process stage preferably includes a final air lift assembly, here designated by reference numeral  56 .  
         [0065]    Initial air lift assembly  52  preferably includes a upstream partition  60  which preferably extends downwardly from a top location above the water level  62  in basin  40  to a bottom location spaced from the bottom  66  of basin  40  and preferably extends fully from side to side of the basin  40 . In the initial air lift assembly  52 , the upstream partition is attached to a deflector plate  68  which extends in a downstream direction from upstream partition  60  at a location preferably generally at the water level  62 . The initial air lift assembly  52  preferably also includes a downstream partition  70  which also extends fully from side to side of the basin  40  but does not extend up to the water level  62  or as close to the bottom  66  as does partition  60 .  
         [0066]    Each intermediate air lift assembly  54  preferably includes an upstream partition  80  which preferably extends downwardly from a top location below the water level  62  in basin  40  to a bottom location spaced from the bottom  66  of basin  40  and preferably extends fully from side to side of the basin  40 . In the intermediate air lift assembly  54 , the upstream partition  80  is separated from a deflector plate  88  which extends in a downstream direction from upstream partition  80  at a location preferably generally at the water level  62 . The intermediate air lift assembly  54  preferably also includes a downstream partition  90  which also extends fully from side to side of the basin  40  but does not extend up to the water level  62  or as close to the bottom  66  as does partition  80 . The top of downstream partition  90  is preferably at the same level as is the top of upstream partition  80 .  
         [0067]    Final air lift assembly  56  preferably includes an upstream partition  100  which preferably extends downwardly from a top location below the water level  62  in basin  40  to a bottom location spaced from the bottom  66  of basin  40  and preferably extends fully from side to side of the basin  40 . The final air lift assembly  56  preferably also includes a downstream partition  110  which also extends fully from side to side of the basin  40  and extends to a top location above the water level  62  and closer to the bottom  66  than does partition  110 . In the final air lift assembly  56 , the downstream partition  110  is attached to a deflector plate  118  which extends in an upstream direction from downstream partition  110  at a location preferably generally at the water level  62 .  
         [0068]    It is noted that in the retrofit of FIGS. 2 &amp; 3 a first plurality of air diffusers  126  are disposed at the bottom of basin  40  intermediate the upstream and downstream partitions of each air lift assembly and a second plurality of air diffusers  128 , typically lesser in number than the first plurality of air diffusers are disposed at the bottom of basin  40  intermediate adjacent air lift assemblies. All of the air diffusers are coupled by air conduits  130  to one or more air blowers  132 .  
         [0069]    Reference is now made to FIGS. 4 and 5, which are simplified illustrations of the retrofit of FIGS. 2 and 3 showing water flows. As seen in FIGS. 4 and 5, the relatively high density of air diffusers intermediate the upstream and downstream partitions of each air lift assembly causes water to flow upward between the upstream and downstream partitions of each air lift assembly, as indicated by arrows  140 . The relatively lower density of air diffusers intermediate adjacent air lift assemblies allows water to flow downward.  
         [0070]    Due to the construction of the initial airlift assemblies  52 , water flows only in a downstream direction at the top of each initial airlift assembly  52 , as indicated by arrows  142 . Due to the different construction of the intermediate airlift assemblies  54 , water flows in both upstream and downstream directions, indicated by respective arrows  144  and  146 , at the top of each intermediate airlift assembly  54 . Due to the construction of the final airlift assembly  56 , water flows only in an upstream direction, indicated by arrows  148 , at the top the final airlift assembly  56 .  
         [0071]    Reference is now made to FIG. 6, which is a sectional illustration corresponding to FIG. 3 and showing particles  150  preferably located in the retrofit of FIG. 2 in the absence of fluid flow. Particles  150  are preferably floating porous plastic particles having a density lower than that of pure water, preferably having a specific gravity between 0.65 and 0.95. Typically, the particles have an irregular shape, whose largest dimension is approximately 4-10 mm and preferably about 6 mm. Preferably the particles have a total porosity exceeding 50 % and a preferred mean pore diameter of pores, whose diameter exceeds 10 microns, of about 20 microns.  
         [0072]    As seen in FIG. 6, preferably 10-40 percent of the volume of the basin is filled with particles  150  in the absence of water flow.  
         [0073]    Reference is now made to FIG. 7, which is a sectional illustration corresponding to FIG. 6 and showing water flows and fluidization of particles thereby. It is seen in FIG. 7, that due to the water flows, typified in FIGS. 4 and 5, the volume of the bed of particles  150  increases substantially, as the bed of particles is fluidized. The particles  150  are generally constrained to reside outside of the air lift assemblies, inasmuch as they generally do not pass underneath upstream partitions  60 . When particles  150  become heavily coated with biomass, they do sometimes pass under downstream partitions  70  or  90  or upstream partition  100  and are sloughed of some of the biomass as they are propelled upwards by the action of the air lift.  
         [0074]    It is noted that in addition to the water flows indicated by arrows  142 ,  144 ,  146  and  148 , there exists a continuous flow of water from the upstream side of the basin  40  from the waste water inlet  42  to the treated water outlet  44 . This flow is an undulating flow and includes passage under upstream partitions  60 ,  80  and  100 , as indicated by arrows  160 . The passage under upstream partitions  60 ,  80  and  100  is of relatively low volume and generally does not carry floating particles  150  into the air lifts, thereby constraining the particles  150  to reside outside of and between the air lift assemblies and preventing migration of particles across air lift assemblies.  
         [0075]    It is appreciated that the provision of first and second pluralities of air diffusers  126  and  128  enables control of flow velocity between adjacent air lifts while providing a high level of aeration to the water in basin  40 .  
         [0076]    Reference is now made to FIGS. 8A, 8B,  8 C &amp;  8 D, which are simplified illustrations of four embodiments of a unidirectional rectangular airlift used in the embodiment of FIGS.  2 - 7 .  
         [0077]    [0077]FIG. 8A illustrates a preferred initial air lift assembly  52 , including upstream partition  60 , deflector  68  and downstream partition  70  as well as first plurality of air diffusers  128 .  
         [0078]    [0078]FIG. 8B illustrates a preferred final air lift assembly  56  including upstream partition  100 , downstream partition  110  and deflector  118 , as well as first plurality of air diffusers  128 .  
         [0079]    [0079]FIG. 8C illustrates an alternative initial air lift assembly  252 , including upstream partition  260 , an adjustable angle deflector  268  and a downstream partition  270  as well as first plurality of air diffusers  328 .  
         [0080]    [0080]FIG. 8D illustrates an alternative final air lift assembly  356  including an integral curved downstream partition and deflector  358  and an upstream portion  360 , as well as a first plurality of air diffusers  368 . The curved design of the integral downstream partition and deflector reduces energy losses.  
         [0081]    It is appreciated that the adjustable configuration of FIG. 8C may be employed additionally or alternatively for a final air lift assembly and the integral configuration of FIG. 8D may be employed additionally or alternatively for an initial air lift assembly.  
         [0082]    Reference is now made to FIGS. 9A, 9B,  9 C &amp;  9 D, which are simplified illustrations of four embodiments of a bidirectional rectangular airlift used in the embodiment of FIGS.  2 - 7 ;  
         [0083]    [0083]FIG. 9A illustrates a preferred intermediate air lift assembly  54 , including upstream partition  80 , deflector  88  and downstream partition  90  as well as first plurality of air diffusers  128 .  
         [0084]    [0084]FIG. 9B illustrates an alternative intermediate air lift assembly  456  including upstream partition  480 , fixed angle deflector  482  and downstream portion  490 , as well as a first plurality of air diffusers  498 .  
         [0085]    [0085]FIG. 9C illustrates a further alternative intermediate air lift assembly  556 , including upstream partition  560 , a two-way adjustable angle deflector  568  and a downstream partition  570  as well as first plurality of air diffusers  578 . FIG. 9C shows the two-way adjustable angle deflector  568  in a flat orientation.  
         [0086]    [0086]FIG. 9D illustrates the intermediate air lift assembly  556  of FIG. 9C in an alternative operative orientation wherein two-way adjustable angle deflector  568  is arranged to have an angled orientation, such as that shown in FIG. 9B.  
         [0087]    Reference is now made to FIG. 10, which is a simplified illustration of a denitrification unit useful in the embodiment of FIGS.  2 - 7 . De-nitrification units such as those shown in FIG. 10 may be installed instead of all of the intermediate air lifts  54  in a given process stage.  
         [0088]    As seen in FIG. 10, a plurality of axial pumps  600  may provide lift without an air flow, as in the air lifts of FIGS.  1 - 9 , thereby to provide an anoxic denitrification process.  
         [0089]    Reference is now made to FIGS. 11 and 12, which are simplified illustrations of a retrofit of a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with another embodiment of the present invention.  
         [0090]    As shown in FIGS. 11 and 12, it is a particular feature of the present invention that an array of air lifts are retrofitted into a conventional waste water treatment system including a basin  740  having a waste water inlet  742  and a treated water outlet  744 .  
         [0091]    In accordance with a preferred embodiment of the invention, an array of cylindrical air lifts  750  is arranged in multiple process stages, typically 4-12 in number, which are separated from each other typically by partitions  752 , which extend from a bottom location  754  spaced from the bottom  756  of basin  740  by a first vertical separation and extend upwardly to a top location  758  above the water level  760  in basin  740 . Partitions  752  preferably extend fully from side to side of the basin  740 . Each cylindrical air lift  750  typically comprises a hollow shaft  762  which extends from a bottom location  764  spaced from bottom  756  by a second vertical separation which exceeds the first separation.  
         [0092]    A deflector  768  is preferably disposed in spaced relationship over each hollow shaft  762  and is disposed at a location preferably at the water level  760 .  
         [0093]    It is noted that in the retrofit of FIGS. 11 &amp; 12 an air diffuser  770  is preferably disposed underlying each hollow shaft  762  to provide an air lift therethrough. All of the air diffusers  770  are coupled by air conduits  772  to one or more air blowers  774 .  
         [0094]    Immediately upstream of each partition  752  there is provided a series of air diffusers  776 , which are preferably coupled by air conduits  778  to one or more air blowers  774 .  
         [0095]    Reference is now made to FIG. 13, which is a simplified illustration of the retrofit of FIGS. 11 and 12 showing water flows. As seen in FIG. 13, the air diffusers  770  underlying the hollow shafts  762  cause water to flow into the hollow shafts  762 , as indicated by arrows  780  and upwardly through the hollow shafts, as indicated by arrows  782 . The presence of deflectors  768  overlying each hollow shaft  762  causes the water exiting the tops of hollow shafts  762  to move sideways and downwardly, as indicated by arrows  784 . The absence or lower density of air diffusers outside of shafts  762  allows water to flow downwardly, as indicated by arrows  786 .  
         [0096]    Reference is now made to FIG. 14, which is a sectional illustration corresponding to FIG. 12 and showing particles  850  preferably located in the retrofit of FIG. 11 in the absence of fluid flow. Particles  850  are preferably floating porous plastic particles having a density lower than that of pure water, preferably having a specific gravity between 0.65 and 0.95. Typically, the particles have an irregular shape, whose largest dimension is approximately 4-10 mm and preferably about 6 mm. Preferably the particles have a total porosity exceeding 50 % and a preferred mean pore diameter of pores, whose diameter exceeds 10 microns, of about 20 microns.  
         [0097]    As seen in FIG. 14, preferably 10-40 percent of the volume of the basin is filled with particles  850  in the absence of water flow.  
         [0098]    Reference is now made to FIG. 15, which is a sectional illustration corresponding to FIG. 14 and showing water flows and fluidization of particles thereby. It is seen in FIG. 15, that due to the water flows, typified in FIG. 13, the volume of the bed of particles  850  increases substantially, as the bed of particles is fluidized. The particles  850  are generally constrained to reside outside of the hollow shafts  762 , inasmuch as they generally do not reside as low in the basin  740  as the openings of shafts  762  at bottom locations  764  thereof.  
         [0099]    When particles  850  become heavily coated with biomass, they do sometimes enter hollow shafts  762  and are sloughed of some of the biomass as they are propelled upwards by the action of the air lift provided thereby.  
         [0100]    It is noted that in addition to the water flows indicated by arrows  780 ,  782 ,  784  and  786 , there exists a continuous flow of water from the upstream side of the basin  740  from the waste water inlet  742  to the treated water outlet  744 . This flow is a partially undulating flow and includes passage under partitions  752 , as indicated by arrows  860 . The passage under partitions  752  is of relatively low volume and generally does not carry floating particles  850  into the air lifts, thereby constraining the particles  850  to reside outside of and between the air lifts and preventing migration of particles across partitions  752 .  
         [0101]    It is appreciated that control of particle movement and prevention of particle migration may be assisted by ancillary air diffusers  870 , disposed upstream of partitions  752 . These air diffusers are connected via valves  872  and air conduits  772  to one or more air blowers  774 .  
         [0102]    Reference is now made to FIG. 16, which is a simplified illustration of a denitrification unit useful in the embodiment of FIGS.  11 - 15 . De-nitrification units such as those shown in FIG. 16 may be installed instead of all of the air lifts  750  in a given process stage.  
         [0103]    As seen in FIG. 16, a plurality of axial pumps  900  may provide lift without an air flow, as in the air lifts of FIGS.  11 - 15 , thereby to provide an anoxic denitrification process.  
         [0104]    Reference is now made to FIGS. 17A, 17B,  17 C,  17 D and  17 E, which are simplified illustrations of examples of various embodiments of deflectors  768 , useful in the embodiment of FIGS.  11 - 15 .  
         [0105]    [0105]FIG. 17A shows a flat deflector  910 , while FIG. 17B shows a curved deflector  912 . FIG. 17 shows a conical deflector  914 , while FIG. 17D shows a finned conical deflector  916 , having fins  918 . FIG. 17E shows a pyramidal deflector  920 .  
         [0106]    It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove as well as variations and modifications which would occur to persons skilled in the art upon reading the specification and which are not in the prior art.