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

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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 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 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/ 349 350 Journal of Water and Environment Technology, Vol. 18, No. 6, 2020 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 con- troller 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.
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