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Numerical‐graphical model forcontinuous horizontal flow settlingtank designNegib Harfouche a & Dilara Sencan aa Bogazici University, Institute of EnvironmentalSciences, Department Water Pollution Technology , P.K.2–80815‐Bebek, IstanbulPublished online: 23 Feb 2007.

To cite this article: Negib Harfouche & Dilara Sencan (1990) Numerical‐graphical modelfor continuous horizontal flow settling tank design, International Journal of EnvironmentalStudies, 36:4, 303-314, DOI: 10.1080/00207239008710608

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Intern. J. Environmental Studies, 1990, Vol. 36, pp. 303-314 © 1990 Gordon and Breach Science Publishers Inc.Reprints available directly from the Publisher Printed in the United KingdomPhotocopying permitted by license only

NUMERICAL-GRAPHICAL MODEL FORCONTINUOUS HORIZONTAL FLOW SETTLING

TANK DESIGN

NEGIB HARFOUCHE and DILARA SENCANBogazici University, Institute of Environmental Sciences,

Department Water Pollution Technology, P.K. 2-80815-Bebek (Istanbul)

(Received in final form: March 16, 1990)

In terms of prototype research-studies, a simple straightforward numerical-graphical model (FNGM) forcontinuous horizontal flow settling tank design (circular or rectangular) is suggested.

The experimental results, obtained from Imhoff cones, jar test and settling column test, are interpretedby computer programmation and developed graphical methods, yielding settler final design data andperformance. For comparison, recommended design criteria4 are also shown.

Thereby, with the proposed algorithm, the settling tank design, process performance and efficiency arefacilitated and rendered faster, as well as more accurate.

The model (FNGM) determines the settling behavior of a laboratory coagulated-flocculated oily-detergent wastewater, of class 2,t and composition 5 mg/kl detergent (20% LAS) and 100 mg/1 oil,considered by SENCAN2 and permits estimation of settling area and other important design factors, for85% LAS removal, at 0.95 1/s, on a 24 h-day basis (medium scale).

From the design obtained values, the actual system operating characteristics performed on a pilot-rectangular unit, revealed 30% detergent and 60% oil removal, for 200 mg/1 lime dosage, only.KEY WORDS: Chemical coagulation-flocculation-sedimentation, continuous horizontal flow settlerdesign and operating-data, detergent-oil removal.

INTRODUCTION

Gravity separation of suspended material from aqueous solution is the oldest, mostwidely and least expensive used process in water treatment and wastewater recla-mation: suspensions in which particulate matter is heavier than water tend to settle tothe bottom as a result of gravity forces, in the process of sedimentation.

Prediction of performance of a typical sedimentation tank design, for a givenquality of raw water or wastewater effluent, can to some extent be understood withthe help of sedimentation theories; on the other hand, if sedimentation equipment isdesigned without experimental studies, on the particular suspension, unsatisfactoryperformance will often result.

Sedimentation has several uses in both water and wastewater treatment: in watertreatment: it removes impurities that have been rendered settleable by coagulationand flocculation, as when removing turbidity and colour, e.g. water softening.

In Wastewater Treatment

a) Domestic: it is the main process in primary treatment; 50 to 70% of thesuspended solids (containing 25-40% of the BOD) are removed. It is also used after

† Class 2: relatively low solids concentrations of flocculant material. An example of this type of materialis found in water-wastewaters subjected to chemical coagulation-flocculation.

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304 N. HARFOUCHE AND D. SENCAN

biological treatment (secondary) i.e. activated sludge or trickling filtration, toremove the biological floe in these processes.

b) Industrial: beyond its importance as a primary and secondary treatment,whether associated to biological treatment or not, sedimentation is particularly usedefficiently in the following industries:

Food-products wastes industries: meat, fish, poultry and eggs, canned foods.Manufactured product wastes industries: inorganic chemicals, metal finishing,pulp and paper, leather.Mineral product wastes industries: coke and gas (limited), iron and steel non-ferrous metals, petroleum (as oil-separator).Mining wastes: coal, metal and non-metal (as settling pond).

Sedimentation is also used for solids concentration in sludge thickeners.In most cases, the primary purpose of sedimentation is to produce a clarified

effluent; it is generally integrated with coagulation-flocculation and identified asprimary treatment system.

The primary objective of this study is the development and evaluation of anappropriate technique, permitting the optimum design of continuous horizontalrectangular channel size, prior to the actual system performance-test on awastewater of 5 mg/1 detergent and 100 mg/1 oil.

FLOCCULANT SETTLING

Flocculant particles are those with a tendency to coalesce during the sedimentationprocess.3

The application of mathematical equations or analytical studies onto sedimen-tation has not been found valuable in practical process unit design or performanceprediction.3

A number of investigators have tried to improve on the design by conductingexperiments with suspensions in settling columns to compensate for the non idealconditions. Various authors3'4 recommend applying a safety factor equal to 0.65—0.85 for the overflowrate and 1.75-2.0 for the detention time.

The settling column technique has been taken a step further, by combining settlingcolumn test results with clarifier hydraulic dye test curves to predict settler perform-ance.4

MATERIALS AND METHODS

First, Imhoff cones and jar test studies are realised in order to determine best choiceof coagulant-flocculant and floes formation for the specific type of wastewater atgiven experimental conditions of pH and temperature, according to StandardMethods, 1985.5

Second, settling column test is performed on best flocculated wastewater sample(-30 1).

Third, total solids (suspended and dissolved) are determined according to Stan-dard Methods, 1985/

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SETTLING TANK DESIGN 305

EXPERIMENTAL RESULTS AND DISCUSSION

An oily-detergent wastewater, containing 5 mg/1 detergent (20% LAS) and 100 mg/1oil, is tested.

Imhoff Cones

Different dosages of aluminum sulfate, ferric chloride, lime, lime + aluminumsulfate, aluminum sulfate + starch and lime + starch are used.

The most effective floe formation is obtained with: 500 mg/1 alum + 2500 mg/1lime, as 40 ml flocs/1 wastewater, forms.

Jar TestDifferent alum + lime dosages are used according to various pH values.

The LAS remaining in solution is computed by the Methylene-blue method. Thebest coagulant dosage and pH are found respectively as: 500 mg/1 alum + 2500 mg/1lime and 11.5 (±0.3). LAS removal is -80%.

Turbidity Measurement2

An on-line turbidimeter, model: HACH CR 2426 (surface scatter 4)), is used.The turbidity measurements are realized twice: first, after coagulation and floccu-

lation, second, after the settling column-test:

after coagulation-flocculationafter sedimentationreference (tap water)

800 FTU43 FTU10 FTU

Settling Column-Test

After coagulation-flocculation, the initial suspended solids concentration is:2000 mg/1.

The settling column-test average results are shown on Table I.The iso-percentages removals (least-squares best fit) are shown in Figure 3.

Table I Observed settling rates

(min)

5102030405090120

Settling depths

0.6 m

4457617481879195

1.2 m

3951596879858894

1.8 m

3136485166808692

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306 N. HARFOUCHE AND D. SENCAN

SETTLING COLUMN-TEST TECHNIQUE

Theory

The wastewater sample containing the flocculated suspended matter is introduced ina cylindrical column of sufficiently large diameter to prevent excessive wall effectsand height equal to or greater than that anticipated for the prototype/actual settlingtank design. Uniform distribution of particle sizes occurs from top to bottom ofcolumn.

At predetermined time intervals, samples are removed from three verticalsampling ports located at definite equal heights and analyzed for total solids removalpercentages which are then plotted on a time-depth graph.

Isopercentage lines are then drawn. The overall actual percentage removal iscomputed at each isopercentage-removal line and settling rate, respectively, corre-sponding to the total depth of a column (Z3), according to the following expressionand as illustrated in Figure 1 (at t2):

%R = R2 + (AZJZ3) * (R3 ~ Ri)l2 + (&Z2/Z3) * (R4 - R3)/2+ (AZ3/Z3) * + (R5 -R4)/2

R2 = initial percentageAZX = Z3 + Za, AZ2 = Za + Zb, AZ3 = Zb + Zc

Ri+1 — R\ = percentage increment (i.e. 5%) / = 1, 2, 3, 4 . . .

The accuracy of estimation can be improved by decreasing the interval betweenisopercentage removal lines, then adding more terms to the above expression.

The design detention times correspond to the settling rates, ti, i = 1, 2, 3, . . . atdepth Z3.

The design overflowrates, QIA {nrVm2 • d}, are calculated as: Z3 * 60 * 2Alt\ atthe respective design detention times.

On the other hand, given a certain percentage removal, the design detention timesand overflowrates are computed grapically from best-fit plots of detention times andoverflowrates vs. actual percentages removals.

To compensate for non-ideal conditions (i.e. short circuiting, inlet-outlet tur-bulence, density and temperature induced currents) the design settling velocity oroverflowrate and detention times, obtained from settling column test, are decreasedby an arbitrary factor of 1.50 and increased by an arbitrary factor of 1.75,respectively.4

. t. t, t ,Figure 1 Isopercentage removal lines as depth vs. settling rate.2

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SETTLING TANK DESIGN 307

Experiment Set-out

A 14 cm inside diameter, 2.5 m total height plexiglas cylindrical column is set up inthe Environmental Technology Laboratory at the Institute of EnvironmentalSciences in Bogazici University, Figure 2.

Figure 2 Experimental settling column.

Four sampling ports are located at 61 cm apart; the bottom port is mainly used fordischarge of accumulated sludge and drainage.

The uniform flocculated wastewater sample (~30 1) is introduced into the columnthrough the bottom port by a pump; this would also provide homogenization of thewhole mixture, just before taking measurements. Floe settlement is easily observed.

From each sampling port [1st, 2nd, 3rd from top] 100 ml samples are taken,simultaneously, at regular time intervals, for a total period of time until percentagesremoval of total solids data remain quasi-constant.

l.S

1.6

1.4

I 1.2

I i;S B . 8

0,4

0.2

' 55:

experimental points(7orerrcval)

23 48 60 89 ISOSettling rates(Hin)

Figure 3 Isopercentages removals lines.2

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308 N. HARFOUCHE AND D. SENCAN

Table II Design detention times and overflowrates

Detention times{min}

9.013.3317.5026.6732.6736.0039.3342.8646.4350.0083.33

110.00

Overflowrates{m3/m2 • d}

288.00194.40148.1197.2079.3572.0065.9060.4855.8351.8431.1023.56

Percentage removals{%}

52.5357.7662.0969.7775.2577.6380.4082.9485.8686.9489.4492.50

The average design detention times and overflowrates corresponding to actualpercentages removals are indicated in Table II.

The least-squares best-fit design detention times and overflowrates are repre-sented graphically in Figures 4 and 5 respectively.

The actual detention time and overflowrate are obtained from Figures 4 and 6 atthe given percentage removal 85%.

The final settling design data, at 85%, and an estimated flowrate of 82.08 m3/d,obtained by the FNGM, are listed in Table III.

The relationships between the various design parameters outlined above could besummarized by Figure 6. * Alternative designs may be quickly compared using thisdiagram and effects of flow variations on critical loading parameters be determined.

The design chart, however is most suitable for large flowrates, thus large tanks.

80

60

20

/

D^sigr. detention ti.r.es

Polynomial least--sq.ia-.res f i t

20 40 60 SODetention tine(Hin)

100

Figure 4 Design detention times.

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SETTLING TANK DESIGN 309

SO

20

* Design cverfIcwrates

~~ polynomial leastsquares f it

50 100 ISO ZOOOverflowrate(M3/M2.D)

Figure 5 Design overflowrates.

Z50

Forward velocity,Vh (mm/i),

in rectangularhorizontal flow tanks •

Det

enti

on

tim

e, I

(houu)

Sealing velocity, Vp (mm/i)11 a i • t • • f,1

i - • - i ' '

Average depth, H(m)

Length of tank, L(m)(or diameter of a circular tank)

^f/ ///

/ / / //>/ / / jf// /'?/ / /

I./ .•// ^ V

/// ii

f " / I

w(or o'CuUr -jnk:

Figure 6 Design chart for sedimentation tanks.1

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310 N. HARFOUCHE AND D. SENCAN

Table III Final settling design data

FlowrateActual detention timeActual overflowrateSurface area of settlerSettler volumeSettler depthSettler lengthSettler widthHorizontal velocityReynolds numberFroude numberNumber of weirsWeir lengthsHead above weir stillWeir loading rate

{m3/d}{min}{m3/m2 • d}{m2}{m3}{m}{m}{m}{m/s}

{m}{m}{m3/m • d}

= 82.08= 94.3424= 31.2533= 2.627= 5.378= 2.048= 3.241= 0.810= 5.726 E-04= 196.60= 1.7275 E-04= 3= 0.15= 0.0111= 182.4

Mixer

r. Adjustablelinlet weir . .

Adjustablecutlet weir

I I, Parallel SETTLING OIL SEPARATION /Inlet plate 0 W plates AREA SECTION *-7 ,*'•'""

INFLUENT—J

'Exit plate

/ Sludge drai OiL-.cirair.

COAGULATICN-FLOCC'JLATICNSECTION

Figure 7 Drawing of the pilot rectangular settler.

Actual System Operating Characteristics

The designed settling unit performance is achieved through a pilot-horizontal flowchannel (Figure 7) of dimensions: L = 2 m and W = 0.4 m, installed in theLaboratory of Environment Technology, at the Institute of Environmental Sciences.

FlowrateDetention timeOverflowrateSurface areaCross sectional areaSettler volumeTank heightWater depthSettler length

{1/min}{min}{m3/m2 • d}{m2}{m2} ,{m3}{m}{m}{m}

= 8.5= 50= 15.3= 0.8= 0.4= 0.8= 1.0 m= 0.5= 2.0

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SETTLING TANK DESIGN 311

Settler widthHorizontal velocityReynolds numberFroude numberNumber of weirs (rect.)Weir length {m}Weir loading rate

Lime dosing rate

The effluent detergent and oil percentage removal are 30% and 60% respectively.

Recommended Design Criteria for Horizontal Flow Sedimentation Tanks

For practical situations and class-2 sedimentation type, the following typical hy-draulic ranges are recommended:4

{m}{m/s}

{m3/m2 • d}{mg/1}

= 0.40= 7.08 E-04= 102.72- 5.06 E-04= 2= 0.12= 51= 200

Surface loading rateMean horizontal velocityWater depthLength/width ratioDetention timeWeir loading rate

30-60 {m3/m2 • d}0.15-0.9 {m/min}3-5 {m}3-5120-240 {min}140-270 {m3/m • d}

Re <2000Fr >10~5

The more stable the circulation, i.e. high Froude numbers, the more uniform thespeed distribution over the full section of the tank, and the better the hydraulicperformance is.

CONCLUSION

An understanding of the principles governing the various forms of sedimentationbehavior is essential to the effective design and operation of sedimentation tanks.

The experimental-graphical-numerical model, FNGM, outlined, is applicable toboth water and relatively low solids-concentrated wastewaters of class 2, in themedium-larget scale flowrates, at high removal efficiencies.

A combination of depth and settling rate, thus, of overflowrate and detentiontime, could be chosen for any required suspended-solids removal efficiency, yieldingoptimum settler final design data.

From the available results, we can conclude that optimum detergent removal, i.e.~85%, which is function of detergent type, occurs at 2500 mg/1 lime and 500 mg/1alum dosage rates. These amounts could be decreased by addition of specificpolyelectrolyte. This would be encountered in a later study.

Tests using the recommended procedures should be tried on prototype size settler,for other wastewaters and mainly in small$ scale flowrates, accounting for scalefactors and low efficiencies, and the results reported.

† Medium scale: 0.63-3.15 1/s; large scale: 0.38-3.8 x 106 1/d.‡ Small scale: 0.063-0.63 1/s.

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312 N. HARFOUCHE AND D. SENCAN

However, there are many problems with this approach, mainly in the case ofwastewaters: it is difficult to obtain representative samples at any single time, whilefor a proposed new scheme, it is usually impossible to obtain samples at all.

Apart from the difficulties associated with sampling, such data must also bemodified to account for differences between batch-settling column tests and practicalcontinuous-flow sedimentation tanks: model testing is limited by scale-up, whichcannot be adequately expressed by principles of similitude, primarily because solidsparticles are not easily scaled down.

These considerations apply to all three types of tanks commonly used for class-2sedimentation, namely rectangular horizontal flow tanks, circular radial flow tanksand square upflow tanks.

Acknowledgements

The authors wish to thank the Bogazici University Research Fund for their financial support of thisresearch, as well as the Institute of Environmental Sciences staff for their cooperation during the course ofthis study.

Nomenclature

ZRt

q)TJ)q)

dthevtheqMDtQIAAVHLWVhWLReFrFTUBLHD

settling column depthremoval percentagesettling rateexperimental time intervalsexperimental percentages removalsactual percentages removalssettling rates corresponding to 1.8 m column depth andactual percentages removalsdesign detention time at actual percentage removaldesign overfiowrate at actual percentage removalflowratekinematic viscosityactual detention timeactual overfiowratesurface area of settlervolume of settlerwater depthlength of settlerwidth of settlerhorizontal velocity in settlerweir loading rateReynolds numberFroude numberFormaztn Turbidity Unitrectangular weir lengthhead above weir sill

{m}

{min}{min}

{min}{min}{m3/m2

{m3/d}{m2/s}{min}{m3/m2

{m2}{m3}{m}{m}{m}{m/s}{m3/m •

{m}{m}

d}

d>

References

1. D. Barnes, P. J. Bliss, B. W. Gould and H. R. Vallentine, "Water and wastewater engineeringsystems." Longman Scientific and Technical (1986).

2. D. Sencan and N. Harfouche, "Coagulation-flocculation-sedimentation prototype studies on oily-detergent wastewaters." M.S. thesis, Bogazici University, Institute of Environmental Sciences(1988).

3. M. J. Hammer, "Water and wastewater technology." John Wiley & Sons, Inc., New York (1986).4. J. M. Montgomery, Consulting Engineers, Inc., "Water treatment: principles and design." John

Wiley & Sons Inc., New York (1985).5. Standard Methods for the Examination of Water and Wastewater APHA, AWWA WPCF, 16th

edition, Washington (1985).

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SETTLING TANK DESIGN 313

APPENDIX

Computer-aided Design

The settling column test data-processing has been computerized. Isopercentagesremoval lines, actual percentages removals, overflowrates, detention times andrelated design parameters on settler performance, are obtained numerically.

It is achieved by two separate procedures: graphical analysis and numericalmethod.

Method

The experimental results obtained from settling column tests are stored intocomputer's memory (SETCODIS). Isopercentages removals lines are plotted:column depths i.e. 0.6 m, 1.2 mand 1.8 mvs. settling rates; then actual percentagesremovals (at column depth 1.8 m) are obtained by graphical analysis (GRAPHICS).

Design overflowrates and detention times are computed numerically (PER-OMACT), then are plotted (least-squares best-fit) vs. actual percentages removals(GRAPHICS). Thus, given an actual percentage removal, the corresponding designoverflowrate and detection time, are determined.

Finally, the actual overflowrate and detention time, and the settling tank dimen-sions, are evaluated, and the settler hydrodynamic performance is estimated throughReynolds and Froude numbers (SETDESIG).

Programs-datafiles

The software is in BASIC language adapted to ATARI model 520 ST.SETCODIS: allows experimental settling column test data to be stored on datafile

"DATAI."GRAPHICS: 1—allows graphical computation of isopercentages removal lines

and subsequent actual percentages removals at corresponding settling rates.2—allows best-fit plots of design overflowrates and detention times vs. actual

percentages removals.PEROMACT: allows determination of design overflowrates and detention times

corresponding to actual percentages removals that are stored on datafile"PERCOVRI."

SETDESIG: allows determination of final settling tank design data that are storedon datafile "DETNOVRI."

The whole procedure is represented by the computational flow diagram below:

Imhoff cones test(best floe formation)

Experimental •{ ,, . , ! ! f A D t r ,r (best coagulant dosage and PH)

iSettling column test(settling tank design)

4

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314 N. HARFOUCHE AND D. SENCAN

Start

1SETCODIS > DATAI

read:T( ;): time intervalsH( j , I): experimental percentages removals

IGRAPHICS

evaluates graphically actual percentagesremovals, P(r, q) for all isopercentagesremovals lines.

4PEROMACT * PERCOVRI

read:P[r, q)Tj(q): settling rates corresponding to 1.8 m

column depth and actual percentagesremovals

evaluate:dthe: design detention timesvthe: design overflowrates

at actual, percentages removals.

ISETDESIG > DETNOVRI

read:Q: flowrate {m3/d}fi: kinematic viscosity {m2/s}dthe and vthe at given actual percentage removal

evaluate:Dt: actual detention time {Min.}QIA: actual overflowrate {m3/m /d}A: surface area of settler {m2}V: volume of settler {m3}H: water depth {m}L: length of settler {m}W: width of settler {m}Vh: horizontal velocity {m/s}WL: weir loading rate {m3/m • d}Re: Reynolds numberFr: Froude number

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