Sedimentation

Sedimentation is the most common physical unit operation in wastewater treatment, more so in primary treatment where sedimentation is the workhorse of the treatment. The term sedimentation is also called settling in some literature. Sedimentation is, in a nutshell, a process by which the suspended solids, which have higher densities than that of water, are re-

Figure 3.5. A photo of a dissolved air flotation system.

moved from wastewater by the action of gravity in the bottom of the settling tank or basin (also called a clarifier) within a reasonable period of time. Sedimentation basins are usually rectangular or circular with a radial or upward water flow pattern. Sedimentation is not limited to primary treatment; there is also secondary sedimentation by which settleable solids in the biological secondary treatment processes are removed. For example, recovery of activated sludge for recycling is achieved with secondary sedimentation.

In a typical wastewater tretment plant, the wastewater stream exiting from screening devices (and after flotation basins) then enters the second section of the primary treatment of wastewater treatment or sedimentation tanks/basins. Here, the sludge (the organic portion of the sewage) settles out of the wastewater and is pumped out of the tanks. Some of the water is removed in a step called thickening and then the sludge is processed in large tanks called digesters.

Sedimentation uses gravitational force to separate unstable and destabilized suspended solids from wastewater. It is based on the density differ ence between the bulk of the liquid and the solids. Stabilized solids such as colloids can be destabilized with flocculants (see the section "Coagulation and Flocculation" below). Sedimentation is a very important primary treatment process; it is, however, also used in the biological treatment, such as activated sludge and trickling filters for solid removal. The settling characteristics of the solids are determined by the types of the settling solids and their concentrations. Sedimentation has four distinct types of settling:

• Discrete settling (Type I), which is settling of a dilute suspension of solids that do not aggregate.

• Flocculent settling (Type II), which is settling of the particulates that aggregate among themselves and/or with added flocculants to form larger particulates and therefore results in faster settling. The sedimentation operation in a typical primary treatment of wastewater operates in this mode.

• Zone settling (Type III, also called hindered settling), which occurs when particulates adhere together, forming a mass that settles as a blanket with a distinguishable interface with the liquid above it.

• Compression zone (Type IV), which occurs when sinking particulates accumulate at the bottom of the sedimentation tank/basin, forming a compressed structure that supports the weight of the particulates that settled in the bottom of the tank/basin.

Although sedimentation basins in primary treatment are characterized by Type II flocculant settling, each of these zones have different characteristics that warrant further analysis.

Discrete settling (Type I)

The settling of nonaggregated solids in a dilute suspension can be described by its settling velocity of individual particulates. In a settling tank/ basin, the settling of a discrete particle is not affected by the other particles and is only a function of the fluid property and the characteristics of the particle; this may be further depicted in Fig. 3.6 when the movement of the particle of interest is subject to the combined effect of the gravitational force downward and the bulk flow toward the outlet (Equation 3.1):

vt = (tank depth)/(residence time)

Figure 3.6. A schematic diagram of discrete settling.

or mathematically (Equation 3.2): vt = H/t

H is the depth of the sedimentation tank and t is the residence time of the particle in the liquid in the tank. If we assume the residence time of the liquid is the residence time of the liquid in the tank, then it is the following (Equation 3.3):

A is the cross-sectional area of the tank and Q is the overall volumetric flow rate through the tank. Here, Q/A is the overflow rate of the liquid passing through the tank. So, we have the following (Equation 3.4):

The terminal velocity of the particle is equal to the overflow rate of the tank.

For readers who have some exposure to fluid mechanics, the above derivation might strike them as being suspicious, since the terminal velocity of a discrete particle in the liquid follows Stoke's law (Equation 3.5):

where d is the size (equivalent diameter of the particle), g is the gravitational acceleration, ^ is the viscosity of the liquid, and (ps — pl) is the density difference between the particle and the liquid.

This suggests that the terminal velocity is a function of size and the density of the particle, which is nowhere to be inferred from Equations 3.2-3.4. Note that we have assumed that the residence time of the particle in the tank is the same as that of the bulk liquid or the equivalent depth for the particle settling in the tank is the same as the depth of the tank. The overflow rate of the tank as the terminal velocity of a particle in the tank represents the critical velocity of an ideal particle in an ideal settling tank assuming

• The number of ideal discrete particles and the velocity vectors of the liquid are uniformly distributed.

• The liquid flows in the tank as an ideal slug.

• Any particle reaching the bottom of the tank is effectively removed (no resuspension) (Canale and Borchardt, 1972)

Any ideal particles having termial velocity v (average velocity among all particles present) greater than vt is 100% removed from the settling tank/basin. For those particles with less than vt average terminal velocity, the portion of the particles removed in the tank is equal to v/vt.

In reality, the discrete settling is more likely associated with settling of hard particulates with high density and size such as grit and sand. This is a rare type of particulate in a typical food wastewater stream, but it may occur in some sources of agricultural wastewater that is subject to intrusion of soil and dirt.

Flocculent settling (Type II)

Flocculent settling is used in primary clarifiers and the upper zones of secondary clarifiers. In the case of flocculent settling, the particles in the relatively dilute suspension coalesce or flocculate to form larger particles or aggregates during settling, thus increasing the mass of settling solids as well as the settling velocity (and removal rate). In many food wastewater treatment situations, except very dilute ones, suspended solids cannot be described as discrete particles of known specific gravity (a quantity that is the ratio of density of particle to density of water). In general, larger particles settle faster and have a greater tendency to collide with other slower-settling particles, resulting in formation of larger particles in a quiescent body of water. However, the wind, hydrodynamic shear, and hydraulic disturbance all contribute to further contacts among particles in the tank.

Sample ports 60cm apart

Time

Figure 3.7. A diagram of settling column and zoning settling process.

Time

Figure 3.7. A diagram of settling column and zoning settling process.

Furthermore, the greater the depth of the tank, the higher the frequency of collisons among particles will be during settling. Therefore, the flocculent settling is dependent on the properties of particles and the liquid as well as depth of the settling tank/basin. The settled solids in the bottom of the tank are usually promptly removed, so the greater rate of settling as a result of aggregation of individual particles translates into a greater rate of solid removal from the wastewater. Evaluation of a wastewater stream slated for a sedimention tank or basin is carried out using a settling column, as depicted in Fig. 3.7. The laboratory of the settling column is about 15 cm (6 in) in diameter and 305 cm (10 ft) tall, and it has several sampling ports 61 cm (2 ft) apart. The settling evaluation is conducted by first placing a known quantity of wastewater sample in the column. The uniformity of particle size from top to bottom of the column in the beginning of the evaluation and the temperature of the liquid throughout the evaluation should be accomplished. The wastewater containing suspended solids is allowed to settle under quiescent conditions; small samples of suspension at different ports with preset depths are drawn and concentrations of particles in the samples are determined over preset time intervals. The frac

Time

Figure 3.8. A fraction of removal of flocculating particles at each depth.

Time

Figure 3.8. A fraction of removal of flocculating particles at each depth.

tion removal of the particles is calculated for each sample analyzed and is plotted against time and depth. The fraction of the particles removed at each depth is constructed as curve lines called isoconcentration lines, as those illustrated in Fig. 3.8. These lines represent the most efficient particle removal loci for a given removal rate. The ratio of the depth to time is the average settling (terminal) velocity of the particles under a given percent removal.

Zone settling (Type III)

Zone settling, also called hindered settling, acquires its name from the fact that aggregated particulates of a concentrated suspension (activated sludge or flocculated colloids) in the sedimentation basin tend to form a massive blanketlike suspension with a distinct interface. Zone settling is mainly used in secondary clarifiers. Many wastewater treatment process designers use a batch settling test to determine the interface.

Chapter 3: Physicochemical Wastewater Treatment Processes 65 Compression zone (Type IV)

Compression settling involves the highest concentration of suspended solids and occurs in the lower reaches of clarifiers. The particles settle by compressing the mass of the particles below. Compression occurs not only in the lower zones of secondary clarifiers but also in sludge thickening tanks.

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