Centrifugation is a separation process which uses the action of centrifugal force to promote accelerated settling of particles in a solid-liquid mixture. Two distinct major phases are formed in the vessel during centrifugation :

The sediment

Usually does not have a uniform structure.

Find below an example of a sediment deposit :

The centrifugate or centrate which is the supernatant liquid.  

Often clear though sometimes cloudy, due to the presence of very fine colloidal particles that are not readily settled. However it may also contain several phases if the mixture’s interstitial liquid contains element with different densities, such as oils for example.

APPLICATION IN WATER TREATMENT

Centrifugal force

In a cylindrical vessel that rotates at an angular speed w(rad/s) or N (rpm) and contains a liquid ring of mean radius R (m) the centrifugal acceleration F_c (m/s) to which the particles are subjected is:

F_c = w²R = 0.011 N²R

The forced exerted on a particle per unit of weight is expressed by:

Fc = 0.011 N²R (rs – rL) x 1/g = G (rs – rl),

with G = w²R / g = 0.11 N²R / 9.81 = 11.2 x 10^-4 N²R
Where         r_s : density of particle
                  r_l  : density of interstitial liquid

Centrifuges

Centrifuges achieve separation by means of the accelerated gravitational force achieved by a rapid rotation. This can either replace normal gravity in the sedimentation of suspension or provide the driving force in the filtration through a filter medium of some kind.
The most common application is separation of solid substances from high concentrated suspensions. Used in this way for the treatment of sewage sludge it enables the dewatering with the production of more or less consistent sediment depending on the nature of the sludge to be treated, and the accelerated thickening of low concentration sludge. 

Principle
The separation is similar in principle to that achieved in a gravity separation process. The driving force is higher because is resulting from the rotation of the liquid: in the case of sedimentation, where the driving force is resulting from the difference in density between the solids particles and the liquid, the separation is achieved with a force from 1000 to 20000 times that of gravity.

Types
Most centrifuges rotate thanks to some kind of motor drive. The types of centrifuge used for sedimentation include:

• hydrocyclone

• tubular bowl

• chamber bowl

• imperforate basket

• disk stack separator

• decanter

Sedimenting centrifuges were invented for liquid solid separation and not for handling solids. It soon became apparent that these machines had wider applications, which would involve the presence of solid impurities, leading to use for separating solids from liquids.

Hydrocyclone

The simplest device to use centrifugal force to achieve separation is the hydrocyclone. It not really a centrifuge: the centrifugal separation is produced by the motion of the slurry, induced by the tangential introduction of the feed material. Its principle of operation is based on the concept of the terminal settling velocity of a solid particle in a centrifugal field. The following picture describes the conditions in an operating hydrocyclone.
 

The feed enters tangentially into the cylindrical section of the hydrocyclone and follows a circulating path with a net inward flow of fluid from the outside to the vortex finder on the axis. The centrifugal field generated by the high circulating velocities creates an air core on the axis that usually extends on the spigot opening at the bottom of the conical section through the vortex finder to the overflow at the top. In order for this to occur the centrifugal force field must be several times larger than the gravitational one. Particles that experience this centrifugal field will tend to move outwards relative to the carrier fluid because of their relatively greater density. The larger, heavier particles will migrate rapidly to the outside walls of the cylindrical section and will then be forced to move downward to the inside of the conical wall. Small particles will, on the other hand, be dragged inward by the fluid as it moves towards the vortex finder. The solid separation occurs in the passage of the suspension along the barrel of the hydrocyclone, to form thickened slurry at the outer wall, which than leaves the hydrocyclone as a continuous stream from its discharge nozzle.
 

Chamber bowl centrifuge

The chamber bowl centrifuge is a number of tubular bowl arranged co-axially. It has a main bowl containing cylindrical inserts that dived the volume of the bowl into a series of annular chambers, which operate in series. Feed enters the center of the bowl and the suspension passes through each chamber in turn, at increasing distances from the axis. The solids settle onto the outer wall of each chamber and the clarified liquid emerges as an overflow from the largest diameter chamber. This device provides also a classification of the suspended solids: the coarse particles deposit in the inner chamber and the increasingly fine particle deposit on the subsequent chambers. The removal of sedimented solids requires the stopping of rotation for manual cleaning.  

Disk stack separator

Imperforate basket centrifuge

Decanter

The decanter centrifuge is the only sedimentation centrifuge designed from the start to handle significant solid concentration in the feed suspension. At the same time it can achieve quite good degrees of clarification of the liquid concentrate. Although a complicated piece of machinery it embodies a simple principle. They consist basically of a horizontal cylindrical bowl (1) rotating at a high speed, with a helical extraction screw (2) placed coaxially. The screw perfectly fits the internal contour of the bowl, only allowing clearance between the bowl and the scroll. The differential speed between screw and scroll provides the conveying motion to collect and remove the solids, which accumulate at the bowl wall.

1. Cylindroconical bowl 2. Helical extraction screw (scroll) 3. Feed 4. Distributor 5. Ring space 6. Settled product 7. Liquid level 8. Drying zone 9. Clarified liquid 10. Adjustable tresholds

The product to be treated (3) is introduced axially into the unit by appropriate distributor (4). It is propelled into the ring space (5) formed by the internal surface of the bowl and the body of the scroll. The separation process basically takes place inside the cylindrical section of the bowl. The relative velocity of the scroll pushes the settled product (6) along into the bowl. The conveyance of the solids into the length of the cone enables the sediment to pass out of the clarified liquid phase. As the feed is continuous a liquid level (7) is established in the unit following a cylindrical surface that constitute the internal surface of the liquid ring. Once the solids have passed out of the liquid ring the remaining section of the cone all the way up to the ejector provide the final draining: this section is known as the drying zone (8). The clarified liquid (9) is collected at the other end of the bowl by flowing through the adjustable threshold (10), which restrict the liquid ring of the unit. A cover that enables the clarified liquid as well as the sediments to be collected protects the rotor. 

The decanter operates mainly by sedimentation a process causing the separation of suspended solids by virtue of their higher density than the liquid in which they are suspended. If the density difference is high than gravity may provide sufficient driving force for the separation to occur in a reasonable time. If the density difference is small, or the particle size is very small, than gravity separation would take too long and the separation force must be increased by the imposition of centrifugal forces many times that of gravity alone.

Advantages

The prime beneficial characteristic of the decanter is its ability to remove separated solids from the separation zone on a fully continuous basis.

By comparison with:

• Gravity sedimentation: the decanter can achieve separations that would be very difficult in a clarifier or lamella separator, and it produces drier solids.

• Hydrocyclones: the decanter has a much higher liquid capacity, can handle much higher slurry concentrations, and producer much drier solids.

• Tubular bowl centrifuges: the decanter offers higher capacities, the ability to handle concentrate slurries, and continuous operation.

• Imperforate basket centrifuges: the decanter operates continuously, can handle much higher solids concentrations, and produces much drier solids.

• Disk stack centrifuge: the decanter is truly continuous in operation, can handle much higher solids concentrations in the feed slurry and produces drier solids.

The advantages of the decanter are its wide range of potential use, coupled with its continuous operation, its ability to accept a wide range of feed concentrations, and its availability in a wide range of feed capacities.

Applications

References

• ‘Water treatment handbook’, Degremont, 1991

• Ken Sutherland, ‘Centrifuge focus: solids removal the option’, Filtration and separation, July-August 2005

• Alan Records, ‘Decanter centrifuge Handbook’, 2001

Related topics

Sludge treament:

Stabilisation
Thickening
Dewaterig
Sludge drying
Copious growth of filamentous organisms - problems and solutions

Filter presses