Abstract:
The inventions add a modified phase separator in the main line of a wastewater treatment process for enhanced BOD and nutrient removal with a membrane system. In addition, treatment methods and systems are described for high flux membrane filtration to meet secondary and tertiary treatment standards. Phase separation and membrane filtration techniques are employed to create concentrated return solids that are recycled in low flow volumes to reduce equipment sizing, reduce the physical space required for treatment and save energy costs without reducing treatment performance.

Description:
FIELD OF THE INVENTION 
       [0001]    The inventions described herein apply generally to wastewater treatment systems that employ biological processes as a treatment step and also employ one or more membranes in a filtration step. More specifically, the inventions are directed to improved methods of wastewater treatment that use phase separation, membrane filtration and recirculation controls to improve the efficiency of membrane filter operations and promote the removal of organics, nitrogen and phosphorus in activated sludge and enhance solids management in anaerobic treatment processes. 
       BACKGROUND OF THE INVENTION 
       [0002]    Since the advent of federal surface water discharge standards in the early 1970&#39;s, wastewater treatment technology has gradually developed to meet an expanding list of environmental objectives. Conventional applications of activated sludge treatment are known to be effective for removal of organic carbon, represented as biochemical oxygen demand (BOD) and, with clarification, the removal of total suspended solids (TSS) from a variety of commercial, industrial and municipal wastewaters. Additionally, selectively subjecting the mixed liquor suspended solids (MLSS) of the wastewater to aerobic (Ae), anaerobic (An) and anoxic (Ax) conditions is known by various processes in the art to be effective at removing forms of nitrogen and phosphorus (commonly referred to as nutrient removal). In most circumstances, the reduction of concentrations of BOD, TSS, nitrogen and phosphorus to predetermined levels set forth in a National Pollutant Discharge Elimination System (“NPDES”) permit grant a wastewater treatment plant operator the necessary authority under the Clean Water Act to discharge the treated waste stream into local surface water such as a river or lake. 
         [0003]    However, many wastewater treatment plant operators are finding that discharge to surface water is not the best use of the wastewater “resource” collected. For various economic, political or environmental reasons, there is a need in the industry for additional treatment technology that improves on conventional treatment. In fact, some state and federal regulatory agencies have developed additional and more stringent treatment standards that, if met, allow other beneficial uses of treated wastewater such as reuse (for example, as irrigation water or cooling water) and pretreatment for recharge (for example, groundwater aquifer replenishment). 
         [0004]    Although originally developed in the treatment of drinking water, it is now known in the art that membrane technology can be employed to completely remove suspended solids and provide significant reductions of certain pathogens, colloidal organic compounds and other organic and inorganic insoluble compounds from wastewater through various microfiltration, ultrafiltration and nanofiltration techniques. However, the benefit of this fine particle removal technology has substantial associated costs. 
         [0005]    Due to the capital costs and energy requirements of membrane technology, membrane filter arrays are optimally installed in a treatment process at a location downstream of primary and secondary solids removal processes. Conventionally, it is desirable to have the influent to the membrane filter array be of low turbidity (5 NTU or less) and low suspended solids concentrations (5 mg/l or less) with little variation over time. Such an arrangement reduces the energy cost of the membrane step, reduces the required membrane filtration area and extends both the cleaning cycle and life cycle of the membranes. One example of this application is the AquaMB Process® of Aqua-Aerobic Systems, Inc. The AquaMB Process® incorporates biological treatment, secondary settling and cloth media filtration to reduce the solids that must be removed by membrane filtration. However, such multiple barrier applications require adequate physical space which may disqualify such systems from use on compact sites. Therefore there is a need in the art for a membrane filtration process that meets current and potential future effluent standards in a compact space with a low capital cost as treatment volumes increase. 
         [0006]    It is noted that there are compact membrane filtration systems for wastewater treatment currently in use such as the Aqua-Aerobic® MBR technology by Aqua-Aerobic Systems, Inc. In such systems, the solids concentration of the influent to the membrane filter array is the same as the solids concentration in the primary treatment bioreactor, and substantially higher than the desired mixed liquor suspended solids (MLSS) concentration that optimizes membrane filtration. Consequently, for any given membrane biological reactor (MBR) system with an influent rate of 1 Q, at least 4 Q (typically 4 Q to 7 Q) is recycled from the membrane system to the bioreactor. This process results in high system wide energy demand, low membrane flux (the rate at which permeate passes through the membrane), high membrane maintenance cost and increased membrane module replacement interval. Therefore, there is a need in the art for a membrane filtration process that combines a compact site footprint with a high membrane flux rate and low energy and maintenance demands. 
         [0007]    U.S. Pat. No. 5,942,108 (Yang) discloses a multi phase separator for concentrating recycled solids to accelerate and enhance nutrient removal within a biological wastewater treatment system. As described in the Yang reference, phase separators are intended for placement on solids-recycle streams drawn from bioreactor vessels as opposed to placement on the main treatment path. Phase separators are typically intended to operate with inlet MLSS concentrations of 4,000 mg/l-6,000 mg/l with short detention times to isolate a supernatant (subsequently treated) from the biomass in order to increase the efficiency of nitrogen and phosphorus removal. In these applications, the supernatant normally has total suspended solids (TSS) concentration of 20 mg/l-50 mg/l. However, it is a feature and an advantage of the inventions described herein that a modified phase separator can be used to condition MLSS influent to a membrane filter system and reduce the membrane recycle rate. 
         [0008]    As discussed further herein, a modified phase separator, decoupled from its mixing element, can be repurposed to function as an additional MLSS control device. Using a modified phase separator in the main treatment path saves space over multi barrier systems by replacing a solids clarification device and a media filter with a small footprint separator at lower capital cost. Also, by reducing or discounting the conventional nutrient removal function of a phase separator, the flow-through capacity can be substantially increased making the system useful at higher hydraulic capacities. The phase separator retains its solids separation function, and reduces the MLSS concentration entering the membrane filter system. Through supplemental piping, the solids return line in a modified phase separator can be directed as needed to one or more of an anaerobic reactor, an aerobic reactor or an anoxic reactor to enhance nutrient removal capabilities. Alternatively or in combination, the wastewater influent upstream of the phase separator can be directed through anaerobic, aerobic and anoxic reactors to obtain effective nutrient removal in advance of its introduction to the phase separator. With these novel modifications, the phase separator can be applied to treat MLSS concentrations not previously thought practical. 
         [0009]    To save additional space, reduce capital costs, and, more importantly, to enhance the total nitrogen removal, it has been discovered that aerobic and anoxic reactors can be staged in a dual use basin by the sequenced operation of aeration equipment. During the aeration phase of the cycle, conditions promote BOD removal and nitrification. During the anoxic phase of the cycle, conditions promote denitrification along with BOD removal. The staged basin can use time based cycling or instrument control based cycling (such as with a DO probe) to create an effluent with low oxidized nitrogen as an average over time. Also, the advantages of the herein described inventions are effective where a conventional sequencing batch reactor (SBR) process is employed upstream of the modified phase separator as a replacement for the staged basin. The recited advantages may be obtained from either a conventional SBR employing sequential fill, react, and discharge phases for aerated and anoxic conditions, or alternatively with a modified sequencing batch reactor (MSBR) which provides filling, reacting and discharging steps without significant water level change or valves necessary to support the batch processing. 
         [0010]    The presently described inventions overcome limitations of current membrane treatment systems. These and other benefits of the various forms of the inventions are described in detail herein. 
       SUMMARY OF THE INVENTIONS 
       [0011]    The present inventions preserve the advantages of known membrane bioreactor techniques and also provide new features and advantages. In a primary aspect, the inventions enhance the operation of membrane filter arrays by controlling the quality of the influent to the membrane chamber. In another aspect, the inventions result in overall reduction in recycle pumping thereby improving the energy efficiency of the membrane system. Hereafter, where the specification refers to treatment reactors, chambers, vessels and the like, it will be understood to be a reference to any form of isolating the location where a treatment step takes place as those forms are known in the art. Hereafter, where the specification refers to a channel, it will be understood to be a reference to any physical conveyance (such as a pipe, trough, ditch, hose, sluice, tunnel, weir box, etc.) known in the art for the purpose of conveying a wastewater from one location to another. 
         [0012]    In another aspect, the inventions describe the modification and repurposing of a phase separator device of the type described in U.S. Pat. No. 5,942,108 (Yang). Within the scope of the inventions described herein, a phase separator, decoupled from its mixing element, can be designed and employed in the main line of treatment between a primary biological treatment reactor and a membrane filtration chamber to control and condition the MLSS concentration that comes in contact with the membrane. Hereafter, all references to a phase separator will be understood to reference the modified version of a conventional phase separator as described above—meaning without a mixing element. The advantages of reduced size and reduced hydraulic retention time for a phase separator over conventional clarification basins also accomplishes the objective of reducing the physical space needed to meet wastewater treatment objectives. For example, the volumetric requirements for conventional secondary clarifiers following an extended-aeration activated-sludge process are often sized based upon a hydraulic retention time of 4-8 hours, whereas a phase separator requires only 0.4-1.0 hours of hydraulic detention. 
         [0013]    In yet another aspect, the phase separator may be optionally fitted with a weir baffle and scum pipe mechanism or other debris collection equipment as is known in the art. In this configuration, the modified phase separator also acts as an added barrier protecting downstream membrane filters against debris (plastics, wood, fiber and the like) and the damaging effects of grit that may pass through the required primary treatment steps of other MBR systems. Peak hydraulic flows and open top biological reactors in conventional systems bypass grit and debris which ends up impacting the membrane filters. The supplemental grit and debris removal properties of the phase separator provide a critical back-up role to reduce membrane maintenance and extend the life expectancy of the sensitive membranes. Similarly, the phase separator may allow the use of certain ballast materials (such as magnetite) which can be used to augment the biological process but can interfere with the proper operation of membrane systems. Where such ballasted materials possess a specific gravity greater than 1.0, the phase separator can retain the ballast material thereby preventing its contact with the downstream membranes. 
         [0014]    In combination with the modified phase separator, certain variations and sequences of anaerobic, aerobic and anoxic reactors arranged within a continuous flow treatment system are proposed for enhanced removal of nutrients and organics. Alternatively, these reactors may, in various arrangements, be implemented in a conventional sequencing batch reactor, or in a constant water level modified sequencing batch reactor or in a conventional flow-through activated sludge system or an anaerobic process. Thus the inventions provide for the treatment of a wastewater flow with membrane technology to meet secondary or tertiary effluent standards in a small physical space at a reduced cost with improved membrane flux rates, reduced operating pressures, lower maintenance costs and augmented reliability with reduced exposure to grit and debris. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The stated and unstated objectives, features and advantages of the present inventions (sometimes used in the singular, but not excluding the plural) will become apparent from the following descriptions and drawings, wherein like reference numerals represent like elements in the various views, and in which: 
           [0016]      FIG. 1  is a schematic representation of a wastewater treatment process using a staged aeration basin and a phase separator, both hydraulically positioned between an anaerobic reactor and a membrane filter array. 
           [0017]      FIG. 2  is a schematic representation of a wastewater treatment process using a staged aeration basin, a phase separator and a membrane filter array, wherein an anoxic reactor conditions returned solids from the phase separator before discharge to an anaerobic reactor. 
           [0018]      FIG. 3  is a schematic representation of a wastewater treatment process using a staged aeration basin, a phase separator and a membrane filter array, wherein a pre-anoxic reactor conditions returned solids from the membrane filter array before discharge to an anaerobic reactor. 
           [0019]      FIG. 4  is a typical graphic representation of the changes in the concentration levels of various nitrogen compounds over time in a staged aeration reactor (SAR), sequencing batch reactor (SBR) or a constant-level modified sequencing batch reactor (MSBR) which utilizes cyclical aeration. 
           [0020]      FIG. 5  is a graphic representation of the changes in the concentration levels of dissolved oxygen in discreet anoxic and dissolved oxygen controlled aerobic periods over time in a staged aeration reactor, sequencing batch reactor or a constant-level modified sequencing batch reactor which utilize cyclical aeration. 
           [0021]      FIG. 6A  is a schematic representation of a wastewater treatment process using a phase separator hydraulically positioned between a sequencing batch reactor system and a membrane filter array during a react/fill phase of a first SBR cell and a react/discharge/recycle phase of a second SBR cell. 
           [0022]      FIG. 6B  is a schematic representation of a second step in the wastewater treatment process of  FIG. 6A  during a react/fill phase of a second SBR cell and a react/discharge/recycle phase of a first SBR cell. 
           [0023]      FIG. 7A  is a schematic representation of a wastewater treatment process using a sequencing batch reactor system and a phase separator, both hydraulically positioned between an anaerobic reactor and a membrane filter array, during a react/fill phase of a first SBR cell and a react/discharge/recycle phase of a second SBR cell. 
           [0024]      FIG. 7B  is a schematic representation of a second step in the wastewater treatment process of  FIG. 7A  during a react/fill phase of a second SBR cell and a react/discharge/recycle phase of a first SBR cell. 
           [0025]      FIGS. 8A and 8B  are schematic representations of a wastewater treatment process using a conventional multi-stage arrangement of aerobic and anoxic reactors with a phase separator, each hydraulically positioned between an anaerobic reactor and a membrane filter array. 
           [0026]      FIGS. 9A and 9B  are a variation of  FIGS. 7A and 7B  which adds an anoxic reactor to the recycle from the phase separator and operates a constant-level, continuous-flow modified sequencing batch reactor (MSBR) with cross connecting channels between the MSBR cells. 
           [0027]      FIGS. 10A and 10B  are a variation of  FIGS. 9A and 9B  which adds an aeration reactor which receives recycle from the membrane tank and is hydraulically positioned between the MSBR reactors and the phase separator. 
           [0028]      FIGS. 11A and 11B  depict a schematic representation of parallel MSBR reactors within a wastewater treatment process that can alternatively isolate each reactor cell from the main line of treatment to temporarily employ batch treatment within a flow-through system. 
           [0029]      FIG. 12  is a schematic representation of an anaerobic wastewater treatment process using a phase separator hydraulically positioned between an anaerobic reactor and a membrane filter array with circulating gas from the anaerobic reactor used as a membrane scouring agent. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0030]    Set forth below is a description of what is currently believed to be the preferred embodiments or best representative examples of the inventions claimed. Future and present alternatives and modifications to the embodiments and preferred embodiments are contemplated. Any alternatives or modifications which make insubstantial changes in function, purpose, structure or result are intended to be covered by the claims of this patent. Where references in the specification are made to a numeric concentration for a specific wastewater characteristic (such as MLSS), the concentration is intended to be understood as an average concentration over time (in hours or days) as opposed to an instantaneous or episodic concentration value. 
         [0031]      FIG. 1  shows a schematic diagram of a wastewater treatment process according to one of the preferred embodiments of the invention. On the primary treatment path, the process employs an anaerobic reactor  11 , a staged aerobic/anoxic or aeration reactor  12 , a phase separator  13  and a membrane filter  14 . Typically a screened and de-gritted wastewater enters anaerobic reactor  11  via influent channel  20  where it interacts with an activated sludge biomass (not shown) in the presence of one of a variety of non-aerating mixing devices as known in the art, such as an AquaDDM® mixer by Aqua-Aerobic Systems, Inc. Anaerobic reactor  11  promotes the growth of phosphorus accumulating organisms (PAO). Enhanced biological phosphorus removal is expedited in the absence of significant levels of dissolved oxygen and oxidized forms of nitrogen. 
         [0032]    Facultative bacteria present in anaerobic reactor  11  produce acetate and other fermentation products which are then used as substrate by the PAO. By increasing the MLSS concentration in sludge return line  33  in comparison to the MLSS concentration in reactor  11 , less treated liquid (containing little or no organic carbon) is returned to the anaerobic cell  11 . 
         [0033]    Increasing the organic carbon concentration (which could, equivalently, be understood as limiting the volume of diluted liquid in sludge return line  33 ) reduces the quantity of oxidized nitrogen being returned to the anaerobic cell  11 , promoting a purer anaerobic condition. 
         [0034]    Limiting the volume of diluted liquid introduced to the anaerobic cell  11 , also increases the actual hydraulic retention time which, in turn, encourages the fermentation of volatile fatty acids (VFA) from the non-VFA organic carbon. A byproduct of this process is the substantial release of phosphorus from the cell mass into a soluble form. Optionally, a monitor can be placed to sample phosphorus concentrations in anaerobic reactor  11  to indicate the rate of increase of phosphorus released into the basin from the interaction over the contribution of phosphorus present in the influent channel  20 . 
         [0035]    The effluent from anaerobic reactor  11  is conveyed to a staged aeration reactor  12  via channel  21 . A fully mixed environment is maintained in the staged aeration reactor  12  by one of a variety of non-aerating mixing devices as known in the art such as an AquaDDM® mixer by Aqua-Aerobic Systems, Inc. In addition, the staged aeration reactor  12  is equipped with an aeration system, preferably a fine bubble aeration system such as one of the Endura® series aeration systems of Aqua-Aerobic Systems, Inc. The staged aeration reactor  12  also receives concentrated return solids from membrane reactor  14  via return channel  34 . The combined mixed liquor sources from channel  21  and return channel  34  preferably are operated to create and maintain a MLSS concentration of approximately 5,000-10,000 mg/l in staged aeration reactor  12 . 
         [0036]    Instrumentation and controls associated with the staged aeration reactor  12  selectively cycle the aeration system on and off in repeating intervals to create alternating aerobic and anoxic conditions in the reactor  12  (see also,  FIGS. 4 and 5 ). Under aerobic conditions in reactor  12 , nitrification is promoted, organic carbon is converted to carbon dioxide, water and additional biomass; and phosphates are taken up by the biomass, particularly through interaction with PAO. Under anoxic conditions in reactor  12 , denitrification is promoted (increasing as MLSS concentration increases), and the mixed liquor solids are phosphate rich. Although BOD 5  reduction is exhibited under aerobic and anoxic conditions, the rate of BOD 5  reduction is greater during the aerated periods of operation. 
         [0037]    The influent of channel  21  enters the reactor with a certain potential oxygen demand. The oxygen demand is created by the aerobic metabolism of the organic constituents (i.e. BOD 5  reduction) and the nitrification of ammonia nitrogen (NH 3 —N). The aeration system is sized to meet this oxygen demand. A dissolved oxygen (DO) concentration profile like that of  FIG. 5  will normally indicate a pattern of increasing DO concentration during aerated periods, followed by decreasing DO values (to near zero) during non-aerated periods. Typically, the DO concentration will reach a peak value at the end of each aeration period as shown in  FIG. 5 . 
         [0038]    Cycling of the staged aeration reactor  12  may be time based or event based. Preferably, time based cycling is employed by switching the aeration equipment on and off at regular intervals. The DO profile can be managed by providing discreet control (on/off) of the aeration system  42  or by use of variable frequency drives (VFD) on the aeration system blowers to target a specific DO value at any given time during the oxic (aerated) periods. Upon termination of the aeration period, the resulting depletion rate of DO concentration can be monitored as representative of the oxygen uptake rate (OUR) of the reactor  12 . DO probes, redox/ORP probes and similar monitoring devices as are known in the art may be installed in reactor  12  or on a sampling line from reactor  12  to track the changes in DO concentration over time. 
         [0039]    For most wastewaters, it is preferred to operate in one hour cycles with approximately 75% of the cycle in aerobic conditions and 25% of the cycle in anoxic conditions. Event based cycling may be linked to concentrations of dissolved oxygen, nitrates or ammonia nitrogen through the use of various probes or sampling of the mixed liquor in the reactor  12 . Whether event based or time based, the treatment objective in the staged aeration reactor is to obtain an effluent in channel  22  that is low in oxidized nitrogen when averaged over time (see  FIG. 4 ). 
         [0040]    The mixed liquor effluent from staged aeration reactor  12  is conveyed to a phase separator  13  via channel  22 . Phase separator  13  is modified from conventional design. Modifications to phase separator  13  include functionally decoupling the unit from any mixing or aeration equipment. Further optional modifications include adding scum removal equipment (not shown) such as a baffle at the outlet weir box and a scum pipe or similar removal equipment as is known in the art. 
         [0041]    The phase separator  13  creates a low energy environment that results in two discharges with different properties. The supernatant overflow drawn off through channel  23  to the membrane reactor  14  is comparatively low in suspended solids with low concentrations of settleable solids. When the optional scum removal equipment is used, the supernatant is also low in scum, grease and floatable debris. Phase separator  13  also has a second discharge via return solids channel  33  which conveys a thickened sludge back to anaerobic reactor  11 . Thickened sludge is typically conveyed by one of a variety of sludge pumps which are well known in the art for that purpose. The phase separator  13  is preferably sized and configured to remove greater than 70% of the total suspended solids from staged aeration reactor  12  through channel  33 . For most typical wastewaters treated by the process described herein, the total suspended solids in channel  23  and subsequently introduced to the membrane reactor  14  represents less than 50-250 mg/l (based on an approximate flow split of 70% exiting phase separator  13  through channel  23  and 30% of the flow through channel  33 ). In applications which may utilize coagulants (such as aluminum sulfate) for supplemental phosphorus removal or other chemicals to enhance membrane flux, introduction through channel  22  prior to the phase separator  13  will reduce the solids and chemical loading to the membranes. 
         [0042]    In an alternative embodiment shown in  FIG. 2 , return solids channel  33  may route the solids from phase separator  13  to an anoxic reactor  17 . Anoxic reactor  17  is maintained in an anoxic condition for additional denitrification and for the reduction of dissolved oxygen prior to returning the solids to anaerobic reactor  11  via channel  27 . Anoxic reactor  17  may also be used to condition a portion of the system influent upstream of anaerobic reactor  11 . Incoming flow in channel  20  may be split with a portion diverted directly to anoxic reactor  17  via channel  200  prior to being introduced into anaerobic reactor  11  for treatment. Diversion through channel  200  is appropriate when nitrate levels in channel  20  are high. At conventional nitrate levels, channel  200  is normally closed. 
         [0043]    The membrane reactor  14  receives the supernatant effluent from phase separator  13  via channel  23 . Preferably, the submerged membrane filtration system of reactor  14  employs a hollow fiber membrane system, (for example, the PURON™ membranes manufactured by Koch Membrane Systems) and is configured for an outside-in flow path. The PURON™ membrane is a polyethersulfone, hollow fiber, membrane cast onto a braided support and potted at one end of each fiber bundle. The supernatant effluent from phase separator  13  is introduced to the outside of the hollow membrane fibers present in membrane reactor  14 . A vacuum pressure is applied to the inside of the fibers by a vacuum pump or other means as are known in the art to draw a filtrate (or permeate) from the outside of the fiber to the inside. Preferably, the nominal pore size of the membrane fibers is approximately 0.05 microns. However, pore sizes may vary through the full range of microfiltration, ultrafiltration and nanofiltration membranes indicated for use in wastewater applications. Other membrane filtration equipment, pumping systems and procedures as are known in the art may be substituted without departing from the scope of the inventions. 
         [0044]    In a preferred embodiment, the potted end of each fiber bundle is fixed in a foot element, with a central air nozzle to inject air into the center of the bundle on the outside of the fibers. The shear force of the injected air scours the membrane surface removing deposits from the membrane. Module sludging and clogging, noted in other systems, is largely avoided. Air injection is in operation during the production mode of the membrane filters, and may be continuously or intermittently operated. Periodically the membranes may be back-flushed to remove accumulated surface-deposits that have reduced the membrane flux rate. During membrane back-flushing, filtered permeate is pumped in a reverse direction through the membranes in conjunction with the air scouring operation. During conditions where the influent flow  20  is below design capacity, the membranes can be operated in a relaxation state where flow is not passing through the membrane in either a forward or reverse direction, for a limited period, as a method for improving membrane performance. During such a membrane relaxation mode, the phase separator  13  can be similarly controlled whereby flow is neither entering nor exiting the basin by providing proper isolation of the membrane recycle function, resulting in improved performance by increasing the concentration of suspended solids in the underflow stream  33 . Chemical cleaning may also be periodically indicated when membrane fouling is attributable to biological films or adsorbed substances. 
         [0045]    The membrane reactor  14  is a physical barrier to suspended solids and microorganisms which replaces a clarification step and/or a filtration step in conventional treatment processes. In a preferred embodiment, channel  23  includes a distribution manifold located at the bottom of reactor  14  so that the flow path is from the bottom to the top of the membrane fiber bundles. Typically, the manifold allows for even distribution of the influent across the full horizontal dimensions of membrane reactor  14 . 
         [0046]    The mixed liquor which does not pass through the membrane of reactor  14  accumulates solids and is discharged as the retentate of the membrane reactor  14  through solids return channel  34  to the staged aeration basin  12 . Given the pore size of the membrane and the higher flux rate obtained by using an influent with a lower MLSS concentration, the solids inventory in membrane reactor  14  increases rapidly and concentrates at a solids collection point (not shown) for discharge through solids return channel  34 . In normal operation of this embodiment, the MLSS concentration in solids return channel  33  is approximately 1.5% to 2.5% suspended solids. Due to the lower MLSS concentration in feed channel  23 , for any given influent Q, the typical recycle rate from membrane reactor  14  is only 0.5 to 2 Q rather than the normal 4 Q to 7 Q at higher feed concentrations of conventional membrane filtration applications. Additionally, the lower solids input to membrane reactor  14  results in a lower suspended solids concentration from the membrane reactor  14  through solids return channel  34  of approximately 600-1,000 mg/l as compared to conventional values of 10,000 to 20,000 mg/l. 
         [0047]    In an alternative embodiment as shown in  FIG. 3 , solids return channel  34  may be routed from the membrane reactor  14  to a pr e-anoxic reactor  15 . Pre-anoxic reactor  15  is maintained in an anoxic condition for additional denitrification, for pre-fermentation in aid of the phosphorus removal process and for the deoxygenation prior to returning the solids to anaerobic reactor  11  via channel  25 . Pre-anoxic reactor  15  includes a non-aerating mixer such as an AquaDDM® mixer by Aqua-Aerobic Systems, Inc. Pre-anoxic reactor  15  may also be used to condition a portion of the system influent upstream of anaerobic reactor  11 . Incoming flow in channel  20  may be split with a portion diverted directly to pre-anoxic reactor  15  via channel  200  prior to being introduced into anaerobic reactor  11  for treatment. Under this alternative arrangement, the solids discharge from phase separator  13  is conveyed to the staged aeration reactor  12  via return channel  33 . In general, the embodiment of  FIG. 2  is preferred over the embodiment of  FIG. 3 . If the influent waste characteristics exhibit a high influent organic acid concentration, the embodiment of  FIG. 3  is preferred over the embodiment of  FIG. 2 . If the solids concentration of channel  23  is normal to high, the embodiment of  FIG. 2  is preferred over the embodiment of  FIG. 3 . 
         [0048]    In another alternative embodiment, staged aeration reactor  12  may be replaced with a pair of sequencing batch reactors (SBRs)  16 . In the absence of anaerobic reactor  11 ,  FIGS. 6A and 6B  illustrate a pair of SBRs  16 , each operating in three cycled phases including an aerobic phase, an anoxic phase and an anaerobic phase. The SBRs are operated on opposing cycles with SBR 1   16  in react/fill mode while SBR 2  is in react/discharge/recycle mode. As with staged aeration reactor  12 , each SBR unit  16  is equipped with an aeration system  42  (not shown) which operates in the same manner as the aeration system of staged aeration reactor  12 . The react phase of each SBR  16  is operated in a manner consistent with the cycled sequence of aerobic and anoxic phases described for the staged aeration reactor  12 , with the addition of an anaerobic phase to replace the function of anaerobic reactor  11 . 
         [0049]    Where a separate anaerobic reactor  11  is desired or available for use with a SBR process,  FIG. 7A  shows a first SBR 1   16  operating in react/fill mode while receiving influent from anaerobic reactor  11  via channel  21  (shown in solid line). A broken line in  FIG. 7A  from channel  21  to a second SBR 2   16  indicates that flow from anaerobic reactor  11  to the second SBR 2   16  is stopped. At the same time in the treatment process, the second SBR 2   16  is discharging to phase separator  13  via channel  26  (shown in solid line). A broken line in  FIG. 7A  from a first SBR 1   16  to channel  26  indicates that flow out of the first SBR 1   16  is stopped. 
         [0050]    In the embodiment of  FIGS. 7A and 7B , solids return lines  33  and  34  are cross connected via channel  41  through any one of the various means that are generally known in the art. Cross connection channel  41  permits the combination of the solids discharges of phase separator  13  and membrane reactor  14  in various proportions to allow proper control the MLSS concentration returned to the anaerobic reactor  11  and SBRs  16 . 
         [0051]      FIGS. 8A and 8B  represent a conventional multi-stage flow-through activated sludge process which replaces the staged aeration basin with one or more individual aerobic  18  and anoxic  17  reactors between the anaerobic reactor  11  and the phase separator  13  prior to the membrane filter array  14 . For clarity, multiple reactors of the same kind in a single schematic treatment path are ordered from the most upstream reactor (designated “first” or “primary”) sequentially to the most downstream reactor unless the location is otherwise described in relation to a reactor with a known location. In  FIG. 8A , the influent to phase separator  13  comes from a secondary aerobic reactor  18  via channel  28 . The effluent from aerobic reactor  18  is low in oxidized nitrogen, therefore solids discharged from the phase separator  13  through solids return line  33  are returned to anaerobic reactor  11  without the need for a pre-anoxic reactor  15  to condition returned solids as in conventional treatment techniques. The secondary anoxic reactor  17  may accept an additional organic carbon source to promote denitrification. If anoxic reactor  17  is upstream of a first aerobic reactor  18 , the oxidized nitrogen source is preferably from the first aeration reactor  18  via recycle channel  38 . If anoxic reactor  1 . 7  is downstream of the first aerobic reactor  18 , the carbon source is preferably from anaerobic basin  11  via channel  211 . Channel  211  is flow rate controlled by a pump or a valve or other means as are known in the art to deliver a low flow rate to the downstream anoxic basin  17 , preferably at a rate of approximately 0.2 Q. In the embodiment of  FIG. 8A , the discharge from phase separator  13  is preferably split so that channel  23  contains less than 30% of the solids and return channel  33  contains more than 70% of the solids. The high solids concentration in return channel  33  produces a low return flow rate, preferably in the range of 0.3 Q to 0.5 Q. 
         [0052]    A typical example of a flow and solids balance of a preferred embodiment of the invention with respect to the configuration of  FIG. 8A  is described as follows. The embodiment of  FIG. 8A  begins with influent in channel  20  of 1 Q with 200 mg/l TSS and TKN=40 mg/l, and influent of return channel  33  of 0.63 Q at 21,000 mg/l MLSS for 1.63 Q total influent to anaerobic basin  11  at approximately 8,200 mg/l MLSS. At a one hour hydraulic residence time in anaerobic basin  11 , the effluent in channels  21  and  211  will be approximately 8,200 mg/l MLSS. From anaerobic basin  11 , 1.43 Q is conveyed via channel  21  to a first anoxic reactor  17  which also receives 1.5 Q from return channel  38  at 8,200 mg/l MLSS for a total of 2.93 Q into a first anoxic reactor  17  and aerobic reactor  18 . The remaining 0.2 Q is conveyed via channel  211  to a second anoxic reactor  17 . With hydraulic residence times of 1.5 hours in the first anoxic reactor  17  and 3.0 hours in aerobic reactor  18 , channel  28  discharges 1.43 Q to second anoxic reactor  17  which also receives 0.5 Q return from the membrane tank  14  via channel  34  resulting in an MLSS of 6,400 mg/l. Following a 1.0 hour hydraulic residence time in second anoxic reactor  17 , channel  27  conveys 2.13 Q at 6,400 mg/l MLSS to a secondary aerobic reactor  18  with a retention time of 1.0 hours. Optionally, the discharge from the second anoxic reactor may be passed directly to the phase separator  13  to reduce the treatment volume. Phase separator  13  discharges 1.49 Q to membrane reactor  14  at 200 mg/l MLSS and returns 0.64 Q at 21,000 mg/l MLSS to anaerobic reactor  11 . Membrane reactor  14  discharges 1 Q in filtrate via channel  24  and recycles 0.5 Q at 600 mg/l MLSS via solids return channel  34  to the second anoxic reactor  17 . Generally, the treatment process described above is operated to result in a hydraulic retention time of 8 hours and a sludge retention time of approximately 10-15 days. 
         [0053]    In another embodiment,  FIG. 8B  shows a third anoxic reactor  17  with 0.5 hours detention placed between the secondary aerobic reactor  18  and the phase separator  13 . In this embodiment, the return flow through channel  34  from membrane  14  is discharged into the third anoxic reactor  17 . The option presented in  FIG. 8B  effectively limits the potential for oxygen introduction into the secondary anoxic reactor  17  as a means to improve denitrification in the system. 
         [0054]      FIGS. 9  A&amp;B and  10  A&amp;B depict a modified sequencing batch reactor process where a first and second MSBR reactor  19  are operated in an alternating series configuration. In this operational mode, each MSBR reactor  19  receives continuous inflow and outflow resulting in a fixed water level. As with staged aeration reactor  12 , each MSBR unit  19  is equipped with an aeration system  42  (not shown) which operates in the same manner as the aeration system of staged aeration reactor  12 . The react phase of each MSBR unit  19  is operated in a manner consistent with the cycled sequence of aerobic and anoxic phases described for the staged aeration reactor  12 . 
         [0055]    In  FIG. 9A , an anoxic reactor  17  is added between return solids channel  33  and the anaerobic reactor  11  to remove oxygen, reduce forms of oxidized nitrogen (nitrates and nitrites) and initiate volatile fatty acid production in the MLSS prior to introduction of the MLSS into the anaerobic reactor  11  via channel  27 . Also, the first and second MSBRs  19  are cross connected by channels  290  to provide operational flexibility and additional flow equalization capability. The channels may be open (solid line) or closed (dashed line) as needed with appropriate gates, valves or other flow control equipment as is known in the art. Controls on the channels of  FIGS. 9A&amp;B  may be either time based or probe based at the convenience of the operator.  FIGS. 9A and 9B  show the alternating flow paths through a pair of MSBRs designated MSBR I and MSBR II when certain channels are alternatively opened and closed. In the flow path of  FIG. 9A , MSBR I operates with an elevated supply of carbon and other substrate in the wastewater (driving nutrient removal) while MSBR II provides polishing treatment. In  FIG. 9B  changes in the channels that are opened and closed reverse the roles of MSBR I and MSBR II. 
         [0056]      FIGS. 10A  &amp; B include the anoxic reactor  17  of  FIGS. 9A  &amp; B and add an aerobic reactor  18  in communication with the MSBRs  19  and phase separator  13 . Instead of returning the recycle from membrane reactor  14  directly to the MSBRs  19 , solids return channel  34  first enters aerobic reactor  18  to provide a secondary biological oxidation step prior to introduction to the phase separator  13 . Use of an aerobic reactor  18  offers a barrier of treatment which allows the MSBR reactors  19  to operate with a constant liquid level thereby reducing head-loss across the system. Channel  290  opens downstream of the second MSBR (alternatively MSBR I or MSBR II) to discharge into aerobic reactor  18 . 
         [0057]    The flow-through treatment process embodiments of  FIGS. 11A and 11B  use a pair of MSBR reactors  19  downstream of an anaerobic reactor  11 , anoxic reactor  17  and aerobic reactor  18  and upstream of the phase separator  13  and membrane reactor  14  in a configuration that allows for the isolation of an MSBR reactor  19  from the main line of treatment.  FIG. 11A  illustrates a process condition where the first of two MSBR reactors  19  is operating in continuous flow mode on the main line of treatment, while a second MSBR reactor  19  is hydraulically isolated from the line of treatment and is operating in batch mode. It is noted that an MSBR that is isolated from the main line of treatment and operated in batch mode is actually functioning temporarily in the same manner as a conventional SBR in that biological treatment can occur in the absence of incoming wastewater.  FIG. 11B  illustrates the second phase of the same treatment process wherein the second MSBR reactor  19  is operating in continuous flow mode on the main line of treatment, while the first MSBR reactor  19  is hydraulically isolated from the line of treatment and is operating in batch mode. Flow control equipment known in the art is employed to alternate between the treatment configurations of  FIG. 11A  and  FIG. 11B . 
         [0058]    As used herein, a MSBR reactor  19  is a treatment chamber equipped with mixing and aeration equipment, along with the control equipment necessary to operate the reactor alternatively in either batch mode or continuous mode. Each MSBR reactor is capable of performing the treatment steps of nitrification and denitrification with an added benefit of improved filterability characteristics attributed to the polishing treatment resultant from batch, isolated treatment. Therefore the MSBR applications described in  FIGS. 11  A&amp; B are more effective in terms of nitrate and nitrogen removal in comparison to the MSBR applications of  FIGS. 9  A&amp;B. However, the MSBR applications of  FIGS. 9  A&amp;B are considered to be more economical in terms of capital and operational costs than the MSBR applications described in  FIGS. 11  A&amp;B. 
         [0059]    The system includes a primary anoxic reactor  17  in the main line of treatment and a secondary anoxic reactor  17  fed by the return channel  33  from the phase separator  13 . In a typical operation of the system of  FIGS. 11A and 11B , screened, raw influent is introduced to the anaerobic reactor  11  by the influent channel  20 . The anaerobic reactor  11  also receives flow from the secondary anoxic reactor  17  via channel  27 . Flow from the anaerobic reactor  11  is introduced to the primary anoxic reactor  17  through channel  21 . In addition, recycle flow is received from the primary aerobic reactor  18  through return channel  38 . The flow routed through channel  38  will be operator adjustable based upon the actual operating conditions. As known in the art, channel  38  functions as a nitrate and nitrite recycle line from a primary aerobic reactor  18 . The ability of the primary anoxic reactor  17  to reduce the overall nitrate and nitrite levels is proportional to the ratio between the rate of flow in channel  38  to the incoming flow in channel  20 . The flow in channel  38  will vary from 0 to 100% of the incoming raw flow depending upon the effluent nitrate and nitrite levels in conjunction with other anoxic actions taken in the MSBR reactors  19  and the secondary anoxic reactor  17 . 
         [0060]    The primary aerobic reactor  18  receives input from channel  27  while discharges include the nitrate/nitrite recycle  38  and discharge channel  28 . A minimum of two MSBR reactors  19  will be fed sequentially through channel  28  in a manner that isolates cells for batch treatment while maintaining a constant water level. In this respect, the process schematic illustrated in  FIGS. 11A and 11B  offers hydraulic and design benefits attributed to flow-through processes with process conditioning advantages typically offered in batch type systems. 
         [0061]    Similarly, discharge from the MSBR reactors  19  will be sequentially discharged to the phase separator  13  through channel  29 . The phase separator  13  is sized to produce a high-solids stream which conveys more than 70% of the suspended solids mass to the secondary anoxic reactor  17  through channel  33 . Conversely, the phase separator  13  also generates a low-solids stream where less than 30% of the suspended solids are introduced to the membrane tank  14  through channel  23 . The permeate is discharged from membrane tank  14  through effluent channel  24 . Suspended solids which are rejected by the membrane tank  14  are returned through channel  34  to the primary aeration basin&#39;s discharge channel  28 . 
         [0062]    Another variation of the treatment processes generally described above with respect to  FIGS. 1-3  is shown in  FIG. 12 . The treatment process of  FIG. 12  is an anaerobic system that does not include an aeration step. Influent channel  20  conveys a wastewater to anaerobic reactor  11 , which can be operated in either a batch or continuous-flow mode of operation. Effluent from anaerobic reactor  11  is conveyed via channel  21  to a phase separator  13 . Also, gas (primarily methane) released during treatment in anaerobic reactor  11  is captured and conveyed to the membrane tank  14  by blower  44 . The blower  44  and gas line  51  are of conventional design for anaerobic gas transfer in wastewater applications and employ materials and components which are well known in the art. Gas line  51  terminates at membrane tank  14  with a suitable diffuser or other means for producing bubbles of a size that are effective for scouring accumulated debris from the inlet side of the membranes. 
         [0063]    The effluent from anaerobic reactor  11  is discharged to phase separator  13  via channel  21 . As described in previous embodiments, the phase separator is used to reduce suspended solids loading to membrane tank  14  to lower the energy requirements within the membrane system. The membranes of this system can be of the submerged form or preferably modular rack systems such as those offered by Norit&#39;s Airlift™ MBR Membrane Technology. On the effluent side of membrane tank  14 , the scouring gas is recovered via gas return line  52 , which is routed back to anaerobic reactor  11  to complete a closed loop. It is understood that the collection and circulation of gas in the system of  FIG. 12  may be accomplished by other configurations of blowers, piping, valves and control units as are known in the art. 
         [0064]    Preferably, return channels  33  and  34 , along with gas return line  52 , are jointly connected to anaerobic reactor  11  via a jet nozzle  61 . The use of jet nozzle  61  to combine recycle lines  33  and  34  with gas return line  52  substantially aids mixing of the return flows with the contents of anaerobic reactor  11 . Alternatively, separate diffusers and supplemental mixers can be provided to convey all recycle flows and gas return. 
         [0065]    The treatment process of  FIG. 12  may be further modified to provide operation of anaerobic reactor  11  in a batch mode with a constant water level. In this variation, two anaerobic reactors arranged in parallel are connected to the main line of treatment with appropriate valves and controls that alternatively isolate either the first or second anaerobic reactor  11  from the treatment path. 
         [0066]    The embodiments of  FIG. 12  are typically applicable to highly variable industrial-strength wastewater, as opposed to domestic sewage. In such applications, the BOD can vary between, for example, 3000 mg/l and 30,000 mg/l BOD or higher and TSS between, for example, 0 mg/l and 50,000 mg/l TSS or higher. As a general approach, phase separator  13  may remove 70% of the TSS and 30% of the flow received from channel  21  through return channel  33 . Conversely, the phase separator  13  may discharge approximately 30% of the TSS and 70% of the flow received from channel  21  through the discharge channel  23 . 
         [0067]    The above description is not intended to limit the meaning of the words used in or the scope of the following claims that define the invention. Rather, it is contemplated that future modifications in structure, function or result will exist that are not substantial changes and that all such insubstantial changes in what is claimed are intended to be covered by the claims. Thus, while preferred embodiments of the present inventions have been illustrated and described, it will be understood that changes and modifications can be made without departing from the claimed invention. In addition, although the term “claimed invention” or “present invention” is sometimes used herein in the singular, it will be understood that there are a plurality of inventions as described and claimed. 
         [0068]    Various features of the present inventions are set forth in the following claims.