CA2497391A1

CA2497391A1 - Static screens suited for use in a waste water treatment system

Info

Publication number: CA2497391A1
Application number: CA002497391A
Authority: CA
Inventors: Pierre Lucien Cote; Doug Joseph Thompson; Minggang Liu
Original assignee: Zenon Environmental Inc
Current assignee: Zenon Technology Partnership
Priority date: 2005-02-17
Filing date: 2005-02-17
Publication date: 2006-08-17

Abstract

A static screen suited for use upstream of a membrane assembly within a water or waste water treatment system includes a screening surface, having a number of openings distributed over its area. The static screen extends across an interior of a tank such that liquid must flow through the screening surface to reach the membrane assemblies. Cleaning of the screen may be by aeration or backwashing. The screen may have an undulating surface.

Description

-TITLE: STATIC SCREENS SUITED FOR USE IN A WASTE WATER
TREATMENT SYSTEM
FIELD OF THE INVENTION
[0001 The present invention relates to water or waste water treatment systems using membranes and, more particularly, to a process for reducing concentrations of hair, trash and other fibrous materials within a water or waste water treatment system or for protecting membwane assemblies incorporated in a treatment system.
BACKGROUND OF THE INVENTION

[0002] Some waste water treatment systems include a number of membrane assemblies that are each made up of a number of membrane fibers or sheets. The membrane fibers or sheets for a particular membrane assembly are held in place, typically through headers or panel frames, within a respective cassette housing or frame. Each membrane fiber or sheet is relatively delicate and can be damaged by roped hair, trash and other fibrous materials that 'f5 become entangled with the membrane fcber or sheet. Moreover, hair, trash and other fibrous materials are difficult to remove from membranes because, in addition to being delicate, the membrane fibers or sheets are arranged relatively close to one another_ [0003 Reducing the build-up and entanglement of hair, trash and ether fibrous materials within membrane assemblies is desirable for efficient operation and longevity of a waste water treatment system.
[0004] A conventional process for reducing the build-up of hair, trash and other fibrous materials includes pre-screening a raw sewage stream before it eaters a waste water treatment system. However, pre-screening the raw sewage stream is typically only effective in reducing the concentrations of hair, trash and other fibrous materials that are roped or balled together in the feed. Pre-screening the raw sewage stream does not adequately remove individual strands ar small bundles of hair, trash and other 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 (and the like) to easily pass into a waste water treatment system.
Once inside the waste water treatment system the individual hairs (and the like) are prone to roping and balling together. The roped hairs (and the like) become entangled with the membrane fibers causing unnecessary wear and damage.
[0005] Furthermore, pre-screening the raw sewage stream typically requires that the pre-screen employed be designed to accommodate peak raw sewage flow rates that are typically many times higher than the average flow rats Q through the waste water treatment system. Even well designed screens are prone to performing poorly due to inadequate sizing, poor installation and maintenance. Additionally, recontamination of the pre-screened raw sewage is common since the raw sewage is typically left in open aeration tanks included in ~5 many waste water treatment facilities. Debris such as leaves from nearby trees or other contaminates brought by the wind frequently blows into the open tanks.
SUMMARY OF THE INVENTION
[0008] Conventional processes for filtering raw sewage, such as, for example, using a pre-screening filter, permit individual strands and small bundles 2d of hair (and the like, e.g., other fibrous materials) to easily pass into a waste water treatment system. Once inside the waste water treatment system the individual hairs are prone to roping and balling together. Open tanks in the treatment system can also be re-contaminated, for example, by wind-blown leaves or trash. Membrane assemblies that are included in a waste water 25 treatment system aro not currently protected from direct contact with these roped and balled bundles of hair (and the like) that form within the waste water treatment system. According to aspects of various embodiments of the present invention there is provided a water or waste water treatment system or process that reduces wear and damage to membrane fibers caused by roped and balled bundles of hair and the like. In related embodiments the concentration of roped hairs and other fibrous materials is also reduced.
[0007] According to aspects of the invention, there is provided a static screen suited for use in combination with a membrane tank within a treatment ~ system. The static screen includes a screening surface, having a number of openings distributed over its area, for screening water In the treatment system.
The static screen is sealtngly connectable to the membrane tank and extends across an interior width of the membrane tank and from the bottom of the tank to the maximum water level such that mixed liquor must flow through the screening surface to reach the membrane assemblies.
[0008] In some embodiments, the effective size of each of the number of openings is less than approximately 3 mm.
[0009) In some embodiments, the screening surface is a Bat wire mesh. (n some other embodiments, the screening surtace comprises flat screen.
Alternatively, the screening surtace may comprise an undulating panel of material. In some related embodiments, the undulating panel of material is made up of a number of smaller panels connected together.
[0010] In some embodiments, the screening surface is arranged at an angle to a vertical axis.
[0011] In some embodiments, an aerator is provided below the screening surface for providing air scouring of the screening surface during operation or during screening surface cleaning steps.
r0092j In some embodiments a static screen is arranged within a housing that is sealingly mountable within the membrane tank.
[0013] According to other aspects of the invention, there is provided a treatment system having a tank fitted With a static screen upstream of the membrane assemblies. The static screen includes a screening surface, having a number of openings distributed over its area, for screening water in the treafiment system. The static scr~aen extends across an interior width of a tank and from the bottom of the tank to the maximum water level to intercept the flow of mixed liquor tv the membrane assemblies. The system may also include an aerator for cleaning the screen, means to backwash the screen, for example, by draining the tank rapidly from upstream of the screen or aerating the area upstream of the screen, or means to float screenings or remove floated screenings from the tank or from the area of the tank near the screen, for example by a re-cycle to other parts of the system. Same of these elements may be combined. For example, an aerator may simultaneously clean the sreen, float screenings to assist in their removal or recycle, and cause a backwash of the screen.
[0014] In other aspects of the invention, a process includes screening water with a static screen located within a membrane tank or bioreactor upstream or one or more membrane assemblies. Other aspects of the invention include processes for cleaning a static screen, for example, by aeration or backwashing or both.
(0015] Other aspects and features of the present invention, which may reside in a combination or subcombinativn of elements or steps described above or in other parts of this document, will become apparent upon review of the following description of the exemplary embodiments of the invention or are described in the claims.
BRIEF nESCRIPTION OF THE DRAWINGS
[a01B] For a better understanding of the present Invention, and to shave more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, which illustrate examples of embodiments of the present invention and in which:
[0017] Figure 1A is a schematic plan view diagram illustrating a waste water treatment system;

[0018] Figure 1 B is a schematic plan view of a tertiary filtration system;
(0019a Figure 1 C and 1 D are schematic plan views of alternate waste water treatment systems;
[0020] Figure 2 is a schematic diagram illustrating a side view of a 5 membrane tank shown in Figure 1A;
[0021] Figure 3 is a schematic diagram illustrating various views of a flat panel static screen suited for use with a membrane tank similar to the membrane tank illustrated in Figure 2;
[0022 Figure 4A is a schematic diagram illustrating various views of a undulating panel static screen; and [0023] Figure 4B is a schematic diagram illustrating an enlarged portion of the undulating panel static screen shown in Figure 4A.
[0024] Figure 5 is a schematic diagram in elevation of another waste water treatment system.
[0025] Figure 6 is a schematic diagram in isometric view of the water treatment system of Figure 5.
[0026] Figure 7 is a schematic diagram in elevation view of another waste water treatment system.
[0027] Figure 8 is a schematic diagram in isometric view of the treatment system of Figure 7.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0028] Referring to Figure 1A, shown 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 92 rasp~ctfully arranged in series. Briefly, raw sewage (i.e.
influent) 18 flows into the waste water treatment system 10, optionally through g the pre-screen fitter 11; and treated water (i.e. permeateleffluent) 24 flows out of the waste avatar treatment system 10 through the membrane zone 12.
[0029] In some embodiments the pre-screen filter 11 is designed to screen raw waste water (i.e. raw sewage) to an input level acceptable in a conventional activated sludge plant, which typically means that debris (e.g. trash, hair and fiber bundles, etc.), larger than 3 mm to 6 mm in cross-section, is 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 prescreen filter is adapted to meet the requirements for a particular facility that it is employed in. Consequently, debris smaller or larger than described above is permitted to pass through the particular pre-screen filter.
[0030] Generally, a bioreactor 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 specifrc example illustrated in Figure 1, the bioreactor 14 is made up of an upstream anoxic zone l5~that flows into a downstream aerobic zone 16. In same 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 bloreactor 14_ Details relating to the side-screen filtering system 32 are provided within the applicant's co-pending application U.S. Serial No. x91892860 (filed an June 28, 2001), which is hereby incorporated in its entirety by this reference to it_ X0031] Additionally, according to the specific example illustrated in Figure 1, the membrane zone 12 is fluidly connected to the bioreactor 14 by exit stream 22. 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 a respective inlet flow path for the Corresponding membrane tank 21, 23 and 25 so that the amount of hair (and the like) that passes into the membwane tanks 21, 23 and 25 is substantially reduced during operation. Moreover, as will be described in detail further below with reference to Figure 2, each membrane tank 21, 23 and 25 contains one or more respective membrane assemblies 37, 38 and 38. 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 that, for example, may have a width from 0 to 60% wider than the width of the respective membrane assemblies 37, 3$ and 39.
[0032] A first number of respective outlets of the membrane assemblies 37, 38 and 38 are fluidly connected to the effluent stream 24, which is the treated wafier 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 2S; and, similarly, a third number of respective outlets of the membrane tanks 21, 23 and 26 are fluidly connected to a common secondary RAS stream 28. The primary and secondary RAS streams 2B and 28 are Combined and flow back into the bloreactor 14. Specifically, 1n the example of Figure 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 flows, 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.
[0033] In operation the influent stream 18 {i.e. raw sewage) enters the waste water treatment system 10 through pre-screen filter 1 ~ which screens the influent stream 18 so that larger pieces and bundles of debris are kept out of the waste water treatment system 10. Again, a typical pre-screen filter is designed to prevent material larger than 3 mm to fi mm in cross-section from passing into a waste water treatment system.

g (pp34~ The screened influent stream 1$ 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 and merges into and becomes an aerated mixed liquor.
[0035) 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 tv protect the membrane assemblies 37, 38 and 39 within tJie respective membrane ta~ics 21, 23 and 25 from roped and balled bundles of hair (and the like) that have formed together within the bloreactor 14 from smaller strands andlor smaller bundles that passed through the prescreen filter 11 and other trash that has re-contaminated the bioreactor 14. As will be described in detail below with further reference to Figure 2, 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 bioreacfior ~14 via the secondary common RAS stream 28. In some embodiments, the peak flow rate through the secondary RAS stream 28 is about the same as the average flow rate Q, for example between 0.5 and 1.5Q, of the waste water treatment system.
However, flow in the secondary t~AS stream 28 may not be at a constant rate and the flow rates in the sentence above may be averages over time. For ~xample, where the screen 25 is backwashed in a way that causes backwashed liquid or solids to flow to the secondary I~AS stream 2$, as will be described further below, the flow rate in the secondary RAS stream 28 rnay be minimal or zero while liquid flows In a forward direction through the screen and 4-6 C~
during a backwash of the screen 35. 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).
10036) A treafied 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 bioreacfior 14 via the primary RAS stream 26. In some embodiments, the peak flow rate through the primary RAS stream 26 is about three times the average flow rate Q, 113 for example between 2.5Q and 3.5Q, of the waste water treatment system.
Alternatively or addlflonally, waste sludge may be removed from tha waste water treatment system 10, for example as described further below, and disposed of accordingly.
~003Tj independently, the optional side-screen filtering system 32 removes a portion of the mixed liquor from the bior~actor 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 Figure 1, the 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 18.
[0038y 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's co-pending application U.S. Serial No. 091892$60. 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.
~0039~ 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 5 pumped from the bioreactor 14 to the membrane zone 12; and, gravity is employed to circulate the combined RAS stream back to the biareactor 14. The level of the mixed liquor in one or more of the membrane tanks 21, ~3 and 25 is controlled by overflow weir to the primary RAS stream 2G. Advantageously, floating foam and/or scum is passively delivered back to the bioreactor 14 from 10 the membrane zone 12 over the overflow weir, 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.
[00401 Referring to Figure 1 B, a second waste water treatrnent system 9D
has a conventional activated sludge plant (typically Including a settling or clarifying Step and an internal RAS line) 91 upstream of a membrane zone 12 that provides tertiary filtration of the effluent from the plant 91 through conduit 95_ The membrane zone 12 of Figure 1 B is generally similar to that In Figure 1A
and like reference numerals denote the same elements as in Figure 1A. However, the reject stream is returned to other parts of the plant 91. Primary reject stream 97 also carries only the membrane reject, which may be about 0.05 to 0_1 Q.
Secondary reject stream 9fi may be omitted or used only intermittently, for axample, to return solids floated during aeration after backwashing the screens 31, 33, 35, as will be described further l~;low.
[00411 Figures 1 C and 1 A show further embodiments of waste water treatment systems. in Figure 1C, treatment system 92 has a large screen 93 extending across the width of the bioreactor 14, and from the bottom of the tank to the maximum water level, at th~ downstream end of the last zone (aerobic zone 16 in the embodiment of Figure 1 C), and just upstream of the outlet to exit stream 22_ Secondary RAS stream 28 is omitted since retained screenings stay in the bioreactor 14. In Figure 1 D, third system 94 has a common tank for part of the bioreactor 14, the aerobic zone 16, and the membrane zone 12. The screens 31, 33, 35 act as screening partition walls between the aerobic zone 16 and the membrane tanks 21, 23, 25. Again, secondary RAS stream 28 is omitted. In both Figures 1C and 1D, an aerator in the screens 72, 31, 33, 35, to be described further below, may help provide oxygen to the aerobic zone 16.
[0042 Referring now to Figure 2, illustrated is a schematic diagram of a side view of the membrane tank 25 of Figure 1A, 1 B and 1 D that is arranged with the corresponding static screen 35, which is positioned close to the inlet side of the membrane tank 25. Specifically, the static screen 35 extends across the width of th~ membrane tank 25, extending from the bottom of the membrane tank to at least the design maximum mixed liquor level, and generally sealingly cooperates with the bottom and sides of the membrane tank 25. In such an arrangement, the static screen 35 divides the membrane tank 25 into two portions. The first portion is fluidly connected to the un-screened exit stream 22, ZO and the second portion contains the membrane assernbiies 37, 38 and 39 (described below). Membrane tanks 21 and 23 are s~rbstantially identical to membrane tank 25. The arrangement of the embodiment at Figure 1 C also has the features described above except that the large screen 93 extends across the entire aerobic zone 16. The membrane tank 25 is one example ~of how a 25 membrane tank can be arranged in accordance with aspects of an embodiment of the invention although other arrangements may also be used.
(0043 The static screen 35 includes a course bubble aerator 38 for gas scouring, which is coupled tv receive pressurized gas (typically from an air blower) through aeration stream 40. Details relating to two specific exampl~s of static screens according to aspects of embodiments of the inverltian are provided further below with reference to Figures 3, 4A and 4B. Large screen 93 may be constructed like the embodiments of Figures 3, 4A and 4B but at an increased width.
[0044 The membrane tank 25 houses a number of membrane assemblies 37a, 37b, 37c and 37d 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 Ltd.
j0045] The membrane tank 25 also includes two drains and fluid connections to the primary and secondary RAS streams 28 and 28. 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 fluid connection to the primary RAS stream 28 is located downstream of the static screen 35 and the membrane assemblies 37a, 37b, 37c and 37d. The fluid connection to the secondary IZAS stream 28 is located upstream of the static screen 35 on the inlet side of the membrane tank 25 (i.e. in the first portion of the membrane tank 25).
jD046] 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 Figure 1, the common sump 58 (not shown in Figure 1) receives drainage from each of the three membrane tanks 21, 23 and 25. The common sump 56 is in fluid communication with a oommon drain pump 59. The common drain pump 59 is arranged to output a RASIWAS (Waste Activated Sludge} stream from the collection of membrane tanks 21, 23 and 25 via the common sump 56.
j804Tj 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 first portion 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 through the membrane assemblies 37a, 37b, 37c and 37d. The roped and balled bundles of hair and the like that are caught by the static screen 35 are flushed through the fluid connection to the common secondary RAS stream 28, which may be designed, for example, to support a flow generally equal average inlet flow rate Q of the waste water treatment system 10, for example between 0.5 and 1.SQ. Moreover, periodic reverse flows to clean the static screen 35 may also take place employing the fluid connection to the common secondary RAS
stream 28, or direct mixing with the aerobic zone 16 in Figure 1 D, to return sludge flowing in a reverse direction through the screen to the bioreactor 14_ The embodiment of Figure 1 C operates similarly but with adjustments for the location Of the large screen 93.
[OD4$] The mixed liquor that flows through the static screen 35 or large screen 93 flows through the membrane assemblies 37a, 37b, 37c and 37d that are each made up of a number of membrane fibers. Consequently, the static screen 35 or large screen 93 protects the membrane assemblies 37a, 37b, 37c and 37d by continuously screening the mixed liquor directly before the mixed liquor is introduced to the membrane assemblies 37a, 37b, 37c and 37d. The membrane fibers are hollow and porous, which allows clarified water, known as permeate, from the mixed liquor to flow into the hallow interiors of the membrane fibers. The filtered permeate water is then drawn from the membwane tank 25 via a permeate str~am Into the effluent stream 24 illustrated in Figure 1.
[0049] The aeration stream 40 is delivered to each of the membrane assemblies 37a, 37b, 37c and 37d. The aoration stream 40 is coupled to the bottom of each of the membrane assemblies 37a, 37b, 37e and 37d and releases bubbles tv provide air scouring far the respective membrane fibers (not shown).
The aeration stream 40 is also connected to coarse bubble aerators 3$ 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 37a, 37b, 37c, 37d and the static scr~en 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 andlor 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 during intense aeration may flow to the secondary RAS channel 28 dr 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.
~0050~ For example, a screen 35 is an embodiment as shown in Figure 2 may be operated with a maximum head loss to flaw through the screen of 15 to 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 Ions 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 5 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 10 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 sufficient to cause the liquid to flow backwards through the screen 35. Simultaneously, solids and liquid are floated or flow upwards upstnoam of the screen 35 and overtlow into the secondary RAS stream 28. After the backwash period, the rate of aeration 15 returns to the lower level to resume normal forward flow of liquid through the screen 35.
[0051] Sludge that is not extracted through the membrane fibers from the membrane tank 25 generally flows through the fluid connection to the common primary I~AS stream 26, although some is wasted through the drains 51, 52.
[0052] 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 prin7ary 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. 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 IZ4S
stream 2$, are drained out of the area upstream of the static screen 35.
Altemativeiy, this operation ~n be facilitated by pumps that can be controlled td cause a reversal in the normal direction of a mixed liquor flow through one or mare of the membrane tanks.
[0053 Provided as a first example of a static screen shown in Figure 3 Is a schematic diagram illustrating various views of a flat panel static screen 60.
Specifically, a top view of the flat panel static screen 60 is indicated by 61a, a front view of the flat panel screen is Indicated by 61 b and a side view is indicated at 81 c.
[0054] The flat panel static screen 60 is designed to be housed in a stainless steel frame having dimensions that fit snugly, and preferably with ar allowing for a perimeter seal, to a membrane tank 21, 23, 25 which in turn fits closely to the membrane assemblies 37. The dimensions specified herein are provided for example only and relate to ZW-500d cassettes available from Zenon Environmental Inc. The thickness of the flat panel static screen 60, as seen in the fop view 61a, is 0.3 m. The front view 61b as illustrated in Figure 3 shows that the height and width of the screen are 2.6 m and 3.0 m, respectfully. Other sizes may be used as appropriate for other membrane assemblies 37, 38 and 38 which are employed in various facilities.
(0055 The flat panel static screen 60 consists of one ur mare flat panels having punched holes or a sheet of wire mesh 67 positioned at an angle to the vertical with the top of the one or more panels leaning upstream. The holes in the flat panels) or wire mesh are sized to filter roped and balled bundles of hair or trash and the like in a waste water treatment system. In some embodiments the holes may be 0.5 mm to 1.0 mm in diameter. In other embodiments the holes may be specified to be smaller than 3 mm.

[0036] In svmc embodiments the effective area of the flat panel static screen is about 90°~ of the available area of the front surface of the static screen.
In the specific example Illustrated in Figure 3, the effective area is approximately 7.0 m2 or 75 ftz.
[0057] The flat panel static screen 60 also includes a coarse bubble aerator 62_ The coarse bubble aerator 62 provides air scouring to reduce the build-up of trash, debris, grime, etc. on the one or more flat panels or wire mesh 67 during operation and to help float solids to the secondary RAS stream 28.
[0058 Flat pan$I static screens are preferably used in facilities that have membrane tanks that are relatively wide in comparison to the membrane surtace area or have low recycle rates, for example Figure 1B. Accordingly, in such a facility the flat static screen 60 can be sized such that it provides a large effective area for screening mixed liquor.
~0459~ Provided as a second example of a static screen, shown In Figure 4A is a schematic diagram illustrating various views of an undulating panel static screen 70. Specifically, a top view of the undulating panel static screen 70 is indicated by 71a, a front view of the undulating panel screen is indicated by 71b and a side view is indicated at 71c. Moreover, shown in Figure 4B is a schematic diagram illustrating an exploded view, generally indicated by S0, of a portion of the undulating panel static screen shown in Figure 4A indicated by B in Figure 4A.
[0060] Similar to above the undulating panel static screen 70 is designed to be housed in a stainless steel frame having dimensions to generally fill the vertical cross-section of the tank 21, 23, 25 and preferably to abut or be sealable to the insid~s of the walls of the tanks 21, 23, 25.
[0061] The undulating panel static screen 70 consists of one or more flat panels having punched holes or a sheet of wire mesh 77 positioned at an angle to the vertical, with the top of the one or more panels leaning upstream. The holes in the flat panels) or wire mesh are sized to filter roped and balled bundles of hair or trash in a waste water treatment system. In some embodiments the hales may be 0.5 mm to 1.0 mm in diameter. In other embodiments the holes may be speafied to be smaller than 3 mm.
[0062] Vllith further reference to Figure 4B, the undulating panel static screen 70 is made up of a number of flat panels or panel sections 81 (each having holes as described above) arranged in an undulating or zigzag pattern.
In the specific example illustrated in Figure 4B each of the flat panels or panel sections 81 is 30 cm wide and the regular intervals behnteen the flat panels is 3 rm- By arranging the flat panels 89 in this fashion a significantly larger effective surface area is prravided by the undulating panel static screen 70 in comparison to the flat panel static screen 60 illustrated in Figure 3. For example, the surface area m2~y be 5 or 10 times or more greater than the surtace area of a flat panel In the specific example illustrated in Figure 4A, the effective surtace area is approximately 88 m2 or 950 ft~.
[0063] The holes in the flat panels 81 or wire mesh are sized to filter roped and balled bundles of hair and the like that have passed through into the waste water treatment system. In some embodiments the holes are 0.5 mm to 1.0 mm in diameter. In other embodiments the holes are specified to be smaller than 3 mm.
[DOG4] The undulating panel static screen 70 also includes a coarse bubble aerator 72 located at the bottom of the frame. The coarse bubble aerator 72 provides air scouring to reduce the build-up of trash, debris, grime, etc.
on the panels 81 (shown by example in Figure 4B).
[0065] Undulating panel static screens provide mare effective area for screening mixed liquor for a given frxed cross-sectional area. Undulating panel static screens are preferably used in facilities where membrane tanks closely confrne respective membrane assemblies, as described above with respect to Figures 1 and 2, and are narrow compared to the membrane surface area or recycle rate, particularly as in Figures 1A, 1C and 1D. The undulating surface permits a higher flow-through rate for a given pressure drop and is fouled at a slower rate than a comparable flat panel static screen having the same cross-sectional area.
[0066] With reference to the example of static screens ~0 and ~0 illustrated schematically in Figure 3, 4A and 4B, respectfully, in some embodiments the static screens fi0 and 70 can be advantageously pre-fabricated- Thus, each static screen can be designed and installed as a package with an associated membrane assembly that has a cassette structure having a defned set of available dimensions, such as for example, the ZW 600d noted above. Moreover, static screens according to aspects of embodiments of the invention can be sized and pre~manufactured to be installed in existing waste water treatment systems with little or minimal changes to existing membrane tanks.
~Oa6T~ Shipboard MBR systems today are essential to treat grey and black water, excluding bilge water. In principle, these systems are similar to the ZeeWeed MBR process, consisting of a primary screen, a bioreactor and a membrane filtration unit. However, this process train faces serious challenges when applied tv small ships, such as naval warships, coast guard vessels and small cruise ships, because of the following constraints: 1 ) the limited space available with a maximum deck height of 7-8 ft; 2) the extema,l screen undesired for smell concern and solid handling; 3) pure oxygen not preferred especially for the naval ships and 4) no sludge wasting for 2- 45 days. Most importantly, the system simplicity and compactness are so crucial.
[0068] In one shipboard system a grinder pump was applied to replace the external screen, hoping that the trashes could be chopped into small pieces so that membranes would be prevented from sludging, with the help of horizontal ZeeWeed orientation, enhanced air scouring and low permeate flux (2.5 gfd).

This system did not include a mechanism to control the trash, oil & grease, scum and foam. Tests showed that foaming was a concern.
(0069 Another design shown in Figures 5 and 6 aims to treat all the shipboard grey, black and bilge waters in one system. This system has the 5 following features' 1) a coalescing step included to remove free oil from the bilge water; 2) a static screen according to any of the static screens described herein and any of the air cleaning or backwashing processes described herein for a screen applied to remove trashes and solids; 3) a mechanism implemented to collect and waste the trash, ail and grease, scum and foam; 4) a mechanism 10 included to secure the sufficient sludge left after sludge wasting to prevent the system from control failure ar any wrongdoing and 5) the vertical ZeeWeed membranes applied with cyclic air souring.
(0070j This design was originated for ships but the concept may be applicable to other small MBR plants, especially the 1SO containerized.
15 I0071j The system consisfis of a process tank, a sludge transfer pump, a free oil discharge pump, a blower, a permeate pump and a UV unit. The process tank is partitioned into a bilge water coalescing chamber, a trash and O&G
collection chamber, a bioreac#or chamber and a membrane chamber. The trash and O&G collection chamber is further divided into an upper part and a~lower part 20 by a slopped baffle and separated from the bioreactor chamber by a dividing weir that also divides the bioreactor volume into an upper portion and a lower portion.
(007Za The bilge water is pumped by a positive displacement pump from the bilge sump to the bilge water coalescing chamber where the large oil globules are separated by gravity in the first section of the chamber and the residual oil is separated in the second section where the coalescing materials assist the oil globules to join together and migratelrise tv the surface. The free oil is collected in the upper part of the chamber and discharged periodically back to the bilge sump by the free oil discharge pump upon the inlet bilge water flow rate while the decanted water flows Into the trash and 08~Ci collection chamber, blended with the incoming grey ~& black water.
(0073 In the trash and O~G collec~on chamber, the oil & grease stay in the upper part while the trashes and large solids settle down to the lower part of the chamber. The blended water passes through the underneath of the slapped baffle and enters into the bioreactor chamber aver the dividing weir. Due to the low velocity cross the weir and the sufficient height of the weir, the settled trashes and solids won't be carried over to the bioreactor chamber such that the large solids content in the bioreactor chamber is minimized.
(0074] The aerobic bioreactor chamber is aerated by a means of medium bubble aerators in order to compromise the oxygen transfer rate with foaming potential. Because of the short water depth, the aerobic chamber is sized to ensure the oxygen transfer rate for carbonaceous BODICOD nrmoval. Antifoam may be added to control foaming, if necessary.
[0075) The sludge transfer pump transfers the mixed liquor from the bioreactor chamber to the stafiic screen channel in the membrane chamber where the mixed liquor penetrates the static screen and enters into the membrane zone while the solids rejected by the screen are carried back to the trash and O&G collection chamber during the screen backwash period. The oxygen-enriched sludge with the trash from the screen channel prevents the trash and 08G collection chamber from becoming anaerobic and also performs organic biodegradation. Supplement air may ~be added periodically or continuously to the trash and 08~G collection chamber for gentle mixing and oxygen supply, if necessary.
(007 In the membrane zone, the clean water is drawn out through the membranes end the excess trash-free sludge overflows back to the bioreactor chamber. Vertical membrane modules may be applied with cyclic air scouring.

In Case of limited tank height, the membrane modules will be shortened to the available water depth.
r0077] The accumulated sludge is wasted on a regular basis upon the requirement, directly from the trash and oil and grease (O8G) collection chamber. The sludge transfer pump is specified as grinder pump for trash handling. Prior to sludge wasting, the sludge transfer pump is operated in a closed loop within the trash and O8~G collection chamber to mix the settled trashes and solids and then this chamber is completely emptied to discharge the collected trashes, oil and grease with sludge wasting. in the mean time, the scum and foam in the aerobic chamber, if any, flow through the dividing weir back to the trash and 08~G collection chamber and are also wasted with the sludge.
X0078] Note that the weir height is such determined upon the sludge holding time and the design mixed liquor concentration that the total volume of the trash and O&C3 chamber plus the upper portion of the bioreactor volume above the dividing weir is equal to the sludge volume to be wasted and the sufficient sludge is kept in the biareactor chamber for system operation after sludge wasting.
00079] An air blower is included to provide air for the operations of ZeeWeed membranes, the static screen and the bioreactor chamber. 'A UV unit may further disinfect the discharged effluent.
[0080] The invented system treats all the shipboard grey ~ black and bilge waters. It ocansists in the embodiment shown of a process tank, a blower and three pumps and a UV unit.
(0081] The process tank includes a bilge water caalescing chamber, a trash and 08~G collection chamber, a bioreactor chamber and a membrane chamber.

[D082] The bilge water-coalescing chamber removes the free oils and most emulsified oils from the bilge water by means of gr2ivity and coalescing mechanisms. If the bilge water is not included to treat, this chamber can be removed and the system becomes a grey and black water treatment device. The O&G and trash collection chamber is divided by a slopped bafNe into an upper part where the oil and grease are collected and a lower part where the trash and solids are accumulated. The dividing weir between the bioreactor chamber and the trash and O&G collection chamber prevents the settled trashes and solids from being carried over to the bioreactor chamber for protection of the air 'f0 aerators and the static screen. The height of the dividing weir between the bioreactor chamber and the trash and O~G collection chamber is pre-set to keep fihe sufFcient sludge after sludge wasting to prevent the system from control failure and any wrongdoing. The static screen is Included in the membrane chamber to remove the solids carried with sludge and bring the solids back to the trash and iDB~G collection chamber during the backwash period so that the membranes are prevented from sludging.
[0083] The excess trash-free sludge from the membrane zone overtlows back to the bioreactor chamber. The static screen actually serves as a side screen tv transfer the trashes from the bioreactor to the trash and OS~G
collection ZO chamber. Water, O~G and trash separation performances in the trash and 08~G
collection chamber is improved because of reduced hydraulic load and solid concentration.
[00$4] The accumulated sludge is directly wasted from the trash and O&G
collection chamber on a regular basis. The collected trashas and oil and grease, and the scum and foam, if any, in the bioreactor chamber are completely discharged so that ttte trash/soiids accumulation and the foaming potential are minimized.

[0085) A grinder pump is used as the sludge transfer pump that continuously chops the solids carried to the bioreactor chamber and discharges the trashes when wasting sludge.
[OD86] Vertical membrane modules may be applied with cyclic air scouring. For a short tank, the membran~ mod~cles may be made upon the available water depth.
[0087] In one example for use in a naval ship, the grey ~ black arid bilge water treatment on board ship is shown in Figure 5. The system is to treat 22 mild grey and black water and 2_5 mild bilge water within the limited space.
No sludge wasting for 2-8 days. The combined influent is assumed with BOD5 of 960 mglL and TSS of 900 mglL.
[0088] The proposed system is designed to fit a 20' ISO container for general purpQSes, as shown in Figure 6. The process tank is sized as 13.5' L x 7' W x 7.1' H, with the overall volume of 18.9 m3. At a water depth of 1_72 m in the aerobic chamber, the total aerobic volume is 13.2 mg, which gives 2~ HRT
pf 12.9 hrs. The process tank Is partitioned as:
BioreactorMembrane Trash 8~
OS~G

Chamber Chamber Chamber Chamber Bilge water Length, m 2.87 1.25 1.25 1.25 Width, m 2.13 0.'75 1.38 0.25 Water depth, 1.72 1.87 1.T2 0.97 m Liquid volume,10.5 1.75 2.25 0_30 m~

[0088] The dividing weir height is set to1.4 m, giving the upper portion of the aerobic chamber a volume of 1.93 m3. The volume of this portion plus the volume of the trash and 08G chamber is ~29°~ of the total liquid volume. At a MLSS concentration of 14 gIL before sludge wasting, this partition results in a MLSS concentration of 10 gIL after sludge wasting. The system has a SRT of 5.5 -10 days and is able to held sludge for 3 days.
[0090] pue to the limited water depth of 1.87 m in the membrane chamber, the standard Z1N 500 modules are not applicable. Therefore, ZW-5t)Od modules 5 by Zenon Environmental Inc. are cut to fit the ZW chamber and the ZW chamber itself will serve as the support frame- With six (6) such modified ZW-500d modules, the average permeate flux of 4.3 gfd is achieved.
[0091] Oxygen transfer rate with air aeration is the limiting factor in siaing the process tank. Any improvement of oxygen transfer techniques, which may 10 include adding a gas transfer membrane, will make the process tank smaller.
[0092] For most small systems, especially the ISO containerized, standard membrane modules are too tall to install. Without extending the membrane tank above the container, the modules should fit #~e water depth of X1.8 m (or 6').
These modules may be side mounted directly to the membrane chamber walls.
15 [0093 Another small system is shown in Figures ~ and 8 which may b~
called a primary screen-Clarifler. Primary screen-clarifier uses static screen according to any of the embodiments and air cleaning or backwashing processes described herein for a screen to replace the external primary screening equipment for shipboard or other applications. This principle may be applicable 20 to other small IV113R systems. The advantages of this application include;
substantial removal of trashes and BOD / TSS, low hydraulic load to the screen (a Q instead of 4-5 Q), low solid load to the screen (infiluent TSS instead of MLSS), na screening solids to handle since the settled solids are pumpable, no smell concern because the clarifier tank can be fully closed, and compact and 25 low cost, X0094] The rotary drum screens have shown a higher hydraulic capacity with raw sewage than with mixed liquor and, therefore, similar results are expected with the static screen in the primary screen-clarifier. A combination of low hydraulic load with the high hydraulic capacity will significantly reduce the screen sizes.
X0096] As shown in Figures 7-8, the primary screen-darifier consists ofi a clarifier chamber (1), a static screen chamber (2) and a screen passant collection chamber (3). The screen chamber is located between the clarifier chamber and the screen passant collection chamber and formed with a screen overflow baffle and a static screen. A bundle of inclined plates are installed in the clarifier chamber to minimize the hydraulic turbulence and lead the settled solids to the bottom of tank. in the mean time, a screen protection baffle is installed in the screen passant collection chamber to ensure the Screen is always submerged and provide the sufficient water for screen backwash. Backwash may be by flow of air to the aerator sufficient to cause a temporary reverse flow through the screen. The optional hole in the screen overflow baffle in place of the raw water conduit helps draw water from clean side of screen rather than O&G zone during 'I b backwash.
[0096] Once raw wastewater enters into the clarifler chamber, the trash and large particulates settle down to the bottom of tank while~oil & grease float to the top. The medium size solids may be suspended and Carried over with the water stream to the screen chamber through a water conduit. The water conduit is installed behind the inclined plates at a reasonable height to avoid taking any oil ~ grease to the screen chamber. The water stream penetrates the static screen white the suspended solids ar~ rejected and returned during the backwash period back to the ciarifier chamber where the solids settle onto the inclined plates and drop to the bottom of the tank. Screen passant flows to a membrane bioreactor or other downstream treatment stage.
(0097 The trashes and the solids accumulated in the clarifier chamber are discharged periodically to maintain a reasonable solid concentration in the clarifier chamber. The entire clarifier chamber will be fully emptied once a while to dispose of the accumulated oil 8~ grease an the top of the chamber.

(0098] In one example, a system is designed for a naval ship that generate 40m31d grey and black water with TSS of 1000 mglL on average. The IVIBR
system is designed to hold solids and sludge for 45 days without discharge.
Therefore, the bioreactor is sized to allow the mixed liquor concentration to build up during this period of time, from 5 gIL to 35 glL.
[0099] The main concern at such a high MLSS concentration is trash accumulation. Therefore, a primary screen-clarifier is applied to remove the trashes and reduce the BODITSS load to the bioreactor. The primary screen-clarifier is sized as 1.6 m W _ 2.0 m Q _ 2.0 m H, with total vplume of 8.4 m3.
Assuming 25% TSS removal and 3% settled trashlsolids (DS) at the clarifter tank bottom, the clarifier will discharge 333 Ud (or 10 kg DSId) clarified solids to a trash holding tank during the 45 days period.
[00100] The tank partition is listed below:
[00101] Clarifier chamber 1.33 m W x 2 m D x 2 m H
Liquid volume: 4.28 m3 Q 1.8 m H
HRT: 2.55 hr cQ 40 mild influent flow [0010ZJ Passant collection chamber. 0.2 m W x 2 m D x 2 m H
Liquid volume: 0.56 m3 [~ 1.4 m H
HRT: 9.8 min ~ 15 gpm transfer pump X00103] Screen chamber: 0.075 m W x 2 m D x 2 m H
Screen size: 1.8 m x 1.0 m H
Screen surface area: 1.8 m2 Hydraulic load: 0.47 gpmlftz ~ 80% effective area [00104] What has been described is merely illustrative of embodiments of the invention. Other arrangements of elements or steps can be implemented by those skilled in the art, without deparlnng from the scope of the invention, which is definad by the following claims.

Claims

1. A static screen suited for use in combination with a membrane tank containing membrane assemblies within a water treatment system, the static screen comprising:
a screening surface, having a number of openings distributed over its area for screening water in the water treatment system;
wherein the static screen is sealingly connectable to the membrane tank and extends across an interior width of the membrane tank to intercept the water-to-be-filtered flowing through the membrane tank upstream of the membrane assemblies.

2. A static screen according to claim 1, wherein the effective size of each of the number of openings is approximately less than 3 mm.

3. A static screen according to claim 1, wherein the screening surface comprises a flat wire mesh.

4. A static screen according to claim 1, wherein the screening surface comprises a flat screen.

5. A static screen according to claim 1 wherein the screening surface is arranged at an angle to a vertical axis such that the top of the screening surface is further upstream than the bottom of the screening surface.

6. A static screen according to claim 1, wherein the screening surface comprises an undulating panel of material.

7. A static screen according to claim 6, wherein the undulating panel of material is made up of a number of smaller panels arranged at regular intervals and connected end to end.

8. A static screen according to claim 1 further comprising an aerator near the bottom of the screening surface for providing one or more of air scouring over the screening surface during operation of the aerator, floating solids upstream of the screen or inducing a backwash through the screening surface.

9. A static screen according to claim 1 arranged within a cassette housing that is sealingly mountable within the membrane tank.

10. A membrane tank containing membrane assemblies fitted with a static screen, the static screen comprising:
a screening surface, having a number of openings distributed over its area;
wherein the static screen extends across an interior width of the membrane tank from the bottom of the tank to the design maximum water level of the tank.

11. A membrane tank according to claim 10, wherein the effective size of each of the number of openings is approximately less than 3 mm.

12. A membrane tank according to claim 10, wherein the screening surface comprises a flat wire mesh.

13. A membrane tank according to claim 10, wherein the screening surface comprises a flat screen.

14. A membrane tank according to claim 10, wherein the screening surface comprises an undulating panel of material.

15. A membrane tank according to claim 10, wherein the undulating panel of material is made up of a number of smaller panels arranged at regular intervals and connected end to end.

16. A membrane tank according to claim 10 further comprising an aerator near the bottom of the screening surface for providing air scouring to the screening surface during operation of the aerator.

17. The membrane tank of claim 10 having a RAS outlet upstream of the screening surface.

18. The membrane tank of claim 10 having a drain upstream of the screening surface.

19. A water treatment system having membrane assemblies comprising, a static screen directly upstream of the membrane assemblies, the static screen further comprising:
a screening surface, having a number of openings distributed over its area, for screening liquid in the water treatment system;
wherein the static screen is positioned to intercept the flow of the liquid to the membrane assemblies.

20. A water treatment system according to claim 19, wherein the screening surface comprises an undulating panel of material.

21. A water treatment system according to claim 19, wherein the undulating panel of material is made up of a number of smaller panels arranged at regular intervals and connected end to end.

22. A water treatment system according to claim 19 further comprising an aerator near the bottom of the screening surface for providing air scouring to the screening surface during operation of the aerator.

23. The system of claim 19 having a RAS outlet upstream of the screening surface.

24. The system of claim 19 having a drain upstream of the screening surface.

25. Every system and process described herein.