Physical Treatment

8/9/2019 Physical Treatment

1/21

UNIT 8 SEDEMENTATION,FLOCCULATION AND FILTRATIONStructure

8.1 IntroductionObjectives

8.2 Sedimentation8.2.1 Principle of Sedimentation8.2.2 FundamentalTheory8.2.3 Sedimentation in Practice8.2.4 Common Design Criteria

8.3 Flocculation8.4 Flotation8.5 Flow Through Porous Media

8.5.1 Porous Bed Hydraulics8.5.2 Cleaning of Deep Bed Filters8.5 .3 Deep Bed Filtration in Practice

8.6 Chemical Aided Sedimentation8.7 Important Lessons for Physical Treatment Processes8.7.1 Flow Distribution

8.7.2 Subsidence8.7.3 Flow Pattern

8.8 Summary8.9 Key Words8.10 Answers to SAQs8.11 Further Reading

8.1 INTRODUCTIONScreen and grit chambers remove most of the floating materials and heavy inorganicsettleable solids from the sewage. A part of the suspended organic solids which are tooheavy t~re removed as floating matters and too light to be removed by grit chambers aregenerally removed by the sedimentation tanks. Hence, sedimentation tanks are designedto remove a part of the organic matter from the sewage effluent coming out from the gritchambers.In a complete treatment of sewage, the sedimentation is carried out twice. Once beforethe biological treatment (known as primary sedimentation) and next after the biologicaltreatment (known as secondary sedimentation). When chemicals are used for flocculatingthe organic matter during the process of sedimentation, the process is known assedimentation aided with coagulation.ObjectivesAfter going through this unit, you should be able to

calculate settling velocity for a discrete particle,estimate removal efficiency of a settling tank,appreciate the characteristics of different settling basins,calculate the power required for flocculation, andunderstand the differences between different types of filter.


2/21

Physical Treatment 8.2 SEDIMENTATION8.2.1 Principle of SedimentationSedimentation is a natural process by which solids with higher density than the fluid,settle under the action of gravity. The settling velocity of a particle in a fluid is a functionof its density, size and shape as well as the density and viscosity of the fluid. The organicmatter present in sewage has specific gravity greater than that of water. In still sewagethese particles tend to settle down by gravity. Ina flowing sewage they are kept insuspension, because of the turbulence in water. As soon as the turbulence is retarded bymaking storage of sewage, these impurities tend to settle down at the bottom of the tankoffering such storage. The basin in which the flow of sewage is retarded is known assettling or sedimentation tank. If the tanks are big, they are also known as sedimeritatConbasin.8.2.2 Fundamental TheoryDiscrete suspensions are made up of particles with a fixed rigid shape, sand grains forexample, which do not coalesce when brought into contact. Such a suspension thus has aconstant settling velocity under specified conditions. Flocculent suspensions arecomposed of particles with spongy adherent characteristics which tend to agglomerate oncontact and produce fewer, but larger, particles with increasing settling velocity withtime. Simple settling theory considers the situation'in which a descrete particle is placedin a fluid of lower density. The particle wiH accelerate under gravity until a terminalvelocity is reached when the gravitational force is balanced by an equal and oppositefrictional drag force. Mathematical analysis of this situation leads to the classical Stoke'sLaw expression for the terminal settling velocity of a discrete particle in water underlaminar flow conditions :

where, v, = descrete particle terminal velocity,g = acceleration due to gravity,d = particle 'diameter',V = kinematic viscosity of water, andS, = specific gravity of particle.

Table 8.1 :Discrete Particle Settling VelocitiesSpecinc Gravity Diameter TemperatureOC Settling Velocity, mm m/ s

2.5 0.1 10 6.25 x20 8.15 x30 1.02 x lo-2

0.01 10 6.25 x20 8.15 x30 1.02

1.5 0.1 10 2.09 x20 2.72 x30 3.41 x

0.01 10 2.09 x20 2.72 x lo-'30 3.41 x lo-'

1 .1 0.1 10 4.17 x20 5.43 1 0 - ~30 6.81 x lop4

0.01 10 4.17 x20 5.43 x 1 0 - ~30 6.81 x


3/21

Table 8.1 gives some exam ples of terminal settling velocities calculated from E q. (8.1)for a number of coditions, including different temperatures w hich affect the viscosity ofthe water and hence the settling velocity.Practical consideration of the settlement of discrete suspensio ns involves the concept ofthe ideal settling basin (Figure 8.1) in which it is assumed that there is :

- quiescent settlement in the settling zone- uniform flow through the settling zone- uniform solids concentration entering the settling zone- solids entering the sludge zone are not resuspended.

Inlet Zone Outlet Zone

In Flow

Sludge ZoneIigure 8.1 :The Ideal Settling Basin

A discrete particle w ith settling velocity v, enters the settling zone at the su rface and justreaches the sludge zone at the outlet end of the basin. Th is particle falls through a depthh, in the retention time of the tank to.Hence v, = h o /tobu t to = volum e I flow per unit time = V / Qthus v, = h, x Q N =where A = surface area of basin.Thu s critical settling velocity vo = Q / A . . . 8.2)The term Q / A s termed as the surface overflow rate and has units of velocity. Th us fordiscrete particles, removal is in dep en den ~o f epth, and in theory, all particles with asettling velocity of v, or greater will be removed if the surface overflow rate is equal tothis critical velocity, v,. If a suspension of discrete particles with a range of settlingvelocities is fed to the tank all those with v, equal to or greater than v o will be removedand for particles with v, < v,, the removal will be in the ratio v, /v,. If a particle withv, < v o enters the tank at a height of not mo re than v, x to from the bottom, i t will beremoved. If it enters at a higher level than this, it will not reach the sludg e zone and hencewill not be removed.Example 8.1Calculate the settling velocity in water of a spherical discrete particle, 0.06mm

diameter and specific gravity 2.5, if the kinematic viscosity is 1.01 x at 20C.Solution

From Eq. (8.1)

Note : q. (8.1) only ac ; rcrs in laminar flow conditions, i.e., w hen R eynoldsNumber is < 1. ia , therefore, important to check that the velocitycalculated falls within the lamin ar flow range.

SedimentationFlocculation andFiltration


4/21

PhysicalTreatment Reynolds Number (R) = v, d2 v

i.e., the flow is laminar and hence, Eq. (8.1) is applicable.In practice, the settling velocities encountered in w ater and wastewater treatmentare almost invariably scch that laminar conditions occur. For calculation of settlingvelocities in the transition or turbulent flow regions, other relationships which arecovered in texts listed in the Further Reading section should be consulted.

Example 8.2Determine the surface area required in an ideal se ttling basin to ensure removal ofall discrete particles with a settling velocity of 0.0029m Is from a flow of500 m 3 I h.

SolutionFrom Eq. (8.2).

Example 8.3Determine the theoretical removal in the tank in the previous example for discreteparticles with a settling velocity of 0.001 m I s.

SolutionRem oval of particles with settling velocity less than the critical velocity is givenby vs v*i.e., % removal = (0.001 x 100) / '0.0029

= 34.5%Insertion of a series of trays or false bottoms in a tank at a spacing of the lowest v s x towould in theory, ensure complete removal of suspended matter. In practice, there arelimitations to this concept because of the difficulties of ensuring uniform flowdistribution and also o f removing the d eposited solids. In locations wh ere land area is at apremium, it may be possible to utilise sedimentation tanks w ith two, or som etimes threefloors, although the tanks are usually somew hat deeper than conventional units.An extension of the tray concept is that of the inclined tube or plate settlers whichprovide large surface areas for settlement within a sm all space. Depending upon thearrangement of the tubes or plates, it is possible to obtain effective surface areas of asmuch a s ten times the plan area occupied by an inclined settler. A inclined tube settler isdepicted in Figure 8.2.

Inlet

Outflow

OutletBox

Bo

Figure 8.2 : nclined%be or PI& S e w


5/21

8.2.3 Sedimentation in Practice Sedimentationlocculation andThe basic sedimentation theory described in the previous section is for low Filtrationconcentrations of discrete particles. As indicated in Figure 8.3 , the settling characteristicsof flocculent suspensions are non-uniform. In addition, with SS concentrations in excessof around 2000 mg / 1, the phenomena of hindered settlement can complicate theprediction of settling basin performance. An indication of the potential for settlement in asuspension can be obtained by determining the settleable solids content in a sample. Thisinvolves the use of a graduated imhoff cone or determination of SS before and after a 30minute period of settlement in a conventional measuring cylinder. More detailedinformation about the settling characteristics of a suspension can be determined using asettling column. The SS content of samples withdrawn from the column at known depthsand time intervals can be used to produce a settling characteristics curve which isanalogous to that derived in a sieve analysis determination for sand or soil samples.

Top Surfice

,FlocculentSuspension

DiscreteSuspension

1 Top Surface

Hindered Settling

Bottom I --- -TmFipre 8.3 : ettling BehaviourExample8.3A settling column test on a suspension of discrete particles gave the followingresults from a sampling depth of 1.3m.

Sampling time (min) 5 10 20 40 60 80% of initial SS in 56 48 37 19 5 2sample

Determine the theoretical removal of suspended solids from this suspension in ahorizontal flow tank with a surface overflow tank with a surface overflow rate of200 m/d.

SolutionIt is first necessary to convert the sampling time and the depth of collection intovelocities which the solids in each sample have not exceeded:thus 1.3m/5min = 4.33 x 1 0 - ~ m / s

Settling velocity 4.33 2.16 1.08 0.54 0.36 0.271 0 - ~ m / s% SS with v, < v 56 48 37 19 5 2


6/21

PhysicalTreatment These data are plotted in Figure 8.4 which sho ws the percentage of SS with asettling velocity less than or equal to a specified settling velocity. Readers familiarwith a sieve analysis curve will at once see the similarity between this settlingcharacteristic curve and the way in which sieve analysis are expressed.Th e surface overflow rate of 200 m I d = 200 / (60 x 6 0 x 24 )

= 2.38 x m 1 sFrom Figure 8.4, 50 % of the SS have a settling velocity of greater than2.38 x lob3m I s and will thus be removed. In a horizontal flow tank, there willbe an ad ditional removal of S S with settling velocities less than 2.38 x m I sin the ratio v,/v,. This additional removal would be given by the integral from 0 to50% of v, / v, with respect to percent of S S. Since the equa tion for the settlingcharacteristic curve is not normally known, the solution can be obtained byarithmetic integration of the appropriate area as shown in Figure 8.4. Thus theoverall removal of SS from the suspension in a horizontal flow tank will be5 0 + 17 per cent. It should be noted that in a vertical flow tank the theoreticalremoval would be 50% only, i.e., removal only of those S S with v , of equivalent orgreater than v,. Particles with lower settling velocities will be washed out of thetank. However, onc e a sludge blanket is formed, this will se rve to trap someparticles with lower settling velocities in a form of filtration. The removalefficiency of the tank will thus increase as the blanket develops. This growth inremoval efficiency is not readily predictable and depends on the nature of thesuspension being treated.

All RemovedBecauseVs > 2.38 x,---Settling Velocity lo-) m s

Figure 8.4 : ettling Characteristic CurveWh en dealing with flocculent suspensions, which include s most of those encountered inwater and w astewater treatment, the basic theory and techniques described above cannotbe applied directly. In the case of flocculent suspen sions, it is difficult to express theirchanging settling characteristics in a mathematical relationship. Because of theagglomeration which takes place with flocculent suspensions a s particles collide with oneanother, the depth through which particles settle does ha ve an influence on the suspensionsettling velocity. Thu s, with floccu lent suspensions, it is necessary to carry out settlingcolum n tests with samp les collected at a range of depths up to the full scale tank depth.The results obtained then plot as a series of curves w hich can b e used to establish theprobable removal performance over a range of depths and retention times.As ou tlined earlier, the hydraulic characteristics of continu ous flow systems are neverideal and flow distribution is a major factor in the design of efficient sedimentation tanks.The problem i s complicated by the fact that many tanks ha ve to operate over quite a large


7/21

range of flows and under a range of climatic conditions with varying tem peratures andwind ac tion. It should therefore not be surprising that sed imentation tanks do not alwalloperform as w ell as might be pred icted by theory. It is also im portant to appreciate thatsolids separated from the flow by a settling tank must be removed from the sludge zoneas rapidly and effectively as possible. Failure to ensure effective sludge removal willseriously hinder the overall performance of a settling basin and possibly that ofassociated treatment units. In design, it is therefore, necessary to consider the sp eed w ithwhich settled sludge can be scraped from the floor into a collecting hopper. Someconcentration of sludge will occur in the hopper and it is important to ensure that thismore conce ntrated sludge can be withdrawn at a rate which will at least balance, the rateat which it accumu lates. If this does not occu r, sludge levels will build up in the tank,leading to anaerobic breakdown in the cas e of was tewater sludges, causing a deteriorationin effluent quality. Th e hydraulic design of sludge remov al system s is important s inceconcentrated sludg e has a much higher viscosity than water. If the velocity of sludgeremoval is excessive, it is likely that 'piping' will occur in which the liquid is drawnthrough the sludge mass w ithout removing m uch sludge. If this happens for any length oftime, blocking of the sludge withdrawal sy s te r is likely.

Collector Effluent adiustable

(a) Horizontal Flow Rectangular 'lgnk

(b)Radial Flqw Circular lgnk

Influent*

Sludge drain(c) Vertieal Flow Clreular lbnk

(d) Flocculation and Sedimentation

V - notchVJcum board(e) V - notch Weir - Scumboard Ou tlet Arrangement

Figure 8.5 : pes of Sedimentation TanksSedimentation tanks take a variety of forms and som e common types are show ndiagrammatically in Figure 8.5. Although early treatment plants sometimes used batch o r

Sediment-+;--. -...t~uvnandFiltration


8/21

Physical Treatment 'fill and draw' operation for sedimentation, virtually all installations now employcontinuous flow systems. A sedimentation tank (or a clarifier) may be rectangular,circular or square in plan and the flow through the tank may be horizontal, vertical or

, radial. Thus sedientation tanks may be classified as horizontal flow rectangular tank,radial flow circular tank, vertical flow circular tank, etc. (see Figure 8.5). Rectangularhorizontal-flow units provide the most effective use of land area but because of theirconfiguraiton they have hydraulic problems at inlet and outlet sections in relation toestablishing quiescent conditions in the settling zone. Sludge deposited on the bottom ofthe tank must be scraped to a hopper using a reciprocating blade mechanism which can beprone to operational difficulties. A particular problem with rectangular tanks is therelatively short length available for the effluent discharge weir. A simple weir across theexit end of the tank will cause relatively high local velocities with the potential for scourof settled deposits. A better solution is to utilise inset weirs to provide greater dischargelength and hence lower velocities, as shown in Figure 8.6. Circular sedimentation tanksoperates as horizontal-flow units with a baffled inlet at the centre and discharge overperipheral weirs which provide ample length to ensure low discharge velocities.Adjustable weir plates with small 'Vee' notches provide an effective discharge systemwhich is not affected by surface tension at low flows. A rotating scraper mechanismoperating continuoulsy directs sludge to a central hopper. For primary sedimentation ofwastewater, it is essential to provide some form of device to trap surface scum and greaseso that this can be prevented from escaping with the effluent. Vertical-flowhopper-bottom tanks are used in some small wastewater settlement units since they havethe advantage of not requiring mechanical desludging mechanisms.In water treatment the nature and concentration of suspended solids is such that chemicalcoagulation is often employed to improve settling characteristics. In general, however,suspensions in water treatment are of lower density and more flocculent in nature thanthose found in raw wastewaters. Sedimentation in water treatment is thus generallyundertaken in sludge-blanket units. The sludge blanket once established provides a formof filtering action which removes particles with lower settling velocities than the upwardvelocity in the blanket region of the tank. Vertical-flow sludge blanket units are popular,although in more recent designs the hopper bottom has been replaced by a float floor withthe inflow distributed by a pipe system. It should be appreciated that vertical flow tanksare designed to operate at a specified velocity. If the solids do not attain this velocity orthe hydraulic loading on the tank is increased, its performance will deteriorate.

Inset Geir cantileveredfrom walls

FIgure 8.6 : nsetWeirs for Reduced Ovemow Velocities8.2.4 Common Design CriteriaAlthough waters and wastewaters vary quite widely in their settling behavioor, it ispossible to establish general design criteria for sedimentation processes. Table 8.2 setsout such general design criteria for conventional wastewater treatment. If there is anyreason to beIieve that a particular suspension has a typical settling characteristics it wouldbe prudent to cany out laboratory studies to establish its settling behaviour and henceamve at appropriate design criteria. Table 8.3 shows typical design features ofrectangular and circular sedimentation tanks.


9/21

Table 8.2 :Typical Design Criteria for Sedimentation Tanks(at maximum flow)Wastewater TreatmentPrimary SedimentationHorizontal and radial flow units :Surface overflow rate 1 - 1.5 m I hRetention time 2 hOutlet weir loading < 12.5 m3 m hWidth :Length (rectangular units) 1 :4 to 1 : 8Vertical flow u nits :

Surface overflow rate 1 - 1. 8 mhRetention time 2 - 3 hOutlet weir loading c 12.5 m3 I m hFinal Settlement after biological TreatmentSurface overflow rate 1.5 m I hRetention time 2 hOutlet weir loading c 10 m 3 m I hTable 8 3 :Design Features of Sedimentation Tank for Wastewater

SedimentationFlocculation andFi ration

ExampleDesign a rec tangula r hor izonta l -f low se t t l ing tank for the pr imary sed imenta t ion ofa m a xim um r a w se wa ge f low o f 0 .25 m 3 I s.

SolutionFrom T able 8 .2S ur f a c e ove r f low r a t e = 1.2 m I hTan k a rea required = 0.25 x 6 0 x 60/ 1.2 = 7 5 0 m2

Parameter

Max. LengthMax. WidthDepthRange of lengthtwid th ratioRange of lengthldep th ratioBottom slopeMax. DiameterInlet

Outlet

Peak velocityScraper arms velocity8.4

'ljpes of SedimentationTankRectangular

90 m30 m2 - 2.5 rn1.5 - 7.55 - 251%

-Multiple pipes on thewidth side with baffleboards of depth 0.5 mand 0.8 m in front of thepi e inlets and extending2.P m below watersurface for scumpassoverOverflow w eir withV-notches to provideuniform flow at lowheads. Scum b afflesprovided ahead of weirfor wastewaterinstallationsdepend s upon feed0.2 m / min

+

Circular-

-2 - 3.5 m-

-

7.5 - 10% (fromperiphery to centre)30 mCentral inlet ipe withconcentric i&t baffleof diameter 15% of thetank diameter andextending about 1 mbelow water surface

Peripheral weirrovided with!-otches. Scumbaffle extending 0.3mbelow Water su rfaceprovided ahead ofeffluent weir forwastewater installation-

1.5 m / min


10/21

Physical Treatment Hence breadth B = 13.69 m and length L = 54.77 mTwo hours flow = 0.25 x 60 x 60 x 2 = 1800 m3Hence average depth = 1800 750 = 2.40 mMinimum length of weir = 0.25 x 60 x 60 / 12.5 = 72 mClearly, a single weir across the outlet end of the tank will be insufficient.Adouble-sided 'U ' shaped inset weir will be necessary for which the total width willbe approximately 2 x tank width plus 4 x the extension back up of the sides of thetank.

Hence e = 11.15 mThus the weir channel will need to be inset about 1m from the outlet end of thetank and the sides extending about 11 m back up the tank.In contrast, using a circular tank for the same dutyDiameter for 750 m2 surface area

Length of periphery = 3.14 x 30.9 = 97.1 mThus a single-sided peripheral weir on a circular tank more than satisfies themaximum weir loading constraint.Example 8.5Calculate the necessary design data for a secondary settling tank of an activatedsludge treatment plant receiving a peak daily flow of 50,000 m3 of domesticsewage and operating with a mixed liquor suspended solids MLSS) of 300 mg I 1.!Assume a peak factor of 2.25; a surface loading rate of 20 m 1m2 d at mean flowand solids loading of 125 kg 1 m2d at mean flow.

SolutionPeak flowAverage flow= = - - -M'OOO23,000m3 IdPeak factor 2.25

Adopting a surface loading rate of 20 m3 1 m2d at average flow.23000Surface area required =- 1150 m20Check surface loading for peak flow :-

This is within the prescribed range (see Table 8.2)For solids loading of 125 kg I m2 d at average flow, the area required is

Area needed for peak flow at a solids loading of 250 kg 1 m2 is

The higher surface area based on volumetric loading is adopted for designpurposes. Adopting a circular tank, the diameter of the tank, d, is calculated thus :-738.26 metres3.142Weir loading is calcualted as follows :-

The weir loading is higher than the prescribed value of about 150m3 I m d and soon trough instead of a single weir should beprovided at the periphery.


11/21

8.3 FLOCCULATIONWith small suspended solids and those having low specific gravities the actual settlingvelocities can become so low that removal by sedimentation is not a practical option. Inwater and wastewater treatment, this usually occurs with particles of less than about5 0 pm in size. When high concentrations of flocculent particles are present, the creationof velocity gradients in the suspension causes collisions between particles withconsequent agglomeration. This natural flocculation process can be enhanced by theapplication of controlled velocity gradients through hydraulic turbulence or mechanicalstirring. The number of collisions in a suspension is proportional to the velocity gradientand the power input necessary to produce a particular velocity gradient is given by :

where, P = power input per unit volume,p = absolute viscosity of fluid, andG = velocity gradient in basin.

For hydraulic turbulence in a baffled basin

where, mf = mass density of fluid,h = head loss in tank, andt = retention time in tank.

In a tank stirred by rotating paddles

where, CD = Newton's drag coefficient,A = cross-sectional area of paddles,v, = velocity of paddles relative to fluid, andv = peripheral velocity of paddles.2V = volume of tank

Example 8.6A flocculation tank is 10 m long, 3 m wide and 3 m deep with a design flow of0.5 m3 s. Flocculation is done by three paddle wheels each with two blades2.5 m x 0.3 m with the centre line of the blades being 1 m from the shaft which isat mid depth of the tank. The paddles rotate at 3 revs / min and their velocityrelative to the water in the tank is three-quarters of the rotational velocity.Newton's drag coefficient for the paddle blades is 1.8 and the kinematic viscosity

6 3of the water is 1.01 x 10- m / s at 20C.Solution

Paddle velocity = 2 x 3.14 x 1 x 3 / 6 0 = 0 . 3 1 4 m I sRelative velocity= 0.75 x 0.314 = 0.236 m / sPaddle area = 3 x 2 x 2.5 x 0.3 = 4.5 m2

Total power input from equation 8.6

From equation 8.4 G = 70.8/ 1.01 x x lo 3 x 10 x 3 x 3 = 27.9s-( ,"'In many cases the concentration of suspended matter is not high enough for significantagglomeration to occur under tha action of flocculation alone. It is thus frequentlynecessary to introduce a chemical coagulant which precipitates in the water and enmeshesthe suspended matter.

SedimentationFlocdation andFiltration


12/21

PhysicalTreatment For most flocculent suspensions in water and wastewater treatment, the optimum velocitygradient is usually in the range of 25 - 75 s-'. There is some benefit in providing taperedflocculation such that high G values at the inlet encourage collisions and as the flocparticles grow the agitation is reduced to prevent shearing of the floc.

8.4 FLOTATIONWith suspensions of particles whose mass density is close to or less than that of thesurrounding fluid, settlement will be impractical. In these circumstances, the particles canbe.more readily removed by encouraging them to float to the surface where'they can beremoved as a scum. Particles with a density less than the fluid will wish to float in anyevent and those only slightly denser than the fluid can be given positive buoyance by theaddition of a flotation agent. Small air bubbles make excellent flotation agents and thedissolved air flotation (DAF) process as shown in Figure 8. 7 makes use of this property.About 10 per cent of the flow is recycled through a saturator operating at high pressure(up to 400 kPa). The pressurised flow is returned to the inlet of the flotation tank where itis mixed with the incoming flow at the bottom of the tank. The sudden drop in pressurecauses the release of clouds of fine air bubbles from the supersaturated portion of theflow. These air bubbles become attached to suspended particles and thus cause them tofloat to the surface. DAF is particularly useful in water treatment for the removal of ironand manganese and with coloured low-turbidity water following chemical coagulation.Rise rates of up to 12 m / h can be achieved as compared with typical settling rates ofaround 2 - 4 m / h. Flotation units are thus much smaller than conventional settling unitsand can usually produce lower turbidities than sedimentation. The process can be rapidlybrought into operation when required in contrast to floc blanket settling units which cantake 24 hours or longer to achieve stable operation. The scum removed from the surfaceof flotation units usually has a significantly higher solids content than the sludge fromgravity settlement of the same water.

Floatauon Zone

Figure 8.7 : issolved Air Flotation UnitFlotation has applications in wastewater treatment for the separation of suspended solidsin activated sludge systems and also for the thickening of activated sludges produced byconventional gravity settlement.- -8.5 FLOW THROUGH POROUS MEDIABeds of sand are frequently employed to provide tertiary treatment of wastewatereffluents. In this situation, the main purpose of the bed is to remove fine suspendedsolids. Other forms of porous uncompacted solids in deep beds are used to provideadsorption and ion exchange processes.8.5.1 Porous Bed HydraulicsThe hydraulics of flow through porous beds, which applies to clean filters and to granularactivated carbon (GAC) and ion exchange beds is usually described by empiricalrelationships of which that based on the Carman Kozeny equation is probably the mostused :


13/21

where, h / l = head loss per unit depth of bed,F = porosity of bed,d = bed grain 'diameter',s = particle shape factor = A, /A,*A, = surface area of sphere volume, V,,A = surface area of bed grain volume V,s = 1 for a sphere,s = 0.70 - 0.90 for sand grains,E = [150(1 - F ) / R ] + 1.75,R = Reynold's Number = vd/u, andu = Kinematic viscosity of fluid.

Example 8.7A filter bed is composed of 900 mm of unit-size spherical sand, 0.5 mm diameterwith a porosity of 40%. Determine the head loss when the clean bed is operated ata rate of 140 m / d. Kinematic viscosity of water at 20C is 1.01 x 10- I5 m2/ s.

SolutionFiltration rate 140 m 1d = 140/24 x 60 x 60 m / s

Reynolds Number = 1.62 x x 5 x l0 -~ /1 .0 l x 10-I5 = 0.8In Eq. (8.7)

E = [I50 (1 - 0.40) /0.8] + 1.75 = 114.25S = 1 for spherical particles

Hence head loss / unit depht

i.e., head loss 1unit depth = 0.573or head loss in 900-mm deep bed = 0.515 m

The relationship in Eq. (8.7) applies for a bed of uni-size grains and for sand beds whichare usually graded in arithmatic integration of head loss across, sieve sizes must beundertaken.In a bed which receives suspended matter, the porosity is continually changing due to theposition of the solids and thus the head loss behaviour is dynamic. It is usually assumedthat the rate of suspended matter with depth into the beds is a function of the inlet particleconcentration. If all the suspended particles are retained by the bed, the overall head loss(H ) for a unit size medium is thus made up of the 'clean-bed head loss' (h) as calculatedfrom Eq. (8.7), plus an additional head loss caused by reduction in porosity due todeposition.

where, c, = influent suspended particle concentration,t = duration of filter run, andk = a constant depending upon bed and solids.

The build up of head loss with time can be pictured as shown in Figure 8.8 whichillustrates the way in which negative pressure can be produced in a bed with detrimentaleffects on the rate of low. Dual media beds using sand and anthracite are often used toincrease the length of filter runs without affecting filtrate quality. For optimum operationof a deep bed filter, it is desirable for the limiting head loss to be reached at about the



14/21

Physical Treatment same time as the filtrate quality approaches the allowable limit. Such a situation can intheory be achieved by selection of appropriate bed depth and filtration rate for a givenwater quality and filtration rate. Such optimisation does, however, become difficult toachieve if the influent quality is not reasonably constant.

I I 1\ tatic pressureb

. . - . .. . . . Pressure wh en filteringat Times tl . t2, t3 .. . . . - . .

Nagative pressureregionFigure 8.8 :Head Loss Build up in a Deep Bed Filter

8.5.2 Cleaning of Deep Bed FiltersFor deep beds operating at low flow rates (up to 0.3 m / h) such as slow sand filters thereis little penetration of solids into the bed. With flow rates in range 2 - 20 m / h, as typifiedby rapid filter beds, solids are carried deep into the bed and may eventually penetratethroughout the full depth causing a deterioration in filtrate quality.Low-flow rate deep bed units are cleaned when necessary by removing the top fewcentimetres of medium and washing it. The washed medium can be replaced on the bedwhen its depth becomes insufficient.High-flow rate beds can clog in 24 hours or less with turbid feed. Waters and solidspenetrate deeply into the bed. Cleaning must therefore be carried out in-situ using a backwashing process. This introduces previously filtered water into the base of the bed to givean upward velocity sufficient to fluidise the bed and produce an expansion of10-20 per cent. During backwashing, the bed grains are violently agitated so that trappedand attached particles are released and carried upwards through the enlarged pores. Theintroduction of compressed air immediately prior to or at the same time as the wash watermoves upward is common since it provides more effective cleaning. The backwash wateris usually taken to a settling basin where the solids are concentrated for disposal and thewater can be returned to the works inlet.The head loss per unit depth of an expanded bed during backwashing is given by

where, 1, = expanded bed depth, andf, = expanded bed porosity.

The expansion which can be produced by a given backwash rate is a function of thesettling velocity of the bed grains and the bed porosity

where, v b = backwash rate (based on superficial area of bed),v, = bed grain settling velocity (from Eq. 8.1), andn = an experimental constant (commonly 0.22).

Example 8.8Determine the expansion produced when a bed with unit-size sand grains with asettling velocity of 100 mm / s and unexpanded porosity of 40% is backwashed ata rate of 36 m / h.

SolutionUnexpanded porosity =40% fe= 0.4Wash rate 36 m / h = 10 mm / s


15/21

From Eq. (8.10)1,/1 = (1 - 0.4)/[1 - ( l o / 100)O.~~] 1.51

i.e., bed expansion = 5 1%Both of the above expressions are for beds of uni-size grains so that for a gradedbed, arithmatic integration between sieve sizes is necessary.

8.5.3 Deep Bed Filtration in Practice (IThe essential characterisitcs of the two main types of filters used in wastewater treatmentare sumrnarised in Table 8.4 and their main figures are shown in Figure 8.9. Conventionalfilters use beds of graded sand as the filtration medium and the bed grain characteristics

Table8.4 :Technical Features of the Conventional Slowand Rapid Gravity Flow Sand Filters

Preparatory treatment of water Generally none

Rapid Sand FiltersarametersISide of bed Large, 2000 m2 Small, 200 - 400 m2

Depth of bed 0.3 m of gravel; 1.2 m of sand,usually reduced to not less than0.6 m by scraping

Size of sand Effective size 0.25 to 0.3 to 0.35 0.45 mm and higher-coefficientmm; coefficient of uniformity 2 of uniformity 1.5 and lower,to 2.5 to 3 depending on underdrainagesystemGrain size distribution of sand in Unstratified Stratified with smallest or lightestfilterUnderdrainage system Split tile laterals laid in coarse (1) Perforated pipe lateralsstone and discharging into tile or discharging into pipe mains;concrete main drams (2) porous plates above inletbox;(3) porous blocks with includedchannelsLoss of head 0.15 m initial to 1 m final 0.3 m initial to 2.5 m finalLength of run between cleanings 20 to 60 days 12 to 72 hrs.Penetration of suspended matter Superficial- DeepMethod of cleaning (1) Scraping off surface layer Dislodgin and removingof sand and washing and suspendedmatter by upward flowstorin cleaned sand for or backwashing, which fluidisesperio8c resanding of bed the bed. Possible use of water or(2) Washing surface sand in air jets, or mechanical rakes toplace by washer travelling , improve scourover sand bed

Coagulation, flocculation and Isedimentation

Slow Sand Filters

Amount of wash water used incleaning sand

Rate of filtration

are usually specified by two parami-:ers : effective size -th e aperture size which will pass10 per cent of the grains by weip ~ r , nd uniformity coefficient - he aperture size whichpasses 60 per cent of the particles by weight divided by the effective size. These two

3m3/m2/d I 125m31m2/d

0.2 to 6% of water filtered

Washwater rate m / h -


1 to 4 to 6% of water filtered

Air scour rate m / hSupplementary treatment of waterCoct of constructionCost of operation

IDepreciation

-DisinfectionModerate to highRelatively low where sand iscleaned in place or labour costsare low--elatively low

20- 40DisinfectionRelatively lowRelatively high

Relatively high.


16/21

PhysicalTreatment parameters thus give a measure of the 'average' size of the grains and the 'width' of thegrading. Slow filters normally use finer grains with a som ewh at wider range of sizes thanrapid filters as indicated in Table 8.4.Slow sand filters were the original form of filtration used in po table water treatment andare sometimes felt to be obso lete because of their large area and inability to deal withhighly turbid w aters. Nevertheless, for raw waters with less than abo ut 30 NTU they canprovide a very effective form of treatment which is particularly good at removing harmfulmicro-organisms. T his latter property is of great value in develop ing countries wheredisinfection using chlorine may not always be possible. Much of the purification whichtakes place in a slow filter is achieved in the surface layers of the bed and the biologicalactivity which produces a su rface layer know n as the schm utzde ke contributes to theremoval of fin e particulate matter and also causes so me removal of taste and odou rforming o rganic compounds. S low filters will usually opera te for severa l months beforesurface clogging grow s to the point where the flow rate can no longer be maintained. Atthis point, surface scraping, manually or by machine, w ill restore the flow and the bedcan be put b ack into service. To prevent disturbance of the bed surface, a depth 1m ofwater above the bed is used and this provides th e head required fo r flow through the unit.

Underdrains(a) Slow Sand Filter

TWL ---- - . - - 3- wahswater troughsandgravelfilter bonom andunder drains

If?Washwater andqir scour (a)RapidGravityFilter

Inflow--a s h i c Crain > ressurevessel. . - : .. - : . . . . . . . . . . . . . I .. . . . . . . . . . . . . . . . . .. . . . . . . . ............... ..._,..,.. -.....-- sand. . . . . . . . .. . . . . . . . -- ilterbottom anaunderdrains

~ a s h w a t i rndAir Scour

(b) Rapid ProssureNlterFigure 8.9 :Qpes ofb p ed Filter

8.6 CHEMICAL AIDED SEDIMENTATIONVery fine suspended p articles present in waste water so metim es can no t be remo ved byplain sedim entation. Such fine particles are settled by increasin g their size by changingthem int o flocculated particles with addition of som e chemicals to the waste waters. Th echemicals are known coagulants and mostly used chemical compou nds are ferricchlorid e, f em c sulphate, alum, chlorinated copperas etc.


17/21

The chemicals when added to waste waters and mixed thoroughly, form a gelatinousprecipitation know n as floc. The fine particles and colloidal matters present in wastewater get absorbed in flocs, forming the bigger sized flocculated particles. Thecoagulated sewage is then passed through sedimentation tank where flocculated particlesafter getting settled are removed. T he addition and m ixing of chemicals is known ascoagulation. The coagulation process of sewage is similar to that of water and has beendescribed in detail in the "Pollutants and Water Supply" course. But in modem plants ofsewage treatment coagulation is not s o common due to the following demerits :(a) Th e secondary biological treatments, which are used now a day s are completein themselves and d o not need coagulation.


(b ) The coagulation and subsequent sedimentation produces larger quantities ofsludge, adding to the problems of sludge disposal.(c ) Chem icals used in coagulation react with sewag e and during these reactions,they destroy certain micro-organ isms, which are helpful in digestion of thesludge, thus causing difficulties in sludge digestion.(d) Cost of chemicals is added to the cost of treatment.

Still in certain special case s, it is adopted as discussed below :(i) Using som e special chemicals for treating sewage from a particular industry.(ii) Whe n there is large seasonal variation (such as hill station where flowconsiderably increase during seasons) in sewage flow an d the sedimentation

tank gets overloaded . Th e addition of chemicals in such condition acceleratessedimentation.(iii) When there is space constraints for treatment plants. Coagulated settling tankrequires less spa ce than that is required by an ordinary plain settling tank.(iv) Wher e better effluent with lesser BO D and suspended solids is required.

8.6.1 Properties of Some Common Coagulants Used in SewageTreatmentFerric Chloride

Ferric chloride is a widely used coagulant. It fo rms a de nse heavy floc settlingrapidly. The sludge formed is not bulky and is digested and dewatered easily. BODremoval is 8 0 to 90%. Removal of suspended solids is around 90 to 95%. pHrequired for best result is 5.5 to 7.0 and dose require and in ppm is around 25 to 35.

Ferric Sulphate with LimeFerric sulphate is more effective than chlorinated copperas if used with lime. BODremoval is 8 0 to 90%. Suspended solids removed is 9 0 to 95% . It gives best resultin the pH range of 8.0 to 8.5 when added w ith dose of 3 5 to 40 ppm .

Chlorinated Coppera sFerrous Sulphate (C opperas) with chlorine is called chlorinated copperas. T hiscoagulant is effective for producing sludge for activated sludge process. BO Dremoval is 70 to 80%. 8 0 to 90% of suspended solids are removed. Best result isobtained when 3 5 to 80 ppm is added in the pH range of 5.5 to 7.0 and 9.0 to 9.5.

AlumAlthough alum is very common coagulant used in treating water supply, it isgenerally not used in sewage because they form spongy floc which settles slowlyand volume of sludge produced is large. BOD removal is around 60% and around80% of suspended solids are removed.

8.7 IMPORTAN T LESSONS FOR PHYSICALTREATMENT PROCESSESIt will be apparent from the preceding sections that much of the design of physicaltreatment processes is based o n somewh at idealised concepts sup plemented by prag maticand empirical considerations. Failhire o understand the underlying mechanisms of thevarious processes can result irt i r~ dt m en t nits which perform inefficiently or unreliably.


18/21

Physical Treatment 8.7.1 Flow DistributionIn all but the sma llest works, the total area or volum e of treatment un its need to bedivided into multiple units. This allows for treatment to continue when on e unit is beingcleane d or maintained. In such a situation , it is, how ever, vital to ens ure that flowdistribution between the individual units is accurate. Many treatment plants performbadly because the designers have assumed that the principles of hydraulic similarity canbe applied to a large works. Minor differences in elevation, pipe length, or valvecharacteristics can have a major influence on the flow received by different units. Studieshave show n that because of poor flow-splitting arrangements in a gro up of fourapparently identical settling tanks, one unit received almost 75% of the total flow. Notsurprisingly, its performance was poor. As a general rule, flow splitting should beachieved by individual free-falling weir discharges as sketched in Figure 8.10.

Outflow

1 I(U T Free Dischaze T

A

0lnBaar

Weirs

-- Sbut off- - - -+---- Penstock------------------ --- = Outflow

A[+f

(b)Flow Dividing Structure (SecUonA-A)Figurr 8.10 : low D ividing Structurr

7

(a) 14an of the Flo w Dividing Structure

8.7.2 SubsidenceMajor units like sedim entation tanks are prone to differential settlement due to groundconditions during and after construction. It is therefore commo n to see tanks where slightsubsidence has affected the effluent discharge over weirs so that m ost or all of thedischarge takes place over a fraction of the available length. This cause s high localvelocities with the likelihood of sco uring of settled solid s in vicinity. As outlined earlier,the problems of differential subsidence in sedimentation units can be greatly alleviated byusing mov able weir plates, preferably with 'Vee' notch es, which can be re-levelled asnecessary (see Figure 8.5).8.7.3 Flow PatternIt will be recalled that settlemen t theory assumes quiescent lam inar flow conditions in thesedimentation basin. Horizontal and radial flow units for primary settlement of sewage, inparticular, suffer from the fact that they ha ve to ope rate under a w ide range of flowconditions (perhaps 0.4-3 wfl with a feed varying considerably in its SS content and inthe settleability of those s olids. Flow through studies often dem onstra te that the residencetime of the flow is much less than the theoretical detention time. With some rectangularhorizontal-flow tanks tracer residence times of only 40minutes or so have been recordedunder conditions when the theoretical detention time was 10 hours! It is perhaps fortunatethat most of the settleable solids in raw sewage settle out rapidly but clearly when actualresidence times are very much shorter than expected, the design of the tank must be inquestion. C areful baffling of the inlet to settling tanks can do something to reduce the'submerged waterfall' effect which is inevitable when a flow containing suspended matterenters a tank whose contents have a lower density du e to the settlement which has takenplace. In the same way, the provision of am ple discharg e weir length and the extension ofweirs over a large part of the tank surfac e can reduce short-circuiting effects to som eextent.Flow distribution and flow patterns are also of considerable importance in filterinstallations, particularly in relation to the backw ashin g of rapid gravity filters. Unevenbackwash flow distribution can result in the formation of 'mud balls' in the bed. Theseoccur where deposited material is not scoured because of reduced backw ash flow in anarea of the bed, wh ich if not rectified can quickly reduce the effectiv e capacity of the bed.


19/21

An important point w hen flocculation is being carried out to improve the settlingcharacteristics of a suspen sion is to ensure that the floc particles wh en formed are notsubjected to excessive shear. If this happens the floc particles will be broken up andsedimentation will be ineffective. On e particular plant un dertook chem ically-aidedflocculation in a stirred chamber sep arated from the sedimentation chamber by a portedbaffle wall. At the down stream end of the flocculation chamb er large floc particles werevisible but the settled effluent was of poor quality with a large number of fine suspend edsolids. Closer inspection sho wed that after passing through the ports in the baffle wall thelarge floc particles had been broken up. Th is was becau se the designer had made the portsin the baffle of sufficient area to pass the flow withou t checking the G value which wouldbe produced. In fact the port area was very restricted and the G value was an order ofmagnitud e higher than would hav e been appropriate to ensure safe transport of the flocparticles. Enlargemen t of the ports transformed the performance of the settlemen t stagethus highlighting that although turbulence is necessary fo r floc formation, excessiv eturbulence can be counter-productive.

SAQ 1

B y plotting a settling c i i a v c . -nccz\sar) to pb .>\, de LI 9ii7,' - .horizcnta! frob srdiment:~!~


20/21

Physical Treatment SA Q 5Why chemical aided sedimentation is not so common in case of waste waters?

SAQ 6Discuss properties of som e common coagulants used in sew age treatment.

8.8 SUMMARYThis unit describes in the brief the process of sedimentation with its fundamental theoryand design criteria of sedimentation tanks. Flocculation and coagulation also play animportant role in the process of sedim entation. In fixed or attached growth system s thesewag e is made to pass through filter beds also. The unit describes porous bed hydraulicsand details of filters which explains the filtration process. B asic principles an d designcriteria with examples of sedimentation flocculation, flotation, filteration and coagulationhave been g iven in this unit.8.9 KEY WORDSConcentration : A general term referring to the qua lity of a material orsubstance contained in unit quantity of a given medium.When the term concentration is used without furtherqualification, it now mean amou nt of substanceconcentration.Flow Rate, Q : This is amount of liquid passing a plant per hour o r perday.Filter Resistance : This is equal to the head difference between inflow andoutflow (head loss). It increases as the voids in the filter

medium gets clogged by retained particles.Filtration Rate : This is als o called filtration ve locity o r flow velocity,through a filter area of 1Sedimentation Tank : A tank in which sewer containing sediment is retained fora sufficient time at a sufficiently low velocity to removepart of the settlement by gravity.Suspended Solids : The solids which are suspended in a sewage.--- -8.10 ANSWERS TO, SAQs

SAQ 1Refer Example 8.3.SAQ 2

Refer Example 8.4.SAQ 3



Refer Section 8.6.SAQ 6

Refer Sub-section 8.6.2.


21/21

FURTHER READING SedimentationFlocculation andFiltration

(1 ) Nathanson, Jerry A.: "Basic Environmental Technology",John Wiley and Sons; 1986.(2 ) Metcalf and Eddy Inc,; "WastewaterEngineering", Collection and Pumping ofWastewater,McGraw Hill, New York, 1981 .(3) Geyer, J. C.; and Lentz, J. J.; "Evaluationof Sanitary Sewer System Designs", TheJohns Hopkins University School of E ngineering, 1962.(4) Garg, S. K., "Sewage Disposal and Air Pollution Engineering", Khanna Publishers,Delhi, 1988.

Documents

Physical Treatment