CA1176181A - Annular filter - Google Patents
Annular filterInfo
- Publication number
- CA1176181A CA1176181A CA000378862A CA378862A CA1176181A CA 1176181 A CA1176181 A CA 1176181A CA 000378862 A CA000378862 A CA 000378862A CA 378862 A CA378862 A CA 378862A CA 1176181 A CA1176181 A CA 1176181A
- Authority
- CA
- Canada
- Prior art keywords
- filter bed
- backwash
- filter
- bed
- passing
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D24/00—Filters comprising loose filtering material, i.e. filtering material without any binder between the individual particles or fibres thereof
- B01D24/46—Regenerating the filtering material in the filter
- B01D24/4631—Counter-current flushing, e.g. by air
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D24/00—Filters comprising loose filtering material, i.e. filtering material without any binder between the individual particles or fibres thereof
- B01D24/02—Filters comprising loose filtering material, i.e. filtering material without any binder between the individual particles or fibres thereof with the filter bed stationary during the filtration
- B01D24/10—Filters comprising loose filtering material, i.e. filtering material without any binder between the individual particles or fibres thereof with the filter bed stationary during the filtration the filtering material being held in a closed container
- B01D24/105—Filters comprising loose filtering material, i.e. filtering material without any binder between the individual particles or fibres thereof with the filter bed stationary during the filtration the filtering material being held in a closed container downward filtration without specifications about the filter material supporting means
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D24/00—Filters comprising loose filtering material, i.e. filtering material without any binder between the individual particles or fibres thereof
- B01D24/001—Making filter elements not provided for elsewhere
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D27/00—Cartridge filters of the throw-away type
- B01D27/10—Safety devices, e.g. by-passes
- B01D27/108—Flow control valves; Damping or calibrated passages
Abstract
ABSTRACT OF THE DISCLOSURE
A method of filtration comprises passing an in fluent through a cylindrical filter bed of annular cross-section in the direction of the thickness of the filter bed; and subsequently passing a backwash through the filter bed axially thereof.
A method of filtration comprises passing an in fluent through a cylindrical filter bed of annular cross-section in the direction of the thickness of the filter bed; and subsequently passing a backwash through the filter bed axially thereof.
Description
~.76~8~
The present invention relates to filtration systems and methods of filtration for removing impurities from water.
One conventional filter system consists of a bed of granular material such as sand or crushed coal supported on a bed of gravel. When water passes through the filter, suspended particles and flocculant material come in contact with the granular material and adhere to it. This reduces the effective size of the water passages through the filter and produces a straining action within the filter. With time, more and more material becomes trapped in the filter media, the pores clog, and the hydraulic headloss through the bed becomes excessive.
When the headloss becomes excessive, the granular media is cleaned by backwashing. During backwashing, previously filtered water is passed upwards through the filter at a sufficient velocity to force a fluidization of the granular material which results in an expansion of bed depth by about 30 to 50 percent. Material which has been filtered out and adheres to the media is dislodged by the shearing action of this backwash water and is carried off to waste.
It is generally considered that about 5 percent of the filtered water is required for backwashing the dirty filter material. However, one survey of water filtration systems has shown that the volume of backwash water has been reported to be as much as 27 percent of the filtered water. The turbidity of this backwash water requires that it be treated prior to disposal to receiving waters.
During backwashing, filter medias become hydraulically graded to leave the finest grain sizes at the top and the
The present invention relates to filtration systems and methods of filtration for removing impurities from water.
One conventional filter system consists of a bed of granular material such as sand or crushed coal supported on a bed of gravel. When water passes through the filter, suspended particles and flocculant material come in contact with the granular material and adhere to it. This reduces the effective size of the water passages through the filter and produces a straining action within the filter. With time, more and more material becomes trapped in the filter media, the pores clog, and the hydraulic headloss through the bed becomes excessive.
When the headloss becomes excessive, the granular media is cleaned by backwashing. During backwashing, previously filtered water is passed upwards through the filter at a sufficient velocity to force a fluidization of the granular material which results in an expansion of bed depth by about 30 to 50 percent. Material which has been filtered out and adheres to the media is dislodged by the shearing action of this backwash water and is carried off to waste.
It is generally considered that about 5 percent of the filtered water is required for backwashing the dirty filter material. However, one survey of water filtration systems has shown that the volume of backwash water has been reported to be as much as 27 percent of the filtered water. The turbidity of this backwash water requires that it be treated prior to disposal to receiving waters.
During backwashing, filter medias become hydraulically graded to leave the finest grain sizes at the top and the
- 2 -1~76~
coarsest grain sizes at the bottom. This grain size distribution through the filter depth is the reverse of what is desired if the filtered solids are to penetrate deeply into the filter bed. Poor filter bed utilization occurs if the fine grains of the upper surface of the filter clog. ThiS problem can be partially avoided by using a multimedia filter.
Multimedia filters use different ~ranular materials of different specific gravities so that che hydraulic grading places these lower mass, larger diameter particles on the upper surface of the filter. Obvlously, a properly, hydraulically graded filter can only be achieved by using a large variety of media.
An extensive investigation of the effect of various filtration parameters on the backwash characteristics of single and multimedia filters has been comp]eted and published ("Filter Backwash Water Economy", J.D. ~ur~e, M.Enq. thesis, Dept. of Civil Engineering, Royal Military College, Kingston, Ontario.~
In that investigation, marginal reductions in the backwash water required to clean filters were achieved~ Some of these reductions can be attributed to the careful control exercised during the test program. Nevertheless, a dramatic deviation from conventional filtration techniques will be required if water filtration wastes are to be minimized.
It is accordingly an object of the present invention to provide a novel and improved filtration system and method of filtration which reduce filter backwash waste disposal.
According to the present invention, there is provided a method of filtration, which comprises the steps of:
passing an influent liquid throu~h a single cylindrical D
117~i18~
filter bed of annular cross-section~ said filter bed having particles of substantially uniform grain size of between ~50 and 400 ~m in the direction of thickness of the filter bed;
subesquently passing a backwash of filtered liquid through the filter bed in a second direction transverse to the first direction at a sufficlent velocity to fluidi.ze the filter bed and dislodye the filtered solids from the filter bed to form a suspen-sion of solids in the backwash;
~, repeatedly recycling the backwash through the filter bed;
and finally purging the filter bed with clean water.
Also according to the present invention, there is pro-vided a filtration system, comprising:
a single cylindrical fllter bed of annular cross-section having particles of substantially uniform grain size of betweer 250 and 400 ~m;
means for passing an influent liquid through said filter bed in the direction of the thickness of said filter bed;
means for passing a backwash of fi.ltered liquid through said filter bed in a direction perpendicular to the thickness of the filter bed at a sufficient velocity to fluidize said filter bed and dislodge filtered solids from the filter bed to form a suspension of solids in the backwash; and means for passing fresh water through said filter bed in said perpendicular direction.
This invention will be more readily understood from the following description of a conventional filtration system and a filtration system embodying the present invention, taken in conjunction with the accompanying diagrammatic drawings, in which:
D
6~81 Figure 1 shows a perspective view of a conventional filter bed;
Figure 2 shows a perspective view of a filter bed embodying the present invention;
Figure 3 shows a graph i3.1ustrating the penetration of i solids into a filter bed; and Figure ~ shows a filtration system.
A soLid cylindrical core from a conventional filter bed indicated generally by reference numeral 9, is shown in Iligure 1 cotnprising a bed of coal 10 on a bed of sand 11, whic~ in turn D
~L~76~8~
overlies a bed of gravel 12. Influent is passed downwardly through the filter bed 9 as indicated by arrow 14, the r~sultant effluent leaving the bottom of the filter bed 9 a5 indicated by .arrow 15, and a backwash is subsequently passed upwardly through the bed as indicated by arrow 16. According to Darcy's law, the flow through this filter bed i~ given as:
Qc = KCAc ~h Lc (Equation 1.0) where Qc = flow rate (m3/s~
Ac = filter area (m ) Xc = coefficient of permeability (m/s) ~h = head loss through filter (m) ~c = depth of filter bed (m) In operation, the filter bed is backwashed by applying a reverse flow of water up through the filter bed at a sufficient velocity to fluidize the granular media. This reverse flow is maintained for a fixed period of time (usually five minutes) to allow the shearing action of the fluid to dislodge the filtered out solids. An expression for the backwash volume of water required for the conventional filter bed is given as:
Vc ~ VcActb (Equation 2.0) where Vc = volume of backwash water (m3) Vc = backwash velocity (m/s) tb = required backwash time (s~
L ..
6~
Now consider a filter section which occupies the same volume as that shown in Figure 1, but constructed as a hollow cylinder as shown in Figure 2, in which there is illustrated a hollow cylindrical filter bed 20 of annular cross section, referred to below as "the annular filter bed 20~, the influent, effluent and backwash being indicated by reference numerals 14a, 15a and 16a. The flow is now permitted to pass through the cylindrical wall, i.e. in the direction of the thickness of the annular filter bed 20, and, according to Darcy's law, is given as:
QA = 2 x Lc KA dy (Equation 3.0) dx where QA = flow through annular section (m3/s) x = any radius limited between ri and rO (m) Lc = length of annular section (m) ~ = slope of hydraulic surface from rO to ri x quation 3.0 is of the orm dx = 2~L KA d X C _, y hich when integrated between the limits of rO and ri gives 2~L. K
Ln (ro/ri) = ~A- ~- (YO Yi But (yO - Yi) equals the head loss, ~h, across the filter.
Thus QA = 2 Lc KA ~h Ln (rO/ri) (Equation 4.0) When this annular filter bed is backwashed vertically, i.e. axially, in the same manner as the conventional filter, then the annular area through which the backwash water must flow l ~
1~L76~81 is obviously smaller than ~he cylindrical cross-sectional area of the conventional filter cylinder. The volume of backwash water required is VA vA1~rO ri) tB
(Equation 5.0) where VA = volume of backwash water (m3) VA = backwash velocity (m/s) Since backwashing is an erosion process which results from the viscous shear of the washing fluid, the time requi/red for backwashing is reasonably consistent between filters. The required velocity of the backwash water, however, will vary directly as the square of the diameters of the fil~er media particles. In comparison to the larger particles, the lower viscous shear experienced by the smaller particles is balanced by the thinner deposits found on their surfaces, thus requiring similar backwashing t~mes.
It is generally believed that the deposition of solids with regard to penetration depth into the approach face of a filter, follows a logarithmic function. This implies that the greatest mass of filtered solids will be deposited on the approach face of the filter, and will logarithmicaily diminish with depth into the filter. ~n practice, it is recognized that excessive headloss and plugging of the approach face of filters ~B - 7 6~81 signals the requirement for filter cleaning. Thus the mass of filtered sol~ds assumed to be trapped by a f$1ter media would be approximately estimated by the $ntegral of the concentration and depth logarithmic relationship, multiplied by the area of the filter approach face. Much of the filter volume is unused unless solids penetrate deeply into the filter bed.
Tw~ ~n~ of sand were tested bo deb~ne a suitable uniform ~n size for an ~ ular filber and n~m~l grain si~s of 250 bo 400 m gave satisfacbory results. No~l sizes have an a¢ ~ 1 bole ~ oe of -0 +5% and so the ach~l size of gr ~ lies in the range 250 to 420~m.
various fil ~ bed dep ~ ranging from 5 cm to 40 cm were ~nd bo be satisfactory for solids penetration resulting from the f~tratl'on of solutions of bentonite and alu~. Bentonite dosage ranged from 12 to 24 mg/L producing a turb~dity up to 5.0 FTU and alum dosage was reduced from lS to 5 mg/1. The headloss across the filter bed was maintained constant, allowing the flow rate to diminish with t$me. When the flow rate was reduced to approximtely one half of the original flow rate, filtration was terminated and the f~lter bed was analysed for solids concentration at various depths. Filter beds which were too thick showed no util$zation of void space for solids depos~tion in the lower strata. These thicker beds also had a lower in$tial iltration rate than the shallower filter beds. Good filter bed utilization for solids depos$t$on ~as found for beds approx$mately 10 cm deep. This depth of bed also gave effluent turbidities of less than O.S FTU, and initial f$1trat$on rates of up to 3Q0 ~ m2 min w$th a head loss of 30 cm. Typical illustrations of the solids deposition w~th depth in the 10 cm deep filter bed are shown in Figure 3 for 300~m sand. These B tests have shown that a good quallty effluent can ~1761~31 be produced from a uniform sized! fine grain, filter media which does not surface plug.
To backwash this filter media, a closed circuit recycling reservoir system is used as illustrated in Figure 4~
The system shown in Figure 4 comprises a filter bed 20 contained in a housing 21 having at its bottom an outlet 22, which communicates through pipe 23 with a backwash water recycle reservoir 24, through pipe 25 with a clean water reservoir 2~, and through pipe 27 with tank 28, the pipes 23, 25 and 27 being provided with respective flow control valves 30, 31 and 32. The tank 28 is connected by pipe 34 to a turbidimeter 35, which is connected in turn by pipe 36 to the clean water reservoir 26.
Dirty water is supplied to the filter bed 20 through pipe 38 from a holding tank 39 by a pump 40 and a further pump 41 is provided between the outlet 22 and the pipes 23 and 25~
Pipe 42 connects the backwash water recycle reservoir 24 to the filter bed 20 in the housing 21 and is provided with a pump 43. The backwash reservoir has a bottom outlet 44 provided with a flow control valve 45 ànd communicating with a sludge waste tank 46.
In operation, dirty water is fed by the pump 40 to the filter bed 20 and the effluent from the latter passes through the turbidimeter 35 to the clean water. When the filter bed 20 re~uires cleaning, water from the backwash reservoir 24 is pumped sufficient velocity to fluidize the filter bed 20 and dislodge the filtered solids from the filter bed 20. The filtered solids, once dislodged, stay in suspensio~ in the recirculating backwash water, even during the reverse flow 11~7611~
through the expanded filter bed, and are fed by the pump 43 to the backwash reservoir. To purge this murky water from the filter bed prior to recommencing a filtration cycle, a plug of clean, filtered water is drawn from the filtered water reservoir by the pump 41, the valve 31 being opened and the valves 30 and 32 being closed, and is pumped at a fluidizativn velocity through the filter bed 20. When the interface between the clean water and the murky water emerges from the top of the filter bed 20, the backwash cycle is stopped. ThUS, the volume of clean water reguired for filter cleansing is equal to the pore space of the expanded filter bed. An expression for this volume of water for a 30 pércent bed expansion is given as:
V 1 3 n V (~quation 62 where VF = volume of filtered water required (m3) n - filter media porosity Vm ~ volume of filter media bed (m3) The uniform grain size of the filter bed 20 ensures that the coefficient of permeability measured horizontally through a wall section of filter remains constant through the vertical depth at the annular section. Non-uniform grain sizes would be hydraulically redistributed during backwashing and produce a horizontal coefficient of permeability gradient which would increase with depth. Therefore, sand is used for the filter bed which has a high sphericity, giving more consistent sieved grain sizes than angular beach sand. The no~inal 300 ~um sand used has a size range between 300 and 315 ~m or B
1~
~76181 300,+5%,-0% r~ grain size, and a sphericity of about 0.7 as measured by Stokes terminal settlin9 velocityO
Lc ~ 2.0 m r9 = 0.5 m ri ~ 0.4 m KA = 0~3 x 10 3 m/s Rc ~ 4.0 x 10 3 m/s The ratio of flows through the annular filter bed 20 and a conventional filter contained in the same 5pace can be found from the ratio of e~uation 4.0 to equation 1.0 as follows:
2~LC XA ~h ./Qc ~ ~ (Equation 7.0) c Applying the same head loss across both filter beds and substituting appropriate di~ension and permeability values gives QA/QC = 10.75 Thus the rate of flow through the present annular filter bed is 10.75 times greater than that through the conventional filter bed occupying the same volume of space.
The ratio of the approach face area of the annular filter bed to that of the conventional filter bed contained in the same space is:
AA 2~ rOLc (Equation 8.0) AC ~rO
= 8 for ~le measur~ts ~iven.
~B 11 ~17618~
Since the solids deposition into the approach face controls the frequency of backwash cycles, it can, for the present, be considered that the conventional filter bed requires bac~washing, due to headloss, approximately 8 times more frequently than the annular filter bed. Thus, the volume of water filtered by the annular filter bed is 8 times greater than that produced by the conventional filter bed between respective backwash cycles.
Under similar headloss conditions, it is found that a 10 cm thick test section of the filter bed 20 has the same filtration ability to assimilate solids as the thicker section found in the conventional filter bed. During backwashing, however, the required velocity of backwash water varies as the square of the diameter of the filter media particle size. For the annular filter, a preferred particle size of 300t'm is used.
Similarly, average particle size for conventional filter sand beds is about 500r~m with a uniformity coeeficient of about 105.
The ratio of the required backwash water velocities for the conventional and annular filters is:-v 500 = 2.77 Thus, to filter an equivalent amount of water, the ratio of thebackwash volume for the conventional filter bed in comparison to an annular filter bed occupying the same volume is:-~761~
c = VC ~ rO tb AA (Equation 9.0) VA vA ~ (rO ri ) b Ac 2.77 rO
rO ~ ri = 61.5 Without considering the recirculation feature to be used with the backwash water in the annular filter bed, it can be shown analytically that the conventional filter bed will require approximately 60 times more water to clean it, than an annular filter bed producing the same quantity of clean wster.
For the conventional filter, a backwash velocity of about 1.0 m/min is required during a backwash cycle lasting 5 minutes and the conventional filter bed requires backwashing 8 time to produce the same volume of filtered water as the annular filter. Thus the conventional filter requires a backwash volume of approximately Vc ~ 1.0 x rO 2 x 8 x 5 2 31.5 m3 ~1761~3~
Cons~der~ng the recycling feature for the backwash water in the annular filter, the volume of backwash water required as given by equation 6.0 for n equal 0.42 is:
VF = 1.3 x 0.42 x ~rO 2 _ ri 2) Lc _ .31 m3 The present system may b,e used to produce filtered water for cities, towns etc. by replacing the conventional sand filters in ~se, and can also be used for treating other liquid aqueous wastes once further developed.
~B
~L7~
Thus, when recirculation is considered, the ratio of the backwash water required for the conventional filter in comparison to the present annular filter is:
V = ~i-- = 100 ~quation 10.0) It has been found that, with the above-described filtration system:-a. the filter media size is prefereably of 250 to400~m particle size b. the filter media is preferably uniform in grain size c. bed depth (wall thickness) is preferably greater than 5 cm and less than 40 cm d. scouring water ùsed to clean the filter may be very dirty yet still function appropriately e. traces of dirty scouring water may be flushed from the media with a plug flow of clean water f. the cleaning of the filter is water velocity dependent, and the present filter permits reduction of the cross section through which this water velocity must pass, thus reducing the volume required.
~B
coarsest grain sizes at the bottom. This grain size distribution through the filter depth is the reverse of what is desired if the filtered solids are to penetrate deeply into the filter bed. Poor filter bed utilization occurs if the fine grains of the upper surface of the filter clog. ThiS problem can be partially avoided by using a multimedia filter.
Multimedia filters use different ~ranular materials of different specific gravities so that che hydraulic grading places these lower mass, larger diameter particles on the upper surface of the filter. Obvlously, a properly, hydraulically graded filter can only be achieved by using a large variety of media.
An extensive investigation of the effect of various filtration parameters on the backwash characteristics of single and multimedia filters has been comp]eted and published ("Filter Backwash Water Economy", J.D. ~ur~e, M.Enq. thesis, Dept. of Civil Engineering, Royal Military College, Kingston, Ontario.~
In that investigation, marginal reductions in the backwash water required to clean filters were achieved~ Some of these reductions can be attributed to the careful control exercised during the test program. Nevertheless, a dramatic deviation from conventional filtration techniques will be required if water filtration wastes are to be minimized.
It is accordingly an object of the present invention to provide a novel and improved filtration system and method of filtration which reduce filter backwash waste disposal.
According to the present invention, there is provided a method of filtration, which comprises the steps of:
passing an influent liquid throu~h a single cylindrical D
117~i18~
filter bed of annular cross-section~ said filter bed having particles of substantially uniform grain size of between ~50 and 400 ~m in the direction of thickness of the filter bed;
subesquently passing a backwash of filtered liquid through the filter bed in a second direction transverse to the first direction at a sufficlent velocity to fluidi.ze the filter bed and dislodye the filtered solids from the filter bed to form a suspen-sion of solids in the backwash;
~, repeatedly recycling the backwash through the filter bed;
and finally purging the filter bed with clean water.
Also according to the present invention, there is pro-vided a filtration system, comprising:
a single cylindrical fllter bed of annular cross-section having particles of substantially uniform grain size of betweer 250 and 400 ~m;
means for passing an influent liquid through said filter bed in the direction of the thickness of said filter bed;
means for passing a backwash of fi.ltered liquid through said filter bed in a direction perpendicular to the thickness of the filter bed at a sufficient velocity to fluidize said filter bed and dislodge filtered solids from the filter bed to form a suspension of solids in the backwash; and means for passing fresh water through said filter bed in said perpendicular direction.
This invention will be more readily understood from the following description of a conventional filtration system and a filtration system embodying the present invention, taken in conjunction with the accompanying diagrammatic drawings, in which:
D
6~81 Figure 1 shows a perspective view of a conventional filter bed;
Figure 2 shows a perspective view of a filter bed embodying the present invention;
Figure 3 shows a graph i3.1ustrating the penetration of i solids into a filter bed; and Figure ~ shows a filtration system.
A soLid cylindrical core from a conventional filter bed indicated generally by reference numeral 9, is shown in Iligure 1 cotnprising a bed of coal 10 on a bed of sand 11, whic~ in turn D
~L~76~8~
overlies a bed of gravel 12. Influent is passed downwardly through the filter bed 9 as indicated by arrow 14, the r~sultant effluent leaving the bottom of the filter bed 9 a5 indicated by .arrow 15, and a backwash is subsequently passed upwardly through the bed as indicated by arrow 16. According to Darcy's law, the flow through this filter bed i~ given as:
Qc = KCAc ~h Lc (Equation 1.0) where Qc = flow rate (m3/s~
Ac = filter area (m ) Xc = coefficient of permeability (m/s) ~h = head loss through filter (m) ~c = depth of filter bed (m) In operation, the filter bed is backwashed by applying a reverse flow of water up through the filter bed at a sufficient velocity to fluidize the granular media. This reverse flow is maintained for a fixed period of time (usually five minutes) to allow the shearing action of the fluid to dislodge the filtered out solids. An expression for the backwash volume of water required for the conventional filter bed is given as:
Vc ~ VcActb (Equation 2.0) where Vc = volume of backwash water (m3) Vc = backwash velocity (m/s) tb = required backwash time (s~
L ..
6~
Now consider a filter section which occupies the same volume as that shown in Figure 1, but constructed as a hollow cylinder as shown in Figure 2, in which there is illustrated a hollow cylindrical filter bed 20 of annular cross section, referred to below as "the annular filter bed 20~, the influent, effluent and backwash being indicated by reference numerals 14a, 15a and 16a. The flow is now permitted to pass through the cylindrical wall, i.e. in the direction of the thickness of the annular filter bed 20, and, according to Darcy's law, is given as:
QA = 2 x Lc KA dy (Equation 3.0) dx where QA = flow through annular section (m3/s) x = any radius limited between ri and rO (m) Lc = length of annular section (m) ~ = slope of hydraulic surface from rO to ri x quation 3.0 is of the orm dx = 2~L KA d X C _, y hich when integrated between the limits of rO and ri gives 2~L. K
Ln (ro/ri) = ~A- ~- (YO Yi But (yO - Yi) equals the head loss, ~h, across the filter.
Thus QA = 2 Lc KA ~h Ln (rO/ri) (Equation 4.0) When this annular filter bed is backwashed vertically, i.e. axially, in the same manner as the conventional filter, then the annular area through which the backwash water must flow l ~
1~L76~81 is obviously smaller than ~he cylindrical cross-sectional area of the conventional filter cylinder. The volume of backwash water required is VA vA1~rO ri) tB
(Equation 5.0) where VA = volume of backwash water (m3) VA = backwash velocity (m/s) Since backwashing is an erosion process which results from the viscous shear of the washing fluid, the time requi/red for backwashing is reasonably consistent between filters. The required velocity of the backwash water, however, will vary directly as the square of the diameters of the fil~er media particles. In comparison to the larger particles, the lower viscous shear experienced by the smaller particles is balanced by the thinner deposits found on their surfaces, thus requiring similar backwashing t~mes.
It is generally believed that the deposition of solids with regard to penetration depth into the approach face of a filter, follows a logarithmic function. This implies that the greatest mass of filtered solids will be deposited on the approach face of the filter, and will logarithmicaily diminish with depth into the filter. ~n practice, it is recognized that excessive headloss and plugging of the approach face of filters ~B - 7 6~81 signals the requirement for filter cleaning. Thus the mass of filtered sol~ds assumed to be trapped by a f$1ter media would be approximately estimated by the $ntegral of the concentration and depth logarithmic relationship, multiplied by the area of the filter approach face. Much of the filter volume is unused unless solids penetrate deeply into the filter bed.
Tw~ ~n~ of sand were tested bo deb~ne a suitable uniform ~n size for an ~ ular filber and n~m~l grain si~s of 250 bo 400 m gave satisfacbory results. No~l sizes have an a¢ ~ 1 bole ~ oe of -0 +5% and so the ach~l size of gr ~ lies in the range 250 to 420~m.
various fil ~ bed dep ~ ranging from 5 cm to 40 cm were ~nd bo be satisfactory for solids penetration resulting from the f~tratl'on of solutions of bentonite and alu~. Bentonite dosage ranged from 12 to 24 mg/L producing a turb~dity up to 5.0 FTU and alum dosage was reduced from lS to 5 mg/1. The headloss across the filter bed was maintained constant, allowing the flow rate to diminish with t$me. When the flow rate was reduced to approximtely one half of the original flow rate, filtration was terminated and the f~lter bed was analysed for solids concentration at various depths. Filter beds which were too thick showed no util$zation of void space for solids depos~tion in the lower strata. These thicker beds also had a lower in$tial iltration rate than the shallower filter beds. Good filter bed utilization for solids depos$t$on ~as found for beds approx$mately 10 cm deep. This depth of bed also gave effluent turbidities of less than O.S FTU, and initial f$1trat$on rates of up to 3Q0 ~ m2 min w$th a head loss of 30 cm. Typical illustrations of the solids deposition w~th depth in the 10 cm deep filter bed are shown in Figure 3 for 300~m sand. These B tests have shown that a good quallty effluent can ~1761~31 be produced from a uniform sized! fine grain, filter media which does not surface plug.
To backwash this filter media, a closed circuit recycling reservoir system is used as illustrated in Figure 4~
The system shown in Figure 4 comprises a filter bed 20 contained in a housing 21 having at its bottom an outlet 22, which communicates through pipe 23 with a backwash water recycle reservoir 24, through pipe 25 with a clean water reservoir 2~, and through pipe 27 with tank 28, the pipes 23, 25 and 27 being provided with respective flow control valves 30, 31 and 32. The tank 28 is connected by pipe 34 to a turbidimeter 35, which is connected in turn by pipe 36 to the clean water reservoir 26.
Dirty water is supplied to the filter bed 20 through pipe 38 from a holding tank 39 by a pump 40 and a further pump 41 is provided between the outlet 22 and the pipes 23 and 25~
Pipe 42 connects the backwash water recycle reservoir 24 to the filter bed 20 in the housing 21 and is provided with a pump 43. The backwash reservoir has a bottom outlet 44 provided with a flow control valve 45 ànd communicating with a sludge waste tank 46.
In operation, dirty water is fed by the pump 40 to the filter bed 20 and the effluent from the latter passes through the turbidimeter 35 to the clean water. When the filter bed 20 re~uires cleaning, water from the backwash reservoir 24 is pumped sufficient velocity to fluidize the filter bed 20 and dislodge the filtered solids from the filter bed 20. The filtered solids, once dislodged, stay in suspensio~ in the recirculating backwash water, even during the reverse flow 11~7611~
through the expanded filter bed, and are fed by the pump 43 to the backwash reservoir. To purge this murky water from the filter bed prior to recommencing a filtration cycle, a plug of clean, filtered water is drawn from the filtered water reservoir by the pump 41, the valve 31 being opened and the valves 30 and 32 being closed, and is pumped at a fluidizativn velocity through the filter bed 20. When the interface between the clean water and the murky water emerges from the top of the filter bed 20, the backwash cycle is stopped. ThUS, the volume of clean water reguired for filter cleansing is equal to the pore space of the expanded filter bed. An expression for this volume of water for a 30 pércent bed expansion is given as:
V 1 3 n V (~quation 62 where VF = volume of filtered water required (m3) n - filter media porosity Vm ~ volume of filter media bed (m3) The uniform grain size of the filter bed 20 ensures that the coefficient of permeability measured horizontally through a wall section of filter remains constant through the vertical depth at the annular section. Non-uniform grain sizes would be hydraulically redistributed during backwashing and produce a horizontal coefficient of permeability gradient which would increase with depth. Therefore, sand is used for the filter bed which has a high sphericity, giving more consistent sieved grain sizes than angular beach sand. The no~inal 300 ~um sand used has a size range between 300 and 315 ~m or B
1~
~76181 300,+5%,-0% r~ grain size, and a sphericity of about 0.7 as measured by Stokes terminal settlin9 velocityO
Lc ~ 2.0 m r9 = 0.5 m ri ~ 0.4 m KA = 0~3 x 10 3 m/s Rc ~ 4.0 x 10 3 m/s The ratio of flows through the annular filter bed 20 and a conventional filter contained in the same 5pace can be found from the ratio of e~uation 4.0 to equation 1.0 as follows:
2~LC XA ~h ./Qc ~ ~ (Equation 7.0) c Applying the same head loss across both filter beds and substituting appropriate di~ension and permeability values gives QA/QC = 10.75 Thus the rate of flow through the present annular filter bed is 10.75 times greater than that through the conventional filter bed occupying the same volume of space.
The ratio of the approach face area of the annular filter bed to that of the conventional filter bed contained in the same space is:
AA 2~ rOLc (Equation 8.0) AC ~rO
= 8 for ~le measur~ts ~iven.
~B 11 ~17618~
Since the solids deposition into the approach face controls the frequency of backwash cycles, it can, for the present, be considered that the conventional filter bed requires bac~washing, due to headloss, approximately 8 times more frequently than the annular filter bed. Thus, the volume of water filtered by the annular filter bed is 8 times greater than that produced by the conventional filter bed between respective backwash cycles.
Under similar headloss conditions, it is found that a 10 cm thick test section of the filter bed 20 has the same filtration ability to assimilate solids as the thicker section found in the conventional filter bed. During backwashing, however, the required velocity of backwash water varies as the square of the diameter of the filter media particle size. For the annular filter, a preferred particle size of 300t'm is used.
Similarly, average particle size for conventional filter sand beds is about 500r~m with a uniformity coeeficient of about 105.
The ratio of the required backwash water velocities for the conventional and annular filters is:-v 500 = 2.77 Thus, to filter an equivalent amount of water, the ratio of thebackwash volume for the conventional filter bed in comparison to an annular filter bed occupying the same volume is:-~761~
c = VC ~ rO tb AA (Equation 9.0) VA vA ~ (rO ri ) b Ac 2.77 rO
rO ~ ri = 61.5 Without considering the recirculation feature to be used with the backwash water in the annular filter bed, it can be shown analytically that the conventional filter bed will require approximately 60 times more water to clean it, than an annular filter bed producing the same quantity of clean wster.
For the conventional filter, a backwash velocity of about 1.0 m/min is required during a backwash cycle lasting 5 minutes and the conventional filter bed requires backwashing 8 time to produce the same volume of filtered water as the annular filter. Thus the conventional filter requires a backwash volume of approximately Vc ~ 1.0 x rO 2 x 8 x 5 2 31.5 m3 ~1761~3~
Cons~der~ng the recycling feature for the backwash water in the annular filter, the volume of backwash water required as given by equation 6.0 for n equal 0.42 is:
VF = 1.3 x 0.42 x ~rO 2 _ ri 2) Lc _ .31 m3 The present system may b,e used to produce filtered water for cities, towns etc. by replacing the conventional sand filters in ~se, and can also be used for treating other liquid aqueous wastes once further developed.
~B
~L7~
Thus, when recirculation is considered, the ratio of the backwash water required for the conventional filter in comparison to the present annular filter is:
V = ~i-- = 100 ~quation 10.0) It has been found that, with the above-described filtration system:-a. the filter media size is prefereably of 250 to400~m particle size b. the filter media is preferably uniform in grain size c. bed depth (wall thickness) is preferably greater than 5 cm and less than 40 cm d. scouring water ùsed to clean the filter may be very dirty yet still function appropriately e. traces of dirty scouring water may be flushed from the media with a plug flow of clean water f. the cleaning of the filter is water velocity dependent, and the present filter permits reduction of the cross section through which this water velocity must pass, thus reducing the volume required.
~B
Claims (8)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of filtering a liquid which minimizes filter backwash waste disposal, which comprises the steps of:
passing an influent liquid through a single cylindrical filter bed of annular cross-section, said filter bed having particles of substantially uniform grain size of between 250 and 400 µm,in the direction of thickness of the filter bed;
subsequently passing a backwash of filtered liquid through the filter bed in a second direction transverse to the first direction at a sufficient velocity to fluidize the filter bed and dislodge the filtered solids from the filter bed to form a suspen-sion of solids in the backwash;
repeatedly recycling the backwash through the filter bed;
and finally purging the filter bed with clean water.
passing an influent liquid through a single cylindrical filter bed of annular cross-section, said filter bed having particles of substantially uniform grain size of between 250 and 400 µm,in the direction of thickness of the filter bed;
subsequently passing a backwash of filtered liquid through the filter bed in a second direction transverse to the first direction at a sufficient velocity to fluidize the filter bed and dislodge the filtered solids from the filter bed to form a suspen-sion of solids in the backwash;
repeatedly recycling the backwash through the filter bed;
and finally purging the filter bed with clean water.
2. A method as claimed in claim 1 in which the influent is passed through the filter bed from the exterior to the interior thereof.
3. A method as claimed in claim 2, wherein the step of passing the backwash through the filter bed comprises passing the backwash upwardly in a vertical direction through the filter bed.
4. A liquid filtration system, comprising:
a single cylindrical filter bed of annular cross-section having particles of substantially uniform grain size of between 250 and 400 µm;
means for passing an influent liquid through said filter bed in the direction of the thickness of said filter bed;
means for passing a backwash of filtered liquid through said filter bed in a direction perpendicular to the thickness of the filter bed at a sufficient velocity to fluidize said filter bed and dislodge filtered solids from the filter bed to form a suspension of solids in the backwash; and means for passing fresh water through said filter bed in said perpendicular direction.
a single cylindrical filter bed of annular cross-section having particles of substantially uniform grain size of between 250 and 400 µm;
means for passing an influent liquid through said filter bed in the direction of the thickness of said filter bed;
means for passing a backwash of filtered liquid through said filter bed in a direction perpendicular to the thickness of the filter bed at a sufficient velocity to fluidize said filter bed and dislodge filtered solids from the filter bed to form a suspension of solids in the backwash; and means for passing fresh water through said filter bed in said perpendicular direction.
5. A filtration system as claimed in claim 4, wherein said grains have a particle size of about 300 µm.
6. A filtration system as claimed in claim 4, wherein said bed has a bed depth, in the direction of said thickness, of about 10 cm.
7. A filtration system as claimed in claim 4, 5 or 6, wherein the filter bed is of high sphericity sand.
8. A filtration system as claimed in claim 4, 5 or 6, which includes means for passing the backwash upwardly in a vertical direction through the filter bed.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000378862A CA1176181A (en) | 1981-06-02 | 1981-06-02 | Annular filter |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000378862A CA1176181A (en) | 1981-06-02 | 1981-06-02 | Annular filter |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1176181A true CA1176181A (en) | 1984-10-16 |
Family
ID=4120122
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000378862A Expired CA1176181A (en) | 1981-06-02 | 1981-06-02 | Annular filter |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1176181A (en) |
-
1981
- 1981-06-02 CA CA000378862A patent/CA1176181A/en not_active Expired
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US3814247A (en) | Method of filtering | |
| US7374676B2 (en) | High rate filtration system | |
| US3814245A (en) | Method of filtering | |
| US3698554A (en) | Method for backwashing filters | |
| CA1184854A (en) | Process for filtration of oil and gas well treatment fluids | |
| US3459302A (en) | Apparatus and method of filtering solids from a liquid effluent | |
| US3276585A (en) | Filtration process | |
| Darby et al. | Depth filtration of wastewater: particle size and ripening | |
| US3542674A (en) | Method for removing solids suspensions in liquids | |
| US3900395A (en) | Method of and apparatus for filtering | |
| US3631983A (en) | Method for backwashing filters | |
| CA1176181A (en) | Annular filter | |
| US3516930A (en) | Method of treating liquid waste effluent | |
| US3897339A (en) | Filter assembly having a precompacted filter medium | |
| US3674146A (en) | Method of regenerating filter aids | |
| Bourgeous et al. | Performance Evaluation of a Cloth‐Media Disk Filter for Wastewater Reclamation | |
| Ives et al. | Filtration of water and wastewater | |
| JP3436947B2 (en) | Suspension filtration equipment | |
| GB2124921A (en) | Filtering method and apparatus | |
| Adin et al. | The role of particle characterization in advanced wastewater treatment | |
| EP0949947B1 (en) | High rate filtration system | |
| DE102020104877B4 (en) | FILTER UNIT FOR PREPARATION OF A LIQUID PROCESS MEDIUM AND METHOD FOR OPERATING A FILTER UNIT | |
| Delmas et al. | Filtration of clayey sludge by the use of geotextiles: experimental study | |
| KR20020085964A (en) | The backwashing Method of Filter by Water Containing Supersaturated Dissolved Air | |
| KR102666856B1 (en) | Multi-layer media bed filter with improved backwash capability |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |