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 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 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. A biomass support including a plastic biomass support element having a maximum dimension which does not exceed 50 mm and having a specific gravity of between approximately 0.70-0.91, a method of manufacture of a biomass support and a waste water treatment system employing the biomass support are also disclosed.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. patent application Ser. No. 09/866,886, filed on May 29, 2001, entitled “Method and Apparatus For Biological Wastewater Treatment” and from U.S. patent application Ser. No. 10/041,524, filed on Jan. 7, 2002, entitled “Biofilm Carrier, Method of Manufacture Thereof and Waste Water Treatment System Employing Biofilm Carrier”. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to water treatment generally and more particularly to systems and methodologies for biological water treatment and the use of biofilm supports.  
         BACKGROUND OF THE INVENTION  
         [0003]    The following patents and publications are believed to represent the current state of the art:  
           [0004]    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 and 6,036,863.  
           [0005]    French Patent FR 2,707,183.  
           [0006]    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;  
           [0007]    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;  
           [0008]    UPGRADING TO NITROGEN REMOVAL WITH THE KMT MOVING BED BIOFILM PROCESS by Bjorn Rusten, Jon G. Siljudalen and Bjornar Nordeidet, Wat. Sci. Tech. Vol 29, No. 12 pp 185-195, 1994;  
           [0009]    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;  
           [0010]    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.  
           [0011]    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.  
           [0012]    The following U.S. patents are believed to represent the current state of the art in biofilm supports and related technologies.  
           [0013]    U.S. Pat. Nos. 5,980,738; 5,981,272; 5,985,148; 5,993,650; 6,063,268; 6,156,204; 5,948,262; 5,871,674; 5,783,066; 5,783,069; 6,126,829; 5,543,039; 5,458,779; 5,486,292; 4,985,182; 4,333,893; 5,217,616; 4,814,085; 4,814,125; 4,842,920; 5,168,058; 4,385,988; 4,522,767 and 4,537,731.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention seeks to provide improved systems and methodologies for biological water treatment.  
           [0015]    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.  
           [0016]    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.  
           [0017]    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.  
           [0018]    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.  
           [0019]    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.  
           [0020]    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.  
           [0021]    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.  
           [0022]    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.  
           [0023]    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.  
           [0024]    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.  
           [0025]    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.  
           [0026]    Further in accordance with a preferred embodiment of the present invention the upstream partition extends fully from side to side of the basin.  
           [0027]    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.  
           [0028]    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.  
           [0029]    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.  
           [0030]    Further in accordance with a preferred embodiment of the present invention the vertical partitions each extend fully from side to side of the basin.  
           [0031]    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.  
           [0032]    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.  
           [0033]    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.  
           [0034]    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.  
           [0035]    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.  
           [0036]    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.  
           [0037]    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.  
           [0038]    Further in accordance with a preferred embodiment of the present invention the air lift includes an adjustable angle deflector.  
           [0039]    Still further in accordance with a preferred embodiment of the present invention the air lift includes an integral curved downstream partition and deflector.  
           [0040]    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.  
           [0041]    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.  
           [0042]    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.  
           [0043]    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.  
           [0044]    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.  
           [0045]    The present invention also seeks to provide an improved biofilm support as well as an improved waste water treatment system and methodology using the biofilm support.  
           [0046]    There is thus provided, in accordance with a preferred embodiment of the present invention, a biofilm support, including a plastic biofilm support element having a maximum dimension which does not exceed 50 mm and having a specific gravity of between approximately 0.70-0.91.  
           [0047]    There is additionally provided, in accordance with a preferred embodiment of the present invention, a biofilm support, including a plastic biofilm support element having a generally cylindrical configuration and including a plurality of radially extending surfaces extending outwardly from a generally solid center.  
           [0048]    There is further provided, in accordance with a preferred embodiment of the present invention, a biofilm support, including a unitary plastic biofilm support element having a maximum dimension which does not exceed 50 mm and includes a plurality of roughened biofilm adherence surfaces integrally formed as one piece therewith.  
           [0049]    There is still further provided, in accordance with a preferred embodiment of the present invention, a waste water treatment system, including a basin, at least one airlift operating in the basin and a multiplicity of plastic biofilm support elements, having any of the above characteristics, disposed in the basin for cooperation with the airlift.  
           [0050]    There is yet further provided, in accordance with a preferred embodiment of the present invention, a method of manufacturing a plastic biofilm support element including:  
           [0051]    extruding a plastic material mixed with a foaming agent to produce an elongate extruded plastic material having a specific gravity of between approximately 0.70-0.91;  
           [0052]    cooling the elongate extruded plastic material; and  
           [0053]    cutting the elongate extruded plastic material to have a maximum dimension, which does not exceed 50 mm.  
           [0054]    There is additionally provided, in accordance with a preferred embodiment of the present invention, a method of manufacturing a plastic biofilm support element including:  
           [0055]    extruding a plastic material mixed with a foaming agent to produce an elongate extruded plastic material having a generally cylindrical configuration and including a plurality of radially extending surfaces extending outwardly from a generally solid center;  
           [0056]    cooling the elongate extruded plastic material; and  
           [0057]    cutting the elongate extruded plastic material to have a maximum dimension, which does not exceed 50 mm.  
           [0058]    There is yet additionally provided, in accordance with a preferred embodiment of the present invention, a method of manufacturing a plastic biofilm support element including:  
           [0059]    extruding a plastic material mixed with a foaming agent to produce an elongate extruded plastic material having a plurality of roughened biofilm adherence surfaces integrally formed as one piece therewith;  
           [0060]    cooling the elongate extruded plastic material; and  
           [0061]    cutting the elongate extruded plastic material to have a maximum dimension, which does not exceed 50 mm.  
           [0062]    Preferably, the plastic biofilm support element has a generally cylindrical configuration and includes a plurality of radially extending surfaces extending outwardly from a generally solid center.  
           [0063]    In accordance with a preferred embodiment of the present invention, the plastic biofilm support element has a plurality of roughened biofilm adherence surfaces integrally formed as one piece therewith.  
           [0064]    Preferably, the plurality of radially extending ribs includes between 5 and 9 ribs.  
           [0065]    In accordance with a preferred embodiment of the present invention, each of the plurality of ribs has a thickness of between 0.5 and 2 mm.  
           [0066]    Preferably, the plastic biofilm support element includes a strip extending along an outwardly facing edge of each of the radially extending ribs.  
           [0067]    In accordance with a preferred embodiment of the present invention, the plastic biofilm support element is formed of a plastic material selected from the following plastic materials: polyolefin, polystyrene, polyvinyl chloride and polyurethane.  
           [0068]    Preferably, the plastic biofilm support element is formed of a plastic material mixed with a foaming agent.  
           [0069]    In accordance with a preferred embodiment of the present invention, the plurality of ribs and the strips are configured so as to prevent interdigitation between ribs of two separate biofilm support elements.  
           [0070]    Preferably, the support is configured so as to prevent mechanically retained joining of two separate biofilm support elements.  
           [0071]    Preferably, the plastic biofilm support element has a specific gravity of between approximately 0.75-0.89 and more preferably between approximately 0.81-0.87.  
           [0072]    In accordance with a preferred embodiment of the present invention, the roughened biofilm adherence surfaces have a roughness average (Ra) in the range of 100-800 microns and more preferably in the range of 200-500 microns.  
           [0073]    Preferably, the plurality of radially extending surfaces are defined by a plurality of radially extending ribs.  
           [0074]    There is also provided in accordance with a preferred embodiment of the present invention 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 the 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 biomass support elements, supplying waste water to at least one of the plurality of treatment stage regions and allowing the waste water, but generally not the biomass support elements, to flow from 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 biomass support elements.  
           [0075]    There is further 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 the 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 biomass support elements, supplying waste water to at least one of the plurality of treatment stage regions and allowing the waste water, but generally not the biomass support elements, to flow from 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 biomass support elements.  
           [0076]    There is further provided in accordance with yet another preferred embodiment of the present invention a retrofitted waste water treatment apparatus including 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 biomass support elements 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 biomass support elements, to flow from 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 biomass support elements.  
           [0077]    There is also provided in accordance with another preferred embodiment of the present invention a waste water treatment apparatus including 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 biomass support elements 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 biomass support elements, 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 provides aerobic waste water flow therein in operative engagement with the floatable porous biomass support elements.  
           [0078]    Further in accordance with a preferred embodiment of the present invention 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.  
           [0079]    Still further in accordance with a preferred embodiment of the present invention the air lift includes at least one air diffuser disposed underlying a peripheral enclosure which defines a column of water which is lifted by air diffusing upwardly from the at least one air diffuser therethrough.  
           [0080]    Additionally in accordance with a preferred embodiment of the present invention the peripheral enclosure includes a rectangular cylindrical enclosure.  
           [0081]    Moreover in accordance with a preferred embodiment of the present invention 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.  
           [0082]    Typically, the generally vertical partitions divide the basin into between 4 and 12 process stages.  
           [0083]    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.  
           [0084]    Preferably, the lift includes a plurality of air lift assemblies and wherein at least one of the plurality of air lift assemblies include 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.  
           [0085]    Still further in accordance with a preferred embodiment of the present invention the vertical partitions each extend fully from side to side of the basin.  
           [0086]    Additionally in accordance with a preferred embodiment of the present invention the air lift assembly also includes a downstream 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.  
           [0087]    Still farther 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 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 upstream and downstream partitions of the plurality of air lift assemblies.  
           [0088]    Preferably, the first plurality of air diffusers intermediate the air lift assemblies cause water to flow upward between the air lift assemblies.  
           [0089]    Further in accordance with a preferred embodiment of the present invention the second plurality of air diffusers intermediate the upstream and downstream partitions of each air lift assembly allows water to flow downward between the upstream and downstream partitions.  
           [0090]    Typically, the loading includes loading 10-40 percent of the volume of the basin with biomass support elements.  
           [0091]    Further 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.  
           [0092]    Additionally in accordance with a preferred embodiment of the present invention the flow includes passage under stage separation partitions which does not carry floating biomass support elements across the stage separation partition, thereby constraining the biomass support elements of each stage to reside within that stage and preventing migration of biomass support elements across stage partition assemblies.  
           [0093]    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.  
           [0094]    Still further in accordance with a preferred embodiment of the present invention the method further includes installing a de-nitrification unit in at least one of the plurality of treatment stage regions.  
           [0095]    Typically, the de-nitrification unit includes at least one axial pump, which provides lift generally without an air flow, thereby to provide an anoxic de-nitrification process.  
           [0096]    Typically the de-nitrification also includes unit includes at least one agitator which provides lift generally without an air flow, thereby to provide an anoxic de-nitrification process.  
           [0097]    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 rectangular cylindrical air lifts arranged in the plurality of treatment stage regions and separated by the vertical partitions which extend from a bottom location which is spaced from a bottom of the basin by a first vertical separation.  
           [0098]    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 and a plurality of air diffusers which are disposed intermediate the hollow shaft to provide an air lift therethrough, thereby causing water to flow into the hollow shafts and downwardly through the hollow shafts.  
           [0099]    Typically, the step of operating produces fluidization of the biomass support elements.  
           [0100]    Still further in accordance with a preferred embodiment of the present invention the vertical partitions include a first generally vertical partition having respective upstream and downstream surfaces, the first generally vertical partition extending downwardly from a top location above the level of the water in the basin to a bottom location spaced from the bottom of the basin and extending from side to side of the basin, second and third generally vertical partitions disposed adjacent and in spaced relationship with respect to the upstream and downstream surfaces of the first generally vertical partition, the second and third Generally vertical partitions extending from side to side of the basin, and extending upwardly from the bottom of the basin to a top location below the level of water in the basin and upwardly inclined flow director panels disposed on respective upstream and downstream surfaces of the first generally vertical partition and being disposed above and spaced from the second and third generally vertical partitions.  
           [0101]    Additionally in accordance with a preferred embodiment of the present invention the first plurality of air diffusers intermediate adjacent air lift assemblies and intermediate adjacent airlift assembly and stage partition assembly causes water to flow upward between the adjacent air lift assemblies and between adjacent airlift assembly and stage partition assembly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0102]    The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:  
         [0103]    [0103]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;  
         [0104]    [0104]FIG. 2 is a simplified illustration 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;  
         [0105]    [0105]FIG. 3 is a sectional illustration taken along lines III-III in FIG. 2;  
         [0106]    [0106]FIG. 4 is a simplified illustration of the embodiment of FIGS. 2 and 3 showing water flows;  
         [0107]    [0107]FIG. 5 is a sectional illustration taken along lines V-V in FIG. 4, showing water flows;  
         [0108]    [0108]FIG. 6 is a sectional illustration corresponding to FIG. 3 and showing particles located in the embodiment of FIG. 2 in the absence of fluid flow;  
         [0109]    [0109]FIG. 7 is a sectional illustration corresponding to FIG. 6 and showing water flows and fluidization of particles thereby;  
         [0110]    [0110]FIGS. 8A, 8B,  8 C and  8 D are simplified illustrations of four embodiments of a unidirectional rectangular airlift used in the embodiment of FIGS.  2 - 7 ;  
         [0111]    [0111]FIGS. 9A, 9B,  9 C and  9 D are simplified illustrations of four embodiments of a bidirectional rectangular airlift used in the embodiment of FIGS.  2 - 7 ;  
         [0112]    [0112]FIG. 10 is a simplified illustration of a denitrification unit useful in the embodiment of FIGS.  2 - 7 ;  
         [0113]    [0113]FIG. 11 is a simplified illustration of a embodiment of a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with another embodiment of the present invention;  
         [0114]    [0114]FIG. 12 is a sectional illustration taken along lines XII-XII in FIG. 11;  
         [0115]    [0115]FIG. 13 is a sectional illustration corresponding to FIG. 12 and showing water flows;  
         [0116]    [0116]FIG. 14 is a sectional illustration corresponding to FIG. 12 and showing particles located in the embodiment of FIG. 11 in the absence of fluid flow;  
         [0117]    [0117]FIG. 15 is a sectional illustration corresponding to FIG. 14, showing water flows and fluidization of particles thereby;  
         [0118]    [0118]FIG. 16 is a simplified illustration of a denitrification unit useful in the embodiment of FIGS.  11 - 15 ;  
         [0119]    [0119]FIGS. 17A, 17B,  17 C,  17 D and  17 E are simplified illustrations of various deflectors useful in the embodiment of FIGS.  11 - 15 ;  
         [0120]    [0120]FIGS. 18A and 18B are respective simplified pictorial and sectional illustrations of a biofilm support constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0121]    [0121]FIGS. 19A and 19B are respective simplified pictorial and sectional illustrations of a biofilm support constructed and operative in accordance with another preferred embodiment of the present invention;  
         [0122]    [0122]FIG. 20 is a simplified illustration of a methodology for forming a biofilm support in accordance with a preferred embodiment of the present invention;  
         [0123]    [0123]FIGS. 21 and 22 are simplified illustrations of a portion of a waste water treatment system and methodology employing a biofilm support in accordance with a preferred embodiment of the present invention;  
         [0124]    [0124]FIG. 23 is a simplified illustration of a embodiment of a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with another preferred embodiment of the present invention;  
         [0125]    [0125]FIG. 24 is a sectional illustration taken along lines XXIV-XXIV in FIG. 23;  
         [0126]    [0126]FIG. 25 is a simplified illustration of the embodiment of FIGS. 23 and 24 showing water flows;  
         [0127]    [0127]FIG. 26 is a sectional illustration taken along lines XXVI-XXVI in FIG. 25, showing water flows;  
         [0128]    [0128]FIG. 27 is a sectional illustration corresponding to FIG. 24 and showing particles located in the embodiment of FIG. 23 in the absence of fluid flow;  
         [0129]    [0129]FIG. 28 is a sectional illustration corresponding to FIG. 27 and showing water flows and fluidization of particles thereby;  
         [0130]    [0130]FIGS. 29A and 29B are simplified illustrations of two embodiments of a stage partition assembly including a carrier barrier employed in the embodiment of FIGS.  23 - 28 ;  
         [0131]    [0131]FIG. 30 is a graph illustrating preferred parameters of the stage partition assembly of FIGS. 29A and 29B;  
         [0132]    [0132]FIG. 31 is a simplified illustration of a embodiment of a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with another preferred embodiment of the present invention;  
         [0133]    [0133]FIG. 32 is a sectional illustration taken along lines XXXII-XXXII in FIG. 31;  
         [0134]    [0134]FIG. 33 is a simplified illustration of the embodiment of FIGS. 31 and 32 showing water flows;  
         [0135]    [0135]FIG. 34 is a sectional illustration taken along lines XXXIV-XXXIV in FIG. 33, showing water flows;  
         [0136]    [0136]FIG. 35 is a sectional illustration corresponding to FIG. 32 and showing particles located in the embodiment of FIG. 32 in the absence of fluid flow;  
         [0137]    [0137]FIG. 36 is a sectional illustration corresponding to FIG. 35 and showing water flows and fluidization of particles thereby; and  
         [0138]    [0138]FIGS. 37A and 37B are simplified illustrations of two embodiments of a stage partition assembly including a carrier barrier employed in the embodiment of FIGS.  31 - 36 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0139]    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.  
         [0140]    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.  
         [0141]    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.  
         [0142]    Reference is now made to FIGS. 2 and 3, which are simplified illustrations 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. The system of FIGS.  2  and  3  may or may not be a retrofit of an existing system. As shown in FIGS. 2 and 3, it is a particular feature of the present invention that a series of air lifts are fitted into a conventional waste water treatment system including a basin  40  having a waste water inlet  42  and a treated water outlet  44 .  
         [0143]    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 .  
         [0144]    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 .  
         [0145]    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 .  
         [0146]    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 .  
         [0147]    It is noted that in the embodiment of FIGS. 2 and 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 .  
         [0148]    Reference is now made to FIGS. 4 and 5, which are simplified illustrations of the embodiment 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.  
         [0149]    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 .  
         [0150]    Reference is now made to FIG. 6, which is a sectional illustration corresponding to FIG. 3 and showing particles  150  preferably located in the embodiment 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.  
         [0151]    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.  
         [0152]    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.  
         [0153]    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.  
         [0154]    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 .  
         [0155]    Reference is now made to FIGS. 8A, 8B,  8 C and  8 D, which are simplified illustrations of four embodiments of a unidirectional rectangular airlift used in the embodiment of FIGS.  2 - 7 .  
         [0156]    [0156]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 .  
         [0157]    [0157]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 .  
         [0158]    [0158]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 .  
         [0159]    [0159]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.  
         [0160]    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.  
         [0161]    Reference is now made to FIGS. 9A, 9B,  9 C and  9 D, which are simplified illustrations of four embodiments of a bidirectional rectangular airlift used in the embodiment of FIGS.  2 - 7 ;  
         [0162]    [0162]FIG. 9A illustrates a preferred intermediate air lit assembly  54 , including upstream partition  80 , deflector  88  and downstream partition  90  as well as first plurality of air diffusers  128 .  
         [0163]    [0163]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 .  
         [0164]    [0164]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.  
         [0165]    [0165]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.  
         [0166]    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.  
         [0167]    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 de-nitrification process.  
         [0168]    Reference is now made to FIGS. 11 and 12, which are simplified illustrations of a embodiment of a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with another embodiment of the present invention.  
         [0169]    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 .  
         [0170]    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.  
         [0171]    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 .  
         [0172]    It is noted that in the embodiment of FIGS. 11 and 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 .  
         [0173]    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 .  
         [0174]    Reference is now made to FIG. 13, which is a simplified illustration of the embodiment 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 .  
         [0175]    Reference is now made to FIG. 14, which is a sectional illustration corresponding to FIG. 12 and showing particles  850  preferably located in the embodiment 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.  
         [0176]    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.  
         [0177]    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.  
         [0178]    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.  
         [0179]    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 .  
         [0180]    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 .  
         [0181]    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.  
         [0182]    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 de-nitrification process.  
         [0183]    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 .  
         [0184]    [0184]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 .  
         [0185]    Reference is now made to FIGS. 18A and 18B, which are respective simplified pictorial and sectional illustrations of a biofilm support constructed and operative in accordance with a preferred embodiment of the present invention. As seen in FIGS. 18A and 18B, there is provided a biofilm support element  1010  formed of plastic, having a maximum dimension which does not exceed 50 mm and having a specific gravity of between approximately 0.70-0.91.  
         [0186]    Preferably, biofilm support element  1010  has a generally cylindrical configuration and includes a plurality of radially extending surfaces  1012  extending outwardly from a generally solid center  1014 . In accordance with a preferred embodiment of the present invention surfaces  1012  are integrally formed as one piece with the solid center  1014 , preferably by extrusion, and define opposite side surfaces of a plurality of radially extending ribs  1016 , preferably between five and nine in number. In accordance with a preferred embodiment of the present invention, each of ribs  1016  has a thickness of between 0.5 and 2 mm.  
         [0187]    In accordance with a preferred embodiment of the present invention, a transverse strip  1018  is provided along an outwardly facing edge of each rib  1016 . Additional transverse strips may also be provided along each rib. In the embodiment of FIGS. 18A and 18B, the width of each strip is preferably equal to approximately 15-60 percent, and more preferably equal to approximately 20-40 percent, of the overall circumference of the cylindrical biofilm support element  1010 , divided by the number of ribs  1016 .  
         [0188]    It is a particular feature of the present invention that the biofilm support element  1010  and specifically ribs  1016  and strips  1018  are configured so as to prevent retained interdigitation between ribs of two separate biofilm support elements. In the embodiment of FIGS. 18A and 18B, interdigitation can occur, but upon such interdigitation, two separate biofilm support elements readily disengage. Accordingly, the biofilm support element  1010  of FIGS. 18A and 18B is preferably configured so as to prevent mechanically retained joining of two separate biofilm support elements  1010 .  
         [0189]    In accordance with a preferred embodiment of the present invention, biofilm support element  1010  is formed of a plastic material selected from the following plastic materials: polyolefin, polystyrene, polyvinyl chloride and polyurethane. Polypropylene having a melt flow index typically in the range of 0.5-10 is the preferred material.  
         [0190]    In accordance with a preferred embodiment of the present invention, biofilm support element  1010  has a specific gravity of between approximately 0.75-0.89 and most preferably between approximately 0.81-0.87.  
         [0191]    It is a particular feature of the invention that the surfaces  1012  of ribs  1016 , as well as all other exposed surfaces of biofilm support element  1010 , are roughened. Preferably, some or all of the roughened biofilm adherence surfaces have a roughness average (Ra) in the range of 100-800 microns and most preferably in the range of 200-500 microns.  
         [0192]    Reference is now made to FIGS. 19A and 19B, which are respective simplified pictorial and sectional illustrations of a biofilm support constructed and operative in accordance with a preferred embodiment of the present invention. As seen in FIGS. 19A and 19B, there is provided a biofilm support element  1020 , similar to that of FIGS. 18A and 18B, formed of plastic, having a maximum dimension which does not exceed 50 mm and having a specific gravity of between approximately 0.70-0.91.  
         [0193]    Preferably, and similarly to biofilm support element  1010  (FIGS. 18A and 18B), biofilm support element  1020  has a generally cylindrical configuration and includes a plurality of radially extending surfaces  1022  extending outwardly from a generally solid center  1024 . In accordance with a preferred embodiment of the present invention, surfaces  1022  are integrally formed as one piece with the solid center  1024 , preferably by extrusion, and define opposite side surfaces of a plurality of radially extending ribs  1026 , preferably between five and nine in number. In accordance with a preferred embodiment of the present invention, each of ribs  1026  has a thickness of between 0.5 and 2 mm.  
         [0194]    In accordance with a preferred embodiment of the present invention, a transverse strip  1028  is provided along an outwardly facing edge of each rib  1026 . Additional transverse strips may also be provided along each rib. In the embodiment of FIGS. 19A and 19B, the width of each strip is preferably equal to approximately 60-90 percent of the overall circumference of the cylindrical biofilm support element  1020 , divided by the number of ribs  1026 .  
         [0195]    It is a particular feature of the present invention that the biofilm support element  1020  and specifically ribs  1026  and strips  1028  are configured so as to prevent interdigitation between ribs of two separate biofilm support elements. In the embodiment of FIGS. 19A and 19B, interdigitation cannot occur. Accordingly, the biofilm support element  1020  of FIGS. 19A and 19B is preferably configured so as to prevent mechanically retained joining of two separate biofilm support elements  1020 .  
         [0196]    In accordance with a preferred embodiment of the present invention, similarly to biofilm support element  1010  (FIGS. 18A and 18B), biofilm support element  1020  is formed of a plastic material selected from the following plastic materials: polyolefin, polystyrene, polyvinyl chloride and polyurethane. Polypropylene having a melt flow index typically in the range of 0.5-10 is the preferred material.  
         [0197]    In accordance with a preferred embodiment of the present invention, biofilm support element  1020  has a specific gravity of between approximately 0.75-0.89 and most preferably between approximately 0.81-0.87.  
         [0198]    It is a particular feature of the invention that the surfaces  1022  of ribs  1026 , as well as other exposed surfaces of biofilm support element  1020 , are roughened. Preferably, some or all of the roughened biofilm adherence surfaces have a roughness average (Ra) in the range of 100-800 microns and most preferably in the range of 200-500 microns.  
         [0199]    Reference is now made to FIG. 20, which is a simplified illustration of a methodology for forming a biofilm support in accordance with a preferred embodiment of the present invention. As seen in FIG. 20 an extruder  1030 , which may be a conventional extruder, receives a mixture of materials, preferably including a plastic material  1032  selected from the following plastic materials: polyolefin, polystyrene, polyvinyl chloride and polyurethane. Polypropylene having a melt flow index typically in the range of 0.5-10 is the preferred material.  
         [0200]    In accordance with a preferred embodiment of the invention, one or more foaming agents, and preferably the following foaming agents, are supplied to the extruder together with the plastic material:  
         [0201]    an exothermic foaming agent  1034 , preferably azodicarbon amide; and  
         [0202]    an endothermic foaming agent  1036 , preferably sodium bicarbonate or a derivative thereof  
         [0203]    Additionally, in accordance with a preferred embodiment of the present invention, a filler  1038 , preferably limestone or talc, is also added.  
         [0204]    Preferred proportions of the foregoing constituents by weight, for each one unit of plastic by weight, are as follows:  
                                                       exothermic foaming agent 1034   0-2%           endothermic foaming agent 1036   0-3%           filler 1038   0-10%                      
 
         [0205]    Most preferred proportions of the foregoing constituents by weight, for each one unit of polypropylene by weight, are as follows:  
                                                       exothermic foaming agent 1034   0.3-1.5%           endothermic foaming agent 1036     0-2.5%           filler 1038     0-5%                      
 
         [0206]    The foregoing constituents are preferably premixed together prior to being supplied to the extruder  1030  and are preferably supplied in a granulated form.  
         [0207]    The extruder  1030  is preferably operated so as to have a bell shaped temperature profile along a longitudinal axis  1040 , such that the highest temperature in the extruder  1030  is at a location intermediate the flowpath of material therethrough.  
         [0208]    The extruder  1030  is preferably formed with a nozzle  1042 , across which there is provided a pressure drop of at least 1500 psi.  
         [0209]    A roughened extruded elongate profile  1044  exits nozzle  1042  into a cooling bath  1046 . The profile  1044  is drawn by a puller (not shown) and is cut into appropriate lengths by a cutter  1048 .  
         [0210]    Reference is now made to FIGS. 21 and 22, which are simplified illustrations of a waste water treatment system and methodology employing a biofilm support in accordance with a preferred embodiment of the present invention. As seen in FIGS. 21 and 22, biofilm support element  1010  (FIGS. 18A and 18B) or biofilm support element  1020  (FIGS. 19A and 19B) may be advantageously employed in an air-lift type waste water treatment system and methodology. A preferred such system is described in applicants&#39; co-pending U.S. patent application Ser. No. 09/866,886, filed May 29, 2001, entitled “METHOD AND APPARATUS FOR BIOLOGICAL WASTEWATER TREATMENT”, the disclosure of which is hereby incorporated by reference.  
         [0211]    As seen in FIG. 21, an air-lift waste water treatment system and methodology employs a pressurized air supply, typically including nozzles  1050 , located near the floor of a basin  1052 , which are supplied with pressurized air from a compressor (not shown) via pipes  1054 . Waste water  1056  fills part of basin  1052 , and a multiplicity of biofilm supports  1058 , such as biofilm support element  1010  (FIGS. 18A and 18B) or  1020  (FIGS. 19A and 19B) described hereinabove, float at the top of the waste water  1056 , as shown. Preferably, generally cylindrical upstanding air lift enclosures  1060  are provided overlying nozzles  1050 .  
         [0212]    As seen in FIGS. 21 and 22, the air-lift waste water treatment system and methodology employs pressurized air from nozzles  1050  to cause an upward flow of waste water  1056  through air lift enclosures  1060 . This causes biofilm supports  1058  to be inversely fluidized in waste water  1056 , thereby providing enhanced turbulence and mass transfer for efficient waste water treatment.  
         [0213]    Reference is now made to FIGS. 23 and 24, which are simplified illustrations of a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with another preferred embodiment of the present invention, which may or may not be a retrofit. As shown in FIGS. 23 and 24, it is a particular feature of the present invention that a series of air lifts are fitted into a conventional waste water treatment system including a basin  1140  having a waste water inlet  1142  and a treated water outlet  1144 .  
         [0214]    In accordance with a preferred embodiment of the invention, a series of air lift assemblies  1154  is arranged in multiple process stages, typically 4-12 in number. Each process stage includes at least one air lift assembly  1154 . The process stages are separated by stage partition assemblies  1155 , preferred embodiments of which are described hereinbelow with reference to FIGS. 29A and 29B.  
         [0215]    Each air lift assembly  1154  preferably includes an upstream partition  1156  which preferably extends downwardly from a top location below the water level  1162  in basin  1140  to a bottom location spaced from the bottom  1166  of basin  1140  and preferably extends fully from side to side of the basin  1140 . The air lift assembly  1154  preferably also includes a downstream partition  1168 , which preferably also extends fully from side to side of the basin  1140  and extends below the water level  1162  and as close to the bottom  1166  as does partition  1154 . The top of downstream partition  1168  is preferably at the same level as is the top of upstream partition  1154 . Alternatively, some or all of partitions  1156  and  1168  need not extend fully from side to side of the basin  1140 .  
         [0216]    It is noted that in the embodiment of FIGS. 23 and 24 a first plurality of air diffusers  1226  are disposed at the bottom of basin  1140  intermediate the upstream and downstream partitions  1156  and  1168  of each air lift assembly and a second plurality of air diffusers  1228 , typically greater in number than the first plurality of air diffusers are disposed at the bottom of basin  1140  intermediate pairs of adjacent air lift assemblies  1154  and intermediate air lift assemblies  1154  and stage partition assemblies  1155 . All of the air diffusers  1226  and  1228  are coupled by air conduits  1230  to one or more air blowers  1232 .  
         [0217]    Reference is now made to FIGS. 25 and 26, which are simplified illustrations of the embodiment of FIGS. 23 and 24 showing water flows. As seen in FIGS. 25 and 26, the relatively high density of air diffusers intermediate pairs of adjacent air lift assemblies  1154  and intermediate air lift assemblies  1154  and stage partition assemblies  1155  causes water to flow upward between intermediate pairs of adjacent air lift assemblies  1154  and intermediate air lift assemblies  1154  and stage partition assemblies  1155 , as indicated by arrows  1240 . The relatively lower density of air diffusers intermediate the upstream and downstream partitions of each air lift assembly allows water to flow downward as indicated by arrows  1242 .  
         [0218]    Due to the construction of the airlift assemblies  1154 , water flows in both upstream and downstream directions, indicated by respective arrows  1244  and  1246 , at the top of each airlift assembly  1154 .  
         [0219]    Reference is now made to FIG. 27, which is a sectional illustration corresponding to FIG. 24 and showing particles  1250  preferably located in the embodiment of FIG. 23 in the absence of fluid flow. Particles  1250  are preferably floating biomass support elements having a density lower than that of pure water, preferably having a specific gravity between 0.7 and 0.91. Typically, the biomass support elements have a generally cylindrical configuration and include a plurality of radially extending surfaces. Preferred particles  1250  are described hereinabove with reference to FIGS.  18 A- 19 B.  
         [0220]    As seen in FIG. 27, preferably 10-40 percent of the volume of the basin is filled with particles  1250  in the absence of water flow.  
         [0221]    Reference is now made to FIG. 28, which is a sectional illustration corresponding to FIG. 27 and showing water flows and fluidization of particles thereby. It is seen in FIG. 28, that due to the water flows, typified in FIGS. 25 and 26, the volume of the bed of particles  1250  increases substantially, as the bed of particles is fluidized.  
         [0222]    It is noted that in addition to the water flows indicated by arrows  1240 ,  1242 ,  1244  and  1246 , there exists a continuous flow of water from the upstream side of the basin  1140  from the waste water inlet  1142  to the treated water outlet  1144 . This flow is an undulating flow and includes passage under stage partition assemblies  1155 , as indicated by arrows  1260 . The passage under stage partition assemblies  1155  is of relatively low volume and generally does not carry floating particles  1250  across the stage partition assemblies  1155 , thereby constraining the particles  1250  of each stage to reside within that stage and preventing migration of particles across stage partition assemblies  1155 .  
         [0223]    It is appreciated that the provision of first and second pluralities of air diffusers  1226  and  1228  enables control of flow velocity between adjacent air lifts while providing a high level of aeration to the water in basin  1140 . The first plurality of air diffusers  1226  is of principal importance during start up of operation of the system.  
         [0224]    Reference is now made to FIGS. 29A and 29B, which are simplified illustrations of two embodiments of a stage partition assembly including a carrier barrier employed in the embodiment of FIGS.  23 - 28 .  
         [0225]    Turning to FIG. 29A, there is seen a stage partition assembly  1270  comprising an upstanding generally vertical partition  1272 , a top edge  1274  of which extends above the level of water in basin  1140  and a bottom edge  1276  of which is separated from the bottom  1166  of basin  1140 . Disposed adjacent partition  1272  in spaced relationship therewith on both sides thereof are respective upstream and downstream generally vertical partitions  1278  and  1280 , having respective top edges  1282  and  1284  which lie below the level of water in basin  1140  and preferably at a level which is less than half of the height of the water in basin  1140  and respective bottom edges  1286  and  1288  which are preferably sealed to the bottom  1166  of basin  1140 . Preferably the height of each of partitions  1278  and  1280  is approximately one meter and more generally between approximately 0.5 and 1.5 meters.  
         [0226]    Disposed on respective upstream and downstream sides of partition  1272  above and spaced from top edges  1282  and  1284  of respective partitions  1278  and  1280  are inclined flow director assemblies  1290  and  1292 , comprising respective pairs of panels  1294  and  1296  and  1298  and  1300 . Panels  1294  and  1296  preferably are each inclined with respect to partition  1272  and are mutually angled by 90-120 degrees. Similarly, panels  1298  and  1300  preferably are each inclined with respect to partition  1272  and are mutually angled by 90-120 degrees.  
         [0227]    In accordance with a preferred embodiment of the present invention, partition  1272  is spaced from each of partitions  1278  and  1280  by a distance which is selected such that the water flow velocity therethrough is significantly lower than the free rise velocity of the biomass support elements  1250 , in water. Preferably, the flow velocity of water between partition  1272  and partitions  1278  and  1280  is less than one-half of the free rise velocity of the biomass support elements  1250 . Determination of the separation distance of the partitions  1278  and  1280  for a given flow velocity made be readily made from the graph presented in FIG. 30, for different water flow rates.  
         [0228]    The stage partition assembly  1270  preferably is operable to allow water flow therethrough, as indicated by arrows  1302 ,  1304 ,  1306 ,  1308  and  1310 , while generally preventing the passage therethrough of biomass support elements  1250 .  
         [0229]    [0229]FIG. 29B illustrates an alternative embodiment of a stage partition assembly  1320  which is similar to assembly  1270  other than in that panels  1294  and  1298  are eliminated. The operation of assembly  1320  is substantially similar to that of assembly  1270 .  
         [0230]    Reference is now made to FIGS. 31 and 32, which are simplified illustrations of a waste water treatment system of the type of FIG. 1A or FIG. 1B in accordance with a further preferred embodiment of the present invention, which may or may not be a retrofit. The embodiment of FIGS.  31 - 32  is distinguished from that of FIGS. 23 and 24 in that upstream and downstream partitions are eliminated. As shown in FIGS. 31 and 32, it is a particular feature of the present invention that a series of air lifts are fitted into a conventional waste water treatment system including a basin  2140  having a waste water inlet  2142  and a treated water outlet  2144 .  
         [0231]    In accordance with a preferred embodiment of the invention, a series of air lift assemblies  2154  is arranged in multiple process stages, typically 4-12 in number. Each process stage includes at least one air lift assembly  2154 . The process stages are separated by stage partition assemblies  2155 , preferred embodiments of which are described hereinbelow with reference to FIGS. 31 and 32.  
         [0232]    It is noted that in the embodiment of FIGS. 31 and 32 a plurality of air diffusers  2228  are disposed at the bottom of basin  2140  intermediate pairs of adjacent air lift assemblies  2154  and intermediate air lift assemblies  2154  and stage partition assemblies  2155 . All of the air diffusers are coupled by air conduits  2230  to one or more air blowers  2232 .  
         [0233]    Reference is now made to FIGS. 33 and 34, which are simplified illustrations of the embodiment of FIGS. 31 and 32 showing water flows. As seen in FIGS. 33 and 34, the relatively high density of air diffusers  2228  intermediate pairs of adjacent air lift assemblies  2154  and intermediate air lift assemblies  2154  and stage partition assemblies  2155  causes water to flow upward between intermediate pairs of adjacent air lift assemblies  2154  and intermediate air lift assemblies  2154  and stage partition assemblies  2155 , as indicated by arrows  2240 . The relatively lower density of air diffusers intermediate the upstream and downstream partitions of each air lift assembly allows water to flow downward as indicated by arrows  2242 .  
         [0234]    Due to the locations of the airlift assemblies  2154 , water flows in both upstream and downstream directions, indicated by respective arrows  2244  and  2246 , at the top of each airlift assembly  2154 .  
         [0235]    Reference is now made to FIG. 35, which is a sectional illustration corresponding to FIG. 32 and showing particles  2250  preferably located in the embodiment of FIG. 31 in the absence of fluid flow. Particles  2250  are preferably floating biomass support elements having a density lower than that of pure water, preferably having a specific gravity between 0.7 and 0.91. Typically, the biomass support elements have a generally cylindrical configuration and include a plurality of radially extending surfaces. Preferred particles  2250  are described hereinabove with reference to FIGS.  18 A- 19 B.  
         [0236]    As seen in FIG. 35, preferably 10-40 percent of the volume of the basin is filled with particles  2250  in the absence of water flow.  
         [0237]    Reference is now made to FIG. 36, which is a sectional illustration corresponding to FIG. 35 and showing water flows and fluidization of particles thereby. It is seen in FIG. 36, that due to the water flows, typified in FIGS. 33 and 34, the volume of the bed of particles  2250  increases substantially, as the bed of particles is fluidized.  
         [0238]    It is noted that in addition to the water flows indicated by arrows  2240 ,  2242 ,  2244  and  2246 , there exists a continuous flow of water from the upstream side of the basin  2140  from the waste water inlet  2142  to the treated water outlet  2144 . This flow is an undulating flow and includes passage under stage partition assemblies  2155 , as indicated by arrows  2260 . The passage under stage partition assemblies  2155  is of relatively low volume and generally does not carry floating particles  2250  across the stage partition assemblies  2155 , thereby constraining the particles  2250  of each stage to reside within that stage and preventing migration of particles across stage partition assemblies  2155 .  
         [0239]    It is appreciated that the provision of air diffusers  2228  enables control of flow velocity between adjacent air lifts while providing a high level of aeration to the water in basin  2140 .  
         [0240]    Reference is now made to FIGS. 37A and 37B, which are simplified illustrations of two embodiments of a stage partition assembly including a carrier barrier employed in the embodiment of FIGS.  31 - 36 .  
         [0241]    Turning to FIG. 37A, there is seen a stage partition assembly  2270  comprising an upstanding generally vertical partition  2272 , a top edge  2274  of which extends above the level of water in basin  2140  and a bottom edge  2276  of which is separated from the bottom  2166  of basin  2140 . Disposed adjacent partition  2272  in spaced relationship therewith on both sides thereof are respective upstream and downstream generally vertical partitions  2278  and  2280 , having respective top edges  2282  and  2284  which lie below the level of water in basin  2140  and preferably at a level which is less than half of the height of the water in basin  2140  and respective bottom edges  2286  and  2288  which are preferably sealed to the bottom  2166  of basin  2140 . Preferably the height of each of partitions  2278  and  2280  is approximately one meter and more generally between approximately 0.5 and 1.5 meters.  
         [0242]    Disposed on respective upstream and downstream sides of partition  2272  above and spaced from top edges  2282  and  2284  of respective partitions  2278  and  2280  are inclined flow director assemblies  2290  and  2292 , comprising respective pairs of panels  2294  and  2296  and  2298  and  2300 . Panels  2294  and  2296  preferably are each inclined with respect to partition  2272  and are mutually angled by 90-120 degrees. Similarly, panels  2298  and  2300  preferably are each inclined with respect to partition  2272  and are mutually angled by 90-120 degrees.  
         [0243]    In accordance with a preferred embodiment of the present invention, partition  2272  is spaced from each of partitions  2278  and  2280  by a distance which is selected such that the water flow velocity therethrough is significantly lower than the free rise velocity of the biomass support elements  2250 , in water. Preferably, the flow velocity of water between partition  2272  and partitions  2278  and  2280  is less than one-half of the free rise velocity of the biomass support elements  2250 . Determination of the separation distance of the partitions  2278  and  2280  for a given flow velocity made be readily made from the graph presented in FIG. 30, for different water flow rates.  
         [0244]    The stage partition assembly  2270  preferably is operable to allow water flow therethrough, as indicated by arrows  2302 ,  2304 ,  2306 ,  2308  and  2310 , while generally preventing the passage therethrough of biomass support elements  2250 .  
         [0245]    [0245]FIG. 37B illustrates an alternative embodiment of a stage partition assembly  2320  which is similar to assembly  2270  other than in that panels  2294  and  2298  are eliminated. The operation of assembly  2320  is substantially similar to that of assembly  2270 .  
         [0246]    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.