Sedimentation W ATER TREATMENT

WATER TREATMENT WATER TREATMENT Sedimentation

100 100 time [s] 80 600 80 900 1200 60 1800 60 2700 3600 40 40 h= 0.75 m 5400 7200 h= 1.5 m 20 20

h= 2.25 m suspended solids content [%] content solids suspended cumulative frequency distribution [%] frequency cumulative h= 3.0 m 0 0 0 1 2 3 4 5 0 0.5 1 1.5 2 2.5 3 h/t [m/h] distance under water surface [m]

trap t = 0 t = $ t = 2$

1 tube

3 constant water temperature

4 sample of the solution

5 silt sedimentation water treatment

Framework This module represents sedimentation.

Contents This module has the following contents:

1. Introduction 2. Theory 2.1 Sedimentation of discrete particles 2.2 Horizontal flow settling tanks in practice 2.3 Settling efficiency of a suspension 3. Influences on settling in a horizontal flow tank 3.1 Influence of turbulence 3.2 Influence of stability 3.3 Influence of bottom scour 3.3 Influence of flocculant settling 4. Practice 4.1 Determination of the dimensions of an ideal settling tank 4.2 Inlet constructions 4.3 Outlet constructions 5. Settling tank alternatives 5.1 Vertical flow settling tank 5.2 Floc blanket clarifier 5.3 Tray settling tanks 5.4 Tilted plate settling

52 water treatment sedimentation

1 Introduction Q Q V0 B Sedimentation is a treatment process in which suspended particles, like flocs, sand and clay are inlet sedimentation zone L outlet zone zone re-moved from the water. Q Q Sedimentation can take place naturally in reser- V0 H voirs or in compact settling installations. slib zone Examples of settling installations are the horizontal flow settling tanks, the tilted plate settlers and Figure 2 - Horizontal flow settling tank the floc blanket installations. Sedimentation occurs because of the difference Sedimentation is frequently used in surface water in density between suspended particles and wa- treatment to avoid rapid clogging of sand filters ter. after coagulation and floc formation (Figure 1). The following factors influence the sedimentation Sedimentation is applied in groundwater treat- process: density and size of suspended particles, ment installations for backwash water treatment. water temperature, turbulence, stability of flow, bottom scour and flocculation: In horizontal flow settling tanks (Figure 2) water - density the greater the density of the par- is uniformly distributed over the cross-sectional ticles, the faster the particles set- area of the tank in the inlet zone. tle A stable, non-turbulent, flow in the settling zone - size the larger the particles are, the takes care of the settling of suspended matter in faster they settle the settling zone. - temperature the lower the temperature of the The sludge accumulates on the bottom or is con- water is, the higher the viscosity, tinuously removed. so the slower the particles settle In the outlet zone the settled sludge must be pre- - turbulence the more turbulent the flow is, the vented from being re-suspended and washed out slower the particles settle with the effluent. - stability instability can result in a short-circuit flow, influencing the settling of particles Reservoir - bottom scour during bottom scour, settled Fe (III) particles are re-suspended and Floc formation washed out with the effluent - flocculation flocculation results in larger parti- Floc removal by sedimentation cles, increasing the settling velocity. Ozonation

Filtration 2 Theory Activated carbon filtration

Cl2/ClO2 2.1 Sedimentation of discrete particles Clear water reservoir Discrete particles do not change their size, shape or weight during the settling process (and thus do not form aggregates). Figure 1 - Process scheme of a surface water treat- A discrete particle in a fluid will settle under the ment plant influence of gravity. The particle will accelerate

53 sedimentation water treatment

in which: Fdown = downward directed flow by gravity [N] 3 Fup [N] ρs = specific density of particle [kg/m ] 2 sedimentation speed g = gravity constant [m/s ] V [m/s] s V = volume of particle [m3]

Fdown [N] Equality of both forces, assuming a spherical particle, gives as the settling velocity:

Figure 3 - Forces on a settling particle 4 ρs − ρ w vs = ⋅ ⋅g ⋅ d 3⋅ cD ρ w until the frictional drag force of the fluid equals the value of the gravitational force, after which the vertical (settling) velocity of the particle will be in which: constant (Figure 3). d = diameter of spherical particle [m]

The upward directed force on the particle, caused The settling velocity is thus dependent on: by the frictional drag of the fluid, can be calcu- - density of particle and fluid lated by: - diameter (size) of particle ρ - flow pattern around particle. F= c ⋅w ⋅ v2 ⋅ A up D2 s The flow pattern around the particle is incorporat- in which: ed in the drag coefficient. The value of the drag Fup = upward directed force by friction [N] coefficient is not constant, but depends on the cD = drag coefficient [-] magnitude of the Reynolds number for settling. 3 ρw = density of water [kg/m ] For spherical particles the Reynolds number is vs = settling velocity [m/s] given by: 2 A = projected area of the particle [m ] v⋅ d Re = s ν The downward directed force, caused by the difference in density between the particle and the in which: water, can be calculated by: ν = kinematic viscosity [m2/s]

= ρ − ρ ⋅ ⋅ Fdown( s w ) g V In drinking water treatment practice, laminar settling normally occurs. The Reynolds number for

D 1,000 laminar settling of spheres is Re<1, resulting in observed relationship the following relationship between the Reynolds 24 3 c = + + 0.34 100 D Re Re number and the drag coefficient: c = a D Ren 10 Substitution of this relationship in the equation for the settling velocity gives the Stokes’ equation: resistance coefficient c coefficient resistance 1 1 g ρ − ρ v = ⋅ ⋅s w ⋅ d2 0.1 s ρ 0.1 1 10 100 1,000 10,000 100,000 18 v w Reynolds number [-]

Figure 4 - Relationship between Reynolds number The settling velocity is thus dependent on the vis- and drag coefficient cosity of the fluid and also the temperature.

54 water treatment sedimentation

10,000 T=10oc The relationship between kinematic viscosity and temperature is: 100

497⋅ 10−6 v = 1 1.5 (T+ 42.5)

in which: [mm/s] velocity settling ρs - ρw = 5000 o 0.01 500 T = temperature [ C] 50 5 1 When the Reynolds number Re > 1600, settling 0.0001 0.0001 0.001 0.01 0.1 1 10 100 is turbulent and when 1

The flow velocity in a horizontal settling tank is: In special cases, when the settling velocity equals Q the surface loading, the particle reaches the end vo = of the tank. This settling velocity is called the criti- BH⋅ cal velocity vso. in which: It can be concluded that a particle will only be vo = horizontal flow velocity [m/h] removed if the settling velocity is greater than or Q = flow [m3/h] equal to the critical settling velocity (Figure 7). B = width of the tank [m] H = height of the tank [m]

The surface loading of a settling tank is deter- H, t2 v0 mined by: q Q q = L, t1 BL⋅ Figure 6 - Settling in a horizontal flow settling tank

55 sedimentation water treatment

vs > vso - all particles settle completely h v⋅ T v =s = s v0 H vso⋅ T v so vs

in which: v = v - all particles settle completely s so T = residence time of water in the settling tank[s]

H v0 vs The residence time of water in the settling tank is expressed as T and equals t1 from Figure 6. vs < vso - some of the particles settle completely

h v 0 2.3 Settling efficiency of a suspension vs In a suspension the fraction of particles with a L settling velocity higher than the surface loading Figure 7 - Settling of a suspension in a horizontal flow settle completely. The fraction with a lower settling velocity settles partly. The efficiency is determined from the cumulative frequency distribution After determining the settling velocity of a particle of settling velocities obtained from a settling test. during a settling test, the surface loading and thus the dimensions of the tank can be determined. The settling test is executed in a cylindrical con- It is remarkable that, in theory, settling in a hori- tainer (column) filled with a homogeneous sam- zontal flow settling tank is only determined by the ple of the suspension to be tested (Figure 8). At flow and the surface area of the tank and is inde- different time intervals samples are taken at dif- pendent on the height of the tank. ferent depths and analyzed for suspended solids, turbidity or any other index that can be reduced The fraction of the particles that settle in case vs by settling. The depth is measured with the water < vso is (Figure 7): surface as reference. In Table 1 the analyses of a settling column test at depth h=1.0 m are represented (Figure 8).

trap t = 0 t = $ t = 2$

1 tube

3 constant water temperature

4 sample of the solution

5 silt

Figure 8 - Settling column and representation of different settling velocities

56 water treatment sedimentation

Table 1 - Particle concentration and relative particle concentration from a settling test at a depth of h = 1.0 m t (s) 0 666 900 1800 2700 3600 5400 7200 c (ppm) 86 84 79 57 41 29 7 3 p=c/co (%) 100 98 92 66 48 34 8 4

In Figure 9 the cumulative frequency distribution From the particles with a lower settling velocity of the settling velocities is represented. The ratio than vso, only the particles that enter the tank at a of sampling depth and time is given as a func- reduced height will be removed. tion of the relative solids concentration. The sol- From the fraction of particles dp with settling ve- ids with the lowest settling velocity determine the locity vs, only the fraction h/H or vs/vso will be residence time of a settling system. removed. This part of the efficiency (partial removal) can be described by: The particles with a settling velocity higher than p p the critical settling velocity v are removed com- o v 1 o so r=s dp = v dp pletely. This is represented in Figure 9 by the red 2 ∫v v ∫ s 0 so so 0 arrow. Expressing the relative solids concentration for a settling velocity of vso as po, the first part in which: of the settling efficiency is: r2 = part of the efficiency caused by partial settling [-] r1= 1 − p o The efficiency caused by partial settling is rep- in which: resented by the blue surface in Figure 9 divided r1 = part of the efficiency caused by complete by the critical settling velocity. Graphically, this settling [-] part of the total efficiency can be determined as po = relative solids concentration at surface shown in Figure 10. loading so [%]

100 100 p [%] p [%] 1-po

80 80 po po 1 po vs dp vso ∫0 60 60

po vs dp °0 40 40 dp equal-sized surfaces 20 20

0 0 0 0.5 1 1.5 0 0.5 1 1.5 -3 -3 vso vs [10 m/s] vso vs [10 m/s]

Figure 9 - Cumulative frequency distribution of settling Figure 10 - Efficiency of partial settling velocities

57 sedimentation water treatment

2 3 4 6 8 2 3 4 6 8 2 3 4 6 8 100 1.0 Vs Vso 0.9 80 2.0 1.5 0.8 60 1.2 0.7 1.1 1.0 40 0.9 0.6 0.8 efficiency r [%] r efficiency 0.7

20 0.5 0.6

efficiency [-] efficiency 0.5 0 0.4 0.4 0 1 2 3 4 5 0.3 vso [m/h] 0.3

Figure 11 - Removal efficiency in a horizontal flow set- 0.2 0.2 tling tank 0.1 0.1

0 The equation of the total settling efficiency be- 0.001 0.01 0.1 1 Vs comes: Vo p Figure 13 - Influence of turbulence on the efficiency of 1 o r=( 1 − p) + v dp settling o v ∫ s so 0 3.1 Influence of turbulence For different values of vso the efficiency is calcu- In laminar flow in a horizontal flow tank, a particle lated and the results are represented in Figure follows a straight line. 11. In turbulent flow, eddies will transport particles in It can be concluded that with increasing surface a random direction, influencing the settling of the loading of the settling tank (by increasing flow), particles (some settle faster and others slower) the settling efficiency decreases. (Figure 12).

With the Reynolds number the flow characteris- 3 Influences on settling in a hori- tics can be determined: zontal flow tank - laminar flow: Re < 2000 - turbulent flow: Re > 2000. In the preceding paragraphs an ideal flow and discrete settling were assumed. The Reynolds number for flow in a tank can be In practice, however, the ideal situation does not calculated with: exist and the efficiency is influenced by: v⋅ R Re = o - turbulence of flow ν - instability of flow - bottom scour in which: - flocculation. R = hydraulic radius of a settling tank [m]

The hydraulic radius of a rectangular tank can be calculated with:

BH⋅ R = B+ 2 ⋅ H

Figure 12 - Influence of turbulence on settling in a hori- With the expression vo=Q/(B•H) the Reynolds zontal flow settling tank

58 water treatment sedimentation number can be rewritten as:

Q 1 Re = ⋅ down νB + 2 ⋅ H

In Figure 13 the settling efficiency for turbulent flow is represented as a function of vs/vso and vs/vo. Figure 14 - Short-circuit flow caused by wind

In practice turbulence is not always a disadvan- From Figure 15 it can be concluded that the lower tage because, in general, flocculant settling oc- the Camp number is (and thus more short-circuit curs (section 3.4). Turbulence increases the col- flow occurs), the shorter the minimal and average lision frequency of particles, thus increasing the residence times become. This is due to the de- efficiency of the flocculant settling. crease in the effective cross-section of the settling tank and, therefore, to an increase in flow velocity. 3.2 Influence of stability The efficiency of a settling tank, therefore, will be Flow is called stable when short circuiting does lower than is the case in a stable flow condition. not occur. In Figure 14 an example of a short-circuit flow caused by wind effects is illustrated. The wind 3.3 Influence of bottom scour creates a dead zone (or eddy) in the corner of In theory, a particle is removed from the water the settling tank. The water flow can then flow, when it reaches the bottom of the settling tank. locally, in the opposite direction from the general In practice, however, it is possible that resuspen- flow through the tank. sion of already settled particles occurs.

Stability of flow is characterized by the Camp In Figure 16 the forces on particles at the bottom number cp: of the tank are shown. The shear force of water on a spherical particle 2 v is: c = o p g⋅ R l t = × r × v 2 8 w sc Substituting the equations for vo and R for a rectangular tank, the Camp number becomes: T 1.0 2 Q B+ 2 ⋅ H T0 Ta cp = ⋅ g BH3⋅ 3 0.8 T0

-5 0.6 cp > 1• 10 stable flow -5 cp < 1• 10 unstable flow 0.4 Ti T0 In Figure 15 the minimal residence time (Ti) and 0.2 the average residence time (Ta) of water droplets are represented in comparison with the theoreti- 0 -7 -6 -5 -4 -3 -2 -1 cal residence time (To) for different values of the 10 10 10 10 10 10 10 Camp number. Cp Figure 15 - Short-circuit flow

59 sedimentation water treatment

t=0 t=$ t=2$ t=3$

v0 N T d sediment

f bottom of tank

Figure 16 - Bottom scour in which: τ = hydraulic shear [N/m2] λ = hydraulic friction factor (λ = 0.03) [-] Figure 17 - Flocculant settling vsc = critical scour velocity [m/s] v0 <= vsc no bottom scour The shear force of particles at the bottom (mechanical friction) is proportional to the submerged Given the surface loading, the width and depth of weight of the sludge layer: a settling tank can be determined based on this criterion. f = β . (ρs - ρw ) . g . d in which: 3.4 Influence of flocculant settling f = mechanical shear [N/m2] During settling, aggregates are formed as a result β = mechanical shear factor (b = 0.05) [-] of collisions between particles, and settling velocities will increase. This phenomenon is called In equilibrium the hydraulic shear equals the me- flocculant settling (Figure 17). chanical shear and the critical scour velocity can be calculated: In Table 2 the results of a settling test of a floc- 40 ρs − ρ w culant suspension are shown.. vsc = ⋅ ⋅g ⋅ d 3 ρw

When the flow velocity in a settling tank is lower In Figure18 the cumulative frequency distribu- than the scour velocity, bottom scour will not oc- tion of settling velocities is given at different tank cur: depths. From the fact that the distributions differ

Table 2 - Relative particle concentration from a settling test h = 0.075 m h = 1.5 m h = 2.25m h = 3.0 m t = 0 s 100 100 100 100 t = 600 s 93 96 98 99 t = 1200 s 81 86 88.5 89.5 t = 1800 s 70.5 77.5 81 83 t = 2700 s 28 38 46.5 53 t = 3600 s 13.5 22 31 40 t = 5400 s 3 8 13.5 20 t = 7200 s 1.5 3 6 9.5

60 water treatment sedimentation

100 100 time [s] 600 80 80 900 1200 1800 60 60 2700 3600 40 40 5400 h= 0.75 m 7200 h= 1.5 m 20 20 h= 2.25 m cumulative frequency distribution [%] frequency cumulative h= 3.0 m [%] content solids suspended 0 0 0 1 2 3 4 5 0 0.5 1 1.5 2 2.5 3 h/t [m/h] distance under water surface [m] Figure 18 - Cumulative frequency distribution of settling velocities at different tank depths Figure 19 - The relative particle concentration

Example A horizontal flow settling tank has a height of 2 The Reynolds number is higher than 2000 and m, a width of 20 m and a length of 45 m. The the flow will be turbulent. The Camp number is flow through the tank is 0.5 m3/s and the water about 1•10-5 and no short-circuit flow will occur. temperature is 10 0C Determine the efficiency of the settling tank con- Check if the tank meets the hydraulic require- sidering the suspension from the settling test in ments. Table 2.

The horizontal flow velocity and the critical set- In Figure 19 the suspended solids content is tling velocity are: represented as a function of water depth at the Q 0.5 different sampling times from Table 2. Between v = = =12.5 ⋅ 10−3 m / s o B⋅ H 20 ⋅ 2 0 and 0.75 meters the progress of the graph is estimated (dotted line). The residence time is: Q 0.5 v = = =0.56 ⋅ 10−3 m / s SO B⋅ L 20 ⋅ 45 B⋅ H ⋅ L 20 ⋅ 2 ⋅ 45 The hydraulic radius of the tank can be calcu- τ = = = 3600s lated with: Q 0.5 B⋅ H 20 ⋅ 2 R = = = 1.67 B+ 2 ⋅ H 20 + 2 ⋅ 2 From Figure 19 the efficiency can be determined, assuming a residence time of 3600 sec- The Reynolds number and the Camp number onds and a height of 2 meters. The blue surface are: above the line indicates the amount of solids that are settled. The green surface below the v⋅ R 12.5⋅ 10−3 ⋅ 1.67 Re =o = = 15935 line indicates the amount of solids that are still ν 1.31⋅ 10−6 in suspension after 3600 seconds. It can be concluded after measuring the surfaces that the −3 2 v 2 (12.5⋅ 10 ) settling efficiency is 80%. c =o = =0.954 ⋅ 10−5 p g⋅ R 9.81 ⋅ 1.67

61 sedimentation water treatment

≈ 0.82 due to the amount of space they occupy. In practice, a tank will be a compromise between ≈ 0.41 the Reynolds and Camp numbers, on the one hand, and the construction costs, on the other, Vo limiting the length/width/depth ratios.

Figure 20 - Settling tanks with laminar and stable 4.2 Inlet constructions flows In the preceding paragraphs it was assumed that the water is uniformly distributed over the cross- over the height of the tank, it can be concluded section of the tank, but in practice this assump- that flocculant settling occurs. tion is not totally accurate. For an even distribution of the water over the From Figure 19 it can also be concluded that the width (and depth) of the tank, inlet constructions efficiency increases with the increasing depth. are introduced. For flocculant settling, in contrast to discrete set- In Figure 21 an example of an inlet construction tling, the height of the tank is of importance to the is represented. The inlet velocity is reduced by settling efficiency. introducing several inlet channels, followed by a diffuser wall that distributes the water over the entire cross-section of the tank. 4 Practice A diffuser wall (Figure 22) has openings to dis- 4.1 Determination of the dimensions of tribute the water over the width (and depth) of the an ideal settling tank tank. At the end of the wall, the flow velocity in In ideal settling tanks the flow is stable (cp > 10-5) the inlet channel is zero and so is the velocity without turbulence (Re < 2000). head. The head loss caused by friction, however, At a temperature of 10oC these conditions are is lower than the decrease in velocity head, re- met with a horizontal flow velocity and a hydrau- sulting in an increase in the piezometric level. lic radius of: The water level at the end of the inlet channel -3 vo = 6.4•10 m/s is, thus, higher than the level at the beginning. R < 0.41 m The result is that at the end of the inlet channel more water enters the tank than at the beginning Tanks that meet these conditions are short, wide of the inlet channel. To avoid this uneven distribu- and shallow or long, narrow and deep (Figure tion, the head loss over the openings in the dif- 20). fuser wall must be larger than the difference in These constructions, however, are expensive piezometric level induced by the decrease in flow

vi v

Q diffuser wall Q Q 4 Q sedimentation zone 0

2 energy li v0 ne 2g h z h z+$

Figure 21 - Inlet construction Figure 22 - Diffuser wall

62 water treatment sedimentation

H Vo

Vso VH

Clifford inlet Stuttgarter inlet L Figure 23 - Clifford and Stuttgarter inlet Figure 24 - Upward velocity to overflow weir velocity. L < 5 H In practices, more as well as alternative inlet constructions exist, like the Clifford and the Stuttgar- Most horizontal flow settling tank have an L/H>5 ter inlet (Figure 23). and thus:

Q <5 ⋅ H ⋅ v 4.3 Outlet constructions n⋅ B so The outlet construction is situated at the end of the settling tank and generally consists of an Therefore, the length of the overflow weir must overflow weir. be several times the width of the tank. At the outlet construction, re-suspension of set- To create sufficient length for the overflow weir, tled solids must be prevented and the flow ve- several troughs are placed parallel to each locity in an upward direction will thus be limited other(Figure 25). (Figure 24).

The flow velocity in an upward direction is: 4.4 Sludge zone and removal In the sludge zone the solids are accumulated. 1 Q v = ⋅ < v The removal of the sludge can be done hydrauli- H 5 B⋅ H so cally and mechanically. in which: vH =outflow velocity in an upward direction [m/s] Hydraulic sludge removal is done at regular intervals by dewatering the tank and flushing the Resulting in: sludge with pressured water (from hydrants) to a hopper at the bottom of the tank from where it is removed by gravity or by pumping.

Figure 25 - Overflow weir for effluent discharge

63 sedimentation water treatment

Mechanical sludge removal is frequently applied s < s0 does not settle when sludge volumes are large or the sludge is unstable, resulting in anaerobic decomposition The settling efficiency is entirely determined by during storage in the sludge zone. the particles that settle completely (see Figure Mechanical sludge removal consists of scrapers 9): that transport the sludge to a hopper in the mid- = − dle of a round settling tank or near the inlet of a r 1 po rectangular tank. From the hopper, the sludge is removed. The settling efficiency of discrete particles in vertical flow settling tanks is lower than in horizontal flow tanks, and vertical flow tanks are therefore 5 Settling tank alternatives not used for discrete, totally flocculated, suspen- sions. 5.1 Vertical flow settling tank In the case of flocculant settling, vertical flow In vertical flow settling tanks the inlet of the wa- tanks are used (e.g., in the form of floc blanket ter to be treated is situated at the bottom of the clarifiers). tank and the water flows in an upward direction (Figure 26). The flow velocity equals, in this case, the surface 5.2 Floc blanket clarifier loading: The floc blanket clarifier consists of a (conical) vertical flow tank (Figure 27). Q v= = s Coagulant is dosed at the inlet of the clarifier and o BL⋅ o floc formation occurs in the installation. Small, The result is that only particles with a settling ve- light flocs with a settling velocity lower than the locity higher than the upflow velocity will settle upflow velocity are transported with the water and others will be washed out: flow in an upward direction and collide with larger, heavier flocs. After attachment, the settling ve- s >= s0 settles completely locity increases until they reach the bottom of the

Q/A

V0 floc blanket H sludge sludge discharge discharge

A Vs Q

heavy sludge disposal possibility

Figure 26 - Vertical flow settling Figure 27 - Floc blanket installation

64 water treatment sedimentation

Vo a V‘ V /2 e so = so

b d Vo f c V‘so = Vso/2

a raw water feed Figure 29 - Tray settling b stirring mechanism c blending space d floc blanket The Reynolds number decreases and the Camp e clear water discharge number increases. The flow becomes less turbu- f sludge discharge lent and more stable. Figure 28 - Floc blanket installation at Berenplaat tank, where a sludge removal device is installed. 5.4 Tilted plate settling In tilted plate settling tanks, water passes baffles Flocs that do not form aggregates are transport- that are placed at a steep angle. ed to the top of the installation where the surface area is the largest and the upflow velocity the In Figure 30 an example is given of a counter- lowest, and a floc blanket is formed. current tilted plate settling tank. The water flows The floc blanket is manipulated by a drain at the in upward direction and the sludge settles on the required thickness. The floc blanket has a filter- plates and slides down. The angle of the plates ing effect for even small flocs. Therefore, high must be about 55o to 60o to secure sludge re- efficiencies can be realized with relatively short moval. residence times. In Figure 31 the flow in a counter-current tilted plate settling tank is pictured. The plant at Berenplaat in Rotterdam is where Using geometry the following surface loading can one of the floc blanket clarifiers in the Nether- be derived: lands is located (Figure 28). The inlet of the water w+ t is at the top of the installation. A stirring device so '= s o ⋅ H⋅ cos α + w creates turbulence in the floc formation chamber to increase the collision frequency. After leaving in which: the mixing chamber, the flocs form a blanket and so’ = vertical surface loading [m/h] the effluent water is drained by troughs. w = distance between plates [m] t = thickness of plates [m] H = height of plates [m] 5.3 Tray settling tanks α = angle of plates with the horizontal [o] The efficiency of discrete settling can be in- creased by applying horizontal baffles (false In a similar way the surface loading for a co-cur- floors or trays) (Figure 29). rent flow can be derived: In section 2.2 it is shown that the settling efficiency for discrete particles is independent of the height of the settling tank. The application of horizontal baffles gives a double surface area and half of Q the surface loading, resulting in an increase in efficiency. sedimentation zone, surface A Horizontal baffles also improve the flow pattern. Figure 30 - Counter-current tilted plate settling

65 sedimentation water treatment

Table 3 - Design parameters q Design parameters Value counter-current 550 - 600 co-current 300 - 400 H 1 - 3 m Vo w 3.4 - 8 cm

L t 5 mm H -5 V‘ In Figure 32 the stability boundary, cp > 10 , and t so the turbulence boundary, Re < 2000, are given. w In addition, the combinations of hydraulic radius and horizontal flow velocity of horizontal flow and A tilted plate tanks applied in practice are shown. From the graph it can be derived that the flow w sin A L cos A in horizontal flow tanks is turbulent and in some Figure 31 - Flow through a tilted plate settler cases instable (and short-circuit flow can occur). The flow in tilted plate tanks, however, is favora- w+ t ble. The Reynolds number is always smaller than so '= s o ⋅ H⋅ cos α − w 2000, resulting in laminar flow; and the Camp number is always higher than 10-5, resulting in a In Table 3 the design parameters of Figure 31 stable flow without short-circuiting. that are applied in practice are given. The angle of the plates in co-current systems can -5 be gentler than in counter-current systems with- 100.000 Cp > 10 <0.05 m/s out deteriorating the sludge removal. s 10.000 Re < 2,000 v Substituting the values of Table 3 into the equa- horizontal flow 1.000 sedimentation tanks tions for surface loading for both co-current and counter-current systems results in: 0.100 tilted plate

hydraulic radius [m] radius hydraulic sedimentation s s ' ≈ o 0.010 o 20 0.001 0.0001 0.001 0.01 0.1 1 10 The space occupied by tilted plate settling tanks horizontal flow speed v0 [m/s] is thus a factor 20 smaller than is needed for hori- Figure 32 - Hydraulic conditions for optimal settling zontal flow tanks.

Both the Camp number and the Reynolds number depend on the hydraulic radius and the horizontal flow velocity.