Abstract:
Enhancements to the Double Drain technology used for separation of settle-able solid wastes previously developed. Anti-vortex fins are added to the central clarified effluent water manifold to minimize re-suspension of settled solids. A further aspect includes the addition of a slotted horizontal cover plate to a solid waste collection sump below the tank floor. These additions significantly increase the effectiveness of the Double Drain system for separation of settle-able solid waste from fish culture water. The enhanced Double Drain Tank design with the anti-vortex feature could also be used in non-fish culture applications to enhance the separation of settle-able solids from any liquid waste water flow.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/065,700, filed on Feb. 14, 2008, which application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In previous patents, such as U.S. Pat. Nos. 5,055,186 and 5,593,574 to Van Toever, which are incorporated herein by reference, a Double Drain system is described, which takes advantage of the natural tendency for the spinning circulation flow in circular fish tanks to move and concentrate settle-able solids, consisting of excess uneaten fish feed and fish feces, to the center of the tank floor. This phenomenon is commonly referred to as the “tea cup effect.” 
         [0003]    The tanks are operated to achieve this effect by injecting the inlet fish culture water, tangentially into the tank to slowly rotate the water column. The fish swim into the current created and spread out uniformly through the water column. The constant gentle swimming provides exercise for the fish and the rotating flow provides relatively uniform water quality for each fish. This primary rotating flow induces a secondary flow of water in a radial direction across the tank floor toward the center of the tank. With sufficient spin, this radial bottom flow will transport waste particles that settle to the tank floor to the center of the tank. 
         [0004]    Before the Double Drain system was developed, the combined fish culture water and wastes were continually flushed from the tanks through a single screened, central bottom drain or central vertical screened pipe. The Double Drain design provides two strategically placed exits from the fish tanks, one for clarified waste water or effluent and one for the settled solids that concentrate at the center of the tank floor, the Double Drain innovation provides a very simple, inexpensive and effective mechanism for separation of settle-able solid waste from the fish culture water. 
         [0005]    Rapid removal of the intact solid wastes reduces the cost and complexity of subsequently separating them from a single effluent stream and minimizes mechanical breakdown of the particles and secondary leaching of dissolved wastes from the solid waste particles. This is a very important first step in filtering fish culture water to enable re-use of the fish culture water and to minimize the environmental impact of fish culture operations wastewater discharge. 
         [0006]    The previous designs included two basic versions. One is the “lateral” flow design, which is sometimes referred to as the “side box” design. Clarified waste water exits laterally through the tank sidewall through a screened pipe or external screened side box and concentrated solid wastes exit at the center of the tank floor. The second configuration is the “axial” flow design. Clarified waste water exits through a pipe manifold located vertically at the center axis of the tank through slots in the pipe located at the mid-upper water column level of the tank and concentrated solid wastes exit from a concentric opening at the base of the clarified water manifold at the center of the tank floor. 
         [0007]    Experience with operating circular fish culture tanks together with laboratory studies of circular Double Drain tanks have demonstrated that the axial flow design (clarified fish culture water exits at the center of the tank) provides a stronger “tea cup effect” than the lateral flow design. (See Veerapen, J. P., Brian J. Lowry and Michel F. Couturier; Solids Separation in Double-Drain Fish Tanks; Recirculation Aquaculture Research Group, Department of Chemical Engineering, University of New Brunswick, Canada.) This makes intuitive sense as the bulk of the flow with the lateral design is injected into the tank at the outer wall and exits at the outer wall essentially short-circuiting the flow. With the axial flow design in contrast, water moves more uniformly through the tank from the periphery where it is injected to the center where it exits. Despite this deficiency, the lateral flow design is popular especially for larger fish tanks such as those greater than 3 meters in diameter. 
         [0008]    Double Drain Systems have usually been operated by splitting the effluent flow into two continuous streams. The larger flow is the clarified water flow, which normally exits the tank and undergoes several other filtration processes before being returned to the tank. The so-called “under-drain” split, which carries the solids out of the tank at the center of the floor, is controlled so that the flow is just sufficient to extract the solids from the tank. Since the under-drain fraction is generally directed to waste and not re-circulated to the fish tank, it is desirable to minimize this flow. 
         [0009]    Since all of the clarified water flow exits through the tank sidewall with the lateral flow design, and as noted, the “tea cup effect” is weaker with this configuration, to compensate for this, the under-drain flow split must be increased to enhance the “tea cup effect.” With increasing tank diameter, more water must be diverted out the under-drain to move solids across the tank floor to the center or else they will settle and accumulate on the tank floor. Unfortunately this means that increased culture water is lost along with the solid wastes. The more dilute under-drain flow therefore usually has to be further treated to concentrate the solids content and recover some of the culture water before flushing the concentrated solids to waste. The added cost for the extra filtration technology can be significant, especially with larger fish culture tanks which have large under-drain flows. 
         [0010]    It has been found that the axial flow design (clarified water exiting at the center of the tank) has a stronger “tea cup effect” and the solids, which settle to the tank bottom move to the center even if the under-drain flow is reduced to zero. Since the solids can be removed as a concentrate with little or no under-drain flow, the upper clarified flow is larger and a greater percentage of the fish culture water can be re-circulated with less secondary filtration required to do so. 
         [0011]    Another phenomenon, which commonly occurs when operating circular fish culture tanks with higher water exchange rates (generally greater than 2 exchanges per hour) is the formation of a vortex at the center of the tank. A vortex in a fish tank is the equivalent of a tornado in the atmosphere. As the vortex increases in definition and intensity, solids which have settled and migrated to the center floor of the tank are re-suspended into the water column just as a tornado lifts objects off the ground. The solids are broken down into smaller particles and are re-distributed though-out the tank water column, significantly deteriorating the water quality for the fish. A small, low energy vortex will only have sufficient velocity to re-suspend the smallest particles but as the spin on the tank increases and the vortex intensifies, larger settled solids and eventually all the settled solids will be lifted off the bottom and re-suspended in the water column, therefore negating the “tea cup effect” completely. 
         [0012]    As noted, the axial flow design has the strongest “tea cup effect” but because all of the clarified water exits at the center, it is also more prone to the establishment of vortices. 
         [0013]    An exemplary embodiment of the previous axial flow design is shown in  FIG. 1  which schematically illustrates fish tank  10  on supports  12  with cylindrical side  14  and floor  16 . Water level in the tank is illustrated by dotted line  18 . The tank  10  incorporates a single, larger slotted pipe  20  in the center of tank  10  with two sets of outlet slots, one upper set of slots  22  and a second set of slots  24  adjacent tank floor  16 . Upper set of slots  22  are for the clarified water effluent which will be processed further (not shown or described herein) before it is returned to the tank. A secondary, smaller vertical pipe  28  is located concentrically in the larger slotted pipe  20 . Opening  30  of smaller pipe  28  is adjacent the upper set of slots  22  in outer pipe  20  and it serves as the clarified water effluent outlet for tank  10 , which water is processed further as noted above. The second set of slots  24  located at the base of larger pipe  20  provides the outlet for solids transported to this point by the “tea cup effect.” All the slots  24  are sized to retain the fish in the tank, but they allow water and solids to exit. A horizontal plate  34  located inside the larger slotted pipe  20  divides the pipe  20  into an upper clarified water chamber  36  and a lower solids chamber  38 . Plate  30  prevents flow between lower solids chamber  38  and upper clarified water chamber  36  so that solids are not drawn into the clarified water flow shown by arrow  40 . 
         [0014]    Lower solids cylindrical chamber  44  is molded or otherwise secured at  46  onto tank floor  16  which forms a socket to install the larger slotted pipe  20  within the chamber  44 . Small concentric space  46  between the lower end of smaller pipe  28  and base of larger pipe  20  within chamber  44  allows for the addition of horizontal solids outlet pipe  50  exiting the side of chamber  44 . Solids or particulates are drawn through the lower bottom set of slots  24  by the under-drain flow and exit chamber  44  out pipe  50  which has flow control valve  52 . For small fish culture tanks with low densities of fish, there may be sufficient volume available in the small concentric space  46  to store the small amounts of solids waste produced so that no continuous under-drain flow is required out pipe  50 . Solids can be stored in the concentrated space  46  and periodically flushed to waste by opening valve  52  on the end of the pipe  50 . With greater fish densities and waste generation a small, more continuous under-drain flow out pipe  50  is required to continually extract solids or wastes to external storage. Clarified water effluent within smaller pipe  28  passes out conduit  60  to external level control stand pipe  62  as is known in the art. As is evident from  FIG. 1 , the clarified water effluent in the stand pipe passes upwards between outer pipe  64  and inner pipe  66  and then into inner pipe  66  through upper opening  68 , from which the clarified water effluent exits at  70  to be further processed (not shown) as is known in the art. By connecting the stand pipe  62  with conduit  60  by a rotatable coupling  72 , the stand pipe  62  can rotate or pivot about coupling  70  and control the level  18  of water in tank  10 , as is known in the art. 
       SUMMARY OF THE INVENTION 
       [0015]    The invention in one broad aspect pertains to apparatus for use in association with removing clarified waste water from a fish tank comprising, at least one vertical fin structure adapted to extend in an axial direction along a periphery of a clarified waste water manifold centrally located in the fish tank. 
         [0016]    Another broad aspect pertains to a fish tank system wherein water is injected into the fish tank having a wall and floor to cause spinning circulation flow to water in the tank, and wherein the flow causes settable-solids in the tank to move to the center of the tank. A clarified water effluent outlet pipe concentric with the tank has openings adjacent an upper end through which clarified waste exits the tank for further processing. A sump below the tank floor, adjacent the center of the tank, has a substantially horizontal cover plate with openings sized to permit the settable-solids to gravitate into the sump but prevent fish from passing through the openings. 
         [0017]    Another aspect of the invention pertains to a method of reducing suspension of settable-solid particulates in a fish tank system having a wall and floor wherein water injected into a fish tank causes a spinning circulation flow in the water in the tank and wherein the settable-solid particulates in the tank are caused to move to the center of the tank, the method comprising:
       a. providing a clarified water effluent outlet pipe concentric with the tank having openings adjacent an upper end through which clarified water effluent exits for further processing;   b. providing a sump adjacent to the bottom of the tank with a cover plate substantially in line with the tank floor and through which the settable-solids particulate can gravitate into the sump; and,   c. providing a fin assembly in association with the clarified water effluent pipe, the fin assembly comprising at least one fin extending along and outwardly from the clarified water effluent pipe.       
 
         [0021]    More particularly, with a new inventive feature herein, the larger extended slotted pipe  20  has been eliminated and an anti-vortex assembly is provided to reduce vortex agitation of solids adjacent the clarified water effluent pipe. Preferably this anti-vortex can take the form of at least one fin attached directly to the clarified water effluent pipe or manifold which as noted above in the prior art embodiment, was concentrically inserted in a larger slotted pipe. The clarified water effluent pipe or manifold extends to the top of the fish tank and one set of upper slots provides for the clarified water effluent outlet. Although a preferred anti-vortex assembly is shown in the detailed embodiment which follows, any anti-vortex assembly of baffles or fins associated with the tank or clarified water effluent outlet is contemplated by this invention. 
         [0022]    A preferred embodiment of the anti-vortex assembly comprises at least one and preferably two anti-vortex fins associated with and extending the length of the clarified water effluent pipe, the fins or baffles effective in providing a quiet collection zone adjacent the fins at the junction of the clarified water pipe or manifold with the tank floor. 
         [0023]    Another preferred aspect of the invention is that a larger cylindrical sump chamber molded onto the tank bottom to provide increased solids storage capacity, generally large enough to store one day&#39;s solid waste production. A slotted cover plate allows solid waste to gravitate into the sump chamber. Still further, the tangential alignment of a sump outlet pipe or manifold with the periphery of the sump chamber enhances the flushing of the chamber when the under-drain is activated through opening of the valve on such pipe or manifold. 
         [0024]    Another broad aspect pertains to an enhanced Double Drain Tank design with an anti-vortex feature of the invention used for enhancing separation of settle-able solids from any liquid waste water flow. An aspect of the design including an apparatus for use in association with removing clarified waste water from a holding container having at least one vertical fin structure adapted to extend in an axial direction along a periphery of a clarified waste water manifold centrally located in said container. An even further aspect of the design including an apparatus for use in enhancing separation of settle-able solids from a liquid waste water flow having at least one vertical fin structure adapted to extend in an axial direction along a periphery of a clarified waste water manifold centrally located in a holding tank or container. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a schematic perspective view of a Prior Art Axial Flow Double Drain System; 
           [0026]      FIG. 2  is a schematic side view of an embodiment of the invention with anti-vortex fins and slotted sump cover plate; 
           [0027]      FIG. 3  is a schematic perspective view of the embodiment of  FIG. 2 , the stand pipe being omitted for clarity; 
           [0028]      FIG. 4  is a top view of a slotted storage sump cover plate with the anti-vortex fins; 
           [0029]      FIG. 5  is a schematic side view of a second embodiment of the invention wherein the waste collection sump outlet is offset and aligned tangentially with the sump wall; and, 
           [0030]      FIG. 6  is a schematic perspective view of the second embodiment of  FIG. 5 , the stand pipe being omitted for clarity. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0031]    With respect to  FIGS. 2-4 , structural features being common relative to those in  FIG. 1  are noted with the letter “a”. 
         [0032]    To take advantage of the stronger “tea cup effect” of the axial flow design in higher flow configurations, consideration was given to attempting to lessen the formation of vortices. The most effective preferred configuration has been the addition of an anti-vortex fin assembly  60 , preferably with two relatively narrow vertical fins  62 ,  64  along the sides of the central clarified water manifold  28   a  extending the entire depth of the water manifold as seen in  FIGS. 2-4 . This new anti-vortex axial flow configuration allows for a strong “tea cup effect” while minimizing the re-suspension of settled solids. It creates a “quiet zone” at the base of the vertical clarified water manifold  28   a  adjacent tank floor  16 , where solids can be easily removed from the fish tank. 
         [0033]    A second aspect of the invention allows passive removal of settled solid particles into an enlarged storage chamber  44   a  located below the center of the tank from which concentrated solids can be periodically flushed in small concentrated batches with no significant water loss. No continuous under-drain split flow is required with this design. 
         [0034]    As previously described, the prior art design used a second set of slots  24  at the base of the larger double slotted pipe  20  for removal of solids. The solids in that structure moved horizontally through the slots  24  into the small concentric collection area. The under-drain flow transported the settled particles through the slots into the chamber  44  and out the horizontal waste pipe  50 . 
         [0035]    With the design as illustrated in  FIGS. 2-4 , there is larger storage sump  44   a  and a slotted horizontal sump cover plate  80  provided to prevent fish from entering the sump  44   a . Solids are drawn onto the surface of the plate  80  by the “tea cup effect” and simply fall by gravity through slots  82  into the storage sump  44   a  below. No under-drain flow is required to move the solids through the horizontal openings or slots  82  so the solids can accumulate in a concentrated form in chamber  44   a  and are simply batch dumped through pipe  50   a  to a larger centralized waste storage area (not shown) once or twice daily with an insignificant amount of associated culture water exiting with the solids. No further treatment is required to extract re-usable culture water from the solids. 
         [0036]    The anti-vortex fin assembly  60  is preferably fabricated by attaching the fins  62 ,  64 , to upper and lower sleeves  86 ,  88 , which slide down over the waste water effluent outlet manifold  28   a . The sump cover plate  80  is preferably connected to the base sleeve  86  of the anti-vortex fin assembly  60 . A set screw  90  is located in the top sleeve  84  which is used to attach the anti-vortex fin assembly  60  to the slotted clarified water outlet manifold  28   a . This allows removal of the assembly of fins  62 ,  64 , and plate  80  as a unit for cleaning or changing the slot sizes of plate  80  to accommodate different fish sizes. The cover plate  80  can be detachably connected to the fin assembly. 
         [0037]    Although the anti-vortex fin assembly  60  preferably has two diagonally opposite fins  62 ,  64 , it will be appreciated that one fin would still be effective and more than two fins are possible. Further, the sump plate  80  need not be attached or secured to the anti-vortex fin assembly  60 , but could be separate and removed and replaced by other means separately over the manifold or pipe  28   a.    
         [0038]      FIGS. 5 and 6  illustrate a modification to the tank  10   a  utilizing the anti-vortex fin assembly  60  and sump cover plate  80 . Features which are similar to those of  FIGS. 2 and 3  are labeled with “a”. The major difference between the first inventive embodiment of  FIGS. 2  to  4  and the second modified embodiment of  FIGS. 5 and 6  is in the location of the solid waste collection sump outlet  50   a . It is aligned tangentially to the side wall  90  of the chamber  44   a  rather than on the sump side wall in line with the center of sump  44   a . With the tangential design when the waste valve  52  is opened to purge the waste stored in chamber  44   a , the waste and water are caused to swirl around the inside of sump  44   a  so the solids are flushed quickly from sump  44   a  and do not hang up on the walls  90  of the sump  44   a.    
         [0039]    Although the sump cover plate  80  is shown with slots to allow the settled particulate to fall by gravity into chamber  44   a , circular holes or openings or other shaped openings are contemplated as long as the openings are large enough to allow all particles through, but small enough to prevent fish passing through. Accordingly, the changing of the sump cover plate  80  to one having varying sized openings may be appropriate depending on the size of the fish in the tank. 
         [0040]    Although  FIGS. 2 and 5  illustrate an external water level control stand pipe, it is technically not necessary with the new system to control the water level in the tank. The slots  22  in the clarified water manifold  28   a  are placed near the water column surface  18  and therefore the water level  18  is now determined by these slots. Generally the water covers several of the lower slots  22  and the actual level depends on the flow of water entering the tank in relation to the size of the slots and flow out manifold  28   a . With smaller tanks up to about  1 . 5 m diameter, the manifold  28   a  may be reached from outside the tank and lifted vertically to drain the tank. Alternately the tank may be drained out though the solid waste sump  44   a  which usually is required for harvesting fish. 
         [0041]    Although a preferred embodiment of the invention is set forth above, with respect to the baffle design, different baffle designs and sizes of baffles are contemplated in association with the tank or the clarified water effluent pipe or manifold. Applicant found that with no baffles of any design and with sump cover plate  80  in place and with water in the tank caused to slowly rotate by treated water introduced into the tank, particles settled to the floor of the tank, moved to the center and although larger particles gravitated through the openings  82  in the cover plate  80 , smaller particles were re-suspended and drawn vertically upwards and exited out the slots  22  in the manifold or pipe  28   a.    
         [0042]    With horizontal baffles at the water surface extending from the outer tank wall  14  to the center water manifold  28   a , trials were made with baffles across half of the tank, and also across the full tank diameter. These surface baffles slowed the spin at the tank water surface and prevented formation of an obvious vortex in the upper region of manifold  28   a  but at the tank floor  16  vortexes continued to spin rapidly and fine particulates were still re-suspended. Nevertheless, use of the horizontal baffles did decrease the vortex overall in comparison to the trials with no baffles and more of the particles in the mid-size range fell into the sump rather than being re-suspended so there was an improvement in the quantity of wastes removed. 
         [0043]    In another embodiment, the addition of vertical baffles on the inner walls  14  of the fish tank  10  generally slowed the spin of the water column somewhat but were not effective in reducing the spin over the sump  80  sufficiently to prevent re-suspension of fine particulates. However, the baffles did improve and increase the overall quantity of waste removed. 
         [0044]    Various numbers, lengths and widths of vertical baffles were added to the walls of the clarified water manifold  28   a  and the impact on solids re-suspension was observed. Baffles added to the sides of the manifold  28   a  only along the upper portion of the pipe affected solids removal in a similar manner to the horizontal surface baffles noted above. The spin in the upper water column was decreased but the spin in the lower water column continued to be excessive and the finer particulates were re-suspended. However, again, solids removal was superior to the trials without any baffles. 
         [0045]    Vertical baffles were then added to the sides of only the bottom half of the clarified water manifold  28   a . A visible vortex was still created at the water surface  18  but the spin of the water column at the bottom was reduced. It was observed that a quiet zone was created on the downstream side of the baffles while the water column beyond the upper edges of these baffles continued to spin relatively quickly and some of the smaller particulates continued to be re-suspended although less than in the trials without baffles. 
         [0046]    As illustrated in the preferred embodiment, vertical baffles  62 ,  64  were added along the entire vertical height of the sides of the clarified water manifold,  28   a . Extending the baffles and adding the upper attachment collar  84  above the water level  18 , provided easier to access the set screw  90  for installation and removal of the anti-vortex fin assembly  60 . Further, this most preferred design was the most effective in preventing vortex creation and excessive water column spin velocity throughout the water column adjacent the manifold  28   a  and had the desired effect of increasing removal efficiency of the smaller particulates. This design is effective in creating a “quiet” zone, downstream of the baffles, especially over the sump cover plate which allows even the smallest settled particle sizes to fall through the slots into the storage sump without being re-suspended. In a tank wherein the clarified water effluent was about 2 inches in diameter the baffles  62 ,  64  were each about  2  inches in width whereas the cover plate  80  was about 8 inches in diameter. A width of each baffle is about 25% to 33% of the diameter of the cover plate  80  is effective. The baffles need not extend as far as the peripheral edge of the cover plate to be effective. 
         [0047]    Accordingly, Applicant&#39;s invention encompasses those embodiments not specifically described as preferred but within the scope of the invention as claimed.