STUDY OF THE SEDIMENTATION PROCESS OF SUSPENDED SOLIDS IN HORIZONTAL SEDIMENTATION TANKS FOR WASTEWATER

ABSTRACT

Sedimentation is the simplest and most commonly used method of separating coarse impurities from wastewater, which, under the influence of gravitational force, settle to the bottom of the settling tank or float to its surface. Depending on the required degree of wastewater treatment, sedimentation is used either for pretreatment before treatment at other, more complex facilities, or as a method of final purification, if local conditions require only undissolved (precipitating or floating) impurities to be isolated from wastewater.

АННОТАЦИЯ

Осаждение - это самый простой и наиболее часто используемый метод отделения крупных примесей от сточных вод, которые под действием силы тяжести оседают на дно отстойника или всплывают на его поверхность. В зависимости от требуемой степени очистки сточных вод осаждение используется либо для предварительной обработки перед очисткой на других, более сложных установках, либо в качестве метода окончательной очистки, если местные условия требуют выделения из сточных вод только нерастворенных (осаждающихся или плавающих) примесей.

Keywords: Settling, horizontal sump, solid particles, wastewater, flocculation, Reynolds number.

Ключевые слова: Отстаивание, горизонтальный отстойник, твердые частицы, сточные воды, флокуляция, число Рейнольдса.

Introduction. The primary sedimentation tank is usually located after the grain chamber. This is where as much of the settling undissolved particles as possible are separated. This sludge is called primary sludge [11-9]. In some plants (oxidation ditch types) where no primary sedimentation tank is installed, the undissolved particles are trapped in the activated sludge and stabilized there. The sand particles are basically called granular particles, which settle unchanged (undetectable sedimentation) [10-2]. It is also well known that during the settling process, particles become larger as a result of particle agglomeration or flocculation (flocculent settling). This process can be stimulated by agitation, the addition of chemicals and biological agents. Flocculation occurs at higher concentrations of undissolved particles; it is clearly the primary sedimentation.

Problem Statement and Solution Method. Deposition is a treatment process that removes suspended particles such as flakes, sand, and clay from water. Sedimentation is often used in surface water treatment to avoid rapid clogging of sand filters after coagulation and flake formation. Sedimentation is used in groundwater treatment plants for backwash treatment. In horizontal-flow settling tanks (Fig. 1), water is evenly distributed across the cross-sectional area of the tank in the inlet area. Stable, non-turbulent flow in the sedimentation zone ensures sedimentation of suspended solids in the sedimentation zone. The sediment accumulates at the bottom or is permanently removed.

Figure 1. Horizontal flow sump

Sedimentation occurs because of the density difference between the suspended solids and the water.

The following factors influence the settling process: density and size of suspended solids, water temperature, turbulence, flow stability, bottom cleaning and flocculation:

• density - the higher the particle density, the faster the particles settle.
• size - the larger the particles, the faster they settle.
• temperature - the lower the temperature of the water, the greater the viscosity, therefore the slower the particles settle.
• turbulence - the more turbulent the flow, the slower the particles settle.
• stability - instability can lead to overflow, affecting particle settling
• bottom cleaning - during bottom scour, settled particles resuspended and washed
• flocculation - with wastewater flocculation results into larger particles, increasing the rate of settling.

Deposition of discrete particles

Individual particles do not change their size, shape, or weight during settling (and therefore do not form aggregates).

A discrete particle in a liquid will settle by gravity [8, 3-5].  The particle will accelerate as long as the drag force of the fluid in friction does not equal the value of gravity, after which the vertical (settling) velocity of the particle will be constant (Fig. 2).

Figure 2. Forces acting on settling particle

The upward force acting on the particle, caused by the resistance of the fluid during friction, can be calculated using:

Where:

Fup  directions of upward force due to friction,        [N]

drag coefficient                                                     -

ρ   water density []

 settling velocity [  мs ]

A  – projected particle area []

The downward force caused by the difference in density between the particle and the water can be calculated using:

Where:

 downward flow under the action of gravity     [Н]

            specific density of particles []

              gravitational constant [м²]

       particle volume []

Figure 3. Relationship between Reynolds number and drag coefficient

The equality of both forces, assuming a spherical particle, gives as settling velocity:

where:

d - diameter of the spherical particle

Thus, the rate of deposition depends on:

• particle density and liquid density
• particle diameter (size)
• the flow pattern around the particle.

The flow pattern around the particle is accounted for in the drag coefficient. The value of the drag coefficient is not constant, but depends on the value of the Reynolds number for settling. For spherical particles the Reynolds number is given by the formula:

where:   - kinematic viscosity    [м²]

In the drinking water treatment practice, laminar settling usually takes place. The Reynolds number for laminar deposition of spheres is Re<1, which leads to the following relationship between the Reynolds number and drag coefficient:

Replacing this relationship in the equation for deposition rate gives the Stokes equation:

Thus, the deposition rate depends on the viscosity of the fluid as well as the temperature. The relationship between kinematic viscosity and temperature is:

Where:

T – temperature [°С]

When the Reynolds number Re > 1600, the deposition is turbulent, and when 1<Re<1600, the deposition is in a transition state between laminar and turbulent. Figure 3 shows the relationship between drag coefficient and Reynolds number. The horizontal flow sedimentation tank is 2 m high, 20 m wide, and 45 m long. The flow rate through the tank is 0.5 m³/s and the water temperature is 10 °C. We have to check if the tank meets the hydraulic requirements. The horizontal flow rate and the critical settling velocity are equal:

The hydraulic radius of the tank can be calculated with:

Reynolds number and Camp number are equal:

The Reynolds number exceeds 2000, and the flow will be turbulent. The Camp number is approximately 1-10^-5, and no short-circuiting will occur. Let's determine the efficiency of the settling tank with the slurry from the sedimentation test in Table 1. Figure 5 shows the suspended solids content as a function of water depth at different sampling times from Table 1. Between 0 and 0.75 meters, the progress of the graph (dashed line) is estimated [9, 6-7].  The residence times are:

Table 1.

Relative particle concentration as a result of the precipitation test

Figure 4. Distribution of the total frequency of setting velocities velocities at different tank depths

Figure 5. Relative concentration of particles

Conclusions. Based on the calculations, we can conclude that, assuming a residence time of 3600 seconds and a height of 2 meters, from Figure 19, the blue surface above the line indicates the amount of deposited solids, and the green surface below the line shows the amount of solids that are still in suspension. After measuring the surfaces, it can be concluded that the settling efficiency is more than 80%.

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