Coagulation and

Flocculation

Coagulation and Flocculation

In coagulation operations, a chemical substance is added to an organic colloidal suspension to cause its destabilization by the reduction of forces

that keep them apart.

It involves the reduction of surface charges responsible for particle repulsions.

This reduction in charge causes flocculation (agglomeration).

Particles of larger size are then settled and clarified effluent is obtained.

The Process..

Negatively charged particles repel

each other due to electricity.

Neutrally charged particles

attract due to van der Waal's

forces.

Particles and coagulants join

together into floc.

Coagulation-Flocculation and Settling in a

Wastewater Treatment

Rapid Mixing and Flocculation

In the rapid-mix basins, intense mixing or agitation is required to disperse the chemicals uniformly throughout the basin and to allow adequate

contact between the coagulant and the suspended particles.

By the time the water leaves the rapid mix basins, the coagulation process has progressed sufficiently to form microfloc.

Rapid Mixing and Flocculation

In the flocculation basins, the fine microfloc begins to agglomerate into larger floc particles.

The aggregation process (flocculation) is dependent on duration and amount of gentle agitation applied.

By the time water leaves the flocculation basins, the floc has agglomerated into large, dense, rapid-settling floc particles.

The Agitator

Mechanical agitators (most common)

Pneumatic agitators

Baffle basins

Rapid Mixing and Flocculation

Based on T.R. Camp (1955), rapid mixing and flocculation are basically mixing operations, governed by the same principles and require similar design parameters.

Degree of mixing is based on the power imparted to the water, which is measured by velocity gradient.

Velocity Gradient for Mechanical or Pneumatic

Agitation

Velocity Gradient for Baffle Basins

Velocity Gradient

The rate of particulate collisions is proportional to the velocity gradient (G), therefore the gradient must be sufficient to furnish the desired particulate collisions.

The velocity gradient is also related to the shear forces in the water.

Large velocity gradients produce appreciable shear forces.

If the velocity is too great, excessive shear forces will result and prevent the desired floc formation.

Velocity Gradient

The total number of particle collisions is proportional to the

product of velocity gradient

(G) and the detention time

(T).

Thus, the value of GT is important in design.

Rapid Mixing

Mechanical agitation is the most common method for rapid mixing since it is reliable, very effective and extremely flexible in operations.

Usually rapid mixing employs vertical shaft rotary mixing devices such as turbine impellers, paddle impellers and propellers.

All of the rotary mixing devices impart motion to the water in addition of turbulence.

Types of Rapid Mixing Chambers or Basins

Most

common

used

Types of Rapid Mixing Chambers or Basins

Variable Speed Drives

Since the optimum velocity gradient may vary respect to time, it is desirable to have equipment with variable speed drives.

A speed variation of 1:4 is commonly used.

Mixing Basin

If only one chemical is added, a mixing basin with only one compartment may be used.

If more than one chemical is required, sequential application and dispersion of each chemical is desirable, necessitating multiple compartments.

Mechanical mixing basins are not affected to any extent by variations in the flow rate and have low head losses.

Detention Time and Velocity Gradient

Detention times from 20-60 sec are generally used. (range 10 sec 5

min)

To obtain high velocity gradients (700-1000 fps/ft), requires relatively

high mixing power levels.

Mixing Basins

Single compartment mixing basins are usually circular or square in plan view.

Fluid depth is 1.0 to 1.25 times the basin diameter or width.

Tanks may be baffled or unbaffled.

Small baffles are desirable since they minimize vortexing and rotational flow.

Turbine Impellers

Most

common

used

Turbine Impellers

The stationary vanes of the shrouded turbine prevent rotational flow.

The impeller blades maybe pithed and vertical (most common).

The diameter of the impeller is usually 30 to 50 percent of the tank diameter or width.

The impeller is usually mounted one impeller diameter above the tank bottom.

The speed ranges range from 10-150 rpm and the flow is radially outward from the turbine.

Flow Pattern

Small baffles extending into the tank a distance of 0.1 times the tank width or diameter will:

1. Minimize vortexing and rotational flow

2. Cause more power to be imparted to the liquid greater turbulence.

Turbine Impellers

Turbines are the most effective of all mechanical agitation or mixing devices because the produce high shear, turbulence and velocity

gradients.

Paddle impellers usually have two or four blades.

The blades may be pitched or vertical (most common).

The diameter is usually from 50 to 80 percent of the tank diameter or width.

The width of paddle is usually 1

6 to

1

10 of the diameter.

The paddles are usually mounted one-half of paddle diameter above tank bottom.

The paddle speeds 20 to 150 rpm.

Types of Paddle Impellers

Flow Regime

The flow regime for two-blade paddle is similar to the turbine impeller.

Baffling is required to minimize vortexing and rotational flow except at very slow speeds.

Paddle Impellers

The paddle is not as efficient as the turbine type since it does not produce as much turbulence and shear forces.

Propeller Impeller

May have two or three blades.

The blades are pitched to impart axial flow to the liquid.

Usually the pitch is 1.0 or 2.0 and the max propeller diameter is 18 inch.

Flow Regime

The rotation of a propeller traces out a helix in the liquid and the pitch is defined

as the distance the liquid moves axially

during one revolution, divided by the

propeller diameter.

The axial flow strikes the bottom of the tank and divides and imparts a flow

regime.

Propeller Impellers

For deep tanks two propellers may be mounted on the same shaft.

The propeller speed is ordinarily 400 to 1750 rpm.

Baffling is required in large tanks.

In small tanks the propeller may be mounted off center to avoid rotational flow.

Propeller agitators are very affective in large tanks because of high velocities imparted to the liquid.

Power Imparted to The Liquid

For turbulent flow (NRe >10.000), the power imparted by an impeller in a baffled tank is given:

Power Imparted to The Liquid

If the flow is laminar (NRe >10 to 20), the power imparted by an impeller in either a baffled or unbaffled tank is given:

Power Imparted to The Liquid

The reynolds number for impellers:

Power Imparted to The Liquid

For laminar flow, the power imparted in a tank is independent of the presence of baffles.

In turbulent flow, the power imparted in an unbaffled tank may be as low as one-sixth the power imparted in the same tank with baffles.

KL and KT

Power Imparted

For turbulent flow, the power required for agitation in a baffled vertical square tank is the same as in a baffled vertical circular tank having a diameter equal to the width of the square tank.

In an unbaffled square tank the power imparted is about 75 percent of that imparted in a baffled square of circular tank.

Two straight blade turbines mounted one turbine diameter apart on the same shaft impart about 1.9 times as much power as turbine alone.

Pneumatic Mixing Basins

Variation of velocity gradient may be obtained by varying the air flow rate.

Not affected to any extent by variations in the influent flow rate.

Hydraulic head losses are relatively small..

Pneumatic Mixing Basins

Power required can be determined by equation given.

The basin volume (V) may be determined from the flow rate and detention time (T).

Pneumatic Mixing Basins

The air flow rate to impart the desired power to the water may be determined by:

Baffle Type Basins

This type depends on hydraulic turbulence to furnish the desired velocity gradient.

The head loss usually varies from 1 to 3 feet.

These basins have very short circuiting.

Baffle basins are not suitable for conditions where there are wide variations in the flow rate.

It is not possible to vary the velocity gradient to any extent.

Because of that, baffle basins are not widely used.

Velocity Gradient in Baffle Type Basins

Flocculation

Mechanical agitation being the most common for flocculation.

Formerly, baffle basins were used, but since the available range of G and GT values is limited, they are not employed at present to any extent.

Most mechanical agitators are paddle wheels.

Flocculation

Flocculation

The degree of completion of the flocculation process is dependent on the floc characteristic, the velocity gradient, and the value of GT.

GT is related to total number of collisions during aggregation in flocculation process.

High GT indicates a large number of collisions during flocculation.

GCT where C is the ratio of the floc volume to the total water volume being flocculated.

Flocculation

If the velocity gradient is too great, the shear forces will prevent the formation of large floc.

If velocity gradient is insufficient, adequate inter particulate collisions will not occur and proper floc will not be formed.

If water coagulates readily, a high strength floc usually results and final velocity gradient may be as large as 100 fps/ft.

Flocculation Basins

Flocculation basins are frequently designed to provide for tapered flocculation.

The flow is subjected to decreasing G values as it passes trough the flocculation basin.

This produces a rapid buildup of small dense floc the aggregates at lower G values into larger, dense, rapid settling floc particles.

Accomplished by providing a high G value in the first third of flocculation period, a lower G during the next third and much lower G during the last third.

Horizontal Shaft Paddle Wheel Flocculator

(Cross Flow Pattern)

Horizontal Shaft Paddle Wheel Flocculator

(Axial Flow Pattern)

Vertical Shaft Paddle Wheel Flocculator

Drag Force

Power Imperted

CD

Peripheral velocity should range from 0.3 to 3 fps.

The velocity of a paddle blade relative to the water is three-fourths the peripheral blade velocity.

The total paddle-blade area on a horizontal shaft should not exceed 15 to 20 percent of total basin cross sectional area (or excessive rotational flow will result!).