SEDIMENTATION

Dr Vesna MarjanovićHIGH BUSINESS-TECHNICAL SCHOOL

2013.

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Aims and outcomes:

• To provide the general information of the technologies ofsedimentation and relationship of this operation within awastewater treatment plant facility.

• Students will obtain a good understanding of the practicalaspects of sedimentation practices for a municipal watertreatment facility.

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INTRODUCTION

• The principles presented in this lecture could be applied to the class of unit operations aimed at separation, handling and processing of heterogeneous or two-phase system. These principles are the basic principles that underlie the traditional ex-situ remediation method.

• The widespread and successful application of these hydrodynamic processes is based on ability to take advantage of one or a combination of five primary forces: gravity, centrifugal , buoyancy, pressure, and electric.

• Examples of operations:- These operations involve the simultaneous flow of gas and solid,

or liquid and solid phases

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- and found applications where heterogeneous system are included:(a) sedimentation of dust in chambers and cyclone separators, (b) separationof suspensions in settlers, (c) separation of liquid mixtures by settling and centrifuging, (d) hydraulic and pneumatic transport, (e) hydraulic and air classification, flotation, mixing by air and

others.• Gravity primary forces:

- Gravity is the controlling force for separations achieved in settlers.

- Gravity thickening process involves the concentration of thin sludges to more dense sludge in special circular tanks.

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• Applications:- Its use is largely restricted to the watery excess sludge from the

activated sludge process, and in large plants of this type where the sludge is sent direct to digesters instead of to the primary tanks.

- It may also be used to concentrate sludge to primary tanks or a mixture of primary and excess activated sludge prior to high rate digestion.

• Equipment and operation- The thickening tank is equipped with slowly moving vertical

paddles built like a picket fence.- Sludge is usually pumped continuously from the settling tank to

the thickener which has a low overflow rate so that the excess water overflows and the sludge solids concentrate in the bottom.

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- A blanket of sludge is maintained by controlled removal which may be continuous at a low rate.

- A sludge with a solids content of ten percent or more can be produced by this method.

- This means that with an original sludge of two percent, about four-fifths of the water has been removed, and one of the objectives in sludge treatment has been attained.

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GRAVITY SEDIMENTATION

• Sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling.

• This operation is divided into:- thickening, i.e., increasing the concentration of the feed stream,

and- clarification, i.e., removal of solids from a relatively dilute

stream.• A thickener is a sedimentation machine that operates according to the

principle of gravity settling.

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THICKENERS AND CLARIFIERS

• Thickener’s principal advantages are:- simplicity of design and economy of operation;- its capacity to handle extremely large flow volumes; and- versatility, as it can operate equally well as a concentrator or as a

clarifier.• In a batch-operating mode:

- a thickener normally consists of a standard vessel filled with a suspension;

- after settling, the clear liquid is decanted and the sediment removed periodically.

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• Figure illustrates a cross-sectional view of a standard thickener.

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• The operation of a continuous thickener is also relatively simple:(1) A drive mechanism powers a rotating rake mechanism.(2) Feed enters the apparatus through a feed well designed to

dissipate the velocity and stabilize the density currents of the incoming stream.

(3) Separation occurs when the heavy particles settle to the bottom of the tank.- Some processes add flocculants to the feed stream to enhance

particle agglomeration to promote faster or more effective settling.

(4) The clarified liquid overflows the tank and is sent to the next stage of a process.

(5) The underflow solids are withdrawn from an underflow cone by gravity discharge or pumping . do ovog dela – jedna strana

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• Thickeners can be operated in a countercurrent fashion :- involves streams of liquid and thickened sludge moving

countercurrently through a series of thickeners- applications are aimed at the recovery of soluble material from

settleable solids• On Figure is shown the basic scheme of a three-stage continuous

countercurrent decantation (CCD) system:

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- the thickened stream of solids is depleted of soluble constituents as the solution becomes enriched

- in each successive stage, a concentrated slurry is mixed with a solution containing fewer solubles than the liquor in the slurry and then is fed to the thickener

- as the solids settle, they are removed and sent to the next stage- the overflow solution, which is richer in the soluble constituent, is

sent to the preceding unit- solids are charged to the system in the first-stage thickener, from

which the final concentrated solution is withdrawn- wash water or virgin solution is added to the last stage, and

washed solids are removed in the underflow of this thickener

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• Continuous clarifiers handle a variety of process wastes, domestic sewage and other dilute suspensions.

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• Figure shows example of cylindrical clarifier - thickener in that sedimentation tanks or basins whose sludge removal is controlled by a mechanical sludge-raking mechanism:

- most cylindrical units are equipped with peripheral weirs; however, some designs include radial weirs to reduce the exit velocity and minimize weir loadings

- the unit shown also is equipped with adjustable rotating overflow pipes

- the feed enters up through the hollow central column or shaft (a siphon feed system)

- the feed enters the central feed well through slots or ports located near the top of the hollow shaft.

- siphon feed arrangements greatly reduces the feed stream velocity as it enters the basin proper - this tends to minimize undesirable cross currents in the settling region of the vessel

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RECTANGULAR SEDIMENTATION TANKS

• An essential prerequisite for upgrading sedimentation tanks is: - Beside the clarified water discharge, the feeding method for the

sludge/water mix and the skimmer system have an essential influence on the separation efficiency in tanks.

- The inlet height is approx. 2/3 to 1/3 of the tank depth and the skimming direction in the counter flow. This concept is based on the empirical knowledge of normally minor turbulence in the tank.

- However, changed process concepts with a bottom-near inlet and concurrent skimming are able to minimize such turbulences to such an extent that the sludge load and thus the separation efficiency can be increased.

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• Figure shows a rectangular settling basins in operation

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• Of particular importance is the design of the inlet section (as turbulences are generated there by mixing with the wastewater inflow which may have an intense influence on the sedimentation process).- The density of flows has a strong influence on the separation

efficiency. - To increase the separation efficiency of tanks, the density flow

should be minimized.• Important of inlet dimensions

- The density flow can be substantially influenced by the inlet structure by minimizing the potential and kinetic energy of the wastewater stream with a suitable feed design.

- The inlet should have bottom-near feed openings to have an as small intermixing zone as possible between the sludge/water mix and the tank content.

- The velocity gradient in the inlet section should be small to avoid floc disturbance by shearing forces.

- The inlet section sludge scraping also influences the separation efficiency, most notably it impacts on the degree of thickening in the sedimentation tank.(a) Practice has shown that scraping the sludge in the direction of

the density flow works best.(b) The scraping velocity should be low in order to prevent re-

suspension of the activated sludge flocs.

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THE LAMELLA CLARIFIER

• Thickening of the sludge- To increase the degree of thickening and to minimize the volume

flow of the return sludge, a minimum sludge residence time in the tank must be provided.

- Although a sludge hopper for thickening the sludge is not necessary, a sludge hopper at the tank end tends to increase the surface load of the tank.

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• Cross-flow lamellar clarification is a technology used in industrial environments to remove oils and solids from residual water.

• Applications:- The process is mainly used for dealing with oils and grease

present in residual liquids emanating from industrial activities in the petrochemical, chemical, mechanical, metallurgical and food-processing sectors.

- This technology is designed to handle effluents containing a maximum of 10,000 mg/l of grease, and 3,000 mg/l of solids.

• This process is the combination:- of natural flotation ( natural tendency of oils to float) and- clarification techniques (decantation principle for suspended

solids that are denser than water)

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• Equipment• The cross-flow lamellar clarifier consists of the following basic

units:- primary screening chamber- separator plate cell- sludge silo- oil and grease storage chamber

• Operations:- First, water is pretreated in the primary screening chamber to

remove part of the floating oil and grease and to allow sedimentation of large solid particles.

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- Then the effluent feeds through the plate cell where separation of phases is accomplished as follows:(a) oils are deflected upwards by the plates to form a film on the

surface of the water,(b) sludges settle to the bottom and the purified water flows

horizontally to the reservoir outlet.• Treatment of sludge

- The sludge is kept apart from water to be treated, so as not to draw it back into the process flow.

- Sludges are recovered in a conical extraction silo which aids their compaction and provides easier handling.

- Dryness rates of 1 to 5% are achieved depending on sludge type.

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• Treatment of oilsFloating oils are recovered from the water surface and channeled into a storage reservoir located beneath the sludge silo.

• Coalescer- If the oils are partially or completely emulsified, the cross-flow

lamellar clarifier can be equipped with a coalescer.- The coalescer uses a physical process to trigger separation of the

oil and water phases.- The coalescer is filled with various elements (rings, plastics,

honeycombs, other appropriate materials...) which maximize the potential contact surface.

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• Removal of free oils and greases, and of suspended solids- It varies from 90 to 99%.- With no chemical amendment (i.e. demulsifying agent), 20-40%

removal of emulsified oils and greases can be achieved.- The addition of an agent enables the process to achieve 50-99%

removal, depending on the application.

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REFERENCES:

HANDBOOK OF WATER AND WASTEWATER TREATMENT TECHNOLOGIES,Nicholas P. Cheremisinoff, Ph.D., Butterworth-Heinemann, 2002, ISBN: 0-7506-7498-9, Chapter 8

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RECOMMENDED REFERENCES:Water Treatment - Principles and Design, By: Crittenden, John C.; Trussell, R. Rhodes; Hand, David W.; Howe, Kerry J.; Tchobanoglous, George, 2005 John Wiley & Sons John Wiley & Sons, ISBN 471110183

Karolien Vanbroekhoven, Francis Van den Broeck, Karel Feyaerts, Eric Van den Broeck, Paul Verkaeren, Jan Verstraelen, Ludo Zeuwts, Johan Gemoets and Ludo Diels, In situ precipitation for remediation of heavy metal contaminated groundwater at a non ferrous industrial site, IMWA Symposium 2007: Water in Mining nvironments, R. Cidu & F. Frau (Eds), 27th - 31st May 2007, Cagliari, Italy

THERMAL DESORPTION

1

Dr Vesna MarjanovićHIGH BUSINESS-TECHNICAL SCHOOL

2013.

Aims and outcomes:

• To provide overview of different methods and combinations oftechniques which used heating of polluted soil and/orgroundwater for the physical treatment of pollutants.

• Students will understand the basic principles of differentthermal treatment remediation methods and will be able tochoose appropriate remediation method for specific scenarios.

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Introduction

• The thermal conduction process can be used to heat soil and/or groundwater for either:

(1) in-situ or

(2) ex-situ remediation.

• In either application, it can be carried out on large volumes of contaminated materials in a single batch.

• The heat is injected either from areal surface blankets or from vertical or horizontal wells.

• The fundamental processes, including heat flow, fluid flow, phase behavior, and chemical reactions are similar for each method.

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• In each case, heat is applied to a soil from a high-temperature surfacein contact with the soil, so that radiation heat transfer is effectivenear the heater, and thermal conduction and convection occur in thebulk of the soil volume.

• Heat can be introduced to the subsurface by electrical resistanceheating, radio frequency heating, dynamic underground stripping,thermal conduction, or injection of hot water, hot air, or steam.

• Thermal conduction accounts for over 80% of the heat transfer.• A significant feature of the process is the creation of a zone of very

high temperature (>500°C), which causes destruction of manycontaminants before they exit the soil.

• Thermal methods can be particularly useful for dense or lightnonaqueous phase liquids.

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In-Situ Thermal Treatment

• In-Situ Thermal Conduction Heating is a soil remediation process inwhich heat and vacuum are applied simultaneously to subsurfacesoils,

- with surface heater blankets or- with an array of vertical heater/vacuum wells.• Both thermal blankets and thermal wells have been proven to be

highly effective in removing a variety of contaminants includingpolychlorinated biphenyls (PCBs), pesticides, chlorinated solvents,and heavy and light hydrocarbons.

• Radiation heat transport dominates near the heaters (operated at 800to 900°C), however, thermal conduction accounts for most of theheating at greater distances into the soil.

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• Compared to fluid injection processes the conductive heating process

- is very uniform in its vertical and horizontal sweep,- transport of the vaporized contaminants is improved by the creation

of permeability (resulted from drying and shrinking of the soil),- flow paths are created even in tight silt and clay layers, allowing

escape and capture of the vaporized contaminants.• As soil is heated, contaminants in the soil are vaporized or destroyed

by a number of mechanisms, including(1) evaporation into the air stream,(2) steam distillation into the water vapor stream,(3) boiling,(4) oxidation, and(5) pyrolysis.

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• The vaporized water, contaminants, and natural organic compoundsare drawn by the vacuum in a direction countercurrent to the heatflow into the vacuum source at the blankets or wells.

• The heat can destroy or volatilize organic chemicals. As thechemicals change into gases, their mobility increases, and the gasescan be extracted via collection wells for capture and cleanup in anex-situ treatment unit.

• Contaminants that have not been destroyed in-situ are removed fromthe produced vapor stream at the surface with an air pollution controlsystem.

• The vapor treatment train consists of a thermal oxidizer, heatexchanger, carbon bed absorbers, and vacuum blowers (destructionand removal efficiencies in excess of 99.9999% have been achieved).

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Thermal Blankets

• Surface heating and vacuum extraction is achieved by evacuating thesoil under a flexible, impermeable sheet and heating the soil surfaceup to as much as 900°C with a relatively flat electric blanket heater(Figure 1).

• Each heat treatment of the soil requires 2 to 10 days, depending onthe desired depth of treatment, water content of the soil, and otherfactors.

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Figure 1

• The heat flows downward by radiation and thermal conduction, andthe consequent increase in soil temperature results in removal ofcontaminants from the soil by a number of mechanisms, includingboiling, evaporation, steam distillation, pyrolysis, oxidation, andother chemical reactions.

• Contaminant vapors or volatile decomposition products areconvected by the vacuum to the surface, where they are collectedinto the vapor treatment facility.

• At remediation sites where a large amount of water vapor isproduced, it is sometimes preferable to maintain 100% vapor phasethroughout the vacuum treatment system.

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• Air, moisture, and contaminants in the soil below the heater arepulled almost vertically to the surface.

• Atmospheric air, which enters the soil from outside the impermeablesheet, is also produced.

• Entry of outside air into the central vacuum system is restricted,however, since the air must travel some distance horizontally throughthe soil.

• The flow of air through the high temperature soil serves to evaporateand oxidize contaminants in-situ, thereby supplementing the boilingand steam distillation mechanisms.

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Thermal Wells

• For soil contamination at higher depths, heating with surfaceblankets is ineffective and thermal wells are needed to attain hightemperatures in the soil.

• The principle of in-situ thermal desorption with heater/suction wells is shown in Figure 2.

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Figure 2

• A standard arrangement is a regularly spaced arrayof heaters emplaced in screened holes in the soil.

• The space between wells at the surface is covered with animpermeable sheet that enables a vacuum to be imposed by the wellson the entire targeted soil region.

• In most applications to date, vertical wells are used; however, slantedor horizontal wells offer attractive alternatives for remediation underbuildings, foundations, roads, or other inaccessible areas.

• Before beginning heating in a remediation project, it is advisable toproduce any liquids that can be pumped or drawn by vacuum fromthe screened wells.

• Groundwater can enter a remediation site from the edges, from thebottom, or from the top. Inflow from the top can be prevented byproper drainage design of the impermeable sheet covering the area.Often groundwater problems are seasonal, and in those cases, theproject should be scheduled to avoid the rainy season.

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• Heating at the thermal wells is continued until the targettemperatures (based on contaminant properties) arereached at the coldest point between the wells.

• The temperature history of the soil consists of three periods:(1) heat-up,(2) boiling water, and(3) superheating.• During the first period, the soil minerals and fluids (mainly water)

are heated to the boiling point of water. (This heat-up is fairly rapid, especially if the soil is fairly dry, since the heat capacity of silicate and carbonate minerals is small.)

• During the second period, the temperature stays at the boiling point until all the pore water is boiled off. (The duration of this phase depends on the amount of pore water to be boiled; if additional groundwater flows into the target zone during heating, the boiling time is extended even further.)

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• When all the water has been vaporized, the dry soil can be superheated.During this third period, the soil temperature rise is even more rapid thanduring the first period, since only the soil minerals remain to be heated.

• As soil temperatures increase, water, volatile organic compounds (VOCs),semivolatile organic compounds (SVOCs), PCBs, petroleum aromatichydrocarbons (PAHs), and volatile metals contained in the soil matrix arevolatilized by the dynamic heat front and drawn to the wells by the vacuum.

• If air is present, the contaminants are rapidly oxidized in the hot soil nearthe heating elements.

• Remaining contaminants in the product stream (which is composedprimarily of air, water, and oxidation products) are drawn through themanifold system to the Process Trailer for further treatment.

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Ex-Situ Thermal Treatment

• Ex-situ thermal desorption treatment technology utilizes heat toincrease the volatility of contaminants such that they can be removedfrom a solid matrix: typically, soil, sludge, filter cake, or drillcuttings.

• Unlike other ex-situ processes that are carried out in reactors whereresidence times are only a few seconds or minutes, the modified in-situ thermal conduction heating allows treatment times ranging fromhours to many days.

• A thermal desorption system has two major components:

- the desorber itself and

- the off-gas treatment system.

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Two Basic Designs

• Desorbers are designed as a separation technology to remove organic compounds from such matrices without thermally destroying them. The volatilized contaminants are then either:

- collected or- thermally destroyed in secondary treatment units.• Thermal desorption systems can be stationary facilities or mobile

units and are comprised of two general categories: (1) Direct-Fired, and(2) Indirect-Fired. • Determining the best suited unit for a particular application requires

an understanding the characteristics of the material to be treated and the applicable regulatory requirements.

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Applying Thermal Desorption

• As a rule of thumb, most contaminants can be reduced to less than 1.0 mg/kg (ppm) if the starting level is less than 500 mg/kg.

• Another way to make a preliminary estimate is to assume a removal efficiency of 98 to 99.99%.

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Typical Contaminant Reduction

- Most volatile organic compounds and solvents can easily be reduced to less than 0.1 mg/kg and frequently to below 0.01 mg/kg.

- Most semi-volatile compounds can be reduced to less than 1.0 mg/kg and frequently to less than 0.1 mg/kg.

- The various PCB Aroclors can be reduced to less than 2.0 mg/kg and frequently to less than 0.5 mg/kg.

- Most pesticides will have a 99.5 to 99.99% removal.

Basis for Selecting a Treatment System

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• The following characteristics of a site will determine how suitable thermal desorption is for specified project.

1) Quantity of material requiring treatment2) Types and concentrations of contaminants3) Material classification – Hazardous, special waste, or non-

hazardous4) Physical characteristics of the material5) Cleanup objectives 6) Site logistics

• Thermal desorption units come in various sizes and configurations. Thermal desorption units can be direct fired, indirect fired and either be continuous flow or a batch type processor.

Direct Fired ThermalDesorption Systems

• Direct fired thermal desorbers consist of an inclined rotating cylinder with a burner injecting an open flame from one end. Materials are fed into the desorber at the inclined end and as the desorber rotates, the materials travel to the discharge point at the lower end. Solids are typically discharged at temperatures of 260 to 427°C. An exhaust fan maintains a negative draft on the desorber, removing vaporized hydrocarbons, combustion gasses, and entrained dust. Contaminants are typically destroyed in downstream organic vapor control devices such as a thermal oxidizer.

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Basic Thermal Desorber Design

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Thermal desorption History and Status• Direct fired rotary desorbers have been used extensively over the years

for petroleum contaminated soils and soils contaminated with RCRA hazardous wastes as defined by the USEPA. A 1992 paper on treating petroleum contaminated soils estimated that between 20 and 30 contractors have 40 to 60 rotary dryer systems available. Today, it is probably closer to 6 to 10 contractors with 15 to 20 portable systems commercially available.

Thermal Desorber Operation• Processed materials are discharged at a defined temperature and re-

moisturized for reuse. Production rates for direct fired thermal desorbers range from 5 to 120 tons per hour and is dependent upon equipment size, configuration, operating temperature, and moisture content of the feed material.

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The majority of these systems utilize a secondary combustion chamber (afterburner) or catalytic oxidizer to thermally destroy the volatilized organics. A few of these systems also have a quench and scrubber after the oxidizer which allows them to treat soils containing chlorinated organics such as solvents and pesticides. The desorbing cylinder for full scale transportable systems is typically four to ten feet in diameter with heated lengths ranging from twenty to fifty feet. The maximum practical solids temperature for these systems is around 400 to 480°C depending on the material of construction of the cylinder. Total residence time in this type of desorber normally ranges from 3 to 15 minutes.

Indirect Fired Thermal Desorption Systems • Indirect fired thermal desorption systems are comprised of two

general categories: Continuous feed and batch type. Although some designs utilize electrical resistance heating, most use a petroleum fired heat source. These systems are constructed in such a way that the flame and its products of combustion are unable to come in contact with the soil or the vaporized contaminants.

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Continuous Feed Indirect Fired Thermal Desorbers• Most continuous feed systems utilize an externally heated rotating

cylinder as the desorber, where heat is conducted through the cylinder wall and into the soil on the other side. Treatment capacities generally range from 4 to 25 tons per hour for transportable units. The second most common design consists of a jacketed screw conveyor where heat transfer fluids are pumped through the device and indirectly heat the conveyed material. These systems have been most successful in removing petroleum hydrocarbons from solids at capacities up to approximately 4 tons per hour.

Batch Type Indirect Fired Thermal Desorbers

• There are two basic variations of batch processors: Mixing and static. One benefit of a batch unit is that the processing retention time can be infinitely adjusted as needed to meet the treatment criteria. Otherwise, these units typically produce lower treatment capacities compared to continuous feed systems.

• Mixing type units generally consist of a sealed rotating drum enclosed within an insulated furnace jacket. Internals of these drums contain paddles that provide additional mixing during rotation. These desorbers also typically operate under a vacuum which increases the volatility of the compounds being vaporized. These systems can be applied to most any waste profile including mercury and radioactive material.

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• Static desorbers are usually of rectangular construction and heat the contaminated media via direct steam contact, or infrared heaters.

• Direct contact steam units provide a stripping, or washing effect that aids in treatment performance, but are limited to treatment temperatures of 100°C unless super heating is employed. These units are best suited for media contaminated with low boiling point compounds such as chlorinated solvents.

• Typical steam unit batch durations range from 1 to 3 hours. Infrared systems can achieve higher treatment temperatures and be applied to higher boiling point contaminants, but generally rely more on conduction rather than convective heat transfer. These units can require batch times of up to 24 or 48 hours in order to achieve treatment goals.

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In-Site Treatment of Hazardous Waste

• Environmental consultants are routinely faced with environmental remediation scenarios comprised of soils impacted by hazardous constituents.

• Many times these projects are classified as difficult to remediate via in-situ methods due to factors relating to moisture content, permeability, hydraulic conductivity, non-uniform particle sizing, and soil type.

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The Problem...

Comparation of the thermal desorption systems

• Generally thermal desorption systems are differentiated into two classes by their primary chamber heating methods:

1) Direct – Fired and2) Indirect – Fired

• Historically, direct-fired systems have been less costly and simpler to operate than their indirect-fired systems, but these systems can be difficult to permit for sites such as those involving soils classified hazardous.

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• Additionally, direct-fired units are inherently subject to producing much larger vent flows, and are faced with the issue of managing Hydrochloric Acid that is created when chlorinated off-gasses are treated by thermal oxidation prior to discharge.

• Although indirect-fired rotary desorbers are very well equipped to meet most soil remediation project performance and regulatory standards with a high degree of confidence, these systems are typically very large and technically more complex to operate than direct-fired units. Mobilization, setup and commissioning of a unit of this type can not be equitably justified at the majority of smaller sites.

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Advantages and disadvantagesof thermal methods

• The main advantage of in-situ thermal methods is that they allow soil to be treated without being excavated and transported, resulting in significant cost savings.

• However, in-situ treatment generally requires longer time periods than ex-situ treatment, and there is less certainty about the uniformity of treatment because of the variability in soil and aquifer characteristics and because the efficacy of the process is more difficult to verify.

• Thermal desorption has successfully been used for just about every organic contaminant found to date. (This does not mean that thermal desorption is the best choice for every contaminated soil project.)

• With the exception of mercury, thermal desorption cannot be used to remove heavy metals. (However, stabilizing agents can be added to the treated soil prior to discharge to decrease the leach ability of most metals.)

REFERENCES:

THERMAL CONDUCTION HEATING FOR IN-SITU THERMAL DESORPTION OF SOILS by George L. Stegemeier and Harold J. Vinegar. Reprinted with permission from Hazardous & Radioactive Waste Treatment Technologies Handbook, Ch. 4.6-1, Oh, Chang H. (Ed.) Copyright 2001, CRC Press, Boca Raton, Florida.

http://www.midwestsoil.com/thermal-desorption/off-gas-systems/

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RECOMMENDED REFERENCES:

1. Stegemeier, G.L. and Vinegar, H.J. (1995), “Soil Remediation by Surface Heating andVacuum Extraction,” SPE 29771, SPE/EPA Exploration and Production EnvironmentalConference, Houston, Texas, March 27–29.

2. Iben, I.E.T, Edelstein, W.A., Sheldon, R.B., Shapiro, A.P., Uzgiris, E.E., Scatena,C.R., Blaha, S.R., Silverstein, W.B., Brown, G.R., Stegemeier, G.L., and Vinegar, H.J.(1996), “Thermal Blanket for In-Situ Remediation of Surficial Contamination: A PilotTest,” Environmental Science and Technology, Vol. 30, No. 11, November, pp. 3144–3154.

3. Sheldon, R.B., Iben, I.E.T., Edelstein, W.A., Vinegar, H.J., de Rouffignac, E.P., Carl,F.G., Menotti, J.L., Coles, J., Hirsch, J.M., Silverstein, W.B., Blaha, S.R., Scatena, C.R.,and Siegel, G.W. (1996), “Field Demonstration of a Full-Scale In Situ ThermalDesorption System for the Remediation of Soil Containing PCBs and OtherHydrocarbons,” HAZWaste World Superfund XVII, Washington, D.C., October 15-17.

4. Vinegar, H.J., de Rouffignac, E.P., Rosen, R.L., Stegemeier, G.L., Bonn, M. M.,Conley, D. M., Phillips, S.H., Hirsch, J. M., Carl, F.G., Steed, J.R., Arrington, D.H.,Brunette, P.T., Mueller, W.M. and Siedhoff, T.E. (1997), “In Situ Thermal Desorption(ISTD) of PCBs,” HAZ Waste World Superfund XVIII, Volume 2, pp. 453-462,Washington, D.C., December 2–4.

5. Conley, D.M., Hansen, K.S., Stegemeier, G.L., Vinegar, H.J., Fossati, F.R., Carl, F.G.,and Clough, H.J. (2000), “In-Situ Thermal Desorption of Refined PetroleumHydrocarbons from Saturated Soil,” Proceedings of the 2nd International Conference ofRemediation of Chlorinated and Recalcitrant Compounds, Physical and ThermalTechnologies, pp. 197-206, Edited by Wickramanayake, G.B. and Gavaskar A.R.,Battelle Press, Columbus, OH, May 23.

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6. Vinegar, H.J., Menotti, J.L., Coles, J.M., Stegemeier, G.L., Sheldon, R.B., and Edelstein, W.A. (1997), “Remediation of Deep Soil Contamination Using Thermal Vacuum Wells,” SPE 39291, 1997 Soc. Pet. Eng. Annual Technical Conference, San Antonio, Texas, October 5–8.

7. Newmark, RL., Aines, R.D., Leif, R., and Knauss, K., (1998), “Thermal Treatment: Dynamic Underground Stripping and Hydrous Pyrolysis Oxidation,” paper presented at the EPA Technical Innovation Office Seminar on In-Situ Thermal Treatment, Atlanta, GA, December 16.

8. Uzgiris, E,E., Edelstein, W,A., Philipp, H.R., and Iben, I.E.T. (1995), “Complex Thermal Desorption of PCBs from Soil,” Chemosphere, Vol. 30., No. 2, pp. 377–387.

9. Stegemeier, G.L. (1998), “Design Equations for In-Situ Thermal Desorption,” presented at the U.S. EPA, Technical Innovation Office Seminar on In-Situ Thermal Treatment, Atlanta, GA, December 16.

10. Conley, D.M. and Lonie, C.M. (2000), “Field Scale Implementation of In SituThermal Desorption Thermal Well Technology.”, Physical and Thermal Technologies: Remediation of Chlorinated and Recalcitrant Compounds, pp. 175-182, Edited by G.B. Wickramanayake and A.R. Gavaskar, Battelle Press, Columbus, OH.

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8. Uzgiris, E,E., Edelstein, W,A., Philipp, H.R., and Iben, I.E.T. (1995), “Complex Thermal Desorption of PCBs from Soil,” Chemosphere, Vol. 30., No. 2, pp. 377–387.

9. Stegemeier, G.L. (1998), “Design Equations for In-Situ Thermal Desorption,” presented at the U.S. EPA, Technical Innovation Office Seminar on In-Situ Thermal Treatment, Atlanta, GA, December 16.

10. Conley, D.M. and Lonie, C.M. (2000), “Field Scale Implementation of In SituThermal Desorption Thermal Well Technology.”, Physical and Thermal Technologies: Remediation of Chlorinated and Recalcitrant Compounds, pp. 175-182, Edited by G.B. Wickramanayake and A.R. Gavaskar, Battelle Press, Columbus, OH.

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FILTRATION

Dr Vesna MarjanovićHIGH BUSINESS-TECHNICAL SCHOOL

2013.

1

Aims and outcomes:

• To provide an overview of filtration terminology and basicengineering principles, as well as calculation methods thatdescribe the filtration process in a generalized way.

• The students will understand the basic principles of filtrationprocess.

2

INTRODUCTION

• A traditional method of ex-situ remediation is pump-and-treattechnology that uses filtration and incineration.

• Filtration operates by passing the solution or suspension (granular,incompressible, free-filtering materials to slime-like compositions,finely divided colloidal suspensions) through a porous membrane ormedium, upon which the solid particles are retained on the medium'ssurface or within the pores of the medium, while the fluid, referredto as the filtrate, passes through.

• It can be used:(1) for the recovery of valuable products (either the suspended solids

or the fluid),(2) for purify the liquid stream (thereby improving product quality) -

or both.

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• Applications of filtration:(1) Adsorption, chromatography, ion exchange, operations involving

the flow of suspensions through packed columns are examples of some processes that rely on filtration.

(2) Some specific applications of filtration are in petroleum engineering, for the separation of water and miscible solvents, in reservoir flow, in hydrology, in soil physics, in biophysics, but waster treatment has historically and continues to be the largest general application of filtration.

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TERMINOLOGY

• Essentially four important physical parameters that characterize a filter media and are used as a basis for relating the characteristics of the material to the system flow dynamics are:(1) Porosity(2) Permeability(3) Tortuosity and(4) Connectivity.

• The term porosity refers to the fraction of the medium that contains the voids.

• Porous medium is a solid matter containing many holes or pores, which collectively constitute an array of tortuous passages.

• The manner in which holes or pores are embedded, the extent of their interconnection, and their location, size and shape characterize the porous medium.

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• Porous media are classified as:(1) unconsolidated (examples: sand, glass beads, catalyst pellets,

column packing materials, soil, gravel and packing such as charcoal) and

(2) consolidated (examples: most of the naturally occurring rocks, such as sandstones and limestones, and manmade consolidated media, such as, concrete, cement, bricks, paper and cloth).

and/or as:(1) ordered - regular packings of various types of materials, and(2) random - have no particular correlating factor

• Porous media can be categorized in terms of geometrical or structuralproperties (best represented by average properties) as they relate to the matrix that affects flow and in terms of the flow properties that describe the matrix from the standpoint of the contained fluid.

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• The fraction of the medium (i.e., the pores) that contributes to the flow (when a fluid is passed over the medium) is referred to as the effective porosity of the media.

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PORE STRUCTURE• The objective of a pore-structure analysis is to provide a description

of the structure of the pores that relates to the macroscopic or bulk flow properties.

• In studying different samples of the same medium, it becomes apparent that the number of pore sizes, shapes, orientations and interconnections are enormous.

• A common approach to defining a characteristic pore size distribution is to model the porous medium as (i) a bundle of straight cylindrical or (ii) rectangular capillaries.

• Pore structure for unconsolidated media is inferred from a particle size distribution, the geometry of the particles and the packing arrangement of particles.

• Information on particle size, geometry and packing theory allows us to develop relationships between pore size distributions and particle size distributions.

• A simplified approach is to assume the medium to be ideal - meaning homogeneous, uniform and isotropic.

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On this figure it can be seen SEM of pores on surface of activated carbon particle

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GOVERNING EQUATIONS NOMENCLATURE USED IN EQUATIONS

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A = area (m2)b = parameter in slip flow

expression for K ((sec2∙m)/kg)c = shape factor, known as

Kozeny constantD = diameter (m)Dp = particle diameter (m)g = acceleration due to gravity

(m/sec2)h = hydraulic head (m)k = intrinsic permeability (k2)K = hydraulic conductivity (m/sec)L = characteristic macroscopic

length (m)n = number of pore layersp = pressure (kg/(sec2∙m)

q = seepage velocity (m/sec)Q = volumetric flowrate (m3/sec)Qm = volumetric flowrate at

average pressure pm (m3/sec)r = radiusRe= Reynolds numberS = specific surface (m2)V = volume (m3)ν∞ = velocity of approach (m/sec)x = coordinate (m)z = coordinate (in direction of

gravity) (m)μ = viscosity (kg/(m∙sec))ρ = density (kg/m3)τ = tortuosityϕ = porosity

PERMEABILITY AND DARCY’S LAW

• A reservoir composed of a section of coarse gravel and a section of fine sand, where these two materials are separated and have significantly different permeabilities, is heterogeneous in nature.

• Defining dimensions, locating areas and establishing average properties of the gravel and sand constitutes a reservoir description.

• Darcy’s law:- the governing flow equation describing flow through as porous

medium - a relationship between the volumetric flow rate of a fluid flowing

linearly through a porous medium and the energy loss of the fluid in motion

- considered as valid for creeping flow where the Reynolds number is less than one

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( )Re

1pD ν ρ

µ φ∞=−

• Where:

( )1 2K A h hQ

h−

=∆

ph z constρ∆

∆ =∆ + +

the parameter, K:- a proportionality constant that is known as the hydraulic

conductivity

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- depends on the properties of the fluid and on the pore structure of the medium

- temperature-dependent, since the properties of the fluid (density and viscosity) are temperature-dependent

- can be written in terms of the intrinsic permeability, k of the porous medium (as a function only of the pore structure, and not temperature-dependent) and the properties of the fluid

k gK ρµ

=

The differential form of Darcy's equation is:

Q k dpqA dxµ= =−

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Where:- the term q is known as the seepage velocity (and is equivalent to

the velocity of approach ν∞, which is used in the definition of the Reynolds number)

- the minus sign results from the Δ p = p2 – p1, which has negative quantity

• Permeability is determined using linear flow in the incompressible(a liquid is used as the flowing fluid) or compressible (gas is used as the flowing fluid)

• The volumetric flowrate Q (or Qm), which is determined at several pressure drops, is plotted versus the average pressure pm.

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• The slope of this line will yield the fluid conductivity K and, for known the fluid density and viscosity, the intrinsic permeability k can determined.

• For gases, the fluid conductivity depends on pressure, so a straight line results (as with a liquid), but it does not pass through the origin; instead it has a slope of bK (where b depends on the fluid and the porous medium) and intercept K.

• The explanation for this phenomenon is that gases do not always stick to the walls of the porous medium.

EFFECTS OF HETEROGENEITY, NONUNIFORMlTY AND

ANISOTROPY ON PERMEABILITY

• Permeability is the conductance of the medium and has directrelevance to Darcy's law.

• Heterogeneity, nonuniformity and anisotropy (defined in the volume-average sense) are based on the probability density distribution ofpermeability (expressed by Darcy's law) of random macroscopicelemental volumes selected from the medium

• Permeability - reflects the conductance of a given pore structure.• Permeability is related to the pore size distribution, since the

distribution of the sizes of entrances, exits and lengths of the porewalls constitutes the primary resistance to flow.

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• Anisotropy in natural or manmade packed media may result fromparticle (or grain) orientation, bedding of different sizes of particlesor layering of media of different permeability.

TORTUOSITY AND CONNECTIVITY• Permeability and porosity are related to each other (if the porosity is

zero the permeability is zero).• Permeability cannot be predicted from porosity alone, additional

parameters (tortuosity and connectivity) that contain moreinformation about the pore structure are needed.

• Tortuosity is defined as the average length of the flow paths to thelength of the medium.

• It is a macroscopic measure of both(1) the sinuosity of the flow path and(2) the variation in pore size along the flow path.

• Connectivity is a term that describes the arrangement and number ofpore connections.

• Connectivity represents a macroscopic measure of the number ofpores at a junction.

• Tortuosity and connectivity are different features of the pore structure and are useful to interpret macroscopic flow properties (such as permeability, capillary pressure and dispersion).

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THE KOZENY EQUATION

• Since the porous media is typically characterized as an ensemble ofchannels of various cross sections of the same length, the Navier-Stokes equations can be used for all channels passing a cross sectionnormal to the flow:

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2 c krφ

=

• Where:- S is the specific surface area of the channels- parameter c, known as the Kozeny constan, is interpreted as a

shape factor that is assigned different values depending on theconfiguration of the capillary

• For a circular capillary, c = 0.5 and, for other than circularcapillaries, a shape factor is included

2 cSkφ

=

• Tortuosity t is basically a correction factor applied to the Kozenyequation to account for the fact that in a real medium the pores are not straight (i.e., the length of the most probable flow path is longer than the overall length of the porous medium):

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32 cS

kφτ

=

• To determine the average porosity of a homogeneous but nonuniform medium, the correct mean of the distribution of porosity must be evaluated.

• The average porosity of a heterogeneous nonuniform medium is the volume-weighted average of the number average:

• For a homogeneous but nonuniform medium, the average permeability is the correct mean (first moment) of the permeability distribution function.

• The correct average for a homogeneous, nonuniform permeability, assuming it is distributed log-normally, is the geometric mean, defined as:

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1

1

m

i i

m

i

V

V

φφ =

∑

∑

1

1

nn

ik k = ∏

• For flow in heterogeneous media, the average permeability depends on the arrangement and geometry of the nonuniform elements, each of which has a different average permeability.

• In order to explain this - consider the flow into the face of a rectangular element (with overall dimensions of height H, width W and length L).Within that rectangular system - consider a series of smaller, parallel rectangular conduits (such that the cross-sectional area of each flow element is A1, A2, A3, etc.)Since flow is through parallel elements of different constant area and assuming the overall length of each element is equal, Darcy's law for each element is:

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The flowrate through the entire system of elements is:

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1 11

A k pQ

Lµ∆

=

1 22

A k pQ

Lµ∆

=

1 2 ...Q Q Q= + +

1 1 2 2 ...pA k A k A k= + +

This means that the average permeability for this heterogeneous medium is the area-weighted average of the average permeability of each of the elements:

Reservoirs and soils are usually composed of heterogeneities that are nonuniform layers, so that only the thlckness of the layers varies. This means that simplifies to:

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1 1 2 2 ...p

A k A kkA

+ +=

1 1 2 2 ...ph

h k h kkh

+ +=

and, if all the layers have the same thickness, then:

1

n

ii

p

kk

n==∑

FILTRATION DYNAMICS

where n is the number of layers.• The operation of filtration may be performed with either

incompressible fluids (liquids - which volume decreases linearly with pressure) or slightly to highly compressible fluids (gases -volume of gases is found to be inversely proportional to pressure).

• The physical mechanisms controlling filtration, although similar, vary with the degree of fluid compressibility.

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• A filter medium is: (1) by nature inhomogeneous, (2) with pores nonuniform in size, (3) irregular in geometry and (4) unevenly distributed over the surface.

• When a suspension of solids passes through a porous media, the solid particles are collected on the feed side of the plate while the filtrate is forced through the media and carried away on the leeward side.Since flow through the medium takes place through the pores only, the micro-rate of liquid flow may result in large differences over the filter surface.This implies that the top layers of the generated filter cake are inhomogeneous and, furthermore, are established based on the structure and properties of the filter medium.As a result, the cake and filter medium influence each other.

• Pores with passages extending all the way through the filter medium are capable of capturing solid particles that are smaller than the narrowest cross section of the passage.

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• The interaction between the particles in suspension and the filter medium determines specific mechanisms responsible for filtration.

• This is generally attributed to the phenomenon of (1) particle bridging or, (2) physical adsorption.

• Adsorption – is the grouping together of molecules on the surface of a solid or liquid, which is the result of attractive forces between molecules.

• On the figure it can be seen examples of particle bridging:

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• Due to the particular filtration technique, different filter media can be employed: sand, diatomite, coal, cotton or wool fabrics, metallic wire cloth, porous plates of quartz, chamotte, sintered glass, metal powder, and powdered ebonite.

• Pore characteristics are greatly influenced:(1) by the fabrication method of the filter medium:

- they are altered when fibrous media are first pressed together;

- they also depend on the properties of fibers in woven fabrics, as well as on the exact methods of sintering glass and metal powders.

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(2) by the operating conditions- under the same operating conditions some filter media (such

as cloths, especially fibrous layers) undergo considerable compression, while other filter media (such as ceramic, sintered plates of glass and metal powders) are stable

(3) by the separation process occurring within the pore passages, as this leads to a decrease in effective pore size and consequently an increase in flow resistance

• For practical reasons:- filter medium openings are designed to be larger than the average

size of the particles to be filtered- filter medium should be capable of retaining solids by adsorption- interparticle cohesive forces should be large enough to induce

particle flocculation around the pore openings29

PROCESS CLASSIFICATION

• The process of filtration may be characterized as a hydrodynamic process in which the fluid's volumetric rate is directly proportional to the existing pressure gradient across the filter medium, and inversely proportional to the flow resistance imposed by the connectivity, tortuosity and size of the medium's pores, and generated filter cake.

• The pressure gradient constitutes the driving force responsible for the flow of the suspension.

• There are two major types of filtration:(1) “cake” - solid particles generate a cake on the surface of the filter

medium(2) “filter-medium” - solid particles become entrapped within the

complex pore structure of the filter medium - the filter medium consists of granular media (sand or anthracite coal)

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• In practice, cake filtration is used more often than filter-medium filtration. Upon achieving a certain thickness, the cake must be removed from the medium:(1) by the use of various mechanical devices(2) by reversing the flow of filtrate back through the medium

• There are two important parameters that must be considered, when specifying filtration equipment:(1) the method to be used for forcing liquid through the medium(2) the material that will constitute the filter medium

• Gravity filter used - when the resistance opposing fluid flow is small, gravity force effects fluid transport through a porous filter medium.

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• Vacuum filter used - when gravity is insufficient to induce flow, the pressure of the atmosphere is allowed to act on one side of the filtering medium, while a negative or suction pressure is applied on the discharge side.

• If still greater force is required, a positive pressure in excess of atmospheric can be applied to the suspension by three ways:(1) motive force may be in the form of introduced compressed air(2) the suspension may be directly forced through a pump acting

against the filter medium(3) centrifugal force may be used to drive the suspension through a

filter medium

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DYNAMICS OF CAKE FORMATION

• The rate of filtration increases more slowly than the rate at which the pressure gradient rises due to the pores of filter medium and cake are compressed and consequently the resistance to flow increase.

• Filtration is often used in combination with clarification (remove undissolved substances from wastewater, which is dependent upon density differences and is often enhanced by chemical means)

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• Filtration operations of finely divided colloidal suspensions, in which the cakes are incompressible, tend to contaminate or foul the filter medium.

• To prevent the formation of muddy filtrate:- at the beginning of the subsequent filtration cycle, a thin layer of

residual particles is sometimes deposited onto the filter medium;- the filtration cycle is initiated with a low, but gradually

increasing pressure gradient at an approximately constant flowrate.

• The structure of the cake formed (and its resistance to liquid flow) depends on:(1) the properties of the solid particles(2) the liquid phase suspension(3) the conditions of filtration.

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• Cake structure is established (1) by hydrodynamic factors- cake porosity- mean particle size- size distribution,- particle specific surface area and sphericity

and is strongly influenced (2) by physicochemical factors- the rate of coagulation or peptization of solid particles,- the presence of tar and colloidal impurities clogging the pores,- the influence of electrokinetic potentials at the interphase in the

presence of ions, which decreases the effective pore cross section,

- the presence of solvate shells on the solid particles.

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• The influence of physicochemical factors is especially manifested by the presence of small particles in the suspension, while, for presence of large particle sizes result in an increase in the relative influence of hydrodynamic factors.

• Attention must be given to those methods that minimize high cake resistance:- one option is the use of a large number of filter modules - which

increasing the physical size of equipment;- a more flexible option is the implementation of process-oriented

enhancements that intensify particle separation, by two different methods:(1) in the first method, the suspension to be separated is

pretreated (by addition of filter aids, flocculants or electrolytes to the suspension) 36

FILTRATION CONDITIONS

(2) in the second method, employing pure initial substances or performing a prefiltration operation under milder conditions tends to minimize the formation of tar and colloids during the period which suspensions are formed

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• There are two significant operating parameters influence the process of filtration:(1) the pressure differential across the filtering plate, i.e., the particle

inhomogeneity and the ability of the particles to undergo deformation when subjected to pressure and settling characteristics due to the influence of gravity

- small particles retained on the outer layers of the cake are often entrained by the liquid flow and transported to layers closer to the filter medium, or even into the pores themselves which results in an increase in the resistances across the filter medium and the cake that is formed

- the addition of coagulating and peptizing agents can greatly improve filterability:(a) by prevent the penetration of fine particles into the pores

of a filter plate when processing low concentration suspensions,

(b) by build up a porous, permeable, rigid lattice structure that retains solid particles on the filter medium surface, while permitting liquid to pass through.

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WASHING AND DEWATERING

(2) the temperature of the suspension influences the liquid-phase viscosity, which subsequently affects the ability of the filtrate to flow through the pores of the cake and the filter medium.

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• If contaminated, polluted, or valuable suspension liquors are present, it becomes necessary:- washing the filter cake to effect clean separation of solids from

the mother liquor or to recover the mother liquor from the solids- dewatering the filter cake to recover residual liquid retained in

the pores ( by forcing a clean fluid through cake) directly after filtering or washing

REFERENCES:

HANDBOOK OF WATER AND WASTEWATER TREATMENT TECHNOLOGIES, Nicholas P. Cheremisinoff, Ph.D., Butterworth-Heinemann, 2002, ISBN: 0-7506-7498-9, Chapter 2

RECOMMENDED RESOURCES:

REVERSE OSMOSIS A PRACTICAL GUIDE FOR INDUSTRIAL USERS First Printing 1995, ISBN 0-927188-03-1; By Wes Byrne

PRACTICAL PRINCIPLES OF ION-EXCHANGE WATER TREATMENT Second Edition 1995, ISBN 0-927188-00-7; By Dean L. Owens

BASIC PRINCIPLES OF WATER TREATMENT 1st Printing 1996; ISBN 0-927188-05-8 By Cliff Morelli

COAGULANTS AND FLOCCULANTS--Theory and Practice 1st Printing 1995; ISBN 0-927188-04-X By Dr. Yong H. Kim

FUNDAMENTALS OF FLUID FILTRATION - A TECHNICAL PRIMER 2nd edition 1998; ISBN 0-927188-01-5; By Peter R. Johnston

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