Patent Publication Number: US-2009218299-A1

Title: Inverted aerated immersed screen, screen assembly and operating process

Description:
For the U.S.A., this is an application claiming the benefit under 35 USC 119(e) of U.S. Ser. No. 60/797,773 filed May 5, 2006; U.S. Ser. No. 60/798,294 filed May 8, 2006; and U.S. Ser. No. 60/876,134 filed Dec. 21, 2006. All of the applications above are incorporated herein, in their entirety, by this reference to them. 
    
    
     FIELD 
     This invention relates to an immersed or static screen, to a method of making an immersed or static screen, to a process of operating or cleaning a screen and to a water treatment apparatus or process using screens, for example a water treatment apparatus or process using membranes. 
     BACKGROUND 
     The following description of background is not an admission that anything discussed in the description is citable as prior art or part of the knowledge of persons skilled in the art in any country. 
     Some water treatment systems include a number of membrane assemblies that may contain a number of membrane fibers or sheets. The membrane fibers or sheets are held in place, typically through one or more headers or frames, within a larger assembly which may be called an element, module or cassette. The membrane fibers or sheets can be damaged by trash, roped hair or other fibrous materials that may become entangled with or around the membrane fiber or sheet. Moreover, trash, hair or fibrous materials are difficult to remove from membranes. 
     Reducing the build-up and entanglement of trash, hair or fibrous materials within membrane assemblies is desirable for efficient operation and longevity of a water treatment system. 
     One process for reducing the build-up of hair, trash or fibrous materials includes pre-screening a raw feed stream before it enters a membrane bioreactor. However, pre-screening the feed stream is typically only effective in reducing the concentrations of trash or fibrous materials that are roped or balled together in the feed. Pre-screening the raw sewage stream does not adequately remove individual strands or small bundles of trash or fibrous materials that can later come together to form relatively thick roped lengths or balled bundles inside the waste water treatment system. That is, a pre-screening filter permits individual strands of hair, for example, to easily pass into a water treatment system. Once inside the water treatment system the individual hairs are prone to roping and balling together. The roped hairs become entangled with the membrane fibers causing wear and damage. Additionally, recontamination of the pre-screened water is common since the water may pass through open tanks included in many water treatment facilities. Debris such as leaves from nearby trees or other contaminates brought by the wind frequently blows into the tanks. Further, the mechanical design of screens themselves may make them expensive or difficult to install or operate, particularly at high flows and fine mesh sizes. 
     U.S. Pat. No. 6,814,868 describes a process for reducing a trash or fibrous materials concentration in a wastewater treatment system having a membrane filter in conjunction with a bioreactor. The process comprises flowing a portion of mixed liquor through a screen in a side stream. The flow rate of the mixed liquor through the screen is about no more than the average design flow rate of the wastewater treatment system. The screenings can be either treated or disposed of directly or in combination with the waste activated sludge. The openings of the screen are between about 0.10 mm and about 1.0 mm in size as can be provided by, for example, a rotary drum screen. 
     U.S. patent application Ser. No. 11/168,405 filed on Jun. 29, 2005, and published as US Publication No. 2006-0008865 describes, among other things, a number of possible configurations for a static or immersed screen and a method of cleaning such a screen which involves inducing a backwash through the screen, for example by aerating an upstream section of the screen. US Publication No. 2006-0008865 is incorporated herein, in its entirety, by this reference to it. 
     SUMMARY 
     The following summary is intended to introduce the reader to the invention but not to limit or define any claimed invention. Inventions may reside in a combination or sub-combinations of the apparatus elements or process steps described below or in other parts of this document. The inventors do not waive or disclaim their rights to any other invention or inventions disclosed in this specification merely by not describing such invention or inventions in the claims. 
     A screening apparatus for use in a water treatment system may have an upstream area under ambient pressure with a first static head and a downstream area under ambient pressure with a second static head. The screening apparatus may comprise: 
     one or more generally static screening surfaces in the form of a three-dimensional figure with a discharge port near the bottom of the figure;
 
a structure for holding the screening surface in communication with the upstream and downstream areas such that the screening surface intercepts water flowing between the upstream and downstream areas; and,
 
one or more aerators in communication with the upstream area.
 
     An apparatus may comprise:
         one or more fluidly connected tanks;   an inlet to the one or more tanks;   a membrane assembly immersed in one of the tanks;   a static screen in the form of an open-bottomed three-dimensional figure separating a volume of water containing the membrane assembly from the inlet;   a permeate outlet connected to the membrane assembly; and,   a membrane retentate outlet in communication with the volume of water containing the membrane assembly.       

     A screening surface may be in the shape of a three-dimensional figure, for example a cylinder having an opening near its bottom. The opening may be, for example, an open bottom of the figure or a port in another surface near the bottom of the figure. The opening may be fluidly connected to one or more conduits, for example pans or pipes, which may be fluidly connected to a downstream area, for example a membrane tank or zone. One or more of the three-dimensional figures may be held in a frame. The frame may also hold aerators. The frame may have guardrails or other restraining elements to constrain the movement of uppers ends of the screening surfaces. The screening surface may have an area that is twice the cross-sectional area of the screening apparatus or more. The screening surface may be cleaned without the use of moving mechanical parts acting directly on the screening surface. A static screen may have a screening surface and a non-porous surface. 
     An upstream aerator may provide air scouring of a screening surface during forward operation or cause a backwash of the screening surface during a cleaning or deconcentration procedure. The screening apparatus may further have an overflow weir or drain upstream of the screening surface for removing solids retained by the screen, for example during deconcentration or cleaning procedures. Solids retained by the screen in an upstream area may be sent to a waste stream or re-cycle to other parts of the system. Some of these elements may be combined. For example, an aerator may simultaneously scour the screening surface with bubbles, float screenings in the upstream area to an overflow to assist in their removal or recycle, and cause a backwash of the screen. 
     A two-part screen assembly may provide a high SSA ratio  (ratio of total screen surface area of one or more screen assemblies to the area of a vertical cross-section of a tank holding the screen assemblies), for example 5 or more or 10 or more. The screen assembly may be generally in the shape of an elongated three-dimensional body, for example having a height of five times or more than the diameter of a circle having the same area as its base. The screen assembly may also have an internal passage, the cross-sectional periphery of which is mostly, or generally, surrounded by a separating layer. The screen element may comprise a supporting structure and a separation layer. The screen assembly may be prismatic, for example tubular. The screen assembly may be connected to a collector, for example a pan or a conduit. The collector may be in communication with a downstream container. Water being filtered may flow through the separating layer to the internal passage, then flow through the internal passage to the collector and then to the downstream container. 
     A method for cleaning an immersed static screen may involve lowering down the water level in a section upstream of the screening surface by partially or completely draining the upstream section. The upstream section may be drained through a weir set at a height near the minimum water level in a membrane tank downstream of the screening surface. The drained water may be returned to an upstream process tank. Flow from a process tank upstream of the upstream section of the screen may be inhibited or stopped while the upstream section of the screen is drained. 
     One or more other apparatuses or processes may be provided by combining any one or more apparatus elements or process steps selected from the set of all apparatus elements and process steps described in this summary or in other parts of this document. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view diagram illustrating a waste water treatment system; 
         FIG. 2A  is a schematic diagram illustrating a side view of a membrane tank shown in  FIG. 1A ; 
         FIG. 2B  is a schematic plan view of an alternate membrane tank. 
         FIG. 2C  is a schematic side view of a further alternate membrane tank and wastewater treatment system with a screening apparatus. 
         FIG. 3A  is a schematic isometric drawing of another screening apparatus with screening elements removed; 
         FIG. 3B  is an isometric cross-section of  FIG. 3A  with screening elements attached; 
         FIG. 4  is a schematic side view of a screening apparatus. 
         FIG. 5  is a schematic representation of static screens of various configurations. 
         FIG. 6  is a photograph showing examples of rigid tube used as inner part of a cylindrical screen assembly. 
         FIG. 7  is a schematic diagram showing an example of a 2-part cylindrical screen assembly. 
         FIG. 8  is a schematic diagram of a vessel containing a static screen and immersed membranes. 
         FIG. 9   a  is a schematic diagram of HSM collectors with screen assemblies. 
         FIG. 9   b  is a schematic diagram of a flat pan collector. 
         FIG. 9   c  shows a U-pan collector. 
         FIG. 10  is a schematic diagram of an immersed screen installation based on the HSM conduits. 
         FIG. 11  is a schematic diagram of an immersed installation based in a pan (U-pan or flat pan). 
         FIGS. 12   a  and  12   b  are schematic diagrams of MBR configurations. 
         FIG. 13  is a schematic plan view diagram of an MBR layout with sump. 
     
    
    
     DETAILED DESCRIPTION 
     Various apparatuses or processes will be described below to provide an example of an embodiment of each claimed invention. No embodiment described below limits any claimed invention and any claimed invention may cover processes or apparatuses that are not described below. The claimed inventions are not limited to apparatuses or processes having all of the features of any one apparatus or process described below or to features common to multiple or all of the apparatuses described below. It is possible that an apparatus or process described below is not an embodiment of any claimed invention. The applicants, inventors and owners reserve all rights in any invention disclosed in an apparatus or process described below that is not claimed in this document and do not abandon, disclaim or dedicate to the public any such invention by its disclosure in this document. 
       FIG. 4  shows a screening apparatus  100  having a static screen  35  mounted in a vessel  102 . The vessel  102  may be, for example, a tank, trough, channel or other conduit or holding means for water. The vessel  102  has a bottom  104  and a pair of opposed sides  106 , the closer of the two opposed sides  106  not shown, defining a pathway for water to flow through the vessel  102  by generally open channel flow. The sides  106  may be curved, as in a round tank. The static screen  35  spans between the opposed sides  106  either directly or by spanning between partitions or other non-porous elements attached to the sides  106 . The static screen  35  also extends from the bottom  104  of the vessel to above a surface level  108  of the water in the vessel  102 , either directly or by extending between non-porous elements attached to the bottom  104  or across a higher elevation of the vessel  102 . In particular, the static screen  35  may have a screening surface  35   a  and a non-porous surface  35   b . Water passing through the pathway, or from one end of the vessel  102  to another, is made to pass through the static screen  35 , particularly the screening surface  35   a . In this way, the static screen  35  separates the vessel  102  into an upstream section  110  and a downstream section  112 . Either the upstream section  110 , the downstream section  112  or both may be shared with other elements of a water treatment system. For example, the downstream section  112  may function as a membrane tank. 
     The non-porous surface  35   b  may extend from below the downstream water level  108   b  to above the upstream water level  108   a . The non-porous surface  35   b  may cover between about 5% to 25% of the height of the static screen  35 . The non-porous surface  35   b  serves to prevent water in the upstream section  110  above the water level  108   b  in the downstream section  112  from flowing to the downstream section  112 . This assists in creating an airlift in the upstream section  110  when the upstream section  110  is aerated and is believed to improve the effectiveness of the backwash, particularly in upper parts of the static screen  35 . In the absence of a distinct non-porous surface  35   b , trash or other solids etc. may accumulate on an upper section of the screening surface  35   a  and eventually act as a non-porous section  35   b . It is not necessary to use moving mechanical parts in contact with the screening surface  35   a  to clean the static screen  35 . 
     During forward operation, a difference in static head between the water level  108   a  in the upstream section  110  and the water level  108   b  in the downstream section  112  drives the flow of water through the static screen  35 . This head difference may be low, for example 30 cm or less, or between 15 and 30 cm. The water level  108  may be generally in the range of 2 to 4 metres. 
     The screening apparatus  100  may have an upstream barrier  114  which may be a partition or, as shown, an end wall of the vessel  102 . The barrier  114  and the most downstream surface of the screen  35  may be located near each other, for example between 15 cm and 2 m apart, such that the upstream section  110  may have a relatively small volume compared to the downstream section  112 . For example, the upstream section  110  may have a volume that is 30% or less than the volume of the downstream section  112 . Particularly where the downstream section  112  contains membrane assemblies, the upstream section  110  may have a volume between about 2% to 20%, for example about 10%, of the volume of the downstream section  112 . The specific size of upstream and downstream sections  110 ,  112 , or their relative volumes, may be designed by noting that if all flow to the membrane assemblies pass through the static screen  35 , then the flow to the membranes (in m 3 /d) is equal to (a) the product of screen specific surface area (m 2  screening surface  35   a  per m 3  upstream section  110  volume), the screen flux (m/d) and the volume of the upstream section (m 3 ) which is in turn equal to (b) the membrane specific surface area (m 2  membrane surface area per m 3  volume of the downstream section  112 ) the membrane flux (m/d) and the volume of the downstream section  112 . Membrane specific surface areas and fluxes may range from, for example, about 50-400 m 2 /m 3  and 0.5-2.0 m/d respectively. Screen specific surface area may range from, for example, about 3-30 m 2 /m 3 , or be typically about 10 m 2 /m 3 , and screen flux may range from about 50-200 m/d, with a typical value about 100 m/d. Alternately, or additionally, the dimensions of the upstream and downstream sections  110 ,  112  may be designed noting that between about 15 and 150%, for example 20-70%, of the volume of the upstream section  110  may flow through the static screen  35  from the downstream section  112  during a backwash, to be described below. This flow should not decrease the water level  108   b  in the downstream section  112  excessively, for example by not more than about 20 cm or 10 cm or 7% of the ordinary water level  108   b  of the downstream section  112 . 
     An inlet  116 , which may be, for example, a pipe or hole or space below a partition, allows influent water or feed to enter the upstream section  110 , for example from near the bottom of the upstream section  110 . An overflow  118 , which may be a low wall, weir, pipe, channel, or other feature, may allow water containing retained screenings, which may form a waste, reject or recycle stream  120 , to leave the upstream section  110  other than by passing through the static screen  35  when the water level  108   a  in the upstream section  110  rises to above the bottom of the overflow  118 . Primary  122  and secondary  124  drains may allow the upstream section  110  and downstream section  112 , respectively, to be drained. The drains  122 ,  124  may be valved collectively, as shown, or individually to allow the drains  122 ,  124  to be opened separately. An aerator  38 , for example a coarse bubble aerator, may be located in the upstream section  110 , for example near the bottom  104  of the vessel  102  and near the static screen  35 . The aerator  38  may be fed at different times by a filtration gas flow  126  or a backwash gas flow  128  or both. The gas flows  126 ,  128  may come from a single source, for example a variable speed blower, multiple independently controlled blowers, or flow control valves connected to a source of pressurized air. The filtration gas flow  126  may be in the range of between no flow and one half of the rate of the backwash gas flow  128 . 
     The screening apparatus  100  may operate in repeated cycles of screening and backwashing. The screening may be dead end screening, that is with a volume of water generally equal to the volume of water entering the upstream section  110  passing through the static screen  35  during a filtration period. Alternately, there may be a flow of reject  120  during some or all of a filtration period, either over the overflow  118 , through the primary drain  122  or through another outlet, but with water continuing to flow to the downstream section  112  through the static screen  35 . The filtration gas flow  126  may be provided continuously or intermittently at a low level during filtration to decrease the rate of reject build up on the static screen  35  while still permitting water to flow forward, that is towards the downstream section  112 , through the static screen  35 . As rejected materials build up on the static screen  35 , the head difference between the water levels  108   a ,  108   b  will increase if a constant flow through the static screen  35  is maintained, or flow through the static screen  35  will decrease. In either case, performance may be fully or partially restored by backwashing the static screen  35 . Backwashing can be, for example, at fixed intervals, for example as controlled by a timer, or triggered by reaching a preset water level  108   a  in the upstream section  110 , or a decline in flow or another parameter. 
     The required backwash frequency is related to screen loading rates, trash tolerance, screen surface area and upstream section  110  volume. For example, a pilot system had a screen surface area of 5.4 ft 2  operating at a screen loading rate of 5.5 gpm/ft 2  which allowed for a trash tolerance of 3 g/L. The volume of the upstream section  110  was 75 L. The feed flow was 30 gpm (5.5 gpm/ft2×5.4 ft2) and the maximum allowed trash accumulation in the upstream area  110  was 225 g (3 g/L×75 L). With dead end screening, and a trash concentration of 150 mg/L in the feed  116 , and assuming complete rejection of trash by the static screen  35 , the maximum trash loading is reached in about 13 minutes, requiring backwashing every 13 minutes. Backwashing frequency may vary between 2 and 60 minutes or between 5 and 30 minutes. 
     Backwashing may be performed, for example, by applying the backwashing gas flow  128  to the aerator  38 . The backwashing gas flow  128  may reduce the density in the water in the upstream section  110 , floats solids, creates an air lift or performs a combination of two or more of these effects. For example, applying air at a rate of between 2 and 10 scfm into a 67.5 L upstream section  110  produced air to liquid rates of 3 to 20% in the water in the upstream section  110  and approximately corresponding reductions in the density of the fluid on the upstream section. The air to liquid ratio varied generally linearly with air flow rate. The backwashing gas flow  128  causes a flow reversal through the screen  35 . During the flow reversal, water is removed from the upstream section  110 , for example through primary drain  122  or by increase of the water level  108   a  in the upstream section  110  above the overflow  118 , or further increase of upstream water level  108   a  above the overflow  118  if the water level  108   a  was previously above the overflow  118 , to remove accumulated solids entrained in the backwash flow. At the end of a period of forward screening, the driving head may have increased to 10 to 30 cm of water column. The backwashing gas flow  128  rate may be such that the air hold-up, or the amount of air trapped in the liquid column, reduces the density of the mixture such that the static head in the upstream section  110  is below that of the downstream section  112 . The backwashing gas flow  128  may be in the range of 10-50 scfm/ft 2  of footprint, or plan view area, of the upstream section  110 . Backwash periods may last between 5 and 60 or 10 and 20 seconds. During a backwash, water entering the inlet  116  may continue to flow to, but by-pass, the static screen  35  and assist in recovering retained or rejected solids from the upstream section  110 . Alternately, feed flow through the inlet  116  may be stopped during a backwash. For example, feed flow through the inlet  116  during a backwash may be between 0% and 100% or between 10% and 100% of the volume of the upstream section  110 . Thus, considering feed flow and backwash flow from the downstream section  112 , between 25% and 250% or between 40% to 150% of the volume of the upstream section  110  may be discharged during a backwash. 
     Rates of gas flows  126 ,  128  and allowable head through the static screen  35  are related so as to allow both forward filtration and backwashing. For example, maximum head differential, overflow  118  elevation, downstream water level  108   b , and backwash gas flow  128  are related in that backwash gas flow  128 , in combination with other conditions, must be sufficient to cause a backwash, with water in upstream section  110  at the overflow  118  if aeration and an overflow  118  are the method of water removal during backwash. In contrast, filtration gas flow  126  is made high enough to scour the static screen  35  and prevent quick plugging, but not so high as to reduce the effective head unnecessarily or excessively given a desired range of head differential between upstream and downstream areas  110 ,  112  during forward screening, overflow elevation  118  or downstream water level  108   b  constraints. 
     If the vessel  102  contains membrane assemblies in the downstream section  112 , relaxing the membrane assemblies, that is reducing the rate of permeation, or stopping permeation, may be done to reduce the reduction in downstream water level  108   b  caused by permeation during a screen backwash. Further, backwashing the membrane assemblies may be done during a screen backwash to add water to the downstream section  112  and may temporarily raise the water level  108   b  in the downstream section  122 . In some systems, and optionally with feed  116  to the upstream section  110  temporarily stopped, backwashing the membrane assemblies can cause a backwash of the static screen  35  alone or assist in keeping the water level  108   b  in the downstream section  112  high during a backwash. To use this effect, a controller controlling the screen backwash process, for example by controlling when the backwash gas flow  128 , may communicate with a controller controlling the membrane permeation or backwash processes such that screen backwashing and membrane relaxation or backwashing occur wholly or partially sequentially, simultaneously or generally near each other in time, for example with the membrane backwash or relaxation starting slightly before or with the screen  35  backwash. In this case, the screen  35  backwash frequency may match a fraction or multiple of a membrane backwash or relaxation frequency. Parameters, such as screen opening size, screen loading rate, upstream section recirculation flow, screen aeration rate during filtration, fixed solids loading, etc. may be adjusted to make an even fraction or multiple of the membrane backwash or relaxation frequency acceptable as the screen backwash frequency. 
     The screening apparatus  100  is useful, among other things, for combination with a membrane water treatment system. The screening apparatus  100  protects downstream membranes. The screening apparatus  100  may be placed directly in front of the membranes to protect them from contamination in upstream parts of the treatment system, for example by placing membrane assemblies in the downstream section  112 . In addition to protecting the membranes, the screening apparatus  100  may allow the membranes to be packed at a higher density or operated at increased flux or reduced cleaning or aeration. The screening assembly  100  may replace, remove or reduce the need for head works screening. The static screen  35  may have openings of 3 mm or less. Round or square openings are preferred although other shapes may also be used. 
     Opening size of punched holes is taken as the diameter of round holes or the smallest width of the opening of holes that are not circular. Opening size of an opening in a mesh is taken as the width between edges of the mesh fibers if using a square mesh, or across the shortest width if the openings are rectangular. Non-round punched holes or rectangular mesh openings preferably do not have a width of opening in any direction more than 5 times, or more than 2 times, the smallest width of opening. 
     For the purposes of this document, the word “trash” refers to solid particles of 1 mm or more in any dimension. However, a screening apparatus  100  may also protect membranes from other undesirable solids. The words “undesirable solids” refer in this document to any solid having any dimension of 20 μm or more. Trash and undesirable solids may be originally present in the feed water, be introduced into a water treatment system after its inlet or form in the water treatment system by combination of smaller particles. Trash may include roped or balled hair, bits of plastic, vegetation debris, or other solids. Undesirable solids may include sand particles, eggs, or other solids. In general, trash tends to be more damaging to membranes than other undesirable solids. An opening size of 3 mm or less may offer significant protection against trash. Further, the inventors have observed that solids smaller than the opening size may still be caught by a static screen. However, a smaller opening size may help operation with backwash and air scouring as the only cleaning operations. For example, openings of 1 mm or less may avoid stapling with feeds containing hair or short fibres and so reduce cleaning and maintenance needs of the static screens  35 . But, much smaller openings may be difficult to clean and provide unnecessary removal of solids. For example, in the context of a membrane bioreactor where mixed liquor is screened, an opening size of 1 mm or less removes significant amounts of hair, even though the hair has a diameter of much less than 1 mm. However, an opening size of 0.5 mm or less will also remove significant amounts of paper fibers although paper fibers appear to readily pass through larger openings. The paper fibers are much less damaging than hair and may also biodegrade in the system. There may be an insufficient protection advantage to justify the increased screen head loss and maintenance of a screen surface  35   b  with openings of 0.5 mm or less caused by retention of paper fibers. For these reasons, the inventors prefer opening sizes of between 0.5 and 1 mm for screening mixed liquor. However, when screening surface water, for example, the solids loading is lower and biodegradation of undesirable solids does not occur and so smaller opening sizes may be used. For example, opening sizes of 250 μm or less or 100 μm or less provide enhanced protection with acceptable screen head loss and maintenance. Even smaller openings, for example 50 μm or less, or between 20 μm and 50 μm, may advantageously also remove algae or other such items and so offer increased membrane or system performance sufficient to justify further increases in screen head loss and maintenance. 
     The backwash or reject stream water is a diluted suspension of rejected materials and may be sent to an upstream process tank or a side stream or branch process, for example a backwash water collection tank, a clarifier, a hydrocyclone, or directly to waste. The downstream section  112  is preferably of sufficient volume such that the backwashing lowers the water level  108   b  on the downstream sections by only a fraction, for example ½ or less, of the maximum head differential through the static screen  35 , for example by about 15 cm or less or about 10 cm or less. The backwash gas flow  128  requires a fairly large flow for a short period of time and may be provided by diverting air from an existing source or a source with other uses, for example membrane scouring air or aerobic tank air. 
     The attributes of the screening apparatus  100  make it ideal for the protection of membranes by continuously screening mixed liquor which will be the primary application described below. However, the screening assembly  100  may also be used for other applications. Such other applications include screening raw sewage, particularly in shipboard applications where there is a low loading rate and tankage to store feed and filtered water, or other small waste water treatment systems. The screening apparatus  100  may also be used to protect membranes filtering surface or other water to create potable or process water or performing tertiary filtration. In this case, smaller openings in the static screen  35 , for example 250 microns or less or 100 microns or less, may be used to remove undesirable particles such as sand, Barnacle eggs etc. The screening assembly  100  can also be used to remove algae or floc in surface water or enhanced coagulation filtration applications. In these cases, openings in the static screen  35  may be 50 microns or less and the screening assembly  100  may provide an active separation step. 
     The static screen  35  may be made in a variety of shapes or configurations, for example as shown in  FIG. 5 . Design (a) is a simple flat screen laid across a section of a vessel  102  with proper reinforcement. Designs (b), (c) and (d) aim at increasing the screening surface area for a given cross-sectional area of the static screen  35  or a tank that the static screen  35  fits into, as defined by a “Specific Surface Area” parameter: 
     
       
         
           
             
               SSA 
               ratio 
             
             = 
             
               
                 Screening 
                  
                 
                     
                 
                  
                 surface 
                  
                 
                     
                 
                  
                 area 
               
               
                 
                   Tank 
                    
                   
                       
                   
                    
                   cross 
                 
                 - 
                 
                   sectional 
                    
                   
                       
                   
                    
                   area 
                 
               
             
           
         
       
     
     SSA ratio  may be about 1 in situations where a simple screen is sufficient. In more demanding applications, static screens with SSA ratio  of 2 or more, 5 or more or 10 or more, for example between 2 and 15, may be used. Sample designs and screen areas for each of the four designs of  FIG. 10  are presented in Table 1. It was assumed for this Table that the screens would be located across the front of a standard tank specified for ZeeWeed™ 500d modules by their manufacturer Zenon Environmental Inc. these tanks have a width of 3 m (10 ft) and operate at a water depth of about 2.75 m (9 ft). To simplify comparison, the 3 non-flat screens have been designed to the same SSA ratio  of 9. Larger screening surface areas could be provided at the same SSA ratio  by locating the static screen along the side of the tank rather than across the front of it. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Surface Area 
                   
               
               
                 Screen Concept 
                 Key Dimensions 
                 m 2  (ft 2 ) 
                 SSA ratio   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Flat screen (a) 
                 Tank width: 3 m 
                 7.4 (80)   
                 0.9 
               
               
                   
                 Water depth: 2.75 m 
               
               
                   
                 Screen fraction: 0.9 
               
               
                 Corrugated 
                 Corrugation depth: 300 mm 
                 74 (800) 
                 9 
               
               
                 screen (b) 
                 Corrugation pitch c/c: 50 mm 
               
               
                   
                 # of corrugations: 60 
               
               
                   
                 Screen height: 2.3 m 
               
               
                   
                 Screen fraction: 0.9 
               
               
                 Vertical cylinders 
                 Cylinder diameter: 100 mm 
                 74 (800) 
                 9 
               
               
                 screen (c) 
                 Cylinder length: 2.0 m 
               
               
                   
                 # of cylinders: 117 
               
               
                   
                 c/c spacing: 125 mm 
               
               
                   
                 Top plate dimensions: 0.6 m × 
               
               
                   
                 3.0 m 
               
               
                 Horizontal 
                 Cylinder diameter: 60 mm 
                 74 (800) 
                 9 
               
               
                 cylinders 
                 Cylinder length: 0.5 m 
               
               
                 screen (d) 
                 # of cylinders: 785 
               
               
                   
                 c/c spacing: 100 mm 
               
               
                   
               
            
           
         
       
     
     Flat or corrugated screens may be made, for example, of wire, plastic or textile fibers, woven or welded into a mesh or fabric, or perforated plates. Cylindrical screens may also be made of, for example, wire mesh, plastic mesh or punched or molded parts. Other materials and structures may also be used. 
     Tests on a flat screen, as in design (a) of  FIG. 10 , with 0.75 mm openings, indicate that such a static screen can handle 3-6 gpm/ft 2 , depending on cleaning frequency, trash concentration and whether there is a recirculation flow, for example of about 1 Q through the upstream section  110 . Such a recirculation flow, which may flow across the face of the static screen  35  and exit through the overflow  118  or another outlet, has been found to increase acceptable loading rates by 1.5 to 2.5 gpm/ft 2 . With a 10 ft wide tank and 9 ft water depth, and providing 3″ for structural support on all 4 sides, the flat panel static screen  35  has an area of about 80 square feet. Such a screen is suitable for applications having up to about 2 Q of flow with a tank holding a cassette 48-64 ZeeWeed™ 500 membrane elements or flows of 1 Q with two such cassettes. Suitable applications could include filtration plants, small sewage systems, or shipboard or military wastewater systems. Changing to a corrugated static screen  35  allows a higher flow or more membrane elements to be placed in the tank. For example, a corrugated static screen  35  may have a depth of 300 m, pitch of about 60 mm, height of 2.6 m, and 50 loops for a total area of about 78 m 2  or 845 ft 2 . Such a screen would allow flows of 3-6 Q to be provided to tanks containing about 192 to 384 elements of ZeeWeed™ 500 membranes, or  3  to  8  cassettes, with flows through the static screen  35  of 5 gpm/ft 2  or less. Such a static screen  35  would be suitable, for example, for larger wastewater treatment systems. 
     Similarly, designs according to options (c) or (d) of  FIG. 10  also allow increased flow. For example, 16 cylindrical screens of 9′ height and 12″ diameter spaced at 14.5 inches centre to centre provide a screening surface area of 465 square feet. This should be sufficient to allow flows of 3-6 Q to 64 to 224 ZeeWeed™ 500 elements or 1 to 4 cassettes. In all of the cases discussed above, the number of membrane elements or flow, as a multiple of Q, can be increased by altering the plan view shape of a membrane tank. For example, if the membranes and tank walls are rearranged to make the tank larger in one dimension, the static screens  35  can be placed across the larger dimension with the inlet  116  to the tank moved to feed into the upstream area  110 . For example, a static screen  35  may run down one or both edges of a long tank rather than across the front of such a tank, for example as shown in  FIG. 2B . The use of one or more static screens  35  with large SSA ratio , locating the static screen  35  across the length of a tank, or having a cross or recirculating flow across the face of the static screen  35  may be appropriate for using the screening assembly  100  in large municipal wastewater treatment plants or other intense applications. 
     In operation, a repeated cycle of forward filtration and backwashing is the ordinary operation mode. During this mode of operation in a bioreactor, trash or undesirable solids of a size caught by the screening apparatus  100  build up in the biomass to a concentration generally equal to the ratio of SRT to HRT multiplied by the concentration of such solids in the feed. During an optional mode of operation, used for example at night or other periods when the flow rate is reduced, the screening apparatus  100  is run for an extended period of time, for example 1 hour or more, without backwashing. This causes the trash or undesirable solids concentration to increase in the upstream section  110 . At the end of this period, the trash or undesirable solids are wasted by overflow or drain, for example to a waste activated sludge holding tank. This removes large amounts of trash or undesirable solids from the system in excess of that ordinarily removed with wasted sludge. The process may be repeated, if desired, to remove more trash. The average concentration of solids retained by the screening apparatus  100  may thus be less than the concentration described above under ordinary operation. Using this additional concentration and wasting procedure may reduce or eliminate the need for head works or side stream screening. 
       FIG. 1  is a schematic diagram illustrating an example of a waste water treatment system  10 . The waste water treatment system  10  includes an optional pre-screen filter  11 , a bioreactor  14  and a membrane zone  12  respectfully arranged in series but with some recycle. Briefly, raw sewage  18 , alternately called influent or feed, flows into the waste water treatment system  10 , optionally through the pre-screen filter  11  and treated water  24 , alternately called permeate or effluent, flows out of the waste water treatment system  10  through the membrane zone  12 . 
     In some embodiments the pre-screen filter  11  is designed to screen raw waste water  18  (i.e. raw sewage) to an input level acceptable in a conventional activated sludge plant, which typically means that debris (e.g. wood, fish, trash, hair and fiber bundles, etc.) larger than 3 mm to 6 mm in cross-section are stopped by the pre-screen filter  11 , whereas smaller pieces of debris (including hair and the like) are permitted to pass through into the waste water treatment system  10 . In alternative embodiments, a pre-screen filter  11  is adapted to meet the requirements for a particular facility that it is employed in. Consequently, debris smaller or larger than described above may be permitted to pass through a particular pre-screen filter  11 . 
     Generally, the bioreactor  14  is made up of, without limitation, alone or in various combinations, one or more anaerobic zones, one or more anoxic zones, or one or more aerobic zones. According to the specific example illustrated in  FIG. 1 , the bioreactor  14  is made up of an upstream anoxic zone  15  that flows into a downstream aerobic zone  16 . In some embodiments the sewage in one or both zones  15  and  16  is continuously stirred. The bioreactor  14  also includes an optional side-screen filtering system  32  that is provided to further reduce the concentration of hair, trash and other fibrous materials in the bioreactor  14 . Details relating to a side-screen filtering system  32  are provided within the applicant&#39;s U.S. Pat. No. 6,814,868 issued on Nov. 9, 2004, which is hereby incorporated in its entirety by this reference to it. 
     Additionally, according to the specific example illustrated in  FIG. 1A , the membrane zone  12  is fluidly connected downstream of the bioreactor  14  through exit stream  22 . Flow through the exit stream  22  may be by gravity flow or pumped. The membrane zone  12  may be made up of one or more membrane tanks  21 ,  23  and  25  which may be separate tanks or partitioned areas of a larger tank. Membrane tanks  21 ,  23 ,  25  each have a respective static screen  31 ,  33  and  35 . Each static screen  31 ,  33  and  35  sealingly covers and intercepts a respective inlet flow path for the corresponding membrane tank  21 ,  23  and  25  so that the amount of fibers and trash that pass into the membrane tanks  21 ,  23  and  25  is substantially reduced during operation. Moreover, as will be described in detail further below with reference to  FIG. 2A , each membrane tank  21 ,  23  and  25  contains one or more respective membrane assemblies  37 ,  38  and  39 . Each membrane tank  21 ,  23  and  25  is preferably designed to closely confine the respective membrane assemblies  37 ,  38  and  39  to reduce the required area of the membrane tanks  21 ,  23  and  25 . For example, the membrane tanks  21 ,  23 ,  25  may have a width from 0 to 60% wider than the width of the respective membrane assemblies  37 ,  38  and  39 . 
     A first number of respective outlets of the membrane assemblies  37 ,  38  and  39  are fluidly connected to the effluent stream  24 , which is the treated water or permeate stream. A second number of respective outlets of the membrane tanks  21 ,  23  and  25  are fluidly connected to a common primary Return Activated Sludge (RAS) stream  26 ; and, similarly, a third number of respective outlets of the membrane tanks  21 ,  23  and  25  are fluidly connected to a common secondary RAS stream  28  or RAS by-pass. The RAS stream  26  may carry a flow of 3-5 Q. The secondary RAS stream  28  may carry flow only from backwashing the static screens  31 ,  33 ,  35 , or may also carry a continuous recirculating flow of, for example, 0.5-2 Q. The primary and secondary RAS streams  26  and  28  are combined and flow back into the bioreactor  14 . Specifically, in the example of  FIG. 1A , the combined primary and secondary RAS streams  26  and  28  are fed back into the anoxic zone  15 . In other embodiments, the feed back of RAS from any number membrane tanks may flow, without limitation, to a suitable combination of one or more anoxic zones, one or more anaerobic zones, and one or more aerobic zones or to a point upstream of the bioreactor  14 . 
     In operation the influent stream  18  enters the waste water treatment system  10  through pre-screen filter  11  which screens the influent stream  18  so that larger pieces and bundles of debris are kept out of the waste water treatment system  10 . 
     The screened influent stream  18  then enters the anoxic zone  15  of the bioreactor  14  where it is processed accordingly and becomes and merges with mixed liquor. Mixed liquor from the anoxic zone  15  flows to the aerobic zone  16 , where it is again processed accordingly into, merges into and becomes an aerated mixed liquor. 
     The aerated mixed liquor exits the bioreactor  14  through exit stream  22 , which is, in turn, fed into the membrane zone  12 . Within the membrane zone  12  the mixed liquor is delivered into the membrane tanks  21 ,  23  and  25  by first passing through the corresponding static screens  31 ,  33  and  35 , respectively. The static screens  31 ,  33  and  35  serve to protect the membrane assemblies  37 ,  38  and  39  within the respective membrane tanks  21 ,  23  and  25  from, for example, trash such as roped and balled bundles of hair that have formed together within the bioreactor  14  from smaller strands, smaller particles that passed through the pre-screen filter  11 , or trash that has re-contaminated the bioreactor  14 . As will be described in detail below with further reference to  FIG. 2A , one way of dealing with the screenings that cannot pass through the static screens  31 ,  33  and  35  is to flush them back into the bioreactor  14  via the secondary RAS stream  28 . In some embodiments, the flow rate through the secondary RAS stream  28  is about the same as the average flow rate Q, for example between 0.5 and 1.5 Q, of the waste water treatment system. However, flow in the secondary RAS stream  28  may not be at a constant rate and the flow rates in the sentence above may be averages over periods of time. For example, where the screen  25  is backwashed in a way that causes backwashed liquid or solids to flow into secondary overflow weir  29  to join the secondary RAS stream  28 , as will be described further below, the flow rate in the secondary RAS stream  28  may be minimal or zero while liquid flows in a forward direction through the screen and 4-6 Q during a backwash of the screen  35 . As mentioned above, a constant flow, for example of 0.5-2 Q, through the secondary RAS stream  28  may also be superimposed onto these flows. Flow in the secondary RAS stream  28  may be by gravity, for example when the membrane zone  12  is at a higher elevation than the bioreactor  14 , or by pump, optionally after flowing by gravity into a well, sump or channel, for example if the bioreactor  14  is at a higher elevation than the membrane zone  12 . Alternatively or additionally, screenings may be removed from the waste water treatment system  10  and disposed of as Waste Activated Sludge (WAS). 
     A treated effluent stream  24  exits from the permeate side of the membrane assemblies  37 ,  38  and  39 . RAS, including material rejected by the membrane assemblies in the membrane zone  12 , is fed back to the bioreactor  14  via the primary RAS stream  26 . In some embodiments, the flow rate through the primary RAS stream  26  is about three or four times the average flow rate Q, for example between 2.5 Q and 4.5 Q, of the waste water treatment system. Required flow through the static screens  31 ,  33 ,  35  may be 3.5-5.5 Q. Alternatively or additionally, waste sludge may be removed from the waste water treatment system  10 , for example as described further below, and disposed of accordingly. 
     Independently, an optional side-screen filtering system  32  may remove a portion of the mixed liquor from the bioreactor  14  in order to remove trash, hair and other fibrous materials from the mixed liquor before re-introducing the screened mixed liquor into the bioreactor  15 . Specifically, as shown in  FIG. 1 , the optional side-screen filtering system  32  is coupled to remove a portion of the mixed liquor from the aerobic zone  16  of the bioreactor  14  and re-introduce the screened mixed liquor into the aerobic zone  16 . 
     In some embodiments, a side-screen filtering system operates at a constant flow rate that may be 25% to 75% of the average flow rate Q through a waste water treatment system. In some related embodiments one or more side-screen filtering systems can be placed at various other locations within a waste water treatment system for screening the mixed liquor and subsequently re-introducing it to the same location or another location within the waste water treatment system. Again, details relating to side-screen filtering are provided within the applicant&#39;s U.S. Pat. No. 6,814,868. The side screen filtering system reduces the concentration of roped or balled hair or similar materials and other trash in the bioreactor  14 , but does not eliminate them. 
     The flow of mixed liquor through waste water treatment system  10  can be facilitated in a number of ways. According to a first option mixed liquor is pumped from the bioreactor  14  to the membrane zone  12 ; and, gravity is employed to circulate the combined RAS stream back to the bioreactor  14 . The level of the mixed liquor in one or more of the membrane tanks  21 ,  23  and  25  is controlled by the height of overflow weir  27  to the primary RAS stream  26 . Advantageously, floating foam and/or scum is passively delivered back to the bioreactor  14  from the membrane zone  12  over the overflow weir  27 , although other means for RAS recirculation and foam or scum control can be used. Alternatively, according to a second option, mixed liquor passively flows (e.g. assisted by gravity) from the bioreactor  14  to a membrane zone  12 ; and, the combined RAS stream is circulated to the bioreactor  14  using a pumping mechanism. Advantageously, in accordance with the second option, the RAS pump does not have to process the permeate flow, reducing the peak pumping requirements of the system. 
     Referring now to  FIG. 2A , illustrated is a schematic diagram of a side view of the membrane tank  25  of  FIG. 1  that is arranged with the corresponding static screen  35  to provide an integrated screening apparatus  100 . The static screen  35  is positioned close to the inlet side of the membrane tank  25  to provide an upstream section  110 . Specifically, the static screen  35  extends across the width of the membrane tank  25 , extending from the bottom of the membrane tank  25  to at least the design maximum mixed liquor level, and generally sealingly cooperates with the bottom and sides of the membrane tank  25 . A non-porous surface  35   b  may extend from the top of a frame around the screen surface  35   a  below the downstream water level  108   a  to above the upstream water level  108   a . In such an arrangement, the static screen  35  divides the membrane tank  25  into two portions, the upstream section  110  and downstream section  112 . The upstream section  110  is fluidly connected to the exit stream  22  which is an inlet to the upstream section  110  bringing in mixed liquor, either by pumped or gravity flow. The downstream section  112  contains the membrane assemblies  37 ,  38  and  39  (described below). Membrane tanks  21  and  23  are substantially identical to membrane tank  25 . The static screen  35  includes a coarse bubble aerator  38  for gas scouring and gas flow induced backwashing. Aerator  38  is coupled to receive pressurized gas (typically through an air blower, through aeration screen  40 . 
     The membrane tank  25  houses a number of membrane assemblies  37   a ,  37   b ,  37   c  and  37   d  that are placed downstream of the static screen  35  (i.e. in the second portion of the membrane tank  25 ). In some embodiments the membrane assemblies are in a cassette form, such as, for example, a ZW-500d cassette available from Zenon Environmental Inc. now GE Water &amp; Process Technologies. As shown in  FIG. 2B , the membrane tank  25  may also be re-arranged, for example by providing a static screen  35  along one or both lengths of the membrane tank  25  to provide larger static screens  35  for membrane assemblies  37  of the same membrane surface area. Optionally, the static screens  35  may surround the membrane assembly  37  on all four sides in plan view with primary RAS  26  withdrawn through the floor of the membrane tank  25  below the membrane assemblies  37 . Further optionally, the static screens  35  may encapsulate the membrane assemblies  37 , for example by providing screening surfaces  35  or non-porous surfaces  35   b  on all 6 sides of a rectangular cassette of membrane assemblies  37 , preferably with primary RAS  26  withdrawn by pipe passing through a static screen  35  and with screen backwashing by backwashing the membrane assembly  37 . 
     The membrane tank  25  also includes two drains  51 ,  52 . A larger primary drain  51  is located upstream of the static screen  35  and a smaller secondary drain  52  is located downstream of the static screen  35 . The primary and secondary drains  51 ,  52  share a fluid connection to a drain valve  54 , which is in fluid communication with a common sump  56 . With further reference to  FIGS. 1A ,  1 B,  1 C and  1 D, the common sump  56  (not shown in these Figures) may receive drainage from all or a plurality of the membrane tanks  21 ,  23  and  25 . The common sump  56  is in fluid communication with a common drain pump  59 . The common drain pump  59  is arranged to output a RAS/WAS (Waste Activated Sludge) stream from the collection of membrane tanks  21 ,  23  and  25  via the common sump  56 . 
     In operation, mixed liquor enters the membrane tank  25  on the inlet side of the membrane tank  25  upstream of the static screen  35  (i.e. in the upstream section  110  of the membrane tank  25 ). The static screen  35  serves to filter out a substantial portion of roped and balled bundles of hair and the like from the mixed liquor entering the membrane tank  25  before the mixed liquor is permitted to flow to the membrane assemblies  37   a ,  37   b ,  37   c  and  37   d . The roped and balled bundles of hair and the like that are caught by the static screen  35  and are flushed eventually through the fluid connection to the common secondary RAS stream  28 , which may be designed, for example, to support a flow generally equal to average inlet flow rate Q of the waste water treatment system  10 , for example between 0.5 and 1.5 Q. Moreover, periodic reverse flows to clean the static screen  35  may also take place employing the fluid connection to the secondary RAS stream  28  to return sludge flowing in a reverse direction through the screen to the bioreactor  14 . 
     The mixed liquor that flows through the static screen  35  or large screen  93  flows through the membrane assemblies  37   a ,  37   b ,  37   c  and  37   d  that are each made up of a number of membrane fibers. Consequently, the static screen  35  or large screen  93  protects the membrane assemblies  37   a ,  37   b ,  37   c  and  37   d  by continuously screening the mixed liquor directly before the mixed liquor is introduced to the membrane assemblies  37   a ,  37   b ,  37   c  and  37   d . The membrane fibers are hollow and porous, which allows clarified water, known as permeate, from the mixed liquor to flow into the hollow interiors of the membrane fibers. The filtered permeate water is then drawn from the membrane tank  25  via a permeate stream into the effluent stream  24 . 
     The aeration stream  40  is delivered to each of the membrane assemblies  37   a ,  37   b ,  37   c  and  37   d . The aeration stream  40  is coupled to the bottom of each of the membrane assemblies  37   a ,  37   b ,  37   c  and  37   d  and releases bubbles to provide air scouring for the respective membrane fibers (not shown). The aeration stream  40  is also connected to coarse bubble aerators  38  below the static screens  35  to provide bubbles which contact and rise past the static screens  35 . This helps reduce and delay fouling of the static screens  35  and to float retained solids to the secondary RAS stream  28 . Alternately, separate aeration streams  40  may be provided to the membrane assemblies  37   a ,  37   b ,  37   c ,  37   d  and the static screen  35 . Air, or other gases, in the one or more aeration streams  40  may be provided continuously, intermittently or cyclically. Air valves  41  may be operated to allow air, or other gases, to be provided to the screen  35  or membrane assemblies  37 , or both, at any given time. For example, the supply of gases may be provided to the membrane assemblies  37  for most, for example between 50% and 95%, of operation time, and intermittently diverted to the screen  35 . Alternately, gases may be supplied to the membrane assemblies  37  without regard to the needs of the screen  35 , which is aerated when desired without regard to the needs of the membrane assemblies  37 . However, since aerating the screen  35  reduces the density of water upstream of the screen  35 , which interferes with flow of liquids to the membrane assemblies  37 , the screen  35  may be aerated only periodically, for example directly before and/or during a screen  35  backwash as described below. Alternately, or additionally, the screen  35  may be aerated periodically with sufficient intensity to cause a backwash of the screen  35  by reducing the density of water upstream of the screen  35 . Liquids backwashed through the screen  35  during intense aeration may flow to the secondary RAS channel  28  or mix with an upstream zone or other part of the total system. These comments, and others referring to one screen  35 , apply to the other screens  31 ,  33 ,  93 . 
     For example, a screen  35  in an embodiment as shown in  FIG. 2A  may be operated with a maximum head loss to flow through the screen of 15 to 30 cm. During normal operation of the screen  35 , liquid flows through the screen  35 . While liquid flows through the screen, air is provided to the aerator  38  of the screen  35  at a rate between about 0.5 and 2.0 scfm per horizontal linear foot of screen  35 . This provides some cleaning of the screen  35  without causing an unacceptable head reduction for flow though the screen  35 . During this time, very little, if any, liquid or solids overflows into the secondary RAS stream  28 . Air may also be provided to the membrane assemblies  37  during this time as desired. Periodically, for example between about once a minute and once an hour, the screen  35  may be backwashed by providing a higher rate of aeration. For example, air may be provided to the aerator  38  of the screen  35  at a rate between about 8 and 12 scfm per horizontal linear foot of screen  35 , for a backwash period of between about 5 to 20 seconds. If necessary, the air valves  41  may be operated to divert air from the membrane assemblies  37  to provide the increased airflow to the screen  35 . This higher rate of aeration causes a decrease in the density of the liquid upstream of the screen  35 , or otherwise causes liquid to flow backwards through the screen  35 . Simultaneously, solids and liquid are floated or flow upwards upstream of the screen  35  and overflow into the secondary RAS stream  28 . After the backwash period, the rate of aeration returns to the lower level to resume normal forward flow of liquid through the screen  35 . The membrane assemblies  37  may be backwashed just before or while the increased airflow is provided to assist in backwashing the screen  35 . In water filtration systems which typically have larger membrane surface areas in relation to the influent flow Q, of flow into screening apparatus  100 , than wastewater plants, the volume of water added to the downstream section  112  during a membrane backwash may be significant and may even be sufficient to backwash the screen  35  alone. 
     Sludge that is not extracted through the membrane fibers from the membrane tank  25  generally flows through the fluid connection to the common primary RAS stream  26 , although some is wasted through the drains  51 ,  52 . 
     In an additional, optional, cleansing process, the static screen  35  (as well as static screens  31  and  33 ) can be purged by backwashing and draining solids from upstream of the static screens  31 ,  33 ,  35 . In order to do this the drain valve  54  is opened and the mixed liquor flows out through the primary and secondary drains  51  and  52 , respectively. Since the primary drain  51  is larger than the secondary drain  52  a larger amount of the mixed liquor flows through the primary drain  51  causing the mixed liquor in the membrane tank  25  to flow in the opposite direction through the static screen  35  than it normally flows when the drain valve  56  is closed. At this time flow of mixed liquor through exit line  22  may be slowed or stopped or the drain flow rates may be made to exceed the mixed liquor flow rate through exit line  22 . Reversing the flow of the mixed liquor through the static screen  35  removes at least some of the trash, debris, grime, fibers, etc. that have collected on the upstream side of static screen  35 . At least some of this released material, as well as solids too dense to be floated to secondary RAS stream  28 , are drained out of the area upstream of the static screen  35 . Alternatively, this operation can be facilitated by pumps that can be controlled to cause a reversal in the normal direction of a mixed liquor flow through one or more of the membrane tanks  25 . The membrane assemblies may be backwashed directly before or during the draining to assist in backwashing the screen  35 . 
       FIG. 2C  shows another treatment system  400  having a bioreactor  14  and a membrane zone  12  and a screening apparatus  402  integrated into a membrane tank  25 . The screening apparatus  402  may alternately be in a separate tank upstream of the membrane tank  25 . The size of the downstream section  112  of the membrane tank  25  containing the membrane assemblies  37  and the bioreactor  14  have been reduced in the figure to allow an upstream section  110  of the membrane tank  25  to be drawn larger. Flow from the bioreactor  14  to the membrane zone  12  is by pump  302  in the exit line  22 . RAS  26 ,  28  flows by gravity back to the bioreactor  14  through pipes (pipe from RAS  26  not shown) with check valve  304 . Alternately, a “pump from” arrangement may be used as described in U.S. Patent Application No. 60/798,294 filed May 8, 2006 to Theodoulou et al. or as descried in relation to  FIG. 12 , part b. and  FIG. 13  herein. The upstream and downstream sections  110 ,  112  of the combined membrane tank  25  and screening apparatus  100  are separated by a partition  300  that also acts as a non-porous surface  35   b  of a static screen  35 . Static screen  35  rests on legs  404  on the floor of membrane tank  25 . Optionally, static screen  35  may rest directly on the floor of membrane tank  25  or be held in a frame suspended for the top of membrane tank  25 . The static screen  35  has a set of open bottomed and closed or screened capped cylinders  306 , having 0.75 mm openings, which function as screening surfaces  35   a . Optionally, screening surfaces of other shapes, materials or opening sizes may be used. The cylinders  306  may be made in two parts as described in U.S. Patent Application No. 60/797,773 filed on May 5, 2006 by Cote et al. or as described herein with reference to  FIGS. 6 to 11 . The cylinders  306  are connected to a header  308  having an outlet  310  passing through the partition  300  to the downstream section  112 . The bottom 10-20 cm of the cylinders  306  may, optionally, have non-porous sections  312 . These non-porous sections  312  may be a product of the construction of the header  308  or may be provided to enhance a horizontal flow of water to aerators  38 . A valve  406  may be provided in outlet  310  and can be closed when desired to allow for draining the upstream area  110  without draining the downstream area  112  or vice versa. The upstream section  110  may be aerated to scour or shake material from the static screen  35  before draining. Such a process may be useful, for example as an alternate regular cleaning method, as a method used from time to time to remove solids from the upstream section  110  that cannot be floated over the overflow  118  or at the end of a night or low flow operation mode described earlier in which solids have been allowed to accumulate in the upstream section  110 . Drained material may be, for example, further processed, wasted or recycled. 
       FIGS. 3A and 3B  show an alternate screening apparatus  602  for use in treatment system  400  of  FIG. 2C . In screening apparatus  602 , bottom collection pipes  604  have inlets  606  for receiving the bottoms of screen cylinders  306 , not shown, oriented vertically. The open bottoms of the cylinders  306  are placed over and attached to inlets  606 , for example by being clamped over the inlets  606 , to connect cylinders  306  to a collection system  608  comprising the tubes  604  and, optionally, a header  610 . The collection system  608  directs the screened water into the membrane portion for the tank via header  610  which may pass through a hole in tube wall  300  of  FIG. 2C  into downstream section  112 . This design utilizes continuous downward water velocity in the cylinders  306  to minimize the ability for solids to settle or otherwise collect within the cylinders  306 . 
     An enclosure frame  612  captures the screen cylinders  306  as well as houses the collection system  608 . In addition the screen aerators  38  are incorporated into the frame  612 . Spaces between collection pipes  604  allow bubbles to rise from aerators  38  to the screen cylinders  306 . The frame  612  may be bolted to the top or walls of a tank  25  through the attachment fittings  614  at the top of the frame  612 . One or more of a guardrail  616 , a divider  618  or other support structures may be used to keep the cylinders  306  oriented vertically. 
     Referring to  FIGS. 6 to 11 , a screen assembly can be built from an elongated screen. The elongated screen may have a round section but other shapes are also possible, including square and star-shaped. The cross-section may or may not vary along the length, for example a cylinder or rectangular paralleliped may be used but a conical or pyramid shape is also possible. 
     A screen assembly (SA) may be made as a 1, 2 or 3 or more part assembly to be self-supporting and provide the required mesh opening, for example on the outside. A two-part SA may be built as follows: 
     The inner part may be a rigid tube built from coarse netting material which provides mechanical support for the outer part while minimizing resistance to flow. Rigid tubes are available from several suppliers including InterNet (http://internetplastic.com/filtration.htm), in a variety of dimensions, for example as shown in  FIG. 6  or in even longer tubes. A suitable tube available from InterNet is part RN7480 with the following specifications: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Inside diameter (inches) 
                 2.865 
               
               
                   
                 Outside diameter (inches) 
                 3.045 
               
               
                   
                 Wall thickness (inches) 
                 0.090 
               
               
                   
                 MD strands 
                 35 
               
               
                   
                 Cross stands (# per inch) 
                 3.75 
               
               
                   
                 Hole size (inches) 
                 0.2 × 0.2 
               
               
                   
                 Opening (%) 
                 50 
               
               
                   
                   
               
            
           
         
       
     
     The outer part may be a plastic netting that surrounds the rigid tube. The outer part may be, for example welded, glued, stitched or clamped to the inner part, for example at one or both of it ends or in a line along the length of the SA. Plastic netting are available in diamond, rectangular and square opening shapes in a variety of dimensions from several suppliers. A suitable tube available from InterNet is part XN6070 with the following specifications: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Nominal hole size (inches) 
                 0.021 × 0.027 
               
               
                   
                 Thickness (inches) 
                 0.014 
               
               
                   
                 Roll width (inches) 
                 43.5 
               
               
                   
                 Strands per inch (MD × CD) 
                 27.6 × 25.0 
               
               
                   
                 Opening (%) 
                 35 
               
               
                   
                   
               
            
           
         
       
     
     The SA can be made from any plastic. For example, polyolefins (PP or PE) may be used and are low cost and can be welded. 
     The diameter, or the diameter of a circle of equivalent average cross-sectional area, of a SA can vary, for example from 5 cm to 15 cm, or from 7 cm to 12 cm. A CA built from parts RN7480 and XN6070 is shown in  FIG. 7 . 
     A SA could also be build from a single rigid tube which has to be selected to provide the required hole size for the target application. Other materials, such as metals, can also be used. 
     A SA could also be built from 3 parts, where each part can play the following roles: 
     Part 1: inner coarse tube to provide mechanical support 
     Part 2: middle layer to provide required hole size 
     Part 3 outer coarse layer to support middle layer during backwash, to protect a fragile middle layer, and/or to promote the formation of a cake layer on the outer surface of the middle layer 
     One end of a SA may be capped or covered with netting, for example by sewing an end covering piece of netting to the outer part of the netting at one end of the SA, while the other end is in fluid communication with the downstream section of the immersed screen through a collector as described below. 
     The purpose of the collector is to hold one or more SAs into place and transfer the screened liquid to the downstream section of the immersed or static screen. The screened liquid may then travel to another downstream vessel or area, for example a membrane tank or zone. The collector may be used to install the SAs into a section of a tank separated by a vertical dividing wall from a section containing an immersed membranes ( FIG. 8 ). 
     The collector can be, for example, a HSM conduit or a pan as illustrated in  FIG. 8 . 
     A hollow structural member (HSM) is a conduit, for example with a round, square or rectangular cross-section, to which SAs are attached ( FIG. 9   a ). HSMs may be laid out horizontally across the screen section of a tank with the SAs facing down, up or horizontally or laid out vertically with the SA&#39;s horizontal. The HSM conduits go through the vertical dividing wall and put the downstream side of the immersed screen in fluid communication with the immersed membrane section. Multiple HSM conduits may be laid out side-by-side with a gap to allow trash, air and water to rise and overflow during a backwash. Example dimensions are as follows: 
     Width of the HSM: 5-25% larger than the OD of the SA 
     Height of the HSM is determined by length and cross-section required for flow (typically 2-3 times width) 
     From 5 to 50 SAs per HMS, spaced by a distance “x” 
     Gap between HSMs: “y” 
     Distances “x” and “y” may be 4 to 10 times larger than the largest piece of trash to be removed by the immersed screen. For example, if the largest piece of trash is 6 mm (i.e., the opening of a head-works coarse screen in a MBR), “x” and “y” may be between 24 to 60 mm. 
     An immersed screen installation based on HSM conduits is shown in  FIG. 10 ; the liquid above the HSM is un-screened. The feed is introduced to the bottom portion of the screen section. During backwash, the trash, air and water first flow vertically up above the HSM conduits and then flow horizontally to an overflow trough. 
     A pan collector is a horizontal structure that holds the SAs and screened fluid directly above it. The pan can be built as a flat plate with SA distributed in one or both directions across the plate or as a series of elongated U-shaped pans ( FIG. 9   c ) that can be assembled side-by-side. Gaps between SAs are based on the criteria given above. 
     An immersed screen installation based on a pan collector is shown is in  FIG. 11 ; the liquid above the pan is screened. The feed is introduced in the bottom portion of the screen section. During backwash, the trash, air and water remain below the pan and flow horizontally to an overflow trough. 
     In both collector concepts, a section of SA close to the collector may be solid (without screening surface or other openings) to inhibit air from escaping to the downstream side. This section can also be of a smaller diameter than the screen section to increase the cross-sectional area for horizontal flow under the pan during backwash. The length of the collector may be as long as required to support enough SAs across the width of a tank, or provide the required SSA ratio  to meet the required flow through the static screen. 
     SA can be attached and sealed to the collector by a number of means: O-ring, gasket, glue, welding, etc. They can be removable or permanently fixed. For example, pieces of solid (without openings) tubes may be threaded, welded etc. to a pan or HSM. These tubes may have a rubber ring slipped over their outside surfaces near their ends. The rubber ring may have an inside diameter like the outside diameter of the solid tube and an outer diameter like the inside diameter of a SA. A SA may then be slipped over the rubber tube and clamped, for example with a band pipe clamp, in place. 
     Referring to  FIGS. 12 and 13 , membrane bioreactors can be designed hydraulically as “pump-to” or “pump-from” configurations, referring to the method used to circulate mixed liquor through the membrane tank ( FIG. 12 ). While a MBR treats a flow rate of 1.0×Q, a much higher flow rate needs to be recirculated through the membrane tank for the following reasons:
         1. To prevent excessive build-up of MLSS in the membrane tank upon withdrawal of permeate   2. To recycle the mixed liquor (ML) to the to head of the plant for biological nutrient removal (as illustrated in  FIG. 12 )       

     The ML flow rate to the membrane tank may range between 3 and 10×Q, for example 5Q as shown in  FIG. 12 . 
     The pump-from configuration is sometimes used because the flow rate of ML pumped is lower by 1 Q (4 Q versus 5 Q in  FIG. 12 ), translating in lower energy consumption. 
     A significant difference between the 2 configurations is how the levels in the MBR tanks vary as in reaction to the changes in wastewater flow rate to the plant. In a typical MBR, the flow rate extracted by the permeate pumps is varied to maintain a target constant level in the biological or membrane tanks. However, to a certain extent, the biological or membrane tanks are used for equalization and their level can vary by up to 50 to 100 cm. 
     Backwashing an immersed screen by exposing the upstream side of the screen to a large flow rate of air to induce reverse flow through the screens and airlift the backwash ML containing the trash into the biological tanks (directly or through a channel) is limited by the maximum lift, for example about 20-40 cm, that can be generated by aeration. 
     The air-induced backwash method can easily be used in a pump-to configuration ( FIG. 12   a ) because the system can be designed so that the minimum level in the immersed screen upstream compartment is always above the maximum level in the biological tanks. 
     However, the air-induced backwash method may be difficult to use in a pump-from configuration ( FIG. 12   b ) if the level variation in the tanks (50-100 cm) is much larger than the maximum lift of the air-induced backwash (20-40 cm). In other words, the weir for the air-induced backwash has to be located at a pre-selected elevation; given the level variation in the tanks, a chosen elevation may not ensure backwash in all situations. 
     In another overflow method and apparatus, a submerged overflow weir is provided in the upstream section of the immersed screen at an elevation that will ensure backwash of the screen ( FIG. 12   b ). This elevation is selected to be slightly, for example a few cm, below the minimum operating water level of the membrane tank. The discharge line from this overflow is equipped with a valve which is normally closed. When the immersed screen needs to be backwashed, this valve is opened and the level in the upstream section of the immersed screen is suddenly lowered to the weir level, inducing backwash of the screen. This backwash operation will last 20-60 s and can be scheduled to take place at the same time as the backwash or relaxation of the membranes. In this method, the airflow rate to the screens does not need to be increased during backwash but optionally can be allowing the weir to be higher, possibly above the minimum operating water level of the membrane tank but still at a level that causes water to backwash through the screening surface when the area upstream of the screening surface is aerated. 
     The immersed screen backwash ML may be discharged into a sump. A pump (which can be designed to run continuously) transfers this ML back to the biological tanks. For example, there may be 5-10 backwashes per hour; each backwash may last 20-40 seconds. To provide efficient backwash, the instantaneous backwash flow rate may be 5-10% larger than the flow from the biological tanks to the immersed screen. Based on these conditions, it can be calculated (example below) that the required volume for the sump is less than 1% of the volume of the biological tanks or less than 10% of the volume of the membrane tanks. The sump pump (if run continuously) may be designed for a flow rate of 0.25-0.75×Q. 
     As an option, the inlet gate to the membrane tank, including the upstream section of the screen, can be partially or totally closed as part of the backwash sequence in order to reduce the backwash flow rate, volume of the sump and size of the sump pump. 
     An MBR may have multiple membrane tanks in parallel for a given set of biological tanks in series ( FIG. 13 ). Multiple membrane tanks may allow isolation of a subset of the membranes for cleaning or maintenance without disrupting operation of the plant (multiple biological tanks may be used to satisfy biological nutrient removal requirements). There are two options in the design of the submerged overflow screen backwash for such an MBR:
         1. Simultaneous backwash. A common overflow pipe from all membrane tanks can be equipped with a single valve to backwash all the immersed screens at the same time. In this situation, the volume of the sump is large.   2. Individual backwash. Overflow pipes from individual tanks can be each equipped with a valve to allow separate backwashing of the screens in each tank. In this situation, the volume of the sump can be made smaller.       

     The design approach for the submerged backwash will have an impact on the distribution of air to the immersed screens. In the individual backwash, the lowering of the water level in the upstream section of the immersed screens could lead to a significant imbalance in the airflow rate to the screens when all are fed from a unique source (i.e. all the air could be diverted to the tank under backwash, where the static head above the aerators is the lowest). To counter this, one solution is to equip the air supply with a valve to turn off the air to the tank that is backwashed or, alternately, to all of the tanks with screens that are not being backwashed. This is not an issue with the simultaneous backwash. 
     Example 1 
     Inlet Gates Open 
     This example is given for a pump-from system as represented by  FIG. 12   b  and  FIG. 13 . 
     Average daily flow: 24,000 m3/d 
     Hydraulic retention time: 8 hours 
     Calculate total volume of biological tanks: 24,000×8/24=8,000 m 3    
     Average flux: 0.5 m/d (8.94 gfd) 
     Calculate membrane surface area: 24,000/0.5=48,000 m2 
     Use ZeeWeed 500d cassettes with surface area of 1680 m2/cassette 
     Calculate number of cassettes: 48,000/1680=28.5 cassettes 
     Design system with 6 tanks containing 5 cassettes each. 
     Membrane tank volume for one cassette: 3 m×3 m×2.5 m: 22.5 m3 
     Calculate the volume of one membrane tank (membrane section): 22.5×5=112.5 m3 
     Volume required for immersed screen (per cassette): 3 m×3 m×0.3 m=2.7 m3 
     Calculate volume for immersed screen per membrane tank: 2.7×5=13.5 m3 
     Calculate total volume of membrane tank: 112.5+13.5=126 m3 
     Calculate volume of all membrane tanks: 126×6=756 m3 
     Calculate volume of anoxic tank: 8,000×1/3=2664 m3 
     Calculate volume of aerobic tank: 8,000−2664−756=4,580 m3 
     Assume backwash duration of 30 s 
     Assume draining 10% volume of the screen section: 13.5×6×0.1=8.1 m3 
     Assume combined backwash flow rate of 6 Q (5 Q in plus 1 Q backwash): 24,000×6/3,600/24=1.66 m3/s 
     Calculate total backwash volume: (1.66×30)+8.1=58 m3 
     Calculate volume of sump as 25% larger: 58×1.25=72.5 m3 
     Assume 10 backwashes per hour 
     Calculate sump pump flow rate: 58/6×1440=13,920 m3/d 
     Calculate fraction of Q: 13,920/24,000=0.58 Q 
     The volume of the backwash can be significantly reduced by restricting the flow to membrane tanks during a backwash. This can be achieved by closing partly or completely the inlet gate to the membrane tanks prior to initiation of the backwash. 
     Example 2 
     Inlet Gates Partially Closed 
     This example is identical to Example 1 with the exception that the inlet gate is partially closed to reduce the inlet flow to 1 Q. 
     Assume combined backwash flow rate of 2 Q (1 Q in plus 1 Q backwash): 24,000×2/3,600/24=0.55 m3/s 
     Calculate total backwash volume: (0.55×30)+8.1=24.6 m3 
     Calculate volume of sump as 25% larger: 24.6×1.25=30.8 m3 
     Assume 10 backwashes per hour 
     Calculate sump pump flow rate: 24.6/6×1440=5,904 m3/d 
     Calculate fraction of Q: 5,904/24,000=0.25 Q 
     What has been described above is merely to give one or more examples. Other arrangements of elements or steps can be implemented by those skilled in the art, without departing from the scope of the invention, which is defined by the following claims.