GB2239192A

GB2239192A - Filter bed

Info

Publication number: GB2239192A
Application number: GB8928570A
Authority: GB
Inventors: Edward Andrew Seymour
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-12-19
Filing date: 1989-12-19
Publication date: 1991-06-26
Also published as: WO1991008817A1; AU6907691A; GB8928570D0; EP0468001A1

Abstract

A liquid treatment tank (1) with a liquid inlet (2) contains a layer of components (4) having buoyancy in the liquid. During treatment the liquid is directed so that it premeates through the layer. During treatment the layer is stabilised by restricting the buoyancy of the layer in the liquid. This may be done by maintaining the level of the liquid below the surface of the layer with the outlet stand pipe (7). The layer filters the liquid of particles alone, or together with soluble material. When the apparatus is to be serviced for removal of particles from the layer the buoyancy of the layer is released. This provides for loosening of the components and the particles can be freed from the interstitial spaces of the layer. The particles are released to a position below the layer and may be discharged from the tank. The invention can for example be applied to the mechanical and biological filtration of impurities from waste water.

Description

TITLE: TREATING LIQUIDS IN FILTERS SPECIFICATION The present invention relates to treatment systems for liquids.

More particularly the invention is directed to the treatment of liquids by mechanical and biological filtration.

In the treatment of municipal or industrial waste water for example, there is a requirement for the water to be filtered mechanically of suspended particles and also for it to be purified by the removal of undesirable dissolved substances. The mechanical filtration may be conducted in a tank whereupon the water is passed through a layer composed of a plurality of individual components which serve to retain particulate, material in the interstitial spaces and therefore filter them from the water.

The removal of dissolved substances can be conducted in a biofilter comprised of either a separate tank or the same tank as that used for mechanical filtration. The biofilter may contain a plurality of individual components forming a layer of substrate which supports micro-orcanisms on its surface. These micro-organisms biologically remove impurities when the water to be treated is passed in close contact. Chemical processes may also require similar treatment and in this case the liquid may be a particular chemical and the substrate may for example be composed of, or comprise a catalys; or other substance beneficial for a chemical reaction. In this particular case the treatment tank could more appropriately be termed a reactor.

Filters and reactors which use immobile substrate layers during treatment of a liquid require a plug flow through the layer for the substrate to be used effectively. This may be disturbed by the blocking of the layer with solids either as particles carried in with the inlet liquid, or as a result of their generation inside the layer by chemical or biological processes. In biological processes for example the solids may originate from the growth of microorganisms and the accumulation of organic debri. Accumulation of high levels of organic material and micro-organisms can lead to a slowing of the reaction because of increase oxygen demand which creates anaerobic regions where toxic or other undesirable products are generated.

Layers of stones, sand or gravel have been used to provide immobile substrate layers in both biofilters and mechanical filters. In such arrangements the liquid to be treated is passed through the layer. As the liquid permeates the layer the suspended solids are trapped in the interstitial spaces. In biofilters the liquid may pass in close contact with micro-organisms on the surface of the components of the layer which remove smaller particles and dissolved substances. Accumulation of micro-organisms and organic material in the layer reduces the size of the effective interstitial spaces with time and the effectivity of the mechanical filtration may thus improve. However these substrates can become rapidly clogged and require frequent cleaning.For cleaning, large volumes of water alone or together with gases may be passed through the substrate at high pressure to dislodge the excess particles and allow them to be flushed from the tank. UK Patent No.

14,473 to Joseph describes a method for backwashing a layer of normally immobile components whereupon fluid is pumped up through the layer to wash particles from the layer to a position above the layer for discharge.

The requirement for this so termed, backwashing, is reduced in biofilters with immobile substrate layers by providing for the substrate layer to have a relatively high interstitual space for example above 90%. This allows the liquid to flow freely through the layer and it also facilitates the cleaning process. Plastic components are commonly used in such biofilters to provide a large interstitial space and a high surface area for the reaction.

Other types of biofilters dispense with the immobile substrate layer to overcome these difficulties. Thus for example, the activated sludge process for the treatment of waste water provides for the waste water to be agitated together with a suspension of the micro-organisms and sufficient oxygen for the reaction to proceed aerobically. Several patents deal with this system such as the "oxidation ditch" system as described in US patent No. 3,846,292 to Lecompte and US patent No. 4,199,452 to Mandt. After this process the particles resulting from the reaction need to be separated. A further technique involves movement of the substrate layer in the liquid to be treated. In this technique the substrate may consist of a number of disks placed perpendicular to the liquid surface which are rotated about an axis parallel to the liquid surface.

A disadvantage of rotating disk type systems and systems with immobile substrate layers and large interstitual spaces is that there may occur spontaneous release of particles into the treated liquid. This may mean that the particles need to be separated in a subsequent stage, such as with the activated sludge process.

This invention relates to a novel way of overcoming the present difficulties. A liquid treatment apparatus comprising a tank containing a plurality of components, said components arranged in a layer, said layer having buoyancy in said liquid, means to supply said liquid to said tank, means to direct said liquid to pass through said layer, means to restrict the buoyancy of said layer for treatment of said liquid, means to release the buoyancy of said layer and means to discharge particles from below said layer.

The invention can be applied to both the mechanical filtration of suspended particulate material and to the biofiltration of waste water. It may also be applied to the treatment of other liquids and in other liquid processes. The invention provides a convenient way of achieving both of these objectives at the same time. In the description the term liquid is used in a general sense and in many applications this liquid is water.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a vertical section of the treatment tank (1). An inlet (2) provides liquid to be treated adjacent the top of the tank.

In this case the inlet liquid is distributed over the surface by spreading the inlet liquid from a series of channels to spread the liquid. The tank contains a plurality of spheroid components (3) which form a layer (4). The liquid is directed to permeate through the layer by the outlet (5) which is situated on the opposite side of the layer. The layer is confined in the path of the liquid by the walls of the tank and a grid (6) near the base of the tank through which the liquid can pass but not the components. This grid helps encourage plug flow through the layer and guards against the components being carried from the tank with the treated liquid.

The components and the interstitual spaces of the layer are stabilized by restricting the buoyancy of the components and therefore the layer. This is achieved here by the outlet stand pipe (7) which maintains the level of the liquid during treatment below the surface of the layer, in this instance the outlet stand pipe is petat a level (A). The restriction of the buoyancy stabilises the interstitual spaces of the filter and helps prevent spontaneóus release of particles. In biofilter applications it also allows the growth of micro-organisms in the interstitual spaces of the layer and provides for mechanical filtration When it is desired to remove the accumulated particulate material from the interstitual spaces of the layer (4), the level of the liquid in the tank is raised by closing a valve (8) on the outlet stand pipe (7).With the rise in level of the liquid the buoyancy of the component layer in the liquid is released. This provides for a loosening of the layer such that the interstitial spaces may release their contents. This process may be assisted by blowing air or other gas into the tank through an inlet (9). These particles are negatively buoyant and settle in the lower region of the tank below the layer where they may be concentrated, in this instance in a cone (10). In this example the concentration is assisted by the provision of a horizontally rotating current provided by liquid pumped into the bottom region of the tank through an inlet (11) positioned to direct liquid tangentally to the wall of the tank. When the reactor is sufficiently serviced the particles are discharged through the outlet sump (12) by opening tap (13). The reactor is then returned to normal treatment mode by opening the tap (8) on the outlet stand pipe (7) such that the liquid in the tank again falls to below the surface of the layer. The actual level of the liquid in the tank may be slightly higher than the level (A) of the stand pipe (7) owing to resistance to passage of liquid through the layer.

In this description the reference to tank is meant to include a vessel, container, reservoir or any other structure which can contain liquid. The stand pipe outlet (7) or (16) may also comprise a weir alone or connected to a channel such that it allows passage of. treated liquid. The means to agitate the liquid may comprise fluid jets of liquid or gas or any combination thereof, directed to discharge down, up or horizontally into the layer. This agitation means could for example comprise a supply of air and this could also be operated continuously during the treatment of the liquid to supply oxygen for micro-organisms in a biofilter application. The means may also be located inside the layer to create for example a swirling and rotating movement whereupon the particles are dislodged and released from the interstitial spaces.It may also be sufficient to use a downward stream of fluid such as the liquid from the inlet (2). This would serve to flush the particles in the direction of the cone (10) and discharge sump (12). Mechanical means may also be used. The agitation method may also include structures protruding into the layer such that the layer is disturbed when its buoyancy is released. The disturbance and mixing may accordingly be brought about by arranging for the repeated rise and fall of the liquid in the chamber.

The filter may also be arranged for self cleaning such that particles are allowed to accumulate in the layer. This will increase the resistance to liquid flow through the layer with a consequent increase in the level of the liquid in the tank relevant to the level of the outlet stand pipe (7). When the liquid rises to the surface of the layer, the layer or components of it, will automatically have their buoyancy released. The interstitial spaces of the layer will be increased and the accumulated particles responsible for blocking the flow will be disturbed. When the passages for liquid flow are sufficiently clear, liquid will again permeate through the components and the level of the liquid will drop to below the surface of the layer such that normal treatment is resumed. In this case the particles might be released into the treated liquid.This could however be controlled if desired by installment of suitable compensatory provisions.

The release of the buoyancy of the layer may be conducted by any convenient means, it may for example be conducted by the raising of the level of the outlet stand pipe (7) or it may be brought about by arranging for a lowering of the grid (6). If for example the components are resting on the floor of the tank instead of the grid (6), it may be brought about by arranging for a lowering of the floor of the tank. It may also be brought about by arranging for the components to be held below the surface of the liquid during the treatment by a grid which allows the passage of liquid but not the components. In this instance the grid could be raised to allow the buoyancy of the layer to be released and the interstitual spaces to be opened. The layer could thus be elevated and could become more dispersed in the tank.

Figure 2 shows a different arrangement where the liquid enters the tank (14) through an inlet (15) situated below the layer and leaves from an outlet (16) situated towards the top of the tank. The outlet (16) is screened to prevent the escape of components from the layer. In this case it is screened by connection to a series of perforated or slotted pipes (17) placed inside the layer below the upper surface of the layer. The components may also be prevented from leaving the tank by providing a grid or screen across the surface of the layer. In this case the layer could be held below the liquid surface by the grid and this might assist the flow of treated liquid to the outlet. It could also encourage uniform flow through the layer. The outlet sets the level of the liquid in the tank which in this example may be held constant.The layer of components (18) is supported from below by a moveable screen (19), the position of which may be controlled from above by an arm (20) to which it is connected. During treatment of the liquid the screen is situated at a level such that the top of the component layer is above the level of the liquid, in this instance above the level of the outlet (16). The layer is confined by its own weight and the buoyancy of the layer is restricted. For servicing the screen (19) is lowered. This allows the layer to be agitated with the release of particles from the interstitial spaces of the layer and components. The particles are discharged through the sludge outlet (21) which is situated below the component layer.

This last arrangement may be preferred because the particles or suspended particles in the liquid will be concentrated in the lower region of the layer and may be released more easily from below the layer during servicing. The release of the particles from the layer on the opposite side of the layer to that of the outlet of treated liquid might also be preferable to reduce risk of contamination of the outlet liquid. This arrangement might also be preferred where it is desirable to avoid changes in the hydraulic pressure parameters. This arrangement could be considered advantageous in a recycled water system supporting aquatic organisms because new water in such systems may be expensive and may also effect temperature changes which increase demand for heating. In this instance only the water lost in the discharge of the particles as a sludge need be replaced.The invention may also be beneficial in this application because of the low interstitial space this means that the total volume of the water recycled is lower and the temperature may be controlled more easily.

I have found that this invention operates very well with a filter substrate layer composed of spheroid globules with an outer skin and an inner porous structure to provide the required buoyancy.

These may be graded to regular size ranges. Sizes ranging from 1 mm to 35 mm in cross section are usable and sizes ranging from 4 mm to 30 mm in cross section provide a convenient size in predominantly biofilter-type operations. Sizes between 10 and 25 mm in cross section are considered to give best results for biofilter applications because they provide a high surface area for bacteria relative to volume and in a layer they provide a low interstitial space for filtration. Other components may be employed and these may be of any other suitable size, configuration and shape. They might also possess buoyancy through their density relative to the liquid, or through their porous or hollow structure, such that a gas or other fluid is trapped and provides the buoyancy, or they may be a combination of any of these factors.In other applications such as for example where mechanical filtration of particles is the main objective, or where very tiny interstitial spaces are required, or where high contact to volume ratios are required, then the components may have much smaller dimensions such as that of sand. The components may also for example be microscopic and may comprise microscopic polystyrene beads or comprise structures of any other material and configuration. The optimum size of the components will depend upon the particular application.

It is preferable that all of the components constituting the layer have buoyancy and tend to float in the liquid. However the components may lose their buoyancy with time and a proportion may tend to sink in the liquid. These may however be easily lifted in the liquid by using fluid streams such as that from the inlet (9).

The components which loose buoyancy in the liquid could inhibit the passage of particles downwards from the layer and are therefore undesirable. Means could be provided in the tank to remove these components. Their loss of buoyancy could be inhibited by operating the apparatus so that the components are not immersed in the liquid during treatment. In this case the tank could be arranged such that the liquid trickles downwards over the surface of the components and the liquid level is set below the layer during treatment. It is preferable that at least 95% of the components forming the layer tends to float in the liquid. However 75% may be sufficient and 50% could prove operable.

The invention is advantageous because the servicing technique

reduces the energy costs associated with pumping backwash fluids into conventional filters. jLower levels may restrict the flow of Iliquid for treatment and higher levels wi(l increase the liquid for treatment and higher levels will increase the volume o#) 25 and-45 -'' rnsidered to be I space in the layer of between 25 an % is considered to be optimum during treatment but the apparatus may be operated with interstitial spaces of any other practical level.

Claims

Claim 1. A liquid treatment apparatus comprising a tank containing a plurality of components, said components arranged in a layer, said layer having buoyancy in said liquid, means to supply said liquid to said tank, means to direct said liquid to pass through said layer, means to restrict the buoyancy of said layer for treatment of said liquid, means to release the buoyancy of said layer and means to discharge particles from below said layer.
Claim 2. Apparatus as described in Claim 1 where the buoyancy of said layer is released by raising the level of the liquid in said, tank Claim
3. Apparatus as described in Claim 1 where the buoyancy of said layer is released by lowering a moveable screen supporting said layer Claim
4. Apparatus as described in Claim 1 where the buoyancy'of said layer is released by raising the level of a screen over said layer Claim
5. Apparatus as described in Claims1 to 4 where the components are between 1 and 35 mm in cross section Claim
6. Apparatus as described in Claimsl to 4 where the components are between 4 and 30 mm in cross section Claim
7. Apparatus as described in Claims1 to 4 where the components are between 10 and 25 mm in cross section Claim
8. Apparatus as described in Claimsl to 7 where the flow of liquid to be treated passes downwards through said layer Claim
9. Apparatus as described in Claimsl to 7 where the flow of liquid to be treated passes upwards through said layer Claim
10. A liquid treatment tank substantially as described herein with reference to Figures 1-2 of the accompanying drawings.