ThammasatInt. J. Sc.Tech.,Vol.5, No.2, May-August2000 How to Estimateand Designthe Filter Run Duration of a Horizontal-FlowRoughing Filter Dome Sittivate Dept. of Rural Technology, Thammasat University,Pathumthani,Thailand' Abstract methodsusedbeforeslow sandfilters(SSF),horizontal-flowroughing Among pre-treatment for applicationin developing filters(HRF) have beenfound to be the most effectiveand appropriate parts, have low capitalcost,can be mechanical require no countries.This is becausethey are simple, with a wide varietyof raw capacity, and retention solids their high to long time due for a operated on turbidity (solids)removal Most researchon HRFs has concentrated surfacewater characteristics. whereasthe way to estimateand designthe run durationtime beforefilter (HRF) cleaningnecessary a largegravel(size l0-20 mm) filter mediawere placed hasreceivedlittle attention.In this research, filtersused as a singlepackage,uncoveredand outletat the top. It was foundthat the laboratory-scale over time, of filter run to the estimation with respect physical model in this study provideda useful This modelis practical field experience. with agreement in reasonable be found to were andthe results basisfor the designof full scaleHRFsand is easyto apply. a reasonable Keywords: Algae removal, motile algae,HRF (horizontal-flow roughing filter), filter run duration, turbidity l.Introduction U n d e rt r o p i c acl o n d i t i o n so,n ep o s s i b i l i t y which has been used as pre-treatmentbefore slow sand filtration for reducing the algae contentof raw wateris horizontal-flowroughing f i l t r a t i o n( H R F ) t l l . H R F i s n o t o n l y u s e df o r improvingthe physicalwaterquality in orderto but also meetthe slow sandfilter requirements ranging viruses and bacteria for rernovingsome pm and 0.4 20 l0 to in sizefrom approximately rnost cornrnon The to 0.02 pm, respectively[2). type of HRF, which has beentrsedwidely, was d e v e l o p ebdy W e g e l i n[ 3 ] i s s h o w ni n F i g u r el . FurlhermoreSittivate [4] found that the large gravelmedia(size l0-20 rnm) which were ttsed in the filterswith the outletat the top of an HRF ( s i m i l a rt o W e g e l i nt 2 l , t 3 l ) p r o d L r c et dh e b e s t resultsfor algae and turbidity retroval (tnore than 95Yo and 90o/ootl average.respectively) and sedimentationwas consideredto be the for the rernoval principalmechanismresponsible of algaeand turbidity in raw water at the low filtrationrate(0.3 m/h) The objectiveof this researchwas to a devise simple method by which horizontalflow roughingfilterscould be designedfor field applicationunder tropical conditionsand their long-term performance predicted' This essentiallyrequires an understandingof the processvariables,knowledgeof the perlinent of removal of particulatematter, rnechanisms includingalgae,andthe availabilityof equations to describe the time-space variation and accumulationof materials inside the filter rnedia. Many theoreticalequationshave been developedto describethe filtrationprocessand, particularly,to try to estimatethe time of run duration for HRF design However, some parameters and constantsin theseequationsare ruotuniversaland will, no doubt,vary with each 2000 Int.J. Sc.Tech.,Vol.5,No.2,May-August Thammasat combination of influent characteristicsand fi ltrationconditions[5]. Moreover,when designershaveapplied such equationsthey have had to make many assumptions of those parameters for the estimationof results. It is known that the diversityof removal mechanismscauses the flow of suspended particlesthrough porous media to be a very In the presentstudy,the complexphenomenon. run durationtime of the HRF model was rather short. Therefore,it is logical to considerthe main removal mechanismis sedimentationof the incomingsolidsand algal massin the large gravelmediafilter [4]. In spite of the difficulty of accounting for all variables,it was decidedto usethe results obtained in the experimentsof this study to developanotherapproachto designthe HRF anc estimatethe run duration before cleaning is necessary. i,li___f<-ffii__+_ffi_l A H + I-+a a I" Drainvalves List of symbols designguidelines dg (mm) gravelsize vr: H (m) filter depth Vd: Lr,z.t W A a Qd VF Vd (m) (m) (rn') (m3/h) (m3/h) (m/h) (m/h) filter length filter width area filter cross-section flow rate drainagerate filtration rate drainagerate Q : a :0.3-1.5m/h A -60-90m/h Q (Lr+L2+L3).W H.W AH-30cm H -0.8-1.20m Figure l: Layout and Designof a Horizontal-flow Roughing Filter (Wegelin,1996) 2. Experimental Procedure 2.1 Pilot Plant The pilot plant system [4] slrown in of the following:(l) A plastic Figure2 consisted filter box composed of two symmetrical of 1.6m x channelswith dirnensiorrs rectangular 0.39 m x 0.195 m each:the lateralwalls of the filter box were fitted with sampling ports. (2) Two plastic cylindrical tanks (total volume of one cubic metre) with an internaldiameterof I .60 m and heightof 0.50m; thesewerethe feed tankswhich containedalgaeandclay to simulate ThammasatInt. J. Sc.Tech.,Vol.5, No.2, May-August2000 tropical surface water. (3) Two stirrers for mixing the suspensionin the large tank. (4) A light sourceconstructedfrorn lightweightsteel bars to carry fluorescent tubes; this was suspendedover the two large tanks. (5) Two peristaltic pumps were used to feed the from the algaeinoculationtanksinto suspension the filters. The suspensionof turbidity and algae from the largetankswas pumpedinto the inlet chambersof each filter, passedthrough the gravel media and dischargedinto the outlet chambersof each filter. During the filter runs, effluentwater sampleswere analysedfollowing the method of Standard Methods for Examinationof Water and Wastewater[6], mainly to determine the DO concentration, turbidity and pH. The chlorophyll a concentrationwas also determinedas a measure ofthe algaeconcentration. The pilot studyplant was installed in a specific room where the temperaturewas controlled between 19 and,23o so as to maintaingood environmental condition for algal growtlr in the largetanks [4]. Light Figure 2: A schematicdiagram of filtration equipment(not to scale) 2.2 Design of Synthetic Raw Water with a high algae To preparea medir"rm contentin tlre preparationof an appropriateraw water, one factor which was consideredwas turbidity. High turbidity in water inhibits photosynthesis and thus reducesthe production of oxygenby algaeas well as algal growth [7]. Two types of motile green algae (Euglena gracilis and Chlamydontonasreinhardtii) were chosenas representative algaefor this research Wegelin suggested that the possibilityof [4]. [3] annualturbiditymaximumin raw watersources in tropicalareasis normallynot higherthan 100 NTU. As a result, it was decidedto set the tLrrbidityof the syntlreticraw waterat 100NTU for every run in this research. Algae inocLrlation in the largetank usedthe rnethodssuggested by The Freshwater Ecology Institute of ( A m b l e s i d eU , K ) . T h e r e q u i r e dt u r b i d i t yw a s achievedby the additionof a known weight of clay, which would produceturbidity 100 NTU in the water in the largetank and maintainedin suspension by continuousstirring [4]. The type of clay usedwas kaolin clay (Speswhitechina clay) supplied by ECC International Ltd, C o r n w a l lE , ngland. 2.3 ExperimentalPlanning Experiments as shown in Table I and in the Filter diagram (Figure 3) were performed on the 1.60 m long channel. The filtration rate was 0.3 m/h throushout the study. Thammasatlnt. J. Sc.Tech.,Vol 5, No.2, May-August2000 Table 1: Planningof exPeriments Filter Run no. lnfluent turbiditY Algae (NTU) type B J 4 125 to 1 9 5 E E E 2 3 0 to 2 3 5 C E&C tz5 to 1 9 5 lnfluent algaecontent Gravel Outlet during the run (pg/l) size(mm) position L T t' 6.49to 3'7.66 uell 6.49ro 37.66p.p.ll M L 145to 198 l 9 . 4 8 t o1 5 1 . 9 5 ue/l 8 6 1 0t o 3 6 3 1 2 $ e / l I 50to 260 23883to 360ue/l L L T T reinhardtii' Notes:A: Filter A, B = Filter B. E= Euglenagracilis,C= Chlamydomonas - l0 mm' 5 s i z e d g r a v e l , M e d i u m M : r n m . L : L a r g e g r a v e ls, i z e d1 0 2 0 T : Outletat the top, 0.I 6 m from the bottomof the filter bed' solids(SS)andgravelmedia by suspended Cloggedfilter volume= Filtervolumeaccumulated : by algae [4]' Inleilurbidity(NTU) clay turbidity+ tLrrbiditvproduced fFtffil -+ I h tlter tt I -> Outlets Run nos.l to 4. passthrough filters Figure 3: Plan view of filter showingflow of syntheticraw water 3. Results Throughoutthe study'the filter volumes i n w h i c h s u s p e n d esdo l i d sa n d m i c r o o r g a n i s m s accumulatedin the pore spacesof the filter media over time were measured Linear of the dimensionsof the clogged measurements areaprofile along the side of both filters were made at various times in every rlln and converted into volumes of the filter b;' multiplicationof the filter width (0 195 m) Thesedataare given in Tables2 to 6 Note that this total volume is composedof the volume of solids and media Particles gravel filter the occupying clay) and fmicroorganisms mediaporespaces. media (size-l0-20 mm) filter Table 2: Accumulationof solidsmassin the large gravel : volume in Run No'1: Algal culture Euglena gracilis Accumulated time (h) JJ )o 96 126 242 365 484 55'7 Cloggedfilter volume (crn') Time difference (h) Clogged filter volume difference(cmi) 4,09).UU 7,020.00 9 . 74 0 2 5 1 3 . 3 0785 t4,820.00 22.985.63 28.255.50 35 , 0 1 2 . 3 5 23 40 30 Ir6 lL) I l9 I J 2.925.00 2,720.25 3,568.50 1 . 5I 1 . 2 5 8 ,r 6 5 . 6 3 5,269.87 5 6.'756.'7 average : Clogged filter volume difference / differencetime (cm3/h) Influent turbidity (Nru) 1 2 ' 7t 7 68.01 r1 8 . 9 5 13 . 0 3 40 t70 44.28 92.56 t20 t45 125 15.17 152.14 66.19 ThammasatInt. J. Sc.Tech.,Vol.5, No.2, May-August2000 Table 3: Accumulationof solidsmassin the medium gravel media (size5-10 mm) filter volume in Run No.l: Algal culture = Euglena gracilis Accumulated time (h) 33 56 96 126 242 365 484 557 605 Clogged filter volume (.,n') Time difference (h) Clogged lilter volume difference (cm') Clogged filter volume difference / difference time (cm3/h) 3.968.25 6 1t1.t'l z) 6.669.00 40 30 l16 I 1,407.50 14,430.00 17.788.88 2t ,498.7 5 23.75t.00 26,822.2s I z) ll9 48 2.344.88 355 . 8 7 4 , 7 38 . 5 0 3.022.50 3 , 35 8 . 8 8 3.709.87 2.252.25 3 , 0 7.12 5 average = r01.95 8.90 t57.95 26.06 27.31 31 t 8 30.85 Influent turbidity (NTU) 95 70 40 70 zv 63.98 45 25 20 56.02 148.13 Table 4: Accumulationof solidsmassin the large gravel media (size10-20mm) filter volume in Run No.2: Algal culture = Euglena gracilis Accumulated time (h) 28 52 82 9'7 t45 Clogged filter volume (cm') 25.072.13 32,048.2s 44,850.00 49.530.00 6t,t61.75 Time difference (h) Clogged filter volume difference (cmr) Clogged filter volume/ difference time (cm3/h) Influent turbidity (Nru) 235 24 30 l5 48 69' 76.12 12,801.75 4.680.00 II,631.75 average: 290.67 426.73 3t2.00 z+z.) ) 230 225 230 3r7.93 230 Table 5: Accumulationof solidsmassin the large gravel media (size10-20mm) filter volume in Run No.3 : Algal culture : Chlamydomnasreinhardtii Accumulated time (h) 53 79 t27 223 3t9 415 487 Clogged filter volume (.'n') Time difference (h) Clogged filter volume difference (cmr) r6.32r .50 2t,67 4.25 29.250.00 33.783.75 38 . 2 8 8 . 2 5 55,282.s0 6 0 , 0 9 4l 3. 26 Clogged filter volume difference / difference time (cm3/h) Influent turbidity (Nru) I 57 . 8 3 98 45 50 4.533 . 75 lt.zJ 45 4.504.50 46.92 16.994.25 t77.02 66.83 45 45 50 116.95 54 5 '157 75 7 5 7 57 5 48 96 96 96 72 4 , 8 11 . 6 3 average = 205.88 2000 ihammasatInt.J. Sc.Tech.,Vol.5,No.2,May-August Table 6: Accumulation of solids massin the large gravel media (size 10-20mm) filter volume in Run No.5: Algal culture = Euglens & Chlamydomonos Accumulated time (h) 78 05 30 50 9l Time difference (h) Clogged filter volume difference (cm3) 5.533.r3 42.237.00 47,s80.00 27 25 20 6l.l6l.7s 4l Clogged filter volume (.'nt) Clogged filter volume difference / difference time (cm3/h) (Nru) 260 23,868.00 29.40r.13 Influent turbidity l2.835.88 5.343.00 l 3 . 5 8.17 5 average = 204.93 513.44 267.15 33t.26 2r0 329.20 201 200 185 150 b) Simplifiedword statement: 4. Discussion 4.1 Progressionof Filter Volume Occupied by SuspendedSolids and Filter Media In HRFs, the filtration rate is assumedto remain constantthroughouta filter run and the type of flow in filter media is plug flow [3].Tchobanogloust8l suggested that, in of the general,the mathematicalcharacterisation within matter particulate of removal time-space the filter is based on a considerationof the equation of continuity, together with an auxiliary rate equation. The equation of continuity for the filtration operation may be developedby consideringa suspendedsolids mass balance for a section of filter of crosssectionalarea,and ofthe thickness,measuredin the directionof flow. The massbalancewould be as follows: = [inflow - outflow] - Accumulation (2) degradation However, it was found that the degradationin HRF occurred very little [a]. Hence, the last term in equation(2) is negligible. of equation(1) [8]: c) Symbolicrepresentation d V ac d :-a 0c dx t (3) A where, differential volume of reactor or dV filter (L3) dcldt = rate of change of suspendedsolids within the containeror concentration filter (ML-3Tr) a) Generalword statement: a Rateof accumulation of solidswithin the : volumeelement Rateof flow of solidsinto the volumeelement Rateof flow of solids out of the volume element (1) : dCl& : dx C : : volumetric rate of flow -into and out of the reactoror filter (L' T-') changein concentrationofsuspended solids in fluid stream with distance (MLr.L) differentialdistanceof filter (L) concentrationof suspendedsolids in reactoror filter (ML") Thammasat Int.J. Sc.Tech.,Vol.5,No.2,May-August 2000 If A dx is substituted for dV. the resulting This can be rewritten; expressionis vz) (ya),f" - (vrXy,'),f" A d x 0 C : - a d C dx dt T (4) [(cssrz- cssorz)tz- (cssrr- cssorr)tr] where; I = cross-sectional areaof filter (L2) Therefore,equation(4) may be rewritten: A&dc :- Qac dt (s) Vr V2 Ttz where; "f" ytr At : At : weight of suspendedsolids retained in the filter mediaat any time (M) differenttime (T) Therefore, equation(6) can be rewritten in a form suitablefor numericalanalysis: AP, tz-tr = QAC. or - ( C s s , rC P-rz-:&': Q [(Css,z- sso,r)] Csso,z) tz-tr (7) where; = weight of suspendedsolids retained Pt in the filter mediaat any time (g) = flow rate through the cross-sectional a areaof filter (m'/h) Css,r : concentration of suspende-d solidsin the influentat time tr (g/m') Css,z = concentration of suspended solids in the influent at time tz(g/m') Csso,r = concentration of suspended solids in the effluentat time t, (g/mt) Csso,z= concentration of suspende^d solids in the effluentat time h (g/m') : cross-sectional A areaof filter (m') = filtration rate (m/h) , t: time (h) Vp (g) where; It can be said that the first term ofequation(5) is the differentweight of solidsaccumulatedin the pore spaceof the filter media over the different time. Hence,equation(5) may be rewrittenas an ordinarydifferentialequation: dP:- Qdc (6) Pr A vpx : filter volume occupied by suspended solidsandfilter mediaat time t1(cmr) : filter volume occupied by suspended solidsand filter mediaat time t2 (cm3) = total unit weight of solids mass accumulatedin the pore spaceof filter mediaat time ty(g/ cmr) = total unit weight of solids mass accumulatedin the pore spaceof filter mediaat time t2(g/ cmr) = porosity of the bed when clean (Vo / v) The time difference between t1 and t2 is not much.Hence,it can be said that Ttz-Ttt.=ft Therefore,equation(8) may be rewritten; .f" (yJ [Vz - Vr] = A vr [(Cssrz- Cssorz)rztr-tr (Cssrr- Cssor,)tr] (e) where; It total unit weight of solids massaccumulated in the pore space of filter media The filter volume(Vz , V,) and porosity (/" ) are easily measuredbut it is very difficult to measurethe total unit weight of solids mass accumulatedin the pore spaceof filter media (yJ. This is becausethe suspendedsolidscontent will be differentat any depth or at any length in the pore spaceof the filter media of the HRF, and at any time as influencedby the drag force of fluid, velocity of flow throughthe pore space of the filter media,gravity and the characteristic diameterof the solidsparticles(Fair et al., 1968) during the run duration. So the first term of equation (9) cannot be used to estimate the solids mass retainedin the filter. However, all Int. J. Sc.Tech.,Vol.5,No.2, May-August2000 Thammasat parametersin the last term of equation(9) can be measuredand used to estimatethe solids massretainedin the filter media Hence, it follows that (V2 - V1) is occupiedby gravelmediaand solidsmassA vp tu- (Css,r- Csso,r)t1].Therefore, [(Css,,- Csso,z) the rate of filter volume occupiedby gravel mediaand solid mass,per time, is equivalentto the differencein weight of suspendedsolids mass accumulatedin the pore space of filter mediaover the time change,dividedby porosity of gravel media and total unit weight of solids mass accumulatedin the pore spaceof filter the media.Hence,equation(9) afterrearranging terms.vields: Vz - Vr tz- tr = - Csso,z) tz- (Css'rA vr [(Css'z (y,) ,f" Hence,with the outlet at the top and a filtration rate of 0.3 m/h (at average influent turbidify = [230 + 154 + 201] I 3: 195 NTU), the averagechangeof filter volume over time accumulatedby gravel media and suspended solids mass(clay and algal mass)in the large gravel media (size l0-20 mm) filter is 250 cm'/h. Hence, the average change of filter volume accumulatedby gravel media and solidsmass(clay and algal mass)in suspended the large gravel media filter pore space over t i m e ( S e e e q u a t i o n( 1 0 ) ) i s 1 1 0 c m ' / h ( : l J Q crn'/h * porosityof gravelmedia= 250 cmrlhv 0.4372) This rate could be the basisof both the design of HRFs, and estimatesof the run durationtimesfor scaledup HRFs. 4.2 Confirmationof Results (10) c s s o , rt ), l Sittvate [4] indicated that under the same operatingconditionsas adoptedfor the large gravel media filter, the medium gravel (size 5-10 mm) filter with lesspore spacethan the largegravelmediafilter, was blockedfaster than the largegravelmediafilter. This resulted in the duration of a filter run for a rnedir.rm gravel media filter becomingshorterthan that for a large gravel media filter. Furthermore, becausetaking many samplesalong the bed of the large gravel filter over time drew out clay from the pore spaces,as observedin the first run it was decided,therefore,not to use the result from Table 2 and 3 in this analysis.For the largegravelmediafilter model,the filter volume solidsand filter mediaat occupiedby suspended differenttimes,from tables4 to 6, were 317.93, I I 6.95 and 329.20 cm'/h, respectively.Irr addition,Sittivate[4] found that the patternsof the removal characteristicsfor Euglenagracilis and Chlamydomonas reinhardtii in HRF were similar. Hence, the average filter volume solidsand filter rnediaat occupiedby suspended differenttimes 3t7.93+116.95+329.20 3 254.69 - 250cm3lh C h e c k i n gt h i s v a l u e( l l 0 c m r / h ) w a s achievedby estimatingthe run durationtime of the large gravel filter model which normally failed by solidsblockingalong the filter length i n l 0 d a y s[ 4 ] ; The effective volume pore space of filter modeloccupiedby largegravelmedia : 1 9 . 5x 2 0 x 1 5 3x 0 . 4 3 7 2 : 26,087.72cm3 .. R u nd u r a t i o nt i m e = 2 6 . 0 8 7 . 1 2 1 1 1x0 2 4 : 4,3 9 . 8 8d a y s - l 0 d a y s Scale-upConsiderations As shown in Section 4.2, for the conditionsof filtration velocity (vr) : 0.3 m/h and with the filter outlet at the top, it was that: calculated R (day) = (Volurneof filter mode-loccupied 116 lsrn'/h) x 24 by gravelmedia(cmr)(/") ) (11) where: Run duration (days) before filter cleaningis necessary. Thammasat Int. J. Sc.Tech.,Vol.5,No.2, May-August2000 where; Let, A,n = L,n w,n : D r : Qo : The cross-sectional areaof filter model ( : 1 9 . 5x 2 0 c m 2 ) The lengthof filter model(internal) The width of filter model(internal) The depthof filter model(internal) porosityof the bedwhenclean(V"i V1) flow-rate through the cross-sectional areaof filter prototype(m3/h) Q,n = flow-rate through the cross-sectional areaof filter model(mr/h) Equation( l4) / Equation( 15)gives, 0; ( 1 1 )w i l l b e ; H e n c ee, q u a t i o n Ao.vr : Qo = n*u' Ao A- therefore; Qo = (AolA,,).Q, Ap/A,n = rnr 1 1 0( c m 3 / hx) 2 4 or : (W,n.D,n.)x L,,,(/o) 2,640(cm3lh) a therefore, Qo and, Q,n = (17) (12) where; This filter model will also fit the prototype (the large-scale design for field application), on the basis of an irnplied confidencein the principlesof geometricaland dynamicalsimilarity. Henderson[9] suggested that the detailed interpretationof model measurementsrequires that scale ratios be available for translating model values for variousquantities,e.g.,velocity,discharge, etc., prototypevalues.Scales into the corresponding can be deducedfor all physical quantitiesif scalesare known for mass,length,tirne,and the physical propertiesof prototype and rnodel fluids. The prototype will have conditions similar to the filter model in this study, for instance the low filtration rate (0.3 m/h), position of the outlet, gravel media size, and irrfluerrtturbidity.The scaleratio of this rnodel and the prototype can be obtained frorn the cross-sectional areaof the prototypedividedby the cross-sectional area of the model. This is becausethe filtration rate (ve) in the designof the prototype is equal to that irr the model and the depth of the filter is not significantin the f i l t e r b e h a v i o r s[ 0 ] . H e n c e .t h i s s c a l er n o d e l ratiocan be calculatedas follows: From (l6) : rn1 scaleratio of protofypeand model Hence, for the large scale of HRF (prototype) with tlre sarneconditionsas in the model, the run durationcan be estimatedby; : R Ao.Lp (,f")p (t8) 2,640 (m1) wnere: : the lengthof filter prototype(m) Lp (,f")o = porosity of the bed of filter prototype whenclean(V"/ Vr) It was found that R ccli vr. and R cc l/ influentturbidity(NTU) [2], therefore,equation (18) could be modified by these proportional relationships for use in the designof a HRF, as f o l l o w s[ 9 ] : R : Ao.Lo (.f")o (NTU)p/(NTU)," 2,640 rn1(v1)o/(ve)," when; Av (13) trz Ap.vr ( 14) rn: : A,n.Vp ( 15) = (ve)o/(vp),,, (NTU)p/(NTU,. (le) ThammasatInt. J. Sc.Tech.,Vol.5,No.2, May-August2000 the filter media are assumedto not change. This equationshould be used with the low filtration rate of 0.3 m/h for the scaled up designas in this study,becauseit was found that, with thesefilters used in this study, both algae and turbidity removal were more than 90oh at this filtration rate. Moreover, this filtration rate was found to be suitable for operationand has producedturbidify removal efficiencies more than 80% in field i n s t a l l a t i o n( [s3 ] a n d[ 1 l ] ) . Equation( l9) becomes, R : Ao.Lo.(/")o 2-640. mr "t" m, (20) 4.4 Model validation From equation(20), it can be saidthat the filtration rate and turbidity are the main parameterswhich affect the run duration and turbidity removal efficiency as Wegelin [2] found. If the gravel media is chosenas in Wegelin [3]'s designguidelines,the porosity of the gravel media size 10-20 mm will not affectthis equation.As a result,this model is valid for use in HRF design,for the filtration rateadoptedand up to the influentturbidityof raw water usedin the studY. 4.5 Confirmation of the Physical Modelling Although equation (20) has been developedto be a simple tool for the design and estimationof the run durationtime of a HRf', it is still not proven on the larger scale. However,the horizontal-flowroughingfilters usedin the Blue Nile HealthProjectin Sudan [2], were similarto this modeland the results reportedby Brown can be usedto confirm the applicabilityof equation(20). A plan view of the HRF seriesusedin the Sudanis shown in F i g u r e4 . Brown (1988) rePortedon two runs wherethe run durationof the filter (Figure4) in the when solidsaccumulated was terminated filter media pore spaces to the point of breakthrough.In the first run, with a filtration rate of 0.4 m/h and averageinfluent turbidity 300 NTU, the run durationtime was 20 days. In the secondrun (after cleaning),with average filtration rate 0.45 m/h and averageinfluent turbidity 312 NTU, the run durationtime was I 4 days. Lebcir[10] alsofoundthat: - Particles are removed mainlY bY and the removal is slowed by sedimentation increasingthe filtration rate or Reynolds Number. -Laminar flow conditions prevailed when the filtration ratewas lessthan I m/lr and 2 mlh for large gravel media filter (LGF) and small gravel filter (SGF), respectively(both outlet at the top). - The patternof the turbiditydistribution insidethe bed is in conformitywith that of the sedimentationtanks at flow velocitiesbetween 0.5 - I m/h. This is becausethese flow velocities are low enough to not cause turbulencein the gravelfilter media. - For the depthofbed channel,studiesto date merely state the problems of structr-rral constraints that can be faced with deep channelsandthey do not give any indicationof whether the depth will influence filter behaviour. Hence,equation(20) could be valid if the filtration rate doesnot exceedI m/h for the largeand medium gravelfilter mediaand the other conditionsare as in tlris study.This is within this rangethe conditionsinside because From equation(20); for the first run, Ao.Lo (,f")o 2,640 m1.Flz.lrlr np 1Tl1 l0 1 x 0 . 9 5x 1 0 , 0 0 0c m 2 0 . 9 5 x1 . 0 0 x1 0 , 0 0 0 : 2 4 . 3 6 19.5x 20 Ill2 ( V F ) p / ( V F ) *= 0 . 4 / 0 . 3 : lTlr ( N T U ) p / ( N T U ) .: 3 0 0 / 1 9 s : 1.33 1.54 ThammasatInt. J. Sc.Tech.,Vol.5,No.2, May-August2000 EE BROKENNILE BRICKS f.:0.575 {-VF -rsffirsffi,,ffi,ul*_ Figure 4: Plan view of the Horizontal Flow Roughing Filter used in Blue Nile Health Project in Sudan [12] (not to scale) Note: The depth of this filter was 1 m. Substituting thesevaluesin equation(20), R : W providesthe estimatedrun durationtirne for the first run; R : I x 0 . 9 5x 1 , 0 0 0 , 0 0 0 ( 2 . 9 x 0 . 5 715x+ 0.40+1x0.37) I x 0 . 9 5x 1 , 0 0 0 , 0 0 0 x ( 2 . 9 x 0 . 5 7I 5x + 2 , 6 4 0x 2 4 . 3 6x 1 . 3 3x 1 . 5 4 : 15 days (cf. 14 daysin Brown's results) 0.40+1x0.37) : 17.58days (cf. 20 days in Brown's results) For the secondrun, rnz : ( V e ) p (i V r ) . : 0 . 4 5 l 0 . 3 : 1.5 rn: = (NTU)'/(NTU),': 3l2l 195= 1.6 Substituting thesevaluesin equation(20), providestlre estimatedrun durationtime for the secondrun; It would appearthat equation(20) gives a reasonable agreement with practical field experienceand the estimation of HRF run duration obtained from the equation is a reasonable basisfor design.As a resultof this. Figure 5 was constructedin order to provide someguidelinesfor designand estimationof the run durationfor the large gravel media (10-20 mm) HRF operatingunder the conditionsas shown in Figure5. For eachgiven turbidityand filter length,the run durationin dayscan be read off. For example,with inputturbidity(including algaeand suspended solid particles)150 NTU and a filter lengthof 7 m, the corresponding run durationtime is sixty days(Figure5). 95 ThammasatInt. J. Sc.Tech.,Vol.5, No.2, May-August2000 Run duration (daY) 360 340 320 300 ____ (T__________1_ ll t n t . lt l _____7 HRF ll llouttet 280 260 240 220 200 180 160 140 120 100 80 60 t ---- 40 20 0 9 1 0 1 1 1 2 1 3 1 4 15 16 17 18 19 Filter length (m) 5: Designguidelinesfor large gravel media filter (sizel0-20 mm) with outlet at the Top' Figure porosity (/") = 0.43,filtration velocity:0.3 m/h, depth of water level: 1.0 m' grave|mediafi|terdepth-l.05m,andwidthofthefi|ter=1.00m. 5. Conclusion Due to the fact that there was not sufficient time over the period of the programsto covera full statistical experimental evaluation of all design variables such as areaof the filter,the filtrationrate.cross-section the size of the filter media, filter the of length of raw water rnedia,the irrfluentcharacteristics ( t u r b i d i t ye) t c . ,a c o m p r e h e n s i vd e s i g nm o d e li s n o t p o s s i b l e .H o w e v e r .b a s e do n e x p e r i m e n t a l findings and theoretical considerations,the physicalmodelswhich have beendevelopedin this study can provide significantimprovement on previousdesign and could be used in the designand to estimatethe run durationtime of HRFs. 6. References W e g e l i n ,M . , R o u g h i n gG r a v e l F i l t e r sf o r SuspendedSolids Removal,In Slow Sand Filtration, Recent Advances in Water TreatmentTechnolog,',Edited by N' J' D' Graham,Ellis Hardwood,UK, pp. 103-122, I988. M' Horizontal Flow Roughing Wegelin, t2l Filtration, a Design, Construction and Operation Manual, IRCWD, Switzerland, 1986. t3] Wegelin, M., SurfaceWater Treatmentby RoughingFilters:A Design.Construction and Operation Manual, SANDEC Report, 2196, Swiss Centre for DeveloPment Cupertinoin Technologyand Management (sKAT), 1996. t4l Sittivate, D., Algae Removalfrom Surface Roughing Il'ater by Horizontal-flow ill Thammasat Int. J. Sc.Tech.,Vol.5,No.2, May-August2000 Filtration,Ph. D. Thesis,Civil Engineering Department,University of Newcastleupon T y n e ,U K . , 1 9 9 9 . l-51 Mohammed,S.T., Roughing Filtration of Surface Water for Village Supplies in Developing Countries, Ph. D. Thesis, Universityof Newcastleupon Tyne, UK., I987. t6l APFIA, Standard Methodsfor Examination of Water and LVastewater, (19't' edition), AmericanPublic Health Association,New York, 1995. 17l Carncross, S and Feachem. R.G., Environmental Hectlth Engineering in the Tropics,John Wiley and Sons,New York, 1993. t8l Tchobanoglous,G. and Burton, F.L.. Biofogical Unit Process in Wastewater Engineering Treatment, Disposal and Reuse,McGraw Hill, New York, pp. 88-90, 1991. f - I t_l [9] Henderson, F.M., Open Channel Flow, MacmillanPublishingCo., Inc.,New york, 1966. [10] Lebcir, R., Factors Controlling the Performance of HorizontalFlow Roughing Filters,Ph. D. Thesis,The University of NewcastleuponTyne,UK., 1992. l ] L l o y d , 8 . , P a r d o n ,M . a n d W h e e l e r .D . . I Final Report on the Development, Evaluationand Field Trials of a Small Scale Multi-stage, Modular Filtration Systemfor the Treatmentof Rural Water S u p p l i e sO. D A , L o n d o n U , K., 1986. 2] Brown, D., Horizontal Flow Roughing | Filtration as an Appropriate Pretreatment before Small Slow Sand Filters in Developing Countries,M.Sc. Dissertation, Civil EngineeringDepartment,University of NewcastleuponTyne,UK., 1988.

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