AU694681B2 - Filtration medium - Google Patents

Description

WO 97/07063 ICT/AU96/00504 FILTRATION MEDIUM TECHNICAL FIELD The present invention relates to an improved filtration medium for use in the biological and mechanical treatment of water for a range of uses and, more particularly, for water used in recirculating systems.

BACKGROUND ART The management of recirculating aqueous systems has an important bearing on the aquaculture, aquarium and water purification industries.

The so-called "biological treatment" of waters in recirculating aquaculture systems consists of entrapment of suspended solids and the subsequent or simultaneous exposure of the recirculated water to the action of various bacterial organisms. These bacterial organisms convert certain contaminants in the water into less harmful substances.

In aquaculture systems with recirculating water, marine or aquatic animals produce toxic ammonia as a waste product either directly through respiration and elimination or through the decomposition of solid wastes. When water containing these waste products is passed over porous medium containing substantial colonies of the genera Nitrosomonas and Nictrobacter as well as other bacteria, in the presence of dissolved oxygen, this ammonia is converted to less harmful nitrites and thence to comparatively harmless nitrates. This process is referred to by those familiar with the art as "nitrification", Nitrates produced as the end products of the nitrification processes are then disposed of either by the regular replacement of a proportion of the water in the system, by the harvesting of plant material which feeds on the nitrates, or through the further anaerobic conversion of nitrates to free nitrogen by other bacteria.

Nitrification gives rise to the formation of certain dust-like particulate material which is referred to by those with knowledge of the art as "biofloc". This material accumulates SUBSTITUTE SHEET (Rule 26) I -I WO 97/07063 PCT/AU96/00504 2 within the system, and more particularly, within the media and requires periodic removal in order that the performance and flow characteristics of the biological media might be maintained.

Similar principles are applied to the treatment of a range of waste waters using suitable bacterial colonies, in an appropriate environment, in order to accomplish whatever decomposition process is required.

The varied and numerous solutions currently used in the treatment and purification of waste water or recirculated water used in the aquaculture and other industries suggests that the optimal solution has yet to be achieved. The treatment of the water generally involves four phases, which are now listed as follows: the entrapment of suspended solids; the removal from or decomposition of solids within the system; the exposure of the water to nitrifying and or denitrifying bacteria, and periodic or continuous removal of biofloc from the system.

Various types of media have been used for bacterial colonisation. These media usually take the form of a fibrous, lattice-like or porous bed through which the water is passed in any direction. The media may be fully submerged, partly submerged or free draining.

Gravel, oyster shells, sand, woven materials, plastic shapes and so on are all employed for water treatment and the efficacy of such media depends, among other things, on the total surface area offered within the media bed for colonisation by nitrifying bacteria and the ease with which the flowing water finds exposure to these surfaces.

Tightly packed media such as sand tend to channel unevenly and to clog due to the presence of fine introduced solids and biofloc and uniform flow throughout the media is therefore prevented.

SUBSTITU SHEET (Rule 26) cl WO 97/07063 PCT/AU96/00504 3 Porous materials, such as activated charcoal, scintered glass and materials with scratched or stippled surfaces have also been used in an attempt to maximise the surface area available for colonisation by bacteria. Such materials are quickly colonised but, after a time, the very small voids and pores that attracted the bacteria become filled with biofloc, dead unicellular algae and other small particles. Water flow through these pores is then restricted; biological action deteriorates and the biological performance of the medium begins to approximate that of a non-porous medium of similar shape and size. Often within a matter of months, such porous materials require aggressive cleaning, which also removes the bacteria, or partial replacement.

There is thus a dual requirement for the biological media; it must have a large surface area and must have the maximum possible open space and aperture size to accommodate fluid flow, air movement (if the filter is free draining) and simultaneously provide for the accumulation and dispersement of solids.

There have been two techniques used in respect of the handling of particulate solids in th6 system. One approach has been to regularly remove accumulated solids in order to minimise the biological load on the system resulting from the decomposition of these solids. Such filter systems tend to be comparatively compact but require regular cleaning.

The other approach has been to provide sufficient capacity in the system to retain the accumulated solids until they have decomposed and then to treat the products of decomposition (ammonia and so on) by exposure to other species in the bacterial colony.

Filters such as this tend to be comparatively large but, over the longer term, require vigorous cleaning to remove accumulated biofloc, which can be time consuming for the operator and damaging to the bacterial films.

Solids can be entrapped and or removed from the system in a number of ways. Solids can be collected in a settling chamber, of any design, in which the velocity of the water is reduced sufficiently for the solids to fall to the bottom where they can accumulate for decomposition or removal. Solids can also be entrapped in any type of mesh or gauze which can be removed for regular cleaning or replacement or may remain in place to SUBSTITUTE SHEET (Rule 26)

I

WO 97/07063 PCI'/AIJ96/00504 4 provide a site for decomposition. Solids may also be trapped in a medium which can be cleaned in situ. Some systems attempt to use the biological medium for the dual purpose of biological treatment and solids capture, with the solids either remaining in the media until ultimate decomposition, or until the media is removed in whole or in part for cleaning, or is cleaned in situ, The excessive accumulation of organic particulates within the media can substantially increase the biological oxygen demand of the system, with decaying solid matter and beneficial aerobic bacteria competing for the available oxygen, The efficacy of biological filtration depends to a considerable extent on the delivery of an adequate supply of dissolved oxygen to the aerobic nitrifying bacteria, and the periodic or continuous removal of decaying solid wastes is regarded as desirable practice by those with knowledge of the art. If the biological media is relied upon for the collection of decaying solids there arises the difficulty of removing such solids without excessive damage to the delicate bacterial films on the surfaces of the media.

Another approach has been to intercept suspended solids prior to the biofilter by any of the methods herein described and to provide a continuous flushing action on the media in order to prevent the accumulation ofbiofloc. So-called "tricking filters", which drip water downward through a suspended and aerated body of media are one solution which allows biofloc to fall away from the media for later collection by a strainer or other device. These filters require the use of a medium which is specifically designed to shed solids and the surface area available for bacterial colonisation is therefore restricted. So called "fluidised bed filters" use very fine granular media which is kept fluidised in a water column to prevent the accumulation ofbiofloc. Filters ofthis type offer relatively high surface area, but require a comparatively high power input for pumping and have been troublesome in cases of pump or power failure.

An ideal filtration medium comprises cylindrical or tubular elements, because of the large available surface area, and because of the ease of cleaning by back washing or the like, with minimal damage to any bacterial films that are being relied upon to treat the water.

SUBSTITUTE SHEET (Rule 26) WO 97/07063 1PCT/AUt96/0050 Japanese patent No. JP59-123596 describes the use of extruded plastic foam cylinders 3in diameter, 3-10mm in length and with a wall thickness of 0.1-0.14mm. The outer surfaces of the cylinders described therein are scratched, using an apparatus described, to provide an enhanced environment for colonisation by bacteria and to ameliorate the ongoing deterioration of porous media described previously herein, Japanese patent No. JP07-068288 describes a technique for the rapid biological treatment of waste waters in which are used cylindrical, toroidal or disc-shaped elements manufactured from porous polyvinyl formal of SG 1.0--1.25. One embodiment therein describes the use of cylindrical elements of 0.5-10mm diameter and 0.5-10mm length, with preferred dimensions 2-5mm diameter and 2-5mm length. Pore size is described as no more than 2,000 microns, DISCLOSURE OF THE INVENTION It is an object of this invention to provide an improved filtration medium for use in the biological treatment of water for any purpose, including the treatment of domestic and industrial effluents.

It is another object of this invention to provide an improved filtration medium which goes at least some way towards overcoming or at least minimising the prior art problems or limitations.

It is a further object of this invention to provide an improved filtration medium which is universally adaptable in its applications.

It is yet another object of this invention to provide an improved filtration medium which is relatively simple and cost effective in its applications.

These and other objects of the invention will become more apparent from the following description: SUBSTITUTE SHEET (Rule 26)

II

WO 97/07063 IPCMTAU96/00504 6 According to one aspect of the present invention there is provided a biological and mechanical filtration medium comprising a plurality of randomly disposed hollow cylindrical or tubular elements of predetermined size and density.

Ideally, the filtration medium comprises a plurality of massed cylindrical or tubular elements which are randomly dispersed in any plane. Preferably, the cylindrical or tubular elements have an outside diameter in the range of about 2.7 to 3.5mm, a length in the range of about 3.0 to 4.5mm, a wall thickness of about 0.2 to 0.4mm, and a ratio of length to diameter of about 1.5:1.

Preferably, the cylindrical or tubular elements are fabricated from plastics material having a specific gravity of about 0.85 to 0.95, and have a relatively smooth non-porous surface finish, which is amenable to the establishment .of a bacterial film thereon over a relatively large surface area. The bacterial film which forms on the inner surfaces of the tubular elements is protected from mechanical damage during agitation.

Ideally, the tubular elements are fabricated from extruded plastics material, cut offto the required length(s), the ends of which may be at any angle to the longitudinal axis of the tubular element. Ideally, the filtration medium of massed tubular elements comprises about 90% of void or open space, providing minimal resistance to fluid flow.

The filtration medium is contained within a larger volume filtration vessel for filtration, backwashing and rinsing procedures. The filtration medium can be agitated by mechanical hydraulic or pneumatic means such that the cylindrical elements, and the particulate material entrapped within and between the elements are separated therefrom and dispersed.

BEST MODE OF CARRYING OUT THE INVENTION The invention will now be further described by way of example only with reference to the following non-limiting examples thereof SUBSTITUTE SHEET (Rule 26) WO 97/07063 IC/MAU96/00504 7 According to this preferred embodiment of the invention, the filtration medium comprises a mass biological medium comprised of small plastic cylinders measuring approximately 2.7-3.5mm in diameter and 4,0-5.0mm long with a wall thickness of approximately 0.2- 0.4mm. A preferred size is 3.0mm and 4.5mm length. The cylinders have a relatively smooth surface and are manufactured from any plastic material with a specific gravity in the range of about 0.85-0.99 (preferably 0.89-0.91). The optimal ratio of length to diameter of 1.,5 (4.5mm:3.Omm) of the preferred cylindrical elements provides a medium exhibiting a large surface area, a high proportion of open space, large aperture size when the cylindrical elements are massed in great numbers and the ability to separate and move freely when agitated in a column of moving water.

The specific gravity of the material from which the cylindrical elements are manufactured and the wall thickness thereof are varied within the limits described herein in order to achieve an appropriate combination of physical strength and the desired buoyancy for the particular task to which the media is to be put.

The cylinders are randomly packed in large numbers into a containment vessel, of any design, through which the water to be filtered is passed. The geometry of the cylindrical shapes prevents close-packing or jamming of the cylindrical elements and yields a bulk material of which volume is approximately 90% void space. This allows the material to retain a large quantity of solids without blocking or channelling. The high proportion of void space and large average aperture size also reduce the velocity of the water against the surface of the media, reducing water speed and pumping losses, increasing retention time, promoting bacterial growth and encouraging the retention of solids.

Suspended solids introduced to the media by the incoming water stream are removed by a number of mechanisms of which three are now described: SUL TlTUTE SHEET (Rule 26) le IL Ir WO 97/07063 ICC:T/A 0196100504 physical straining whereby comparatively large particles and long stringy or fibrous particles accumulate, mostly at or close to the leading face of the massed media; adhesion, whereby small particles adhere to the surfaces of the media, a process which is enhanced by the presence of sticky bacterial films; and coalescence, whereby fine solid particles accumulate in those tubular elements of the media which are randomly disposed in close proximity to the horizontal plane and through which the flow of water is significantly less than the flow through those tubular elements which are aligned more closely with the vertical plane.

The cylindrical elements are slightly buoyant, but become either neutrally buoyant or negatively buoyant as the build-up of introduced solids and biofloc progresses on the smooth surfaces of the medium. At some point, the massed cylindrical elements require cleaning in order to remove this accumulated material.

The position and orientation of each cylindrical particle with respect to those surrounding it changes drastically when the massed material is agitated or back-washed. The material then moves freely, expanding in volume as the particles separate. Because the surfaces of the cylindrical elements are relatively smooth, accumulated solids are quickly shed into tilhe surrounding water stream, which is then diverted to waste. Additional agitation may be provided by aerating the massed material, mechanically agitating it with a mixing device, or pumping it in a fluidised state through another separation apparatus.

Bacteria housed on the inner surfaces of the cylindrical elements are protected from excessive damage during agitation.

The preferred embodiment of the invention is for cylindrical shaped herein described and exhibiting the following physical properties: SUBSTITUTE SHEET (Rule 26) l r l-l- WO 97/07063 I'T/A l9 l0004 Uase material specific gravity 0.9 Void space in base material Nil Dimensions of cylindrical elements 3.0mm diameter x 4.5mm long Wall thickness 0.2-0.4mm Surface finish Smooth Bulk density of massed cylindrical elements: Approx. 65-140 Kg per cu m Percentage void space of massed cylindrical elements: 87-91.5% Maximum aperture size of massed cylindrical elements 3mm diameter Minimum aperture size of massed cylindrical elements: 0.5mm diameter Specific surface area of massed cylindrical elements: 1,400 sq m per cu m From the above description of the invention, illustrated by exemplary but non-limiting embodiments thereof, it will be apparent that the present invention relates to a mechanical medium which provides a substantial surface area, a large proportion of void space, large aperture size and the ability to be cleaned by simple agitation or back-washing.

Whilst the present invention arose from work conducted in the aquaculture industry, its application is not limited to that field nor is the invention confined or limited to any particular device, technique or apparatus in which the invention might be used. On the contrary, this invention relates to any circumstance in which the described material may find a use and includes all applications which demand the entrapment of solids contained in any fluid or the exposure of any fluid to a large surface area for any purpose.

Although an exemplary embodiment of the present invention has been shown and described it will be apparent to those having ordinary skill in the art that a number of changes, modifications or alterations to the invention described herein may be made, none SUBSTTUTE SHEET (Rule 26) WO 97/07063 ff"VAU96/00.104 of' which depart from the Spirit of' the present invention. All such chianges, modifications and alterations should therefore be seen as being within thle scope of the present invention.

It should be appreciated that the present invention provides a substantial advance in biological and mechanical filtration medium providing all of the herein-described advantages without incurring any relative disadvantages.

SUBSTITUT SHEET (Rule 26)

Claims (6)

1. A biological and mechanical filtration medium comprising a plurality of discrete randomly disposed hollow cylindrical or tubular elements comprised of a material having a specific gravity less than 1,00 randomly dispersed in any plane.

2. A biological and mechanical filtration medium as claimed in claim 1, wherein said cylindrical or tubular elements are comprised of a non-porous material having a specific gravity in the range of about 0.85 to 0.95.

3. A biological and mechanical filtration medium as claimed in claim I or claim 2, wherein each said element has an outside diameter of about 2.7 to 3.5mm, a length of about 3.0 to 4.5mm, and a wall thickness of about 0.2 to 0.4 mm,

4. A biological and mechanical filtration medium as claimed in any one of the preceding claims, wherein the ratio of length to diameter is in the ratio of about 1.5:1. A biological and mechanical filtration medium as claimed in any one of the preceding claims wherein about 90% of the massed volume of the said elements comprises void or open space.

6. A biological and mechanical filtration medium as claimed in any one of the preceding claims, when contained in a filtration apparatus having a containment volume greater than the massed volume of the said filtration medium contained therein.

7. A biological and mechanical filtration medium as claimed in claim 6, further RZ ^comprising mechanical, hydraulic or pneumatic means to agitate the filtration jmedium within the filtration apparatus. AMENDED SHEET IPEA/A I INTERNATIONAL SEARCH REPORT International Application No. PCT/AU 96/00504 A. CLASSIFICATION OF SUBJECT MATTER Int Cl 6 C02F3/10, B01J 19/30, B01D 39/02 According to International Patent Classification (IPC) or to both national classification and IPC B. FIELDS SEARCHED Minimum documentation searched (classification system followed by classification symbols) IPC 6 C02F 3/10, B01J 19/30, B01D 39/02 Documentation searched other than minimum documentation to the extent that such documents are included in the fields searched AU IPC as above Electronic data base consulted during the international search (name of data base and, where practicable, search terms used) DERWENT C. DOCUMENTS CONSIDERED TO BE RELEVANT Catcgory* Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No. X WATER TREATMENT HANDBOOK, volume 2, published 1991 by Lavoisier 1 Publishing Page 729 X US,A, 4985182 (HARTWIG) 15 January 1991 1,5,6 See whole document X GB,A, 2160856 FLOCOR LIMITED) 2 January 1986 1,5, 6 See whole document SFurther documents are listed in the continuation of Box C See patent family annex Special categories of cited documents: later document published after the international filing date or document defining the general state of the art which is priority date and not in conflict with the application but cited to not considered to be of particular relevance understana the principle or theory underlying the invention earlier document but published on or after the document of particular relevance; the claimed invention cannot international filing date be considered novel or cannot be considered to involve an document which may throw doubts on priority claim(s) inventive step when the document is taken alone or which is cited to establish the publication date of document of particular relevance; the claimed invention cannot another citation or other special reason (as specified) be considered to involve an inventive step when the document is document referring to an oral disclosure, use, combined with one or more other such documents, such exhibition or other means combination being obvious to a person skilled in the art document published prior to the international filing document member of the same patent family date but later than the priority date claimed Date of the actual completion of the international search Date of mailing of the international search report 8 January1997 /7 'l¢7d 7 Name and mailing address of the ISA/AU Authorized 6fficer 61 AUSTRALIAN INDUSTRIAL PROPERTY ORGANISATION PO BOX 200 1- U WODEN ACT 2606 M. OLLEY AUSTRALIA Facsimile No.: (06) 285 3929 Telephone No.: (06) 283 2143 Form PCT/ISA/210 (second sheet) (July 1992) cophin r INTERNATIONAL SEARCH REPORT International Application No, PCTIAU 96100504 C (Continuation) DOCUMENTS CONSIDERED TO BE RELEV Category* I Citation of document, with indication, where appropriate, of the relevant passages TRelevant to claim No. Denv~ent Abstract Accession No. 88-288524/4 1, Class A97 D 15, JP,A, 63209788 (FUJI KASUI) 3] August 1998 Derwent Abstract Accession No. 86-329290/50 Class D15, JP,A, 61245895 (FURUKAWA) 1 November 1986 Form PCTIISA/2 10 (second sheet) (July 1992) cophin INTERNATIONAL SEARCH REPORT Information on patent family members literntliional Applicatlon No. PCT/AU 96/00504 This Annex lists the known publication level patent family members relating to the patent documents cited in the above-mentioned international search report. The Australian Patent Office is in no way liable for these particulars which are merely given for the purpose of information. Patent Document Cited in Search Patent Family Member Report US 4985182 AT 68994 DE 3723804 DE 3865912 DK 3544/88 EP 301237 END OF ANNEX Form PCT/ISA/210 (extra sheet) (July 1992) cophin