CA1060351A - Microscreening method and apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Drop-back of solids into the drum pool of a rotary microscreen is reduced by applying a limited gas pressure differential across an unsubmerged portion of the screen cloth. The pressure is not for the purpose of increas-ing the driving force, .DELTA. H, of the liquid passing through the cloth. For purposes of this disclosure, .DELTA. H is the pressure differential existing across the screen below the surface of both the drum pool and tank pool. In fact, assuming other factors remain equal, application of a gas pressure differential to an unsubmerged portion of the screen cloth, with consequent reduction in drum pool suspended solids concentration, will normally reduce .DELTA. H even though the gas pressure also acts on the surface of the drum pool. More significantly, the invention enables operation of a microscreen unit at increased flow capacity at a given .DELTA. H. Whatever pressure is applied, there will be some reduction of torque, bearing load and wear at any given .DELTA. H.

Description

;03Sl The invention is directed to microscreening. Mlcro-screening refers to separation of minute particles from dilute aqueous suspension in a rotary drum having micron-sized screen-ing media (referred to as a screen), a spray arrangement for washing the screen and a trough to catch liquids and solids displaced from the interior of the drum. The operation is con-ducted with a substantial percentage of "holes" in the screening medium "open" while the majority of the flow is occurring, e.g.
there is free or unobstructed flow through the open holes. Micro-screening is for example used in the final separation of micro-biological bodies from extremely dilute suspensions, prior to discharge of the water to a water course.
To properly understand the present invention, one must carefully distinguish between microscreening and filtration in general.
The present invention provides a method of microscreening in which a dilute aqueous suspensLon of suspended solids is fed to the interior of a drum having a screening medium on its peri-pheral surface, said drum containing a drum pool of said aqueous suspension and an overlying drum gas space, said drum being rotated to cause successive portions of the medium to emerge from sald drum pool and to pass under a spray of back flush fluid from outside the medium while over a solids collecting zone within the drum, and in which a positive pressure differential, from inside to outside, is applied to said successive portions between their emergence from the drum pool and their passage over the solids collecting zone, said pressure differential being sufflclent to reduce the quantity of particles which drop back into said pool from said successive portions.
The present invention also provides microscreen apparatus 10&i0351 comprising a tank-mounted rotary drum having a screenlng medium ~n its peripheral surface, back flush spray means mounted ou~side the drum above solids co:llection means which are within the drum for catching solids captured by the medium and removed therefrom by the spray means on rotation of the drum, and means for maintaining a pool of dilute aqueous suspended solids and an overlying gas space in said drum, said apparatus also including pressure producing means connected with said gas space or gas exhausting means connected with a hooa surrounding the exterior of said drum, for producing a differential gas pressure through an unsubmerged portion of said medium from said gas space to the exterior of said drum.
Other features and advantages of the invention will become apparent from the embodiment given herein by way of exam-ple with reference to the accompanying drawings wherein:
Figures 1 and 2 are schematic diagrams of liquid-solids separation media illustrating the difference between microscreening and filtering generally;
Figures 3a and 3b are sectional schematic diagrams of microscreen unit illustrating the problem of drop-back;
Figure 4 is a schematic diagram of a section of the screen of a microscreen unit illustrating a theory of operation in respect to the present invention;
Figure 5 is a perspective view, partly broken out, of a microscreen unit embodying the invention;
Figure 6 is an end view of the microscreen unit of Figure 5;

Figures 7 and 8 are side views; partially broken out, of the microscreen unit of Figure 5 and a modification thereof;
Figure 9 is a graph illustrating load conditions of a domestic sewage plant to which the present invention may be responsive;
Figure 10 is a schematic diagram of a control system or use in the present invention;
Figures 11 and lla are sectional schematic diagrams of an additional embodiment of the invention;
Figure 12 is a schematic diagram of a control circuit for the embodiment of Figures 11 and lla;
` Figures 13, 14 and 15 are sectional schematic diagrams of three additional em~odiments of the invention;
Figure 16 on the same sheet as Figures 9 and 10 is a graph illustrating microscreen performance at varying drum speeds with and without the invention.
Figure 1 shows a woven wire filtration medium 1, which is filtering a liquid 2. The diagram shows particles 3 of various sizes dispersed in the liquid,-and many particles 4 of the same material which have become packed into layers or a "cake" on the filtering medium~ The cake or layered particles 4 act as a filter to remove the particles 3 from liquid 2. In order that the layers of particles 4 may be the principal factor in filter-ing out suspended particles 3, the filtration operation is con-ducted so as to pass the majority of the liquid through after the multi-particle layer is in place on medium 1. Liquid which has been separated from the solids departs through the "clean" side 5 of the filter às indicated by arrows 6.

In filtering operations, with the object of enhancing the volume rate at which liquid can be handled without undue es-cape of small particles, filtration aids have been used. Also, the liquid 2 has been pressurized in the attempt to force it through the filter at a higher volume rate.
Microscreening is illustrated by Figure 2. In microscreening, one provides such a large area of screen as compared to the amount of solids which are to be captured on that area between cleanings, that the majority of the liquid is processed through the screen before a cake has formed, The weight of solids (dry basis) captured on a given area of screen per pass ~between washings and ignor-ing any losses due to "drop-back" discussed hereinafter) is referred to as "solids loading". Loading levels indi-cative of screening as opposed to filtering are typically less than 0.5 mg/cm2 but in the case of solids having favourable shape, density and other characteristics, may be as high as about 1 mg/cm2, With relatively dense solids which have absorbed relatively little water, and when using screens in the upper end of the screen aperture range given below the solids loading may be up to 2 mg/cm2 Thus, for instance, in the case of clarified biological sewage treatment plant effluent 2, there are minute particles 3 in suspension, some of which may be agglomerated particles 3a. The openings in the screening cloth 1 may be in the range of, for example, up to about 140 microns, the particular dimensions being selected to be smaller than the overall size of the majority of the agglomerates and discrete particles dispersed in the liquid.

10~;0351 The microscreening influent may contain substantial per-centages of discrete particles on the order of 10 microns or less in size and substantial percentages of agglomerates on the order of 50 microns and larger. The concentration of suspended solids in the influent may be, for example, 20 or more parts per million; and microscreening may, for example, reduce this concentration to 10 parts per million or less.
Because of the inordinately large volumes of liquid in relation to solid material in the influent, and notwithstanding the small sizes of the particles, it is impractical to form and maintain a "cake" to help capture the small particles and some particles 9 and 9a escape through the screen. Therefore, the problem of trying to force liquid through a cake at a high volume rate does not arise. In facb, before or shortly after all of the open holes 8 are blocked, the screening cloth is cleaned to remove particles 4 which have blocked the holes.
Figure 3a illustrates the conventional tech-nique for cleaning the screening cloth and certain pro-blems associated therewith. The figure shows a micro-screen unit which has been cut open perpendicular to the axis of rotation 11 of its drum 12.
In general, the influentl that is the dilute aqueous suspension which is to be subjected to micro-screening, enters the interior of the drum through any suitable inlet 13. Liquid flows from a drum pool 14 established inside the drum through the screening cloth 1 to a tank pool 15 in tank 16, leaving particles 4 deposited on the cloth 1, generally as shown in Figure 2. Effluent liquid in tank pool 15 gathers in 106035~

tank 16 and overflows the upper edge of side wall 17 or any suitable weir or other collection device. It is-conventional to control the level 25 of the tank pool 15 to a predeter-mined level by appropriate adjustment of the collection device, which may include an automatic level control system (not shown), The level of the drum pool surface 23 dif-fers from the level 25 of the liquid in tank 16 and by a distance "d" which varies in accordance with the solids loading, drum speed, the size, shape and number per unit area of cloth openings, extent of agglomeration of the particles in suspension and other factors.
I one observes any given section of screening cloth 1 as it carries captured particles from near the bottom of tank 16 to the apex 28 of the circle in which the drum rotates, it will be seen that the screening cloth inverts, Thus, although the particles 4 tsee Figure 2) are above the screening cloth 1 when it is near the bot-tom of tank 16, thése same particles are hanging on the underside of the cloth when the latter reaches apex 28 ~as shown in Figure 3a). Generally, there is sufficient adherence between the captured particles 4 and the cloth 1 so as to retain appreciable numbers of these particles on the cloth in the inverted position.
This makes it convenient to clean the screening cloth by a reverse-flushing technique. A nozzle 18, shown in Figure 3a, is used to impinge a forceable stream 19 of water on the exterior surface of the cloth, normally at the apex 28. Portions 20 of this stream which penetrate cloth 1 dislodge the captured particles and carry them down into a subjacent trough 21 having side walls 22 above the ~0~0351 surface 23 of the drum pool 14. The solids and wash water collected in trough 21 are removed through a suitable outlet (not shown). As the drawing shows, the drum pool level is maintained below the upper edges of trough side walls 22.
The capacity of conventional microscreen units is impaired, in part, by a problem referred to herein as "drop-back". As each successive portion of screening cloth 1 lifts clear from the surface 23 of the drum pool 14 at 26 some of the captured particles 4 stay in pool 14 or drop back into the pool as sho~n at 27. Since screening capacity is in part a function of the concentration of solids in the drum pool, the concentration increase resulting from this drop back reduces the screening capa-city. The complete explanation of why drop back occurs in micro-screening ~-and how to solve it -- were not widely understood in the art prior to the present invention and the need for a simple and effective solution still exists.
The foregoing is illustrated by Figures 3a and 3b.
Figure 3a shows screen cloth 1 rotating in the direction indicated by arrow r, toward the point 26, at which each successive portion of the cloth emerges or diverges from the drum pool 14. Figure 3b is an enlarged portion of Figure 3a in the vicinity of point 26. Figure 3b shows the screen 1 as having warp threads 30 and weft threads 31, emerging from the drum pool surface 23 at point 26. The same pressure PO prevails outside screen 1 and within the gas space between screen 1 and drum pool surface 23. In the region S' - S",the velocity vectors of the water film which emerges ~tith the screen and of the water which remains in drum pool 14 diverge. The resultant currents "C" weaken the adherence of cer-tain captured particles 4a and draw others 4b away fromthe ~0 screen. There is some tendency for hydraulic pressure in zone S' - S" to retain particles 4b against the screen 1, but this pressure diminishes to zero at S' and point 26 where divergence occurs.
The emerged portion of the screen 1 has many of its openings blocked by captured particles 4, but there may be unob-structèd and partially obstructed openings 8, some of which are the result of drawing away of particles 4b. Films of water 32e and 32i are drawn upwardly with the screen as the drum rotates.
Due to the progressively increaslng inversion of a given portion of the cloth as it rises further from the drum pool 14, gravity causes exterior film 32e to flow over and sag through the screen as indicated by arrows "g", through open holes 8 and elsewhere.
This flow back into the drum swells the interior film 32i at various places 34, and dislodges still other captured particles 4c. Where the flow creates droplets 35 which fall from the screen, these carry particles 4d with them back into drum pool 14 creating the capacity limiting concentration effect discussed above.
There is anothar source of exterior and interior flow-ing water films, which is illustrated by Figure 3a. A portion ofthe back-flush water spray 19 does not go directly through the filter cloth. Rather, it flows down the exterior and interior of the filter cloth from the apex of rotation as indicated by arrows 24i and 24e. The flows 24i and 24e which are opposite to the direction of drum rotation are believed to be capable of dis-lodging or weakening the adherence of captured particles in much the same manner as the films 32i and 32e discussed in connection with Figure 3a. However, when the peripheral speed of the drum is great enough, these opposite direction flows, with their accompany-ing contribution to the concentration of solids in the drum pool, 10~i0351 apparently do not develop sufficiently to create a problem.Whether these opposite direction flows of shower water develop --or prove harmful -- may also depend on shower placement.
Summary of the Invention A limited differential gas pressure is applied across an unsubmerged portion of the screening cloth of a microscreen unit, while rotating the drum, screening and back-flushing. Thus, the invention is clearly distinguishable from pressure filtering techniquQs in which pressure is applied against the submerged area of a fil~er by pressurizing a confined body of liquid against a filter cake.
Thus, at least in respect to those unsubmerged portions of the screen where one wishes to improve captured particle ad-herence, one will provide a higher absolute gas pressure against the inside of the screen than against the outside of the screen.
Thus, the pressure differential from the inside to the outside is positive, The controlled pressure differential is preferably provided adjacent to the screen from the point of screen emer-gence 26 to a point where the screen is over a collection meanssuch as trough 21~ This can be done, for instance, by dividing the space above drum pool 14 so that at least a portion of the space which is between the aforementioned points (and in communi-cation with the screen) can be maintained at a different pressure than the remainder of the space and applying the controlled pressure differential to said portion. But in microscreen units of the type depicted in Figure 3a, wherein there is free communi-cation of air throughout the air space above drum pool 14, such controlled pressure differential may be provided throughout that entire space.

_ g _ Although the pressure differential may be applied in any desired manner, it can be applied very conveniently with a blower or pump having an outlet within the drum, preferably above the drum pool surface. However, if one encloses the ex-terior of the drum with a properly sealed housing, the differ-_ ential pressure may be applied by a suction pump capable of re-ducing the pressure within the housing.
Pressure is not applied indiscriminantly. It is be-lieved beneficial in preventing or reducing drop-back if one applies enough pressure to eliminate or retard entry of exterior-ly flowing water films, e.g. films 32e of Figure 3b and 24e ~left side) of Figure 3a, into the drum through screen 1. At the same time, however, the applied differential is limited for restricting the flow of the air from the space above the drum pool outwardly through the screen, at least in those areas of the screen from which there is danger of drop-back. Thus, in the usual case, the pressure differential is controlled at a level less than that required to break through the segments of water film or other liquid which are maintained by surface ten-sion across unblocked or partially blocked openings in the cloth.
Thus, in accordance with the invention, one appliesa pressure differential which is sufficient to substantially reduce the quantity of captured particles which are returned from the screen to the drum pool by the drop-back problem dis-cussed above, In this connection, pressure differentials in the range of about 0.1 to about 6.0 inches of water gauge and preferably aboùt 0.1 to about 3 inches are typical when the invention is applied to conventional, commercially available -microscreen units. However, operation in the range of from more than six to about 10 and above is also contemplated.

The invention provides advanced methods and apparatus which are of particular interest for dealing with agglomerated solids. These involve: applying the differential pressure only during periods of peak load on the microscreen, and/or boosting shower pressure during at least a substantial portion of the time during which the differential pressure is being applied and/or off-setting, inhibiting or preventing the applica-tion of the applied differential pressure in the zone above the collection means.
A number of advantages of the invention are described below, Among these are reduction of the concentration of solids in the drum pool with consequent increase in the throughput of the microscreen operation and apparatus, Theory of the Invention The theory of operation of the invention can be ex-plained by comparison of Figures 3a, 3b and 4. The latter is similar to Figure 3b and shows how application of a pressure dif-ferential in accordance with the invention is believed to alter the occurrences described in connection with Figure 3b. Thus, as in Figure 3b, Figure 4 shows a screen 1 having warp threads 30 and weft threads 31 emerging from the drum pool surface 23 at point 26. However, in Figure 4, the pressure Pl within the gas space between screen 1 and drum pool surface 23 is larger than P~.
In region S' - S", where the hydraulic pressure formerly approached zero at S', the hydraulic pressure at S' is now aug-mented due to the pressure differential Pl - PO pro,vided acr~ss the unsubmerged portion of the screen. This is true whether the drum pool surface 23 remains above the level of tank pool or is driven below the level of tank pool, The result of this augmenta-tion is to provide added holding force to the captured particles 10~0351 in region S' - S". This apparently increases the tendency for particles 4b which were formerly dislodged by currents c, to remain on the screen. Compare Figures 3b and 4.
The pressure may also reduce or eliminate the currents indicated by arrows g in Figure 3b, or reverse their flow to the direction i~dicated by arrows h in Figure 4. Although the emerged portion of screen 1 may still have unobstructed openings 8, the pressu~e differential Pl - PO prevents or retards sagging of the e~terior film 32e through the screen 1.
Thus, more of the particles remain in place until they arrive over the collection trough (Figure 3a), so they may be deposited therein by the back-flush spray. To the extent that this occurs, there are fewer particles dropping back into the drum pool, and the tendency towards development of excessive concentra-tion in the drum pool is reduced.
In the practice of the invention, pressure is used in a different way to accomplish a different result as compared to conventional filtering techniques. Consider that in a micro-screen unit a H is the liquid driving force across that portion of the screen which is at or below the level of both the tank pool and the drum pool. Struct~ral, effluent quality and other considerations make this ~ H a limiting factor in determining unit throughput capacity. Application o differential pressure in accordance with the invention, which is accompanied by a re-sultant decrease in drum pool concentration, will actually re-duce the ~ H or liquid driving force which is required to screen liquid at a given flow rate. Even more significantly, applica-tion of the invention makes more flow capacity available at a given ~ H, other factors remaining equal.
Specific Embodiment Referring now to Figures 5-7 there is illustrated a 10~03Sl microscreen unit 41 adapted for operation in accordance with this invention. Such a system generally comprises a tank or tub means 42 which forms in its internal portion a tank pool or reservoir 43 (Figure 6). Located in end walls 45a and 45b of tank 42 and its hood 49 are annular drum axle bearing means 47a and 47b which form stationary retaining means about which the microscreen drum 53 (hereinafter described) may rotate. The hood or lid 49 is on top of tank 42 and may extend over the entire top portion of the system so as to prevent splattering of fluid and solids from within the system.
Located within the end wall 45a of tank 42 is inlet conduit 51 which communicates between a point external of the system and the drum pool zone 54, Located within drum 53, positioned longitudinally above its axis and directly below its highest point or apex of rotation, generally illustrated as at point 28 (see Figure 6), is sludge collecting means 55. It includes a collecting trough 57 and a conduit 59 which connects the trough through the end wall 45b with a point external to the tank, ~0 Located at about the apex of rotation 28 and internally of hood 49 are spray means 61. Spray means 61 may assume any con-ventional configuration which generally would comprise fluid conduit 63 and a plurality of axially spaced spray nozzles 65.
While nozzles 65 may be any conventional nozzle currently em-ployed in the art, it is particularly preferred to employ nozzles of the self-purging type such as those prcduced by Lodding Engineer-ing Coxporation of Auburn, Massachusetts. Such nozzles are often known as "self-cleaning showers" and generally comprise a spring actuated plunger which closes down the orifice to form a spray when water pressure is applied behind the plunger. When water - 10~;0351 pressure is eliminated or reduced, such as by turning o~f the water, the plunger retracts and the nozzle opens thus purging it.
While back flush fluid (e.g. wash-water) may be pro-vided by a source external to the system, it is preferred as illustrated in Figures 5-7 to supply thus back flush fluid by means of a pump 67 which draws, for its source of fluid from reservoir 43 and which then sends this fluid under pressure by way of conduit 69, fluid conduit 63 and nozzles 65 to the screen.
In this respect, it is often convenient to provide a manual throttle valve 71 for manually adjusting the pressure to the nozzles 65, For the purposes of this invention, pump 67 may be any conventional type such as a centrifugal pump. However, pump 67 is preferably capable of delivering back flush fluid to con-duit 69 at two or more different pressures, one relatively lower than the other.
Filter drum 53 is rotatably driven by motor 73 which drives the drum 53 by way of rotating axle 75 linked to pinions 77 which are connectingly associated with gear wheels 79 on both ends 81 of drum 53.
While screen 83 may be of any conventional design, the preferred microscreen compxises a perforate supporting member, filter cloth of interwoven strands or filaments defining aper-tures ~therebetween, the apertures being smaller than the open-ings in the pèrforate supporting member, and a locking layer for locking the cloth in engagement with the supporting member. In addition, the loc~ing layer is usually formed of solid material which, at least prior to locking, is soft or softenable under conditions which do not distend or impair the material of the filter cloth and the locking layer has an outer portion fixedly - 14 ~

10~i035~

secured to the supporting member and having an inner portion which includes integral extensions extending through and at least partially overlapping a sufficient number of the filaments or strands of the cloth to securely lock the cloth to the perfor-ate supporting member.
As best illustrated in Figure 5 side wall 42b of tank 42 has therein a spill weir 85 for removing the filtered `'purified" liquid from the system and sending it either to drain or to further processing. Such a spill weir communicates with a spill tank 87 ~Figure 6) and an outlet conduit 89.
In accordance with the invention, the microscreen unit of Figures 5-7 is provided with a shelf 90 mounted on the cover plate 91 which covers the central aperture 92 in annular drum axle bearing 47b. Secured upon shelf 90 is blower 93 having drive motor 94, inlet 95 and outlet duct 96. The duct extends via a water-tight connection through cover plate 91 to a trans-verse offset pipe 97 which in turn connects with riser pipe 98, elbow 99 and outlet 100.
~ rum 53 is structurally and operationally sealed against fluid flow at its ends by walls 81 and a water-tight fit or pack-ing at the joint between the drum axles lOla and lOlb and the bearings 47a and 47b, respectively. In general, there is no direct communication between the interior of the drum and the surrounding atmosphere via drum inlet conduit 51 or sludge trough outlet conduit 59. This can be accomplished, for instance, by providing traps in these conduits. Moreover, the openings in the screen 83 are sufficiently small, and the tenacity of the water films which bridge these openings is great enough, so that a limiteddl~har~e of air from outlet 100 on operation of blower 93 will apply a positive pressure differential 10~0351 across screen 83 from the interior of the drum to the exterior.
This pressure differential is applied to the air above the water in the drum pool and is separate from the hydraulic head ~ H (Figure 3a) which exists across the screen beneath the tank and drum pool surfaces, The output of the blower is preferably controlled to prevent escape of air through screen 83.
Alternatively, the positive pressure differential can be applied in the manner shown in Figure 8, which is in many respects similar to Figure 7, and in which like parts have like reference~numerals. This embodiment also uses a blower 93, but in this case it functions to reduce the pressure in the space between the drum and its housing. Thus, blower 93 can be mounted upon end wall 45b with its inlet duct 95 communicating with the space between the outside of the drum and the microscreen unit hood through an aperture 102 in wall 45b. Operation of the blower causes air to follow the path indicated by arrows 103, thus reducing the pressure on the outside of the screen 83 rela-tive to the pressure of the air within the drum gas space, which is vented to the atmosphere.
As can be seen by a comparison of Figures 7 and 8, use of blower 93 to generate suction ~as shown in Figure 8) reduces the amount of piping required, and generally simplifies the operation. On the other hand, it may necessitate sealing the ends 81 of the drum housing and the conduits, and c~eates the potential for corrosion of the blower by the mist from spray no~zles 65 which is present in the air drawn in through inlet duct 95~ Thus, even though an adequately corrosion-proofed blower 93 may be available, the Figure 7 embodiment is neverthe-less preferred, Preferably, outlet 100 of Figure 7 and inlet duct 95 of Figure 8 are located above the highest expected water level.

10603Sl The spray means 61 and collecting means 55 are fea-tures found in conventional microscreen units. They are usually designed with the object of passing the majority of the wash water from spray nozzles 65 directly through the cloth and into the collecting trough 57. However, in the operation of such conventional units, some of the wash water, and some of the solids which are loosened from the screen, do not reach the trough.
Some of the water does not penetrate the screen, but rather forms an exterior film which clings to the exterior of the screen and eventually reaches the tank pool 43. Some of the water penetrates the screen but forms an interior film which clings to the in-terior of the screen and eventually reaches the drum pool 54.
As each successive portion of the screen passes under the shower, solid particles clinging to the inside of the screen are loosened.
Some of these are dislodged and carried into the trough by that portion of the shower water which reaches the trough.
However, some of the solids loosened by the shower are not dis-engaged from the cloth and may become dispersed in the interior water film. Application of a differential pressure in the em-

2~ bodiment of Figures 5-7 may increase the quantity of water which does not penetrate the cloth and may also force a portion of the interior film to the exterior of the cloth. Such effect is acceptable in most cases, and particularly where the influent is substantially free of "agglomerated" solids. These are solids which are actually clusters of adherent particles which can be broken apart by the shower water, significant quantities of the separable particles being smaller than the screen openings.
A flow of interior water film to the exterior of the drum can carry some of these smaller particles with it, resulting in a limited loss of screening eficiency. Screening efficiency 10603S~

is the percentage of inlfuent suspended solids which is removed.
Provided the quantity of such smaller particles is not too great, units constructed and operated in accordance with Figures 5-7 can collect a greater weight of solids per unit time than they could without the applied differential pressure One might describe this situation in simplified fashion by saying that the increased throughput and solids removal are of greater im-portance than the loss in screening efficiency resulting from tlle presence of the smaller particles.
However, where the influent contains sufficient agglo-merated solids to justify the effort, the screening efficiency of the microscreen units of the invention may be enhanced by one or more techniques described below with the aid of Figures 9 through 15. These techniques include applying the differential pressure only during periods of peak load on the microscreen, and/or by boosting shower pressure during at least a substantial portion of the time during which the differential pressure is being applied and/or by inhibiting, offsetting and/or preventing the application of the applied differential through successive portions of the screen as they pass through the shower.
Application of differential pressure only during peak load conditions provides very suitable design and operational advantages for microscreen units intended for municipal sewage plants. When a microscreen unit is provided with means to apply or increase the differential pressure during periods of peak load and eliminate or reduce the differential pressure during periods of reduced load, it reduces the amount of excess capacity (and therefore capital investment and operating cost) which must be designed into the microscreen unit to handle sel-dom or less frequently experienced peak loads. Consider for 10t;0351 instance the hypothetical time vs. load graph of Figure 9.
The base line of the graph is divided into hours in accordance with the twenty-four hour clock. The vertical axis registers the load imposed on a microscreen unit in units of millions of gallons of liquid input per 24 hour day (MGD). In the early morning hours, load is low. As more and more people arise, load increases. Operation of domestic clothes washers and other morning activities eventially produce a morning peak load, indicated by diagonally cross-hatched area "M" in Figure 9.
After an afternoon lull, there is an evening peak, indicated by diagonally cross-hatched area "e", followed by a lower level through midnight (2400 hours). Each plant has a characteristic pattern which may differ in detail from the hypothetical load curve in the graph, but in the absence of precipitation the same general pattern normally repeats from day to day. However, a sudden heavy rainstorn can produce an unusually high peak indi-cated by hoxizontally and vertically hatched area R, especially where there have been a substantial number of legal or illegal connections of roof downspouts into the sewage system, or where ~0 the sewer line joints inadequately bar infiltration of ground water, In general it has been most common practice to design sewage treatment plants with sufficient capacity to handle the highest peaks in the usual diurnal flow to the plant. In design-ing a plant based on this criterion to handle the load depicted in Figure 9, one would perhaps design for 1.5 MGD. This means that during much if not most of the day the plant operates well below its design capacity. By application of the present inven-tion to a conventional microscreen, it is not unreasonable to expect a one and a half to two fold increase in available capacity lO~iV351 without proportionally increasing unit costs. Thus, a microscreen unit with an effective area and cost approximately consistent with a 1 MGD load can serve the load depicted in the graph. Assuming the influent contains sufficient agglomerated solids to justify it, the system may be equipped with automatic controls to provide the applied differential pressure only when the plant is operat-ing at peak loads indicated by hatched areas M, R and e. Thus, ~hen the load is below a 1 MGD rate, the applied differential pressure may be eliminated and the plant will operate at maximum screening efficiency~ The overall operation, considered on a daily average basis, can provide an overall acceptable effluent quality.
Moreover, as illustrated by the rain peak R, a 1. 5 MGD
conventional plant could occasionally suffer peak loads in ex-cess of capacity, resulting in by-passing of unscreened waste water~to a river or lake. The chances for such an occurrence are reduced by a microscreen unit in accordance with the inven-tion, as it has, at a very reasonable level of capital invest-ment, a considerable latitude in operating capacity. Thus, even where one is applying maximum differential pressure to handle maximum load, resulting in some loss of screening efficiency, the over-all results will be better than if it had been necessary to by-pass the filter and send unfiltered solids to the river or lake~
A unit of the type referred to in the preceding dis-cussion is shown in Figure 10. Figure 10 depicts schematically a microscreen tank 105, fitted with rotatable drum 106. The tank confines a tank pool 107 having a surface 109 while the drum contains, for screening, a drum pool 108 having a surface 110 which is higher than the surface 109 by a distance d. As 10~0351 previously described, the drum screen is washed by spray 113 from nozzle 112, and wash water and solids are collected by subjacent trough 111. Differential pressure is provided by blower 114 having discharge duct 115 and an outlet 116 to th~
air space within the drum 106 above drum pool 108.
The control system includes means responsive to one or more indicators of load (influent solids concentration and/or volume rate of influent liquid). They may for instance take the form of a vent pipe 117 having a closed end 120, an entrance ori-fice 119 and an outlet to the atmosphere 118~ The effective area of the entrance orifice 119 is controlled by the surface 110 of the drum pool 108 thus causing a greater pressure drop across the orifice 119 upon a rising level of surface 110.
This increased pressure drop results in an increase in the pres-sure in the air space within the drum 106. The blower 114 is chosen so that it has a cutoff pressure equal to the maximum pressure desired when the surface 110 has risen to completely block the entrance orifice 119 of vent pipe 117. As the liquid level 110 in the drum pool 108 is affected by load, generally rising in response to increased load, and this same level in-crease will increase the pressure drop acxoss orifice 119, in-creasing the differential gas pressure across the unsubmerged portion of the screen, the system continuously increases and decreases the applied pressure differential in response to in-creasing and decreasing load. Thus, extra capacity is provided when it is needed most, and when the load is relatively low, the applied pressure differential is reduced or eliminated.
Many other types of control systems may be applied to regulating the amount of differential pressure in relation to system load. This may for instance take the form of a tank pool level sensor having a float and a float arm which followa the tank pool level and establishes an electric signal represen-tative thereof. This sensor may connect to a controller via an appropriate circuit. The controller may be any device capable of controlling the applied pressure differential, such as by regulating the blower 114 in a variable speed and/or on-off mode to vary or commence and discontinue application of the applied pressure differential between the inside and outside of the drum. The controller may receive line current via an appropriate cirQuit and ~eèd cu~rent to the blower via another circuit.
The blower may be maintained at low speed or off at lower liquid levels, the blower being energized or speeded up to commence or increase application of the differential pressure as liquid level rises~ On the other hand, the controller could ~ctuate a pressure regulating valve on the inlet or discharge side of a constant speed blower 114 in an open-closed, stepped flow or infinitely variable flow mode to control the applied dif-ferential pressure.
The foregoing are only a few examples of how the applied ~0 differential pressure is reduced (including elimination thereof) at or in response to lower operating loads and increased tinclud-ing being turned "on") at or in response to higher operating loads. Also the operation of blower 114 may be controlled in rasponse to other factors indicative of load, such as for instance, iniluent volume flow rate or suspended solids concentration, viscosity of the slurry discharged from the collecting trough, effluent ~`'clean wàter") suspended solids concentration, and the like.
A particularly preferred control system, depicted in Figures 11, lla and 12, also embodies application of 1~)6~)3S~

differential pressure only during periods of increased load.
This is combined with operating the shower at increased pressure during at least a substantial portion of the time when differen-tial pressure is applied, and operating the shower at decreased pressure during at least a substantial portion of the time when differential pressure is not applied. However, periodic boosting of shower pressure may also be practiced for other pur-poses, such as to occasionally provide more thorough cleaning of the screen. Thus, boosting of shower pressure may be prac-ticed irrespective of whether differential pressure is applied continuously or di scontinuous ly.
Figures 11 and lla illustrate a microscreen unit 141 having tank or tub means 142, tank pool or reservoir 143 with end walls 145a and 145b and annular drum axle bearing means 147a and 147b. In tank 142 is rotatably mounted microscreen drum 153. Means ~not shown) are provided for rotating the drum.
As in prior embodiments, there are a spill weir, spill tank and outlet conduit (not shown) to handle over-flow of "clean" water from tank 142 ~not shown).
One of the annular axles of drum 153 extends through end wall 145a of tank 142 forming a conduit 151. It communicates between a head box 152 and drum pool zone 154. The top of head box lS2 is open to the atmosphere. Supported and suspended within head box 152 is a pair of switches 175 and 177. Through suitable actuating arms these switches are connected respectively with floats 175a and 177a. The floats hang at two different eleva-tions above the top of conduit 151 and in position for floating upwardly on any water which may be rising in head box 152 above the top of conduit 151, thereby operating switches 175 and 177.
Within drum 153 is sludge collecting means 155 with collecting trough 157 and a conduit 159 which connects the trough through end wall 145b with a sludge disposal point with a seal, e.g. a trap, (not shown) outside the system. A blower 190 has its outlet connected with conduit 159. As this conduit is only partly full in normal operation, the outlet of the blower is in communication with the gas space 173 above the drum pool or reservoir 154 through the airspace in the conduit and the open top of trough 157, Blower 190 operates whenever the micro-screen is working.
As in previous embodiments, there are spray means 161, which may include, for instance, a conduit 163, nozzles 165, and a pump 167 (FIGURE 12), This pump may be of any type which is cap-able of delivering back flush fluid to conduit 163 at two or more different pressures. Thus, for instance, the motor 179 may be provided with low speed windings 183 and high speed windings 185. For example, one might choose a pump and motor combination which, in conjunction with other system characteristics, would produce shower pressures of 30 psi and 120 psi respectively.
Figure 12 is merely an example of the type of circuitry which may be employed in conjunction with other systems elements for boosting shower pressure during at least a portion of the time when differential gas pressure is being applied across un-submerged portions of the screen. Thus, for instance, low speed windings 183 may be connected through a master switch l91 with a source of line current 189 so that they are energized at all times when master switch 191 is in the on position. A controller 18~, connected to the line through the same main switch, can be used to control high speed windings 185 with the assistance of the switches 175 and 177 in the head box 152.
Although one of the switches 175 or 177 would suffice 10~351 to control high speed windings 185, certain advantages may be obtained by using two switches. For instance, the controller may include appropriate relay circuitry so that windings 185 will be energized by the raising of float 177a and de-energized by the descent of float 175a. This avoids unwanted "on-off" cycl-ing of the high speed windings, a problem sometimes experienced with single switch controls.
In the operation of this embodiment, the drum pool 154 has a certain level 192 (see Figures 11 and lla) which may be characterized as low level operation. With the level of water in the head box 152 and drum pool 154 at level 192, conduit 151 is partially open. Thus, the gas space 173 of drum pool 154 is in communication with the atmosphere through the open top of head box 152. An increase in load, e.g. influent flow and/or influent suspended solids concentration, will force an increase in liquid level 192. If the load increase is sufficient so that the liquid rises to level 193, the liquid will close conduit 151.
The air delivered by blower 190 through conduit 159 and the open top of trough 157, which formerly escaped through conduit 151, when it was partially open, now pressurizes gas space 173.
If the load increases still further, there can be still further increases in the liquid level to levels 194 and 195. When the liquid level reaches 195 in head box 152, thus raising float 177a, switch 177 will energize high speed windings 185 through controller 187. This, in turn, will increase the sho~er pressure from a first level of 30 psi to a second and higher level of 120 psi. The increase or boost in shower pres-sure is of assistance in recovering more of the solids which the screen transports to the shower ~one. This is accomplished both through more thorough cleaning of the screen and more complete 10~0351 recovery of shower water in the collection trough. With this further improvement, the drum solids concentration is reduced, diminishing ~ H, enabling the unit to handle an even larger liquid flow, while maintaining reasonable concentrations of suspended solids in the effluent. ~hen load decreases to level 194, float 175a actuates switch 175 and controller 189 to de-energize high speed windings 183. Shower pressure returns to 30 psi Further decreases in load -- which cause the liquid level to drop below level 193, unseal conduit 151, open the drum gas space 173 to the atmosphere, and eliminate the pressure differential -- return the unit to low level operation.
The techniques of and apparatus for inhibiting, off-setting and/or preventing the application of differential pressure through successive portions of the screen as they pass through the shower are illustrated by Figures 13-15. The punpose of these embodiments is to apply differential pressure where it is needed, while reducing, eliminating or even reversing the dif-ferential pressure where it is not needed, e.g. across successive portions of the screen passing between the spray means and the trough. Once the concept is understood, many different embodi-men~s can be formulated by those skilled in the art. However, for Simplified illustration and discussion, and to provide a showing of their preferred form, the microscreen units of Figure 13-15 are each individually identical to the embodiment of Figures 5-7, except as otherwise shown and described.
Figures 13-15 disclose the rotary drums 253, screens 283, drum pools 254, drum gas spaces 273, tank pools 243 and spray means 261 of three microscreen units which are alike in all respects,except in respect to certain modified collection troughs and hood arrangements discussed below. Although not 1~0351 shown in Figures 13-15, these three embodiments also include the same kind of tank, drum axle bearings, inlet conduit, sludg-outlet conduit, nozzles, back flush fluid pump, drum driving means, spill weir, spill tank, "purified" liquid outlet conduit, differential pressure blower, associated pipes and ducts for the blower, and sealing means disclosed in Figures 5-7 and the accompanying description.
As shown in Figure 13, one may provide a pressure in-dependent zone 273a between the inside of screen 283 and trough 257a by extending the trough side walls 222 and ends (not shown) upwardly until they touch or nearly touch the screen. Provided the space 230 between the edges of the trough and the inner sur-face of screen 283 is narrow enough, there will be a sufficient pressure drop across this space to make the trough zone 273a at least partially pressure independent from drum pool gas space 273.
In this embodiment, as in Figures 14 and 15, the pre-vailing pressure conditions outside the drum are represented by Pl; in these three Figures the pressure conditions in the drum gas space between the point 226, at which screen 283 emerges from drum pool 254, and the point 235, at which it passes over the edge of collection trough 257a, is represented by P2; and, in Figures 13 and lS, where there is an at least partially pres-sure independent zone 273a for the trough, the pressure conditions therein are represented by P3.
The appropriate pressure conditions for the embodiment of Figure 13 are given by the expression: P2 ~ Pl and P3 ~ P2.
~owever, preferably; P2 ~ Pl ~ P3. The pressure P3 in zone 273a can be provided in any suitable way. Where it is feasible to provide a sufficiently close fit between the trough edges and 10~3Sl the inner surface of screen 283, P3 may be maintained at less than P2 by venting zone 273a to the atmosphere or other zone of reduced pressure relative to P2; or, if desired, the inlet of the blower which pressurizes zone 273 may be connected to the zone 273a to exhaust air therefrom. With blowers of substan-tial capacity, relatively large spacings between screen 283 and trough sidewalls 222 are permissible.
As shown in Figure 14, there may be free communica-tion between the interior of trough 257 and the remainder of the drum pool gas space 273. In such case, the unit may be provided with a hood 210 over spray means 261. One may provide a pressure independent zone 218 in hood 210 between the outside of screen 283 and spray means 261 by extending the hood sidewalls 215 and ends (not shown) downwardly until they nearly touch the screen. When the space 240 is sufficiently narrow, it will pro-vide a sufficient pressure drop to make the hood zone 218 at least partially independent from the remainder of the gas space surrounding the drum, in which pressure Pl prevails.
In this embodiment, as in Figure 15, the-pressure con-ditions prevailing in the hood zone 218 are represented by P4.The appropriate pressure conditions for the embodiment of Figure 14 are given by the expression: P4~ Pl and P2 ~ Pl. However, preferably: P4 ~ P2 ~ Pl. The desired conditions may for instance be produced by connecting the outlet of a blower to hood 210.
Figure 15illustrates employment of both the upwardly extended trough 257a of Figure 13 and hood 210 of Figure 14 to provide two zones 218 and 273a which are, respectively, at least partially pressure independent from the drum pool gas space 273 and the remainder of the gas space surrounding the drum, in ~06035~
which pressure Pl prevails. In this embodiment, the appropriate pressure conditions are defined by the expression P3 - P4C P2 ~ P1, where Pl, P2, P3 and P4 are absolute pressures or prPssures based on a common reference. However, preferably: P2~ Pl and P4 ~ P3. The desired conditions may be produced by one or more blowers or combinations of blowers and vents, as indicated by the discussions of Figures 13 and 14 These embodiments are useful in the situation where one wishes to obtain enhanced screening capacity as compared to the embodiment of Figures 5-7. These embodiments are helpful in improving shower water penetration and recovery. However, these embodiments are also beneficial from the standpoints of improving solids recovery and effluent quality, especially when operating on agglomerated solids. While these embodiments are particularly well suited for operating at high differential pressures, they are useful throughout the entire differential pressure range contemplated for the invention.
As indicated above, the differential pressure may be applied in any desired manner. It has already been shown how blowers may be used for this purpose. The inventors are not aware of any common practice of providing for pressure differ-ences between the air space in a microscreen drum above the drum pool and the ambient, e.g. the air space outside the drum.
The inventors are familiar with microscreen units in which por-tions of the drum are sufficiently ill-fitting so that they would not properly develop the differential pressure with a blower of practical size. To apply the present invention to such units, it would therefore be necessary to provide for or improve upon the sealing off of certain drum components, especially in the ends of the drum and around the e~ges of the media, so that the drum air space can be pneumatically isolated from the ambient while differential pressure is being applied. When an adequately sealed drum is available,one may, if desired, apply the differ-ential pressure by entraining air in the influent liquid or the shower water.
Reduction of drop-back problems by application of the above-described differential pressure enables an increase in drum speed to levels not previously considered satisfactory.
In a conventional microscreen at low drum speed and high solids concentrations in the drum pool, an increase in drum speed pro-duces a near proportional increase in screening capacity per square foot of screen area. However, as the peripheral speed of the screen increases to higher values, the gains in capacity per unit of speed increase taper off. Captured solids fail, in progressively higher proportions to reach the collection trough, while the cumulative flow through a unit area of screen per revolution of the drum decreases. Thus, there is a peak level of drum speed beyond which performance reduces, as illus-trated by the curve labelled "WITHOUT" in Figure 16.
In contrast~ with the applied differential pressure, one or more of the drop-back phenomena is reduced in signifi-cance. As a result, the drum can be rotated faster before per-formance tapers off, and a higher peak is attained. This is illustrated by the curve labelled "WITH" in Figure 16.
Where the liquid undergoing microscreening includes agglomerated or flocculated solids, increases in drum speed --with consequent changes in the drum water velocity vector at the point where the screen leaves the drum pool -- may tend to reduce the degree of agglomeration or flocculation The resultant increase in number of smaller particles present in 10~0351 the drum may increase the overall number of particles escaping through the screen into the effluent per unit of liquid through-put, impairing effluent quality. Where the invention appreciably offsets this trend, it enables increasing the peak-performance speed of a microscreening drum even when operating on agglomera-ted or flocculated solids. However, the peripheral speeds which are useful with agglomerated or flocculated solids will generally be lower than the peripheral speeds which can be used with discrete solid particles.
Operation of a rotary microscreen unit at increased xotational speed increases the operating torque, bearing load and wear rate of the unit. The present invention, in addition to providing the greater throughput capacity discussed above, can also reduce wear, especially when operating on discrete solids at the higher end of the useful pressure range. The ad-ditional buoyancy which the drum has when the air above the drum pool is pressurized and/or when the drum is more deeply immersed reduces the bearing loads, torque and wear accordingly.
With this object in mind, one might select an applied differ-ential pressure in the range of about 3 to about 6 inches of water gauge or from above 6 to about 10 and above.
One might expect that in a conventional microscreen unit, improved performance could be attained by increasing the depth of submergence of the drum, e~g. by raising the level of the tank and drum pools relative to the axis of rotation, and thereby increasing effective screening area. This, unfortunately, causes the drum to emerge from the drum pool at a flatter angle, so that gravity is more effective in prematurely separating cap-tured particles from the screen. Still more significantly, this flatter angle increases the thickness of the exterior water film (32e in Figure 3b) which is in part responsible for gravi-tational flow "g". However, the present invention tends to off-set gravity, improving screen operation at deeper levels of submergence.
By way of illustration, and without intention of unnecessarily limiting the invention, a tabulation is set forth below giving some ranges of system parameters within which the invention will most commonly be practiced. In each case where a pressure is expressed as inches of liquid, it will be understood to be the pressure xequired to support a column of the same liquid being processed through the microscreen, the column height being the number of inches specified in the table.
The items listed under the heading "Range B" are the particularly preferr~d or more commonly encountered values.

Item Range A Range B
(about) (about) Peripheral Drum Speed (feet per minute) 30-240 90-180 Drum Diameter (feet) 2-12 2-6 Drum Pool Distance above 0-,4D 0-.25D
Drum centerline, where "D"
is drum diameter in feet.
Screen Openings Size (Microns) 5-140 20-70 Screen Open Area (percent) 10-60 20-40 H - Hydraulic Pressure Head 0-18 0-10 Across Submerged Portion of Screen (inches of liquid) Applied Differential Pressure 0.1-10 0.1-3 (inches of liquid) Shower Pressure (psig) 20-160 25-100 In~luent Solids Concentration (mg/l) 5-1000 15-100 Influent Solids Particle Size 5-200 20-100 (microns) .

Claims (34)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a microscreening process in which microscreen throughput capacity is impaired by drop back, the improved method of microscreening which comprises:
(A) providing a microscreen unit having a tank with sta-tionary walls for containing a tank pool, a drum mounted for rotation in said tank for containing a drum pool, a screen-ing medium having openings of about 5-140 microns which defines the peripheral surface of the drum and provides communication of liquid between said drum pool and tank pool, back flush spray means mounted above and outside the screening medium and collecting means within the drum beneath the back flush spray means;
(B) feeding as influent into said drum pool a dilute liquid suspension containing about 5 to 1,000 milligrams per liter of suspended particles;
(C) during said feeding of influent, controlling the upper surface of the drum pool at a level, relative to the tank pool level, corresponding to a hydraulic pressure head .DELTA. H
across the submerged portion of the screen of up to about 18 inches of said liquid, said level not exceeding about .4D
above the drum center line, where D is the drum diameter, to maintain a gas space above said drum pool surface;
(D) screening said suspension by passing liquid there-from through said medium and depositing said suspended particles on the interior of said drum at a solids loading of up to about 2 milligrams per cm2 while rotating said drum at a peripheral speed in the range of about 30 to 240 feet per minute;

(E) as the screen rotates, transporting said deposited particles on the inside of the screen over that portion of the gas space which extends from the location where said peripheral surface emerges from the drum pool to a location where it passes over said collecting means;
(F) maintaining a pressure differential in the range of about 0.1 to 10 inches of liquid gauge across the medium, which is positive on the inside relative to the outside, in a portion of the medium which is traversing said portion of the gas space, said pressure differential being sufficient to reduce the quantity of particles which drop back into said drum pool from said screening medium; and (G) directing a spray of back flush fluid from outside the medium through said peripheral surface over said collecting means at a pressure of about 20 - 160 psig for dislodging particles from said screening medium and directing the dis-lodged particles, along with liquid added thereto by said back flush spray, into said collecting means.

2. Process in accordance with claim 1 wherein said liquid suspension is a waste water treatment plant effluent containing microbiological particles.

3. Process in accordance with claim 1 wherein the por-tion of the screening medium to which the pressure differential is applied includes the location at which the screening medium emerges from the drum pool.

4. Process in accordance with claim 1 wherein the portion of the screening medium to which the pressure differential is applied includes the location at which the screening medium emerges from the drum pool and the entire portion of the screen-ing medium which extends from said location to where the screen-ing medium passes over the collecting zone.

5. Process in accordance with claim 1 wherein the applied pressure differential is increased in response to higher operat-ing loads in said drum and reduced in response to lower operating loads in said drum.

6. Process in accordance with claim 1 wherein the applied pressure differential is increased in response to higher liquid levels in said drum and reduced in response to lower liquid levels in said drum.

7. Process in accordance with claim 1 wherein the pressure differential is applied only during peak loads on said microscreen.

8. Process in accordance with claim 1 wherein the solids loading is about 1 milligram per cm2 and the pressure differential is in the range of about 0.1 to 6 inches of liquid gauge.

9. Process in accordance with claim 1 wherein the applied pressure differential is more than six inches and up to about ten inches of liquid gauge.

10. Process in accordance with claim 1 wherein the pressure differential is less than that required to break through segments of liquid film extending across openings in said portion of the screening medium.

11. Process in accordance with claim 1 wherein the back flush spray pressure is increased in response to higher loads in said drum and decreased in response to lower operating loads in said drum.

12. Process in accordance with claim 1 wherein the back flush spray pressure is increased in response to higher liquid levels in said drum and reduced in response to lower liquid levels in said drum.

13. Process in accordance with claim 1 wherein said back flush spray is operated at increased pressure during at least a substantial portion of the time when said pressure differential is increased, and said back flush spray is operated at decreased pressure during at least a substantial portion of the time when said pressure differential is reduced.

14. Process in accordance with claim 1 wherein the applied pressure differential is applied discontinuously during rotation of said drum, said back flush spray is operated at increased pressure during at least a substantial portion of the time when said pressure differential is applied, and said back flush spray is operated at decreased pressure during at least a substantial portion of the time when said pressure differential is not applied.

15. Process in accordance with claim 1 wherein a zone is provided in and adjacent to that portion of said medium traversed by the back flush liquid, which provides pressure outside the medium which is greater than or equal to the pressure inside the medium.

16. Process in accordance with claim 1 where a zone is provided in the drum and adjacent to that portion of the medium traversed by the back-flush liquid spray, which zone is at a lower pressure than that of the gas space

17. Process in accordance with claim 1 wherein a zone is provided outside the drum and adjacent to that portion of the medium traversed by the back-flush liquid spray, which zone is at a higher pressure than the atmosphere surrounding the drum.

18. In a microscreening process in which microscreen throughput capacity is impaired by drop back, the improved method of microscreening which comprises:
(A) providing a microscreen unit having a tank with stationary walls for containing a tank pool, a drum mounted for rotation in said tank for containing a drum pool, a screen-ing medium having openings of about 5-140 microns and an open area of about 10-60% which defines the peripheral surface of the drum and provides communication of water between said drum pool and tank pool, back flush spray means mounted above and outside the screening medium and collecting means within the drum beneath the back flush spray means;
(B) feeding as influent into said drum pool a dilute aqueous suspension containing about 5 to 1,000 milligrams per liter of suspended microbiological particles;
(C) during said feeding of influent, controlling the upper surface of the drum pool at a level, relative to the tank pool level, corresponding to a hydraulic pressure head .DELTA. H across the submerged portion of the screen of up to about 10 inches of water, said level not exceeding about .4D above the drum center line, where D is the drum diameter, to maintain a gas space above said drum pool surface;
(D) screening said suspension by passing water therefrom through said medium and depositing said suspended microbiologi-cal particles on the interior of said drum at a solids loading of up to about 1 milligram per cm2 while rotating said drum at a peripheral speed in the range of about 30 to 240 feet per minute;
(E) as the screen rotates, transporting said deposited particles on the inside of the screen over that portion of the gas space which extends from the location where said peri-pheral surface emerges from the drum pool to a location where it passes over said collecting means;
(F) maintaining a pressure differential in the range of about 0.1 to 6 inches of liquid gauge across the medium, which is positive on the inside relative to the outside, in a portion of the medium which is traversing said portion of the gas space, said pressure differential being sufficient to reduce the quantity of particles which drop back into said drum pool from said screening medium, and less than that required to break through segments of water film extending across openings in said portion of the medium; and (G) directing a spray of back flush water from outside the medium through said peripheral surface over said collecting means at a pressure of about 20-160 psig for dislodging particles from said screening medium and directing the dislodged particles, along with water added thereto by said back flush spray, into said collecting means.

19. Apparatus for microscreening dilute liquid suspensions of suspended particles, said apparatus including:
a stationary walled enclosure for containing a tank pool;
a drum, mounted for rotation in said tank pool, said drum having on its peripheral surface a screening medium with openings in the range of about 5-140 microns, for containing a drum pool of said liquid suspension and an overlying drum gas space;
said drum being connected with means for rotating said drum at a peripheral speed in the range of about 30-240 feet per minute to cause successive portions of the medium to emerge from said drum pool bearing said particles;
particle collection means within the upper portion of the drum;

back flush spray means outside the drum, positioned over the particle collection means, for discharging back flush spray liquid through the medium at a pressure of about 20 to 160 psig, for cleaning the medium and for introducing a mixture of said particles and added back flush spray liquid into said collection means;
said drum being connected with means to provide on said screening medium a solids loading of particles of up to about 2 mg/cm2 per pass;
means, including pressure producing means or gas ex-hausting means, in communication with said drum for maintaining across a portion of said medium in said gas space between where it emerges from the drum pool and where it passes over the solids collecting means, a pressure differential in the range of about 0.1 to 10 inches of water gauge, which is sufficient to reduce the drop back of particles from said portion into said pool.

20. Apparatus in accordance with claim 19 wherein the portion of the screening medium to which the pressure differ-ential is applied includes the location at which the screening medium emerges from the drum pool.

21. Apparatus in accordance with claim 19 wherein the portion of the screening medium to which the pressure differential is applied includes the location at which the screening medium emerges from the drum pool and the entire portion of the screening medium which extends from said location to where the screening medium passes over the collecting zone.

22. Apparatus in accordance with claim 19 wherein the pressure producing means or gas exhausting means is connected to and is operative in response to means for sensing the load in said microscreen apparatus, for increas-ing the applied pressure differential in response to higher operating loads in said apparatus and for reducing said pressure differential in response to lower operating loads in said apparatus.

23. Apparatus in accordance with claim 19 wherein the pressure producing means or gas exhausting means is con-nected to and is operative in response to means for sensing the liquid level in said drum, for increasing the applied pressure differential in response to higher liquid levels in said drum and for reducing said pressure differential in response to lower liquid levels in said drum.

24. Apparatus in accordance with claim 19 wherein the pressure producing means or gas exhausting means is con-nected to and is operative in response to means for sensing the load in said microscreen apparatus, and for applying said pressure differential only during peak loads on said microscreen.

25. Apparatus in accordance with claim 19 wherein the pressure producing means or gas exhausting means is for applying a pressure differential which is less than that required to break through segments of water film extending across openings in said portion of the screening medium.

26. Apparatus in accordance with claim 19 wherein said back flush spray means is connected to and is operative in response to means for sensing the load in said microscreen apparatus, for increasing the back flush spray pressure in response to higher operating loads in said apparatus and for decreasing said back flush spray pressure in response to lower operating loads in said apparatus.

27. Apparatus in accordance with claim 19 wherein said back flush spray means is connected to and is operative in response to means for sensing the liquid level in said drum, for increasing the back flush spray pressure in response to higher liquid levels in said drum and for decreasing said back flush spray pressure in response to lower liquid levels in said drum.

28. Apparatus in accordance with claim 19 wherein means are provided for operating said back flush spray at increased pressure during at least a substantial portion of the time when said pressure differential is increased, and operating said back flush spray at decreased pressure during at least a substantial portion of the time when said pressure differential is reduced.

29. Apparatus in accordance with claim 19 wherein means are provided for applying the pressure differential discontinuously during rotation of said drum, operating said back flush spray at increased pressure during at least a substantial portion of the time when said pressure differential is applied, and operating said back flush spray at decreased pressure during at least a substan-tial portion of the time when said pressure differential is not applied.

30. Apparatus in accordance with claim 19 including an enclosure adjacent to the screening medium and defining a region in which back flush liquid traverses the screening medium, said enclosure being connected with means for controlling the pressure in said region, for maintaining the pressure outside the screening medium relatively greater than or equal to the pressure inside the medium.

31. Apparatus in accordance with claim 1 including an enclosure adjacent that portion of the medium which is traversed by the back flush liquid spray, and means for maintaining the pressure within said enclosure the same as or different from that of the gas space.

32. Apparatus in accordance with claim 31, in which the enclosure is connected with means for maintaining the enclosure at a lower pressure than that in the gas space.

33. Apparatus in accordance with claim 31, in which the enclosure is connected with means for maintaining the enclosure at a higher pressure than the atmosphere surrounding the drum.

34. Apparatus for microscreening dilute liquid suspensions of suspended particles, said apparatus including:
a stationary walled tank for containing a tank pool;
a drum, mounted for rotation in said tank pool; said drum having on its peripheral surface a screening medium with openings in the range of about 5-140 microns, and an open area of about 10-60% for containing a drum pool of said liquid suspension and an overlying drum gas space;
said drum being connected with means for rotating said drum at a peripheral speed in the range of about 30-240 feet per minute to cause successive portions of the medium to emerge from said drum pool bearing said particles;
particle collection means within the upper portion of the drum;
back flush spray means outside the drum, positioned over the particle collection means, for discharging back flush spray liquid through the medium at a pressure of about 20 to 160 psig, for cleaning the medium and for introducing a mix-ture of said particles and added back flush spray liquid into said collection means;
said drum being connected with means to provide on said screening medium a solids loading of particles of up to about 1 mg/cm2 per pass;
means, including pressure producing means or gas exhausting means, in communication with said drum for maintaining across a portion of said medium in said gas space between where it emerges from the drum pool and where it passes over the solids collecting means, a pressure differential in the range of 0.1 to 6 inches of water gauge, which is sufficient to reduce the drop back of particles from said portion into said pool.