the fluidization backwash method of filter beds by air

349

Original ArticleThe Fluidization Backwash Method of Filter Beds by

Air-water Bubbly FlowMasao Kuroda Anri Yoshida Emi Obuchi Hironoshin Kawabata Tadao Arai

Yamato Corporation Environmental Technology Research Center Maebashi Japan

ABSTRACTA novel fluidization backwash method by the air-water bubbly flow with air bubbles of various sizes has been proposed for rapid filters The backwash efficiency is closely related to the bubble wake mo-tion Bubble coalescence bed contraction and jet generation caused by the motion of air bubble wakes strikingly enhance the discharge of retained sludge The effect of the bubble wakes on the backwash efficiency is ensured by controlling the fluidizing condition which is easily identified visually The size of air bubbles should be controlled properly and the air bubble size at the dense bed surface must be within several centimeters to prevent the loss of filter media particles from filter beds The backwash efficiency of the filter bed achieved 94 in average by optimizing the air bubble size in the air-water bubbly flow The air-water bubbly flow backwash method was also applied to a self-backwash filter where the backwash flow rate depends on an elevated water tank and the backwash efficiency was as high as that for the constant flow rate backwash method

Keywords backwashing air-water washing self-backwashing bubbly flow granular filtration

INTRODUCTION

The backwashing of the filter layer is essential for the filtration operation A combined air and water backwash method is widely used as an effective backwash method However it is not easy to determine the optimum operating conditions such as the flow rates Therefore many studies have been conducted to determine the optimum air and water flow rates the bed expansion ratio and so on [1ndash11] Amirth-arajah investigated the semi-fluidization backwash method and derived a relational equation to predict the optimum water flow velocity the minimum fluidization velocity of the filter media particle and the air flow rate [1] A recent paper has indicated that the amount of backwash water varied according to the backwash process and the operating con-ditions but the net clean water production rate subtracting the wastewater for washing was almost constant [10] This result suggests that auxiliary water wash may be necessary to obtain sufficiently high backwash efficiency and the equa-tion presented by Amirtharajah might not always express the

optimum air and water flow ratesFluidization backwashing is an efficient backwash method

because the fluidization condition could be readily controlled to discharge the retained sludge Since the size of silica sand and anthracite used as filter media particles is small the water flow rate is limited in a certain specified range which is a little more than the minimum fluidization velocity The fluidized bed may be categorized as a three-phase fluidized bed and will be divided into two parts of a freeboard region and a dense region [1213]

Characteristics of the three-phase fluidized bed are gener-ally affected not only by the air and water flow rates but also by the air bubble and the particle [12ndash14] When the particle size is small and the water flow rate is limited in a certain range used in the fluidization backwash operation the air bubble is important in terms of the behavior of the fluidized bed The air bubble grows by coalescence on the way of rising up the fluidized bed and the rising motion of the air bubble causes vigorous agitation and mixing in the fluidized bed [12ndash15]

Corresponding author Masao Kuroda E-mail mkurodawatvnejpReceived February 15 2020 Accepted July 28 2020 Published online December 10 2020

Open Access

This is an open-access article distributed under the terms of the Creative Commons Attribution (CC BY) 40 License http

creativecommonsorglicensesby40

Journal of Water and Environment Technology Vol18 No6 349ndash358 2020doi 102965jwet20-014

Journal of Water and Environment Technology Vol 18 No 6 2020 350

In the three-phase fluidized bed the air bubble motion is closely related to the air bubbles size Also the fluidizing condition can be clearly identified compared to the collapse pulsing condition under the semi-fluidization state studied by Amirtharajah[1]

Since sludge retained in the filter bed is often distributed in the range of several centimeters under the surface of the filter bed [16-18] the motion of air bubble wakes in the fluidized bed seems to be helpful for removing the retained sludge It also would prevent the formation of mudballs The fluidization backwashing method utilizing the advantage of the characteristics of air bubble motion is considered to be very effective compared to the conventional backwashing methods However the research work in many literatures has focused on the air and water flow rates and the minimum fluidization velocity of the filter media particle There seems to be few researches focusing on the air bubbles

Therefore the effect of air bubbles on fluidization back-wash efficiency was investigated experimentally by varying the size of air bubbles in the air-water bubbly flow to develop an optimum air-water bubbly flow backwash method In ad-dition the fluidization backwash method with the air-water bubbly flow was applied to a conventional self-backwash filter where the backwash flow rate depends on an elevated water tank The effect of the air-water bubble flow on the backwash efficiency was also investigated

MATERIALS AND METHODS

Constant flow rate backwash methodsIn tap water treatment raw water with high turbidity is

generally treated by a combined process of coagulation-flocculationsedimentation followed by bed filtration For low turbidity raw water direct filtration is considered as a suitable process The aim of this study is low turbidity groundwater treatment

A schematic diagram of the experimental apparatus for a constant flow rate operation is shown in Fig 1 PACl in Fig1 shows polyaluminium chloride The filtration column is a transparent polyvinyl chloride pipe with the diameter of 200 mm and the total height of 1600 mm and a scale for measuring the filter bed height is attached to the surface of the filter column The filter media is silica sand (effective size of 06 mm and uniformity coefficient of 15) and anthracite (effective size of 12 mm and uniformity coefficient of 15) and the specified bed height is in the range of 400ndash600 mm The filter media particles are supported on a porous resin plate with the porosity of about 33 and the pore diameter of 1 mm

The raw water line consisted of a raw water tank a poly-aluminium chloride reservoir a pump a static mixer a flow controller and a pressure gauge The backwash line consisted of a backwash water tank a pump an air pump a flow con-troller and a bubble generator The fine bubble generator (A) was a developed venturi-type bubble generator The inlet

Fig 1 Schematic diagram of the experimental apparatus for constant flow rate backwash


angle and the outlet angle were 40deg and 55degrespectively and the outlet-throat diameter ratio was 367

Backwash methods examined are a water flow backwash method (water backwash) a combined air and water flow backwash method (air-water backwash) and an air-water bubbly flow backwash method In the air-water bubbly flow backwash method three methods were examined They were a micro air bubble flow backwash method (micro backwash) a milli air bubble flow backwash method (milli backwash) and a combined micro- and milli air bubble flow backwash method (bubbly backwash)

Raw water was prepared by adding kaolin clay powder into tap water with the concentration range of 15minus25 mgL The coagulant polyaluminium chloride (PACl) (PAC 6010 liquid basicity 55ndash6510ndash11 Al2O3 specific gravity 12 (Taimei Chemical Co Ltd Tokyo Japan)) in dose of 15ndash23 mgL was injected to the raw water line at a certain place just before the static mixer and the raw water was directed to the filter The filtration rate was in the rage of 100ndash160 mday and the filtration operation completed when the head loss reached 20 kP

At the end of filter run the filter was backwashed with the backwash methods mentioned above Tap water was used as the clean water In the air-water backwash operation the water level in the filter was drawn down to just above the bed surface to prevent the discharge of particles and after that a 3 min air scour was carried out After air scouring the bed was washed by the water flow In air-water bubbly flow backwash operations air bubbles were generated as follows Micro air bubbles were generated by the pump1 and milli air bubbles were generated by the developed venturi-type bubble generator (A) For the micro-and milli air bubble flow the micro bubble flow generated by the pump1 was passed through the developed venturi-type bubble generator (A) The air flow rate Va the water flow rate Vw the backwash time t and the bubble diameter db were summarized in Table 1

Solid mass retained in the filter (sludge) will be considered

equivalent to the amount of kaolin clay K and PACl sludge P which were fed for each filter run P was aluminum hy-drate (Al (OH)3) of 2 moles produced from aluminum oxide (Al2O3) of 1 mole K and P were calculated by the equations K = QCk and P = 0153αQCp respectively (Q raw water [m3] Ck turbidity [kgm3] α Al2O3 content [-] Cp PACl dose [kgm3])

The backwash effluent was collected in 200 litter plastic tanks The solid mass contained in the backwash effluent the sludge was determined from the solid concentration of efflu-ent collected in the plastic tank The turbidity measurement was carried out by a turbidity meter (Turbidity meter WA 6000 Nippon Denshoku Industries Co Ltd Tokyo Japan) For the experiments raw water temperature and backwash water temperature varied between 15 and 17degC

Self-backwash methods with variable flow rates de-pending on the head

A schematic diagram of the experimental apparatus for a variable flow rate operation is shown in Fig 2 The filtra-tion column is a transparent acrylic pipe with the diameter of 300 mm and the total height of 1200 mm and a scale for measuring the filter bed height is attached to the surface of the filter column Silica sand (effective size of 06 mm and uniformity coefficient of 15) and anthracite (effective size of 12 mm and uniformity coefficient of 15) are used as filter media particles and each packed height is 270 mm and 200 mm respectively The filter media particles were sup-ported on the porous resin plate with the porosity of about 33 and the pore diameter of 1 mm

The raw water line consisted of a raw water tank a pump and a flow controller The backwash line consisted of a back-wash water tank a pump an air pump a flow controller two nets and two bubble generators Two 5 mm mesh nets were installed 6 cm above the top of the filter bed to prevent the discharge of filter media particles

In Fig 2 the bubble generator 1 was made of a porous tube and the bubble generator 2 was a developed venturi-type

Table1 Summary of the backwash procedures for constant flow rate operations

Water Air-waterAir-water bubbly flow

Micro Milli Bubblyt[min] 5 81 5 5 5Va[NLmin] 24 Air1=05 Air2=10 Air1+Aair2Vw[mmin] 05 05 05 05 05db[mm] 532 00652 322 1852

1 Air scour 3 min Water 5 min 2 Mean diameter


bubble generator (2) The inlet angle and the outlet angle were 40deg and 6deg respectively and the outlet-throat diameter ratio was 23 The porosity pore diameter and length of the porous tube were about 51 200 μm and 300 mm respectively In order to generate small air bubbles under several millimeters in diameter the backwash water was passed through the bubble generator 1 at a certain flow rate and air was injected at the flow rate of 18 NLmin to the bubble generator 1 The backwash water tank was placed at 900 mm height above the filtration column and the effective head was 1500 mm

The filtration rate was 90 mday and the filtration op-eration lasted 181 min Raw water turbidity was adjusted as close as possible to the amount of retained sludge per bed volume measured in constant filtration rate experiments The raw water was prepared by adding kaolin clay powder into tap water and the concentration of kaolin clay was in the range of 40ndash50 mgL The coagulant PACl in dose of 27ndash35 mgL was injected into the raw water and mixed by a mixer with the speed of 150 rpm for first 3 min and after that 80 rpm continuously After coagulation the raw water was directed to the filter At the end of filter runs the filter was backwashed with the average flow rate 04 mmin and the entire backwash operation lasted 3 min

In the water backwash clean water being tap water

flowed into the filter bed through the porous resin plate The backwash effluent was collected in plastic tanks And dur-ing backwashing the backwash effluent samples were col-lected every 15 seconds for measuring the amount of sludge discharged The collected backwash effluent samples were filtered by a glass filter paper The solid residue sludge was dried to weigh

The backwash methods examined were the water back-wash and the milli backwash For the experiments raw water temperature and backwash water temperature were in between 12 and 13degC

Air bubble size measurementThe bubbly flow was flowed through a shallow acrylic

channel at the flow rate about 5 mmsec to take pictures of bubbles For taking pictures of microbubbles the water was flowed through a shallow acrylic channel at a low flow rate and a small amount of the micro bubble flow was intermit-tently added to the water flow and dispersed in the water flow Air bubble pictures were taken by a camera equipped with a micro lens Circles shown in pictures were regarded as the air bubble diameters and the air bubble size distribution was obtained from the picture The picture was not shown in the paper

Another filter column with 100 mm in diameter was prepared to measure the air bubble and the jet height The porous resin plate was used to support filter media particles and the nozzle with 05 mm in diameter was set at the center of the porous resin plate to flow air The air flow rate was controlled to generate certain air bubbles with the range of 05ndash30 mm in diameter The bed height of filter media par-ticles was varied in the range of 100ndash400 mm The water flow rate was 05 mmin

The air bubble and the jet generated at the dense bed sur-face were filmed by a video camera to measure the air bubble size and the jet height

RESULTS AND DISCUSSION

Bubbles generated by bubble generatorsFigures 3b and 3c show typical air bubble size distribu-

tions for bubbly and milli bubble flows The diameter in the figure denotes the volumetric mean diameter defined as d30 = ((Σ6Vbπ)n)13 (d30 volumetric mean diameter of air bubbles n number of air bubbles Vb air bubble volume) [1920] For the milli air bubbles the mean air bubble size was obtained by the above equation as the volume-surface mean diameter of air bubbles was significantly changed by a few large air

Fig 2 Schematic diagram of the experimental apparatus for self-backwash


bubbles The volumetric mean diameter was 185 mm for the bubbly flow and 32 mm for the milli air bubble flow The average diameter of micro air bubbles was 65 μm

Minimum fluidization velocity and fluidized bed ex-pansion

In the bubbly backwash the minimum fluidization veloc-ity was almost the same as that of the water backwash While the expansion ratio of the fluidized bed defined as the ratio of expanded bed height H to static bed height H0 was signifi-cantly different between the bubbly backwash and the water backwash Under the condition of the silica sand bed of H0 = 40 cm height the expansion ratio HH0 was about 112 in the water backwash but HH0 in the bubbly backwash and the milli backwash was about 102 Such a low expansion ratio shows the contraction of the fluidized bed due to the air bubble wake motion

Air bubbles and jetsMicro air bubbles smaller than 100 μm in diameter passed

through the porous resin plate without any resistance and the air bubble size did not change by the porous resin plate And no micro air bubbles grew on the way of rising up the fluidized bed

Air bubbles larger than a few millimeters in diameter ac-companied wakes underneath base of the air bubbles and the air bubbles grew on the way of rising up the fluidized bed Large air bubbles accompanied large wakes and vigorously agitated the fluidized bed On the other hand smaller air bubbles of about 1 mm in diameter little accompanied wakes and such smaller bubbles hardly agitated the fluidized bed

The air bubble size generated at the porous resin plate were affected by the air bubble size entering the filter When smaller air bubbles enter into the base of the filter a small amount of air bubbles was accumulated under the porous resin plate and relatively small air bubbles were generated intermittently through the porous resin plate However en-tering large air bubbles such as 1 cm air bubbles were ac-cumulated in large quantities under the porous resin plate and grew by coalescence Under such conditions air bubbles were generated successively through the porous resin plate and some of the generated air bubbles were coalesced and grew quickly Such phenomena correspond to air bubble generation from perforated plates [19]

The air bubble growth in the fluidized bed also depended on the bed height For the silica sand bed lower than about H0 = 20 cm height the air bubble growth was not much For the silica sand bed beyond H0 = 20 cm height the air bubble growth became more with increasing the bed height The air bubble of about 05 cm at the bottom of the fluidized bed became about 2ndash3 cm at the height of 40 cm Air bubbles except for micro air bubbles grew on the way of rising up the fluidized bed by coalescence and enhanced the agitation of the bed

Figure 4 shows the evolution of air bubbles and particle drift Particle drift induced by the air bubble is called a jet [21] The bubble diameter is about 1 cm The air bubble and the jet rise up the freeboard The jet collapses at a certain height in the freeboard and particles are settled back to the dense bed surface

Jet heights at interfaceFigure 5 shows the relationship between the jet height and

the size of the single air bubble at the dense bed surface The data represented by and were cited from the refer-ence 22 The jet height depends on the size andor density

Fig 3 Bubble size distributiona pump1 b pump1amp developed venturi-type bubble genera-tor (A) c developed venturi-type bubble generator (A)


of the particle as well as the air bubble size The larger the air bubble the higher the jet height Results shown in Fig 5 suggest that the jet height is affected by the particle-water interaction (shear stress on particle surface) [15] The main difference between the referencersquos and the present cases lies in the minimum fluidization velocity As the minimum fluidization velocity in the reference 22 is lower than that of the present case the bed expansion is larger and the solid holdup is smaller

The difference of the jet height between the silica sand bed and anthracite bed was little for air bubbles smaller than about 15 cm However for the air bubble of 3 cm the jet height in the anthracite bed was about twice as much as that of the silica sand bed If the freeboard height is not sufficiently large anthracite particles will be discharged In successive air bubbles it seems that the second bubble moves toward the first bubble due to the presence of the wake of the first bubble and the first and second bubbles are paired Bubble coalescence occurs between two bubbles Therefore for air bubbles generated successively the jet was larger than that of the single air bubble In the case of the silica sand bed for the air bubble of 3 cm the jet height caused by the single air bubble was about 10 cm and the jet height caused by the air bubbles generated successively became more than 20 cm Filter media particles were partially flowed out of the filter The air bubble size and the frequency of bubble generation should be suitably controlled to prevent the loss of filter media particles

In the present experiments it is considered that the de-sirable air bubble size generated at the dense bed surface was about 2ndash3 cm though the large air bubbles vigorously

pulsated the dense bed surface These experimental results show that the air bubble size at the inlet of the fluidized bed and that at the interface between the freeboard and the dense bed should be controlled properly

The surface layer formed by the remnants of sludge at the top of the filter bed becomes cemented into a compact crust and broken pieces form mudballs If there is no layer formed by remnants the mudballs will not be formed In the bubbly backwash no layer was observed at the filter bed surface Therefore the behavior of air bubble wakes is helpful for discharging the retained sludge and preventing the formation of mud balls to ensure the backwash effect

Removal rates of retained sludge by the constant flow rate backwash method

Figure 6 shows the variation of the retained sludge remov-al rate (R) with the water backwash in which five backwash methods are compared Figure 7 shows the average and deviation of R obtained in each backwash method R was defined as the ratio of W to Ws in which W was the amount of sludge removed from the filter bed by backwashing and Ws was the amount of sludge retained in the filter bed In the present filtration runs the turbidity of the filtrate was 005 mgL or less and little sludge was observed in filtrate So Ws was regarded as W0 which is the sum of K and P shown previously R was varied with the backwash method and was increased in the order of the water backwash the micro backwash the milli backwash the bubbly backwash and the air-water backwash

For the air scour operation in the air-water backwash the air bubbles were successively generated and the shape of the

Fig 4 Variation of a bubble wake and jet on the dense bed surface

Fig 5 Variation of jet height with bubble diameter


air bubble was an oblate ellipsoid rather than a sphere The fluidized bed contracted decreasing the fluidized bed height by about 2 cm On the other hand the water level increased 2ndash3 cm from the static water level The bed was vigorously agitated by air bubbles By water backwash after air scour R increased quite rapidly with increasing the backwash time and approached asymptotically the maximum value This re-sult indicates that in the air scour operation the fluidized bed was agitated vigorously and retained sludge was rubbed out so that the agglomerated sludge became fine and the retained sludge was effectively discharged

On the other hand for the water backwash R was much lower than that of the air-water backwash and was 595 in average even at the end of backwash In the water backwash the filter bed was fluidized to remove the retained sludge but the fluidized bed was not agitated strongly enough to separate the retained sludge from filter media particles And moreover after the backwash operation an accumulated layer of the remnants of sludge was observed at the filter bed surface It shows that the agglomerated sludge could not be become fine As the results R by the water backwash was low

For air-water bubbly flow backwash methods R increased successively with the backwash time For the bubbly back-wash R achieved 94 in average and for the milli backwash R achieved 84 in average The difference in R shows that micro bubbles can effectively enhance sludge discharge

Air bubble wakes play an essential role in determining the behavior and performance of the bed as the air bubble wakes induce intimate waterparticle mixing and are responsible for bed contraction Therefore the difference of the maximum

value of R between the bubbly backwash and the water back-wash was considered to be due to the effect of the air bubble wake As these effects caused by the air bubble wake boosted rubbing filter media particles and discharging the retained sludge the retained sludge was successively discharged up to the end of the backwash operation However since there were no air bubbles in the water backwash the sludge dis-charge stopped at the middle of the backwash time

For the micro backwash R reached 74 in average although there was neither agitation nor pulsation in the fluidized bed This result shows the effects of peculiar char-acteristics of micro air bubbles on the floating up motion of sludge The micro air bubble has adsorptivity and low negative electrical potential [23] The zeta potential of flocs (particles) formed by PACl is positive [24] Thus the micro air bubbles will be more likely to adhere to and promote floating up the sludge [25]

The bubbly backwash method could achieve the high re-moval rate by easier operations than the air-water backwash method Thus it is the most efficient backwash method

At the beginning of backwashing there was little dif-ference of R between the bubbly backwash and the water backwash In the bubbly backwash the bubbly flow in the fluidized bed for the first about 1 minute contained mainly small air babbles less than 2 mm and rarely contained large air bubbles of 5 mm or more Therefore as there was no effective agitation of the fluidized bed at the beginning of backwashing R was not so high

Fig 6 Variation of R with time in five backwash methods

Fig 7 Average removal rate and deviation of retained sludge in five backwash methods


Behaviors of the fluidized bed by bubbly flow in the self-backwash method

The backwash flow rate was about 04 mmin which was fairly lower compared with 06ndash07 mmin in conventional water backwash methods And also the expansion ratio HH0 was about 103 which was smaller than about 12 in conventional self-backwash methods Experimental results showed the occurrence of contraction of the fluidized bed The air bubbles were a little larger as shown in Fig 8 and the volumetric mean diameter of air bubbles was 64 mm They were accumulated in large quantities under the porous resin plate and grew by coalescence Then relatively larger air bubbles of about 1 cm were generated successively through the porous resin plate At the dense bed surface large air bubbles of about 4 cm and larger jets of 20ndash30 cm in height were observed frequently Two 5 mm mesh nets installed effectively suppressed the loss of the filter media particles

The removal rate of retained sludge in the self-back-wash method by the bubbly flow

The retained sludge in the bed per unit bed volume was about 012 dry-kgm3 which was smaller than 07 dry-kgm3 of constant filtration rate experiments It was considered that kaolin clay andor PACl were discharged partially as they could not form complete flocs So that the sludge feed amount was regarded as the amount of sludge obtained from the solid concentration of raw water And the net amount of retained sludge Ws was obtained as the difference between the sludge feed amount and the amount of discharged sludge F obtained from the sludge concentration of filtrate

Figure 9 shows a typical change of η and R by backwash time where comparison between the milli backwash and the water backwash is shown η was defined as the ratio of C to C0 in which C was the sludge concentration of backwash ef-fluent and C0 was 2 mgL which was the maximum turbidity allowed for backwash effluent R was defined as the ratio of W to Ws And Fig 10 shows the average and deviation of R obtained in each backwash experiment

As shown in Fig 9 in the milli backwash η decreased slower than that of the water backwash In the water back-wash the lower peak and rapid decrease of η indicate rela-tively less efficient backwashing In the milli backwash R increased successively with the backwash time while R in the water backwash saturated at a certain value

In the milli backwash the rising motion of air bubble wakes agitated the fluidized bed and rubbed filter media particles And also the surface layer of the dense bed was pulsated by jets Thus the retained sludge was discharged successively during the backwash operation As there were no air bubbles in the water backwash sludge discharge

Fig 8 Bubble size distribution generated by bubble genera-tors 1 and 2

Fig 9 Comparison of η and R in the water backwash and the bubbly backwash

Fig 10 Average removal rate and deviation of retained sludge in the water backwash and the milli backwash


stopped at the middle of the backwash time As the result R by the milli backwash achieved 92 in average while R by the water backwash was about 525

CONCLUSIONS

A fluidization backwash method by the air-water bubbly flow with air bubbles of various sizes was investigated focus-ing on the effect of air bubbles The following conclusions are drawn

1 The fluidizing bed in the fluidization backwash by the air-water bubbly flow shows the characteristics of three-phase fluidized beds and the fluidization backwash efficiency is boosted with coalescence bed contraction and jet generation caused by the motion of air bubble wakes For the bubbly backwash the removal efficiency could achieve 94 in average by easier operations than the conventional air water backwash method

2 The behavior of air bubbles is closely related to the specified characteristics of fluidized beds and affect the fluidization backwash efficiency The air bubble size should be properly controlled in terms of boosting the backwash efficiency and preventing the loss of filter media particles

3 The desirable air bubble size moving to the freeboard from the dense bed surface was about 2ndash3 cm and the corresponding air bubble size entering the fluidized bed was about a few mm

The milli backwash method was also effective for a self-backwash filter and the removal efficiency of retained sludge achieved 92 in average

REFERENCES

[1] Amirtharajah A Fundamentals and theory of air scour J Environ Eng 110(3) 573ndash590 1984 doi101061(ASCE)0733-9372(1984)1103(573)

[2] Stevenson D Process conditions for the backwash-ing of filters with simultaneous air and water Water Res 29(11) 2594ndash2597 1995 doi1010160043-1354(95)00041-I

[3] Humby MS Fitzpatrick CSB Attrition of granular filter media during backwashing with combined air and water Water Res 30(2) 291ndash294 1996 doi1010160043-1354(95)00177-8

[4] Ogawa S Sano S Hydraulics in the process of acti-vated carbon filtration systems Proceedings of the Japan Society of Civil Engineers No572II-40 63ndash72 1997 [in Japanese with English abstract] doi102208jscej1997572_63

[5] Hall D Fitzpatrick CSB Specctral analysis of pressure variations during combined air and water backwashof rapid gravity filters Water Res 33(17) 3666ndash3672 1999 doi101016S0043-1354(99)00092-5

[6] Turan M Sabah E Gulsen H Celik MS Influence of media characteristics on energy dissipation in filter backwashing Environ Sci Technol 37(18) 4288ndash4292 2003 PMID14524466 doi101021es020661r

[7] Amburgey JE Optimization of the extended terminal subfluidization wash (ETSW) filter backwashing proce-dure Water Res 39(2ndash3) 314ndash330 2005 doi101016jwatres200409020

[8] Han SJ Fitzpatrick CSB Wetherill A Simulation on combined rapid gravity filtration and backwash models Water Sci Technol 59(12) 2429ndash2435 2009 PMID19542649 doi102166wst2009308

[9] Naseer R Alhail SA Lu X-W Fluidization and opti-mum backwashing conditions in multimedia filter Res J Appl Sci Eng Technol 3(11) 1302ndash1307 2011

[10] Slavik I Jehmlich A Uhl W Impact of backwash-ing procedures on deep bed filtration productiv-ity in drinking water treatment Water Res 47(16) 6348ndash6357 2013 PMID24008223 doi101016jwatres201308009

[11] Hemmings DG Fitzpatrick CSB Pressure signal analysis of combined water and air backwash of rapid gravity filters Water Res 31(2) 356ndash361 1997 doi101016S0043-1354(96)00279-5

[12] Tsutsumi A Chen W Kim YH Classification and char-acterization of hydrodynamic and transport behaviors of three-phase reactors Korean J Chem Eng 16(6) 709ndash720 1999 doi101007BF02698341

[13] Fan LS Yang GQ 27 Gas-liquid-solid three-phase flu-idization In Yang WC (ed) Handbook of Fluidization and Fluid-particle Systems Marcel Dekker New York USA pp 765ndash810 2003

[14] Yang GQ Du B Fan LS Bubble formation and dy-namics in gasndashliquidndashsolid fluidizationmdashA review Chem Eng Sci 62(1ndash2) 2ndash27 2007 doi101016jces200608021


[15] Tsuchiya K Song GH Tang WT Fan LS RampD Note Particle drift induced by a bubble in a liquid-solid flu-idized bed with low-density particles AIChE J 38(11) 1847ndash1851 1992 doi101002aic690381117

[16] Ebie K Fundamental characteristics of high-rate fil-tration through uniform sand filters Kitami Institute Technology Report 3(1) 151ndash164 1971

[17] Hozumi H Yoshida H Jaren M Ioroi H A study on advanced treatment of secondary wastewater effluent by a dual-media filter Proc Environ Eng Res 32 339ndash347 1995 [in Japanese with English abstract] doi1011532proes199232339

[18] Gitis V Rubinstein I Livshits M Ziskind G Deep-bed filtration model with multistage deposition kinetics Chem Eng J 163(1ndash2) 78ndash85 2010 doi101016jcej201007044

[19] Miyahara T Matsuba Y Takahashi T Size of bubbles generated from perforated plates Kagaku Kogaku Ron-bunshu 8(1) 13ndash17 1982 [in Japanese with English abstract] doi101252kakoronbunshu813

[20] Miyahara T Hashimoto S Matsunaga H Akagi Y Bubble splitting by turbulent jet emitted from orifice J Chem Eng of Jpn 32(1) 91ndash96 1999 doi101252jcej3291

[21] Reiter G Schwerdtfeger K Observations of physi-cal phenomena occurring during passage of bubbles through liquidliquid interfaces ISIJ Int 32(1) 50ndash56 1992 doi102355isijinternational3250

[22] Miyahara T Tsuchiya K Fan LS Mechanism of particle entrainment in a gas-liquid-solid fluidized bed AIChE J 35(7) 1195ndash1198 1989 doi101002aic690350715

[23] Serizawa S Fundamentals of micronano bubbles Journal of the JIME 46(6) 56ndash61 2011 [in Japanese] doi105988jime46861

[24] Mogaki T Yoshioka T Mizoguchi T Purification plant sludge concentration using electrophoresis J of Japan Society of Material Recycles and Waste Management 17(1) 42ndash49 2006 [in Japanese with English abstract] doi103985jswme1742

[25] Terasaka K Shinpo Y Separation of fine particles suspended in water using microbubble floatation Japanese J Multiphase Flow 21(1) 77ndash83 2007 [in Japanese] doi103811jjmf2177


In the three-phase fluidized bed the air bubble motion is closely related to the air bubbles size Also the fluidizing condition can be clearly identified compared to the collapse pulsing condition under the semi-fluidization state studied by Amirtharajah[1]

Since sludge retained in the filter bed is often distributed in the range of several centimeters under the surface of the filter bed [16-18] the motion of air bubble wakes in the fluidized bed seems to be helpful for removing the retained sludge It also would prevent the formation of mudballs The fluidization backwashing method utilizing the advantage of the characteristics of air bubble motion is considered to be very effective compared to the conventional backwashing methods However the research work in many literatures has focused on the air and water flow rates and the minimum fluidization velocity of the filter media particle There seems to be few researches focusing on the air bubbles

Therefore the effect of air bubbles on fluidization back-wash efficiency was investigated experimentally by varying the size of air bubbles in the air-water bubbly flow to develop an optimum air-water bubbly flow backwash method In ad-dition the fluidization backwash method with the air-water bubbly flow was applied to a conventional self-backwash filter where the backwash flow rate depends on an elevated water tank The effect of the air-water bubble flow on the backwash efficiency was also investigated

MATERIALS AND METHODS

Constant flow rate backwash methodsIn tap water treatment raw water with high turbidity is

generally treated by a combined process of coagulation-flocculationsedimentation followed by bed filtration For low turbidity raw water direct filtration is considered as a suitable process The aim of this study is low turbidity groundwater treatment

A schematic diagram of the experimental apparatus for a constant flow rate operation is shown in Fig 1 PACl in Fig1 shows polyaluminium chloride The filtration column is a transparent polyvinyl chloride pipe with the diameter of 200 mm and the total height of 1600 mm and a scale for measuring the filter bed height is attached to the surface of the filter column The filter media is silica sand (effective size of 06 mm and uniformity coefficient of 15) and anthracite (effective size of 12 mm and uniformity coefficient of 15) and the specified bed height is in the range of 400ndash600 mm The filter media particles are supported on a porous resin plate with the porosity of about 33 and the pore diameter of 1 mm

The raw water line consisted of a raw water tank a poly-aluminium chloride reservoir a pump a static mixer a flow controller and a pressure gauge The backwash line consisted of a backwash water tank a pump an air pump a flow con-troller and a bubble generator The fine bubble generator (A) was a developed venturi-type bubble generator The inlet

Fig 1 Schematic diagram of the experimental apparatus for constant flow rate backwash















































































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the fluidization backwash method of filter beds by air

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