Rotary disc filtration device with means to reduce axial forces

Rotary disc filtration devices and filtration processes using those devices are disclosed. The devices have one or more fluid filtration gaps into which fluid to be filtered into permeate and retentate is placed. Each fluid filtration gap is defined by a disc and a filter, one of which rotates with respect to the other. The filter is carried on a filter support member. Fresh feed is introduced to each fluid filtration gap near the longitudinal axis of the shaft on which the discs are rotated. Holes through the disc in the active area of the disc, which is the area opposite the filter, counteract the tendency of the disc and filter to move towards one another.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention concerns the field of filtration and more specifically, 
rotary disc filtration devices. 
2. Background Art 
Filtration devices are used to separate one or more components of a fluid 
from other components. Common processes carried out in such devices 
include classic filtration, microfiltration, ultrafiltration, reverse 
osmosis, dialysis, electrodialysis, pervaporation, water splitting, 
sieving, affinity separation, affinity purification, affinity sorption, 
chromatography, gel filtration, and bacteriological filtration. As used 
herein, the term "filtration" includes all of those separation processes 
as well as any other processes using a filter that separate one or more 
components of a fluid from the other components of the fluid. 
Filtration processes make use of the greater filter permeability of some 
fluid components than others. As used herein, the term "filter" includes 
any article made of any material that allows one or more components of a 
fluid to pass through it to separate those components from other 
components of the fluid. Thus, the term "filter" includes metallic and 
polymeric cloth filters, semipermeable membranes and inorganic sieve 
materials (e.g., zeolites, ceramics). A filter may have any shape or form, 
for example, woven or non-woven fabrics, fibers, membranes, sieves, 
sheets, films, and combinations thereof. 
The components of the fluid that pass through the filter comprise the 
"permeate"and those that do not pass (i.e., are rejected by the filter or 
are held by the filter) comprise the "retentate." The valuable fraction 
from the filtration process may be the retentate or the permeate or in 
some cases both may be valuable. 
A common technical problem in all filtration devices is blinding or 
clogging of the filter. Permeate passing through the filter from the fluid 
layer adjacent to the feed side of the filter leaves a retentate layer 
adjacent to or on that side of the filter having a different composition 
than that of the bulk feed fluid. This material may bind to the filter and 
clog its pores (that is, foul the filter) or remain as a stagnant boundary 
layer, either of which hinders transport of the components trying to pass 
through the filter to the permeate product side of the filter. In other 
words, mass transport per unit area through the filter per unit time 
(i.e., flux) is reduced and the inherent sieving capability of the filter 
is adversely affected. 
Generally, fouling of the filter is chemical in nature, involving 
chemisorption of substances in the feed fluid onto the filter's internal 
(pore) and external surface area. Unless the chemical properties of the 
filter surface are altered to prevent or reduce adsorption, frequent and 
costly filter replacement or cleaning operations are necessary. 
One of the most common causes of fouling arises from the low surface energy 
(e.g., hydrophobic nature) of many filters. U.S. Pat. Nos. 4,906,379 and 
5,000,848, which are assigned to Membrex, Inc., assignee of the present 
application, disclose chemical modification to increase the surface free 
energy (e.g., hydrophilicity) of filter surfaces. (All of the documents 
identified, discussed, or otherwise referenced in this application are 
incorporated herein in their entirety for all purposes.) In general, 
however, relatively little attention has been given to modifying surface 
chemistry to reduce filter fouling. 
In contrast to the chemical nature of most fouling problems, the formation 
of a boundary layer near the surface of the filter is physical in nature, 
arising from an imbalance in the mass transfer of feed fluid components 
towards the filter surface as compared to the back-transfer from the 
boundary layer to the bulk feed fluid. Some form of force (for example, 
mechanical, electro-kinetic) must be used to promote the desired mass 
transfer away from the filter surface. Unfortunately, few strategies have 
been developed that promote adequate back-mixing to reduce the boundary 
layer or prevent its formation. 
The most common strategy is called "cross-flow" filtration ("CFF") or 
"tangential flow" filtration ("TFF"). In principle, the feed fluid is 
pumped across (i.e., parallel to) the outer surface of the filter at a 
velocity high enough to disrupt and back-mix the boundary layer. In 
practice, however, cross-flow has several disadvantages. For example, 
equipment must be designed to handle the higher flow rates that are 
required, and such higher flow rates generally require recirculating 
retentate. However, recirculation can injure certain materials that may be 
present in the fluid (e.g., cells, proteins) and make them unsuitable for 
further use (e.g., testing). 
A different approach to eliminating the stagnant boundary layer involves 
decoupling the feed flow rate from the applied pressure. With this 
approach, a structural element of the filtration device, rather than the 
feed fluid, is moved to effect back-mixing and reduction of the boundary 
layer. The moving body may be the filter itself or a body located near the 
filter element. 
Some of the rare moving-body devices that have enhanced filtration without 
energy inefficient turbulence are exemplified in U.S. Pat. No. 4,790,942, 
U.S. Pat. No. 4,876,013, and U.S. Pat. No. 4,911,847 (assigned to Membrex, 
Inc.). These three patents each disclose the use of filtration apparatus 
comprising outer and inner cylindrical bodies defining an annular gap for 
receiving a feed fluid. The surface of at least one of the bodies defining 
the gap is the surface of a filter, and one or both of the bodies may be 
rotated. Induced rotational flow between these cylinders is an example of 
unstable fluid stratification caused by centrifugal forces. The onset of 
this instability can be expressed with the aid of a characteristic number 
known as the Taylor number. Above a certain value of the Taylor number, a 
vortical flow profile comprising so-called Taylor vortices appears. This 
type of secondary flow causes highly efficient non-turbulent shear at the 
filter surface(s) that reduces the stagnant boundary layer thickness and, 
thus, increases the permeate flux. 
In contrast to classic cross-flow filtration, the devices of U.S. Pat. No. 
4,790,942, U.S. Pat. No. 4,876,013, and U.S. Pat. No. 4,911,847 allow the 
shear rate near the filtration surface and the transmembrane pressure to 
be independently controlled. Furthermore, because those two operating 
parameters are independent and high feed rates are not required to improve 
the permeate flux, the feed rate can be adjusted to avoid non-uniform 
transmembrane pressure distributions. Accordingly, mechanically agitated 
systems of this type enable precise control over the separation. 
Rotary disc filtration devices also allow shear rate near the filtration 
surface and transmembrane pressure to be independently controlled. In such 
devices feed fluid is placed between the disc and oppositely disposed 
filtration surface that define the fluid filtration gap and one or both of 
the disc and filtration surface are rotated. See, e.g., U.S. Pat. No. 
5,143,630 and 5,254,250 (both assigned to Membrex, Inc.). Additional 
documents concerning rotating impellers, rotary discs, filtration, rotary 
disc filtration devices, other filtration devices using mechanical 
agitation, and seals include: U.S. Pat. No. 1,762,560; U.S. Pat. No. 
3,455,821; U.S. Pat. No. 3,477,575; U.S. Pat. No. 3,884,813; 
U.S. Pat. No. 4,025,425; U.S. Pat. No. 4,066,546; U.S. Pat. No. 4,132,649; 
U.S. Pat. No. 4,216,094; U.S. Pat. No. 4,311,589; U.S. Pat. No. 4,330,405; 
U.S. Pat. No. 4,376,049; U.S. Pat. No. 4,592,848; U.S. Pat. No. 4,708,797; 
U.S. Pat. No. 4,717,485; U.S. Pat. No. 4,781,835; U.S. Pat. No. 4,867,878; 
U.S. Pat. No. 4,872,806; U.S. Pat. No. 4,906,379; U.S. Pat. No. 4,950,403; 
U.S. Pat. No. 5,000,848; U.S. Pat. No. 5,599,164; Austrian Patentschrift 
258313; European Published Application Nos. 0 226 659, 0 227 084, 0 304 
833, 0 324 865, 0 338 433, 0 443 469, and 0 532 237; German Patentschrift 
343 144; PCT Published Application WO 93/12859; PCT Published Application 
WO 97/19745 (corresponding to U.S. Pat. No. 5,707,517, owned by Membrex, 
Inc.); U.K. 1,057,015; Aqua Technology Resource Management, Inc., "How to 
Keep Your Fluid Processing Budget from Going to Waste," 3-page brochure; 
Aqua Technology Resource Management, Inc., 4-page brochure (untitled) 
discussing "Technology Background," "Overcoming Concentration 
Polarization," etc.; Fodor, "Mechanical Seals: Design Solutions for 
Trouble Free Sterile Applications," Bioprocess Engineering Symposium, The 
American Society of Mechanical Engineers (1990), pages 89-98; 
Ingersoll-Rand, "Upgrade your entire filtering and/or washing operation 
with the new Artisan Dynamic Thickener/Washer," Bulletin No. 4081, 4 pages 
(2/86); Ingersoll-Rand, "Patented filter/wash capability permits 
simultaneous washing and filtering,"Bulletin No. 4060, 4 pages (8/83); 
Lebeck, Principles and Design of Mechanical Face Seals, pages 17-20, 107, 
146 (John Wiley & Sons, Inc. 1991); Molga and Wronski, "Dynamic Filtration 
in Obtaining of High Purity Materials--Modelling of the Washing Process," 
Proceedings of the Royal Flemish Society of Engineers, Antwerp, Belgium, 
October 1988, Volume 4, pages 69-77; Murkes and Carlsson, Crossflow 
Filtration--Theory and Practice, pages 69-99, John Wiley & Sons, New York 
(1988); Parkinson, "Novel Separator Makes Its Debut," Chemical Engineering 
(January 1989), 1-page reprint by Aqua Technology Resource Management, 
Inc.; Rudniak and Wronski, "Dynamic Microfiltration in Biotechnology," 
Proceedings 1st Event: Bioprocess Engineering, Institute of Chemical and 
Process Engineering, Warsaw University of Technology, Warsaw, Poland, Jun. 
26-30, 1989; Schweigler and Stahl, "High Performance Disc Filter for 
Dewatering Mineral Slurries," Filtration and Separation, January/February, 
pages 38-41 (1990); Shirato, Murase, Yamazaki, Iwata, and Inayoshi, 
"Patterns of Flow in a Filter Chamber during Dynamic Filtration with a 
Grooved Disk," International Chem. Eng., Volume 27, pages 304-310 (1987); 
Snowman, "Sealing Technology in Lyophilizers," in Bioprocess Engineering 
Symposium, The American Society of Mechanical Engineers (1989), pages 
81-86; Todhunter, "Improving the Life Expectancy of Mechanical Seals in 
Aseptic Service," Bioprocess Engineering Symposium, The American Society 
of Mechanical Engineers (1989), pages 97-103; Watabe, "Experiments on the 
Fluid Friction of a Rotating Disc with Blades," Bulletin of JSME, Volume 
5, number 17, pages 49-57 (1962); Wisniewski, "Anticipated Effects of Seal 
Interface Operating Conditions on Biological Materials," Bioprocess 
Engineering Symposium, The American Society of Mechanical Engineers 
(1989), pages 87-96; Wronski, "Filtracja dynamiczna roztworow polimerow," 
Inz. i Ap. Chem., number 1, pages 7-10 (1983); Wronski, Molga, and 
Rudniak, "Dynamic Filtration in Biotechnology," Bioprocess Engineering, 
Volume 4, pages 99-104 (1989); Wronski and Mroz, "Power Consumption in 
Dynamic Disc Filters," Filtration & Separation, November/December, pages 
397-399 (1984); Wronski and Mroz, "Problems of Dynamic Filtration," 
Reports of the Institute of Chemical Engineering, Warsaw Techn. Univ., 
T.XI, z.3-4, pages 71-91 (1982); and Wronski, Rudniak, and Molga, 
"Resistance Model for High-Shear Dynamic Microfiltration," Filtration & 
Separation, November/December, pages 418-420 (1989). 
Conventional rotating disc filter devices utilize stacked filter disc 
arrangements. Historically, most of these devices comprise disc filters 
that are rotated by a central drive shaft to which the filter elements are 
attached. Some rotating disc devices utilize stationary filter discs 
separated from each other by rotary elements attached to the shaft. Murkes 
and Carlsson, Crossflow Filtration--Theory and Practice, John Wiley & 
Sons, New York (1988), FIG. 3.15 at page 91. In this type of device a 
unitary stationary filter element surrounds the central rotating drive 
shaft. 
The effectiveness of rotating disc filtration devices depends in large part 
upon the flowpaths of the feed, retentate, and permeate fluids. Means to 
overcome the potential for buildup of rejected species caused by flowpath 
limitations may involve changing either the rotating disc design (e.g., 
adding blades or grooves), or changing the feed pathways, or both. In some 
designs, feed fluid is introduced near the peripheries of the filter(s) 
and disc(s). In other designs, feed fluid is introduced near the axis of 
rotation (longitudinal axis of the filter(s) and disc(s)) and the feed 
fluid may be delivered to the fluid filtration gap(s) via hollow rotating 
shafts having ports (or nozzles) to direct the feed to either or both 
sides of the filter support members. 
It has been found that in some cases during use of a rotary disc filtration 
device, the disc and its adjacent filter defining the fluid filtration gap 
may contact one another, which is highly undesirable (e.g., the "binding" 
or "rubbing" of disc against filter may significantly increase power 
requirements, the filter may be harmed, and the rotary bearings may suffer 
premature wear or failure). Despite all the development work concerning 
rotary disc filtration devices, the need still exists for rotary disc 
filtration devices that can avoid such contact and the ensuing problems. 
SUMMARY OF THE INVENTION 
Such devices have now been developed. In accordance with this invention, it 
has surprisingly been found that providing, in combination with the other 
elements of the invention, second feed means in the active area of a disc 
defining a fluid filtration gap will significantly alleviate these 
problems and provide other benefits. This was particularly surprising 
because placing the second feed means in the inactive (non-active) area of 
the disc does not appear to alleviate these problems or provide the 
benefits of this invention. The active area of the disc is that portion of 
the disc that is oppositely disposed to the filter's active area (which is 
the "active filtration area"). Thus, it is the active filtration area of 
the filter and the active area of the disc that are oppositely disposed 
from one another across the fluid filtration gap that those two active 
areas define. If the preferred spiral grooves are used on the disc, the 
active area of the disc will typically correspond to the grooved area 
because grooves would typically not be placed on the disc except where 
they were directly opposite the filter to define the fluid filtration gap. 
The second feed means are desirably through-holes (holes) in the disc. Use 
of the second feed means in combination with the other elements of the 
device are presumed to reduce the net forces (pressures) acting on the two 
surfaces defining that gap that tend to move those two surfaces together. 
Other presumed unexpected benefits of the invention are that any 
starvation of the filtration process being conducted in that gap is 
avoided and the tendency for fouling of the filter defining that gap is 
reduced or eliminated. These benefits, as well as others, will be apparent 
to those skilled in the art from this disclosure. 
Broadly, in one aspect this invention concerns a rotary disc filtration 
device for filtering feed fluid in a fluid filtration gap into permeate 
and retentate, the device comprising: (a) a filter support member having a 
major face, the major face having a filter with (i) an active filtration 
area, (ii) a peripheral region, and (iii) a longitudinal axis 
substantially perpendicular to the active filtration area; (b) a disc 
having first and second oppositely disposed major faces, the second major 
face having (i) an active area, (ii) a peripheral region, and (iii) a 
longitudinal axis substantially perpendicular to the active area; the 
active area of the disc and the active filtration area of the filter 
defining the fluid filtration gap therebetween, fluid passing from the 
fluid filtration gap through the active filtration area of the filter 
being the permeate and fluid not passing through the active filtration 
area of the filter being the retentate; (c) rotation means for rotating 
either the disc or the filter around the respective longitudinal axis or 
for rotating both so that the disc and filter rotate with respect to each 
other and a pumping action is created that tends to move fluid in the 
fluid filtration gap from near the longitudinal axis of the filter towards 
its peripheral region; (d) first feed means for feeding feed fluid to the 
fluid filtration gap near the longitudinal axis of the filter; and (e) 
second feed means in the disc for feeding fluid adjacent the first major 
face of the disc through the active area of the second face of the disc to 
the fluid filtration gap. 
In another aspect this invention concerns a rotary disc filtration device 
for filtering feed fluid in one or more fluid filtration gaps into 
permeate and retentate, the device comprising (a) one or more filter 
support members each having first and second oppositely disposed major 
faces, each major face having a filter with (i) an active filtration area, 
(ii) a peripheral region, and (iii) a longitudinal axis substantially 
perpendicular to the active filtration area; (b) one or more discs mounted 
on a rotatable shaft and in alternating interleaved relationship with the 
filter support members to define a plurality of fluid filtration gaps, 
each disc having first and second oppositely disposed major faces, each 
major face having an active area and a peripheral region, the shaft having 
a longitudinal axis of rotation; each fluid filtration gap being defined 
by the active area of one of the discs and the active filtration area of 
the adjacent filter, fluid passing from each fluid filtration gap through 
the active filtration area of the one or more filters being the permeate 
and fluid not passing through the active filtration area of the one or 
more filters being the retentate; (c) rotation means for rotating the 
shaft so that the one or more discs rotate with respect to the filters and 
a pumping action is created that tends to move fluid in the fluid 
filtration gaps in a direction away from the longitudinal axis of the 
shaft; (d) first feed means for feeding feed fluid to each of the fluid 
filtration gaps near the longitudinal axis of the shaft; and (e) second 
feed means in at least one of the one or more discs for feeding fluid 
adjacent the active area of the first major face of the disc through the 
active area of the second major face of the disc to the fluid filtration 
gap defined by that second major face. 
In another aspect this invention concerns a rotary disc filtration device 
for filtering feed fluid in one or more fluid filtration gaps into 
permeate and retentate, the device comprising: (a) one or more filter 
support members each having first and second oppositely disposed major 
faces, each major face having a filter with (i) an active filtration area, 
(ii) a peripheral region, and (iii) a longitudinal axis substantially 
perpendicular to the active filtration area; (b) one or more discs mounted 
on a rotatable shaft and in alternating interleaved relationship with the 
filter support members to define a plurality of fluid filtration gaps, 
each disc having first and second oppositely disposed major faces, each 
major face having an active area and a peripheral region, the shaft having 
a longitudinal axis of rotation; each fluid filtration gap being defined 
by the active area of one of the discs and the active filtration area of 
the adjacent filter, fluid passing from each fluid filtration gap through 
the active filtration area of the one or more filters being the permeate 
and fluid not passing through the active filtration area of the one or 
more filters being the retentate; (c) rotation means for rotating the 
shaft so that the one or more discs rotate with respect to the filters and 
a pumping action is created that tends to move fluid in the fluid 
filtration gaps in a direction away from the longitudinal axis of the 
shaft; (d) first feed means for feeding feed fluid to each of the fluid 
filtration gaps near the longitudinal axis of the shaft; and (e) second 
feed means in at least one of the one or more discs for feeding fluid 
adjacent the active area of the first major face of the disc through the 
active area of the second major face of the disc to the fluid filtration 
gap defined by that second major face, the second feed means comprising 
one or more holes through the disc, wherein substantially all of those 
holes in each disc are located at least about 0.1 R from the longitudinal 
axis of the shaft, where R is the equivalent circular radius of that disc. 
In another aspect, the invention concerns a method for reducing the 
tendency for a rotary disc and a filter in a rotary disc filtration device 
to be forced together by the pumping action caused by the rotation of the 
disc or filter during the filtration process, the rotary disc filtration 
device comprising: (a) a filter support member having a major face, the 
major face having a filter with (i) an active filtration area, (ii) a 
peripheral region, and (iii) a longitudinal axis substantially 
perpendicular to the active filtration area; (b) a disc having first and 
second oppositely disposed major faces, the second major face having (i) 
an active area, (ii) a peripheral region, and (iii) a longitudinal axis 
substantially perpendicular to the active area; the active area of the 
disc and the active filtration area of the filter defining the fluid 
filtration gap therebetween; (c) rotation means for rotating the disc or 
the filter with respect to the other, thereby creating a pumping action 
that tends to move fluid in the fluid filtration gap from near the 
longitudinal axis of the filter towards its peripheral region; and (d) 
first feed means for feeding feed fluid to the fluid filtration gap near 
the longitudinal axis of the filter; the method comprising providing 
second feed means in the disc for feeding fluid adjacent the first major 
face of the disc through the active area of the second face of the disc to 
the fluid filtration gap. 
In another aspect, the invention concerns a method for reducing the 
tendency for a rotary disc and a filter defining a fluid filtration gap in 
a rotary disc filtration device to be forced together by the pumping 
action caused by the rotation of the disc during the filtration process, 
the rotary disc filtration device comprising: (a) one or more filter 
support members each having first and second oppositely disposed major 
faces, each major face having a filter with (i) an active filtration area, 
(ii) a peripheral region, and (iii) a longitudinal axis substantially 
perpendicular to the active filtration area; (b) one or more discs mounted 
on a rotatable shaft and in alternating interleaved relationship with the 
filter support members to define a plurality of fluid filtration gaps, 
each disc having first and second oppositely disposed major faces, each 
major face having an active area and a peripheral region, the shaft having 
a longitudinal axis of rotation; each fluid filtration gap being defined 
by the active area of one of the discs and the active filtration area of 
the adjacent filter; (c) rotation means for rotating the shaft so that the 
one or more discs rotate with respect to the filters and a pumping action 
is created that tends to move fluid in the fluid filtration gaps in a 
direction away from the longitudinal axis of the shaft; and (d) first feed 
means for feeding feed fluid to each of the fluid filtration gaps near the 
longitudinal axis of the shaft; the method comprising providing second 
feed means in at least one of the one or more discs for feeding fluid 
adjacent the active area of the first major face of the disc through the 
active area of the second major face of the disc to the fluid filtration 
gap defined by that second major face. 
The specific design of the rotary filtration device is not critical any 
design may be used so long as the benefits of this invention can be 
achieved. Thus, this invention may be used with any of the rotary disc 
filtration devices disclosed, described, or otherwise referenced in the 
documents referenced herein, including the patents and applications owned 
by Membrex, Inc. 
In preferred embodiments, each disc is generally planar and has two major 
faces and a filter is "oppositely disposed" to each major face of a disc, 
thereby forming two fluid filtration gaps with each disc. In other 
preferred embodiments, three or more fluid filtration gaps are defined by 
pluralities of discs and filter support members. In still other preferred 
embodiments, the discs are mounted on a vertical shaft for rotation, the 
fluid filtration gaps are contained within the body of fluid to be 
filtered (which fluid may be contained within a housing), the periphery of 
the filter support members carry retentate restriction means for 
restricting the flow of retentate out of the fluid filtration gaps into 
the body of fluid, and the bottom filter support member has an opening 
through which fluid to be filtered passes upward and into the fluid 
filtration gaps. In yet other preferred embodiments, the one or more 
rotating discs each has one or more spiral grooves in fluid communication 
with the fluid in the fluid filtration gap. 
The term "spiral" may be defined in many ways but one simple definition is 
that a spiral is the path of a point in a plane moving around a central 
point in the plane while continuously receding from or advancing toward 
the central point. A "groove" is a generally elongate depression, hollow, 
or cavity extending from the surface of the disc or filter to below the 
surface of the disc or filter, where the length of the groove is generally 
parallel to the surface. The "spiral groove" need not be a true spiral 
along the entire length of the groove. 
As used herein, the term "oppositely disposed" means that, for example, two 
surfaces are on opposite sides of the same element, for example, the two 
major faces of a sheet of paper are oppositely disposed, or that two 
elements face one another across some gap or boundary, for example, the 
surface of a disc and the surface of a filter on opposite sides of a fluid 
filtration gap (that is, defining a fluid filtration gap) are oppositely 
disposed. 
The term "substantially parallel" means that the two lines or planes or 
elements that are "substantially parallel" do not form an angle with each 
other greater than about 30 degrees ("substantially parallel" is further 
defined below). 
"Closely spaced" means that two lines or planes or elements are not so far 
apart that they can not interact or work together to perform a desired 
function. Thus, in the case of the facing surfaces of the disc and the 
filter, "closely spaced" usually means that those surfaces are not more 
than about 100 millimeters apart, and in that context, "closely spaced" is 
further defined below. 
In some embodiments, the one or more discs and also preferably the one or 
more filter support members are "suspended from" one or more parts of the 
device that may be collectively regarded as being "the first member." One 
or more rotating members (one or more of the disc(s) and/or the filter(s)) 
rotate during filtration. Thus, a "rotatable suspension" must be used for 
rotatably suspending from the first member the rotatable shaft carrying 
those one or more rotating members. The rotatable suspension may be any 
suitable means, for example, bearings, lip seals, dynamic seals, bushings, 
packing, or packing glands. However, the rotatable suspension will 
preferably be above the normal level of the fluid to be filtered, thereby 
eliminating the need for rotary seals and allowing a generally simpler, 
less costly, and less critical type of rotatable suspension (e.g., a 
simple rotary bearing) to be used. 
The term "suspended from" should be understood to include being attached 
to, being secured to, depending from, and/or hanging from; should also be 
understood to include cantilevered suspension; and should also be 
understood to include suspension that results in any spatial orientation 
(whether vertical, horizontal, or diagonal) of the discs and filters; and 
should also be understood to include both direct and indirect suspension 
(e.g., where a first filter support member is directly suspended from the 
first member and the second filter support member is directly suspended 
only from the first filter support member and not from the first member, 
in which case the second filter support member is said to be indirectly 
suspended from the first member). 
For a device in which the discs and filter support members are suspended 
from the same unitary member, it is clear that they are suspended from 
"the first member." However, for some devices, two or more parts (e.g., 
plates, structural beams, gear box, motor) of the device (some or all of 
which may or may not be fastened together) may constitute "the first 
member." 
One indication of whether two or more parts of the device collectively 
constitute "the first member" is whether they can be (but do not 
necessarily have to be) removed together from one or more other 
significant parts of the device (e.g., the rest of the device or the rest 
of the housing or the vessel portion of the device that holds the fluid to 
be filtered) to remove the disc and filter support members together from 
the other parts of the device. Accordingly, if the discs and filter 
support members can be removed together from the device by removing 
together the one or more device parts from which the discs and filter 
support members are suspended, those one or more device parts from which 
the discs and filter support members are suspended can collectively be 
regarded as "the first member" and the discs and filter support members in 
this device are "suspended from the first member." Furthermore, in that 
device "the first member" is considered to be "removable." "Removability" 
of the first member can allow the filter support members and discs to be 
removed as a unit, e.g., for maintenance and without having to disassemble 
the rest of the device. 
There is another way to consider whether the discs and/or filter support 
members are "suspended from the first member," which can be used, for 
example, for a device in which the discs and filter support members are 
suspended from one or more parts of the device and those one or more parts 
of the device are generally not removable from other significant parts of 
the device. Such a device, for example, may be one where the discs and 
filter support members hang from the top of a device and the top (which 
may comprise one or more parts) is non-removable from the rest of the 
device, including the several legs on which it stands (e.g., stands in a 
lake or other body of fluid). In this device, the discs and filter support 
members are also considered to be "suspended from the first member" 
because the discs and filters are all suspended in cantilevered fashion in 
"generally the same direction" (because they all hang down from the top). 
The direction of suspension is the overall direction of suspension from the 
supporting member to the supported member, which direction ignores any 
curves or bends. By "generally the same direction" is meant that the 
direction of suspension of the discs and the direction of suspension of 
the filter support members are at no more than an acute angle to each 
other, i.e., an angle less than 90 degrees, desirably less than 45 
degrees, more desirably less than 30 degrees, preferably less than 15 
degrees, and most preferably are not at an angle to each other exceeding 5 
degrees. 
Suspension of discs or filter support members from the first member is not 
inconsistent with the discs, the filter support members, the assemblage of 
discs and filter support members, and/or the shaft carrying the discs from 
contacting or being stabilized by or being attached in some way directly 
or indirectly to another part of the device or to a part of the "natural 
vessel" (e.g., the bottom of a lake) holding the fluid to be filtered. 
One or more of the filters/filter support members defining a fluid 
filtration gap may (but do not necessarily) have restriction means for 
restricting (and also directing) the flow of retentate out of that fluid 
filtration gap into the body of fluid. Without any restriction means, the 
retentate leaving the one or more fluid filtration gaps flows into the 
body of fluid more radially distant from the axis of rotation 
(longitudinal axis) than the outer periphery of the discs and filter 
support members. The rotational velocity component of the retentate moving 
radially outside of the fluid filtration gap(s), which rotational 
component is imparted by the rotation of the one or more discs or filters, 
causes the fluid in the body of fluid radially outside the fluid 
filtration gap(s) to rotate in the same direction as the discs or filters 
rotate. Rotation of that radially distant fluid, which rotation can be 
quite vigorous, in turn tends to make it more difficult to accomplish 
flotation of less dense materials or settling of denser materials in the 
same vessel, if such flotation is desired. The rotation of that radially 
distant fluid also tends to cause gas (e.g., air) to be sucked into the 
fluid to be filtered. 
Consequently, controlling the effluent flow of retentate from the 
peripheries of the fluid filtration gap(s) is generally desirable. Such 
control may be accomplished by creating a barrier or dam near the outer 
periphery of the filter support member(s) to significantly restrict the 
egress of retentate from the fluid filtration gap(s) into the radially 
distant liquid. A complete barrier would prevent any retentate from 
leaving the gaps and would substantially prevent any rotation of the 
radially distant liquid. Because it is usually desirable to allow some 
retentate to leave the gap(s), openings may be placed in the barrier or 
dam. Also, directing the flow retentate effluent against the direction of 
rotation will tend to counteract that rotational velocity component and 
decrease the tendency of the radially distant fluid to mix or rotate. 
Means for directing the retentate effluent flow may be openings or nozzles 
in or on the barrier pointed against the direction of rotation. The 
openings or nozzles or other means may direct the retentate effluent in 
any other suitable direction. Using the barrier or dam tends to prevent 
undue agitation (e.g., swirling) of the body of fluid and allows 
establishing quiescent zones in the vessel, e.g., to allow flotation of 
less dense material and settling of denser material. The restriction means 
may be thought of as not only substantially decoupling the flow pattern in 
the fluid filtration gap from the flow pattern in the body of feed fluid 
but also substantially decoupling the pressure in the fluid filtration gap 
from the pressure in the body of feed fluid. Thus, the restriction means 
may allow the pressure in the fluid filtration gap(s) to be considerably 
higher than the pressure in the body of feed fluid. 
Desirably, filter support members are used that can be easily put into or 
removed from their position with respect to the rotatable discs to avoid 
the need to remove the discs from the shaft to allow removal of the filter 
support members. Such easily removable filter support members may have any 
shape but will be generally be D-shaped or circular in plan view. In 
either case, a cut-out can provide clearance, e.g., for the rotatable 
shaft on which the one or more discs are mounted. Two generally D-shaped 
filter support members may be put into position (proximate a disc) so that 
their straight sides are facing one another, thereby together forming a 
generally circular assemblage. In that case, each filter support member 
will have located in the middle of its straight side a generally 
semi-circular cut-out for the shaft or suspending sleeve. A generally 
circular easily removable filter support member will usually have a radial 
cut-out that extends from the periphery to the center of filter support 
member to provide the necessary clearance for the shaft or sleeve. No 
matter what the shape of the filter support member, two or more filter 
support members may be mechanically connected to permit them to be moved 
as a unit (a filter support member cartridge) into and out of position 
with respect to the discs. 
Other features, aspects, and advantages of the invention will be apparent 
to one skilled in the art.

These drawings are provided for illustrative purposes only and should not 
be used to unduly limit the scope of the invention. 
DETAILED DESCRIPTION OF THE INVENTION 
The design of the rotary filtration device of this invention is not 
critical and any design may be used so long as the device meets the 
requirements of the claims and affords the benefits of this invention. 
Thus, it is within the scope of this invention to have a rotating disc 
surface itself also be at least in part a filter surface although that is 
not preferred. It is also within the scope of the invention to have two 
oppositely disposed closely spaced filtration surfaces define the fluid 
filtration gap and to have one or the other or both of the surfaces 
rotate, in which case one of the filtration surfaces would be considered 
to be the disc. Accordingly, use of the term "disc" does not preclude its 
surface facing and helping to define the filtration gap from also being a 
filter surface. Similarly, use of the term "filter" to refer to an element 
through which permeate passes and whose surface is the second surface 
facing and helping to define the fluid filtration gap does not preclude 
the filter surface from rotating. Preferably, however, only the discs 
rotate, the discs do not have filtration capability, the filters (and 
filter support members, which carry the filters) are not rotatably 
suspended and therefore do not rotate, and all filtration capability 
resides in the filters. 
If the filter facing and helping to define the fluid filtration gap is to 
have any grooves or blades or other protuberances, the filter should be 
rigid enough to hold the requisite shape. In that case, rigid filter 
materials such as metal (e.g., sintered metal), ceramics, or glass might 
be suitable. It is preferred, however, that the filter itself not contain 
any groove or blades and that the disc surface helping to define the fluid 
filtration gap contain any grooves or blades that are used. 
The filter may be made of any material so long as the filter can perform 
the functions required in accordance with this invention and is otherwise 
chemically and physically suitable under its respective operating 
conditions. Accordingly, the filter may be polymeric, metallic, ceramic, 
or of glass, and may be of any form or shape. Thus, the filter may be 
formed of particles or of a film or of fibers or of a combination of all 
three. The filter may be woven or non-woven. Generally, non-woven metal 
filters have certain advantageous features as compared with polymeric 
filters: they are easier to sterilize; generally have superior chemical 
and heat resistance; may be cleaned more easily; and have significantly 
better structural integrity and rigidity. If two or more filters are used 
in a device, they may be of the same or different material and filtration 
or sieving characteristics. 
The filter used may be an asymmetric surface filter. An asymmetric surface 
filter is a filter whose two major faces have different distributions of 
pore sizes such that the average or median pore size on one face is 
significantly smaller than the average or median pore size on the other 
face. Desirably, the asymmetric surface filter is oriented in a device of 
this invention with the face having the smaller average or median pore 
size facing the fluid filtration gap and the face with the larger average 
or median pore size facing away from the gap. A preferred metal filter of 
this type is the DYNALLOY fiber metal filter marketed by Fluid Dynamics of 
DeLand, Fla. The use of a metal filter may be advantageous if one or more 
electric fields are also being used in the device or if the filter is to 
carry a charge. 
One or more electric fields may be applied in axial, or radial, or 
non-radial non-axial directions. The fields may be useful in aiding 
separation and can be applied using known technology. As used herein, 
"axial" means along or parallel to the axis of rotation of the one or more 
rotating members and "radial" means along or parallel to a radius of the 
plane of a disc or filter (i.e., perpendicular to the axis of rotation of 
the one or more members). The field may be the result of direct or 
alternating voltage, e.g., a high frequency alternating potential. One or 
more fields in different directions may be applied, which together will 
result in a single imposed field. One or more fields may be varied as a 
function of time, e.g., one radial field and one axial field in 
interleaved on/off synchronization. Thus, the term "an electric field" as 
used herein should be understood to include all of the foregoing. 
The key function of a filter is to freely pass the permeate and not pass 
the retentate. To do that efficiently, the permeate should adequately 
"wet" the filter. One indicator of wetting is the contact angle a drop of 
permeate forms when placed on the filter surface (see U.S. Pat. Nos. 
4,906,379 and 5,000,848). Generally speaking, the lower the contact angle, 
the greater the wetting, and, conversely, the larger the contact angle, 
the lesser the wetting. 
A drop of permeate recovered using a device of this invention will usually 
have a contact angle on the filter used in that device of less than 45 
degrees, desirably less than 40 degrees, more desirably less than 35 
degrees, most desirably less than 30 degrees, preferably less than 25 
degrees, more preferably less than 20 degrees, and most preferably less 
than 15 degrees. The contact angle is measured using the method described 
in U.S. Pat. No. 4,906,379 (see, e.g., column 10, line 42 et seq.) and 
U.S. Pat. No. 5,000,848 (see, e.g., column 12, line 46 et seq.). 
Because water is a high energy liquid, principally because of hydrogen 
bonding, and because water is often a permeate in filtration processes, 
hydrophilic filters are preferred for use in the device of this invention. 
Filters whose surface energy has been increased to increase their 
hydrophilicity may be used. Thus, filters having a high surface energy 
(e.g., those of regenerated cellulose and those in accordance with U.S. 
Pat. No. 4,906,379) are a preferred class of filters. Such filters are 
more easily wet by polar substances, such as water, but resist wetting by 
non-polar substances such as organic compounds. Such high energy filter 
surfaces also have a reduced tendency to become fouled by materials having 
low energy properties, such as proteins and other organic substances. 
Preferred filters used in this rotary disc invention are made in 
accordance with U.S. Pat. No. 4,906,379 and are marketed by Membrex, Inc. 
under the trademark UltraFilic.RTM.. The UltraFilic.RTM. membrane is made 
of modified polyacrylonitrile (PAN) and its surface is chemically modified 
to be extremely hydrophilic ("hyperhydrophilic"). 
A device of this invention using a filter that allows water to pass 
(permeate) but rejects oil will find particular use in separating water 
from oil, e.g., in cleaning up oil spills or in recycling aqueous cleaning 
solution in a parts washing system. Alternatively, a filter that is 
relatively hydrophobic (low surface energy) and allows oil to pass and 
rejects water may be used. Other especially advantageous combinations of 
the device of this invention and filters having certain inherent 
properties (e.g., high rejection rate of certain materials but rapid and 
easy permeation of their co-components in the feed fluid) will be apparent 
to those skilled in the art. Use of such filters in combination with the 
device of this invention will provide advantages that may not be 
achievable without the combination. 
The filter may have pores of any size or shape provided they are 
appropriate for the feed fluid and the permeate and can provide the 
separation desired. The filter may have a narrow or broad or other 
distribution of pore sizes and shapes and may be asymmetric and used as an 
asymmetric surface filter. The filter may have a relatively sharp 
molecular weight cut-off point. 
The filter matrix, and particularly a polymeric filter matrix, may also 
have ligands attached to it for selective sorption applications (e.g., ion 
exchange/sorption, affinity sorption, and chelation). Suitable ligands 
include any ligand capable of attaching to the matrix or to a precursor or 
a derivative of the matrix. 
Preferred ligands comprise (a) ion-selective affinity groups (such as 
chelator and cage types) that selectively bind inorganic ions and (b) 
bio-selective affinity groups that selectively bind biologically active 
substances. The inventory of affinity ligands is large and rapidly 
increasing. Most often, such ligands are derived from nature (i.e., 
substances of biological origin) while others are wholly or partially 
synthetic (i.e., bio-mimic substances). Preferred ligands, preferred 
methods for attaching ligands to membrane filters, and preferred membrane 
filters are taught in U.S. Pat. No. 4,906,379. Other useful ligands and 
methods for attaching the ligands to the filter will be known to those 
skilled in the arts of affinity sorption, enzyme immobilization chelation, 
and the like. As used herein the term "selective sorption ligands"includes 
all of the foregoing ligands. 
Almost any fluid to be filtered can be filtered using a device of this 
invention, but it finds particular use in filtering feeds having high 
solids content, mixed phase fluids, and biological fluids. 
High solids content fluids may be, for example, biological fluids, fluids 
containing affinity particles (e.g., selective sorption affinity 
particles), particles of ion exchange resin, catalyst particles, adsorbent 
particles, absorbent particles, and particles of inert carrier. The inert 
carrier particles may themselves carry catalyst, resin, reactants, 
treating agents (e.g., activated charcoal), etc. Mixed phase fluids 
include liquid/solid, liquid/liquid, and liquid/gas systems. The fluid may 
contain more than two phases. The liquid phases may all be aqueous or 
non-aqueous or may be one or more aqueous phases and one or more 
non-aqueous phases together. The phases may be immiscible, e.g., two 
aqueous phases that are immiscible because each phase has a different 
solute. The fluid may have gaseous, liquid, and solid phases. Reaction 
and/or heat transfer may accompany the filtration process of this 
invention and take place inside or outside a device of this invention. 
Biological fluids are fluids that originate from or contain materials 
originating from biological organisms (e.g., from the animal or plant 
kingdoms) or components thereof, including living and non-living things 
(e.g., viruses). Thus, the term "biological fluids" includes blood; blood 
serum; plasma; spinal fluids; dairy fluids (e.g., milk and milk products); 
fluids containing hormones, blood cells, or genetically engineered 
materials; fluids from fermentation processes (including fermentation 
broths and reactant, intermediate, and product streams from beer-making 
and wine-making, and waste water treatment streams); fluids containing or 
consisting of microbial or viral material, vaccines, plant extracts, or 
vegetable or fruit juices (e.g., apple juice and orange juice); fluids 
containing microorganisms (e.g., bacteria, yeast, fungi, viruses); and so 
forth. The device is particularly useful with fluids containing 
pressure-sensitive or shear-sensitive components, e.g., cells (blood 
cells; mammalian hybridomas; pathogens, e.g., bacteria in a fluid sample 
that are being concentrated to allow detection; etc.). It is useful for 
filtering fluids containing drugs and precursors and derivatives thereof 
It is also useful for filtering organic compounds in general (including 
oils of all types, e.g., petroleum oil and food oil) as single or mixed 
phases (e.g., oil/water). It is also useful for filtering fluids 
containing surfactants, emulsions, liposomes, natural or synthetic 
polymers, waste waters from deburring and polishing operations (e.g., 
tumbling and grinding fluids), industrial and municipal waste waters, and 
aqueous, semi-aqueous, and solvent-based cleaners. 
A plurality of discs and/or a plurality of filter support members, which 
carry the filters, may be used in a device according to this invention. 
Thus, it is within the scope of the invention to have a single disc 
disposed between two filters, thereby defining two filtration gaps. In 
such a device, one or both of the major faces of the disc would desirably 
each have at least one spiral groove. It is also within the scope of this 
invention for such a device to have several alternating interleaved discs 
and filter support members, that is, discs and filter support members in 
alternating arrangement, so that several filtration gaps are defined. In 
that case, the discs could be mounted on a common shaft for rotation in 
unison and the permeate from the filters could flow to a common manifold 
for collection. In a device having a plurality of interleaved discs and 
filter support members, each surface defining a fluid filtration gap may 
have one or more spiral grooves. 
Regardless of which elements (that is, the filter(s), the disc(s), or 
combinations thereof) rotate, rotation may be at a constant speed or at 
varying speeds and in a single direction or in alternating directions. If 
two or more members rotate, they may rotate in the same or different 
directions and at the same or different speeds. The rotating member(s) may 
periodically reverse its or their direction(s) of rotation (i.e., 
oscillate). At least one of each disc and filter pair defining each fluid 
filtration gap should rotate with respect to the other. Thus, the filter 
and disc defining a fluid filtration gap should not rotate in the same 
direction and at the same speed. Preferably the filter or filters (and 
therefore the filter support member or filter support members) are 
stationary and the disc or discs rotate and only in a single direction of 
rotation. Withdrawal of permeate that passes through the filters is 
simplified if the filter support members are stationary during filtration. 
The disc(s) and/or filter(s) may translate axially (reciprocate) 
approximately perpendicular to the plane of rotation) whether or not it or 
they are the rotating element(s). The disc(s) and/or filter(s) also may be 
vibrated or oscillated to aid filtration. 
Each filter is desirably mounted on a filter support member, which 
functions to support the filter and/or to provide a collection network for 
the permeate. Such a support is desirable, particularly if the filter does 
not itself have substantial structural rigidity. Preferably, a network of 
permeate collection passageways is disposed in the filter support member 
in fluid communication with the downstream side of the filter (facing away 
from the fluid filtration gap) so that permeate passing through the filter 
flows into the permeate collection passageways. Any method of mounting the 
filter on the filter support member may be used provided it does not 
unduly hinder operation of the device. Preferably, the method of mounting 
the filter does not significantly reduce the active filtration area of the 
filter but such reduction may be necessary in some cases. 
The filter support member may have any size or shape provided that the 
advantages of the invention can be achieved. Two or more filter support 
members may be arranged in a plane to form a filter support member 
assemblage that helps define a fluid filtration gap. Thus, for example, 
two D-shaped members (with semi-circular cut-outs for the shaft, etc.) may 
be placed with their straight sides near each other to define a filter 
support member assemblage having a circular outer periphery. 
Desirably, each of the one or more of the filter support members defining a 
fluid filtration gap can have near its periphery restriction means for 
restricting (and also directing) the flow of retentate out of that fluid 
filtration gap into the body of fluid. If the restriction means are high 
enough (i.e., extend sufficiently away from the plane of the filter 
support member, e.g., perpendicularly or diagonally away from the plane of 
the filter support member), they may come close to or touch the adjacent 
filter support member. In that case, the restriction means may be thought 
of as forming a wall separating a regime of more intense shear and fluid 
movement (the fluid between the discs and filter support members, and the 
fluid between the peripheries of the discs and filter support members and 
the inside surface of the restriction means) from a regime of less intense 
shear and fluid movement (the rest of the body of fluid, including the 
volume radially distant from the outside surface of the restriction means 
and the volume axially remote from, i.e., axially outside of or beyond, 
the two outer filter support members). 
The restrictions means can also be used to separate a region of higher 
pressure (an inner region whose outer boundary is the restrictions means 
and, for example, the two outer filter support members) from a region of 
lower pressure (the region outside of the inner region, i.e., the body of 
fluid to be filtered). A higher pressure can be developed in the fluid 
filtration gap for a given fluid by adjusting the geometry of the device 
and the rotation speed. The geometry of the device includes the size and 
shape of the two surfaces defining the gap, the smoothness of those 
surfaces, the width of the gap, whether there are any grooves or blades or 
other concavities or convexities on either surface, and, if so, their 
number, size, shape, and relative position. 
If the fluid in the appropriate parts of the regime of less intense shear 
and fluid movement moves slowly enough and if the fluid properties (e.g., 
surface tension, viscosity, and density) are satisfactory, flotation and 
settling may be conducted in this regime. That is useful, e.g., in the 
separation subsystem of an aqueous parts washing system, where oil removed 
from the parts by the cleaning solution and particles (e.g., metal 
filings) carried by the cleaning solution into the separation subsystem 
can be separated by flotation (the oil) and by settling (the metal 
filings) from the aqueous cleaner. 
The design of the restriction means (if used) is not critical and any 
configuration, shape, location, or size may be used so long as the 
restriction means can perform its intended function. Although restriction 
means unattached to any filter support member could be placed in the 
device (e.g., a hollow cylindrical member interposed between the periphery 
of the filter support members and the rest of the body of fluid to be 
filtered, i.e., between the periphery of the filter support members and 
the cylindrical wall of the housing), it is preferable for the restriction 
means to be carried by the filter support members (i.e., for the filter 
support members to have the restriction means), for example, so that the 
restriction means can be removed as a unit with the filter support 
members. Restriction means not carried by the filter support members 
(e.g., a cylindrical wall) may be suspended from the first member or may 
be attached to another vessel wall (e.g., the sidewall or bottom of the 
vessel). 
The restriction means may comprise a circular dam or lip located near the 
outer periphery of the filter support member that projects a sufficient 
distance from the plane of the filter support member. Thus, the lip may 
project in only one direction away from the plane of the filter support 
member (e.g., above) or it may project in both directions away from the 
plane of the filter support member (i.e., both above and below). 
Desirably, the filter support members will carry restriction means and 
those means will substantially isolate the fluid in the high-shear zone 
from the fluid in the quiescent zone. Compressible means may optionally be 
used between the restriction means of one filter support member and the 
appropriate portion of the adjacent filter support member to provide a 
fluid-tight seal. If the restriction means is carried by the filter 
support member(s), the restriction means may be but need not be located at 
the periphery of the filter support member(s); the restriction means 
should however be radially distant enough to perform the desired function. 
For example, if the fluid filtration gap is 100 millimeters wide, each 
filter support member may carry restriction means and those means may 
project above and below the plane of the filter support member 
approximately 50 millimeters. Alternatively, the restriction means could 
project 100 millimeters above the plane of the filter support member and 
not at all below the plane of the filter support member. 
In most cases, it is desirable for retentate to remix with the rest of the 
body of fluid to be filtered. That remixing may occur, for example, in the 
body of fluid to be filtered outside the retentate flow restriction means, 
or just prior to being fed to the fluid filtration gap (e.g., in the 
annular region between the disc rotating shaft and the sleeve supporting 
the filter support members), or in the fluid filtration gap itself. Such 
remixing is desirable for several reasons, including preventing extreme 
concentration gradients from arising and "washing out"from the fluid 
filtration gap the solids or other materials that might otherwise tend to 
accumulate and more rapidly blind or clog the filter. 
If the restriction means prevents substantial remixing, it may be necessary 
to provide retentate flow effluent means (e.g., openings) in the "inner 
wall" formed by the restriction means to allow the retentate to leave the 
high shear regime. It may also be desirable to provide retentate flow 
directing means to direct the flow of the retentate leaving the high shear 
regime to counteract any undue agitation (e.g., swirling) of liquid in the 
radially distant volume that would otherwise occur because of the rotation 
of the rotating members (usually the discs). Accordingly, openings in the 
inner wall formed by the restriction means may be angled against the 
direction of rotation of the rotating members or nozzles oriented against 
the direction of rotation may be provided. Those openings and/or nozzles 
may also be oriented so that the retentate flow out of them is at an angle 
to the plane of rotation (e.g., perpendicularly) to achieve other flow 
patterns within the fluid regime of less shear. 
The restriction means for a fluid filtration gap will often block a 
significant portion of the nominal area occupied by the restriction means. 
Thus, the percentage of the nominal area blocked by the restriction means 
will often be at least 85%, usually at least 90%, desirably at least 92%, 
more desirably at least 94%, most desirably at least 95%, preferably at 
least 96%, more preferably at least 97%, most preferably at least 98%, and 
sometimes as much as 99% of the nominal area occupied by the restriction 
means. In other words, the open area defined by the openings in the 
restriction means will often be less than 15%, usually less than 10%, 
desirably less than 8%, more desirably less than 6%, most desirably less 
than 5%, preferably less than 4%, more preferably less than 3%, most 
preferably less than 2%, and sometimes less than 1% of the nominal area 
occupied by the restriction means. For this purpose, the nominal area 
occupied by the restriction means for a fluid filtration gap is taken to 
be the inner periphery of the restriction means (which in the case of 
cylindrical restriction means is its inner circumference) multiplied by 
the height of the fluid filtration gap. The height of the fluid filtration 
gap will be taken as the distance from the mid-plane of the disc to the 
mid-plane of the oppositely disposed filter support member defining that 
gap. 
Feed fluid may be introduced into the fluid filtration gap continuously or 
in batches. Permeate may be removed continuously or in batches. Retentate 
may be removed continuously or in batches. Retentate containing one or 
more species concentrated from the feed fluid may be the desired product, 
e.g., for testing. The permeate product may be feed fluid from which 
particulate or other matter that would interfere with subsequent testing 
has been removed by the filtration device. Testing of the retentate and/or 
permeate may be for the presence of or concentration of any chemical or 
biological species or for one or more physical or chemical properties 
(e.g., pH, temperature, viscosity, extent of reaction, specific gravity, 
chloride ion, antibodies, bacteria, viruses and other microorganisms, 
e.g., Cryptosporidium oocysts and Giardia cysts, DNA fragments, sugars, 
ethanol, and toxic metals, toxic organic materials, and the like). Thus, a 
device of this invention may further comprise means for physically and/or 
chemically testing the retentate and/or the permeate, e.g., for one or 
more of the foregoing species and/or properties (characteristics). 
Any method may be used to place fluid to be filtered into the one or more 
fluid filtration gaps but the fluid will desirably be placed into the gap 
near the longitudinal axis, i.e., the axis of rotation. Thus, for example, 
feed fluid may flow through the rotatable shaft or a sleeve around the 
shaft (forming an annular region between the shaft and the sleeve) and 
pass out into the fluid filtration gaps through ports in the shaft or the 
sleeve, or one or more gaps may be immersed in a natural body of fluid 
(e.g., a pond or lake) or in a body of fluid contained in a vessel (or 
housing), or two or more of those and other flow schemes may be used. 
In a particularly desirable configuration, the retentate leaving one or 
more fluid filtration gaps is recycled to the fluid filtration gaps. For 
example, retentate leaving the fluid filtration gaps may be piped to the 
annular region between (i) the rotatable shaft by which the discs are 
rotated and (ii) a sleeve around the shaft that supports the filter 
support members, which annular region may be in fluid communication with 
one or more of the fluid filtration gaps. The restriction means and 
suitable piping may be arranged to accomplish that recycle of retentate to 
the fluid filtration gaps, and some or all of the retentate leaving the 
fluid filtration gaps may be recycled. Fresh (non-recycle) feed (from the 
body of fluid to be filtered) can enter the fluid filtration gaps by any 
suitable means, including by passing through entry ports in the sleeve (if 
a sleeve is used and is fluidly connected to the fluid filtration gaps) or 
by passing through an opening in one or more filter support members (e.g., 
the filter support member farthest from the first member) or by any 
combination of those and other means. Fresh feed and any recycle retentate 
may or may not be mixed before entering the fluid filtration gaps. For 
example, such mixing may occur in the annular region between the sleeve 
and the shaft or just before entry into one of the fluid filtration gaps. 
The vessel or housing to hold the fluid may be part of the device. The 
housing (including the bottom, top, and/or sides) may be of any size or 
shape and of any material so long as the housing does not adversely affect 
performance of the device of this invention. Generally, the housing will 
be no larger than is reasonably required (1) to house and/or suspend the 
disc(s) and the filter(s), and (2) to provide a sufficiently large body of 
fluid to be filtered (if the housing is used to hold the fluid), and (3) 
to provide sufficient volume for flotation and/or settling (if flotation 
and/or settling are to be accomplished in the same vessel). A housing need 
not be used at all or the housing or a part of its bottom, top, and/or 
sides may be open and the device with the housing may be placed into a 
body of fluid (e.g., a lake, a fermentation tank) to produce a permeate 
and/or retentate product, e.g., for testing. Partial or complete immersion 
of the device can allow fluid to flow into the fluid filtration gap. The 
pumping action of the device (e.g., caused by the rotation of the disc(s)) 
can also be used to move the feed fluid into the filtration gap from the 
body of feed fluid. 
A device of this invention may be used in many different ways, e.g., for 
monitoring a reaction (e.g., by testing, or for producing a testable fluid 
from, the reaction medium in a reactor or a reactor effluent stream), or 
as an integral part of a reactor scheme (e.g., for separating catalyst 
from a reactor effluent stream for recycling to the reactor or for 
regeneration, or for continuously removing product and/or by-products 
and/or continuously replenishing nutrients in a cell culture reactor, or 
in biological waste water treatment (e.g., for retaining the activated 
sludge used to digest organic matter)), or as part of a recovery scheme 
(e.g., for separating products, by-products, contaminants, etc. from a 
reaction or process stream). The device may be located in situ in any type 
of process vessel (e.g., reactor) or pipeline (e.g., reactor effluent 
piping or slip-stream piping) for any purpose (e.g., producing a testable 
fluid) where filtration needs to be performed continuously or 
intermittently. 
Although there are no theoretical upper or lower limits on the diameter of 
the discs and filters, because of the speed of rotation, which may vary 
anywhere from under 100 rpm to 1000 rpm or higher, and because of 
engineering, fabrication, and cost constraints, the rotating member(s) of 
the filtration device will rarely be more than one or two meters in 
diameter. Accordingly, to increase the capacity of a device of this 
invention beyond the capacity provided by discs and filters approximately 
one or two meters in diameter, it is preferred that the filtration 
capacity be increased by adding additional discs and/or filters as needed. 
Regardless of the disc and filter diameters, capacity can always be 
increased by adding more discs and filters to a single device or by 
connecting two or more devices in series or parallel. 
Discs and/or filter support members may be mounted on a plurality of 
different suspension means in a common housing, hanging from a common 
member (e.g., a top), etc. Thus, for example, a housing for containing the 
body of fluid to be filtered could have two or more rotatable shafts in 
it, where one or both shafts are suspended from the top or side of the 
device and each shaft carries one or more discs, and/or one or more sets 
of filter support members could be suspended from the top or side of the 
device. A framework (e.g., a top mounted on several legs for standing in a 
reaction vessel or a lake) could carry two or more rotatable shafts on 
which two different sets of discs are mounted. 
The disc may be made of any material and have any design or shape provided 
it has the requisite physical and chemical properties so that it can 
perform its function according to the present invention. Because the disc 
may be rotated according to the present invention and because it is 
desirable that the disc not deform during the filtration process, the disc 
requires a certain minimum structural rigidity. Also, the disc preferably 
should be relatively inert chemically to the feed fluid. Generally, the 
disc will be made of metal although other materials such as ceramics, 
glass, and polymers may be used. 
Preferably, the surface of the disc facing the filtration gap, including 
the inner surface(s) of any grooves in the disc, is relatively smooth 
(except for the presence of the second feed means). Preferably, the 
surface of the filter, including any grooves used in the filter, is 
relatively smooth. A rough surface favors the onset of turbulent flow in 
the fluid in the filtration gap at lower rotation rates, which flow is 
energy inefficient and may adversely affect one or more components of the 
fluid being filtered. Thus, desirably the flow of fluid in the fluid 
filtration gaps is substantially non-turbulent, preferably essentially 
non-turbulent, and most preferably completely non-turbulent. It is 
surprising that although the presence of second feed means, e.g., holes 
leading from one major face of the disc through the disc to the active 
area of the other major face of the disc defining the fluid filtration 
gap, would seem to promote turbulence in the fluid filtration gap, such 
second feed means are an integral part of this invention, do not destroy 
the desired substantially non-turbulent flow in the fluid filtration gap, 
and help provide the benefits of this invention. 
Generally, the periphery of the disc and of the filter and of the filter 
support member will be circular, although other shapes may be used. The 
center of the filter will desirably coincide with the center of the filter 
support member, the center of the disc will desirably coincide with the 
center of the filter and the centers will desirably lie on the axis of 
rotation of the rotating element(s) and on the longitudinal axes of the 
disc(s) and filter support member(s). The peripheries of the disc and of 
the filter support member will usually be approximately the same radial 
distance from the axis of rotation. Usually one disc surface will face a 
single filter support member and the peripheries of each will be 
approximately the same distance from the axis of rotation. 
Preferably, the surface of the filter is substantially planar. Depending on 
the type of filter and its surface, the surface may have microconcavities 
and microconvexities; however, their presence is not inconsistent with the 
filter surface being considered to be substantially planar. Furthermore, 
if the filter surface contains one or more grooves and even if those 
grooves occupy almost the entire filter surface and have depths of 5 
millimeters or more, that will still not prevent the filter surface from 
being considered to be substantially planar. 
Similarly, the disc surface helping to define the fluid filtration gap is 
preferably also substantially planar, and the presence of 
microconcavities, microconvexities, and grooves with depths of 5 to 10 
millimeters or more will still not prevent the filter surface from being 
considered to be substantially planar. 
Although the disc and filter surfaces are preferably planar (e.g., for ease 
of fabrication), they need not be planar. For example, either or both may 
have axial cross-sections that are conical, trapezoidal, or curved. In 
fact, any shape may be used provided the benefits of this invention can 
still be achieved. Because the width of the fluid filtration gap may vary 
radially (i.e., as the distance from the axis of rotation, which is the 
longitudinal axis of the rotating shaft, varies), the two surfaces 
defining the gap may, for example, be closer to each other at their 
centers or at their peripheries. If both surfaces have the same 
cross-sectional size and shape, they may be oriented so that the gap width 
is constant, e.g., as where both disc and filter are conical and are 
nested. 
It is preferred that neither the disc nor the filter have any significant 
non-spiral protuberances (e.g., non-spiral blades or vanes) extending into 
the fluid filtration gap because their presence will tend to adversely 
affect, for example, energy efficiency by favoring the onset of turbulence 
at lower rates of rotation. 
Preferably, the disc surface and the filter surface defining the fluid 
filtration gap will be "substantially parallel," that is, the planes of 
the two surfaces will not be at an angle to each other exceeding 
approximately 30 degrees, desirably 20 degrees, more desirably 15 degrees, 
preferably 10 degrees, and most preferably will not be at an angle to each 
other exceeding 5 degrees. Even if a member (disc or filter) is, strictly 
speaking, non-planar (e.g., conical discs and filters), the member still 
will be considered to have a major plane of its general orientation, and 
it is that plane which should be used in determining whether the planes 
are substantially parallel. 
A device according to this invention may be oriented horizontally, 
vertically, or diagonally, that is, the axis of rotation of the disc 
and/or rotatable filter support members (if any) may be horizontal, 
vertical, or diagonal. In a vertically oriented device having one disc and 
one filter, the disc may be above the filter or the filter may be above 
the disc. Regardless of the number of discs and filters and the 
orientation of the device, it is desirable that the fluid filtration gap 
be kept filled with fluid during filtration. 
Rotation of the disc(s) and/or filter support member(s) may be achieved 
using any direct or indirect means, for example, an electric motor, a 
motor coupled via pulleys and drive belt or by gear transmission, or a 
magnetic drive. Thus, the rotating members (e.g., the discs) need not be 
mounted on a shaft that rotates them. Axial translation of the disc(s) or 
filter support member(s) and vibratory movement may be accomplished using 
known technology. 
In contrast to classic cross-flow filtration devices, the shear rate near 
the filtration surface and the transmembrane pressure or transmembrane 
pressure differential ("TMP") in a device of this invention may be made 
substantially independent of one another. (Despite the fact that the 
filter used herein need not be a membrane, the term "transmembrane 
pressure" is used because it is a common term.) A filter system of this 
invention enables precise control over the separation and can be operated 
and controlled in a variety of ways. For instance, for a given feed fluid, 
device geometry, filter, and rate of rotation of the rotating member, the 
permeate flow can be controlled by a permeate withdrawal (metering) pump 
(e.g., a peristaltic pump) and the retentate concentration (bulk 
concentration) controlled by setting the ratio of feed to permeate flow 
rates. Control of the system can also be achieved with flow control valves 
and pressure control valves. Some of the advantages of this invention are 
made possible by the fact that key operating parameters (shear rate, 
transmembrane pressure, and feed, retentate, and permeate rates) can to a 
substantial extent be independently controlled and manipulated. 
The control system for the filtration device may provide for continuous or 
batch addition or withdrawal of feed fluid and/or permeate and/or 
retentate. The design of the peripheral equipment used with the filtration 
device is not critical. Off-the-shelf technology may be used for the 
addition, collection, and withdrawal of fluid, for the control system, the 
rotary drive means, etc. The design and selection of all of this 
peripheral equipment are within the skill of the art. 
Generally, the operating pressure and transmembrane pressure in the device 
can be any values that do not interfere with the filtration process or 
adversely affect the feed or product fluids. Thus, a transmembrane 
pressure only slightly above atmospheric pressure may be used or the 
transmembrane pressure may be substantially higher. Generally, lower 
transmembrane pressures are preferred because they tend to minimize solids 
build-up on the surface of and within the filter. Also, lower operating 
pressures are generally preferred because they tend to make the equipment 
less costly. However, in some cases it may be desirable to use higher 
operating pressures to aid filtration. For instance, when processing 
carbonated beverages, the operating pressure must be kept sufficiently 
high to prevent degassing. Higher pressures in the fluid filtration gap 
may also be desirable to help drive the filtration. Higher pressure in the 
fluid filtration gap may also allow dispensing with a vacuum pump for 
removing permeate. It may also be desirable to use other forces, for 
example, electromotive force, to aid filtration in certain cases. 
Desirably one or more spiral grooves are used on one or more of the 
surfaces defining each fluid filtration gap, and preferably the disc 
defining each gap rotates and carries one or more spiral grooves and the 
filter defining each fluid filtration gap does not rotate and does not 
have any groove. 
A groove is a long narrow channel or depression. It may also be thought of 
as an elongate concavity or depression whose length lies in a plane 
parallel to the surface in which the groove is located. The term "spiral" 
may be defined in many ways but one simple definition is that a spiral is 
the path of a point in a plane moving around a central point in the plane 
while continuously receding from or advancing toward the central point. 
The spiral grooves used herein preferably are but need not be continuous. A 
surface may have more than one spiral, in which case the spirals may start 
and/or end at different distances from the center of the surface. If more 
than one spiral groove is used on a surface, the grooves may cross each 
other and need not have the same shape or curvature or central point or 
transverse cross-sectional shape or area. The spirals need not end at the 
periphery of the surface. The spirals need not be on the rotating 
member(s). Preferably, however, the one or more spiral grooves used are 
located on the surface of the disc, the disc rotates, feed is introduced 
to the fluid filtration gap at or near the axis of rotation, the grooves 
are true spirals, start near the axis of rotation, extend to the periphery 
of the disc, and do not cross over each other. 
Preferably the grooves are oriented on the surface and the surface is 
rotated in a direction so that the outer peripheral end of each groove 
points or faces away from the direction of rotation. That tends to reduce 
the force of impact of fluid exiting the groove. 
The grooves desirably used herein are generally concave in transverse 
cross-section and usually do not have any convexities. Preferably the 
inner surface of the transverse cross-section of the groove is a smooth 
continuous curve, for example, a section of an ellipse or circle or 
combinations thereof. The groove may also have straight walls and be, for 
example, triangular, rectangular, or square in cross-section. The 
transverse cross-section may also have straight and curved portions. A 
groove used herein preferably is of constant width and depth but those 
dimensions may vary along the length of the groove. 
The ratio of groove width to disc (or filter) radius will usually be from 
0.001 to 0.6, preferably from 0.01 to 0.5, and most preferably from 0.01 
to 0.4. The width may vary along the groove path length such that the 
ratio of groove width to radial location changes. 
Ratios of groove width to disc (or filter) radius outside the range of 
0.001 to 0.6 may be used if the other parameters (e.g., speed of rotation) 
can be adjusted so that the benefits of this invention are achieved. 
The separation between the two surfaces defining the filtration gap and the 
speed of rotation affect the cleaning action or shear and, hence, the 
flux. The cleaning action, generally speaking, is inversely related to the 
gap width. The effect of varying the gap, at least within a certain range, 
has a measurable but relatively small effect on flux, that is, the 
relationship between gap width and wall shear (i.e., shear rate at the 
membrane surface) is not strong. In any case, at some point, the 
filtration surface and its oppositely disposed disc will be too far apart 
for rotation of at least one of the members to have any beneficial effect 
on flux. On the other hand, because of engineering tolerances, among other 
things, at some point the two surfaces defining the filtration gap will be 
too close together to allow rotation of one or the other or both members. 
Accordingly, there is a useful working range of gap widths for any 
particular filtration device for a given feed fluid. The two oppositely 
disposed surfaces defining the fluid filtration gap should be "closely 
spaced" and thus the gap width will usually be within the range of 1 to 
100 millimeters, often 1 to 50 millimeters, desirably 1 to 25 millimeters, 
preferably 1 to 15 millimeters, and most preferably 1 to 10 millimeters. 
Spacings outside the range of 1 to 100 millimeters may be used if the 
other parameters can be adjusted so that the benefits of this invention 
are obtained. The gap width for a given device may vary, e.g., in the case 
where the disc(s) and/or filter(s) are not planar (for example, two 
conical surfaces that point towards or away from each other). In other 
words, the fluid filtration gap can vary radially. Such variation may be 
useful to help maintain constant average shear stress as feed viscosity 
increases as a result of concentrating one or more species (e.g., as in 
dewatering). 
The speed of rotation affects the flux: higher rotation rates increase the 
cleaning action and lower rotation rates decrease the cleaning action. Any 
speed of rotation may be used that is consistent with the design of the 
equipment and the shear-sensitivity of the fluid being processed. The 
speed will usually be from 50 to 2000 rpm, desirably from 100 to 1500 rpm, 
preferably from 100 to 1200 rpm, and most preferably from 100 to 1100 rpm. 
Values outside the range of 50 to 2000 rpm may be used provided the 
benefits of this invention can still be achieved. 
Other variables affecting the performance of the device of this invention 
include, e.g., the number of spiral grooves on the surface, the length, 
width, and depth of the grooves, their cross-sectional shape, the 
smoothness of the surfaces defining the filtration gap, and the parameters 
defining fluid rheology, including fluid viscosity, density, whether it 
contains particles (e.g., cells), and the size, shape, and concentration 
of those particles. As explained in U.S. Pat. No. 5,143,630, the angle 
subtended by a spiral groove (angle Y in FIG. 1 of that patent) and the 
curvature of the groove (relating to angle T in FIG. 2 of that patent) 
also affect performance. 
Still other variables affecting the performance include the size, shape, 
and location of any retentate flow restriction means, the number, size, 
shape, and location of any retentate flow directing means, and whether 
some or all of the retentate effluent passing through the restriction 
means is recycled to the fluid filtration gap(s) and, if so, how that is 
accomplished. 
With this background, we turn to the accompanying drawings, which 
illustrate various embodiments of the present invention. 
With reference to FIGS. 1-4, rotary disc filtration device 20 comprises 
first plate 22, second plate 24, motor 26, shaft 28 having longitudinal 
axis 30, sleeve 32, two filter support members 34, and rotary disc 36 
having first (upper) major face 94 and oppositely disposed second (lower) 
major face 96. Nut 38 at bottom of shaft 28 locks disc 36 to shaft 28. 
There are two fluid filtration gaps 40, which are parallel to each other. 
Each gap is defined by filter 42, which rests on circular plate 44 (which 
is the major part of filter support member 34), and the corresponding 
oppositely disposed major face of disc 36. Device 20 may be placed on top 
of a container (not shown) holding fluid to be filtered so that second 
plate 24 rests on supports across the top of the container. The upper 
level of the body of fluid to be filtered would be below the bottom face 
of second plate 24. Thus, rotatable suspension 56, in which shaft 28 
rotates, would not also need to seal against fluid. 
Each filter support member 34 has circumferential peripherally located lip 
46, which projects above and below plate 44 of filter support member 34. 
The two lips 46 on the adjacent filter support members meet along a 
circular path that is radially distant from longitudinal axis 30. 
Compressible member (e.g., O-ring) 48 lies along that circular path and 
provides a substantially fluid-tight seal between the two lips 46. 
Alternatively, lip 46 on one filter support member can be designed to nest 
within an adjacent filter support member to provide a barrier or 
fluid-tight seal to restrict the retentate flow. The nesting mechanism may 
also be used to aid in aligning the filter support members in the proper 
configuration during assembly of the filter support member/disc assemblage 
or during assembly of a cartridge of filter support members (described 
below). A compressible member (e.g., the O-ring) need not be used. 
During normal operation, rotation of disc 36 will cause circulation of 
fluid within each fluid filtration gap 40 and an outward pumping action 
(i.e., movement of fluid in the gap from longitudinal axis 30 towards 
circumferential (peripheral) lips 46. Varying pressure differences across 
(perpendicular to) plates 44 as a function of radial distance from 
longitudinal axis 30 will tend to cause plates 44 to deform, which in turn 
will cause the width of the fluid filtration gaps to vary radially. Ribs 
50 tend to prevent this flexing (deformation) of plates 44 and thereby 
tend to maintain relatively constant fluid filtration gap widths. 
Alternatively, ribs may project radially from sleeve 32 to thereby limit 
deformation (flexing) of the filter support members. 
Drive column 52, which is part of shaft 28, is connected at its upper end 
to the rotor of motor 26 and is fixedly attached at its lower end to the 
rest of shaft 28. Annular gap 54 lies between shaft 28 and sleeve 32. 
Because sleeve 32 is centered with respect to the longitudinal axis of 
rotation of shaft 28 (i.e., axis 30) and because upper filter support 
member 34 is connected to and centered with respect to the sleeve, the 
topmost filter support member is centered with respect to the axis of 
rotation of the shaft and disc. The lower filter support member is aligned 
with and connected to the upper filter support member and through its 
connection to the upper filter support member is connected (indirectly) to 
sleeve 32. Therefore both filter support members are connected to the 
"first member," which comprises plates 22 and 24. Sleeve 32 does not 
rotate. Thus, the filter support members remain stationary and the disc 
rotates with respect to them when motor 26 rotates shaft 28 on which the 
disc is fixedly mounted. The rotatable suspension of the rotating member 
(the disc) from the "first member" is indicated by reference numeral 56 
and is above the normal level of the fluid to be filtered when device 20 
is placed in the body of fluid to be filtered. 
Rotatable suspension 56 is for convenience depicted in FIG. 1 as a rotary 
bearing mounted in a plate (here, second plate 24); however, the rotatable 
suspension will often (and sometimes preferably) be the rotary bearing or 
bearings in the gear box, motor, or other motive means that rotate drive 
column 52 (which is part of shaft 28) and there will be no rotatable 
suspension in any of the plates (i.e., the drive column or shaft will pass 
through a hole in the plates without any bearing being located at that 
point). 
Holes 58 in the sidewall of sleeve 32 (typically four holes, only two of 
which are shown, but more or less may be used) allow fluid to flow into 
and/or out of gap or annular region 54 between sleeve 32 and shaft 28 from 
and/or to the body of fluid in which the device is immersed. Annular 
region 54 is in fluid communication with upper fluid filtration gap 40 
and, via means discussed below, is also in fluid communication with lower 
fluid filtration gap 40. 
Centrally located circular opening 60 in each of filter support members 34 
is defined by its inner rim 62. Shaft 28 extends through the central 
opening in the upper filter support member 34, and central opening 60 in 
lower filter support member 34 allows the fluid to be filtered to enter 
the upper and lower fluid filtration gaps 40 (during the filtration 
operation, device 20 is immersed in the fluid to be filtered to a level 
below second plate 24). Fluid entering through opening 60 in the bottom 
filter support member flows readily into the lower fluid filtration gap 
40. Holes 64 are present in the inactive area of disc 36 and holes 66 
(second feed means) are present in the active area of the disc. Dotted 
circle 68 in FIG. 3 indicates the inner periphery of the active area of 
disc 36. In this embodiment, the active area of the disc is the same as 
the grooved area of the disc, since the grooved area is present on each 
disc face directly opposite to the active filtration area of the 
respective filter. 
Semicircular openings in the circumferential lip 46 of upper filter support 
member 34 are aligned with identical openings in the circumferential lip 
46 of lower filter support member 34 to form circular openings 70 in the 
"inner wall" formed when the two filter support members lie adjacent to 
one another with compressible member 48 in between as shown in FIG. 2. 
These openings 70 allow retentate to leave fluid filtration gaps 40. There 
may be a gap in compressible member 48 where each of the circular openings 
70 is formed by the semicircular openings in circumferential lips 46 so 
that the circular openings are not partially blocked by compressible 
member 48 (which would otherwise horizontally bisect them). 
With reference to FIG. 1, inner rim 62 of upper filter support member 34 is 
attached to sleeve 32. That attachment may be made using any suitable 
semi-permanent fastening means (for example, pins, bolts, or screw 
threads) or any permanent fastening means, if desired (e.g., adhesive), 
although semi-permanent fastening means are preferred so that the upper 
filter support member can be detached from the sleeve. Lower filter 
support member 34 is maintained adjacent to upper filter support member 34 
with the semicircular openings in the two filter support members aligned 
properly by bolts (not shown) that pass through corresponding bolt holes 
(not shown) located in lip 46 of each of the two filter support members. 
The openings in each of the two filter support members that form circular 
openings 70 (FIG. 2) are evenly spaced around peripheral lips 46. Ribs 50 
are also evenly spaced around each filter support member 34. 
FIG. 3 shows the bottom face of disc 36 of FIG. 1, and FIG. 4 is a 
cross-sectional view of that disc taken along line 4--4 of FIG. 3. Disc 36 
having rim 72 is attached to the bottom of shaft 28 (FIG. 1) by nut 38. 
The center of disc 36 coincides with the center of each of the two filter 
support members 44 and longitudinal axis 30 of shaft 28 (FIG. 1). The 
bottom face of the disc (FIG. 3 and the right side of FIG. 4) and the top 
face of the disc (the left side of FIG. 4) each have 15 equally spaced 
spiral grooves 74 spaced 24 degrees apart. Dotted line 76 indicates the 
bottom of one of the spiral grooves, which are separated from each other 
by spiral lands 78. Spiral grooves 74 terminate at their outer ends at rim 
(periphery) 72 and at their inner ends at an ungrooved central portion. 
Disc 36 is generally symmetrical about mid-plane 80, with the following 
major exception. Cavity 82 terminates before reaching the top face of disc 
36 (the left side in FIG. 4), otherwise nut 38 would not be able to secure 
disc 36 to the bottom of shaft 28. 
On the lower major face of the disc (FIG. 3), dotted circle 68 separates 
the central portion, which is the disc's inactive or non-active area, from 
the disc's active area, which contains the spiral grooves. With reference 
also to FIG. 1, the disc's upper active area (the active area on the upper 
major face) is oppositely disposed to filter 42 on top filter support 
member 44, and the disc's lower active area (the active area on the lower 
major face) is oppositely disposed to filter 42 on lower filter support 
member 44. On each of the two major faces, the active area is bounded near 
the disc's longitudinal axis 30 by imaginary circle 68 and by outer 
peripheral region (rim) 72. Holes 64, which are within the inactive area 
of the disc, are not oppositely disposed to either upper or lower filter 
42. Holes 66, which are within the active area of the disc, are oppositely 
disposed to both upper and lower filters 42. (So as to not make FIG. 4 
confusing, holes 66, which are shown in FIGS. 1 and 3, are not shown in 
FIG. 4.) 
FIG. 5 shows a schematic plan view of one possible filter support member 
having generally circular periphery 84 and radial cut-out 86 terminating 
at its inner end in opening 88. The radial cut-out allows the filter 
support member to be moved in a direction generally perpendicular to the 
sleeve and shaft on which the discs are mounted, as described in U.S. Pat. 
No. 5,254,250. Thus, each filter support member can be detached from and 
removed from the assemblage of interleaved alternating discs and filter 
support members without having to remove the discs and filter support 
members in alternating sequence. 
FIG. 6 is a schematic plan view of another possible filter support member 
used in the present invention. This D-shaped filter support member has 
semicircular outer periphery 88, centrally located semicircular cut-out 
90, and straight portion 92. Two such D-shaped filter support members may 
be arranged as in FIG. 7 with their straight sides near to or contacting 
one another (a gap between the two straight sides would allow retentate to 
flow from stage to stage). This D-shaped configuration also allows each of 
the filter support members to be added to or removed from the assemblage 
of discs and filter support members without having to remove any of the 
discs from the shaft. Thus, the filter support members need not be unitary 
members and any size and shape may be used to form the filter support 
member surface that supports a filter defining a fluid filtration gap 
(with its oppositely disposed disc). 
FIG. 8 shows a multiplicity of discs 36 mounted on shaft 28. Each disc has 
several spiral grooves 74 on each of its first major face 94 and second 
major face 96. 
FIG. 9 shows assemblage (cartridge) 98 of five D-shaped filter support 
members of FIG. 6. Each D-shaped member has straight side 92, semicircular 
periphery 88, central semicircular cutout 90, first (upper) major face 
100, and second (lower) major face. The rotatable shaft 28 will be located 
in the elongate central passageway defined by the circular holes formed by 
semicircular cutouts 90 when a similar mirror-image cartridge is brought 
next to cartridge 98 (straight sides 92 proximate the straight sides of 
the mirror-image cartridge) in the assembled rotary disc filtration 
device. The five D-shaped filter support members of cartridge 98 are 
mechanically connected to each other by members 104. Two of members 104 
also fluidly connect the filter support members to each other for common 
permeate removal through nozzles 106. Cartridge 98 may be mounted in the 
rotary filtration device using bolts (not shown) that pass through bolt 
holes 108 in two of members 104. Each cartridge of filter support members 
may be moved as a unit into position with respect to the discs that help 
to define the fluid filtration gaps. 
In these drawings, the devices depicted are all vertically oriented (shaft 
28 is vertically oriented), it is the topmost filter support member that 
is attached (directly) to the "first member" (which comprises first plate 
22 and second plate 24), the bottommost and the topmost member is attached 
to a sleeve around the shaft that rotates the disc. However, as previously 
noted, the device need not be vertically oriented. Furthermore, it need 
not be the first filter support member in the assemblage of filter support 
members and discs that is attached to the first member and the one or more 
filter support members need not be attached to such a sleeve. 
EXAMPLE 
Runs were made using a device similar to that shown in FIG. 1, one major 
difference being that the experimental device had two D-shaped filter 
members forming a complete essentially circular filter member surface 
above the rotating disc and two D-shaped filter members forming a complete 
essentially circular filter member surface below the rotating disc (see 
FIG. 7). The outer diameter of each "circular" filter formed by the two 
D-shaped filters was 14.625 inches and the central circular clearance 
region (the opening in FIG. 7 formed by the two semi-circular cutouts 90) 
of each was about 2.687 inches in diameter. The filters used on each of 
the four filter support members were Membrex's UltraFilic.RTM. filter. 
Pressure sensors were placed above and below the rotary disc underneath the 
respective filters at four different radial distances from the 
longitudinal axis (center or axis of rotation) of the shaft on which the 
disc was mounted. Those four radial distances were approximately 2.5 
inches (an imaginary circle of 5-inch diameter whose center is the 
longitudinal axis of the shaft), 3.75 inches (an imaginary circle of 
7.5-inch diameter), 4.75 inches (an imaginary circle of 9.5-inch 
diameter), and 6 inches from the longitudinal axis (an imaginary circle of 
12-inch diameter). Thus, for example, the innermost pair of sensors (one 
under the filter above the upper fluid filtration gap and one under the 
filter below the lower fluid filtration gap) was 2.5 inches from the axis 
of rotation (or on an imaginary circle of 5-inch diameter). 
Each disc had spiral grooves on each of its two major faces, as shown in 
FIG. 3, and was rotated at 600 rpm, with water as the process fluid. The 
pressure in the gaps caused water to pass through the filters (permeate) 
and for both permeate and retentate to try to leave the system. The upper 
gap width (between the top of the disc and the oppositely disposed filter 
surface) was set at 0.36 inches and the lower gap width (between the top 
of the disc and the oppositely disposed filter surface) was set at 0.08 
inches. 
Each disc was approximately 13.75 inches in diameter. The central, 
ungrooved portion of the disc was about 2.75 inches in diameter. In other 
words, the spiral grooves commenced at about 2.75 inches in diameter 
(where the ungrooved central portion ended) and the grooves terminated at 
the periphery of the disc (13.75 inches in diameter). 
Four holes (0.3-inch diameter) were symmetrically located in the central, 
ungrooved portion of each disc at 90 degree angles to one another, with 
the north-south holes being about 2 inches apart and the east-west holes 
being about 2 inches apart. In some runs, the central holes were plugged. 
Different discs had different numbers and locations of 0.3-inch diameter 
holes running from the active (spiral grooved) area of one major face of 
the disc to the active (spiral grooved) area of the other major face of 
the disc. The innermost set of holes (when used) in the active area was 
along an imaginary circle superimposed on the disc and having a diameter 
of 3.21 inches. In other words, the innermost holes were at a radial 
distance from the center of the disc (the axis of rotation) of about 1.6 
inches. The other three sets of holes (when used) were along imaginary 
circles having diameters of 4.97 inches, 7.91 inches, and 10.75 inches. 
The holes along each imaginary circle were evenly spaced. In other words, 
if five holes were used for one of the imaginary four circles, the holes 
were about 72 degrees (360 divided by 5) apart. 
Using the difference between the upper and the lower pressures at each of 
the four radial positions and making various assumption, the net force on 
each disc was calculated. The results are shown in the table below. 
__________________________________________________________________________ 
Pressure Difference 
(Top minus Bottom (PSI) 
Run 
Hole Configuration 
At 5" 
At 7.5" 
At 9.5" 
At 12" 
Calculated Net Force (Pounds) 
__________________________________________________________________________ 
1 no holes in active area; 
1.23 
1.68 
2.28 
0.79 
209.1 
lower face of disc rubs 
2 five holes at 3.21" diameter; 1.12 1.15 1.06 
0.64 128.8 
retentate line plugged 
3 five holes at 3.21" diameter 0.16 -0.15 0.01 
0.26 5.4 
4 five holes at 7.91" diameter 0.20 -0.31 -0.05 
0.17 -3.7 
5 five holes at 3.21"; five 0.15 -0.30 -0.05 
0.26 -3.0 
holes at 7.91"; central holes 
plugged 
6 five holes at 3.21", five 0.17 -0.33 -0.06 
0.23 -4.5 
holes at 4.97"; five holes at 
7.91"; central holes plugged 
7 five holes at 3.21", five 0.10 -0.27 -0.04 
0.16 -4.8 
holes at 4.97"; five holes at 
7.91" 
8 five holes at 3.21"; five 0.21 -0.14 0.04 
0.25 7.8 
holes at 10.75" 
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In run 1, the force pushing the disc down towards the smaller gap is over 
two hundred pounds. Continued operation would put undue stress on the 
bearings in which the shaft rotates. Furthermore, the rubbing of the disc 
against the lower filters would cause premature wear and possible failure 
and significantly reduce the efficiency of the filtration occurring in the 
lower fluid filtration gap. Note that the downward pressure is very high 
despite the presence of the four holes in the central (inactive) area of 
the disc. 
In run 2, addition of just five small holes in the active area of the disc 
but with the retentate line plugged (to prevent retentate removal), which 
in essence increases the back-pressure on the system, still reduced the 
downward pressure on the disc by about 40%. 
In run 3, which is the same as run 2 except that the retentate line is no 
longer plugged, the downward pressure on the disc has been reduced to just 
5.4 pounds. In other words, addition of just five small holes at a 
diameter of 3.21 inches (about 0.23 R, where R is the radius of the disc), 
reduces the downward pressure from 209.1 pounds to 5.4 pounds, a reduction 
of about 98%. 
In run 4, five holes in the active area of the disc are again used but lie 
along an imaginary circle 7.91 inches in diameter (a circle of 0.56 R, 
where R is the radius of the disc). The pressure on the disc, instead of 
being a downward pressure, is now an upward pressure of about 3.7 pounds. 
In run 5, two sets of five holes each are used, one set at 3.21 inches 
(diameter) and the second set at 7.91 inches (diameter). The central holes 
in the inactive area are plugged. The upward pressure on the disc is about 
3 pounds. 
In run 6, a third set of holes in the active area has been added at 4.97 
inches diameter. The holes in the central, inactive area are again 
plugged. The upward pressure on the disc is about 4.5 pounds. 
Run 7 is identical to run 6 except that the central holes are not plugged. 
The upward pressure on the disc is about 4.8 pounds, which is essentially 
the same as the upward pressure on the disc in run 6. This shows that the 
holes in the central, inactive area of the disc make no difference. In 
other words, the problem of uneven axial forces is not alleviated at all 
by the use of holes in the central, inactive area of the disc. 
Run 8 has two sets of holes in the active area of the disc, one set at 3.21 
inches and the second set at 10.75 inches. The pressure on the disc is 
mildly downward, namely, 7.8 pounds. Comparison of this run with run 3 
(downward pressure of 5.4 pounds) suggests that the additional of the 
second set of holes at the larger diameter makes little difference. 
Broadly, the second feed means (holes) in the active area of the disc will 
generally be along an imaginary circle of at least about 0.1 R, where R is 
the radius (or equivalent circular radius, if the disc is not circular), 
desirably along an imaginary circle of at least about 0.25 R, and 
sometimes along an imaginary circle of at least about 0.5 R or sometime 
even 0.75 R. The number of holes along each circle will desirably be at 
least 2, preferably at least 3, and most preferably at least 5. The holes 
are desirably evenly spaced along the imaginary circle. Holes along more 
than one imaginary circle may be used (for example, at about 0.25 R and at 
about 0.5 R). It is surprising that holes in the active area in accordance 
with this invention can provide the benefits of this invention, in view of 
the teachings in the art that the active area of the disc (e.g., the 
grooved area) should not contain any concavities or roughness so as to try 
to avoid turbulence. 
Variations and modifications will be apparent to those skilled in the art 
and the following claims are intended to cover all variations and 
modifications falling within the true spirit and scope of the invention.