2. PIERO M. ARMENANTE
NJIT
Filtration
• Filtration is a process by which suspended
solid particles are separated from a liquid by
passing the liquid through a porous, medium
(e.g., a sand bed) capable of entrapping the
suspended particles.
• A pressure gradient generated across the filter
bed is the driving force for filtration.
3. PIERO M. ARMENANTE
NJIT
Pressure Gradient Generation in
Filtration Operations
The pressure gradient in filtration can be
produced in a variety of ways including:
• gravity
• vacuum
• high pressure
• centrifugal forces
4. PIERO M. ARMENANTE
NJIT
Classification of Solid-Liquid
Separation Processes
Using Density
As a Driving Force
Sedimentation,
Thickening
Flotation Centrifugation
Using Pressure Gradient
As a Driving Force
Deep-Bed
Filtration
Cake
Filtration
Cross Flow
Filtration
Fixed Wall
Centrifugation
Rotating Wall
Centrifugation
Solid-Liquid Separation Processes
5. PIERO M. ARMENANTE
NJIT
Filter Medium
The filter medium is the element that produces the
filtering action. Examples include:
• filter screens and supporting septa (e.g., a
fabric screen);
• beds of particulate materials (e.g., sand, coal);
• beds of solids screened from the solid-liquid
suspension (e.g., biosolids in sludge
thickening) or a slurry (e.g., diatomaceous
earth).
6. PIERO M. ARMENANTE
NJIT
Types of Filtration Operations
• Cross-flow filtration, in which a septum is
responsible for the filtering action (e.g.,
microscreens);
• Depth (or deep-bed) filtration, in which the
particles are removed throughout the filter
bed or in a significant portion of it (e.g.,
sand filters);
• Cake filtration, in which the particles are
removed on the surface of a cake formed
by the solids accumulating on a septum
(e.g., rotary vacuum filters).
7. PIERO M. ARMENANTE
NJIT
Classification of Filtration Systems
Filtration systems can be classified according to:
• type of operation (batch vs. continuous)
• direction of fluid flow with respect of filter
medium (perpendicular vs. parallel)
• type of filter medium (e.g., screen, deep bed,
cake)
• location within the filter medium where particle
deposition occurs
• flow rate or pressure control during filtration
(e.g., constant pressure drop)
8. PIERO M. ARMENANTE
NJIT
Filtration Operations
• Batch or semicontinuous filtration
− Periodical removal of solids is required
(e.g., through backwashing)
− Pressure across and/or flow rate through
filter change with time
• Continuous filtration
− Solids are continuously removed
− Pressure across and/or flow rate through
filter are relatively constant with time
9. PIERO M. ARMENANTE
NJIT
Process Variables Affecting Filtration
• Flow rate of slurry
• Type of slurry and solid particles contained in
it
− Liquid viscosity
− Liquid density
− Solid concentration
− Particle size distribution
− Surface charge of particles
− Type and/or shape or particles
10. PIERO M. ARMENANTE
NJIT
Process Variables Affecting Filtration
• Type and properties of filter medium
− Medium average particle size and shape
− Medium particle size distribution
− Medium surface charge
− Medium density
− Medium void fraction (porosity)
− Mesh size opening
• Height of filter medium
• Allowable pressure drop across filter
11. PIERO M. ARMENANTE
NJIT
Particle Removal Mechanisms
Involved in Filtration
• Mechanical straining
• Sedimentation on filter medium
• Impaction with filter medium
• Interception by contact with filter medium
• Flocculation
• Adhesion
− Chemical adsorption
− Physical adsorption
12. PIERO M. ARMENANTE
NJIT
Particle Removal Mechanisms
Involved in Filtration - Straining
After Metcalf and Eddy, Wastewater Engineering, 1991, p. 262
13. PIERO M. ARMENANTE
NJIT
Particle Removal Mechanisms
Involved in Filtration - Sedimentation
and Impaction
After Metcalf and Eddy, Wastewater Engineering, 1991, p. 262
14. PIERO M. ARMENANTE
NJIT
Particle Removal Mechanisms
Involved in Filtration - Interception
After Metcalf and Eddy, Wastewater Engineering, 1991, p. 262
15. PIERO M. ARMENANTE
NJIT
Particle Removal Mechanisms
Involved in Filtration - Adhesion
After Metcalf and Eddy, Wastewater Engineering, 1991, p. 262
16. PIERO M. ARMENANTE
NJIT
Particle Removal Mechanisms
Involved in Filtration - Flocculation
After Metcalf and Eddy, Wastewater Engineering, 1991, p. 262
17. PIERO M. ARMENANTE
NJIT
Typical Applications of Filtration in
Wastewater Treatment
• Pretreatment of industrial wastewaters
containing low concentrations of suspended
solids (up to 100 ppm)
• Removal of solids after coagulation of colloidal
suspensions
• Final clarification (tertiary treatment) of
effluent wastewater from (secondary)
biological treatment
• Dewatering of slurries
18. PIERO M. ARMENANTE
NJIT
Examples of Filtration Operations in
Hazardous Waste Treatment
• Filtration of the clarified effluent after settling
following neutralization of acid wastewaters
with lime or limestone
• Dewatering of the sludge formed during
neutralization of acid wastewaters with lime or
limestone
• Filtration of the effluent from the clarifier after
heavy metal precipitation as hydroxides,
sulfides or carbonates
19. PIERO M. ARMENANTE
NJIT
Examples of Filtration Operations in
Hazardous Waste Treatment (cont' d)
• Dewatering of the sludge formed during heavy
metal precipitation
• Dewatering of floating sludge after air flotation
of wastewaters containing oily residues prior
to sludge incineration
• Dewatering of (anaerobically or aerobically)
digested activated sludge used in the
treatment of wastewaters containing toxic
organic priority pollutants prior to sludge
incineration
20. PIERO M. ARMENANTE
NJIT
Limitations to the Use of Filtration
Filtration cannot:
• remove solutes in solution (although filters
that are able to retain an active microbial
population can partially operate as bioreactors
and produce some degradation of soluble
materials);
• separate chemical constituents present in the
same phase;
• be used to process viscous materials;
• be used to process solid wastes.
22. PIERO M. ARMENANTE
NJIT
Cross-Flow Filtration
• In cross flow filtration the slurry flows parallel
to the filter medium on one side of it. Only the
clarified liquid can cross the filter medium and
exit on the other side
• Because of the high velocity of the slurry the
level of turbulence intensity on the slurry side
is quite elevated. This prevents the build-up of
a stable cake and reduces the rate of pressure
increase with time across the medium
• Cross-flow filters can be effectively used to
clarify slurries containing up to 0.5% of
suspended solids
