Clean bed head-loss of various filter media

Granüler filtrasyon su arıtımında yaygın olarak kullanılan bir arıtma yöntemi olup bu prosesin tasarım ve işletmesi açısından yük kayıpları büyük önem taşımaktadır. Bu çalışmada laboratuvar ölçekli bir filtrasyon kolonunda sık kullanılan farklı filtre malzemeleri için temiz yatak yük kayıpları belirlenmiştir. Karışık boyutta elde edilen malzemelerin eleme işlemi ile fraksiyonlarına ayrılması sonucu 6 kum, 5 perlit, 8 garnet ve 3 kırık cam fraksiyonu elde edilmiştir. Karşılaştırma amacıyla küreler ile de deneysel çalışma yürütülmüştür. Deney düzeneği filtredeki hidrolik koşulların da etkisini incelemek amacıyla yüksek hızlarda da veri elde edilebilecek şekilde tasarlanmıştır. En yüksek yük kayıpları malzemenin çapına da bağlı olarak 0.1 m/sn filtre hızında 5-6 m olarak gözlenmiştir. Filtrasyon hızı ile yük kaybının lineer olmayan bir şekilde değiştiği tüm malzeme türleri ve her bir fraksiyon için teyit edilmiştir. Aynı zamanda gözeneklilik ve tanecik çapı parametrelerinin yük kaybı üzerindeki etkisi incelenmiştir. Yaklaşık olarak aynı tanecik çapındaki kum, perlit ve garnet yataklarında yük kaybı oluşumu karşılaştırmalı olarak incelenmiş ve gözenekliliğin en düşük olduğu kum yatakta en yüksek yük kaybı elde edilmiştir. Küresellik ile ifade edilen malzeme şeklinin gözenekliliği etkileyen bir unsur olduğu ortaya konmuştur. Tane çapının yük kaybı üzerindeki etkisi kum ve garnetin farklı fraksiyonlarından oluşan kapsamlı bir aralıkta incelenmiş ve her iki malzeme türü için de tane çapı küçüldükçe yük kaybının arttığı gözlenmiştir. Anahtar Kelimeler: Filtrasyon, granüler malzeme, temiz yatak, yük kaybı, gözeneklilik, küresellik.Granular filtration is a process that is widely used for removing particulate matter from water. The granular media filtration process is affected by the properties of the filter media including grain size, bed porosity and specific surface area. Especially, determination of clean bed head loss is important in the design and operation of filters. Clean bed head-loss of various common filter media obtained from several sources was determined in a laboratory scale filter column. The cylindrical column made of plexiglass was 4 cm in diameter and 2 m in height. It was connected to a water tank through a series of pipes and valves. The water tank was filled with tap water and served as a water reservoir for the filtration column. A constant speed centrifugal pump drew water from the tank and pumped it to the top of the column. In addition, the system allowed the water to flow upwards in the column to achieve various porosity ranges as well as to fluidize the media and bleed any residual air. In the filtration cycle the water travelled down the filter column through a bed of media and was then returned to the water tank. A cartridge filter was used to trap possible suspended solids thus ensuring the recirculated water remained clean. To measure the head-loss across the media bed as water passed through it, piezometer taps at the top and bottom of the media were connected to a water-air manometer, as well as a mercury manometer and a differential pressure transducer. Flow rate of the equipment being used determined the choice of the instrument. Flow rate was measured by an electromagnetic flow meter across the range of 0.17-17L/min. Because the density and dynamic viscosity of the fluid changes with temperature, a Pt-100 thermometer was installed on the column and temperature was monitored continuously. Once the media had been loaded in the column, the filter was operated in the down flow mode and the flow rate was gradually increased and then decreased to a minimum value prior to head-loss measurements. As such the compaction of the media during the experiment was prevented. The porosity of the filter bed was determined separately for each run using the weight of the filter media introduced to the column, the height of the filter bed, the inside diameter of the filter column and the specific gravity of the media. The media which were normally composed of different grain sizes were sieved and 6 fractions of sand, 5 fractions of perlite, 8 fractions of garnet and 3 fractions of crushed glass were obtained. Experiments were also conducted on using glass beads for comparison. Experimental set-up was designed so as to obtain data at high filtration rates in order to evaluate the hydraulic behavior in the column. The maximum head-loss was measured as 5-6 m for 0.1 m/s filter rate. Head-loss measurements were made? for a minimum of three porosities for each medium. These porosities corresponded to the maximum compaction that could be obtained by directly tapping on the column, gradual shut off of backwash water and an intermediate value between these. For each type of medium and fraction it was confirmed with literature that there exists a non-linear relationship between filter rate and head-loss. Besides, effect of porosity and grain size on head-loss was assessed. As the bed consists of uniform particles when a sieved fraction of medium is used instead of its mixed form, more robust evaluations were possible. The sensitivity of head-loss to porosity was examined via extensive experiments. The head-loss occurrence in beds of sand, perlite and garnet of approximately same grain size were compared to each other. The highest head-loss was observed for the bed composed of sand due to the lowest porosity value. Also, experiments were conducted with approximately same size of sand and glass beads separately but at the same porosity. It was noticed that sand caused higher head-losses, which were more easily observed at higher filter rates. Different porosities were achieved by directly tapping to the column or gradual shutoff of the backwash water. Also, it was verified that grain shape defined by sphericity affected the porosity. The sphericity of crushed glass being an angular medium was found to be around 0.5. The porosity range obtained with this media showed to be higher than other filter media. To evaluate the effect of grain size on head-loss, several fractions of sand and garnet were compared and an inversely proportional relation was observed independent of the type of the medium. Keywords: Filtration, granular material, clean bed, head-loss, porosity, sphericity

Expansion of non-spherical media during fluidization

Liquid-solid fluidization has a number of applications in engineering. The expansion of granular filter media during backwashing is of particular interest. Another area of application that is of growing importance is fluidized-bed reactors used in wastewater treatment. It is important to have an understanding of fluidization principles and an ability to predict bed expansion as a function of liquid velocity to design such systems properly. More often than not, the media involved are not spherical and it is necessary to have an expansion model that can be applied to beds of non-spherical particles. Numerous equations have been proposed to predict the expansion of liquid fluidized beds of spherical particles. Very few general equations exist, however, for non-spherical media. Furthermore, the accuracies of the expansion models for non-spherical media have not been evaluated or compared in a conclusive manner to this date. This study considers the expansion of beds of possibly non-spherical particles during particulate fluidization. New experimental data with both spherical and non-spherical media are presented. Fluidization experiments have been carried out with glass balls of eight different sizes (1.11, 1.19, 2.03, 2.99, 3.18,  4.03, 4.98 and 6.01 mm), plastic balls of three different sizes (1.97, 2.48 and 2.87 mm), ten sieved fractions of silica sand, eleven sieved fractions of crushed glass, and seven sieved fractions of perlite. Perlite and crushed glass were included in this study because their properties (densities and sphericities) are different than those of silica sand, and as such they can provide additional fluidization data. It may also be noted that both materials have been considered as substitutes for silica sand in rapid filters. To obtain additional fractions of crushed glass material, particles retained in the topmost sieve tray were crushed again and sieved. In this manner sufficient quantities of additional fractions of crushed glass were obtained. Glass fractions obtained by repeated crushing and sieving were observed to have higher sphericity values. Using this procedure, crushed glass fractions with approximately the same size and density but different sphericities were produced. This allowed the collection of additional fluidization data to investigate the effect of shape on expansion behavior.The sand, perlite, and crushed glass fractions were obtained by a manual sieving procedure followed by an additional 1 minute of manual sieving such that the change in weight during the latter was less than 1% for each fraction. Densities were measured by a water-displacement technique. Equivalent diameters have been measured by counting and weighing 200 grains of each fraction. Porosities were calculated from bed weight, bed height, and density values. Sphericity of each material was determined using fixed-bed head loss data in conjunction with the Ergun equation. For all the materials studied in this work, sphericity values calculated using fixed-bed head loss measurements and the Ergun equation allowed successful prediction of the effect of particle shape on bed expansion during fluidization. Sphericity values of the materials studied was found to be 0.74, 0.66, 0.42 and 0.55 for sand, perlite, crushed glass and crushed glass produced by repeated crushing, respectively. For the eight different sizes of glass and plastic balls, the calculated sphericities by using the Ergun equation were always close to 1.0. A new equation is developed by analyzing fluidization data from the literature and the data collected in this work. The proposed equation represents the mentioned data very accurately and can be used to predict the expansion of both spherical and non-spherical media.The non-spherical particle data fall below the curve for spheres on the friction factor versus the modified Reynolds number diagram. For the materials studied, it has been observed that this shape effect depends on the Reynolds number and is considerably stronger than documented previously in the literature.  The proposed equation can be used to predict the expansion of both spherical and non-spherical media. When applied to the non-spherical particle data obtained in this work (bed expansions from 20% to about 100%, sphericities between 0.410 and 0.757), the mean error in the predicted porosity values is 2.45%. The corresponding mean error that results from Dharmarajah-Cleasby (1986) equation is 4.4%.     Keywords: Filter backwash hydraulics, fluidization, filter material, sphericity.Akışkan yatakların genişlemesi sırasındaki hız-gözeneklilik ilişkileri üzerine önerilen pek çok denklem mevcuttur. Bununla birlikte bu denklemlerin tamamına yakını sadece küresel malzemelere uygulanabilir nitelikte olup küresel olmayan tanecikler için önerilen az sayıda modelin büyük çoğunluğu da ampiriktir ve şekil faktörü etkisini açık bir şekilde ortaya koymamaktadırlar. Ayrıca küresel olmayan tanecikler için geçerli genişleme modellerinin doğrulukları sistematik bir şekilde değerlendirilmemiş ve karşılaştırılmamıştır. Bu çalışmada küresel olmayan malzemeler ile ilgili akışkanlaşma deneyleri yürütülmüştür. Bu amaçla 10 farklı kum, 7 farklı perlit ve 11 farklı kırılarak oluşturulmuş cam fraksiyonu hazırlanmış, malzemelerin şeklinin akışkanlaşma sırasındaki davranışları üzerine etkisini belirlemek amacıyla sabit yatak yük kayıpları ve Ergun denklemi kullanılarak elde edilen küresellik katsayıları tespit edilmiştir. Bu şekilde her malzeme için ayrı ayrı hesaplanan küresellik parametresi akışkanlaşma çalışmalarının modellenmesinde oldukça iyi neticeler vermiştir. Çalışılan malzemelerin küresellik katsayıları kum, perlit, tek seferde kırılarak oluşturulmuş kırık cam ve tekrarlı kırma suretiyle oluşturulmuş kırık cam fraksiyonları için sırasıyla yaklaşık 0.74, 0.66, 0.42 ve 0.55 olarak bulunmuştur. Literatürde sıklıkla atıf alan küresel olmayan malzemelerin akışkanlaşma sırasındaki hız-gözeneklilik ilişkisini veren Dharmarajah-Cleasby (1986) denklemi bu çalışmada elde edilen deneysel veriler kullanılarak irdelenmiş ve küresel ve küresel olmayan malzemelerin genişlemiş yatak yükseklikleri ve gözenekliliklerinin tahmininde kullanılacak alternatif yeni bir denklem ile karşılaştırılmıştır. Geliştirilen yeni modelin Dharmarajah-Cleasby (1986) denklemine göre çok daha tutarlı sonuçlar verdiği görülmüştür. Anahtar Kelimeler: Filtre geri yıkama hidroliği, akışkanlaşma, fitre malzemesi, küresellik. &nbsp

Çift tabakalı filtreler: Türkiye şartlarına uygun bir tasarım ve değerlendirme

Silica sand is the most widely used rapid filter medium around the world. The use of dual-media filtration, however, is now widespread in developed countries. Generally anthracite coal and silica sand are used together in dual-media filters. On the other hand, large water treatment plants in Turkey (including all the older and the new plants constructed by İSKİ and DSİ) still employ single-medium sand filters. A sand/anthracite dual media filter design was evaluated as a possible upgrade of the widely used rapid sand filters in Turkey. Pilot scale direct filtration experiments were carried out using raw and synthetic waters. The raw surface water was obtained from the raw water intake at the İkitelli drinking water treatment plant of Istanbul. Two identical filters were operated in parallel in all the experiments. One filter contained the silica sand medium that is currently used by several large municipalities in Turkey, whereas the other filter contained a layer of the same sand plus an anthracite layer that replaced the topmost 40 cm of the mono-medium sand filter. The properties of the dual media filter were selected such that the currently used sand filters in Turkey can be converted to dual media filters with a minimal amount of effort. Experiments were repeated several times as follows: (i) without the use of a coagulant, (ii-iv) with 3 mg/L, 5 mg/L and 10 mg/L of alum, and (v-vi) with 5 mg/L and 10 mg/L of ferric chloride. Turbidity, particle counts, and head losses were measured and compared as functions of time. In all the experiments carried out in this work, the following were observed: (1) Whenever a coagulant was used, the filter containing sand/anthracite dual media produced effluent turbidities and particle counts similar to those obtained with the sand filter. (2) Sand/anthracite medium generated a smaller clean-bed head loss and smaller clogging head losses than those of the sand filter. Considering the experience with dual media filters over many years in other parts of the world, these findings are not surprising. These findings are significant, however, because they demonstrate the applicability of dual media filtration under unique local conditions. The following factors are important in this respect: (1) The sand medium employed in the dual media filter was the same as the currently used sand in local plants. The same sand can be continued to be used after conversion to dual media filtration. All that is necessary is to replace about 40 percent of the sand by anthracite. Furthermore, the currently used media-retaining nozzles and the underdrain system will not have to be changed. (2) The tests are unique in that, the performance of the particular combination of media sizes and depths used in this study –to the best knowledge of the authors- have not been documented in the literature. (3) Many of the studies comparing dual media filters with mono-medium filters were carried out long time ago and without particle counters, using turbidity as the sole indicator of particle removal efficiency. The use of on-line particle counters in this study allowed a better comparison of the effluent qualities achievable in single and dual media filters. It is concluded that the particular sand-anthracite dual media design selected in this study can be employed to decrease and delay filter head loss development. The use of the dual media filter design described here will allow longer filter runs while obtaining essentially the same effluent quality. Another benefit of converting to dual media filtration will be the more widespread use of direct filtration which is currently not preferred by local engineers even when faced with low turbidity raw waters. Keywords: Drinking water treatment rapid filtration, filter material, silica sand, anthracite coal, particle removal.Silika kumu, bütün dünyada en yaygın olarak kullanılan filtre malzemesi olmakla birlikte gelişmiş ülkelerde çift ve daha çok tabakalı filtrelerin kullanımı yaygınlaşmıştır. Çift malzemeli filtrelerde genellikle antrasit kömürü ve silika kumu birlikte kullanılmaktadır. Ancak Türkiye’deki mevcut büyük arıtma tesislerinde sadece silika kumu kullanımı devam ettirilmektedir. Bu çalışmada, İstanbul’un içme suyunun büyük bir bölümünü sağlayan İkitelli içme suyu arıtma tesisi girişinden temin edilen hamsu ve sentetik olarak hazırlanan sular üzerinde pilot ölçekli hızlı filtrasyon deneyleri yürütülmüştür. Çalışmalarda iki paralel filtre kullanılmıştır. Filtrelerden biri Türkiye’de yaygın olarak kullanılan silika kumunu, diğeri ise antrasit kömürü ve silika kumunu içerecek şekilde çift tabakalı olarak hazırlanmıştır. Deneyler bu iki paralel filtrenin eş zamanlı çalıştırılması ile yürütülmüş ve filtrelerin performansı zamana bağlı olarak bulanıklık, parçacık sayımı ile yük kayıpları ölçülerek mukayese edilmiştir. Çalışmanın temel hedefi değişik filtre malzemesi kombinasyonu kullanımının su kalitesi ve yük kayıplarına olan etkilerini tespit etmektir. Bu çalışmada elde edilen temel neticeler şu şekilde özetlenebilir: (i) Uygun dozajda bir yumaklaştırıcı kullanıldığı takdirde, çift tabakalı (antrasit ve kum) filtrede tek tabakalı (silika kumu) filtredekine eşdeğer çıkış suyu kalitesi elde edilmiştir. (ii) Çift tabakalı filtrede hem temiz yatak (filtrasyon başlangıcındaki) yük kayıpları hem de filtrenin tıkanmasından dolayı ortaya çıkan yük kaybı artışları tek malzemeli kum filtreninkine nazaran mühim ölçüde daha az olmuştur. Anahtar Kelimeler: İçme suyu arıtımı, hızlı filtrasyon, filtre malzemesi, silika kumu, antrasit kömürü, parçacık giderimi

Simple formulae for velocity, depth of flow, and slope calculations in partially filled circular pipes

The application of the Manning equation to partially filled circular pipes is considered. Three different approaches based on the Manning equation are analyzed and compared: (1) using a constant value for the roughness coefficient n and defining the hydraulic radius as the flow area divided by the wetted perimeter. (2) Taking the variation of n with the depth of flow into account and employing the same definition of the hydraulic radius. (3) Defining the hydraulic radius as the flow area divided by the sum of the wetted perimeter and one-half of the width of the air-water surface and assuming n is constant. It is shown that the latter two approaches lead to similar predictions when 0.1 less than or equal to WD less than or equal to 1.0. With any one of these approaches, tedious iterative calculations become necessary when diameter (D), slope (S), and flow rate (Q) are given, and one needs to find the depth of flow (h/D) and the velocity (V). Simple explicit formulas are derived for each of the three approaches. These equations are accurate enough to be used in design and sufficiently simple to be used with a hand calculator

An experimental investigation on the hydraulic behavior of declining rate filtration

Hydraulic behavior of declining rate filtration (DRF) was investigated by means of pilot-scale experiments. A bank of four declining rate filters was constructed and operated. The common water level in the filters, individual filter velocities, and head losses in the filters were carefully monitored and recorded as functions of time. Many of the previous studies on DRF employed either a single average filtration velocity or a single coagulant dosage. In this study, however, filter runs were repeated treating the same water at several different filtration rates and coagulant dosages. Using such an experimental matrix of several different rates and coagulant dosages allowed an evaluation of the behavior of the declining rate filtration system under different operating conditions and the effects of the mentioned variables on various hydraulic characteristics of the DRF system. The validity of certain simplifying assumptions used in design calculations was also tested in these experiments. It is believed that the new data presented herein will lead to a better understanding of DRF

Prediction of filter expansion during backwashing

The application of the Ergun equation to predict the expansion of filter media during backwashing is investigated. Fluidization data from the literature have been analyzed and the values k(1) =3.519 and k(2) =0.266 have been found to give a very good fit to the data in the range of Reynolds numbers of interest in filter backwashing. An empirical equation that is applicable over a wider range of Reynolds numbers than the Ergun, equation is also developed. New experiments have been carried out,with glass spheres, plastic. spheres, silica sand, and crushed glass. The effect of particle shape on expansion behaviour is investigated. It is found that the influence of particle shape is larger than previously recognized. Furthermore, the shape effect depends on the Reynolds number based on the backwash velocity. The advantages, limitations, and range of applicability of each method of calculation are delineated

Effect of ozone injection location on filter performance in direct filtration

Two identical pilot scale sand filters were operated in parallel to study the effects of preozonation in direct filtration. No coagulants were used. The influent of one filter was ozonated, whereas the influent to the second filter was aerated. Significantly improved reduction in turbidity, particle count, and iron was observed with the filter receiving preozonated water. The head loss development rate was increased as a result of using ozone. In a second set of experiments, the effects of ozone injection point on filter performance were investigated. Two identical filters both receiving preozonated water were operated. With one of the filters, the raw water was ozonated immediately before it entered the filter. The entrance of the preozonatea water to the other filter was delayed by using a detention tank between the ozone contact chamber and the filter. In addition to effluent turbidity and particle count values, zeta potentials of the waters entering the two filters were measured. Head loss development at several locations within each filter bed was also observed and recorded. Slightly better effluent quality (turbidity and particle count) was observed with the filter receiving preozonated water without delay. It was observed that the zeta potential of the prezonated water became more negative with increasing delay time

A new simple equation for the prediction of filter expansion during backwashing

Fluidization experiments have been carried out with glass spheres, plastic spheres and several sieved fractions of silica sand, garnet sand, perlite and crushed glass. The effect of particle shape on expansion behavior is investigated. Sphericity as determined using the Ergun equation and fixed-bed head loss data is employed to quantify the shape effect. It is found that the influence of particle shape depends on the Reynolds number based on backwash velocity. A new equation that accounts for particle shape is proposed. For the materials studied, the proposed equation gives excellent agreement with both the spherical and the non-spherical particle data

On the backwash expansion of graded filter media

Granular media filtration is one of the most important and commonly used operations in water treatment and is also widely used for advanced wastewater treatment. Successful filter design requires a correct specification of backwash rates for filter cleaning. Significant progress has been made in recent years towards a capability of accurate predictions of backwash expansion of uniform (sieved) fractions of nonporous and porous nonspherical media. Graded filter media, however, have not been studied in a systematic and satisfactory way so far. Current design calculations consider a bed with a size gradation to consist of several layers of approximately uniform size according to the sieve analysis data, and the expansion of each layer is separately calculated. The total expansion is calculated by adding the expansions of all the layers. The present work evaluates the accuracy of this approach and a number of alternative calculation methods by carrying out fluidization experiments with actual filter media including silica sand, garnet sand, perlite, crushed recycled glass, activated carbon, anthracite coal, and zeolite. (C) 2017 Elsevier B.V. All rights reserved