CHAPTER 9SOLIDS/LIQUIDS SEPARATION

Solids/liquids (S/L) separation in water treatment includes the processes forremoval of suspended solids from water by sedimentation, straining, flotation,and nitration; it also includes solids thickening and dewatering by gravity, sedi-mentation, flotation, centrifugation, and filtration, processes that remove waterfrom sludge or liquids/solids (L/S) separation. Suspended solids are defined asthose captured by filtration through a glass wool mat or a 0.45-jum filter mem-brane.Those solids passing through are considered to be colloidal or dissolved.

REMOVAL OF SOLIDS FROM WATER

Selection of the specific process or combined processes for removal of suspendedsolids from water depends on the character of the solids, their concentration, andthe required filtrate clarity. For example, very large and heavy solids can beremoved by a simple bar screen or strainer. Fine solids may require both sedi-mentation and filtration, usually aided by chemical treatment. The approximaterelationship of particle size to the S/L separation devices used in water treatmentis shown by Figure 9.1.

Straining includes such conventional devices as bar screens (Figure 9.2), trav-eling trash screens, and microstrainers. In some plants, instead of using screens,a device called a comminutor grinds the gross solids so they will settle and notinterfere with the sedimentation equipment.

In some applications, microstraining can be used instead of granular mediafiltration for solids reduction. Microstraining has been used for many years foralgae removal in the United Kingdom and is used as a tertiary polishing step insome wastewater treatment plants in the United States.

A typical microstraining system is shown in Figure 9.3. The unit consists of amotor-driven rotating drum mounted horizontally in a rectangular pit or vat. Therigid drum support structure has either a stainless-steel or plastic (polyester)woven screen covering fastened to it. Mesh size is normally in the 15- to 60-jumrange. Sometimes a pleated configuration is used to increase surface area.

Feed passes from the inside to the outside of the drum, depositing solids onthe inner surface. Water jets on top of the screen dislodge collected solids into awaste hopper. Where biological growth is a problem, the units may be equippedwith uv lights. The peripheral drum speed is usually adjustable. Filtration ratesare in the 10 to 30 gal/min/ft2 range. Pressure drop through the screen is 3 to 6 inH2O, while head loss through the complete system is in the 12 to 18 in H2O range.

P a r t i c l e s i z e , mmFIG. 9.1 Approximate operating regions of solids/liquids separation devices in treatingwater.

FIG. 9.2 Bar screen with automatic cleaning mecha-nism. (Courtesy ofEnvirex, a Rexnord Company.)

Stra iners ,s ieves.screens

Fabric and yarnwound f i l t e r s

G r a v i t y sedimentat ion and f l o ta t i on

C y c l o n e s and cen t r i fuga l c leaners

C e n t r i f u g e s

Granu la r media and septum f i l t e r

Membrane f i l t e r s

Microns

FIG, 9.3 Microstrainer used for removal of fine suspended solids from storm water or waste-water. (Courtesy ofCochrane Division, the Crane Company.)

Microstrainer effluent is used to backwash and to remove solids. If grease, algae,or slime are present, hot water wash and industrial cleaners may be required atregular intervals to dislodge these materials.

Sedimentation

Sedimentation is the removal of suspended solids from water by gravitationalsettling. Flotation is also a gravity separation, but is treated as a separate process.To produce sedimentation, the velocity of the water must be reduced to a pointwhere solids will settle by gravity if the detention time in the sedimentation vesselis great enough. The effect of overflow rate on settling is shown in Figure 9.4.

The settling rate of particles is affected by their size, shape, and density as wellas by the liquid they are settling through. As a particle settles, it accelerates until

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the frictional drag of its surface against the liquid equals the weight of the particlein the suspending fluid. The relationship governing particle settling is given bythe following equation:

FacgM-V* K

where F = impelling forceg = gravitational constantK = volume of the particle

51 - density of the particle52 = density of the fluid

Hindered Settling

When particles settle through a liquid in free fall, the liquid displaced by the par-ticles moves upward and the space between the particles is so large that the coun-terflow of water does not interpose friction. When the particles approach the bot-tom of the vessel and begin to form a liquid/solid interface, their free-fall velocityis arrested. The collected solids, or sludge, now slowly compact in a processknown as hindered settling. In hindered settling, the particles are spaced so closelythat the friction produced by the velocity of the water being displaced interfereswith particle movement. Figure 9.5 illustrates the change from free-fall to hin-dered settling. As sedimentation continues, the particles reach a previously estab-lished dense sludge layer; settling then becomes even slower because of the appar-ent increase in density in the liquid through which the particles are settling (Figure9.6).

Settling rate is also affected by water temperature. Raising the temperaturefrom 32 to 850F (O to 290C) doubles the settling rate for a given discrete particlebecause both the density and viscosity of the water are reduced.

In clarification, the major objective of the sedimentation is a clear effluentwater, rather than a dense underflow sludge. Clarification is used for raw waterpreparation and for wastewater treatment. Many process applications also useclarification, e.g., separation of fines from coal preparation tailings.

Perc

ent

rem

oval

•< Free settling >~^— Hindered settling ^^-Compression—^

Time—»-

FIG. 9.5 The steps in settling of participates in water: Particles at first fall freelythrough the water. As they come closer together, their rate of sedimentation isrestricted, and settled sludge volume increases. In the final stages, compaction orcompression becomes very slow.

FIG. 9.6 (a) Hindered settling is reached as particulates become soclose to each other that the passages between them restrict the ability ofwater to escape from the sludge, (b) Compaction occurs naturally, butslowly, by gravity and by dehydration of the particulates; it is aided bygently moving the sludge to develop crevices behind the moving picketsor scraper blades for water release.

Gravity Clarifier Design

There are three major types of gravity clarifiers: plain sedimentation, solids con-tact units, and inclined plane settlers. There are several designs of plain sedimen-tation basins (clarifiers): center feed (the most common), rectangular, and periph-eral feed. The center feed clarifier has four distinct sections, each with its ownfunction (Figure 9.7).

The inlet section of the center feed clarifier provides a smooth transition fromthe high velocities of the influent pipe to the low uniform velocity required in thesettling zone. This velocity change must be carefully controlled to avoid turbu-lence, short-circuiting, and carryover.

The quiescent settling zone must be large enough to reduce the net upward flowof water to a velocity below the subsidence rate of the solids. The outlet zoneprovides a transition from the low velocity of the settling zone to the relativelyhigh overflow velocities, which are typically limited to values less than 12 to 15gal/min per lineal foot of weir or launder.

Solid

s le

vel

FIG. 9.7 Simple centerfeed clarifier with scum skimmer and sludgescraper. (Courtesy of Envirotech.)

The fourth section, the sludge zone, must effectively settle, compact, and col-lect solids and remove this sludge from the clarifier without disturbing the sedi-mentation zone above. The bottom of a circular clarifier is normally sloped 5 to8 degrees to the center of the unit where sludge is collected in a hopper forremoval. Usually, mechanically driven sludge scrapers plow or rake the sludgedown the sloping bottom to the sludge hopper. Some of the collected sludge maybe returned to the feedwell for seeding, if chemical treatment is applied.

The rectangular basin is somewhat like a section taken through a center feedclarifier with the inlet at one end, and the outlet at the other. A typical rectangular

Blades and squeegees

ArmHingedskimmer

Influent pipeDrive unit

Drive control with load•indicator

Motor Scum box

Baffle

Sludge drawoffpipe

Weir

Drive unitWalkway with handrails

Baff leWeir

Feedwell Skimmersupport

ArmScum pipe

influent pipe

Blades with adjustable squeegees

Sludge drawof f pipe

FIG. 9.8 Side-by-side rectangular clarifiers with common wall, each with a traveling bridgesludge collector. (Courtesy of Walker Process Division, Chicago Bridge & Iron Company.)

clarifier has a length to width ratio of approximately 4:1. Sludge removal in rec-tangular sedimentation tanks is normally accomplished by a dual purpose flightsystem. The flights first skim the surface for removal of floating matter and thentravel along the bottom to convey the sludge to a discharge hopper. However, thesurface skimming is not a common feature, being used almost exclusively forwastewater, rather than raw water treatment. This flight system must move slowlyto avoid turbulence, which could interfere with settling. An advantage of this typeof clarifier is that common walls can be used between multiple units to reduceconstruction costs (Figure 9.8).

The peripheral feed (rim feed) clarifier (Figure 9.9) attempts to use the entirevolume of the circular clarifier basin for sedimentation. Water enters the lower

FIG. 9.9 Peripheral feed clarifier with sludge pipe and scum removal device. (Courtesy ofEnvi-rex, a Rexnord Company.)

CoI feet ionchannel Skimmer

Drive me|||nism

Iafluent

Deflector-Skirl

Sludge collecjil pipe

Sludge 4IfPDrive support

i Effluenl

FIG. 9.11 Sludge blanket clarifier providing increasing area for the water rising in the outerannulus, resulting in reducing velocity to match sludge settling rate. (Courtesy of the PermutitCompany.)

FIG. 9.10 Slurry recirculation design clarifier. (Courtesy ofEcodyne Corporation.)

EFFLUENT

SLUDGICONCENTRATOR

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2ONE BAFFtES

FREC!PiTATOR ORAiN:

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SAMPle CQMNS,

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UNE

SKIMMINGSLOT

INFLUENTCHEMfCAL FEED INLETS

AGITATOREFFLUENT COLLECTOR FLUME

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section at the periphery at extremely low velocities providing immediate sedi-mentation of large particles. The velocity accelerates toward the center, thendrops as the flow is reversed and redirected to a peripheral overflow weir. Sincethe flow pattern depends entirely on hydraulics, this type of clarifier is sensitiveto temperature changes and load fluctuations. Recirculation of sludge is very dif-ficult in the peripheral feed design.

A second major category of clarifier is the solids contact unit, available in twobasic types: the slurry recirculation clarifier and the sludge blanket clarifier (Fig-ures 9.10 and 9.11). Both of these combine chemical mixing, flocculation, andclarification in a single unit. In the mixing zone of a solids contact clarifier, thesolids concentration may be as much as 100 times that of the simple clarifier. Thishigh solids level greatly increases the rate of chemical destabilization reactionsand particle growth. Because of these features, the solids contact units are usuallyused in lime softening. In the slurry recirculation unit, the high floe volume ismaintained by recirculation from the flocculation zone to the clarification zone.In the blanket-type clarifier, the floe solids are maintained in a fluidized blanketthrough which the water must flow. Because of the increased solids in a solidscontact unit, clarifier size may be reduced. The even distribution of the inlet flowand the vertical flow pattern of this type clarifier provides better performancethan standard horizontal flow clarifiers. In passing through the sludge blanket, thelarger floe settles to the bottom by gravity and the remaining fine floe is removedby straining and adsorption.

Variable speed mixers are used to control flocculation and solids concentrationin the reaction zone. The solids concentration in the reaction zone is maintainedby bleeding solids out of the system to balance those coming in with the raw waterand the solids produced by chemical reaction. Sludge removal can be accom-plished either by a sludge blowoffpipe as in Figure 9.12, or by a conventionalrake and pump system as in Figure 9.13. Balancing the solids budget—solidsinput versus output—is the most difficult aspect of controlling a sludge blanketunit.

FIG. 9.12 Sludge collection pipe (arrow) for periodic removal of sludge to a collection hop-per. (Courtesy ofEnvirex, a Rexnord Company.)

FIG. 9.13 Circular sludge collectors, with rakes designed for corner cleaning. (Courtesy of FMCCorporation.)

Control of Flow Pattern

Two major problems with gravity clarifiers are short-circuiting and random eddycurrents. These are related in that both can be induced by changes in flow, inletcomposition, temperature, and specific gravity. They are both aggravated bylocalized sludge deposits, which block the normal flow pattern.

It is quite obvious that in the conventional circular clarifier, water must berelatively stagnant in a significant proportion of the total volume in detention.Notably, there is no flow in the annular space below the peripheral overflow laun-der. The actual detention can be determined by measuring effluent chloride con-centrations or conductivity at timed intervals after injecting a measured slug ofsalt into the feed. The results of this should be discussed with the equipmentdesigner if short-circuiting is suspected.

Eddy currents are usually readily observed, showing up as an apparent boilingof the sludge. Often these disturbances can be traced to weather conditions, suchas high winds or bright sunlight which either heats the sludge unevenly or encour-ages algal production of O2.

Floe separators have been a solution to these problems in many gravity clari-fiers. These units, made up in modules for installation in a variety of clarifierdesigns, add just enough frictional resistance to flow to even out the hydraulicpattern and eliminate the problems of short-circuiting and eddy currents. Figure9.14 shows a typical installation of floe barriers in a sludge-blanket type clarifier.Figure 9.15 shows the design of one kind of floe barrier module.

In most gravity clarifiers, the mean water depth through which sludge particlesmust fall is on the order of 5 ft or more. The time required for the sludge to fallthis distance is a critical factor in limiting the clarifier capacity. Two similar mod-ifications to the standard design of gravity clarifiers reduce the distance of fall

FIG. 9.14 Installation of floe separator modules in a sludge blanket clarifier. (Courtesy of EcodyneCorporation.)

EFFLUENT COLLECTOR (G)

INCLINED PLANE (E)SEPARATED

DETENTIONZONE (B)

REACTIONZONE (A)

SETTLINGZONE (C)SLUDGERECIRCULATINGIMPELLER (D)

SLUDGE SLOWDOWN LINESLUDGE SCRAPER ARM

SLUDGE REMOVALSUMP (F)

FIG. 9.16 The basic tube-settler configurations used with flocculation and filtration.(Courtesy of Neptune Micro/Joe, Inc.)

FIG. 9.15 Plastic module of inclined plates simulating inclined tubes.These modules help equalize water distribution. (Courtesy of NeptuneMicrofloc, Inc.)

CHEMICAL COAGULANTS

RAWWATER

FLOCCULATOR TUBE SETTLER

TUBE CONTENTS DRAINEDDURING FILTER BW

ESSENTIALLY HORIZONTAL TUBE SETTLER

LAST PORTION OF FILTERBW REFILLS TUBES

FIRST PORTION OFBW TO WASTE

FILTER

BW TO WASTE

FILTER

CHEMICAL COAGULANTS

RAWWATERFLOCCULATOR TUBE

/SETTLER

SLUDGEr>RAu/nrr

FIG. 9.17 Closely spaced inclined plates multiply the available settling surface in a small volumeand reduce installation space. (Courtesy ofParkson Corporation.)

from feet to inches, increasing the effective rise rate and radically reducing spacerequirements for clarification. These are the tube-settler and the inclined planesettlers.

The so-called tube-settler may in fact be a series of inclined tubes, somewhatlike a heat exchanger bundle connected at the inlet to a flocculation chamber andat the outlet to a clear well (Figure 9.16). The angle of inclination is varied to suitthe required duty. It is affected by the concentration and nature of the solids, andby the ensuing processes of water nitration and sludge thickening. The tube-settlermay also be a vessel packed with floe separators or with parallel inclined plates.

The inclined plate separator (Figure 9.17) is more intricate, but the same prin-ciple applies in that the sludge particles have a very short settling distance, and

FLOW DISTRIBUTION ORIFICES

OVERFLOW BOX

OVERFLOW(EFFLUENT)

SLUDGE HOPPER(REMOVABLE)

UNDERFLOW(SLUDGE)

VIBRATORPACK

LAMELLAPLATES >

FEED(INFLUENT)

COAGULANTAID

FLASH MIXTANK

FLOCCULATION TANK

FEED BOX

DISCHARGE FLUMES

the accumulated sludge is induced to flocculate and concentrate as it rolls downthe inclined surface. These units are ideally suited for localized treatment of indi-vidual waste streams in cramped locations. An example is the installation of theseseparators for treating chemical plant wastes (Figure 9.18).

A final example of a gravity clarifier, which is a modification of the plain sed-imentation unit, is the rectangular drag tank (Figure 9.19). This is designed for

FIG. 9.19 Simple separator for settling and removal of gritty, nonhydrous solids fromwastewater. Some dewatering occurs on the incline above the water line. (Courtesy ofFMC Corporation.)

FIG. 9.18 Installation of an inclined plate settler in achemical plant, clarifying wastewater. (Courtesy ofParkson Corporation.)

Collector driveSteel trough (beach)

Idler Water levelScum pipe

Grit canInfluent Baff le

Chain

Flow DrawoffScum baffle

Effluent

FlightsTravel

FIG. 9.20 Steel mill scale pit, provided with oil skimmer andsludge collector. (Courtesy of FMC Corporation.)

removal of dense solids, such as granulated slag from a foundry cupola. As thesolids are dragged from the vessel by flights moving up the beach, water drainsoff, producing a relatively dry mass. Fragile solids are broken down by the move-ment of the flights up the beach, so the drag tank is limited in the type of solidsthat can be separated. The detention is usually short, and overflow clarity ratherpoor, even when chemicals are used for coagulation and flocculation. However,the drag tank can be modified, such as by providing an hour or so of detentionand preflocculating the feed, to deliver clear water, as in handling scale pit solidsin a steel mill (Figure 9.20).

Flotation Clarification

Solids can also be removed from water using an air-flotation clarifier such as theone shown in Figure 9.21. In this unit, light solids are floated to the surface by airbubbles and skimmed off while heavier solids are settled and removed in the nor-mal fashion.

Air flotation has been used for many years in the mining industry for concen-trating mineral ores, and in the paper industry for treating white water for fiberrecovery and water clarification. The use of dissolved air flotation has broadenedto include treatment of oily waste from refineries, petrochemical plants, steel roll-ing mills, automotive plants, and railroad terminals. In these industries, the oil

FIG. 9.21 Typical horizontal air flotation unit.

in the waste may coat solid particles, giving them a tendency to float rather thansettle. In these applications, the air flotation clarifier is often preceded by anAmerican Petroleum Institute (API) separator for the removal of free oil (Figure9.22).

Another important application of dissolved air flotation is food industry wastetreatment. Meat and seafood processing plants, canneries, and wineries have sig-nificantly reduced BOD and suspended solids using air flotation equipment.

In flotation clarification, the waste flow is usually pressurized and supersatu-rated with air. When the pressure is released, air comes out of solution, formingmicrobubbles, which float the solids to the surface. In some cases, instead of pres-surizing the influent, a portion of the effluent is recycled through an air-saturationtank to meet the feed stream, as discussed later.

In treating wastes containing solids which tend to float, air flotation may be soeffective that it may reduce clarification time to 15 to 20 min of detention time,compared to the several hours typical of gravity sedimentation. As with gravityclarifiers, it is often necessary to add coagulant or flocculant chemicals such asferric chloride and alum to flotation units to aid in floe formation, using lime ifneeded for pH adjustment. Polyelectrolytes have been gaining popularity for this

FIG. 9.22 Typical design of API separator. (Courtesy ofEnvirex, Inc., a Rexnord Company.)

ADJUSTABLEWEIR

EFFLUENTREVOLVING SCUM

/ SKIAAMER

WATER DEPTHVARIABLE

SLUDGE PIPE

SLUDGE HOPPER

RAIL

COLLECTOR TRAVEL

WATER LEVEL

DRIVE UNJT INFLUENT

RIGID FLIGHTS

Inlet

Bock pressure valveon recycle line

Sludgedischargepipe

Sludgestoragesump

Air release zone Scraper flight

Settled soups out

To recycle

Launder

Eff luent

Adjustableweir.

Float conveyor flightFloat conveyor support

Froth layer( f loat)

Adjustablefloat skimmer

application, and in almost all cases, they increase the efficiency of a flotation clar-ifier. Cylinder tests, pilot plant tests, or both are used to select the best chemicalprogram.

Theory

The amount of air that can be dissolved in water is determined by Henry's law,which states that for nonionizing gases of low solubility, the volume dissolved inwater varies with absolute pressure. At 75 lb/in2 abs (5 bars), for example, 5 timesas much air can be dissolved in water as at atmospheric pressure. The quantityof gas that will theoretically be released from solution when pressure is reducedto atmospheric is:

G'-^(iT7-')where GR = gas released, mg/L

GA = gas solubility at atmospheric pressure, mg/L (see Table 9.1)PA = absolute pressure in saturation tank, lb/in2 abs

The above must be corrected for the efficiency of gas absorption in the saturationvessel, which is influenced by mixing and detention time. The efficiency varies inthe range of 40 to 60%, so the gas released would typically be about half of thatdetermined by the above formula.

TABLE 9.1 Gas Solubility at AtmosphericPressure in mL/L

Temperature O2 N2 Air

O0C (320F) 10.3 18.0 28.31O0C(SO0F) 8.0 15.0 23.02O0C (680F) 6.5 12.3 18.83O0C (860F) 5.5 10.5 16.04O0C (1040F) 4.9 9.2 14.15O0C (1220F) 4.5 8.5 13.0

The air bubbles formed in a DAF unit normally carry a slight negative charge.Depending on the type of particulate matter and the degree of agglomeration ofthe solids, the air bubbles can attach themselves by any of the followingmechanisms:

1. Simple adhesion of the air bubble to the solid surface. This can occur eitherthrough collision or by formation of the air bubble on the particle surface.

2. Trapping of air bubbles under sludge floe, such that the waste particle "takesa ride" to the surface. Sometimes referred to as "screening," this implies thatthere need be no real attachment of air bubbles to sludge particles to accom-plish flotation.

3. Incorporation of air bubbles into floe structures. This is believed to be the mostefficient mode of air usage because there is less chance of floe separation from

the air bubble. This process is encouraged by the use of polyelectrolytes, which,when applied correctly, will cause the flocculation of sludge particles at thesites where air bubbles are coming out of solution.

Since the net specific gravity of the air-solid or air-liquid particles is less thanthat of water, they rise to the surface. There, they consolidate to form a float,which can be removed by mechanical skimmers. The clear subnatant is with-drawn from the bottom of the unit. Figure 9.21 shows a cross section of a typicalhorizontal unit.

Usually the size of a flotation unit is selected on the basis of solids loading onthe bottom, expressed as pounds per hour per square foot of floor area. Dependingon the nature of the solids, the floor loading ranges from 0.5 to 5.0 lb/h/ft2 (2.5 to25 kg/h/m2).

The hydraulic load and inlet solids concentration must be balanced to arriveat an acceptable floor loading. For instance, a unit designed to handle a floor load-ing of 2 lb/h/ft2 can handle a hydraulic loading of 0.8 gal/min/ft2 (0.33 m3/min/2)at 0.5% solids (5000 mg/L) or 1.6/gal/min/ft2 at 0.25% solids (2500 mg/L) withabout equal efficiency. A lower flow rate should be maintained as a safety factorto allow for fluctuations in concentrations. A high effluent solids concentrationmay be the result of an overloaded unit; when this happens the unit feed should

(c) A e r a t i o n of recycle

FIG. 9.23 Several operating schemes for air flotation clarification.

Coagulant

AirFlotationclarif ier

Effluent

Raw waste

( a ) Total aeration of raw waste

Pressure release valve

Coagulant

By-passRaw waste

Effluent

(b) Part ial aerat ion of raw waste

Flotationclari f ier

Pressure release valve

Air

Recycle

Eff luentCoagulant

Raw waste Pressurerelease,valve

Flotationclarif ier

Air

FIG. 9.24 Dispersed air flotation, typically used for clarification of oily waste. (Courtesy ofWemco Division, Envirotech Corporation.)

be closed and effluent recycled to allow the tank to clear. If this is a persistentproblem, it may be possible to increase the amount of dissolved air by increasingthe pressure or the flow rate of the pressurized stream. It may be necessary todecrease the unit feed stream to compensate for the overload situation. If the unitis shut down on a daily basis, clear water should be recycled routinely prior toshutdown to remove suspended and floated material.

Types of Flotation Systems

There are three basic types of dissolved air flotation systems in use (Figure 9.23).In direct aeration, the entire waste stream is pressurized and aerated. In this case,the material to be separated must be able to withstand the shearing forces in thebe closed and effluent recycled to allow the tank to clear. If this is a persistentproblem, it may be possible to increase the amount of dissolved air by increasingthe pressure or the flow rate of the pressurized stream. It may be necessary todecrease the unit feed stream to compensate for the overload situation. If the unitis shut down on a daily basis, clear water should be recycled routinely prior toshutdown to remove suspended and floated material.

Effluent recycle is recommended where fragile floe is formed. This floe wouldbe destroyed by the intense mixing which occurs in the pressurization system. Gasis dissolved in the recycle stream. This stream is then combined with the feedstream at a point where the pressure is released. Mixing of these streams prior toentering the flotation zone results in intimate contact of the precipitated gas andsuspended solids to effect efficient flotation. Effluent recirculation is requiredwhen light flocculent solids such as biological or hydroxide sludges are to be thick-

FEEO BOX

FLOTATIONCELLS NOZZLE-AIR

AERATION UNIT

BAFFLE PLATE TRIM VALVES

PRIMARY ANDSTANDBY PUMPS

THROTTLEVALVE

BREATHERVALVE

INSPECTIONDOORS

LAUNDER

WASTEDISCHARGE(TYPICAL)

SKIDS

SKIMRIERPADDLES

DRAINFLANGE

DISCHARGEBOX

CLEAN WATERDISCHARGE

ened. Rotation areas must be large when effluent recirculation is employed, sincehydraulic loading is based on both feed and recycle flows.

Flotation is also practiced by application of dispersed air into a vessel contain-ing water with oily or solid particulates. Air is mechanically entrained and dis-persed through the liquid as fine bubbles in contrast to release of dissolved gasfrom solution. The dispersed air flotation design is especially suitable for treatingoily waste water (Figure 9.24). It is widely used in water-flooding for crude oilrecovery, where natural gas is used in place of air.

Filtration

Granular Media Filtration. Granular media filtration is generally applicable forremoval of suspended solids in the 5 to 50 mg/L range where an effluent of lessthan 1 JTU (Jackson turbidity unit) is required. Sand filters have been used formany years as a final polishing step in municipal and industrial water plantswhere the clarifier effluent contains 5 to 20 mg/L of suspended solids. In areaswhere a very low turbidity raw water source is available, e.g., the gravel-bed riversof the Rocky Mountains that carry snow melt, both industrial and municipalplants use granular media filtration, with minimal chemical treatment, as the onlytreatment process for solids removal. Granular media filters are also being usedto filter cooling water sidestreams to reduce suspended solids buildup whereeffluent clarity is not critical. Granular media filters may handle suspended solidsup to 1000 mg/L and provide about 90% removal.

A number of mechanisms are involved in solids removal by filtration, somephysical and others chemical. These filtration mechanisms include adsorptionand straining.

Adsorption is dependent on the physical characteristics of the suspended solidsand the filter media. It is a function of filter media grain size and such floe prop-erties as size, shear strength, and adhesiveness. Adsorption is also affected by thechemical characteristics of the suspended solids, the water, and the filter media.The amount of surface exposed for adsorption is enormous—about 3000 to 5000ft2 per cubic foot of media. Straining, which occurs in all granular media filters, isthe major factor controlling the length of filter runs. A major objective of goodfilter design is to minimize straining since it leads to rapid head loss. This occursbecause straining causes cake formation on the surface of the filter bed (particu-larly on sand filters), with the deposited cake then acting as the filter media. Thefilter media in essence become finer as the cake forms, and head loss increasesexponentially with time.

Of the several types of filtration media used to remove suspended solids, themost common is silica sand, but crushed anthracite is also widely used. When asingle medium such as silica sand is used, it will classify in the filtration vesselaccording to size, the smallest particles rising to the top. When water flows down-ward through the sand, which is the traditional path, solids form a mat on thesurface, and filtration typically occurs in the top few inches. The sand is cleanedby upward washing with water or with water and air (backwash), and this hydrau-lically classifies the bed, keeping the finest material on top. If the sand could beloaded into a filter with the larger grains at the top and the smaller at the bottom,this coarse to fine grading would allow in-depth penetration. The increased solidsstorage would allow longer filter runs. However, since backwashing fluidizes thebed, the washed sand would again return to a fine to coarse grading.

If a single medium bed is used, the only path to coarse to fine filtration isupflow. Water is applied into the bottom of the bed. Solids can penetrate the

coarser grain medium, resulting in deeper bed filtration. Backwashing occurs inthe same direction as the filtration. The bed is classified fine at the top to coarseat the bottom. Upflow filters operate at up to 5 gal/min/ft2. Some more sophisti-cated designs combine upflow and downflow filtration and provide extra facilitiesfor bed cleaning, resulting in a system that competes with larger clarifiers for treat-ment of turbid river waters.

Typical single medium filters operate downflow at 2 gal/min/ft2 of bed area inpotable water service, and up to 3 gal/min/ft2 in industrial filtration. The filterbed is 24 to 30 in deep, supported on several courses of graded gravel (Figure9.25a and b).

FIG. 9.25 Schematic details of conventional, municipal-type filtration units, (a) Pressure filter, vertical cylindricaldesign, fabricated of steel. Usually limited to 10 ft O indiameter, (b) Gravity filter, usually of concrete construc-tion; used in larger municipal and industrial plants.

Row Water Inlet

Top Baffle

Surface Washer

Filter Media

3 - 4 Layersof CoarseSupport

ConcreteSub-Fill

Approx.5p%

Freeboard

StrainerHeads

FilteredWaternOutlet

Laterals

Supports

Normal Working Level

OperatingFloor

Inlet

BackwashOutlet

Wash Trough'

24"-30" of Filter Media0.50-0.70 mm.

Bottom'Connection

4-5 Support Layers

Silica sand normally has a grain size of 0.5 to 0.8 mm. Anthracite is usuallyabout 0.7 mm. Smaller grains filter better, but filter runs are short. Larger grainsallow longer filter runs, but if the flow is too high, hydraulic breakthrough willoccur. A coarse filter media will produce acceptable effluent and reasonable filterruns if its depth is increased.

Multimedia Filter Beds

A stacked media bed or two layers (dual media) is one answer to providing coarseto fine filtration in a downflow pattern. The two materials selected have differentgrain sizes and different specific gravities. Normally, ground anthracite is used inconjunction with silica sand. The anthracite grains with a specific gravity of 1.6and a grain size of 1 mm settle slower than sand with a specific gravity of 2.65and a grain size of 0.5 mm, so the coarse anthracite rests on top of the fine sandafter backwashing. In a typical dual media bed, 20 in of anthracite is placed above10 in of sand. The coarse anthracite allows deeper bed penetration and provideslonger filter runs at higher filter rates. The finer sand polishes the effluent. Undernormal conditions, this dual media can produce acceptable effluent at flow ratesup to 5 gal/min per square foot of bed area.

Just as coarse-to-fine dual media is more effective than a single medium filter,further improvement can be gained by introducing a third, smaller, heavier mediaunder the sand. Garnet with a specific gravity of 4.5 and a very fine grain sizesettling faster than the silica sand can be used as the bottom layer. A typical mul-timedia contains 18 in of 1.0 to 1.5 mm anthracite, 8 in of 0.5 mm silica sand,and 4 in of 0.2 to 0.4 mm garnet. This filter operates at higher flow rates andprovides deeper penetration and longer filter runs than a single or dual mediafilter.

To design a filter for maximum performance, the first consideration is thedesired quality of effluent. The selection of filter design required to produce aneffluent of 0.1 JTU is different from that required to produce 1.0 JTU.

Flow rate through a filter is critical, since it limits the throughput and dictatesthe number of filters required. Generally, as flow rate increases, penetration intothe filter increases. The flow rate is limited by the head available and the mediasize. As the media starts to load with solids, the net velocity at a given flow rateincreases until shear forces tear the solids apart and they escape into the effluent.Most filters are designed to be backwashed before this breakthrough occurs at apoint determined by head loss. Typically, single media filters are backwashedwhen the head loss reaches about 10 ft. In deep bed filtration, a terminal head lossof 15 to 20 ft is tolerable.

The gradual increase of head loss across a granular filter as solids accumulatein the bed has been used as the means to actuate backwash of the filter bed. Thishas led to development of the automatic-backwash filter (Figure 9.26) to permitreliable operation of a battery of such filters in remote locations where operatorattention may be infrequent.

In a finer grain media, since solids removal is primarily accomplished in thefirst few inches, increasing bed depth is of little value except for improvinghydraulic distribution. But in coarse filters where penetration is wanted, thecoarser the media the deeper must be the bed for equivalent effluent quality.

Water temperature affects filter performance due to viscosity. At 320F, waterviscosity is 44% higher than at 720F. Backwashing, on the other hand, improveswith cold water, since increased viscosity more effectively scours the bed to

FIG. 9.26 Automatic backwash gravity filter. (Courtesy of the Permutit Company.)

remove solids. Floe formation is much slower at low temperatures so the filtera-bility at a given plant may vary seasonally. In the summer, floe may stay on thesurface, but penetrate deeply into the filter in the winter.

The best method of determining filter media selection for a chemical coagu-lation/flocculation program is by operation of a pilot test column. Chemicals canbe fed directly to the column or into a separate flash mix tank ahead of the col-umn. Various laboratory tests have been used to determine filterability, but noneare as accurate as the pilot test column.

Granular media filters have been used for treatment of oil-bearing waters. Anexample is the use of anthracite-bed filters for removal of oil from industrial plantcondensates. In this case, a slurry of aluminum hydrate floe is formed by reactingalum with sodium aluminate; a portion of this (the precoat) is applied to the filterbed at a rate of about 0,2 Ib per cubic foot of bed volume, with the effluent dis-charged to waste during this application, followed by a short rinse to eliminateby-product salts; the balance of the slurry is then fed directly to the incoming oilycondensate (the body feed) at a rate of about 2 to 3 parts of floe per part of oil.This is a modification of the usual feed of a coagulant directly to the feed streamfor charge neutralization.

Significant advances in engineering design have produced sophisticated filtra-tion systems that compete with sedimentation and flotation devices for removalof solids from water even at high suspended solids concentrations. A design of acontinuous filter with a moving, recycling sand bed is shown in Figure 9.27. Thistype unit has been used in such diverse applications as the direct filtration of riverwater and the removal of oily solids from scale-pit waters in steel mills. Perfor-mance is improved by the application of low dosages of polyelectrolyte to the feedstream. Pilot plant testing is required in many potential applications to tie downall of the cost and performance data needed in choosing between direct filtration,sedimentation, flotation, or a combination of these.

InletHeadtank

Backwashpipe

Inlet

Outlettoservice

1. filtering 2. backwashing

Backwashpipe

Siphonbreaker

Towaste

FIG. 9.27 A continuous upflow filter with a recyclingsand filter media and continuous cleaning of a portion ofthe media. (Courtesy ofParkson Corporation.)

FIG. 9.28 Cross-section of a typical septum-typefilter designed for use of diatomite or similar filteraid in water filtration. (Courtesy of Croll Rey-nolds Engineering Company.)

"WEDGE WIRE"^ELEMENTS

TUBESHEET

REJECT COMPARTMENT -SAND/WATER SEPARATOR

TOP OF AIRLIFT PIPE -SAND RETURN

DIRTYWASHWATER

LAUNDER FORFILTERED WATER

COLLECTIONGRAVITY SANDWASHER - SEPARATOR

SANDWASHWATER

SANDDISTRIBUTION

CONE

ANNULAR INLETDISTRIBUTION-

HOOD

RETURN SAND

AIRLIFT PIPE

SAND BED:CONTINUALLYMOVING DOWNWARD

COMPRESSED AIRINLET - TO AIR LIFT

TOPVENT VESSEL DIA.

OUTLETDOMEDRAIN

OVERALLHEIGHT

INLET

DRAIN

Septum Filters. Where suspended solids concentrations are very low, septum ni-tration can be used. These filters are often referred to as DE (diatomaceous earth)filters since this material is usually used as a filter precoat, although other filteraids can be used. The septum filter (Figure 9.28) relies on a thin layer of precoatapplied as a slurry to a porous septum to produce a filtering surface to strain thesuspended solids. In most cases water being filtered is pumped through the filterunder pressure; in special designs where low head loss is possible the water maybe pulled through using vacuum. As the filter becomes plugged, head lossincreases and the solids, including precoat, have to be removed by reversing theflow through the unit. A new precoat is then applied and filtration is resumed.Usually in addition to the precoat, a body feed of filter aid is used. This body feedis simply additional filter aid added to the influent to extend filter runs by contin-ually providing a fresh filter surface. Because the filter aid has a different shape(morphology) than the solids in the water, the heterogeneous mixture is morepermeable than the solids alone.

A relatively high ratio of filter aid to suspended solids is required to operateseptum filters making operating costs fairly high. Therefore, these units are not ascommon as granular media filters in most industrial systems. DE filters are oftenused for applications such as municipal swimming pools, and they are excellentfor removal of oil from industrial plant condensate.

Septum filters can be cleaned of accumulated solids by air-bumping, a proce-dure requiring little or no water and producing a thick slurry or cake of accumu-lated solids. This simplifies solids disposal, and reduces backwash water require-ments. They can also be fitted into a relatively small space, compared to granularmedia filters.

While diatomaceous earth—the fossil remains of diatoms, a type of algae hav-ing a silica skeleton—is the commonly used filter aid (see Figure 9.29), mixturesof DE and asbestos are often used. At the high temperatures encountered in fil-tration of oily condensate, silica dissolves from the DE filter cake, so Solka-Floc,a cellulosic product, is used to avoid this problem if the condensate is to be fedto a boiler.

FIG. 9.29 Photomicrograph of two common types of filter aid. (Left) Diatomite is obtained fromnatural deposits of the siliceous skeletons of diatoms, a variety of algae. (Right) Perlite, a mineralof volcanic origin, is processed at high temperature to produce a variety of forms of glassy slivers.(About 50OX.) Special grades of cellulose and carbon are also used in water filtration.

REMOVAL OF WATER FROM SLUDGE

Thickening

Thickening is a solids/liquids separation method used to increase the solids con-tent of a slurry prior to dewatering. Thickening normally follows a clarificationprocess where the suspended solids have been separated from the liquid. In clar-ification, feed solids are normally in the 10 to 1000 mg/L range, while influent toa thickener is usually in the 0.5 to 10% range.

The purpose of thickening is to increase the solids of the underflow therebyreducing sludge volume and cost of subsequent handling; in clarification, the pur-pose is to remove solids and produce a clear effluent. The clarity of water leavinga thickener is not as critical as the density of the underflow, since the effluentwater normally is recycled back to the head of the plant. The thickened sludgemust remain liquid to the extent that it can be pumped to subsequent dewateringoperations. In municipal waste treatment plants, where digestion follows thick-ening, improved digestion and conservation of digester space is achieved throughthickening.

In the clarifier, solids have separated from the water primarily by free-fall. Thesolids collected in the lower region have encountered the effects of hindered set-tling. In the thickener, there is no free-fall; the process of hindered settling con-trols the design and the final compaction of sludge.

Gravity thickening and flotation thickening are the two major methods.

Gravity Thickening. Gravity thickening is often used in municipal plants for pri-mary sludges and in industrial plants for chemical sludges. A typical thickeningoperation in a steel mill will double the solids concentration. The gravity processworks well where the specific gravity of the solids is much greater than that of theliquid.

A gravity thickener is constructed much like a clarifier: usually it is circularwith a side wall depth of approximately 10 ft and with the floor sloping towardthe center (Figure 9.30). The floor angle is greater than in a clarifier, normally 8to 10 degrees. As in a clarifier, the sludge is moved to a well by a rake assemblyand then pumped out by a positive displacement pump. In a gravity thickener,because the process of hindered settling controls solids compaction, the sludgerake arm has a dual purpose; besides raking the solids to the sludge well, the armis constructed like a picket fence to gently muddle the slurry, dislodging intersti-tial water from the sludge and preventing bridging of the solids.

As the sludge blanket gets deeper, up to about 3 ft, the density of the solidsincreases, after which there is little advantage in increasing sludge depth. Whenthickening municipal sludges, close attention must be paid to the length of timesludge is in the thickener, since it can become septic and produce gas bubbles thatmay upset the system. This is particularly true with thickening biological second-ary sludge. If septic sludge is encountered, chlorine may be added to the feed tothe thickener. The SVR (sludge volume ratio), which is used to monitor sludgeage, is the volume of sludge in the blanket divided by the daily volume of sludgepumped from the thickener. This gives the retention time, which is normallybetween 0.5 and 2 days.

Overflow and solids loading rates are important controls in gravity thickening;if thickener performance is not satisfactory, the operator can alter these rates toimprove solids capture. Overflow rates for gravity thickeners range from 400 to

FIG. 9.30 Sludge thickener designed for thickening pulp mill waste sludge prior to dewater-ing. (Courtesy of Passavant Corporation.)

800 gal/day per square foot of surface area. The solids loading rate, expressed inpounds of solids per day per square foot, depends on the type of sludge beingthickened.

Chemical Treatment. Chemicals may aid gravity thickening. Salts of iron andaluminum have little effect; in some cases they improve the overflow clarity, butdo not provide increased loading. Polymer flocculants are effective aids to gravitythickening, forming larger, heavier floe particles, which settle faster and form adenser sludge. Depending on the system involved, these polymers can be cationic,nonionic, or anionic in character. Effective dosages of polymers usually are in therange of 2 to 20 Ib per ton of sludge solids on a dry solids basis. Two test methodsare used to determine the best chemical program: these are a simple cyclindersettling test and a stirred thickening test shown in Figure 9.31, where the stirrerrotates at only 0.1 to 1.0 r/min. The sludge is mixed with chemical, placed in thethickening apparatus, and stirred. Effectiveness of treatment is determined bymeasuring the density of sludge samples removed from the bottom of the beakerand comparing it to the concentration of unthickened sludge. An alternatemethod is to measure the percentage of supernate in each sample of settled sludgerepresenting untreated and treated conditions.

When polymer is used in gravity thickening, special attention should be givento the application point and to the dilution water rate. Since older gravity thick-eners were not designed specifically to use polymers, feed taps are often not read-ily available. Suitable feed points would be either directly ahead of the thickenerto the feed pump or into the sludge feed line. Dilution water needs to be adjustedfor optimum dispersion of the polymer into the sludge without defeating the goalof water removal.

FIG. 9.31 Sludge thickening test apparatus. Note: Results are appreci-ably influenced by sludge depth, a greater depth hindering compactionrate. The test time varies from minutes to hours, and is best judged bythe plot of sludge level versus time to determine the time required to fall1 to 2 in. (Courtesy ofEimco Division, Envirotech Corporation.)

Thickening by Flotation. An alternate to gravity thickening is flotation thicken-ing. This is usually more effective than gravity thickening when the solids beingthickened have a specific gravity near or less than that of the liquid from whichthey are being removed. Because of its high solids loading, a DAF (dissolved airflotation) thickener normally occupies one-third or less of the space required fora gravity thickener.

Flotation may be either dispersed air or dissolved air. In dispersed air flota-tion, bubbles larger than 100 /urn are generated by mechanical shearing devicesacting on air injected into the water in a flotation cell (Figure 9.24). In the dis-solved air flotation process, discussed earlier as a clarification process, the slurrybeing thickened or a portion of the recycle flow is supersaturated with air underpressure. When the pressure is released, air is precipitated as small bubbles in the10- to 100-jKm size range. The air bubbles attach to the solids, increasing the bouy-ancy of the particles and cause them to rise to the surface and concentrate.

Biological suspended solids are usually difficult to settle; however, with theaddition of dissolved air bubbles and polymer, these particles have a tendency tofloat.

Ring stand

Thickened sludge

Two- l i t e r graduated cylinder

Picket thickener mechanismoperating at 0.1 to 1.0 rpm

•Pickets

The DAF thickening process is often augmented by the addition of chemicalaids. Chemicals that have been used include inorganics, such as ferric chlorideand lime, and organic polyelectrolytes. Of the polyelectrolytes used, the mosteffective have been either moderate molecular weight polyamines or very highmolecular weight flocculants. In mostcases, cationic flocculants are used.They are particularly effective in floc-culating biological solids. Introductionof the polymer into the line at a pointwhere bubbles are precipitating andcontacting the solids normally pro-duces the best results.

Methods of evaluating flotation aidsvary with the type of slurry being thick-ened. A standard cylinder settling testis often used to determine which poly-mers will form stable floe. Floatabilityof a waste sludge can easily be deter-mined using the apparatus shown inFigure 9.32. More sophisticated pilotplant test equipment, such as is shownin Figure 9.33, can also be used todevelop chemical programs. Thisequipment can closely duplicate theconditions found in actual DAFthickeners.

In summary, DAF can be an effec-tive method of thickening materialsthat have a tendency to float. The useof DAF to concentrate sludge offers some advantages over gravity thickening.This is especially true in the case of concentration of activated biological sludge,which is troublesome to concentrate by gravity. Gravity thickening of waste-acti-vated sludge will seldom yield concentrated sludge of more than 2% solids. Dis-solved air flotation of the same sludge will normally yield greater than 4% solidsin the float. During the concentration operation, since air is used, the sludge willremain fresh and not become septic as it can if left in a gravity thickener.

Like other thickeners, flotation devices are designed for specific solids loadingrates and overflow rates. Solids loading rates without chemical addition rangefrom 1 to 2 Ib of dry solids per hour per square foot; chemical addition allows theload to be doubled. The air to solids ratio of a DAF is critical since it affects therise rate of the sludge. The air to solids ratio required for a particular sludge is afunction of the sludge characteristics such as SVI. Typically, the air to solids ratiovaries from 0.02 to 0.05.

The DAF column test can be used to simulate the thickening process on asmall scale. This apparatus can be used to measure the floatability of a particularsludge and to evaluate various chemical flotation aids that improve performance,either solids capture or solids density or both.

Centrifugation. Solids concentration or thickening can also be accomplished bycentrifugation. The three widely used types of centrifuges are basket, solid bowl,and disk-nozzle, but the basket and solid bowl centrifuges are more commonlyused in dewatering of sludges.

FIG. 9.32 Bench apparatus for air flotationstudies. (Courtesy of Infilco DegrementIncorporated.)

FIG. 9.33 Pilot plant for evaluation of dissolved airflotation results on specific wastewater problems.(Courtesy of Komline-Sanderson EngineeringCorporation.)

FIG. 9.34 Schematic of several methods of operating a disk centrifuge: (a) basicscheme of operation; (b) with recycle of solids.

Recycle

Feed

Liquid•Center-lineof rotation

Solids

Center-lineof rotation

Feed

Liquid

Solids

The disk-nozzle centrifuge (Figure 9.34), while primarily a liquid/liquid sepa-ration unit, can be used to thicken slurries for further dewatering. It is suitablefor thickening a slurry with very fine uniform particle size since it creates greatercentrifugal force than a solid bowl centrifuge.

Slurry is fed into the center of the machine at the top and then directed to anarea on the outside of the disks (Figure 934a). The disks are stacked in the cen-trifuge so that they are 0.10 to 0.25 in apart. The solids settle as the feed is forcedthrough this narrow space. Since the distance is so narrow, particles do not havefar to travel. The settled solids slide down the underside of the plates and out ofthe bowl wall where compaction takes place prior to discharge through nozzles inthe periphery. The centrate passes under the sludge and is discharged from thecenter of the centrifuge. Since the disk-nozzle centrifuge has such close tolerancesit is subject to frequent plugging when coarse solids are encountered. For this rea-son coarse solids are often screened from the fluid before it enters the centrifuge.

DEWA TERING-L/S SEPARA TION

Dewatering is normally the final step in a liquids/solids separation. The goal is toproduce a cake of such density and strength as to permit hauling to a final disposalsite as a solid waste. It usually follows clarification and thickening operations. Inwaste treatment, the dewatering method is often dictated by the nature of thesolids being dewatered and the final method of solids disposal. If sludge is beingincinerated, as in the case of a raw or biological sewage sludge, it is necessary toextract as much water as possible to minimize the requirement for auxiliary fuelfor incineration. If solids are being used in a land reclamation program or as land-fill, it may not be necessary to dewater to such an extent.

Centrifugation. Centrifugation has long been used for dewatering as well as forthickening, discussed earlier. Selection of the proper centrifuge is important sincedesign characteristics can be tailored to meet specific application needs. There areseveral inherent advantages in Centrifugation that make it attractive for manydewatering applications. Among the important advantages are compact design,high throughput, and relative simplicity of operation. Auxiliary equipment is verysimple.

Solid Bowl Centrifuge. The type of centrifuge normally used in dewateringapplications as opposed to thickening is the solid bowl unit. These are widely usedin municipal sewage plants, paper mills, steel mills, textile mills, and refineries.They are also used extensively in mining operations, such as processing coal andrefuse. In more moderate climates, they can be installed and operated outdoors.

There are three solid bowl designs: conical, cylindrical, and conical-cylindrical.The conical bowl achieves maximum solids dryness, but at the expense of cen-trate clarity by employing a large beach area over a small centrate pool volume.In comparison, the cylindrical bowl has a deep centrate pool throughout its entirelength and provides good centrate clarity, but relatively wet cake.

The conical-cylindrical design (Figure 9.35) is the most commonly used solidbowl centrifuge. It is flexible in its ability to shift the balance of cake dryness andcentrate quality over a broad range by changing pool depth depending upon thedesired performance criteria. The conical-cylindrical solid bowl centrifuge con-sists of a rotating unit comprising a bowl and a conveyor joining through a specialsystem of gears which causes the bowl and conveyor to rotate in the same direc-tion, but at slightly different speeds. Most solid bowl centrifuges operate at 1500