Investigation Of An Improved Performance Of Secondary Clarifiers Using Computational Fluid Dynamics Model

Transcription

1 Investigation Of An Impoved Pefomance Of Seconday laifies sing omputational Fluid Dynamics Model D. Ali H. GHAWI Depatment of ivil Engineeing, ollege of Engineeing - AL-Qadissiyia nivesity IRAQ. ghawi2000@yahoo.com Abstact Seconday claifie is one of the most commonly used unit opeations in wastewate teatment plants. It is customay designed to achieve solids sepaation fom biologically teated effluent though claification of biological solids and thicening of sludge. As teatment plants eceive inceasingly high wastewate flow, conventional sedimentation tans suffe fom oveloading poblems which esult in poo pefomance. Inlet baffle modification by using an enegy dissipating inlet (EDI) was poposed to enhance the pefomance in the cicula claifies in Al-Dewanyia wastewate teatment plant. A 3- Dimensional fully mass consevative claifie model, based on moden computational fluid dynamic theoy, was applied to evaluate the poposed tan modification and to estimate the maximum capacity of the existing and modified claifies. A omputational Fluid Dynamic (FD) model was fomulated to descibe tan pefomance and design paametes wee obtained based on the expeimental esults. The study evealed that velocity and SS is a bette paamete than TS, BOD, OD to evaluate the pefomance of sedimentation tans, and Removal efficiencies of suspended solids, biochemical oxygen demand, and chemical oxygen demand wee highe in the EDI (Baffle). Keywods: laifie, Sedimentation, Sludge, Wastewate, Solids, FD. الخلاصة ان أحواض الترسیب الثانویة واحدة من أكثر وحدات المعالجة أھمیة في محطات معالجة میاه الصرف الصحي. ان أحواض الترسیب مصممة لا زالة المواد الصلبة و المواد العضویة الناتجة من المعالجة الحیویة في أح واض التھوی ة حیث تتجمع ھذه المواد و تترسب قي قعر حوض الترسیب على شكل خبث. نتیجة لزیادة كمی ات التص ریف الواص لة ال ى محطات المعالجة نتیجة للنمو السكاني أصبحت أحواض الترسیب الكلاسیكیة تع اني م ن قل ة كفاءتھ ا ف ي المعالج ة. ل ذلك یتطلب الا مر تحسین أداء ھزة الا حواض, حیث تم في ھزة الدراسة إضافة مصدات عند م دخل می اه الص رف الص حي ال ى حوض الترس یب لغ رض تحس ین الا داء لھ ذه الا ح واض, حی ث ت م تطبی ق ھ ذا التحس ین ف ي محط ة معالج ة می اه الص رف الصحي في مدینة الدیوانیة و التي تعاني من زیادة كمیة المیاه المتدفقة و بالتالي قلة كفاءة المعالجة. في ھذه الدراسة تم تطویر نموذج ریاضي ثلاثي الا بعاد بناء عل ى حف ظ الكتل ة لغ رض نمذج ھ أح واض الترس یب و بالت الي تحس ین أداءھ ا و زی ادة س عتھا. و ش ملت الدراس ة قیاس ات مختبری ھ للم واد العض ویة و الص لبة و الكیماوی ة لغ رض تحس ین أداء محط ة المعالجة. و قد بینت النتاي ج ان كف اءة أح واض الترس یب و بالت الي كف اءة محط ة المعالج ة ت زداد باس تخدام المص دات و كذلك سعتھا. و ان الدراسة قد كشفت ان سرعة الجریان و تركیز المواد الصلبة ھو أفضل مو شر لتحسین أداء أحواض الترسیب. 82

2 Nomenclatues Symbol Desciption A The scape displacement aea (m 2 ) D ns μ F The concentation of solids (mg/l) The dag coefficient The nonsettleable concentation (mg/l) A model constant A volume foce tem (N/m 3 ) which is zeo in both the x and y diections. The tubulent inetic enegy (m 2 /s 2 ) P h, p t s so V t The aveage pessue (Pa) Induce the domination of the fist and the second tem fo the falling and the ising pat Time (s) The aveage flow velocity vecto (m/s) The settling velocity (m/s) The efeence settling velocity (m/s) The blade-to-fluid elative velocity (m/s) ε The dissipation of tubulent enegy (m 2 /s 3 ) η ν t Dynamic viscosity (Pa s) The tubulent viscosity ρ Density (g/m 3 ) ρ The fluid density (g/m 3 ) σ c The Schmidt numbe (0.7) FD BOD OD EDI HRT MLSS RTD SLR omputational Fluid Dynamics Biochemical Oxygen Demand hemical Oxygen Demand Enegy Dissipating Inlet Hydaulic Retention Time Mixed Liquo Suspended Solids Residence Time Distibution Suface Loading Rate 83

3 SOR SS SVI TS TVS VSS Suface Oveflow Rate Suspended Solids Sludge Volume Index Total Solids Total Volatile Solids Volatile Suspended Solids 1. Intoduction In wastewate teatment plants as well as in a vaiety of industial pocesses, sedimentation tans ae used to sepaate suspended solids fom wate. Sedimentation by gavity is the most common and extensively applied teatment pocess fo the emoval of solids fom wate and wastewate. The inceasing concen which is being voiced as to the destuction and pollution of ou envionment has poduced a gowing woldwide awaeness of the need fo moe effective wastewate teatment. In addition the contibution of wastewate teated effluents and sludge to the spead of many types of human and animal infection is now being quantified. This has emphasized the vital need fo impoved wate supply and sanitation, especially in developing counties. Opeation of wastewate teatment wos ae thus no longe the exclusive domain of the enginee and chemist; multidisciplinay teams of enginees and scientists ae equied in ode to maximize the benefits to the community which should occu fom the installation of sewage teatment. In moden societies pope management of wastewate is a necessity, not an option. Wastewate collected fom municipalities and communities must ultimately be etuned to eceiving wates o to the land. The complex question of which contaminants in wastewate must be emoved to potect the envionment - and to what extent - must be answeed specifically fo each case. The answe to this question equies analyses of local conditions and needs, togethe with the application of scientific nowledge, economic analysis, and engineeing judgment based on past expeience and consideation of national equiements and egulations. pgading of existing wastewate teatment plants (WWTPs) may become necessay fo a vaiety of easons. Gowth within the sevice aea, o the desie to seve additional aeas, may esult in the need to incease the capacity of an existing teatment facility. New, moe stingent equiements may be imposed on a teatment facility, esulting in a need to upgade teatment pocesses. Olde facilities may need upgading to eplace existing equipment that no longe functions as intended o to allow installation of newe, moe efficient and costeffective technology. In this case, the objective of the upgading may be to impove plant eliability and / o educe opeating cost. Of couse, moe than one of these easons may combine fo a paticula plant. The subject of upgading existing wastewate plants is paticulaly impotant at this time. 84

4 It is impotant both because of the lage numbe of existing facilities and because of the inceasing stingent equiements imposed on wastewate teatment facilities. A numbe of studies have investigated sediment distibution and flow pattens in sedimentation tans and claifies. Seveal of the studies [1], [2], [3], [4]. [5], [6], [7], [8], and [9], have been caied by use of omputational Fluid Dynamic (FD) model. The FD study of a seconday claifie at Al-Dewanyia Wastewate Teatment Plant in Iaq was undetaen with a view to impoving its capacity to etain sludge unde high hydaulic load conditions, which has become unde pessue due to the gowth in povision of sevices. The objective of this study is to examine the possibility of upgading conventional seconday claifies in an opeating wastewate teatment plant by applying Enegy Dissipating Inlet (EDI) (baffle) fo claifie inlet. In ode to achieve such objectives, field expeiments and mathematical model (FD model) wee conducted in the main wastewate teatment plant in Al-Dewanyia using sedimentation tans with and without EDI fo seconday claification in the activated-sludge unit. 2. Mateials And Methods Thee ae two seconday claifies at the Al-Dewanyia Wastewate Teatment Plant. They ae cicula, 30 m diamete by 3.0 m wall deep and centally fed. Each unit is nominally designed to handle 250 m 3 /h flow, with an equal flow ate of sludge ecycled to the activated sludge plant. The claifies at these plants ae cente-feed and peipheal-oveflow claifies (Figue 1) designed fo optimum activated sludge seconday claifie pefomance. Tan geomety and opeating conditions fo both claifies ae summaized below: laifie diamete = 30 m Side wall depth = 3.0 m Pea Day onditions: Influent Flow = 18,000 m³/d, MLSS = 3,000mg/L, RAS Flow = 7,500 m³/d Suface Oveflow Rate (SOR) = 2.2 m/h Solids Loading Rate (SLR) = 222 g/m²/day Typical Settling haacteistics (fom an example site with an SVI of appoximately 150 ml/g) The pefomance and capacity of a cente-feed claifie is vey sensitive to the intensity of influent jets enteing into the claifies. A cente-feed claifie natually geneates a stong influent jet due to its small cente-feed aea in a cicula claifie tan as shown in Figue 1. The intensive cente influent often bings significant tubulence into the settling compatment, especially unde high flow conditions. To enhance the hydaulic efficiency and capacity of cente-feed claifies, one of the most impotant ey issues is to develop a centefeed appaatus, which could be used to effectively educe the intensity of the cental influent jet and tubulence unde high-flow conditions. 85

5 To enhance the hydaulic efficiency and capacity of the cente feed claifies, the ey is to develop a new cente feed stuctue, which could be used to effectively educe the stength of the cente influent jet unde high-flow conditions. To estimate pefomance enhancements esulting fom the use of an enegy dissipating inlet (EDI) in a cicula claifie a pai of computational fluid dynamics (FD) calculations wee pefomed (Figue 2). In the fist simulation, the flow though a claifie equipped with a cente inlet pipe and open centewell was calculated. In the second simulation, the cente inlet pipe was eplaced with an EDI. a) oss section in claifie b) Plane of claifie Figue 1. laifie scheme in Al-Dewanyia Wastewate Teatment Plant (a. coss section in claifie, and b. plane of claifie) 86

6 The FD calculations povide estimating the effluent solids concentations, etun activated sludge (RAS) concentations, sludge blanet depth and flow distibutions in the claifies. Pefomance compaisons wee made on the basis of these calculated paametes. sing the taditional influent stuctue (as shown in Figues 2), the jet influent though the slots is vey stong due to the vey small coss sectional aea of the slots. Howeve, if the coss sectional aea of the inlet slots is simply enlaged, flow shot-cicuiting o unevenly distibuted flow may occu among the slots. Fo design the EDI, the flocwell diamete was 7.9 m and the depth 1.5 m. Figue 2 shows the actual EDI geomety that wee tested. The wastewate teatment plant was opeated at diffeent flow ates to detemine the effect of Hydaulic Retention Time (HRT) and Suface Loading Rate (SLR) on the pefomance of the claifie. Influent and effluent samples wee collected at diffeent opeating peiods. The liquid tempeatue anged between duing the expeiments. The samples wee analyzed accoding to pocedues outlined in Standad Methods Fo The Examination of Wate and Wastewate [10] to detemine the following paametes: Suspended Solids (SS), Total Solids (TS), Biochemical Oxygen Demand (BOD), hemical Oxygen Demand (OD), Volatile Suspended Solids (VSS), Total Volatile Solids (TVS) and Settleable Solids. a) Model of Modified cente inlet stuctue (EDI) b) Pictue of Modified cente inlet stuctue (EDI) Figue 2. Modified cente inlet stuctue (a, and b) 87

7 88 3. FD Modelling In this study FLENT 6.3 and the 3D -ε tubulence model in the Envionmental Engineeing Module wee used. Duing this study hydaulic FD modelling began with the definition of the settling tan geomety. Secondly fluid chaacteistics and bounday conditions wee defined. The momentum balance including the tubulence model and continuity equations wee then solved numeically fo the tan using the finite volume method. Finally, the obtained solution was post-pocessed to be popely visualised. ommon mathematical hydaulic model equations used fo FD modelling include the momentum balances fo a non-compessible viscous media and the continuity equation [11]. F T t P = Ñ Ñ ø ö ç ç è æ - Ñ Ñ Ñ ú ú ú û ù ê ê ê ë é. ) ) (.( 2 e m m (1). = 0 Ñ (2) In the settling model an additional scala equation was added to include the concentation of the solids. The convection-diffusion equation in the following fom was used: ø ö ç ç è æ = i x i x i x S t c v t s ) ( (3) The settling velocity was modelled using the exponential settling function of Taács, this expession being intoduced in the esolution of the concentation equation. )] ( exp[ )] ( exp[ 0 0 ns p S s h S S X X = (4) The standad -ε eddy-viscosity model was used to account fo tubulent effects. The tubulent viscosity was defined as function of the tubulent inetic enegy and its dissipation ate ε by the equation: e m m 2 t = The distibutions of and ε wee detemined fom the following tanspot equations: S M Y b G G i x j x i u i x t t - - = ø ö ç ç è æ e s m m ) ( ) ( (5) e e e e e s m e e e m S b G sc G i x j x i u i x t t - = ø ö ç ç è æ 2 2 ) ( ) ( ) ( (6)

8 The model constants ( μ, ε1, ε2, σ, σ ε ) in the above equations have been detemined fom expeimental data and ae set to standad paametes [11]: μ = 0.09, 1ε = , 2ε = 1.92, σ = 0.9, σ ε = 1.3 G b descibes the influence of buoyancy effects and is defined as a function of the suspended solids concentation gadient: G b vt = bg s c x p - w = p w vt g s c x The concentation gadient, which its eaches maximum values at the inteface between the clea fluid and the sludge blanet, hindes tubulence. The souce tem G b intoduced in tubulence equation addesses this matte. The value of 2ε, usually epoted as constant, vaies with the atio of gavity diection paallel flow velocity with espect to pependicula flow velocity: S = tanh v u The late expession yields values close to unity fo unstable aeas, and tends towads zeo fo statified sedimentation. A Boussinesq-type appoach also implies that the effect of sludge gavity is intoduced implicitly as a function of suspended solids concentation. Its implementation in the momentum equations is caied out by means of souce tems: p - w g( p - w ) = g (7) p The dependence of viscosity on concentation is empiically inputted at diffeent concentation anges. The effect of the scape blades has been eithe neglected o intoduced as unifom constant souces, especially in the modelling of cicula sedimentation tan. Howeve, due to the significance of the scape system fo a cicula sedimentation tan, an additional sub-model was incopoated to bette model the effects of solids tanspot. The conveying foce exeted on the fluid was appoximated as a function of fluid velocity including a flow egime dependent dag coefficient: F D 1 2 = D AV (8) 2 Diffeent flow ates wee used in each continuous expeiment whee seveal samples wee collected fom the influent and effluent of the tan. The samples wee analyzed to detemine suspended solids, total solids, biochemical oxygen demand and chemical oxygen demand. In addition, some samples wee taen fom the settled sludge to detemine solids concentation. 89

9 3.1 Bounday onditions All the boundaies coesponding to concete sufaces wee modelled using the wall functions povided by FLENT, with a suface oughness paamete set to 0.5 mm. The fee liquid suface was epesented as a igid fictionless suface. The flow bounday conditions wee set by specifying mass withdawal ates. Thus the oveflow ates wee specified at computational cells, and the undeflow ate was distibuted ove a ow of cells coesponding to the sludge withdawal aea. The feed inlet to the claifie was allowed to satisfy the mateial balance by specifying a fixed pessue at the cells coesponding to the location of the feed slots. 4. Existing laifie Pefomance As shown in Figue 3, the existing seconday claifies at Al-Dewanyia Wastewate Teatment Plant, often expeience vey high effluent TSS due to the impact of a massive sludge inventoy. In the oveloaded claifies, the effluent TSS, and BOD is extemely sensitive to any mino vaiations in plant flow. This is because the top of sludge blanet is close to the suface and can easily be caied ove the effluent weis. The oveloaded conditions can often cause a lage unexpected loss of bio-solids fom the seconday teatment pocess. The flow capacity fo the two existing claifies studied anges fom 500 to 750 m 3 /h due to vaiations of the pocess paametes (MLSS). The claifies ae unable to achieve thei expected design flow of 750 m 3 /h due pimaily to the thicening limitation of claifies. The pefomance and capacity of a cente feed claifie is vey sensitive to the stength of the influent jets into the claifies. A taditional cente feed claifie natually geneates a stong influent jet due to its small cente feed aea. Thus, it often bings significant tubulence into the settling compatment, especially unde high flow conditions. Figue 3. Oveloaded claifie opeations 90

10 [ Jounal of Engineeing and Development, Vol. 15, No. 1, Mach (2011) ISSN The expeiments consisted of five uns with diffeent influent flow ates to simulate actual opeating conditions of the seconday claifie in the plant. Each continuous un lasted fo a minimum of 5 hous. The influent to the claifie was the mixed liquo fom the second compatment of a high ate aeation tan in Al-Dewanyia sewage teatment plant. The opeating conditions duing the testing peiod ae pesented in Table 1. Fom the above table it is clea that thee wee no much fluctuations in the influent chaacteistics, i.e. Mixed Liquo Suspended Solids (MLSS), which could affect the pefomance of the tan duing the testing peiod. Simila to SS emoval efficiency, the BOD and OD emoval efficiencies wee moe o less constant duing the opeating peiod at each flow ate. This emphasizes that the tan pefomance was stable duing the peiod of study. Also the elationship between HRT and the emoval efficiency of both SS and TS ae shown in Table 2 and Figue 4. It is clea fom Figue 4 that while the pecentage of SS emoved is inceased as HRT was inceased, the %TS did not show a simila tend since %TS was almost constant, if not slightly deceasing, as HRT was inceased. This may indicate that biological activities too place in the sedimentation tan especially at longe HRT s thus tansfoming the biological SS into dissolved solids. Such tansfomations would ultimately incease the TS concentation at longe HRT, i.e. deceases the %TS emoval efficiency. This emphasizes the impotance of evaluating sedimentation tan pefomance based on SS (athe than TS) as usually epoted in the liteatue. The effect in the case of the elationship between SLR and emoval efficiency of SS and TS is opposite to that obseved fo HRT as shown in Table 2 and Figue 5. Table 1 Opeating conditions duing expeiment of conventional settling tan Q HRT MLSS SVI Tempeatue, ( ) (m 3 /h) (hou) (mg/l) (ml/g) Liquid Ai Table 2 Pefomance of conventional settling tan in SS and TS emoval Q (m 3 /h) HRT (hou) SLR (m 3 /m 2.h) SS emoval (%) TS emoval (%)

11 The good pefomance of the sedimentation tan duing this study is possibly due to the good settleability of the biological solids as indicated by the Sludge Volume Index (SVI) values being in the optimum ange of ( ml/g) as pesented in Table1. Figue 4. Pefomance Of onventional Settle At Diffeent Hydaulic Residence Times. Figue 5. Pefomance Of onventional Settle At Diffeent Suface Loading Rates. 92

12 5. Result And Dissociation 5.1 Pefomance of laifies with an Optimized Influent Stuctue Figue 6 pesents compaison of the omputational Fluid Dynamic (FD) modelling esults fo flow and solids fields between the cente-feed claifie descibed in Figue 1, in which thee is no enegy dissipating appaatus aound the vetical cente-feed pipe. Figues 6(a) and 6(b) pesent the velocity and solids fields in a selected vetical slice of the tested claifies. In the model pedicted velocity fields, each velocity vecto oiginates at a gid point in the FD model. The length of each vecto is popotional to the magnitude of the velocity detemined by the model fo the coesponding gid point, and is in accodance with the 3.0 cm/s scale indicated in the figues. The figues also pesent the simulated solids fields in an identical vetical section of the model. In this figue the contou lines with inteval of 100 mg/l indicates the Suspended Solids concentation. In a cente-feed claifie, it is not easy to enfoce flow evenly to ente the claifie along the im of an enegy dissipating column unless enough esistance along the adial diection can be ceated within the device. Howeve, the high esistance along the adial diection can not be geneated though simply educing the size o numbe of the inlet pots, which would incease the flow intensity enteing into the claifie. The EDI is able to simultaneously satisfy both of the enegy dissipating pinciples, i.e. a lage accumulative space of inlet pots and a unifom flow distibution among all of the inlet pots due to the multilaye flow impingement. Figue 6 consists of the two pats of 6(a) and 6(b) with espect to the two tested claifies with and with no the EDI, espectively. Figue 6(a), the FD modelling esults fo the claifie equipped with a simple cente influent pipe indicating shows: 1. The stong influent jet though the inlet pots (2) penetates the entie adius of the flocculation well (3) and impinges on the inne side of the well (3) due to the lac of effective momentum/enegy dissipating facilities within the flocculation well. Afte impinging on the flocculation well, the influent flow deflects and foms a vey stong downwad cuent towad the sludge blanet and claifie floo (5). 2. Significant evese flow is pedicted undeneath the stong suface influent jet due to the sheas between them. 3. A pinched claifie influent flow unde the baffle lip (3) can be obseved due to the massive sludge inventoy in the claifie. The density fowad cuent is much close to the wate suface than that pedicted unde a lowe flow condition due to the buoyancy impact of the thic sludge blanet. 93

13 a) Existing laifie b) Modified laifie Figue 6. Pefomance Befoe (A) And Afte (B) ental Inlet Retofit As shown in Figue 6(b), the modelling esults fo the claifie equipped with a EDI (8) indicate: 1. The stong influent jet due to the small influent pots (2) continuously impinges with the multilaye pefoated columns (8) one afte one. The velocities of the influent jets have been substantially educed befoe and afte going though the pots (9) in the last pefoated laye (8). The esistance ceated by the multiple pefoated columns (8) foces the influent jet to be sufficiently distibuted along the vetical and tangential diections befoe it entes into the flocculation well (3). 2. The downwad cuent due to the deflection of the influent jet on the flocculation well (3) has been significantly educed, since the momentum of the influent jet is effectively dissipated by applying the EDI. The cicula bottom (10) foces all of the influent flow going though the staggeed pots (9) and pevents flow shot cicuiting between the inlet pots (2) and flocculation well (3). 3. The pinched flow undeneath the lip of the baffle (flocculation well) (3) has been eliminated and the level of density fowad cuent is much close to the claifie floo (5) due to the loweed tubulence and the well contolled dispesed sludge blanet in the claifie. 4. The significant evese flow undeneath the suface influent jet pedicted in the existing claifies has been almost eliminated, since the significantly slowed influent jet geneates a much weae shea influence on the ambient flow. 94

14 The existing claifies have flow capacities of appoximately 1000 (m 3 /h) unde the nomal pocess condition, which is most of the yea. The optimized claifies can achieve a flow capacity of aound 1300 (m 3 /h), which is 30% highe than that of the existing claifies. The pefomance of the EDI (Baffle) was examined by applying nine diffeent influent flow ates anging fom 150 m 3 /h to 350 m 3 /h in sepaate mathematical model uns. The duation of each continuous un was at least, 5 hous duing which diffeent samples wee collected fom the influent and effluent of the tan. The main paametes (i.e. SS, TS, BOD,...etc.) wee detemined and the emoval efficiencies wee calculated at diffeent influent flow ates. The pefomance was stable duing each opeating peiod studied. The values of HRT in the tan wee calculated fo each mathematical model un as illustated in Table 3 and the coesponding SLR values ae pesented in Table 4. Table 3 Opeating conditions duing expeiment of EDI (Baffle) Q (m 3 /h) HRT (hou) MLSS (mg/l) SVI (ml/g) Tempeatue, ( ) Liquid Ai Table 4 Pefomance of EDI (Baffle) in SS and TS emoval Q (m 3 /h) HRT (hou) SLR (m 3 /m 2.h) SS emoval (%) TS emoval (%)

15 The elationships between HRT and the emoval efficiencies of both SS and TS wee established as pesented in Figue 7, fom which it is clea that the emoval efficiency inceases as HRT inceases. Figue 8 shows the elationship between SLR and emoval efficiencies fo both SS and TS. It is evident that emoval efficiency deceases as SLR inceases. Such tends ae simila to those obseved in the conventional sedimentation tan egading pecentage emoval of SS and TS in elation to HRT and SLR. In these mathematical model uns on the upgaded sedimentation tan, simila obsevations to those made duing the expeiments on the conventional sedimentation tan wee evident egading tends in TS, BOD, and OD emoval. Figue 7. Pefomance Of EDI (Baffle) At Diffeent Hydaulic Residence Times. Figue 8. Pefomance Of EDI (Baffle) At Diffeent Suface Loading Rates. 96

16 5.2 ompaison Between onventional And EDI (Baffle) Sedimentation Tans In ode to pefom such a compaison, the emoval efficiency fo SS has been detemined fo both types of settles at five diffeent influent flow ates anging fom 150 m 3 /h to 350 m 3 /h. ompaing the esults obtained fom opeating the mathematical model of tan as a conventional sedimentation basin and as a high ate settle (EDI), i.e. without EDI (Baffle) and with EDI (Baffle), it is appaent that duing opeation with EDI (Baffle) the SS emoval efficiency is bette than in the conventional tan by 2% - 3% which is a maginal incease in efficiency. Howeve, the tan with EDI (Baffle) was capable of maintaining high emoval efficiencies even when the biological solids had high SVI as shown in Table 1 and 2, nowing that high SVI values (>200 ml/g) ae indicative of poo sludge settleability. The meit with EDI (Baffle) is moe appaent when settling athe than thicening is contolling the tan design. This may indicate that application of EDI (Baffle) in seconday claification of biological sludge may not be as advantageous as thei application in pimay claification of wastewate solids. Howeve, when seconday claifies ae oveloaded o suffe fom ising sludge poblems, upgading of such claifies using EDI (Baffle) is definitely advantageous. This is in addition to savings in costs of land aea coveed by settles which is much less in case of EDI (Baffle) than in case of conventional type gavitational settling tans. Based on the esults obtained fo EDI (Baffle), a statistical model could be fomulated by applying linea egession analysis fo the elationship between SLR and %SS, BOD and OD emoval. Figues 9, 10, and 11 illustate the elationship obtained which could be expessed by the following equation 9, 10, and 11: % SS emoval = SLR... (9) % BOD emoval = SLR... (10) % OD emoval = SLR... (11) Figue 9. Effect Of Suface Loading Rate On SS Removal Of EDI (Baffle). 97

17 Figue 10. Effect Of Suface Loading Rate On BOD Removal Of EDI (Baffle). Figue 11. Effect Of Suface Loading Rate On OD Removal Of EDI (Baffle). 98

18 6. onclusions The following conclusions can be dawn: 1. The EDI (Baffle) has poved effective in impoving the pefomance of seconday sedimentation of biological solids at the studied suface loading ates in the ange of 0.2 to 1.6 m 3 /m 2.h. 2. Removal efficiencies of suspended solids, biochemical oxygen demand, and chemical oxygen demand wee slightly highe in the EDI (Baffle). 3. In compaison with conventional settle, the EDI (Baffle) is less affected by oveloading. If the design suface loading ate citeia fo conventional settling tans is used fo designing high-ate settles, the latte should pefom bette within the ange of suface loading ates nomally used in pactical design. 4. Suspended solids emoval efficiency is a bette paamete to descibe the pefomance of sedimentation tans compaed to total solids. Meanwhile, biological tansfomations of solids in the seconday sedimentation tan could contibute to BOD and OD which esults in highe BOD/SS and OD/SS atios in the effluent than in the influent. This emphasizes the uniqueness of SS as a bette paamete in pefomance evaluation. 5. The main advantage of EDI (Baffle) in seconday sedimentation of biological solids lies in thei capability of coping with plant oveloading conditions. Such settles could be easily installed in existing sedimentation tan as a solution to ising sludge poblems at minimal cost compaed to othe solutions such as inceasing tan depth, addition of chemical coagulants,...etc. Installation o emoval of EDI (Baffle) would not intefee with nomal opeation of existing sedimentation tans. 99

19 7. Refeences 1. Kebs, P.. The hydaulics of final settling tans, Wat. Sci. Tech., 23 Kyoto, (1991) 2. Dahl,. Lasen, T. & Petesen, O. Numeical modelling and measuement in a test seconday settling tan, Wat. Sci. Tech. 30(2), (1994) 3. Kebs, P. Vische, D. Guje, W. Inlet-stuctue design fo final claifies, Jounal of envionmental engineeing, 121(8), (1995) 4. Boucaet.J. Bucley.A. The use of computational fluid dynamics fo impoving the design and opeation of wate and wastewate teatment plants. Wat. Sci. Tech., 40(4-5), pp (1999) 5. Laehal D., Kebs P., Kijgsman J. Rodi W. omputing shea flow and sludge blanet in seconday claifies. J. Hyd. Engg., 125(3), pp (1999) 6. Jayanti S. Naayanan S. omputational study of paticle-eddy inteaction in sedimentation tans, J. Envionmental Eng., 130 (1), (2004) 7. Ghawi A. Hadi Jozef Kis. Design and Optimization of Sedimentation Tan in Slovaia with FD Modeling. 10th Intenational Symposiumon Wate Management and Hydaulic Engineeing 2007 with special emphasis on the impact of hydaulic engineeing constuction on the envionment 4 9 Septembe Šibeni, oatia. (2007a) 8. Ghawi A. Hadi, Jozef Kis. A Numeical Model of Flow in Sedimentation Tans in Slovaia Thid Intenational PhD Symposium in Engineeing Octobe 2007, Hungay nivesity of Pollac Mihály Faculty of Engineeing Pécs, Hungay. (2007b) 9. Ghawi A. Hadi, Jozef Kis. Impoved, Modeling, Simulation and Opeational Paametes of Settling Tan. 6th Intenational onfeence of PhD Students nivesity of Misolc, Hungay August pp (2007c) 10. APHA. Standad Methods fo the Examination of Wate and Wastewate, 17th edition, Ameican Public Health Association, Washington, D.. (1989) 11. Wilcox D.., Tubulence Modelling fo FD, DW Industies, Inc., La anada, alifonia, (1998). 100