FILTRATION MEDIUM

Abstract: A biological and mechanical filtration system for use in the treatment of water used in recirculating systems, such as in the aquaculture, aquarium and water purification industries. The filtration medium comprises a plurality of massed, randomly disposed hollow cylindrical or tubular elements, ideally having a specific gravity of less than 1.0, with each element having a ratio of length to diameter of about 1.5:1.

Description: 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 ofthe 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 ofthe 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 ofthe nitrification processes are then disposed of either by the regular replacement of a proportion ofthe 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 ofthe art as "biofloc". This material accumulates within the system, and more particularly, within the media and requires periodic removal in order that the performance and flow characteristics ofthe 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 ofthe water generally involves four phases, which are now listed as follows: (a) the entrapment of suspended solids; (b) the removal from or decomposition of solids within the system; (c) the exposure ofthe water to nitrifying and or denitrifying bacteria, and (d) periodic or continuous removal of biofloc from the system. Narious 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. 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 ofthe 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 ofthe handling of particulate solids in the 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 ofthe 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 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 ofthe 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 ofthe 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 ofthe 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 of biofloc. 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 suiface 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 of biofloc. Filters of this 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 ofthe large available surface area, and because ofthe 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. Japanese patent No. JP59-123596 describes the use of extruded plastic foam cylinders 3- 10mm in diameter, 3-10mm in length and with a wall thickness of 0.1-0.14mm. The outer surfaces ofthe 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 ofthe invention will become more apparent from the following description: According to one aspect ofthe 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 ofthe tubular elements is protected from mechanical damage during agitation. Ideally, the tubular elements are fabricated from extruded plastics material, cut off to the required length(s), the ends ofwhich may be at any angle to the longitudinal axis ofthe 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. According to this preferred embodiment ofthe 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:3.0mm) ofthe 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 ofthe 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 ofthe cylindrical shapes prevents close-packing or jamming ofthe cylindrical elements and yields a bulk material ofwhich 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 ofthe water against the surface ofthe 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 ofwhich three are now described: (a) physical straining - whereby comparatively large particles and long stringy or fibrous particles accumulate, mostly at or close to the leading face ofthe massed media; (b) adhesion, whereby small particles adhere to the surfaces ofthe media, a process which is enhanced by the presence of sticky bacterial films; and (c) 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 ofthe 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 the 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 ofthe cylindrical elements are protected from excessive damage during agitation. The preferred embodiment ofthe invention is for cylindrical shaped herein described and exhibiting the following physical properties: Base 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 3 mm 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 ofthe 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 ofthe 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 ofwhich depart from the spirit ofthe present invention. All such changes, modifications and alterations should therefore be seen as being within the scope ofthe present invention. It should be appreciated that the present invention provides a substantial advance in biological and mechanical filtration medium providing all ofthe herein-described advantages without incurring any relative disadvantages.

Claims: CLAIMS 1. A biological and mechanical filtration medium comprising a plurality of randomly disposed hollow cylindrical or tubular elements 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 less than 1.00, preferably in the range of about 0.85 to 0.95. 3. A biological and mechanical filtration medium as claimed in claim 1 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 ofthe preceding claims, wherein the ratio of length to diameter is in the ratio of about 1.5: 1. 5. A biological and mechanical filtration medium as claimed in any one ofthe preceding claims wherein about 90% ofthe massed volume ofthe said elements comprises void or open space. 6. A biological and mechanical filtration medium as claimed in any one ofthe preceding claims, when contained in a filtration apparatus having a containment volume greater than the massed volume ofthe said filtration medium contained therein. 7. A biological and mechanical filtration medium as claimed in claim 6, further comprising mechanical, hydraulic or pneumatic means to agitate the filtration medium within the filtration apparatus.

Inventors: BERG KEITH EDWARD, BERG, KEITH, EDWARD
Applicants: BERG BENNETT AND ASSOCIATES PT, BERG KEITH EDWARD, BERG BENNETT AND ASSOCIATES PTY. LTD., BERG, KEITH, EDWARD
Application year: 1996
Main classification: C02F3
IPC codes: B01D39, B01J19, C02F3