Abstract:
Limitation of the number of cassettes in an assembly currently used for the production of permeate is obviated by grouping plural cassettes into racks one or more of which may be isolated while permeate production from the remaining racks is uninterrupted. Ancillary functions such as back-pulsing, chemical cleaning and testing for defects in membranes (“integrity testing”) in one or more racks may proceed while production of permeate continues. The number of racks in a bank is determined by the period of the overall filtration cycle, typically 15 min, in which filtration period the ancillary period is a minor portion, typically 30 sec. The racks make it possible to use automatic on/off valves which open and close quicker than larger ones. The resulting saving in time, energy, and equipment costs, which when back-pulsing with permeate is surprisingly economical, is even greater when a chemical cleaning solution is to be used. In addition, the saving in cost of equipment and avoiding the wear and tear of pumps which are intermittently stopped and started, produces unexpected economic benefits. For even greater savings in operating costs, a bank of cassettes, or a bank of racks may be operated with a “gravity flow” system.

Description:
FIELD OF THE INVENTION 
     This invention relates to a commercial filtration plant in which multiple “modules” of immersed membranes such as hollow fiber membranes (“fibers”) are efficiently and frequently back-flushed (back-washed or back-pulsed) to maintain an economical operating flux; in addition, less frequently, the fibers are chemically cleaned and tested for integrity. A “module” is represented by a “skein” of fibers potted so as to be able to collect permeate (filtrate) filtered into their lumens from a contaminated liquid substrate in which the module or skein is immersed. 
     BACKGROUND OF THE INVENTION 
     To date, commercial filtration of a large quantity of liquid substrate, specifically contaminated water, using immersed fiber membranes, has been prejudiced by the capital cost of construction of a large plant, that is, one for production of at least 3.785 million liters/day (MM L/day) or 1 million gallons per day (MM gal/day) of permeate, and the cost of its operation. To produce permeate efficiently, that is, economically, a skein must be back-pulsed frequently; the more contaminated the substrate, the more frequent the back-pulsing. Because skeins are typically back-flushed several times an hour, and permeate is forced through the fibers during back-flushing during a very short period at a flow rate greater than the rate at which permeate is withdrawn, back-flushing is referred to as “back-pulsing”. Fibers may be back-pulsed with water, or gas, typically air, or a mixture of air and water. Filtering municipal sewage, “dirty” river water and the like, may require back-pulsing every 15 min. In a typical commercial filtration plant, a tank is continuously fed with contaminated water; permeate is recovered and concentrate in which the contaminants are concentrated (typically 0.1% to 15% solids) is purged from the tank. 
     In a conventional filtration plant, multiple modules or skeins are connected for operation in parallel, as if a single skein; and each cassette is provided with an on/off valve on its permeate and back-pulse manifolds respectively, to isolate it from other cassettes, if for example a skein becomes defective and the cassette is to be replaced. On/off valves which are fully open or fully closed are referred to herein because there is no reason to have valves which may be partially opened, and on/off valves are cheaper and operate more quickly. By “isolate” is meant that one or more cassettes is hydraulically disconnected from the permeate collection header for the remaining cassettes. If desired, more than one, but less than all the skeins may be so isolated, at relatively frequent intervals for executing an ancillary function. Functions other than filtration are referred to as “ancillary functions”. An ancillary function is typified by back-pulsing with permeate, chemically cleaning with a chemical solution, or integrity testing (to detect a leak referred to as “an integrity test”). To avoid manually turning valves on and off, automatic on/off valves may be used to serve their designed purpose. 
     Back-flushing with permeate requires flowing it through the lumens of the fibers at sufficient pressure to pass through the pores of the membranes; preferably the pressure is sufficiently high and generated quickly enough to provide a “pulse” which effectively dislodges a build-up of material (“foulant”) which may foul and plug the pores. Since skeins are usually back-flushed often, it is desirable to back-flush one skein while the others produce permeate. Though there is no saving in time compared to operating the entire bank as a single skein, since each skein is isolated for the same amount of time irrespective of whether the bank is operated as a single skein, such a procedure allows close control of each skein in the bank. For example, over a 100 cycles, each skein is back-pulsed 100 times. The total time each skein spends being back-pulsed is the same whether all skeins are operated as a single skein or not. 
     A procedure for simultaneously back-flushing one skein while withdrawing permeate from others in a bank or cluster of skeins is disclosed in German Beschreibung No. 10397/6.11.97 to Prof Chmiel. In his FIG. 1 (reproduced in FIG. 1 herein with valves added) he discloses a tank  4  in which is immersed a cluster of two skeins  2   a  and  2   b  of membranes  3  from which permeate is withdrawn at outlet  6 . Permeate leaves from the skeins&#39; upper headers through a permeate withdrawal manifold connected to the suction of a permeate withdrawal pump (“permeate pump”)  8 ; their lower headers are connected to the discharge of a back-washing pump (“back-pulse pump”). In this configuration, permeate can only be withdrawn from one direction (from the upper header is shown), and the skein can only back-pulsed in the same direction as permeate is withdrawn, that is by back-pulsing through the lower headers. In a skein with relatively long fibers, that is greater than 0.5 meter, it may be desirable to withdraw permeate from both the upper and lower headers for efficiency; and this cannot be done with the FIG. 1 configuration. Skein  2   a  is provided with upper and lower automatic on/off valves A 1  and A 2  respectively, and skein  2   b  is provided with upper and lower automatic on/off valves B 1  and B 2  respectively. Thus to operate the two skeins he requires four on/off valves. A machine program switches valving as required to withdraw peremate from one skein while the other is being back-pulsed as follows: when both skeins are in the production mode, valves A 1  and B 1  are open and valves A 2  and B 2  are closed; when skein  2   b  is to be back-pulsed, valve B 1  is closed and valve B 2  is opened. Back-pulse pump  9  introduces permeate though inlet  5  into the lower header of skein  2   b  and the permeate is discharged through the membranes at  7  into the water in the tank. After skein  2   b  is adequately back-pulsed, it is reconnected by opening valve B 1  and closing valve B 2 ; simultaneously valve A 1  is closed and valve A 2  is opened so skein  2   a  is back-pulsed. 
     In FIG. 2 (reproduced herein), a top plan view, Chmiel illustrates 8 skeins in a radial cluster, and one skein  2   b  is back-washed while the other skeins  2   a  produce permeate. After  2   b  is back-washed, it is re-connected to produce permeate and another skein is back-washed. As in FIG. 1 each skein has an upper and lower valve (not shown) and permeate is withdrawn from the top header of the skein while back-washing with permeate or chemical cleaning is carried out through its bottom header. 
     Because the emphasis in Chmiel is on control of each skein, he failed to realize that he may have been able to operate multiple clusters of skeins without valving the skeins, but valving the cluster, and thus save on the number of valves. The possibility of using multiple clusters did not arise because his goal was to control each skein, not to operate a large commercial filtration plant. 
     A “cassette”, like a cluster, comprises plural modules (skeins) each having opposed upper and lower headers, from at least one of which is discharged permeate in open fluid communication with other headers, the skeins being connected for operation in parallel. Conduits from headers of individual skeins in a cassette are not valved. Each skein is in open fluid communication with a cassette manifold. Preferably a cassette has both an upper and a lower CPM which also functions as a manifold for back-pulsing and chemical cleaning. A cassette is a “movable operating unit” which can be installed in a plant without handling individual skeins. A cassette affords no control over a single skein but facilitates handling, installing and operating multiple skeins as if the cassette were a single skein. A cassette has from 2 to about 40 skeins, depending upon the number of fibers in the skein and theri length. A cassette of long fiber skeins preferably has from 4 to 15, and most preferably from 6 to 12 skeins. In a cassette of vertical skeins, each having upper and lower headers, permeate may be withdrawn from both the upper and lower headers, or only one; similarly, a cassette may be back-pulsed through both the upper and lower headers, or only one. 
     An assembly of cassettes is referred to as a “bank” or “train” of cassettes. A bank of cassettes may be operated as a single cassette. A pump may be used to withdraw permeate from the CPM; the same, or another pump may be used to execute an ancillary function. 
     Even distributing cassettes evenly on either side of a suction line of a cassette permeate manifold (“CPM”), operation of a bank of from 12 to 16 cassettes as a single cassette, results in such a difference in pressure drops between cassettes connected at opposed ends of the CPM and cassettes near the middle, that it greatly affects efficiency both with respect to permeate withdrawal and also to back-pulsing. The term CPM is used specifically for the permeate manifold; the cassette back-pulse manifold is referred to as “CBPM”; and the two together are referred to as “cassette manifolds”. It will be understood that, as used herein, CPM and CBPM refer to manifolds which place each cassette in fluid communication with a permeate pump and a back-pulse pump respectively. 
     Moreover it was not possible to operate other cassettes in the bank while one was being replaced, so that one was required to wait until the cassette was isolated to re-commence operation with the rest of the bank. 
     SUMMARY OF THE INVENTION 
     It has been discovered that, in a large filtration plant, by grouping plural modules (or skeins) of hollow fiber membranes for microfiltration (MF) or ultrafiltration (UF) in a cassette, without selectively isolating one or more, but less than all skeins in the cassette, one can economize on the number of valves used, and allow efficient use of skeins having fibers &gt;0.5 meter long; permeate is withdrawn from both headers of a skein; and back-pulsing liquid is introduced into both headers of a skein. Individual skeins are not provided with automatic on/off valves (“AVs” for brevity). Only the cassette manifold(s) is provided with an AV. Permeate is withdrawn continuously from the bank of cassettes while back-pulsing at least one, but less than all of them, preferably less than half of them, during predetermined back-pulse modes. It is thus possible to generate substantial efficiency and savings in operating the filtration plant. The simple but critical step of grouping modules or skeins into cassettes, each with two AVs, allows efficient and accurate testing for integrity of individual membranes when an isolated cassette is tested. This may be accomplished as often as once a day. 
     It has also been discovered that for an even larger plant, by grouping plural cassettes in “racks” without selectively isolating one or more, but less than all cassettes in the rack, and providing appropriate AVs on the manifolds of these racks, it is possible to generate even greater efficiency and savings than when skeins are grouped into cassettes. A rack is an “immovable operating unit” which is assembled in the field. A rack affords no control over a single cassette but is unexpectedly found to permit accurate integrity testing of all cassettes in the rack. 
     It is therefore a general object to provide a membrane filtration system comprising an assembly of at least 100 immersed modules of fibers, e.g. vertical skeins each having a membrane area of at least 10 m 2 , grouped in cassettes each having from 4 to 30 skeins for operation in parallel in a “bank of cassettes”. Unexpectedly economic operation may be achieved with only two AVs valves on each cassette manifold. The number of skeins in a cassette is limited by the ability to test a cassette for integrity (“integrity test”). The number of cassettes in a bank is limited by the time required for each portion of a complete cycle of permeate withdrawal and back-pulsing. 
     It is also a general object to provide a membrane filtration system comprising an assembly of multiple cassettes, preferably from 2 to 20, grouped in “racks”; plural racks, in turn, are connected for operation in parallel as a “bank of racks”. The number of racks in a bank is limited by the time required for each portion of a complete cycle of permeate withdrawal and back-pulsing. It is essential that multiple, automatically actuated on/off valves be used in combination with means to selectively open and close the valves so as to obtain the desired control of operation of the bank; and, unexpectedly such operation may be achieved with only two automatic on/off rack valves on each RPM. 
     Both cassette AVs and rack AVs are relatively small even for a plant producing 5.284 MM L/day (20 MM gal/day) of filtrate, which plant would typically be constructed with multiple parallel banks (whether of cassettes or racks) to provide operating redundancy. Such valves are relatively inexpensive to purchase and to maintain in operation; and using only two large valves per cassette manifold or rack manifold allows a large plant to be built and operated economically. 
     It is a specific object of this invention to provide a grouping of skeins in a cassette, or cassettes in a rack, which allows one to obtain highly reliable results pertaining to defects in any individual membrane in a module or skein. Particularly in a grouping of plural cassettes as a “rack”, they are simultaneously tested after the rack is isolated. Testing may be carried out for a relatively long time, in the range from 10 min to 1 hr, while continuing to operate those racks which are not isolated in the permeate production mode. 
     It is a specific object of this invention to provide a rack of plural cassettes which produce permeate and discharge it under gravity into a wet well. Flow of permeate from modules is started in the rack&#39;s manifold by generating a vacuum in it, and once started, the flow of permeate is self-regulating. Multiple racks may be operated with a single vacuum generating means in fluid communication with a main manifold, a single large rack valve in each rack manifold, and a single back-pulse on/off valve in a back-pulse conduit to each rack manifold. 
     Other specific objects of this invention include the following: (a) to separate the primary filtration function of the membranes and the ancillary functions by isolating a cassette or rack for any particular ancillary operation, and to do so using AVs to isolate the cassette or rack; (b) to operate a permeate pump continuously so that production of permeate in a bank is not stopped; (c) to operate a back-pulse pump essentially continuously, depending upon the number of cassettes or racks in a bank and the period of an ancillary function in the filtration cycle of a single rack; (d) to use no more than two AVs per cassette or rack, each having an opening/closing cycle less than 30 sec. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The foregoing and additional objects and advantages of the invention will best be understood by reference to the following detailed description, accompanied with schematic illustrations of preferred embodiments of the invention, in which illustrations like reference numerals refer to like elements, and in which: 
     FIG. 1 is a prior art configuration of a cluster of two skeins. 
     FIG. 2 is a prior art configuration of a cluster of eight radially disposed skeins connected so as to allow any one to be isolated while the others produce permeate. 
     FIG. 3 is plan view schematically illustrating a configuration of cassettes in a large filtration plant, only two of the cassettes being shown connected for operation in parallel; the lumens of the membranes are in fluid communication with a CPM through which permeate is intermittently withdrawn. A cassette is isolated when it is to be back-pulsed, or to have some other ancillary function carried out. 
     FIG. 4 is an elevational view schematically illustrating plural modules of hollow fibers in a cassette. 
     FIG. 5 is a process flow diagram schematically illustrating only two of numerous racks of a bank of racks, each rack including four cassettes; each rack may be isolated for an ancillary operation other racks produce permeate; a cleaning system separate from the back-pulsing system is illustrated. 
     FIG. 6 is a plan view schematically illustrating a large filtration plant provided with a single bank of plural racks connected for parallel operation, all racks operating to produce permeate continuously except when one or more individual racks are isolated for an ancillary function. 
     FIG. 7 is a process flow diagram of the invention schematically illustrating a single rack from which withdrawal of permeate relies on gravity and a siphoning effect created by using a deep wet well. 
     FIG. 8 is a process flow diagram of the invention schematically illustrating plural racks in a bank similar to one shown in FIG. 5 except that withdrawal of permeate relies on gravity and a siphoning effect created by using a deep wet well. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     For a large filtration plant having a permeate production in the range from about 3.785 to 18.925 MM L/day (1 to 5 MM gal/day) a method for filtering contaminated fluid through a multiplicity of membrane modules in fluid communication with each other and connected for operation in parallel in a bank, comprises, grouping multiple modules in a cassette without providing valves on conduits connecting skeins in the cassette; connecting multiple cassettes in a bank of cassettes; sequentially isolating at least one cassette, typically from about 6 to 50 percent of cassettes in the bank, for a predetermined period of time sufficient to flow a back-pulsing fluid through the modules; and, continuously withdrawing permeate from cassettes not isolated for an ancillary function. 
     For a larger filtration plant having a permeate production in the range from about 18.925 to 189.25 MM L/day (5 to 50 MM gal/day) filtration comprises, grouping multiple cassettes in a rack; connecting multiple racks in a bank of racks; sequentially isolating at least one rack, typically from about 6 to 50 percent of racks in the bank, for enough time to flow a back-pulsing fluid through the modules; and, continuously withdrawing permeate from racks not isolated for an ancillary function. 
     Multiple cassettes are constructed with vertical skeins of fibers such as are described in U.S. Pat. Nos. 5,914,039; 5,783,083; 5,472,607; and 5,639,373 to Mailvaganam Mahendran et al, and the references cited therein. In such skeins, permeate is withdrawn into a skein discharge line and then into a CPM. Each skein in a cassette is simultaneously back-flushed with permeate at regular predetermined intervals during which permeate withdrawal from those cassettes is necessarily discontinued. More than once a month, and preferably in the range from once a day to once every week, the fibers are back-flushed with an aggressive chemical cleaning solution the composition of which is chosen to destroy foulant deposited on the outer surface of the fibers. Typically a dilute solution of sodium hypochlorite is used. 
     Only in a large filtration system where a bank comprises at least 100 modules or skeins, and typically more than 12 cassettes, does an unexpected economic benefit derive from grouping the cassettes in plural “racks”. Such grouping, in turn, derives from the realization that the capital cost of a large plant, and the cost of its maintenance determine whether the plant is built. Grouping cassettes into racks permits using fewer automatic on/off valves than required if the same number of cassettes were connected individually so that each may be independently isolated. In a smaller plant operating a bank of cassettes may be economical. Fewer valves translates into lower capital costs, and lower maintenance costs. These factors become surprisingly significant in a large filtration plant where economics are critical. 
     In a particular large filtration plant with multiple racks in a bank, permeate from several cassette manifolds is collected in the rack&#39;s permeate manifold (“rack permeate manifold”) (“RPM”); and flow from multiple RPMs is combined in the main permeate manifold (“MPM”) or suction to the permeate pump. The diameters of the manifolds are progressively larger, the skein&#39;s header discharge line being the smallest, the CPM being larger, the RPM being still larger, and the MPM being the largest. This system utilizes automatic control valves and separate pumps—one pump to withdraw permeate and the other to provide the backwash. The key to operation of the novel system is the grouping of cassettes into racks, each comprising multiple cassettes, and the use of fewer automatic on/off valves on the racks than would be required in the prior art system. Sequencing fewer valves provides a saving in down-time, permitting more permeate production time. Carrying out an ancillary function on a rack of cassettes instead of an individual cassette may provide the difference as to whether a plant will be economical. 
     All required operating functions may be performed on all cassettes in the entire bank simultaneously. Because back-pulsing, maintenance cleaning and testing for integrity are to be performed frequently, the economics of doing this are critical. Cassettes are back-pulsed several times an hour; they are chemically cleaned as often as several times a day; and they may be tested for integrity every day. To perform an integrity test, the lumens of the fibers are pressurized with air to a predetermined set-point, typically 20-69 kPa (3-10 psi). Whether a skein passes the integrity test is determined by the measured rate at which the pressure decreases (“pressure decay”) over a fixed time interval, typically 1-15 min. 
     Operation of a bank of cassettes in a tank is analogous to the operation of a single cassette. Preferably both, For a skein with relatively long fibers, typically about 2 meters long, upper and lower headers of each skein are manifolded to a CPM to withdraw permeate for more efficient operation than withdrawal of permeate from only one header. A permeate pump provides desired negative pressure (“vacuum”) for optimum operation of the cassette. After operation for a chosen period from about 1 min to 30 min, typically 15 min, a single cassette is isolated and withdrawal of “production permeate” from that cassette is stopped. A separate back-pulse pump draws permeate from a separate “back-wash or back-pulse” tank, then forces permeate through the lumens of the fibers for a chosen period less than 1 min, typically 30 sec, excluding the time required to open and close the required valves. The back-pulsing is then stopped, the cassette is then reconnected to the bank by opening the shut-off valves which had earlier isolated it, and permeate withdrawal for production is re-started. 
     In the novel filtration plant cassettes may have only two AVs per cassette, as described below, to collect permeate in a CPM. Operation of a prior art plant where only two automatic valves are used on a bank of cassettes, is explained in greater detail below. 
     If the entire bank of cassettes is back-pulsed simultaneously, the valves for isolating a cassette are left open unless the cassette is to be removed and replaced. 
     It is essential to note that sizing the cassette manifold is determined by the required back-pulse flow because this is typically in the range from about 12 to 3 times, preferably from 1.5 times to twice the flow of withdrawn permeate. Therefore it is essential to decide whether ancillary functions will be carried out by isolating an individual cassette, or on all cassettes simultaneously. 
     Referring to FIGS. 3 and 4 there is schematically illustrated a bank, indicated generally by reference numeral  10 , of multiple cassettes, only two of which,  11  and  12  are shown. Each cassette comprises multiple skeins  13  (8 are shown in FIG. 2) of fibers  63  producing permeate which is collected from upper and lower headers  61 ,  62  of the skein in upper and lower permeate manifolds  65  and  66  respectively. Upper permeate manifolds  65  are connected to a CPM  18 ′; lower permeate manifolds  66  are connected to a CPM  18 ″; and the CPMs  18 ′ and  18 ″ of each cassette are connected to branches  15 ′ and  15 ″ respectively of the MPM  15 . When long fibers in the range from 1 to 3 meters long, are used, permeate is withdrawn from both ends as shown in FIG.  2 . AVs  16  and  17  allow one to isolate either cassette for an ancillary function while the other cassettes are in a permeate production mode. Less desirably, permeate from the lower ends of the fibers may be withdrawn through CPM  19  (shown in phantom outline) and teed into CPM  18 ′ beneath valve  16 . Cassette  12  is valved in a manner analogous to cassette  11 . The remaining cassettes are similarly valved and connected at either end of the branches  15 ′ and  15 ″ and in the production mode, manifold  15  is the suction line to permeate withdrawal pump  20 . Permeate from the pump may be returned to back-pulse tank  30  through conduit  33  when valve  38  is opened. When valve  38  is closed, permeate is sent through conduit  21  to the distribution network from the filtration plant. 
     In a plant having 16 cassettes connected, 7 cassettes would be connected for parallel operation on either side of the two,  11  and  12 , shown. 
     For back-pulsing a cassette, permeate is withdrawn by back-pulse pump  25  from a back-pulse tank  30  and led to each cassette through back-pulse conduit  31  branches of which lead to each CPBM through on-off valves  32 . If the fibers are to be back-pulsed with air, an air line is provided with on/off valves. 
     For chemical cleaning, one may choose to clean all cassettes sequentially at some predetermined interval, perhaps once a week. To do so, permeate in the back-pulse tank  30  is replaced with a chemical cleaner which is then pumped through the lumens of the fibers for a period sufficient to accomplish the desired cleaning. When all the cassettes are cleaned, the chemical cleaner is replaced with permeate. 
     For greater flexibility in chemically cleaning the modules in each cassettes it is desirable to have a separate chemical cleaning tank  40  from which cleaning solution is pumped by chemical cleaning pump  27  through discharge line  41  by opening chemicals valve  42  after the cassette is isolated. 
     It is not immediately evident that, in a large plant, the cost of automatic valves on each cassette contribute to the construction of an expensive plant. 
     For integrity testing, air pressure is provided by flow of air through an air line  50  which is split to be connected through air lines  51  and  52  to each CPM. Air lines  51  and  52  are each provided with automatic valves  53  to pressurize a cassette after it is isolated. The rate at which pressure decreases is measured and if too great, the defective fiber(s) is identified and plugged. 
     A “bank” includes at least two racks, typically 4 to 12; each rack preferably includes from 4 to 16, typically 8 to 12 cassettes. If a “rack” included only a single cassette, they would be one and the same. Each “cassette” preferably includes 6 to 15, preferably about 8 modules or skeins. Each skein may have a membrane filtration area in the range from about 10 to 100 m 2 . Permeate from each of the modules is collected in a “skein permeate manifold” and flowed into the “cassette permeate manifold”. In the novel system, at least two cassettes are grouped into a rack which collects permeate in a RPM and the RPMs in the bank collectively flow permeate into the main manifold. 
     Referring to FIG. 5 there is schematically illustrated a single bank  80  of a novel 28 MM L/day system, which bank which includes 12 racks  81  only two of which are shown. Each rack has 4 cassettes  11  (see FIG.  3 ), and each cassette has 8 skeins each having a membrane area of 60 m 2  (645 ft 2 ). Permeate withdrawn from lower headers through un-valved CPMs  82 ′ is coupled with that withdrawn from the upper headers in un-valved CPMs  82 . Each manifold  82  is manifolded to a RPM  83  connected at one end, through an AV  84 , to MPM  85 ; and at the other end, through an AV  86 , to a back-pulse manifold  96 . AVs  84  and  86  place the cassettes  11  of each rack  81  in selective fluid communication with a permeate pump  90  into which permeate flows through MPM  85 . Permeate is discharged through discharge line  88  which is provided with a tee  89  to return permeate through conduit  91  to a back-pulse tank  93 . AV  87  on discharge line  88  may be closed to divert all the permeate to the back-pulse tank and fill it quickly. AV  87  is a large valve which takes 15 sec to open and 15 sec to close. AV  92  shuts off flow of permeate into the back-pulse tank  93  when it is full. The size of valve  92  is dictated by the amount of back-pulsing fluid required for each rack and the rate at which the back-pulse tank is to be replenished. Suction line  94  provides permeate to back-pulse pump  95 . 
     In the permeate withdrawal mode, permeate flows through rack permeate headers  81  into main manifold  85  from all racks with rack valves  84  open, and rack valves  86  and ancillary valves  103  (when a separate chemicals cleaning system is desired) closed. Permeate is discharged from the permeate pump  90  into a permeate distribution network. Valves  84  and  86  are relatively small ATVs which take up to 5 sec to open and 5 sec to close. 
     Back-pulse tank  93  holds permeate which is pumped through conduit  91 , when permeate discharge valve  87  is closed, to replenish the tank. Valve  92  is a medium-size valve smaller than valve  87  which takes up to 15 sec to open and up to 15 sec to close. The quantity of permeate required to back-pulse a single rack being a relatively much smaller amount than that required to back-pulse several racks together, only one rack is back-pulsed while the others continue to produce permeate. 
     When a rack is to be back-pulsed, AV  84  of that rack is closed, and the discharge from pump  95  through discharge line  97  is directed into the isolated rack through its open AV  86 . The discharge line  97  selectively communicates with all racks  81  through AVs  86 . All other rack valves  86  remain closed while the other racks produce permeate drawn through open rack valves  84 . After the chosen amount of back-pulse fluid is pumped through the modules of the isolated rack, rack valve  84  is opened and valve  86  is closed so that rack  81  returns to the production of permeate. The remaining racks can be back-pulsed in sequence without turning off the pump  95 , simply by opening and closing rack vales  84  and  85  as appropriate for each rack. 
     The number of racks in a bank is typically determined by the period of a filtration cycle which includes a production period and a non-production period. The production period is determined by the optimum permeate production over a 24 hour period during which the membranes are to be back-pulsed or otherwise treated several times. 
     A typical filtration period is 15 min, including a back-pulse period of 30 sec.; in addition the non-production period includes the time required to switch the positions of appropriate on/off valves from open to shut, and vice versa (15 sec each switchover for rack valves, two switchovers for each cycle). Net production time is therefore 840 sec out of 900 sec. All on/off valves on the RPMs and on the other larger conduits are necessarily automatic. 
     In addition to back-pulsing, fibers are periodically chemically cleaned (maintenance cleaning) and tested for leaks (integrity testing), each of which functions is to be included in the non-production (non-filtration) period. 
     A chemicals tank  100  holds cleaning concentrate which is pumped through pump  101  into chemicals manifold  102 , branches of which are teed into the RPM  83  of each rack  81 . When skeins in a cassette of a particular rack are to be cleaned, that rack is isolated for back-pulsing by closing valve  84 . While permeate is pumped through open valve  86 , an automatic on/of valve  103  opens and injects concentrate into the manifold  83 . When the cleaning cycle is over, valves  86  and  103  are closed and valve  84  is opened to commence a permeate production cycle. If desired, the chemical cleaning solution may be added in concentrated form to the back-pulse tank  93  with a chemical solution pump  103  through conduit  104  in which an on/off valve  105  is provided. 
     It has been found that an entire rack may be tested to determine the existence of a single incision in a single fiber. The ability to conduct a meaningful integrity test on a single rack is an essential object of this invention. It was not apparent that testing a rack would provide sufficient accuracy and sensitivity of the test. Being able to continue production of permeate from other racks while integrity testing allows frequent testing. 
     To integrity test a rack, branches of an air line  140  are teed into each rack manifold  83 . Each air line is provided with an automatic on/off air valve  141  to pressurize the fibers in each rack. Prior to testing for defects, valves  84 ,  86  and  103  are closed and then air valve  141  is opened. Valve  141  is closed except when integrity testing. 
     The filtration period is a function of the specific substrate being filtered, the specific membrane used, and the required specifications of the permeate. Details of various membranes which may be used in modules are presented in the aforementioned &#39;039 and &#39;607 patents and references cited therein. 
     Allowing for an operating margin so as to allocate 1 min for each ancillary period, the maximum number of racks for sequential switchovers of racks with a filtration cycle of 15 min, is 15. 
     Referring to FIG. 6 there is schematically illustrated a bank, indicated by reference numeral  120 , of 9 racks each identified by reference numeral  122 , such as are illustrated in FIG. 4; each rack includes 4 cassettes each identified by reference numeral  124 , each cassette including 8 vertical skeins each identified by reference numeral  126 . The bank is immersed in a tank  130  which is fed through feed conduit  131 ; concentrate is purged through purge conduit  132 . As in FIG. 5, the bank is provided with corresponding permeate and back-pulse pumps, back-pulse tank, chemical cleaning tank, and air line for integrity testing (not shown for clarity) and on/off valves to provide the functions described in FIG.  5 . It will be evident that the permeate pump and back-pulse pump may be operated continuously which provides longevity for the motor. Further, despite only two AVs per rack, permeate may be withdrawn from both ends of long fibers, and they may be back-pulsed and cleaned from both ends making for minimal pressure drop and efficient operation. 
     When a single one of the 9 racks is isolated for back-pulsing, 11.1% of the bank is isolated. If in addition, a second rack is isolated for integrity testing, then 22.0% of the bank is isolated. For sequential operation of a bank of 15 racks for each of which a 1 minute interval is provided for an ancillary function, 15 racks may be used. When only one rack is isolated, 6.66% of the bank is isolated. The less time accorded for an ancillary function, and the smaller the racks, the more time is available for production of permeate and smaller AVs may be used which are quicker opening and closing. 
     If desired, two or more of the racks may be isolated simultaneously for an ancillary function. For example, one rack may be isolated for once-a-day integrity testing while another may be isolated for routine back-pulsing every 15 min. as long as at least one, but preferably the majority of the racks are producing permeate. 
     In another embodiment in which even fewer rack valves are required, permeate is collected by siphoning it under gravity from modules operated at minimal flux and pressure, in one or more banks, so that they may be operated for a much longer period than is possible by operating the membranes at maximum (or nominal) flux. Operation at very low flux and pressure minimizes fouling. The permeate collection headers from each bank are connected to a common wet well through a siphon arrangement. Air is withdrawn from the permeate collection headers by a vacuum pump to generate a siphoning effect. The transmembrane pressure required for filtration corresponds to the difference between the water levels in the membrane tank and the wet well. Upon evacuating air from the permeate collection headers, permeate flows into the lumens of the fibers under gravity. As a result, permeate flows into a wet well, typically about 10 meters (32.8 ft) deep, which is deep enough to overcome both the transmembrane pressure and losses due to pressure drop in the permeate collection headers, so as to generate the desired siphoning effect. It will be appreciated that the maximum siphoning effect will be generated when the differential head of water is equivalent to one atmosphere, so that the wet well is preferably deep enough not only to provide the desired differential head, but also to store permeate which is not immediately withdrawn from the well. Because flow of permeate is self-regulating, complicated controls are unnecessary. Permeate is pumped from the wet well into the distribution network by a set of high pressure pumps which are controlled by demand. In a large system comprising plural trains operating in parallel, when demand is low and the wet well is full, one or more trains may be shut down. 
     Thus, a gravity-operated system of at least one bank is operated by connecting multiple cassettes for operation in parallel in a rack each cassette comprising plural modules comprising hollow fibers; connecting multiple racks for operation in parallel in a bank of racks; isolating from about 6 to 50 percent of modules in a bank for a predetermined period of time to perform an ancillary operation; providing a wet well to collect permeate; generating a siphoning effect by removing air from the bank&#39;s permeate collection header until the transmembrane pressure and losses due to pressure drop in the permeate collection headers is overcome, and, continuously withdrawing permeate from racks left unisolated. 
     Referring to FIG. 7 illustrates a single rack indicated generally by reference numeral  110 , including 4 cassettes  111  of the “gravity flow” system. Permeate to be withdrawn is combined from each cassette in CPMs  113  which combine the separate discharges from both the upper and lower headers (the coupling loop is not shown). Each manifold  113  is manifolded to a RPM  114  through which permeate is led through an AV  121  to a vacuum generating means  122  such as a pump or steam eductor which generates a sufficient vacuum in the RPM  114  to withdraw permeate into wet well  124  where it is discharged through discharge line  123 . Pump  125  discharges permeate through distribution main  126  into a permeate distribution network. A back-pulse tank  130  is refilled with permeate as necessary (as shown in FIG. 5) and the rack is back-pulsed through back-pulse conduit  132  by back-pulse pump  135 . The conduit  132  is provided with an automatic on/off valve  133  which is closed when the rack is in the permeate production mode. 
     Each rack is isolated and back-pulsed in sequence, and isolated and cleaned or tested for integrity as required, as described hereinabove. When the rack is to be back-pulsed, valve  121  on the rack manifold  114  is closed, and also valves  136  and  141 . Valve  133  is then opened and the rack back-pulsed. 
     As before, it is desirable to provide the rack  110  with a chemical concentrate for maintenance cleaning which is pumped, when required, through chemicals conduit  134  which is provided with AV  136 . Back-pulse tank  130  provides permeate pumped through conduit  132  by back-pulse pump  135 , and conduit  132  is valved with on/off valve  133 . For integrity testing, an air line  140  supplies air which is used to pressurize the fibers in the skeins when AV  141  is opened. 
     Referring to FIG. 8 the “gravity flow” system comprises a bank of plural racks  110  only two of which are shown connected in parallel, each rack including 4 cassettes  111 . Permeate to be withdrawn is combined from each cassette in CPMs  113 . Each manifold  113  is manifolded to a RPM  114  through which permeate is led to a MPM  120  through an automatic on/off valve  121  to vacuum pump  122 , as above in FIG. 6, and permeate is withdrawn into wet well  124  where it is discharged as described. 
     Each rack is isolated and back-pulsed in sequence utilizing equipment described in FIG. 7, and isolated and cleaned or tested for integrity as required, as described hereinabove. In each rack in the permeate production mode, only valve  121  is open. When a specific rack is to be back-pulsed, it is isolated by closing valve  121  on the rack manifold  114 , and valves  136  and  141 . Valve  133  is then opened and the rack back-pulsed. 
     It will now be evident that the benefits of not having to purchase, operate and maintain a large number of automatic control valves for individual skeins in clusters, is unexpectedly great. Savings with fewer AVs realized due to grouping skeins into cassettes for a large plant, and cassettes into racks in an even larger plant makes the difference between a plant being theoretically operable and practically commercial. 
     Set forth in the Comparison Table below is a side-by-side comparison of the Chmiel configuration with its emphasis on isolating individual skeins and that of this invention for a large plant producing 28 MM L/day (7.5 MM gal/day) permeate. Each configuration uses the same number (288) of ZW-650 UF skeins each having a membrane area of about 60 m 2  (650 ft 2 ). Each skein operates at a flux of 60 LMH (40 gal/ft 2 /day). Each cassette has 8 ZW-650 UF skeins. The same number of ZW-650 UF skeins and cassettes are grouped into 9 racks, each having 8 cassettes. 
     Assume a cycle period of 15 min, with 30 sec for back-pulsing and 14.5 min filtration. The 14.5 min for filtration includes 15 sec for opening and closing the appropriate AVs. In the Chmiel configuration, as illustrated in his FIG. 1 or  2 , irrespective of how he clusters the skeins, since a cycle is 15 min, he must program the AVs of 20 skeins (288 skeins divided by 15 min/cycle), two AVs on each skein (a total of 40 AVs). AVs on about 20 skeins must open and close essentially simultaneously in the back-pulse portion of the cycle, while the remaining 268 skeins are in the production mode. 
     For the same 15 min cycle period, with 30 sec for back-pulsing, when the 288 skeins are grouped into 36 cassettes (36 cassettes divided by 15 min/cycle), each cassette with two AVs, the AVs on 3 cassettes (total of only 6 AVs) are required to open and close essentially simultaneously. 
     For the same 15 min cycle period, with 30 sec for back-pulsing, when the 288 skeins are grouped into 36 cassettes, and the 36 cassettes are grouped into 9 racks, only 2 AVs are required to open and close essentially simultaneously because only a single rack need be back-pulsed. 
     It will now be evident that the operation of a multiplicity of AVs reliably at precise intervals is surprisingly simplified when individually unvalved skeins are grouped for operation into cassettes, and for a larger plant, when the cassettes are grouped into racks. Not only are fewer valves required, greatly reducing the capital cost of the plant, but the reliability and simplicity of operating fewer AVs makes the difference between a filtration plant with manageable maintenance requirements, and a plant in which those requirements are not. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 COMPARISON TABLE 
               
             
          
           
               
                   
                 Configuration 
                 Chmiel 
                 Cassettes 
                 Racks 
               
               
                   
                   
               
             
          
           
               
                   
                 No. of banks 
                  1 
                  1 
                 1 
               
               
                   
                 No. of skeins 
                 288 
                 288 
                 288 
               
               
                   
                 No. of cassettes 
                 None 
                  36 
                 36 
               
               
                   
                 No. of racks 
                 None 
                 None 
                 9 
               
               
                   
                 No. of AVs 
                 576 
                 72 
                 18 
               
               
                   
                   
               
             
          
         
       
     
     From the foregoing comparison it is evident that grouping skeins in either “cassettes” or “racks” requires purchasing and operating several-fold fewer AVs than required in the Chmiel configuration, and that grouping will effect substantial savings.