filtration water treatment

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GRANULAR FILTRATION

Settling is not sufficient to remove all particles and flocs from water. Typical overflow qualities fromsedimentation tanks range from 1 to 10 NTU. Filtration, usually rapid sand filtration, is ten employedfor furter !polising", i.e. to get te tur#idity to lower tan 0.$ NTU %as required #y legislation&. 'apidsand filtration after prior sedimentation is te most common configuration worldwide. (ncreasingly,direct filtrationis #eing employed were te tur#idity of te raw water is not ig. Slow sand filtrationisnot very common, #ut tere is a renewal in interest in using tis as a very useful unit process wenorganic removal #y #iological means #ecomes important.

Sand filtration is recogni)ed as one of te essential #arriers in water treatment, a #arrier to patogens.Te oter #arrier is disinfection. *it te focus on proto)oan diseases, giardiasis andcryptosporidiasis, te importance of sand filtration as once more #een reali)ed, due to te failure ofdisinfection to inactivate te cysts and oocysts of te proto)oa. To ensure effective removal of oocysts,some water treatment plants strive to acieve a tur#idity level of 0.1 NTU and initial filtrates immediatelyafter #ackwasing are discarded.

Tere are several oter filtering processes used for te separation of particles from a stream of water orsludge. Precoat filtration is mainly used in pretreatment #efore membrane filtration,altoug one formof precoat filtration, diatomaceous earth filters are commonly used in swimming pools. Tesetecnologies are #eing reconsidered in full+scale water treatment to counteract proto)oan diseases.Vacuum filters and filter pressesare used for removing water from sludges #y te use of a filter clot,wic retains a layer of sludge from wic water is sucked or pressed #y an induced pressuredifference. Tese units operate mainly troug te use of a mecanical straining process. Teempasis in tese notes will #e on sand filtration.

3.1 RAPID SAND FILTRATION

Filtration is used in #ot water treatment and wastewater treatment as a separation process, wicremoves fine inorganic and organic particles from te water. Sand filters are often used in treatment ofwater to remove fine particles, wic cannot #e economically removed #y sedimentation. Sand filtrationis a form of granular medium filtration, in wic te filtering medium consists of granular material sucas sand, antracite, activated car#on or oter grains. Te main applications in water treatment arerapid sand filtration and slow sand filtration.

Tere are a num#er of mecanisms, wic result in te removal of particles from water during rapid

sand filtration as discussed #elow.

Mechanical straining

ranular filters remove particles tat are very muc smaller tan te dimensions of te interstices#etween teir grains. -ltoug tere must #e somemechanical strainingeffect, it accounts for only aminor partof te action of a filter. - granular filter is capa#le of capturing very fine particles, even in tea#sence of particles large enoug to #ridge te interstices.Adsortion

Tere are tree essential parts of te filtration process in a granular material. Tey are

%a& te #ed of fi/ed solids %consisting of te filter medium togeter wit previously deposited impurities&,

%#& te water passing troug te interstices #etween te fi/ed solids, and%c& te solid impurities suspended in te water.

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Adsorptionof particles of impurities onto te fi/ed #ed %tat is, te fine particles stick eiter to a grain offilter material or to previously deposited and adsor#ed impurities& is a maor factor in successfulfiltration troug porous media. -dsorption is a process of wic te efficiency depends on te surfaceproperties of #ot te adsor#ing matri/ and te small particles tat are adsor#ed. Tere are two factorsin te adsorption of a particle

a& its a#ility to stickto te matri/ wen it is #rougt into contact and#& its transportto a position were it eiter contacts te surface or comes close enoug to #e attracted to it.

Te metod of attacment of particles is similar to te process of flocculation. - small particle in closepro/imity to a solid surface is su#ect to eiter electrical attraction or repulsion %depending on tesurface canges developed #y #ot te particle and te surface wen in contact wit te water& and tote attraction caused #y van der *aals forces %see Fig .1&. (t is also su#ect to te ydraulic forcesresulting from te movement of te water. Te electrical forces can eiter ini#it or enance teremoval of fine particles from te water as it passes troug a filter. For most of a filtering period, tegrains of filter material are coated wit a layer of impurities and, terefore, te surface carge producedon te impurities is important in ensuring a prolonged effective filter operation #efore cleaning is

required. (f te water as #een treated to give optimum destabilization of colloids for effectiveflocculation and sedimentation, it is likely tat te remaining particles will #e suita#ly desta#ili)ed foreffective filtering.

Te forces of adesion #etween te deposited impurities and te filter grains can #ecome so strongtat te impurities are not readily removed during #ackwasing. Te impurities and filter grains mayten lump togeter to form mud-balls, wic are very resistant to #eing cleaned #y #ackwasing2 teirydraulic #eavior makes tem settle in a fluidi)ed #ed of sand. 3are must terefore #e e/ercised toensure tat altoug te adesion surface forces are strong enoug to trap and old impurities, teyare weak enoug to allow teir release during #ackwasing.

Transort

For adsorption to occur, particles must #e carried close enoug to te matri/ surface to #ecomeattracted and attaced to it. Tey are carried troug te interstices in te filter matri/ #y te water flow,wic, under normal conditions, is laminar.

Tere are tree main mecanisms #y wic particles are transported into contact wit te filter matri/interception, sedimentation and diffusion %see Fig 4.1&.

a) Interception is te process were#y a particle #eing carried along a streamline cances to come closeenoug to te surface for its attacment. (f a particle of effective diameter dpis moving along a streamlinewic passes witin a distance of 15dpfrom a solid surface, tere is an opportunity for adsorption.

b) Sedimentationis te process in wic a particle is deflected from te streamline pat #y te gravitationaleffect, resulting from te difference #etween its weigt and its #uoyancy. - granular filter as an action tat

could #e considered as an irregular sallow dept sedimentation unit.c) Diffusion is te process were#y particles are randomly deflected #y #uffeting resulting from molecular

activity %6rownian motion&. Tis will occur under laminar flow conditions.

Te effectiveness of interception and sedimentation in a filter increases wit an increase in particle si)e,#ut te effectiveness of diffusion increases wit a decrease in si)e. Tus, altoug a filter may removelarge particles efficiently #y interception and sedimentation, and very small particles #y diffusion, tereis an intermediate si)e of particle for wic te removal efficiency is relatively low. For practical filters,tis is a#out 1 7 $ micron. 3onsidering tat 3ryptosporidium oocysts are a#out $in diameter, teirremoval is difficult.

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Fig!re 3.1 Methods o" article transort

Te desta#ili)ation of colloids wit adequate cemical pre+treatment is an essential aspect of rapidsand filtration.

#ead loss in oerating "ilter

For filter flow rates in te normal operating regime of 4 to 1$ m 4 m+ +1, i.e. 4 to 1$ m5 approacvelocity, and sand wit an effective si)e of 0.$ to 1.0 mm, te error in assuming tat te ead loss isproportional to te flow rate is practically negligi#le. Tus 8arcy9s :aw,

; < =

h

L %4.1&

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can #e used to estimate te ead loss in filter, were is te ead loss2 : is te dept of filter #ed overwic te loss occurs2 ; is te velocity of te water at te filter2 and = is 8arcy9s coefficient ofpermea#ility.

Te coefficient of permea#ility is a function of te density, , and viscosity, , of te water, te si)e and

sape of te grains in te #ed, and teir porosity, f %or as in -**-&.=o)eny9s equation gives an estimate of te ead gradient in a clean #ed of sand filtering clean water as

h

L for water+worn

sand, 0.? for angular sand, and 0.@$ for well+saped crused material. (f k is assumed to #e $, teead loss troug a clean #ed of dept : is given appro/imately #y %Note 1A0< $ / @&

4 mm5s

Oeration o" Raid Sand Filters

'apid filtration is used as te final clarifying step in municipal water treatment plants. (f te raw wateras tur#idity in e/cess of 10+0 NTU, flocculation and sedimentation unit processes sould #eemployed #efore te rapid filters must #e provided. Tere is limited #iological action in conventional

rapid filters. Some nitrification #y cemotrops occurs wen te o/ygen content is adequate, and teretention time is sufficient.

Te working of a rapid gravity filter is e/plained #elow wit reference to Figures 4. and 4.4. *en tefilter is in a working condition, only valves 1 and B are kept open and all oters are kept closed.

'ac&$ashing

*ater tat passes troug te filter medium, supporting layer, and underdrain, e/periences frictionalloss of resistance known as eadloss. *en te ead loss e/ceeds 1.$+.$ m, te filter needscleaning. Te first operation is to close valves 1 and B %Figure 4.& and allow te filter to drain until tewater lies a few centimeters a#ove te top of te #ed. Ten valve $ %Figures 4. and 4.4& is opened,

and air is #lown #ack troug a compressed air unit at a rate of a#out 1 + 1.$ m 4free air5min. mof #edarea for a#out +4 minutes, at a pressure of 0+4$ kN5m . Te water over te #ed quickly #ecomesvery dirty as te air+agitated sand #reaks up surface scum and dirt. Following tis, valves and @ areopened, and an upward flow of water is sent troug te #ed at a carefully designed ig velocity. Tissould #e sufficient to e/pand te #ed %0+$0K & and cause te sand grain to #e agitated so tatdeposits are wased off tem, #ut not so ig tat te sand grains are carried away in te risingupwards flus of water.

-fter te wasing of te filters as #een completed, valves and @ will #e closed, and valves 1 and 4opened. Tis restores te inlet supplied troug valve 1. Te filtered water is wasted to te gutter for afew minutes after tis, until te required quality is acieved. Ultimately, valve 4 is closed and valve B is

opened to get te filtered water again. Te entire process of #ackwasing te filters and restarting tesupplies takes a#out 1$ minutes. Te specified minimum #ackwas time for a rapid filter is $ minutes.

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Te amount of water required to was a rapid filter may vary from 4+@K of te total amount of waterfiltered.

Fig!re 3.( Diagra))atic section o" a raid sand "ilter

Fig!re 3.3 Diagra))atic section o" a %ac&$ash s*ste)Upward was water rates are usually of te order of 0.4+1.0 m5min. (t is usually more economical touse gravity flow from a large storage tank, since te #ackwas flow rates are ig. =eeping suc atank topped up #y a relatively small, continuously running pump drawing from te filtered water supplycan minimi)e energy requirements. Te waswater tank sould ave a capacity to provide a minimumof one filter was of 10 minutes duration. (t sould #e capa#le of #eing refilled in @0 minutes.

(f tere is significant particle retention at te surface of te #ed, it is recommended to ave a surfacewas. Surface was is acieved #y an additional c.0. m45min+mof water etted onto te surface ofsand at 1$0+B$0 kN5m. 8ifferent filter #ackwas metods and te recommended design valves are

summari)ed in Ta#le 4.1.

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Te underdrain system of a filter is an important component in te design and operation. Te selectionis #ased on te filter type and si)e, media caracteristics, and te selected metod of #ackwasing.Te underdrain sould contain a central manifold, wit laterals tat are eiter perforated or aveum#rella+type strainers on top. Lter types tat may #e used include clay tile #locks, precast concretelaterals, false #ottoms wit strainers, and porous plates, etc. Te selected underdrain system, wile

ensuring uniform flow distri#ution of te #ackwas, sould ensure dura#ility, relia#ility, and cost+effectiveness.

Ta%le 3.1. Reco))ended design +al!es "or +ario!s %ac&$ash )ethods

Filter Underdrains

Tere are two metods of keeping a uniform flow distri#ution of te #ackwas water inside te filter cell

as descri#ed #elow.

1. 6y making te orifice or slits in te filter underdrain system small enoug to introduce acontrolled eadloss.

. 6y decreasing te flow velocity of te pressuri)ed conduit upstream of te underdrain system sotat te ydraulic grade and energy lines of te flow entering te underdrain system are fairlyuniform.

False+#ottom under drain systems are preferred due to relia#le performance and low maintenancecosts %=awamura, 1>>1&. Ta#le 4. gives te typical design parameters of an underdrain system%lateral manifold system& for a small rapid sand filter.

Ta%le 3.( Design ,riteria "or Underdrains -NS / D'0 122

,riterion 4al!e

Minimum diameter of underdrains 0 cm8iameter of te perforations @+1 mm %suggested at a sligt angle to te vertical a/isof te pipe&Spacing of perforations along laterals ?.$ cm for @ mm perforations

0 cm for 1 mm perforations'atio of total area of perforation to 0.$ for @ mm perforations total cross+sectional area of laterals 0.$0 for 1 mm perforations'atio of total area of perforation to 0.004 te entire filter area:engt to diameter ratio of te lateral @01Ma/imum spacing of laterals 40 cm3ross+sectional area of te manifold 1.$+.0 times te total area of laterals

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;elocity of te filtered water outlet 1.0+1.A m5s

M!lti)edia Filters

Te conventional rapid filter generally uses sand wit an effective si)e of 0.@ +1. mm and a uniformity

coefficient of 1.$ + . Te uniformity coefficient is te ratio of te screen si)e, wic will pass @0K of tematerial to te si)e, wic will pass 10K.

6ackwasing results in te filter media #eing stratified wit te finer medium remaining at te top andte coarser medium at te #ottom of te filter #ed. Tis makes only te top fine sand portion effective infiltration. To overcome tis pro#lem, two alternatives ave #een proposed dual or multimedia filtration2or coarse+si)e, narrowly graded media filtration.

Te conventional approac to multimedia filtration is to ave tree layers of material coarse %1mm&antracite on top, sand of 0.$mm in te middle and 0.$mm garnet at te #ottom. Te si)e and specificgravity are carefully selected to minimi)e intermi/ing. Te commonly used media are antracite coaland sand. Te difference in densities c. 1$00 kg5m4of antracite, $00 kg5m4of sand and 4400 kg5m4

of garnet, ensures tat tese layers maintain teir position also during #ackwasing.C/tensive researc as #een carried out to study te advantages and disadvantages of intermi/ing offilter media in dual media filtration. *ile some researcers feel tat te grain si)e of coarse antraciteand fine sand sould #e cosen in suc a way tat te intermi/ing at te interface is minimi)ed, oters#elieve tat controlled mi/ing among filter media is #eneficial. - detailed design of media is discussedin te literature %;igneswaran et al., 1>A42 Ma)umdar, 1>AB&.

-s a result of te searc for a dual+media filter in wic interface intermi/ing does not occur, a new type of filtercalled te flotofilter as #een introduced. Te flotofilter uses syntetic plastic #eads as te filter medium.Golypropylene is te coarse medium and polystyrene is te fine medium %*erellagama, 1>>4&. 6ot te mediaave a specific gravity less tat water, ence te #ed floats in water. Golypropylene is muc more dense tanpolystyrene, ence it forms te lower part of te floating filter #ed. -s te coarse medium is at te #ottom, te

filtration is upflow. Tis filter needs an upper retention grid to prevent te loss of media wit filtered water. 8ue tote large density difference, te two media do not intermi/ even under severe agitation, leaving a clear mediainterface after #ackwasing. Tis eliminates te most common operational pro#lem encountered in conventionalmultimedia filters. -lso, as tere is a #uffer )one of water #etween te floating #ed and te filter inlet, te #ed isnot distur#ed due to te tur#ulent conditions in te underdrain system. Tis permits te use of a very simpleunderdrain system, resulting in #ot capital and operational cost savings. Since te two media are floating, teycan #e fluidi)ed using muc lower energy tan conventional dual+media filters. Tis results in considera#leenergy and water saving during te #ackwasing of te filter. C/periments carried out in France %6en -im et al.,1>>4& and in -ustralia %;igneswaran and Ngo, 1>>4& ave indicated tat tis can #e successfully used as a staticflocculator and5or prefiltration unit prior to te direct filtration unit.

I)ro+e)ents on Flo$ Rate3onventional rapid filtration operates at a constant rate of appro/imately $ m 45m.. 'esearc work onte variation of flow rate as indicated tat ig+rate filtration and declining+rate filtration areadvantageous for most cases. (f it is possi#le to acieve te desired filtrate quality wit a iger filterrate at an operational and maintenance cost compara#le to tat of a conventional rapid filter, ten onecould acieve a significant capital saving #y using a ig+rate filter. Similarly, researc as indicatedtat declining+rate filtration produces a #etter effluent quality tan te conventional process. Terefore,if one could o#tain te same amount of filtered water wit equal capital investments, ten declining+ratefiltration would ave a definite advantage over conventional rapid filtration. -noter advantage of usingdeclining+rate filtration is tat it does not require automatic rate control.

Te concept of declining+rate filtration is not new. 6asically, no rate controller is used in tis system,and instead a fi/ed orifice replaces it. Te filtration rate in tis system is allowed to decline from ama/imum value at te #eginning of te run, wen te filter is clean, to a minimum value at te end of

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te filter run, wen te filter is in need of #ackwasing. (n practice, several %a minimum of four& filtersare used in parallel, and te water level is maintained essentially at te same level in all four. -s teywill #e in different stages of #ackwasing, flow rate differences will #e evened out.

Hig+rate filtration, in wic te filtration rate is a#out 10+1$ m 45m., as compared to te rate ofconventional rapid sand filtration, wic is of te order of $.0 m 45m., is useful in upgrading e/istingplants. Suc ig filtration rates are possi#le, tanks to te development of %1& dual+media ormultimedia2 and %& control of flocculation #y polyelectrolytes, wic produce relatively less sludge.Hig+rate filters wit dual or coarse+medium arrangements ave #een successfully used in developedcountries. Tis could #e one of te economic solutions for te e/pansion of e/isting water treatmentplants and for te construction of new plants. Te filters sould #e designed to operate at te igestpractical rate. Te design sould #e suc tat even wit te iger frequency of wasing, tey aremore economical. Here, more attention sould #e paid to te selection of te filter media and filtrationrate so tat te filtrate quality meets te required standard.

U"lo$ "iltration

1. Te flow in conventional rapid filtration is in te downward direction. Te maor disadvantage in tisprocess is tat te flow meets te fine sand #efore reacing te coarse sand tat is found at te #ottomlayer %as a result of non+uniformity and stratification during #ackwasing&. (f one inverts tis flow to anupward direction, a #etter use of te filter media can #e acieved. Lne could ten use ungraded sandto provide a larger range of si)es. Tis type of upflow grid filters as #een e/tensively used for filtrationof water for municipal and industrial supplies in Curope, -frica and -merica %:andis, 1>@@2 Lkun, 1>@?&.Lperation witout grids as also met wit considera#le success in Sout -frica. %;an der Merwe andvan :eeuwen, 1>>1&.

3.( DIR5,T FILTRATION

3onventional water treatment plants generally use unit operations suc as rapid mi/ing, flocculation,sedimentation, filtration, and disinfection. 8epending on te quality of te water, one or more unitoperations can #e eliminated, tere#y acieving a cost+effective water treatment. 8irect filtration is onesuc metod. Filters used in direct filtration tus differ little from tose for conventional treatment inconstruction. Te primary difference in te operation of te two systems is related to solids storagecapacity and #ackwasing requirements. 8ifferent direct filtration flow scemes are presented inFigure 4.B.

8irect filtration was first e/plored during te early 1>00s, #ut tese attempts were not successful, due tote rapid clogging of te sand #eds. Te development of coarse+sand filters and dual+media filters asmade it possi#le to store greater amounts of floc witin te filter #ed witout e/cessive eadloss, andas tus increased te feasi#ility of te direct filtration process. Furter advances in filter design andte availa#ility of a wider selection of cemical coagulants and polyelectrolytes ave resulted in avariety of filtration systems #eing designed in wic coagulating cemicals are employed. Teflocculation #asin is eiter eliminated or reduced in si)e, and te sedimentation #asin is not utili)ed.Suc processes tus ave only screening, rapid mi/ing, and a sort time of flocculation prior tofiltration. -ll suspended solids and flocs formed are deposited in te filter, wic is usually a multi+media, granular #ed containing coal, sand, and peraps oter media.

Te -merican *ater *orks -ssociation %-**-& Filtration 3ommittee %1>A0& did a worldwide survey of?0 operating and pilot plants. Tis indicated tat waters wit less tan B0 units of color, tur#idityconsistently #elow $ NTU, iron and manganese concentrations less tan 0.4 mg5: and 0.0$ mg5:,

respectively, and algal counts of up to 000 per m: %measured in a#sorption units at 1000 nm& appearto #e perfect candidates for direct filtration. Tur#idity and color removals are consistently attained in

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tis process. 6y efficient post+clorination, #acteria and virus removal pro#lems can #e eliminated.Most of te literature favors te use of dual or mi/ed media for direct filtration.

8irect filtration can successfully #e used for low+tur#id waters, #ecause of its lower capital andoperational cost. (t does not require any sopisticated equipment, altoug skilled operators are needin order to monitor te filters. -ttention sould #e paid to te possi#ility of poor #acteriological quality ofte filters in case of igly polluted raw water.

Design Princiles

Since optimum design parameters depend greatly on te nature of te water to #e treated, pilot studiesare required to determine te appropriate type of coagulant and coagulant+aid, and te mediacomposition, si)e, and dept. Some guidelines are given #elow.

!ilter media 3an #e single, dual, or mi/ed media, #ut usually dual and mi/ed media are preferredin direct filtration. adkari et al. %1>A0& recommend dual+media wit #ituminous coal or antracitecoal. 8eeper filters wit greater filter medium depts are preferred %=ing and -my, 1>?>&. Te

finest media possi#le sould #e selected to minimi)e cemical dosages. *itin reasona#le limits,coarse filter #eds can produce te same quality filtrate as finer #eds, #ut more polymer is required.Fine filter media are supported on a gravel #ed. Tis is preferred to direct support on #ottomsequipped wit mecanical strainers or no))les, wic are not recommended %3ulp, 1>??&.

Fig!re 3.6 Direction "iltration "lo$ sche)es

"apid mixing Te rapid mi/ing process for direct filtration usually does not differ muc from tatused for conventional plants.

!iltration rate 10+1$ m45m. %constant+rate operation&. - rate of 1 m45m. is usually adopted.'ecent studies ave sown tat iger filtration rates are possi#le %up to 0 m45m.& wit low tur#id

waters %Murray and 'oddy, 1>>4a&. !locculants Te type of flocculent is te most important parameter and sould #e e/perimentally

evaluated initially. -lum as #een utili)ed wit success in many installations. 8osages of $+10mg5: can #e effective for direct filtration of waters of various tur#idities %-din et al., 1>?>&. However,more recently it as #ecome apparent tat a carefully selected cationic polymer may aveconsidera#le advantages in some situations. Golyelectrolyte doses for uncolored water mayfrequently vary from 0.0$+1.0 mg5: wen used as primary flocculants %essentially cationic&, andmay #e less wen used in addition to alum for te treatment of organic contaminated water %-din etal., 1>?>&. Te general range of cationic polymer dosage used as te primary coagulant is 0.1+$mg5: %3ulp, 1>??&. Murray and 'oddy %1>>4#& indicated tat cationic polymer togeter wit alumwill not only increase flow rate, #ut also significantly increases te filter run time. Nonionic polymerand flocculation time were found to #e very important.

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#ackwash method -ny conventional wasing procedure can #e used. Te usual #ackwasingrate is 4?.$+@0 m45m.. (n te case of dual+media and multi+media filters, te #ackwasing ratesould #e cosen in suc a way as to minimi)e te intermi/ing of media. 8etails are givenelsewere %6aumann, 1>?A2 =awamura, 1>?$&. Te total #ackwas volume can #e reduced #y acom#ined air+water #ackwas metod. 8irect filtration needs frequent #ackwasing. *en a plant

contains more tan four direct filtration units, inter+filter #ackwasing can #e used. 3leas#y %1>>4&recommends an inlet restriction as te metod of flow control wen inter+filter #ackwasing is used.

!ilter-run length Filter+run lengt depends on te raw water caracteristics, te coagulant dose, temi/ing energy input for floc formation %e/tent of pretreatment&, te media si)e, te filtration rate, etc.-s a guideline for design, te design parameters sould #e cosen in suc a way tat te filter runis at least 1 ours.

$ead re%uirement Since te filtration rate declines wit time in a direct filtration plant, te total eadrequirement for a direct filtration plant is less tan tat for a constant+rate filtration plant wit tesame flow rate. Te ead previously consumed in underdrains and piping during te early ig+rate part of te filter cycle decreases wit te square of te filtration rate, and #ecomes availa#le to

overcome te clogging eadloss late in te cycle. -lso, te eadloss due to dirt accumulation isreduced as te rate declines. Te total ead requirement is typically a#out two+tirds of tatrequired for a constant+rate plant %3leas#y, 1>>4&.

&se of a standb filter - stand#y filter in a #ank of declining rate filters is often advisa#le. Tis onefilter remains off+line after it is #ackwased and is #rougt on+line as te ne/t dirty filter needs to gooff+line for #ackwasing. Tis arrangement as two oter advantages #esides avoiding te spikesin level and filtrate tur#idity. First, te filter tat goes off+line for #ackwasing can #e allowed tocontinue to filter after te inlet flow is stopped. (t will slowly drop in level as it filters to te clear well.Tis may even take more tan an our in some cases. 6y doing tis, te pretreated water a#ovete filter is not wasted in te desire to finis te #ackwas operation in a urry. -lso, due to tereduction in spike, te filter #o/ dept can #e reduced. Te clean filter will come online wit a lower

total ead and terefore will start at a lower filtration rate. Suc use of a stand#y filter means tatte e/tra filter is not truly redundant %stand#y& as intended #y te regulations %3leas#y, 1>>4&.

:a# tests for direct filtration ttp55www.pipps#ird.com5filtrat.tml

3.3 SLO SAND FILTRATION

(n rural areas, especially in developing countries were land is plentiful, te slow sand filter can #e usedwit success if te raw water is not igly tur#id. (t is well suited for tur#idities less tan $0 NTU. (ncase of igly tur#id water, one as to ave pretreatment prior to slow sand filtration. Te scematicdiagram of a simplified slow sand filtration plant, serving 1,$00+0,000 people, is presented in Figure

4.$.

Princile and oeration o" slo$ sand "ilters

*ater is purified #y passing it troug a #ed of fine sand at low velocities %0.1+0.4 m 45m.&, wiccauses te retention of suspended matter in te upper 0.$+ cm of te filter #ed. 6y scraping out tistop layer, te filter is cleaned and restored to its original capacity. Te interval #etween two successivecleaning9s varies from a few weeks to a few monts, depending on te raw water caracteristics.

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Fig!re 3.7 Si)li"ied oeration o" slo$ sand "ilter

Te removal mecanisms of particles in a slow sand filter include mecanical straining, sedimentation,diffusion, and cemical and #iological o/idation. 3oarse and fine particles of suspended matter aredeposited at te surface of te filter #ed #y te action of mecanical straining and sedimentation,

respectively, wile colloidal and dissolved impurities are removed #y te action of diffusion. 6ycemical and #iological o/idation, te deposited organic matter is converted into inorganic solids anddiscarged wit filter effluent. Micro#ial and #iocemical processes, and ence te removal ofimpurities, take place mainly in te top )oological layer of te filter #ed %known as te !Scmut)decke"&.

Te important design parameters of a slow sand filter are te dept of te filter #ed, filter media si)e,te filtration rate, and te dept of te supernatant water level. -s far as possi#le, tese designparameters sould #e #ased on e/perience o#tained from e/isting treatment plants tat use a rawwater source of similar quality. (n te a#sence of relia#le data from suc e/isting treatment plants, pilotplants can #e used to determine suita#le design criteria. Te values presented in Ta#le 4.4 can serveas elpful guidelines.

Ta%le 3.3 Design criteria "or slo$ sand "iltersDesign ara)eters Reco))ended range o" +al!es

Filtration rate 0.1$ m45m. %0.1+0. m45m.&-rea per filter #ed :ess tan 00 m%in small community water

supplies to facilitate manual filter cleaning&Num#er of filter #eds Minimum of two #eds8ept of filter #ed 1 m %minimum of 0.? m of sand dept&Filter media Cffective si)e %CS& < 0.1$+0.4$ mm

Uniformity coefficient %U3& < +4Heigt of supernatant water 0.?+1 m %ma/imum 1.$ m&Underdrain system

Standard #ricksGrecast concrete sla#s enerally no need for furter ydraulic calculationsGrecast concrete #locks wit oles on te topGorous concreteGerforated pipes %laterals and manifold type& Ma/imum velocity in te manifolds and in laterals < 0.4 m5s

Spacing #etween laterals < 1.$ mSpacing of oles in laterals < 0.1$ mSi)e of oles in laterals < 4 mm

Since te purification mecanism in a slow sand filter is essentially a #iological process, its efficiencydepends upon a #alanced #iological community in te !Scmut)decke". Terefore, it is desira#le todesign te filters to operate as far as possi#le at a constant rate. However, te operation of slow sand

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filters in most developing countries is intermittent due to te financial difficulties in employing operatorsto run te plants around te clock. 6ut te intermittent operation causes deterioration in effluent quality#ecause during stoppages te microorganisms causing #acteriological degradation of trappedimpurities lose teir effectiveness. (ntermittent operation distur#s te continuity needed for efficient#iological activity.

Lne way of overcoming tis pro#lem is #y allowing te filter to operate at a declining rate after a cycleof constant rate filtration. 8eclining rate filtration gives an additional water production of 0.$ and 0.?m45m %of filter area&, wit a declining rate operation for A or 1@ ours after 1@ or A ours of constantrate, operation respectively. Te effluent acieved during tis operation is generally satisfactory.Moreover, te declining+rate model may #e applied during nigt+time, resulting in significant savings ofla#or.

,leaning o" slo$ sand "ilters

6efore cleaning a slow sand filter, floating matter suc as leaves and algae tat may cause nuisancesould first #e removed #y raising te water level in te unit to flus te floating matter over te weir.

Ten te water level in te #ed is lowered to a#out 0.1+0. m #elow te sand surface #y closing teinlet valve and opening #ot te supernatant water drain valve and te valve on te underdrains. Tefilter #ed is ten cleaned #y rapidly scraping off te top 1+ cm of te #ed, in view of minimi)ing teinterference wit te filter micro#ial activity. *en one unit is sut down for cleaning, te oters are runat a sligtly iger rate to maintain te output of te plant.

-fter cleaning, te unit is refilled wit water troug te underdrains. Tis water can #e o#tained froman overead storage tank or #y using filtered water from an adacent filter. Te temporary reduction ofplant output due to tis metod sould #e taken into account wen te clear+water storage tank isdesigned, assuring tat sufficient water is availa#le for te users.

6efore starting operating, te filter needs a period of at least B ours to allow for re+ripening of te #ed.

-fter tis period, te microorganisms usually reesta#lis to #e a#le to produce an accepta#le effluent.(n cooler areas, te ripening may take a few days. Cven ten, if te tur#idity of te effluent is sufficientlylow, te water supply can #e resumed after a period of one day, after adequate clorination %;isscer,1>>0&.

Ta#le 4.B gives a comparison of te caracteristics of te rapid filters %#ot pressure and gravity types&wit slow sand filters.

,osts o" slo$ sand "ilters

Te construction cost of an open slow sand filter e/cluding pipes and valves as two components,namely, %1& te floor, underdrains, sand, and gravel2 and %& te walls of te filter #o/. Te cost of la#orand land is usually less important. (n rural areas, te cost of land rarely e/ceeds 1K of te totalconstruction cost. However, in densely populated areas, te somewat larger areas required for slowsand filter plants can #e a pro#lem %;isscer, 1>>0&.

Te construction cost of small+and+medium+si)ed slow sand filters are often less tan tat of otertypes of treatment. 3onstruction of a slow sand filter in (ndia is less e/pensive tan a rapid sand filterup to a capacity of 4000 m45d. However, if recurrent costs for operation and maintenance are taken intoaccount, te #alance sifts to A000 m 45d. Using ceaper materials like ferro+cement and lesse/pensive drainage systems will reduce te cost of slow sand filtration %;isscer, 1>>0&.

(n 3olom#ia, te new slow sand filtration plants are competitive wit conventional treatment plants.Cven conversion of conventional plants into slow sand filtration plants as #een cost+effective. Fore/ample, in a su#ur# near 3ali, operation and maintenance of a conventional treatment plant cost U.S.1B05mont. 6ecause of te increasing cost of cemicals, it was decided to reconstruct te plant andadopt slow sand filtration as te main treatment. Most of te e/isting structure could #e used, olding

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te cost of reconstruction to as low as U.S. ?000. Te oversi)ed water storage tanks were cangedinto slow sand filters. Lperation and maintenance costs are now U.S. B05mont %;isscer, 1>>0&.

Ta%le 3.6 ,o)arison o" Slo$ Sand Filters and Raid Sand Filters

Raid "ilter

,haracteristic Slo$ sand "ilter Gra+it* Press!re

Filtration rate +$ m45m.d [email protected]

Si)e of #ed :arge %000 m& Small %100 m&

8ept of #ed 400 mm gravel, 1 m sand $00 mm gravel, 0.?+1.0 m sand,unstratified stratified2 in some cases sand and

antracite are used as dual media

Cffective si)e of sand 0.4$ mm 0.@+1. mm

Uniformity coefficient +.$ 1.$+1.?

Head loss Up to 1 m Up to 4 m

:engt of run 0+>0 days 1+ days

Metod of cleaning Scrape off top layer 6ackwas wit water andand was %or replace air D water scour and in some caseswit new sand& surface scour

*aswater consumption 0.+0.@K of filtered water 4+@K of filtered water

Genetration of suspended solids Superficial 8eeptroug te filter #ed

Gretreatment #y coagulation No Oes Oes

3overed construction No Lptional Oes

;isi#le operation Oes Oes No

3apital cost Hig Hig Medium

Lperating cost :ow Hig Hig

Skilled supervision Not required 'equired

-dusting te quality of filtrate 8ifficult 3an #e done quickly

6acteria removal >>.>>K >0+>>K

Ad+antages and disad+antages o" slo$ sand "ilters

Simplicity of design and operation and minimal requirements of power and e/pensive cemicals makete slow sand filter an appropriate tecnique for te removal of organic and inorganic suspended matteras well as patogenic organisms present in surface waters of rural areas in developing countries.Sludge andling pro#lems are also minimal. 3lose control #y an operator is not necessary, wic is

important to small communities #ecause an operator may ave several responsi#ilities. 8esigne/perience in -merica sows tat te slow sand filter is more tan >>.>K efficient in removing 'iardia

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cysts and coliform #acteria, and provides a sta#le effluent quality wit a low operating #udget %Seelauset al., 1>A@&. (t as te added advantage of #eing a#le to make use of locally availa#le materials andla#or.

However, slow sand filtration as certain limitations. Te requirements of a large area, large quantitiesof filter medium, and la#or for te manual cleaning are te maor disadvantages. Tese limitations donot apply to te rural areas in te developing world were land and unskilled la#or are readily availa#le.

Design aroach to sand "ilters

1. Start all calculations on te #asis of 1mof filter area.. Sand filter pressure drop over a clean filter can #e calculated using eiter te =o)eny or 'ose

equations. Tis is calculated from some predetermined ma/imum rate of filtration.4. Te pressure drop resulting from fouling ten needs to #e added as a function of time. *e now

ave a linear function #etween pressure drop %or trougput& and operating time, increasinguntil some unaccepta#le ig pressure drop %or low trougput&. %Te #racketed parts refer toconditions under constant ead filtration.&

B. Te trougput rate over te operational period can #e determined as te product of %average&trougput rate and time.$. Te #ackwasing volume of water necessary can #e calculated from te #ackwas velocities

required and te period required for #ackwasing.@. Te actual trougput per cycle %net production& is ten calculated #y su#tracting te amount

of water required per #ackwas. (f additional water is wasted, tis also needs to #e su#tracted.?. Te total cycle lengt is te filtration time D #ackwas time D downtime5cycleA. Te average trougput rate can #e calculated from te net production divided #y te cycle

lengt.>. Te water demand divided #y te average trougput provides te required filter area.

I)ortant e)erging iss!esTe removal of proto)oan ova2 direct filtration design2 upflow filtration2 continuous #ackwasing duringfiltration2 floating media2 increased #ackwasing and flusing on water quality2 oter forms of filtration toreplace sand filtration2 #iological rapid sand filtration, etc.

Cryptosporidiumooc*sts

R5F5R5N,5S

-din, -., 6aumann, C. '. and 3leas#y, P.:., Te application of filtration teory to pilot+plant design, P. -**-, ?1%1&, 1?,1>?>.

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-mirtaraa, -., Lptimum #ackwasing of sand filters, P. Cnviron. Cng. 8iv. -S3C, 10B%CC$&, >1?, 1>??.-mirtaraa, -., 8esign of granular+media filter units, *ater Treatment Glant 8esign, '. :. Sanks %Cd.&, -nn -r#orScience, Micigan, 1>?A.-non., S*S unit, *orld *ater, B, May 1>A4.-**- Filtration 3ommittee, Te status of direct filtration, P. -**-, ?%?&, B0$, 1>A0.6aumann, C. '., ranular+media deep+#ed filtration, *ater Treatment Glant 8esign, '. :. Sanks %Cd.&, -nn -r#or Science,

Micigan, 1>?A.6en -im, '., Sanoun, -., 3emin, 3., Han, :., ;isvanatan, 3. and ;igneswaran, S., New filtration media and teir usefor water treatment, Groceedings, *orld Filtration 3ongress, Nagoya, Papan, 1>>4.3amp, T. '., ra#er S. 8. and 3onklin, . F., 6ackwasing of granular water filters, P. Cnviron. Cng. 8iv.Groc. -S3C,>?%CC@&, >04, 1>?1.3ansdale, . S., 'eport on Second 'egional 3onsultancy :ow+3ost Filtration System, 'eport No. S3S( ?>1*G1AB, Sout3ina Sea Fiseries 8evelopment and 3oordinating Grogram, Manila, 1>?>#.3leas#y, P. :., 8eclining rate filtration, *ater Science and Tecnology, ?%?+A&, 11, 1>>4.3onley, *. '. and Hsiung, =., 8esign and application of multimedia filters, P. -**-, @1%&, >?, 1>@>.3ulp, '. :., 8irect filtration, -**- P., @>, 4?$, 1>??%?&.Fair, . M., eyer, P. 3. and Lkun, 8. -., *ater and *astewater Cngineering, ;ol., Pon *iley Q Sons, 1>@A.adkari, S. =., 'aman, ;. and adkari, -. S., Studies of direct filtration of raw water, (ndian P. Cnviron. Healt, %1&, $?$,1>A0.

Hagar, 3. 6. and Clder, 8. 6., Filtration processes, Groc. -**- 1>A1 -nnual 3onference, -merican *ater *orks-ssociation, 1>A1.Hamann, 3. :. and Mc=inny, '. C., Upflow Filtration Grocess, P. -**-, @0%>&, 104+104>, 1>@A.Hutcison, *. and Foley. G. 8., Lperational and e/perimental results of direct fi ltration, P -**-,@@%1&, 1>?B.('3R(nternational 'eference 3enter for 3ommunity *ater Supply and Sanitation, Slow Sand Filtration for 3ommunity*ater Supply in 8eveloping 3ountries, 6ulletin Series No. 1@, Te Neterlands, 1>A1.=awamura, S., 8esign and operation of ig+rate fi ltersRGart 1, P. -**-, @?%10&, $4$, 1>?$.:ogsdon, . S., 8irect filtrationRpast, present, future, 3iv. Cng.+-S3C, ?0%?&, @A, 1>?A.Mc3a#e, *. :. and Smit, P. 3., Unit Lperations of 3emical Cngineering, nd ea., Mcraw+Hill, NO, 1>@?.Lkun, 8. -., 8igest of sanitation engineering researc reports, upflow filtration, Gu#lic *orks, >A%@&, 1>+01, 1>@?.Garamasivam, '., Maisalkar, ;. -. and 6ertoue/, G. M., Slow sand filter design and construction in developing countries,P. -**-, ?4%B&, 1?A, 1>A1.Scul), 3. '. and Lkun, 8. -., Treating surface waters for communities in developing countries, P. -**-, ?$%$&, $B,

1>A4.Seelaus, T. P., Hendricks, 8. *. and Panonis, 6., 8esign and operation of a slow sand filter, P. -**-, ?A%1&, 4$, 1>[email protected], 3. M. and Monscvit), P. T., 8esign and operation of a 00+mgd direct+filtration facility, P. -**-, @@%&, 1?, 1>?B.Sweeney, . C. and Grendiville, G. *., 8irect filtration -n economic answer to a citys water needs, P. -**-, @@%&, @$,1>?B.Tate, 3. H. and Trussel, '. '., 'ecent developments in direct filtration, P. -**-, ?%4&, 1@$, 1>A0.Tate, 3. H., :ang, P. S. and Hutcinson, H. :., Gilot plant tests of direct filtration, P. -**-, @>%?&, 4?>, 1>??.;an der Merwe, G P and van :eeuwen, P %1>>1&. 8irect sand filtration of drinking water treatment in small communities. Proc()th *ational +eeting of the SA Inst hemical ngineers, -ug., 8ur#an.;igneswaran, S., Tam, 8. H. and ;isvanatan, 3., *ater filtration tecnologies for developing countries, CnvironmentalSanitation 'eviews, CNS(3, -(T, 6angkok, 1, 1>A4.;igneswaran, S. and 8armappa H. 6., Unpu#lised Notes on *astes and *astewater Treatment, University ofTecnology, Sydney, 1>>4.

;igneswaran, S. and Ngo, H. H., Trends in water treatment tecnologies, Groceedings of =orea+ -ustralia Point Seminar onte 'ecent Trends in Tecnology 8evelopment for *ater uality 3onservation, Seoul, =orea, Pune 1>>4, 1B$+1$$, 1>>4.;igneswaran, S. and 6en -im, '., (mprovements on rapid filter, *ater *astewater and Sludge Filtration, 3'3 Gress, 6oca'aton, Florida, 1>A>.;isscer, P. T., Slow sand filtration 8esign, operation and maintenance, P. -**-, A%@&, @?, 1>>0.

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