Filtration

PIERO M. ARMENANTENJIT

Filtration


Filtration• Filtration is a process by which suspended

solid particles are separated from a liquid bypassing the liquid through a porous, medium(e.g., a sand bed) capable of entrapping thesuspended particles.

• A pressure gradient generated across the filterbed is the driving force for filtration.


Pressure Gradient Generation inFiltration Operations

The pressure gradient in filtration can beproduced in a variety of ways including:

• gravity

• vacuum

• high pressure

• centrifugal forces


Classification of Solid-LiquidSeparation Processes

Using DensityAs a Driving Force

Sedimentation,Thickening

Flotation Centrifugation

Using Pressure GradientAs a Driving Force

Deep-BedFiltration

CakeFiltration

Cross FlowFiltration

Fixed WallCentrifugation

Rotating WallCentrifugation

Solid-Liquid Separation Processes


Filter MediumThe filter medium is the element that produces thefiltering action. Examples include:

• filter screens and supporting septa (e.g., afabric screen);

• beds of particulate materials (e.g., sand, coal);

• beds of solids screened from the solid-liquidsuspension (e.g., biosolids in sludgethickening) or a slurry (e.g., diatomaceousearth).


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 theparticles are removed throughout the filterbed or in a significant portion of it (e.g.,sand filters);

• Cake filtration, in which the particles areremoved on the surface of a cake formedby the solids accumulating on a septum(e.g., rotary vacuum filters).


Classification of Filtration SystemsFiltration systems can be classified according to:

• type of operation (batch vs. continuous)

• direction of fluid flow with respect of filtermedium (perpendicular vs. parallel)

• type of filter medium (e.g., screen, deep bed,cake)

• location within the filter medium where particledeposition occurs

• flow rate or pressure control during filtration(e.g., constant pressure drop)


Filtration Operations• Batch or semicontinuous filtration

− Periodical removal of solids is required(e.g., through backwashing)

− Pressure across and/or flow rate throughfilter change with time

• Continuous filtration

− Solids are continuously removed

− Pressure across and/or flow rate throughfilter are relatively constant with time


Process Variables Affecting Filtration• Flow rate of slurry

• Type of slurry and solid particles contained init

− Liquid viscosity

− Liquid density

− Solid concentration

− Particle size distribution

− Surface charge of particles

− Type and/or shape or particles


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


Particle Removal MechanismsInvolved in Filtration

• Mechanical straining

• Sedimentation on filter medium

• Impaction with filter medium

• Interception by contact with filter medium

• Flocculation

• Adhesion

− Chemical adsorption

− Physical adsorption


Particle Removal MechanismsInvolved in Filtration - Straining

After Metcalf and Eddy, Wastewater Engineering, 1991, p. 262


Particle Removal MechanismsInvolved in Filtration - Sedimentation

and Impaction



Particle Removal MechanismsInvolved in Filtration - Interception



Particle Removal MechanismsInvolved in Filtration - Adhesion



Particle Removal MechanismsInvolved in Filtration - Flocculation



Typical Applications of Filtration inWastewater Treatment

• Pretreatment of industrial wastewaterscontaining low concentrations of suspendedsolids (up to 100 ppm)

• Removal of solids after coagulation of colloidalsuspensions

• Final clarification (tertiary treatment) ofeffluent wastewater from (secondary)biological treatment

• Dewatering of slurries


Examples of Filtration Operations inHazardous Waste Treatment

• Filtration of the clarified effluent after settlingfollowing neutralization of acid wastewaterswith lime or limestone

• Dewatering of the sludge formed duringneutralization of acid wastewaters with lime orlimestone

• Filtration of the effluent from the clarifier afterheavy metal precipitation as hydroxides,sulfides or carbonates


Examples of Filtration Operations inHazardous Waste Treatment (cont' d)

• Dewatering of the sludge formed during heavymetal precipitation

• Dewatering of floating sludge after air flotationof wastewaters containing oily residues priorto sludge incineration

• Dewatering of (anaerobically or aerobically)digested activated sludge used in thetreatment of wastewaters containing toxicorganic priority pollutants prior to sludgeincineration


Limitations to the Use of FiltrationFiltration cannot:

• remove solutes in solution (although filtersthat are able to retain an active microbialpopulation can partially operate as bioreactorsand produce some degradation of solublematerials);

• separate chemical constituents present in thesame phase;

• be used to process viscous materials;

• be used to process solid wastes.


Cross-FlowFiltration


Cross-Flow Filtration• In cross flow filtration the slurry flows parallel

to the filter medium on one side of it. Only theclarified liquid can cross the filter medium andexit on the other side

• Because of the high velocity of the slurry thelevel of turbulence intensity on the slurry sideis quite elevated. This prevents the build-up ofa stable cake and reduces the rate of pressureincrease with time across the medium

• Cross-flow filters can be effectively used toclarify slurries containing up to 0.5% ofsuspended solids


Examples of Cross-Flow Filters

• Porous tube filters

• Microstraining filters


Example of a Cross-Flow Filter

Tangential filtrationFiltrate

Filtrate

IncomingSlurry

ConcentratedEffluent


Example of a Cross-FlowMicroscreen Filter

After Eckenfelder, Industrial Wastewater Pollution Control, p.383

Filtration

Engineering

Transcript of Filtration