The Fluidization Backwash Method of Filter Beds by Air-Water Bubbly Flow

doi: 10.2965/jwet.20-014 Journal of Water and Environment Technology, Vol.18, No.6: 349–358, 2020

Original Article The Fluidization Backwash Method of Filter Beds by Air-water Bubbly Flow

Masao Kuroda, Anri Yoshida, Emi Obuchi, Hironoshin Kawabata, Tadao Arai

Yamato Corporation Environmental Technology Research Center, Maebashi, Japan

ABSTRACT A 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 motion. 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 optimum air and water flow rates. Fluidization backwashing is an efficient backwash method, The backwashing of the filter layer is essential for the because the fluidization condition could be readily controlled filtration operation. A combined air and water backwash to discharge the retained sludge. Since the size of silica sand method is widely used as an effective backwash method. and anthracite used as filter media particles is small, the However, it is not easy to determine the optimum operating water flow rate is limited in a certain specified range which conditions such as the flow rates. Therefore, many studies is a little more than the minimum fluidization velocity. The have been conducted to determine the optimum air and water fluidized bed may be categorized as a three-phase fluidized flow rates, the bed expansion ratio and so on 1–11[ ]. Amirth- bed and will be divided into two parts of a freeboard region arajah investigated the semi-fluidization backwash method, and a dense region [12,13]. and derived a relational equation to predict the optimum Characteristics of the three-phase fluidized bed are gener- water flow velocity, the minimum fluidization velocity of the ally affected not only by the air and water flow rates but also filter media particle and the air flow rate1 [ ]. A recent paper by the air bubble and the particle [12–14]. When the particle has indicated that the amount of backwash water varied size is small and the water flow rate is limited in a certain according to the backwash process and the operating con- range used in the fluidization backwash operation, the air ditions, but the net clean water production rate subtracting bubble is important in terms of the behavior of the fluidized the wastewater for washing was almost constant [10]. This bed. The air bubble grows by coalescence on the way of result suggests that auxiliary water wash may be necessary rising up the fluidized bed, and the rising motion of the air to obtain sufficiently high backwash efficiency and the equa- bubble causes vigorous agitation and mixing in the fluidized tion presented by Amirtharajah might not always express the bed [12–15].

Corresponding author: Masao Kuroda, E-mail: [email protected] Received: 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) 4.0 License. http:// creativecommons.org/licenses/by/4.0/

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

In the three-phase fluidized bed, the air bubble motion is MATERIALS AND METHODS closely related to the air bubbles size. Also, the fluidizing condition can be clearly identified compared to the collapse Constant flow rate backwash methods pulsing condition under the semi-fluidization state studied In tap water treatment, raw water with high turbidity is by Amirtharajah[1]. generally treated by a combined process of coagulation- Since sludge retained in the filter bed is often distributed flocculation/sedimentation followed by bed filtration. For in the range of several centimeters under the surface of the low turbidity raw water, direct filtration is considered as filter bed 16-18 [ ], the motion of air bubble wakes in the a suitable process. The aim of this study is low turbidity fluidized bed seems to be helpful for removing the retained groundwater treatment. sludge. It also would prevent the formation of mudballs. The A schematic diagram of the experimental apparatus for fluidization backwashing method utilizing the advantage of a constant flow rate operation is shown in Fig. 1. PACl in the characteristics of air bubble motion is considered to be Fig.1 shows polyaluminium chloride. The filtration column very effective compared to the conventional backwashing is a transparent polyvinyl chloride pipe with the diameter methods. However, the research work in many literatures of 200 mm and the total height of 1,600 mm, and a scale for has focused on the air and water flow rates and the minimum measuring the filter bed height is attached to the surface of fluidization velocity of the filter media particle. There seems the filter column. The filter media is silica sand (effective size to be few researches focusing on the air bubbles. of 0.6 mm and uniformity coefficient of 1.5) and anthracite Therefore, the effect of air bubbles on fluidization back- (effective size of 1.2 mm and uniformity coefficient of 1.5) wash efficiency was investigated experimentally by varying and the specified bed height is in the range of 400–600 mm. the size of air bubbles in the air-water bubbly flow to develop The filter media particles are supported on a porous resin an optimum air-water bubbly flow backwash method. In ad- plate with the porosity of about 33% and the pore diameter dition, the fluidization backwash method with the air-water of 1 mm. bubbly flow was applied to a conventional self-backwash The raw water line consisted of a raw water tank, a poly- filter where the backwash flow rate depends on an elevated aluminium chloride reservoir, a pump, a static mixer, a flow water tank. The effect of the air-water bubble flow on the controller and a pressure gauge. The backwash line consisted backwash efficiency was also investigated. of a backwash water tank, a pump, an air pump, a flow controller and a bubble generator. The fine bubble generator (A) was a developed venturi-type bubble generator. The inlet Journal of Water and Environment Technology, Vol. 18, No. 6, 2020 351

Table1 Summary of the backwash procedures for constant flow rate operations. Air-water bubbly flow Water Air-water Micro Milli Bubbly t[min] 5 8*1 5 5 5

Va[NL/min] 24 Air1=0.5 Air2=1.0 Air1+Aair2 Vw[m/min] 0.5 0.5 0.5 0.5 0.5 *2 *2 *2 *2 db[mm] 53 0.065 3.2 1.85 *1 Air scour 3 min, Water 5 min *2 Mean diameter angle and the outlet angle were 40° and 5.5°respectively, and equivalent to the amount of kaolin clay K and PACl sludge the outlet-throat diameter ratio was 3.67. P which were fed for each filter run. P was aluminum hy-

Backwash methods examined are a water flow backwash drate (Al (OH)3) of 2 moles produced from aluminum oxide method (water backwash), a combined air and water flow (Al2O3) of 1 mole. K and P were calculated by the equations 3 backwash method (air-water backwash) and an air-water K = QCk and P = 0.153αQCp respectively (Q: raw water [m ], 3 bubbly flow backwash method. In the air-water bubbly flow Ck: turbidity [kg/m ], α: Al2O3 content [-], Cp: PACl dose backwash method, three methods were examined. They were [kg/m3]). a micro air bubble flow backwash method (micro backwash), The backwash effluent was collected in 200 litter plastic a milli air bubble flow backwash method (milli backwash) tanks. The solid mass contained in the backwash effluent, the and a combined micro- and milli air bubble flow backwash sludge, was determined from the solid concentration of efflu- method (bubbly backwash). ent collected in the plastic tank. The turbidity measurement Raw water was prepared by adding kaolin clay powder into was carried out by a turbidity meter (Turbidity meter WA tap water with the concentration range of 1.5−2.5 mg/L. The 6000, Nippon Denshoku Industries Co., Ltd., Tokyo, Japan). coagulant polyaluminium chloride (PACl) (PAC 6010 liquid, For the experiments, raw water temperature and backwash basicity 55–65%,10–11% Al2O3, specific gravity 1.2 (Taimei water temperature varied between 15 and 17°C. Chemical Co. Ltd. Tokyo Japan)) in dose of 15–23 mg/L was injected to the raw water line at a certain place just before Self-backwash methods with variable flow rates de- the static mixer, and the raw water was directed to the filter. pending on the head The filtration rate was in the rage of 100–160 m/day, and the A schematic diagram of the experimental apparatus for a filtration operation completed when the head loss reached 20 variable flow rate operation is shown in Fig. 2. The filtra- k P. tion column is a transparent acrylic pipe with the diameter At the end of filter run, the filter was backwashed with of 300 mm and the total height of 1,200 mm, and a scale for the backwash methods mentioned above. Tap water was used measuring the filter bed height is attached to the surface of as the clean water. In the air-water backwash operation, the the filter column. Silica sand (effective size of 0.6 mm and water level in the filter was drawn down to just above the uniformity coefficient of 1.5) and anthracite (effective size bed surface to prevent the discharge of particles, and after of 1.2 mm and uniformity coefficient of 1.5) are used as that a 3 min air scour was carried out. After air scouring, the filter media particles, and each packed height is 270 mm and bed was washed by the water flow. In air-water bubbly flow 200 mm respectively. The filter media particles were sup- backwash operations, air bubbles were generated as follows. ported on the porous resin plate with the porosity of about Micro air bubbles were generated by the pump1, and milli 33% and the pore diameter of 1 mm. air bubbles were generated by the developed venturi-type The raw water line consisted of a raw water tank, a pump bubble generator (A). For the micro-and milli air bubble flow, and a flow controller. The backwash line consisted of a back- the micro bubble flow generated by the pump1 was passed wash water tank, a pump, an air pump, a flow controller, two through the developed venturi-type bubble generator (A). nets and two bubble generators. Two 5 mm mesh nets were

The air flow rate aV , the water flow rate w,V the backwash installed 6 cm above the top of the filter bed to prevent the time t and the bubble diameter db were summarized in Table discharge of filter media particles. 1. In Fig. 2 the bubble generator 1 was made of a porous tube Solid mass retained in the filter (sludge) will be considered and the bubble generator 2 was a developed venturi-type 352 Journal of Water and Environment Technology, Vol. 18, No. 6, 2020

flowed into the filter bed through the porous resin plate. The backwash effluent was collected in plastic tanks. And during backwashing, the backwash effluent samples were collected 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 backwash and the milli backwash. For the experiments, raw water temperature and backwash water temperature were in between 12 and 13°C.

Air bubble size measurement The bubbly flow was flowed through a shallow acrylic channel at the flow rate about 5 mm/sec 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 intermittently 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 Fig. 2 Schematic diagram of the experimental apparatus for the air bubble diameters, and the air bubble size distribution self-backwash. was obtained from the picture. The picture was not shown in the paper. bubble generator (2). The inlet angle and the outlet angle were Another filter column with 100 mm in diameter was 40° and 6° respectively, and the outlet-throat diameter ratio prepared to measure the air bubble and the jet height. The was 2.3. The porosity, pore diameter and length of the porous porous resin plate was used to support filter media particles, tube were about 51%, 200 μm, and 300 mm respectively. In and the nozzle with 0.5 mm in diameter was set at the center order to generate small air bubbles under several millimeters of the porous resin plate to flow air. The air flow rate was in diameter, the backwash water was passed through the controlled to generate certain air bubbles with the range of bubble generator 1 at a certain flow rate and air was injected 0.5–30 mm in diameter. The bed height of filter media par- at the flow rate of 1.8 NL/min to the bubble generator 1. The ticles was varied in the range of 100–400 mm. The water backwash water tank was placed at 900 mm height above the flow rate was 0.5 m/min. filtration column, and the effective head was 1,500 mm. The air bubble and the jet generated at the dense bed sur- The filtration rate was 90 m/day, and the filtration op- face were filmed by a video camera to measure the air bubble eration lasted 18.1 min. Raw water turbidity was adjusted as size and the jet height. close as possible to the amount of retained sludge per bed volume measured in constant filtration rate experiments. RESULTS AND DISCUSSION The raw water was prepared by adding kaolin clay powder into tap water, and the concentration of kaolin clay was in Bubbles generated by bubble generators the range of 40–50 mg/L. The coagulant PACl in dose of Figures 3b and 3c show typical air bubble size distribu- 27–35 mg/L was injected into the raw water, and mixed by tions for bubbly and milli bubble flows. The diameter in the a mixer with the speed of 150 rpm for first 3 min, and after figure denotes the volumetric mean diameter defined as d30 = 1/3 that 80 rpm continuously. After coagulation, the raw water ((Σ6Vb/π)/n) (d30: volumetric mean diameter of air bubbles, was directed to the filter. At the end of filter runs, the filter n: number of air bubbles, Vb: air bubble volume) [19,20]. For was backwashed with the average flow rate, 0.4 m/min, and the milli air bubbles, the mean air bubble size was obtained the entire backwash operation lasted 3 min. by the above equation, as the volume-surface mean diameter In the water backwash, clean water being tap water of air bubbles was significantly changed by a few large air Journal of Water and Environment Technology, Vol. 18, No. 6, 2020 353

Air bubbles and jets Micro 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 accompanied 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, entering large air bubbles such as 1 cm, air bubbles were accumulated 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, Fig. 3 Bubble size distribution. and some of the generated air bubbles were coalesced and a: pump1, b: pump1& developed venturi-type bubble genera- grew quickly. Such phenomena correspond to air bubble tor (A), c: developed venturi-type bubble generator (A) 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 bubbles. The volumetric mean diameter was 1.85 mm for the H0 = 20 cm height, the air bubble growth was not much. For bubbly flow and 3.2 mm for the milli air bubble flow. The the silica sand bed beyond H0 = 20 cm height, the air bubble average diameter of micro air bubbles was 65 μm. growth became more with increasing the bed height. The air bubble of about 0.5 cm at the bottom of the fluidized bed Minimum fluidization velocity and fluidized bed -ex became about 2–3 cm at the height of 40 cm. Air bubbles pansion except for micro air bubbles grew on the way of rising up the fluidized bed by coalescence, and enhanced the agitation of In the bubbly backwash, the minimum fluidization veloc- the bed. ity was almost the same as that of the water backwash. While Figure 4 shows the evolution of air bubbles and particle the expansion ratio of the fluidized bed defined as the ratio of drift. Particle drift induced by the air bubble is called a jet expanded bed height H to static bed height H0, was signifi- [21]. The bubble diameter is about 1 cm. The air bubble and cantly different between the bubbly backwash and the water the jet rise up the freeboard. The jet collapses at a certain backwash. Under the condition of the silica sand bed of H0 height in the freeboard, and particles are settled back to the = 40 cm height, the expansion ratio, H/H0 was about 1.12 in dense bed surface. the water backwash, but H/H0 in the bubbly backwash and the milli backwash was about 1.02. Such a low expansion ratio shows the contraction of the fluidized bed due to the air Jet heights at interface bubble wake motion. Figure 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 reference 22. The jet height depends on the size and/or density 354 Journal of Water and Environment Technology, Vol. 18, No. 6, 2020

Fig. 4 Variation of a bubble wake and jet on the dense bed Fig. 5 Variation of jet height with bubble diameter. surface. of the particle as well as the air bubble size. The larger the pulsated the dense bed surface. These experimental results air bubble, the higher the jet height. Results shown in Fig. show that the air bubble size at the inlet of the fluidized bed 5 suggest that the jet height is affected by the particle-water and that at the interface between the freeboard and the dense interaction (shear stress on particle surface) [15]. The main bed should be controlled properly. difference between the reference’s and the present cases The surface layer formed by the remnants of sludge at the lies in the minimum fluidization velocity. As the minimum top of the filter bed becomes cemented into a compact crust, fluidization velocity in the reference 22 is lower than that of and broken pieces form mudballs. If there is no layer formed the present case, the bed expansion is larger and the solid by remnants, the mudballs will not be formed. In the bubbly holdup is smaller. backwash, no layer was observed at the filter bed surface. The difference of the jet height between the silica sand Therefore, the behavior of air bubble wakes is helpful for bed and anthracite bed was little for air bubbles smaller discharging the retained sludge and preventing the formation than about 1.5 cm. However, for the air bubble of 3 cm, the of mud balls to ensure the backwash effect. jet height in the anthracite bed was about twice as much as that of the silica sand bed. If the freeboard height is not Removal rates of retained sludge by the constant sufficiently large, anthracite particles will be discharged. In flow rate backwash method successive air bubbles, it seems that the second bubble moves Figure 6 shows the variation of the retained sludge remov- toward the first bubble due to the presence of the wake of al rate (R) with the water backwash, in which five backwash the first bubble, and the first and second bubbles are paired. methods are compared. Figure 7 shows the average and Bubble coalescence occurs between two bubbles. Therefore, deviation of R obtained in each backwash method. R was for air bubbles generated successively, the jet was larger than defined as the ratio of W to Ws, in which W was the amount that of the single air bubble. In the case of the silica sand of sludge removed from the filter bed by backwashing and bed, for the air bubble of 3 cm, the jet height caused by the Ws was the amount of sludge retained in the filter bed. In single air bubble was about 10 cm, and the jet height caused the present filtration runs, the turbidity of the filtrate was by the air bubbles generated successively became more than 0.05 mg/L or less, and little sludge was observed in filtrate.

20 cm. Filter media particles were partially flowed out of So Ws was regarded as W0 which is the sum of K and P the filter. The air bubble size and the frequency of bubble shown previously. R was varied with the backwash method generation should be suitably controlled to prevent the loss and was increased in the order of the water backwash, the of filter media particles. micro backwash, the milli backwash, the bubbly backwash In the present experiments, it is considered that the de- and the air-water backwash. sirable air bubble size generated at the dense bed surface For the air scour operation in the air-water backwash, the was about 2–3 cm, though the large air bubbles vigorously air bubbles were successively generated, and the shape of the Journal of Water and Environment Technology, Vol. 18, No. 6, 2020 355

Fig. 7 Average removal rate and deviation of retained sludge in five backwash methods. Fig. 6 Variation of R with time in five backwash methods.

value of R between the bubbly backwash and the water back- air bubble was an oblate ellipsoid rather than a sphere. The wash was considered to be due to the effect of the air bubble fluidized bed contracted decreasing the fluidized bed height wake. As these effects caused by the air bubble wake boosted by about 2 cm. On the other hand, the water level increased rubbing filter media particles and discharging the retained 2–3 cm from the static water level. The bed was vigorously sludge, the retained sludge was successively discharged up agitated by air bubbles. By water backwash after air scour, to the end of the backwash operation. However, since there R increased quite rapidly with increasing the backwash time were no air bubbles in the water backwash, the sludge dis- and approached asymptotically the maximum value. This re- charge stopped at the middle of the backwash time. sult indicates that in the air scour operation the fluidized bed For the micro backwash, R reached 74% in average, was agitated vigorously and retained sludge was rubbed out, although there was neither agitation nor pulsation in the so that the agglomerated sludge became fine and the retained fluidized bed. This result shows the effects of peculiar char- sludge was effectively discharged. acteristics of micro air bubbles on the floating up motion On the other hand, for the water backwash R was much of sludge. The micro air bubble has adsorptivity and low lower than that of the air-water backwash, and was 59.5% in negative electrical potential [23]. The zeta potential of flocs average even at the end of backwash. In the water backwash, (particles) formed by PACl is positive [24]. Thus, the micro the filter bed was fluidized to remove the retained sludge, air bubbles will be more likely to adhere to and promote but the fluidized bed was not agitated strongly enough to floating up the sludge 25[ ]. separate the retained sludge from filter media particles. And The bubbly backwash method could achieve the high re- moreover, after the backwash operation an accumulated moval rate by easier operations than the air-water backwash layer of the remnants of sludge was observed at the filter bed method. Thus, it is the most efficient backwash method. surface. It shows that the agglomerated sludge could not be At the beginning of backwashing, there was little dif- become fine. As the results, R by the water backwash was ference of R between the bubbly backwash and the water low. backwash. In the bubbly backwash, the bubbly flow in the For air-water bubbly flow backwash methods, R increased fluidized bed for the first about 1 minute contained mainly successively with the backwash time. For the bubbly back- small air babbles less than 2 mm and rarely contained large wash, R achieved 94% in average and for the milli backwash air bubbles of 5 mm or more. Therefore, as there was no R achieved 84% in average. The difference in R shows that effective agitation of the fluidized bed at the beginning of micro bubbles can effectively enhance sludge discharge. backwashing, R was not so high. Air bubble wakes play an essential role in determining the behavior and performance of the bed, as the air bubble wakes induce intimate water/particle mixing and are responsible for bed contraction. Therefore, the difference of the maximum 356 Journal of Water and Environment Technology, Vol. 18, No. 6, 2020

Fig. 8 Bubble size distribution generated by bubble genera- Fig. 10 Average removal rate and deviation of retained tors 1 and 2. sludge in the water backwash and the milli backwash.

The removal rate of retained sludge in the self-backwash method by the bubbly flow The retained sludge in the bed per unit bed volume was about 0.12 dry-kg/m3 which was smaller than 0.7 dry-kg/ m3 of constant filtration rate experiments. It was considered that kaolin clay and/or 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, Fig. 9 Comparison of η and R in the water backwash and the F, obtained from the sludge concentration of filtrate. bubbly backwash. 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 Behaviors of the fluidized bed by bubbly flow in the C , in which C was the sludge concentration of backwash ef- self-backwash method 0 fluent, and C was 2 mg/L which was the maximum turbidity The backwash flow rate was about 0.4 m/min, which was 0 allowed for backwash effluent. R was defined as the ratio of fairly lower compared with 0.6–0.7 m/min in conventional W to Ws. And Fig. 10 shows the average and deviation of R water backwash methods. And also, the expansion ratio, obtained in each backwash experiment. H/H , was about 1.03, which was smaller than about 1.2 in 0 As shown in Fig. 9, in the milli backwash, η decreased conventional self-backwash methods. Experimental results slower than that of the water backwash. In the water back- showed the occurrence of contraction of the fluidized bed. wash, the lower peak and rapid decrease of η indicate rela- The air bubbles were a little larger as shown in Fig. 8, and the tively less efficient backwashing. In the milli backwash, R volumetric mean diameter of air bubbles was 6.4 mm. They increased successively with the backwash time, while R in were accumulated in large quantities under the porous resin the water backwash saturated at a certain value. plate, and grew by coalescence. Then relatively larger air In the milli backwash, the rising motion of air bubble bubbles of about 1 cm were generated successively through wakes agitated the fluidized bed, and rubbed filter media the porous resin plate. At the dense bed surface, large air particles. And also, the surface layer of the dense bed was bubbles of about 4 cm and larger jets of 20–30 cm in height pulsated by jets. Thus, the retained sludge was discharged were observed frequently. Two 5 mm mesh nets installed successively during the backwash operation. As there were effectively suppressed the loss of the filter media particles. no air bubbles in the water backwash, sludge discharge Journal of Water and Environment Technology, Vol. 18, No. 6, 2020 357 stopped at the middle of the backwash time. As the result, R [4] Ogawa S, Sano S: Hydraulics in the process of acti- by the milli backwash achieved 92% in average, while R by vated carbon filtration systems. Proceedings of the the water backwash was about 52.5%. Japan Society of Civil Engineers, No.572/II-40, 63–72, 1997. [in Japanese with English abstract] doi:10.2208/ CONCLUSIONS jscej.1997.572_63 [5] Hall D, Fitzpatrick CSB: Specctral analysis of pressure A fluidization backwash method by the air-water bubbly variations during combined air and water backwashof flow with air bubbles of various sizes was investigated focus- rapid gravity filters. Water Res., 33(17), 3666–3672, ing on the effect of air bubbles. 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The behavior of air bubbles is closely related to the on combined rapid gravity filtration and backwash specified characteristics of fluidized beds, and affect models. Water Sci. Technol., 59(12), 2429–2435, 2009. the fluidization backwash efficiency. The air bubble PMID:19542649 doi:10.2166/wst.2009.308 size should be properly controlled in terms of boosting [9] Naseer R, Alhail SA, Lu X-W: Fluidization and opti- the backwash efficiency and preventing the loss of filter mum backwashing conditions in multimedia filter.Res. media particles. J. Appl. Sci. Eng. Technol., 3(11), 1302–1307, 2011. 3. The desirable air bubble size moving to the freeboard [10] Slavik I, Jehmlich A, Uhl W: Impact of backwash- from the dense bed surface was about 2–3 cm, and the ing procedures on deep bed filtration productiv- corresponding air bubble size entering the fluidized bed ity in drinking water treatment. Water Res., 47(16), was about a few mm. 6348–6357, 2013. PMID:24008223 doi:10.1016/j. 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