WATER TREATMENT WATER TREATMENT WATER 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 2 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 horizon- tal 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-cir- cuit 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 veloc- ity. 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 par- ticle, 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 dif- ference 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 set- tling 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<Re<1600, settling is in diameter [mm] transition between laminar and turbulent. Figure 5 - Settling velocity of discrete spherical parti- In Figure 4 the relationship between the drag co- cles efficient and the Reynolds number is represent- ed. in which: q = surface loading [m3/(m2•h)] In Figure 5 the settling velocity as a function of L = length of the tank [m] particle size and density is shown. In Figure 6 the trajectory of a particle is repre- sented. After t1 the water leaves the tank and 2.2 Horizontal flow settling tanks after t2 the particle is settled. The particles will in practice settle, therefore, when t2 <t1. In practice, settling occurs in flowing water. An The velocity of the particle is divided into horizon- ideal horizontal flow settling tank has the follow- tal and vertical components and the settling times ing characteristics: can be written as: - at the inlet the suspension has a uniform com- H L H B⋅ H ⋅ L 1 1 position over the cross-section of the tank t2≤ t 1 ⇒ ≤ ⇒ ≤ ⇒ ≤ ⇒vs ≥ q vs v 0 v s Q vs q - the horizontal velocity vo is the same in all parts of the tank - a particle that reaches the bottomH is L definitive H- B⋅ H ⋅ L 1 1 t2≤ t 1 ⇒ ≤ ⇒ ≤ ⇒ ≤ ⇒vs ≥ q ly removed from the process.vs v 0 v s Q vs q 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 set- tling velocity settles partly. The efficiency is deter- mined 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.