Dick Mechanisms of sludge thickening.pdf

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LIBRARY OF THEUNIVERSITY OF ILLINOISAT URBANA-CHAMPAIGN

28Li65 c

no. 57-58

tuGKEwe uwwj

ENGINEERING


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CTClHEERHfc IRRMW

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* s-fi UILUENG71 2012

CIVIL ENGINEERING STUDIESSANITARY ENGINEERING SERIES NO. 58

ENGINEERING LIBRARYUNIVEH

URBANA, ILLINOIS &1U01

MECHANISMS OF SLUDGE THICKENING

HflRHHtt QQMo.

ByRICHARD I. DICK

Supported ByFEDERAL WATER QUALITY

ADMINISTRATIONRESEARCH GRANT 17070 DJR

DEPARTMENT OF CIVIL ENGINEERINGUNIVERSITY OF ILLINOIS

URBANA, ILLINOISFEBRUARY, 1971


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MECHANISMS OF SLUDGE THICKENING

byRichard I . Dick

Professor of Civil EngineeringUnivers i ty of 111 inoi s

FINAL PROJECT REPORTA Summary of Research Conducted

under Research Grant 17070 DJR fromthe Federal Water Quality Administration

Environmental Protection Agency

Urbana , 111 inoi sFebruary, 1971


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Digitized by the Internet Archivein 2013

http://archive.org/details/mechanismsofslud58dick


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ABSTRACTTwo areas were emphasized in this research on gravity thickening of

sludges. One was investigation of fundamental thickening properties ofsludges. The other was consideration of rational criteria for thickenerdesign and operation consistent with the observed fundamental thickeningproperties. Most results have been presented in detail in the professionalliterature and they are summarized and interrelated in this report.

The batch flux curve method of thickener analysis using settling dataobtained with alternative sludge depths is considered to be the most reason-able method of thickener analysis available at present. The approach isbased on experimentally determined sludge settling properties, and permitsconvenient evaluation of alternative design or operating conditions.Extreme care is necessary in measuring settling properties for gross anoma-lies in physical behavior can be created by laboratory test conditions. Themaximum concentration which a sludge can reach by gravity thickening is afunction of its compressive strength. Compressive strength, and hence thedifficulty of thickening, increases exponentially with concentration.Permeability of the sludge bed controls both the rate of escape of clarifiedwater and the portion of the effective weight of sludge solids which areeffective in compressing underlying layers.

Also described in the report are results of work concerning the sig-nificance of sludge volume index measurements, the effect of possible methodsof altering sludge settleabi 1 i ty , the relationship between thickening andsludge rheology, the influence of biological variables on the rheology ofactivated sludge, the changes which occur in activated sludge aggregatesduring thickening, the in situ measurement of suspended solids, the appli-cation of the method of thickener analysis to full scale thickeners, and

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the implications of the work to design of the activated sludge processes.

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TABLE OF CONTENTSPage

ABSTRACT j jTABLE OF CONTENTS j vI. INTRODUCTION 1

Importance of Thickening in Wastewater Management 1Purpose of Project 2Project Organization 3Nature of This Report k

II. THICKENING PROPERTIES OF SLUDGES 6Introduction 6Measurement of Settleabi 1 i ty 6

Enhancement of Sett leabi 1 i ty 8Thickening Mechanisms 10Sludge Rheology 13

III. THICKENER DESIGN 16Introduction 16Analysis of Possible Approaches 16Design Technique 18Applications and Extensions of the Technique 20

IV. SUMMARY AND CONCLUSIONS 22REFERENCES 26

Project Reports and Publications 26Other References 28

APPENDICES 31Appendix I - The Sludge Volume Index - What Is It? .... 31


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PageAppendix II - The Effect of Polymer Floccu lat ion on the . . 39

Settling Behavior of Activated SludgeAppendix III - Thickening Characteristics of Activated . . 52

SI udgeAppendix IV - Aggregate Size Variation during Thickening . 68

of Activated SludgeAppendix V - Distribution of Compressive Forces in ... . 87

Subsiding Sludge MassesAppendix VI - Influence of Biological Variables on the . . 93Physical Properties of Activated SludgeAppendix VII - Thickening 118Appendix VI I I - Role of Activated Sludge Final Settling . . 131

Tanks


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I. INTRODUCTIONImportance of Thickening in Wastewater Managemen t

In most waste treatment processes pollutants are concentrated by physical,chemical, or biological means into a settleable form for removal from theliquid waste stream. The solids might, for example, be chemical precipitantsformed by reaction with ions in the waste stream, carbonaceous material con-centrated in the form of microbial mass (in the case of biological wastetreatment processes), or material contained in the suspended form in the rawwaste.

Effective waste management requires effective treatment, handling, anddisposal of the suspension of solids. The efficiency of virtually all methodsof sludge treatment, handling, and disposal depends on the concentration of

solids in the sludge. For example, the cost of transporting sludges dependsalmost directly on the dilution factor of the solids (AWTR, 1968) as does therequired volume of sludge digesters (Shindala et_ a_l_. , 1970). The performanceof sludge dewatering processes such as vacuum filtration and centr ifugationalso depends upon the degree to which solids can be concentrated in the feed(Sleeth, 1970). Clearly the economy of sludge combustion depends on achievinga high solids concentration so that the process becomes thermally self-sustaining (Hurwitz and Katz, 1959).

The most economical way of obtaining large sludge volume reductions isby gravity thickening. Depending on the nature of the sludge, one to tenfoldvolume reduction might be readily achieved by gravity thickening using rea-sonably simple and low-cost equipment. It normally is much more expensiveto achieve similar volume reductions by mechanical means. The importance ofthickening and waste management may be appreciated by considering that about25 to 50 percent of the total cost of waste management is attributable to

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sludge treatment handling and disposal (Levin, I968)

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In addition to the importance of gravity thickeners in sludge managementschemes, thickening principles are also involved in many waste treatmentprocesses. For example, any sedimentation tank is expected to accomplish adegree of thickening, and failure to consider this requirement in designingthe facility can lead to unsatisfactory performance. Also, the performanceof many related processes depends upon how well thickening is accomplishedin sedimentation tanks. To illustrate, the size of the aeration tank in theactivated sludge process depends upon how concentrated the microorganismsare in the sludge returned from the final settling tank. Similarly, theperformance of flocculators in water and waste treatment processes dependsupon the number of particle collisions which can be caused to occur and under

given conditions, this is related to the concentration of particles present.The problem and cost of sludge treatment may be expected to become more

significant in the future as higher degrees of treatment result in productionof large volumes of flocculent suspensions. In summarizing the importance ofthickening research, Vinton Bacon (1966) has written: Sludge concentrationis the largest unsolved research and development problem. The savings thatcould be effected in this area alone would go a long way in improving othertreatment processes and the effluent.Purpose of the Project

In spite of the importance of gravity thickening and the frequency withwhich the process is used, little is known about how to design a thickeneron the basis of a rational interpretation of the thickening phenomena. Mostof the research which has been done on the process has been descriptive in

i nature and conventional design procedures which have evolved are often basedI on consideration of parameters such as surface loading which do not

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necessarily have an influence on the performance of a gravity thickener.The purpose of this work was to investigate the basic nature of the

thickening process in order to afford an understanding of the performanceof the process and to devise techniques for reliable design of thickenerson the basis of the fundamental properties of suspensions.Project Organization

The work was supported by Federal Water Quality Administration ResearchGrant 17070 DJR from September 1, 1966 through August 31, 1 9 69 . Research inprogress at the end of the grant period continued with support from theUniversity of Illinois. The work was conducted at the Department of CivilEngineering, University of Illinois, Urbana, Illinois, under the directionof Richard I. Dick, Professor of Civil Engineering.

Much of the research was carried out by candidates for the Master ofScience degree in sanitary engineering. The following graduate studentswere employed as research assistants on the projects:

T. R. Wall in September 1 966 - February 1967A. R. Javaheri September 1966 - August 1969S. K Chakrabarti February I967 - May 1968W. R. Gain February 1968 - August 1 969

Other graduate students conducted research related to the project inpartial fulfillment of master's degree requirement but were not employedwith project funds. These students, listed below, derived support from theproject in terms of supplies and equipment but received their personalfinancial support from the training grant program of the Federal WaterQuality Administration or the U.S. Public Health Service:

G. A. Farnsworth September 1966 - February 1 967J. R. Quin June - August 1967

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J. I. Barkman February I969 - August I969Mr. Barkman also received a supply and travel allowance from the Decatur,Illinois Water Company.

Other special studies for which the project provided supplies but notsalaries included graduate projects by M. C. Babb and R. C. T. Wang, andundergraduate projects by J. C. Gratteau, W. R. Gain, and G. L. Caban.Miss Caban's work also received support from the National Science Foundation.

Others associated with the project included part-time laboratory tech-nicians and Dr. B. S. Narang who was employed as a part-time ResearchAssociate during the summer of 1 968 . Faculty members from various depart-ments at the University of Illinois, especially Dr. B. B. Ewing of theDepartment of Civil Engineering, served gratuitously as project consultants.

Nature of this ReportMost of the major accomplishments of the project have been reported in

the published literature, or were in preliminary manuscript form at the timethis project report was prepared. Many of these papers and manuscripts areincluded in the appendices. The purpose of this report is to interrelateand summarize these individual publications, but not to repeat in detail workwhich previously has been described.

To distinguish between project publications and other publications,the reference list at the end of the report is divided into two sections.In the text, numbers preceded by PR are used to refer to references fromthe project publication list, while the author-year system is used to citenonproject publications. Of the 31 project reports and publications listed,18 have appeared in the published literature, of which five are includedhere as appendices

Preparation of this report was delayed to permit inclusion of results ofk


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work in progress at the end of the grant period. The research continued withsupport from the University of Illinois Civil Engineering Department and,later, from the Water Resources Center at the University of Illinois. Muchof the work has been completed. However, results bred new ideas, and researchis still in progress. Some of the continuing work makes use of equipment andconcepts generated by the grant. Under these circumstances, judgments as towhich accomplishments deserve inclusion in the final project report becomesomewhat arbitrary. In general, results of work which was underway at theend of the project period concerning excess hydrostatic pressure in subsidingsludge masses have been included here while those related to analysis of theperformance of continuous thickeners have been excluded.

Because the focus of the research was on interpretation of thickening ofsludges on the basis of observed basic physical characteristics and the rela-tion of this fundamental behavior to the design of field scale thickeners,most of the work logically fell within one of two general categories. Onecategory involved study of the basic physical properties of sludges whichcontrol its thickening behavior and the other category concerned work wherethe emphasis was on design of thickeners.

Research concerned principally with study of the physical characteristicsof suspensions is described here in Chapter II entitled Thickening Propertiesof Sludges. This chapter also includes studies related to the interpretationof results of laboratory batch sedimentation tests and some methodology associ-ated with developments of the research work. Work which was related moredirectly to the design of full scale thickeners based on known sedimentationcharacteristics is included in Chapter III entitled Thickener Design.


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II. THICKENING PROPERTIES OF SLUDGESIntroduct ion

In order to consider rational design and operation of thickeners, it isrecessary to be able to measure the physical properties of suspensions whichinfluence their thickening behavior and to understand factors which controlthe physical properties of the sludge. In addition to providing insight intofactors controlling the design and operation of gravity thickeners, knowledgeof the interrelationship between physical properties and thickening behaviormight permit alteration of the basic properties of suspensions by physical,chemical, or biological methods to improve the amenability of the sludge tothickening by gravitational means. Work related to study of the basic natureof the sludges and to experimental procedures required in evaluating sludgeproperties are reviewed in this chapter.Measurement of Settleab? 1 i ty

The property of a sludge most closely identified with its behavior infull-scale thickeners is its subsidence velocity under the influence ofgravity. The apparent settling velocity of a particular concentration ofsludge can be measured readily be observing the rate of subsidence of theliquid-solid interface following uniform dispersion in a transparent labora-tory settling column. The test is deceptively simple, for serious errorscan result because of conditions imposed by the laboratory test procedure.

Studies on the influence of laboratory test conditions on observedsettleab i 1 i ty were conducted as a part of this work, and were extended bycooperation with a related research program at the University of NorthCarolina. Principal factors found to affect results of laboratory sedimen-

' tation tests were the method for initially dispersing sludge solids, column1 diameter, sludge depth, and the presence or absence of slow speed stirring.

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The extent to which these factors influence settling rate is highly dependenton the nature and concentration of the particular sludge solids.

Results of the work have been reported in detail in two publications(PR 10 and PR 29) and have been summarized in two others (PR 11 and PR 17).In general, with activated sludge, column diameter should be as large aspossible and preferably not less than about 3 in., sludge depth should becomparable to the effective depth of the full scale facility, and slowstirring (with about 10 in./min tip speed) is essential. The greatest needfor additional work in this area is for study of the relationship betweenthe batch settl ing rate in cylinders to the settling rate of the same con-centration of sludge under conditions which exist in a full-scale conti nuousthickener.

Realization of the great anomolies in settling behavior which could becaused by improper laboratory test conditions led to a critical evaluationof the test most commonly used to express the sett leabi 1 i ty and physical con-dition of sludge - the sludge volume index. These results also have beenpublished (PR 1 8) and are included here as Appendix I. Basically, it wasreported that the significance of sludge volume index values is seriouslyrestricted by the nature of laboratory tests. While the test may have valueas a plant operational tool, comparison of sludge volume index measurementsfrom various plants is not meaningful. The test was considered to be whollyinadequate for research purposes, and alternative means for measuring thesettleabi 1 ity and physical nature of sludge solids were suggested.

The experimental program involved with evaluating laboratory settlingi tests required a large number of suspended solids determinations. In order'to select a convenient, reliable, and economical technique and to permit^valuation of the number of duplicate samples required to establish a desired

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degree of precision, a study of alternative methods of determining the sus-pended solids concentration of activated sludge was conducted. Results,which have been published (PR 22) show, as a function of sample size andconcentration, the coefficient of variation for four different methods ofdetermining suspended solids. For the purpose of this study, the glass fiberfilter Gooch crucible method with individual desiccators was considered to besuperior.

Enhancement of Sett leab i 1 i tyStudies of the alteration of settling behavior of sludges were under-

taken for two reasons. One reason was to consider the practicality of alter-ing the physical properties of sludges in full-scale applications. The otherwas to study the mechanisms responsible for the improved sedimentation inorder to gain insight into the basic factors controlling sludge thickening.

The most fruitful of the studies of this type was an evaluation of theinfluence of polyelectrolytes on the settling behavior of activated sludge.The practicality of this technique in full-scale installations under certaincircumstances was, of course, known from the published literature (forexample, Jordan and Scherer, 1970) and the thrust of the study was to evalu-ate the basic change in the physical properties of activated sludge respon-sible for the improved sett leabi 1 i ty

The major portion of the work was the subject of a master's thesis byG. A. Farnsworth (PR 20). Detailed results have been summarized in the formof a draft of a manuscript for publication (PR 21) which is included here asAppendix II. Settling properties were expressed as the ultimate settling

;velocity (the velocity at which the sludge would settle if it behaved as an

> ideal suspension (see Dick and Ewing, 1967b) and the retardation factor (ameasure of the extent of deviation from an ideal suspension'*. Polymers were


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found to influence primarily the ultimate settling velocity and did notappreciably alter the effect of i nterpart i cle contacts (as measured by theretardation factor). Because of this, the apparent effect of polymer addi-tion may be highly dependent on the sludge depth used in laboratory experi-ments .

A preliminary evaluation of the feasibility of increasing activatedsludge settling rates by imposing an electrical gradient was made (PR 23).

While some slight effect could be shown the technique was not considered tobe promising either for full-scale application or for more basic laboratoryexperimentation, and the work was terminated.

More extensive study was made of the effect of ultrasonic vibrations onthe settleabi 1 i ty of activated sludge (PR 30). The beneficial effect ofultrasonic vibrations on sludge sett leabi 1 i ty is well known, although theeconomic feasibility in full-scale applications remains dubious. The inter-est in this work was in learning more about basic settling characteristicsof sludges by inquiring as to why exposure to ultrasonic vibrations improvedsettleab i 1 i ty

It was proposed that the basic mechanism explaining the improved thick-ening characteristics was removal of bound water by ultrasonic vibrationsand then to measure its settling velocity and bound water content (using amodification of the technique described by Heukelekian and Weisburg, 1956).Significant reductions in bound water content and appreciable improvement ininterface subsidence rate could be shown to be effected by exposure toultrasonic vibrations prior to sedimentation. However, results were lesswell ordered than desired and quantitative evaluation was difficult. This

' experimental approach is still considered potentially fruitful, but improve-I ments must first be made in the method for determining bound water content.

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Thickening MechanismsUltimately, it is desirable that rational approach to design and

operation of gravity thickeners be founded on a basic understanding of thefundamental mechanisms involved in thickening. Previous work showed thatthe thickening behavior of activated sludge differed appreciably from thatof the ideal suspension considered in basic thickening theory. Work de-scribed here was undertaken to explore causes of this difference and to

evaluate changes which occur during the course of sludge thickening.In work summarized in proceedings of the 4th International Conference

on Water Pollution Research (PR ]h reproduced as Appendix III), a conceptualmodel of activated sludge thickening was developed. The model was based onmathematical analysis of the fluid and i nterparti cle forces acting on a sub-siding mass of sludge. Model predictions deviated from behavior of an idealsuspension, but agreed closely with the observed settling behavior of acti-vated sludge. The relative magnitude of interpart icle forces as computedfrom laboratory settling data using the model was related to the yieldstrength of the sludge as experimentally determined with a viscometer. Thistended to confirm that the cause of deviation from the description ofthickening offered by Kynch (1952) is the existence in activated sludge ofinterparticle forces.

Further examination of data previously reported (Dick and Ewing, 1967b)led to approximation of the concentrations at which interparticle forcesbegin to cause activated sludge sedimentation to differ from that of idealsuspensions (PR 10). As anticipated the concentration at which a continuous

j structure occurred was highly dependent on the nature of the sludge, but in each of the three plants studied the concentration was exceeded by the mixed

liquor suspended solids concentration, and in one case it was as low as 650 mg/10


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In considering basic mechanisms of thickening, it is of interest to knowwhether water eliminated from sludge during thickening originates predominatelyfrom within flocculent masses of sludge solids or from the intersticial spacesbetween sludge aggregates. To answer this and related questions, activatedsludge settling data were analyzed by use of two mathematical models (PR 25).One model (the Ri chardson-Zaki equation) related the settling velocity of amass of particles to the discrete settling velocity of an individual particle

comprising the mass, and the second (the Carmen-Kozeny equation) related flowthrough a porous bed to the physical properties of the bed. Similar conclu-sions were reached with both models. Results of the work were reported at anannual meeting of the Water Pollution Control Federation, and the completepublished findings (PR 26) are included as Appendix IV.

Briefly, it was found that for activated sludges with good settling pro-perties, thickening occurs primarily by elimination of interstitial water.However, with poorly settling sludges, much of the water removal in the courseof thickening comes from inside the aggregates. The fraction of clarifiedliquid which originated from within aggregates increased as thickening tookplace. During thickening the aggregates which comprise the sludge aresqueezed to eliminate water and split into smaller, more numerous, andmore dense particles. To give some idea of the magnitude of values involved,in thickening a good activated sludge from about 0.5 percent solids to about2.0 percent solids, the effective diameter of sludge solids decreased from3 mm to 0.5 mm and the number of particles increased 30-fold. The ratio ofthe volume of floe particles to the volume of solids in the particles de-

creased from 57 to 72 and floe density increased from 1.0015 to 1.0032. At: first, less than 10 percent of the water being eliminated from the sludge. came from inside the aggregates, but at the end, more than 30 percent was

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from this source.A limitation of the work concerning aggregate changes during activated

sludge thickening arose from the fact that when performing the calculations,allowance could not be made for the effect of structural support due tointerparti cle contracts. Experimentally, the amount of interpart i cle supportin a subsiding suspension can be measured by observing the difference betweenthe effective weight of solids above a point and the excess hydrostatic pres-

sure at the point. However, this approach could not be used successfullywith activated sludge because the extremely light weight of the sludge solidsprecluded accurate determination of excess hydrostatic pressure profiles. Inwork with a denser sludge - that from a water softening plant - it was foundthat allowance for interparticle support could readily be made by measuringconcentration and excess hydrostatic pressure profiles. Preliminary work ofthis type (PR 27) was extended and is to be summarized in PR 28. A discussionof the experimental technique and a presentation of some of the pertinent re-sults of this work is included as Appendix V.

The experimental approach afforded considerable insight into the basicmechanisms of thickening. The final solids concentration of a flocculentsludge depends on its compressibility and can be increased by increasing theweight of solids per unit area. The compressive strength of sludge was foundto vary exponentially with solids concentration. Hence, it becomes exceed-ingly difficult to reach higher solids concentrations by gravity thickening.During the course of thickening the amount of the total weight of sludgejwhich serves to compress underlying layers is a function of the permeabilityof the sludge. Thus, sludge with low permeability not only retards egress ofclarified water, but also reduces the compressive force available to accom-plish sludge consolidation. Stirring on sludge thickening in laboratory

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vessels has been shown to increase compressibility and to reduce permeability,The work on the basic nature of aggregate particles and their behavior

in concentrated suspensions provided the basis for several contributions tothe literature concerning the related work of others. In a discussion ofMueller, Voelkel and Boyle's work ( 1 966) on activated sludge floe diameter(PR 5), the possibility for change of the size and water content of floeexposed to a shear field was discussed. Mechanisms of floe breakdown, sedi-

mentation in laboratory vessels, and characterization of floe propertieswere considered in discussion (PR 12) of work by Ham and Christman (1969).Application of the Ri chardson-Zaki equation to activated sludge solids asdescribed by Edeline, Tesarik, and Vostreil (1970) was discussed at the 4thInternational Conference on Water Pollution Research (PR 14). The effect oftemperature on the rate of escape of water from subsiding sludge masses andon compressibility of sludge solids was considered (PR 13) in discussion ofwork by Reed and Murphy (1969).Sludge Rheology

Advances in fundamental understanding of sludge thickening have beenhandicapped by lack of fundamental measures of the physical properties ofsludges. As discussed in previous pages, the measure most commonly used,the sludge volume index, suffers from being influenced by many differentphysical characteristics of sludges, and it also reflects the nature of lab-oratory test conditions. Use was made in these studies of directly observ-able physical properties such as gravimetric concentration, settling veloc-ity, and specific gravity and of calculated or estimated properties such asvolumetric concentration, aggregate size, aggregate density, porosity, andpermeability. However, none of these parameters gave a measure of the basic.deformation and flow characteristics of sludge. Rheological measurements

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were undertaken for this purpose.It had previously been proposed (Dick and Ewing, 1967a) that basic

Theological measures should prove useful in study of sludge treatment pro-cesses, and a viscometer suitable for measuring the rheological character-istics of activated sludge was described. For purposes of the work de-scribed in this report, the viscometer was improved. The modified versionof the instrument, as described by Wang (PR 30 retained the basic featuresof the former instrument including outer cylinder rotation, roughening ofcylinder surfaces, and use of a wide annular space between cylinders. Im-provements incorporated in the modified version included oil damping ofinner cylinder oscillation, use of calibrated torsion wires for measuringtorque, and improved concentricity of the two cylinders. While the modifiedviscometer represents a vast improvement over the original version, additionalimprovements in range and convenience are desirable.

As discussed on previous pages, it was shown early in the project period(PR ]k reproduced as Appendix III) that reported deviations in the settlingbehavior of activated sludge from Kynch's theory could be interpreted in

terms of the rheological behavior of the sludge. This was done by using amathematical model of thickening to compute the relative magnitude of inter-

'. particle forces. This value, deduced by use of observed sedimentation data,was shown to be related to the yield strength of the sludge as measured in aviscometer.

Later in the project period, development of the procedure for measuringthe absolute values of fluid resistance and interpart icle resistance in dense

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sludges permitted a more direct comparison of settling behavior and rheologi-: cal properties (PR 28 - see Appendix V) . The compressive stress at whichfailure of sludge took place under the confined conditions of laboratory

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batch sedimentation tests was related to the yield strength of the sludge.In related studies of flotation thickening of activated sludge (Wood,

1970) the viscometer was used to measure the rheology of sludges beingfloated. Wood concluded that yield strength and plastic viscosity were thebest parameters for characterizing sludges and for predicting flotationbehavior.

Because activated sludge presents severe thickening problems, becauseits thickening performance had been shown to be related to its rheologicalcharacteristics, and because rheological characteristics were known to beinfluenced by the nature of the waste treatment plant (Dick and Ewing, 1967a),studies were undertaken (PR 2 and PR 3) to investigate how biological vari-ables influenced sludge rheology. Results have been summarized in a pre-publication manuscript (PR 19) which is included as Appendix VI.

The yield strength of a particular sludge was shown to be related tosolids concentration and organic loading, while plastic viscosity was in-fluenced principally by concentration only. Great reduction in yieldstrength was produced by aerobic digestion, and dramatic increases occurredshortly after feeding. However, with a constant organic loading and sus-pended solids concentration, changes in biological population could causepronounced changes in both yield strength and plastic viscosity. Suchchanges were not necessarily accompanied by changes in performance of the

,biological phase of the activated sludge process. In the related studies

by Wood (1970) it was confirmed that changes in sludge rheology under seem-

ingly constant biological conditions were attributable to changes in mor-phological characteristics of the organisms making up the sludge. Such'changes, which significantly affect sludge thickening properties are notreflected in a sensitive or definable way by the conventional sludge volume

1 index.

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III. THICKENER DES I GflI ntroduct ion

An important aim, indeed the ultimate goal, of the research was todevelop procedures for design and operation of gravity thickeners on arational basis founded on knowledge of the fundamental thickening proper-ties of suspensions. The bridge between thickening theory and practice isa long hazardous one resting on unsure abutments. For the body of knowledgeon the basic thickening properties of suspensions is not large and most ap-proaches to thickener design have not been rational.

However, the laboratory studies on basic settling properties affordeda basis for accepting or rejecting possible design approaches on a rationalbasis and study of data in the literature and analysis of the performance

of full-scale thickeners gave additional basis on which to proceed. Thework led to a proposed design technique applicable to the flocculent sludgesencountered in sanitary engineering practice which not only permits rationalapproach to design, but also provides a framework for making the judgments

. required in thickener operation. Results of most of the work have been pub-i lished (PR 7, PR 8, PR 9, PR 11, and PR 17) and are summarized in thischapter.

, Analysis of Possible ApproachesA large number of approaches to thickener design and operation proposed

by other workers were considered. Some of these could be rejected without detailed study whereas others were given careful analysis in order to developa suitable framework for a model for thickener designs and operation.

Among those approaches which could readily be rejected were the commonj

i techniques based on hydraulic loading and sludge volume index. While hydrauliciloading is a common basis for thickener design (Great Lakes - Upper Mississippi

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River Board of Sanitary Engineers, I960), it is only related to solids loadingand is in itself wholly inadequate. The sludge volume index has often beenadvocated as a means for predicting thickener performance - particularly forthe final settling tank in the activated sludge process (for example, Stewart,196*0. However, the work on the nature of the SVI test (PR 18) showed thatthe test would be very unreliable for this purpose.

Another common design approach is based on somewhat arbitrary establish-

ment of retention time and solids loading (American Society of Civil Engineers1959)- This approach is based on parameters which are more closely relatedto thickener performance, but the method does not permit the designer oroperator of thickeners to weigh the consequences of alternative decisions.

Other approaches evaluated included empirical correlations of variousvariables with thickener performance. One such approach, by Pflanz (1970)involved the ise of solids feed which was defined as surface settling ratetimes the feed solids concentration. In discussion of this work (PR 15) itwas argued that interpretation of thickener performance by use of basicclarification and thickening theory led to clearer interpretation of thefull-scale data on which the analysis was based. Fischerstrom et_ aj_. (1967)suggested that the product of the thirty minute sediment volume and thesettling velocity be used to evaluate thickener capacity. A discussion ofpossible inadequacies of this approach was published (PR 6). An extensive

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Istudy of the relationship between laboratory and full-scale thickening byEdde and Eckenfelder (I967) led to development of a design procedure based,on mass loading and two empirical parameters. Study of the method and thenature of the empirical parameters indicated that the approach took Intojaccount those basic variables which influence thickening behavior, but that'the manner in which the factors were considered was indirect. Hence, the

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approach was not considered to provide suitable framework for a rationalth i ckeni ng model

Probably the most important and most frequently cited work related tothickening is that of Kynch (1952). The applicability of this work to wastesludges was evaluated previously (Dick and Ewing, 1967b). It was found thatthe work provided a valuable model of the thickening characteristics of idealsuspensions but that the behavior of waste sludges deviated from that of theideal. Hence, the Talmage and Fitch (1955) geometric procedure for thickenerdesign which is advocated in many text books is not directly applicable towaste sludges because it is based directly on Kynch's work. The approach isfurther limited by the arbitrary procedure commonly used to identify thelimiting concentration.

It was considered that another approach which developed from Kynch'swork showed more promise. This was the use of the batch flux plot as advo-cated by Yoshioka et al . (1957) and Shannon et_ aj_. (1963). A limitation ofthe approach however would be that it does not take into account the effectof depth on settling rates.

Another traditional approach to thickener design which required evalu-ation was the procedure for determining thickener depth or volume. Conven-tionally, this has been based on an empirical description of sludge consoli-dation originated by Roberts (193*0- This approach was not considered to

i be well founded in terms of observed thickening behavior because it involvedindependent determination of thickener area and thickener depth. It seemed

: imperative that the design approach take into account the interdependence ofthe two.Design Technique

Critical analysis of potential design methods led to selection of an18


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approach felt to have the greatest utility given to the present state ofknowledge of basic thickening properties of waste sludges. The approachis based on earlier work of authors such as Shannon and Tory (1966). Thebasis of the approach is the rational statement that in a steady state in acontinuous thickener, the solids flux, G, is

G = c.v. + c.u (1)

where c. is the suspended solids concentration of sludge at any point in thethickener, v. is the gravity settling velocity of the sludge at concentrationc. , and u is the downward velocity due to sludge removal. The required solidsflux through the thickener is determined by the solids loading and thethickener area, A, such that

c Q-2^ (2)where c and Q are the feed concentration and feed rate. Hence, the basiso ofor sizing thickeners is to determine the lowest value of G from equation 1for all concentrations of sludge which could occur in the thickener and toascertain that sufficient area is provided so that the value of G fromequation 2 does not exceed this limiting value.

This method of thickener analysis is particularly valuable because ittakes into account both the settling properties of the sludge and the modeof thickener operation. To explain, the first term in equation 1 is depen-dent entirely on sludge properties while the second term is determined byoperating conditions. The value of the underflow velocity, u, is determinedby the rate of sludge removal which depends, in turn, on the desired degree

I of sludge concentration.The design approach has been described in more detail in PR 17 which is

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included here as Appendix VII. In another publication (PR 11), three pos-sible techniques for solving the basic equations were illustrated, and itwas concluded that the approach making use of a batch flux curve had thegreatest utility. This paper is reproduced as Appendix VIM. An illustra-tive problem using the technique is solved on page 123 of Appendix VII, andan illustration of the way which the method can be used as a guide to thick-ener operation is included on page 12*t.

The design technique does not require separate determination of thick-ener depth. Rather, it is necessary that the sludge settling data used indesign be representative of those to be expected in the full-scale facility.The effect of depth on the performance or required size of a thickener isevaluated by the use of settling data representative of different sludgedepthsApplications and Extensions of the Technique

The basic thickening model described was used by Barkman ( 1 9 69 ) inanalysis of the performance of an existing thickener for waste sludge froma water clarification and softening plant. Predictions based on the modelwere in reasonable agreement with plant operation. The analysis served toreveal that the thickener was being operated at less than its potentialcapacity. That is, the value of u in equation 1 was needlessly high be-cause sludge was being withdrawn at a faster rate (and lower concentration)

1 than necessary.The method of analysis for thickener design and operation was applied

to the final settling tank of the activated sludge process (PR 11 reproducedas Appendix VIM). It was shown that use of conventional final settlingtank design procedures could result in inability to maintain desired sus-pended solids concentrations in the aeration tank. The approach could be

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used as a basis for considering the effects of alternative methods foroperating existing tanks or as a basis for optimizing design of new finalsettling tank.

Results of the laboratory studies of basic sludge thickening mechanismsand the conceptual model of continuous thickener performance afford basesfor interpreting results of full-scale thickening studies and for extendingand refining the model. Equation 1 permits comparison of velocities in smalllaboratory batch conditions with those in full scale continuous thickeners,and interpretation of compression in terms of the combined influences ofpermeability and compressibility (see Chapter II) provides a basis for evalu-ating the influence of sludge depth and for studying the effects of rakes inthickeners. The first stage of this work is being conducted with a laboratorycontinuous thickener equipped with means for concentration and excess hydro-static pressure measurement. This research was inspired by results of workconducted as part of this project, but is being carried out with supportfrom the University of Illinois Water Resources Center.

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IV. SUMMARY AND CONCLUSIONSIn spite of the widespread use of sludge thickening in waste treatment,

and in spite of the potential for reducing treatment costs by improved thick-ening, design and operation of thickeners has not been accomplished on arational basis. The basic goal of this research was to investigate basicthickening properties of sludges and to develop thickener design and opera-tion techniques consistent with knowledge of these properties.

Most results of work carried out in the project have been published inthe professional literature. This report serves to summarize and interrelatethe various individual reports of work but detailed procedures and resultsare not repeated. Similarly, general conclusions stemming from the work arepresented in this section and more complete conclusions are to be found inthe project publications (some of which are included as appendices).

Serious errors can result from the conventional laboratory test formeasuring the settling rate of sludges. The influence of slow stirring inlaboratory settling tests is not necessarily a reflection of the benefitsto be derived by stirring in sludge thickeners, but is caused by the artifi-cial conditions created by the laboratory test. Laboratory tests should beconducted in cylinders as large in diameter as feasible and preferably notless than about 3 in. Sludge depth should be comparable to the effectivedepth in the full scale facility and a slow stirrer with tip speed of about

i

j

10 in./min should be provided.The sludge volume index is an inadequate, indeed, a misleading, indica-

tion of settling characteristics. Its normal use should be restricted tomonitoring of gross physical properties of sludge for purposes of routine

;plant operation. Comparisons should not be made between SVI values of dif-ferent sludges and more basic and meaningful measures of the physical nature

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of sludges should be used in research work.Deviations in the thickening characteristics of flocculent sludges like

activated sludges from those of the ideal slurry considered in Kynch's theoryare caused by i nterpart cle contacts. The magnitude of the deviation isrelated to the yield strength of the sludge.

The final concentration of sludge solids achievable by gravity thicken-ing is controlled by the compressive strength of the sludge. Compressive

strength varies exponentially with suspended solids concentration, and hencehigh concentrations are difficult to attain.

Efforts to increase the final concentration of thickened sludges shouldbe directed at reducing the compressive strength of sludges or increasingthe applied compressive load. Greater compressive loads may be achieved byincreasing the weight of solids per unit of thickener area; however, theportion of these solids effective in compressing underlying solids is afunction of sludge permeability. In work with activated sludge, polymerflocculation did not significantly change the magnitude of interparti clecontacts, but rather it altered the discrete settling velocity of the indi-vidual floe particles which comprise the sludge. Stirring reduces compres-sive strength in laboratory settling equipment but may not be of the samesignificance in full-scale tests.

The rate at which high concentrations are reached in batch sedimentationis a function both of compressibility and permeability. Reduction in sludgepermeability yields dual rewards by increasing settling rates and increasingapplied compressive loads on underlying sludges.

During thickening, aggregate particles are squeezed to eliminate waterand also broken apart. The result is the formation of sludge containingsmaller, more numerous, and more dense particles. With sludge exhibiting

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poor thickening characteristics, much of the water removed by thickeningoriginates from inside sludge particles whereas more of the water fromsludges which thicken well comes from between particles. In either case,the fraction of supernatant water originating from within particles increasesas thickening progresses.

With activated sludge, the yield strength in shear (and hence the com-pressive strength) is increased with increased organic loading intensity.

However, it is also highly dependent on the morphology of the organismscomprising the sludge, and two sludges of equal solids concentration devel-oped on the same waste at the same loading intensity may have quite differ-ent yield values. Aerobic digestion causes appreciable reduction in yieldstrength, and pronounced changes occurred following feeding.

Analyses of proposed methods of thickener design indicated that manyare inadequate because they fail to take into account those factors whichinfluence thickener performance while others are unsatisfactory becausethey are based on suspensions with properties different than those encount-ered in waste treatment. Given the present state of understanding ofthickening behavior, the most reliable approach to thickener analysis is asimple statement of continuity in a full-scale thickener. Analyses based onthis approach take into account both the settling characteristics of thesludge and thickener operating practices. The batch flux curve affords a

I

convenient technique for evaluating alternative thickener designs oralternative operational modes.

Application of the design approach to the final settling tank of the

activated sludge process indicates that use of conventional settling tankdesign practices can lead to unsatisfactory performance of the entire pro-cess. Even when process performance is not affected, failure to consider

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the interrelationships between the biological and physical phases of theprocess leads to uneconomical design.

Full-scale thickener operational practices have an important influenceon results. Use of inadequate underflow velocities will become apparentbecause of solids loss in the effluent. This can be corrected by increasingthe rate of sludge removal. However, use of too high an underflow ratemust be prevented because it results in the use of facilities at less thanf ul 1 capaci ty .

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REFERENCESProject Reports and Publications

PR 1 Babb , M. C., An Instrument for the Determination of Sludge Density, 1 'Unpublished Special Project Report, 11 pp. (Jan. 1968).PR 2 Caban, G. L., Changes in Some Physical Properties of Activated

Sludge under High Biological Loading Conditions, NSF UndergraduateResearch Project Report, University of Illinois, 71 PP- (Aug. I969).

PR 3 Chakrabarti, S. K. , Changes in Some Physical Properties of ActivatedSludge under Different Biological Loading Conditions, CivilEngineering Studies, Sanitary Engineering Series No. k~] , Universityof Illinois, Urbana, 65 pp. (June 1 968)

PR 4 Dick, R. l., Discussionofj_n S i tu Measurement of Solids in FinalClarifiers by A. E. Albrecht, R. E. Wul lsch ieger , and W. J. Katz,Journal of Sanitary Engi neeri ng Pi vis ion American Society of CivilEngineers , 92, SA5, 117-119 (1966).

PR 5 Dick, R. I., Discussion of Nominal Diameter of Floe Related to OxygenTransfer by J. A. Mueller, K. G. Voelkel, and W. C. Boyle, JournalSanitary Engineering Division American Society of Civil Engineers ,92, SA6, 144-146 (1966)

PR 6 Dick, R. I., Discussion of Settling of Activated Sludge in HorizontalTanks by C. N. H. Fisherstrom, E. Isgard, and I. Larsen, J ournalSanitary Engineering Division American Society of Civil Engineers ,93, SA6, 271-273 (1967).

PR 7 Dick, R. I., Gravity Thickening of Sludge, Summer Institute inWater Pollution Control - Biological Waste Treatment, ManhattanCollege, Bronx, New York ( 1968)

PR 8 Dick, R. I., Some Fundamental Aspects of Sedimentation - the Clarifi-cation Function, Water and Wastes Engineering , 6_, 2, 47-50 (1969).PR 9 Dick, R. I., Some Fundamental Aspects of Sedimentation - the Thicken-

ing Function, Water and Wastes Engineering , 6_, 3, 44-45 (1969).PR 10 Dick, R. I., and Ewing, B. B., Discussion Closure to Evaluation of

Activated Sludge Thickening Theories, Journal Sanitary EngineeringDivision American Society of Civil Engineers , 95_, SA2, 333-3^0 (1969)

PR 11 Dick, R. I., Role of Activated Sludge Final Settling Tanks, JournalSanitary Engineering Division American Society of Civi 1 Engineers ,96, SA2, 423-436 (I97O) (See Appendix VIM). ~~ ~

In the text, references to project reports and publications are bynumber preceded by PR whereas other references are cited by useof the name-date system.

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PR 12 Dick, R. I., Discussion of Agglomerate Size Changes in Coagulation byR. K. Ham and R. F. Christman, Journal Sani tary Engi neer i ng DivisionAmerican Society of Civil Engineer? , 96, SA2 , 624-627 (1970)-

PR 13 Dick, R. I., Discussion of Low Temperature Activated Sludge Settlingby S. C. Reed and R. S. Murphy, Journal Sanitary Engineering DivisionAmerican Society of Civil Engineers , 96, SA2 , 638-641 (1970).

PR 14 Dick, R. I., Thickening Characteristics of Activated Sludge, in Advancesin Water Pollution Research , Proceedings of Fourth InternationalConference on Water Pollution Research, Prague, 1969, 625-642 (1970)(See Appendix III).

PR 15 Dick, R. I., Formal Discussion of Sedimentation of Activated Sludgein Final Settling Tanks by P. Pflanz, in Advances in Water PollutionResearch , Proceedings of Fourth International Conference on WaterPollution Research, Prague, 1969, 583~585 (1970).

PR 16 Dick, R. I., and Javaheri, A. R. , Discussion of Fluidization of FloesProduced in Chemical or Biological Treatment Plants by F. Edeline,I. Tesarik, and J. Vostrei 1 , in Advances in Water Pollution Research ,Proceedings of Fourth International Conference on Water PollutionResearch, Prague, 1969, 538 (1970).

PR 1 7 Dick, R. I . , Th ickeni ng, Advances in Water Quality Improvemen t -Physical and Chemical Processes , E. F\ Gloyna and W. W. Eckenfelder,Jr., (editors), University of Texas Press, 358-369 (1970) (SeeAppendix VII).

PR 1 Dick, R. I., and Vesilind, P. A., The Sludge Volume Index - What IsIt? Journal Water Pollution Control Federation , _4j_, 7, 1285-1291(1969) (See Appendix I) . ---

PR 19 Dick, R. I., Chakrabarti, S. K. , and McCutcheon, G. L., Influence ofBiological Variables on Rheological Properties of Activated Sludge,Prepubl ication Manuscript (1970) (See Appendix VI).

PR 20 Farnsworth, G. A., The Effect of Induced Flocculation on the Settlingand Thickening Behavior of Activated Sludge, Civil EngineeringStudies, Sanitary Engineering Series No. 42, University of Illinois,Urbana, 54 pp. (Aug. 1967).

PR 21 Farnsworth, G. A., and Dick, R. I., The Effect of Polymer Flocculationon the Settling Behavior of Activated Sludge, Prepubl icationManuscript (1970) (See Appendix II).

PR 22 Gratteau, J. C, and Dick, R. I., Activated Sludge Suspended SolidsDeterminations, Water and Sewage Works , 1 15 , TO, 468-472 (1968)

PR 23 Gain, W. R. , Electrically Induced Settling of Activated Sludge,Unpublished Special Project Report, 29 pp. (Feb. 1968)

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PR 2k Gain, W. R. , In Situ Measurement of Suspended Solids Profiles inSludge Thickeners, M. S. Thesis, University of Illinois, Urbana(currently being prepared).

PR 25 Javaheri, A. R., Applicability of Two Mathematical Models to theBatch Settling of Activated Sludge, Civil Engineering Studies,Sanitary Engineering Series No. 51, University of Illinois, Urbana101 pp. (June 1969) .

PR 26 Javaheri, A. R., and Dick, R. I., Aggregate Size Variations DuringThickening of Activated Sludge, Journal Water Pollution ControlFederati on, 4j_, 5, Part 2, R197-R214 (1969) (See Appendix IV).

PR 27 Quin, J. R., Role of Structural Support in Sludge Thickening,Civil Engineering Studies, Sanitary Engineering Series No. k$ ,University of Illinois, Urbana (May 1968).PR 28 Shin, B. S., Distribution of Compressive Forces in Subsiding Sludge

Masses, M. S. Thesis, University of Illinois, Urbana (currentlybeing prepared ) (See Appendix V)

PR 29 Vesilind, P. A., and Dick, R. I., Initial Depth as a Variable inActivated Sludge Settling Tests, Effluent and Water TreatmentJournal , 9, 5, 263-268 (1969).

PR 30 Wallin, T. R. , The Influence of Ultrasonic Vibrations upon thePhysical Features of Activated Sludge, Civil Engineering Studies,Sanitary Engineering Series No. ^3, University of Illinois, Urbana,101 pp. (Nov. 1967)

PR 31 Wang, R. C. T., A Viscometer for the Study of the Rheology ofActivated Sludge, Unpublished Special Project Report, 25 pp.(June 1967).

Other ReferencesAmerican Society of Civil Engineers, Sewage Treatment Plant Design, Manual

of Practice No. 36 (1959) .AWTR Summary Report, Advanced Waste Treatment Research Program, July 1964-

July 1967, Federal Water Pollution Control AdministrationPublication WP-20-AWTR- 19 (1968).

Bacon, V. W. , nd Dalton, F. E., Chicago Metro Sanitary District Makes noLittle Plans, Public Works , 97, 11, 66 (1966)

Barkman, J. I., Gravity Thickening and Mechanical Dewatering of Alum-LimeSludge, Decatur, Illinois, M. S. Special Problem, University ofIllinois, Urbana (1969) .

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Dick, R. I., and Ewing, B. B., The Rheology of Activated Sludge, J ournalWater Pollution Control Federation, 39, *f , 5^3-560 (1967aT.

Dick, R. I., and Ewing, B. B., Evaluation of Activated Sludge ThickeningTheories, 1 ' Journal Sanitary Engineering Cof Civi 1 Engineers, 93, SA* , 9~29 (1967b)Theories, 1 ' Journal Sanitary Engineering Divi s ? on American Soc iety

Edde, H. J., and Eckenfelder, W. W., Jr., Theoretical Concepts of GravitySludge Thickening and Methods of Scale up from Laboratory Units toPrototype Design, Center for Research in Water Resources ReportNo. 15, University of Texas, Austin, ]kk pp. (1967).

Edeline, F., Tesarik, I., and Vostril, J., Fluidizat ion of Floes Produced inChemical or Biological Treatment Plants, in Advances in WaterPol 1 ut ion Research , Proceedings Fourth International Conference,Prague, 1969 ,~ S. H. Jenkins (editor), 523 (1970).

Fischerstrom, C. N. H., Isgard, E., and Larsen, I., Settling of ActivatedSludge in Horizontal Tanks, J ournal Sa nitary Engineering Pi vi s ionAmerican Society of Civil Engineers , 93, SA3, 73-83~TT9 677.~

Great Lakes - Upper Mississippi River Board of Sanitary Engineers, Recom-mended Standards for Sewage Works (i960).

Ham, R. K. , and Christman, R. F., Agglomerate Size Changes in Coagulation,Journal Sanitary Engineering Division American Society of Civi lEngineers , 96, SA3 , 48~l-502 (1969).

Heukelekian, H., and Weisburg, E., Bound Water and Activated Sludge Bulking,Sewage and Industrial Wastes, 23 , 558-57** (1956).

Hurwitz, E., and Katz, W. J., Concentrating Activated Sludge to a FuelValue of 4000 BTU per Gallon, Wastes Engineering, 30, 730-733(1959).

Jordan, V. J., and Scherer, C. H., Gravity Thickening Techniques of a WaterReclamation Plant, Journal Water Pollution Control Federation ,kl, 2, 1 80 (1970)

Kynch , G. J., A Theory of Sedimentation, Transactions Faraday Society , 48_,166-176 (1952).Levin, P., Disposal Systems and Characteristics of Solid Wastes Generated

at Waste Water Treatment Plants, P roceedings 10th Sanita ryEngineering Conference , University of Illinois Bulletin 5, 115,21 (1968).

Mueller, J. A., Voelkel, K. G. , and Boyle, W. C, Nominal Diameter of FloeRelated to Oxygen Transfer, Journal Sanitary Engineering DivisionAmerican Society of Civil Engineers , 92, SA2 , 9 20 (1966).

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Reed, C, and Murphy, R. , Low Temperature Activated Sludge Settling,Journal Sanitary Engineer ing Pi vi s ion Amer i can Society of C ivi 1_Engineers , 95, SWTWFWI (1969).

Roberts, E. J., Colloidal Chemistry and Pulp Thickening, Transact ionsAmerican Institute of M i nin g and Metallurgical Engineer s, J_l_2,178-188 (193*0

Shannon, P. T., Stroupe, E., and Tory, E. M., Batch and Continuous Thickening,'I ndustrial and Engineering Chemistry Fundamentals , 2, 203-211 ( 1 9 63)

-

Shannon, P. T., and Tory, E. M., The Analysis of Continuous Thickening,Transactions American Institute of Mining Engineers, 235, 375 _ 382TisSST

Shindala, A., Pust, J. V., and Champion, H. L., Accelerated Pigestion ofConcentrated Sludge, Water and Sewage Works , 1 17 , 9, 329~332 (1970).

Sleeth, R. E., Further Experience in the Use of Polyelectrolytes for SludgeConditioning at Worthing, Effluent and Water Treatment Journal ,j_o, 10, 582-591 (1970) .

Stewart, M. J., Activated Sludge Process Variables - the Complete Spectrum,Water and Sewage Works , R260-262 (196*0

Talmage, W. P., and Fitch, E. B., Oetermining Thickener Unit Areas,Industrial and Engineering Chemistry , kj_, 38-41 (1955).

Wood, R. F., The Effect of Sludge Characteristics upon the Flotation ofBulked Activated Sludge, Thesis submitted in partial fulfillmentof the requirements for the degree of Poctor of Philosophy,University of Illinois, Urbana, H3 pp. (1970).

Yoshioka, N., Hotta, Y., Tanaka, S., Nlaito, S., and Tsugami , S., ContinuousThickening of Homogeneous Flocculated Slurries, Chemical Eng ineering(Tokyo) , 21, 66-7** (1957).

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APPENDIX I

THE SLUDGE VOLUME INDEX - WHAT IS IT?

byRichard I . Dick

andP. Aarne Vesi 1 i nd

Reproduced fromJournal of the Water Pollution Control Federation

Volume 41, No. 7, Pages 1285-1291July, 1969

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Copyright as part of the July 19(59, Journal Watch PoiFederation, Washington, D. C, 20016

Printed in U. S. A.

THE SLUDGE VOLUME INDEXWHAT IS IT?Richard I. Dick and P. Aarne Vesilind

The sludge volume index (SVI),introduced by Mohlman (1) in 1934,has become the standard measure ofthe physical characteristics of acti-vated sludge solids. It is defined asthe volume in ml occupied by 1 gactivated sludge after settling theaerated liquor for 30 min (2). Thesludge density index, introduced byDonaldson (3), is the reciprocal ofthe SVI multiplied by 100. Both ofthese indices originally were intendedto be rough measures of sludge settle-ability to be used in the everydayoperation of waste treatment plants asmeans for monitoring the physical con-dition of activated sludge. Becauseof the simplicity of the SVI test, how-ever, it has been applied widely forpurposes for which it was not intendedoriginally.The general acceptance of this ar-

bitrary parameter as a basic measureof the physical properties of activatedsludge solids is indicated by its wide-spread use both in the operation ofwaste treatment facilities and in re-search on waste treatment. For ex-ample, the SVI commonly is used inresearch applications to evaluate theeffect of biological variables or physi-cal or chemical treatment on the prop-erties of sludge. Also, the SVI hasbeen advocated as a means for estab-Bichard I. Dick is Associate Professor of

Sanitary Engineering, University of Illinois,Urbana, Illinois, and P. Aarne Vesilind isassociated with the Norsk Institutt for Vann-forskning, Oslo, Norway. At the time thispaper was prepared, Br. Vesilind was Re-search Associate, Department of Environ-mental Science and Engineering, Universityof North Carolina, Chapel Hill, North Caro-lina.

lishing the required sludge recircula-tion rate or for calculating the mixedliquor suspended solids concentrationwhich can be maintained in the aera-tion tank. The most common use ofthe parameter, of course, has been inmonitoring waste treatment plant op-eration and in comparing the settlingcharacteristics of various sludges.

In the standard SVI test, sludgevolume is observed after a uniformlymixed sample of sludge has settledquiescently for 30 min in a standard1-1 graduated cylinder. The vol-ume occupied by the sludge after thisperiod of settling depends on both theinitial settling rate and the subsidencecharacteristics at the higher sludgeconcentrations. Two different acti-vated sludges, both of which have thesame initial suspended solids concen-tration and identical 30-min sedimentvolumes, will have identical SVI val-ues. However, the settling propertiesof the two sludges may be grossly dif-ferent (Figure 1). Since the SVI de-fines only one point on the settlingcurve, it is not a precise measure ofsettling characteristics.

If the SVI then is not a measureof the sludge settling characteristics,what is it? What properties of acti-vated sludge influence its magnitude?Does it quantitatively describe physi-cal properties which are indicative ofthe behavior of the full-scale process?Can meaningful comparisons be madebetween the values of the SVI invarious plants?The purpose of this paper is to

propose answers to the above ques-tions. The limitations of the SVItest are discussed and alternate means

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JOURNAL WPCF July ]9G9of describing the basic physical prop-erties of activated sludge are sug-gested.

Factors Influencing the SludgeVolume IndexSuspended Solids Concentration

If the sludge volume index weresome fundamental measure of the sol-ids which comprise activated sludge,then some orderly relationship betweenthe concentration of sludge solids andthe sludge volume index would be ex-pected. Sludge volume index deter-minations were conducted using vari-ous concentrations of sludge from sev-eral activated sludge plants. The re-sults (Figure 2) indicate that no con-sistent relationship seems to exist.The rapid increase of the SVI with

increasing concentrations is because ofthe failure of the sludge to agglom-erate into a coarse, open lattice topermit settling. The formation of thisopen lattice structure, frequently re-ferred to as agglomeration, can be de-termined readily from observation.The failure to agglomerate is an arti-fact of cylinder diameter and does notoccur necessarily in the full-scale plant(4). For very high suspended solidsconcentrations [greater than 6,000mg/1 for plants E and A2 (Figure 2)]the sludge still may not agglomerate

;

but since the maximum possible SVI

10 20TIME.min

FIGURE 1.These two sludges, withgrossly different settling characteristics,have identical SVI values.

0 5000 IOPOO 15,000SUSPENDED SOUDS CONCENTRATION, mg/jtFIGURE 2.There is no consistent re-

lationship between suspended solids con-centration and SVI.

of a sludge decreases as concentrationincreases, the greater concentrationstend to decrease the SVI. Maximumpossible SVI values are shown in Fig-ure 2 as a function of concentration.To illustrate, a sludge of 10,000 mg/1solids, even if it did not settle at all,still would have a maximum SVI equalto 1,000 ml/lOg (or 100 ml/g) whichgenerally is considered to be a desir-able SVI value. Clearly, therefore,the SVI of a sludge is highly depen-dent on its suspended solids concen-tration. SVI values measured at vari-ous solids concentrations vary widely(Figure 2). Yet SVI values com-monly are compared without regard toconcentration.Rheological Characteristics

Rheological characteristics are fun-damental measures of the physicalcharacteristics of a suspension relat-ing to deformation and flow proper-ties. To determine if these propertiesconsistently influence sludge volumeindex values, the relationships betweenthe SVI and the yield strength andplastic viscosity of various sludgeswere determined using the viscometerand procedures described by Dick andEwing (5) (Figures 3 and 4). Sludgeyield strength was not related to SVIvalues in a consistent fashion (Figure

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Vol. 41, No. 7 SLUDGE VOLUME INDEX3). Higher SVI values normally wereassociated with higher plastic viscosi-ties, but the relationship was not thesame for all sludges (Figure 4). Thusit was substantiated that SVI is not areflection of just these basic physicalproperties.

Interface VelocityThe initial sludge interface velocity

obtained in batch settling tests is usedwidely as an indication of sludge set-tling characteristics. The relationshipbetween the sludge volume index ofsludge samples and their initial set-tling velocities in one-liter graduatedcylinders was explored, and the re-sults (Figure 5) indicate that thereis not a consistent, meaningful rela-tionship between the initial settlingvelocity and the SVI. Attempts alsowere made to relate the compressionrate constant described by Roberts (6)to the SVI. Again, no meaningful cor-relation was obtained.Cylinder DiameterBecause the SVI values have been

used to predict possible underflowsolids concentrations and thus requiredrecirculation rates in full-scale plants,it is appropriate to explore the rela-tionship between settling behavior inthe SVI test and in full-scale facili-

600

600500

^400-E_-300r->

200100-

'0 0.1 0.2 0.3 0.4 0.5 0.6YIELD STRENGTH, dynes/sq cm

FIGURE 3.Sludge yield strength doesnot influence SVI in a consistent manner.

002 004 006 008 010 0.12PLASTIC VISCOSITY, dyne sec/sq cm

FIGURE 4.The SVI is not related toplastic viscosity in a consistent manner.

ties. The diameter of the standardone-liter cylinder in which SVI mea-surements are made may influence re-sults. Unless the relationship betweensettling in small cylinders and settlingin full-scale plants can be established,use of SVI values may yield mislead-ing predictions of full-scale settlingbehavior.The results of SVI experiments us-

ing various sized cylinders (Figure 6)indicate that SVI values can be ob-tained that are appreciably greater orless than the value associated with thestandard cylinder, and that the resultsfrom a one-liter cylinder may not beat all indicative of the true sludgesettling characteristics. In addition,it does not seem likely that a consis-tent relationship between SVI and set-tling in prototype tanks is possible.In this light, it is interesting to notethat researchers working with smallvolumes of sludge often conductSVI measurements in 100-ml grad-uated cylinders. The effect of cylin-der diameter on settling characteristicshas been described more thoroughlyby Vesilind (7).Initial Depth

The 1-1 graduated cylinders usedfor the SVI tests are approximately14 in. (35.6) cm tall. Intial depth,

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JOURNAL Wl'CK Julyhowever, has a considerable influenceon the sludge settling rate. Forvarious concentrations of 3 activatedsludges, settling velocities in 14-in.(35.6-cm) columns were found to befrom 18 to 84 percent of the settlingvelocities attainable in much tallercylinders (8). The lower settlingvelocities in short columns are thoughtto be caused by the increased supportprovided by underlying solids. Depthaffects different sludges differently, in-dicating that if the settling character-istics of several sludges are to be com-pared, the tests should be conductedin relatively tall cylinders.

TemperatureThe effect of temperature on the

settling of sludge was studied by Ru-dolfs and Lacy in 1934 (9) and theirdata permit calculation of SVI values(Figure 7). Clearly, the SVI is influ-enced considerably by the temperatureunder which the tests are conducted, aswould be expected because of viscositychanges. Hence, temperature changesalone can be expected to cause appre-ciable changes in SVI values. Twosludges with the same SVI value may

300 i i rPLANT D

0.2 0.4 0.6 0.8 10 1.2INTERFACE VELOCITY, iaAnin

FIGURE 5.The initial sludge interfacevelocity, used widely as an indication ofsludge settling properties, is not relatedconsistently to SVI.

0.5 1.0 1.5 2.0 2.5 3.0 3.5CYLINDER DIAMETER, in.

FIGURE 6.The effect of cylinder diam-eter on SVI is shown.

not be similar if they are taken fromtwo treatment plants with differentwaste temperatures. Also, seasonalchanges in SVI, not related to sludgesolids properties, may occur within agiven plant.StirringThe effect of stirring on sludge set-

tling is complex. Stirring is thoughtto (a) aid in the agglomeration of thesludge and (6) destroy the bridgingwithin a sludge bed in small cylinders.Both of the effects of stirring resultin better settling (4).

Because actual settling basins arenot quiescent, stirring in test cylindersmay tend to yield more realistic re-sults. The results of a series of ex-periments using slow stirring (1 rpm)(Figure 8) indicate that stirring re-duces SVI values significantly. Notethat in the stirred tests the SVI stillwas not independent of the concen-tration of suspended solids.The relationship between the quies-

cent and stirred SVI values is shownon Figure 9. Here the SVI valuesare expressed as a percent of quiescenttest SVI with a value less than 100indicating a beneficial effect of stir-ring. All of the sludge tested ex-hibited better settling under stirredconditions. However the degree ofimprovement varied with differentsludges, and no meaningful correla-tion existed between the stirred andquiescent tests.

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Vol. 41, No. 7 SUJPOE YOUJUK INDEXDiscussion

The data indicate that sludge vol-ume index is a very nonspecific, arbi-trary measure of the physical charac-teristics of activated sludge. The testdoes not provide an indication of thesettling velocity, since only one pointon the settling curve is recorded.Neither is the test a measure of thecompactability of the sludge since itis conceivable that, at the end of 30min, the interface still would be set-tling at a constant rate. It thereforeis difficult to say exactly what the testdoes measure, and therein lies its majordifficulty. It is not related consist-ently to any basic physical property,but rather represents the combinedinfluence of all of the various physicalproperties of the sludge. Regrettably,the relative influence of particularphysical properties on the SVI changesfrom sludge to sludge and, indeed, be-tween various concentrations of thesame sludge.

In addition, the sludge volume in-dex is not a representative measureof the settling characteristics of sludgein full-scale settling basins because of

300

200-

100

'0 10 20 30 40TEMPERATURE C

50

300250200150

100

500.

PLANT 0-NOTSTIRRED

] rPLANT D-STIRRED/ / ^ rPLANT E-NOT

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JOURNAL WPCF July LOGOUse of the SVI test as an operational

tool to monitor changes in sludge char-acteristics in a given plant would seemto be the only valid application of themeasurement. Sludge volume indexvalues based on pilot-plant data can-not be used to calculate the thickeningperformance of final settling basins orto determine required recycle rates.It would seem particularly inappropri-ate to use the SVI in research appli-cations. More fundamental measuresof physical characteristics are requiredin these cases.

It is suggested that alternate, moremeaningful measurements of the phys-ical characteristics of activated sludgebe used where possible. One basicmeasure which can be determined withabout the same ease as the .SVI is theinitial settling velocity associated withvarious concentrations of activatedsludge solids. This is determined byfinding the slope of the interface sub-sidence curve of activated sludge solidsin a comparatively large stirred set-tling column. It also may be possibleto relate the interface velocities to theconcentration by an equation such asv =ae bC where v and C are the inter-face velocity and concentration, re-spectively, e is the base of the natural

90y 8i 70S 60-

50-40-

2010-

PLANT DPLANT C

STIRRING RATE

0 4000 8000 12,000SUSPENDED SOLIDS CONCENTRATION, mg/tFIGURE 9.AU of the sludges tested

exhibited lower SVI values under stirredconditions.

log, and a and /; are constants (7).Use of these constants may afford amethod by which the settling charac-teristics of different sludges may becompared.

Another possibility, suggested byDick and Ewing (,9), is that ideologi-cal measures, such as yield strengthand plastic viscosity, be used to do-scribe fundamental physical propertiesof activated sludge.

It is realized that some of thesealternate methods of measuring sludgeproperties are influenced by variablessuch as temperature and concentrationjust as the sludge volume index is.However, the influence is direct andpredictable, whereas the effect of suchvariables on the SVI is indirect andsubtle. This is because the sludge vol-ume index may be influenced to vary-ing degrees by several physical prop-erties of the sludge, and each of theseproperties is influenced in a differentway by a change in temperature orconcentration. A thorough investiga-tion is required, with many differentsludges, to develop a test which will bea true measure of the sludge settlingcharacteristics and physical proper-ties. Until better methods are devel-oped, SVI still is a useful test for in-plant control. The value of SVI inresearch and design applications, how-ever, seems to be limited, and other,more basic measures should be used.The disadvantages of the alternateparameters suggested for measuringsludge characteristics is that 35 yr ofexperience are not available to aid ininterpreting the meaning of individualmeasurements as is the case with thesludge volume index. Ultimately,adoption of measures more meaning-ful than the sludge volume index maybe helpful for use in control and im-provement of the activated sludgeprocess. Accumulation of experiencewith more basic measures of sludgeproperties, perhaps obtained alongwith SVI measurements, would seemdesirable.

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Vol. 41, No. 7 SLUDGE VOLUME INDEXSummary

The sludge volume index test doesnot measure basic physical propertiesof activated sludge. As an operationaltool for in-plant control, the test isuseful, but comparisons of sludge vol-ume index measurements from vari-ous plants are not meaningful. Re-sults of SVI tests cannot be used withcertainty to predict settling behaviorin full-scale plants.Even for in-plant control, alternate

measures of settling characteristicsprobably would be more useful. Forresearch applications, alternate mea-sures of physical properties of acti-vated sludge such as settling velocity,yield strength, and plastic viscosityshould be used.

AcknowledgmentsThis work was supported at the Uni-versity of Illinois by Research GrantWP 01011 from the Federal Water

Pollution Control Administration andat the University of North Carolina byResearch Grant WP 00569 from theNational Institutes of Health.

References1. Mohlman, F. W., The Sludge Index.

Sew. Works Jour., 6, 1, 119 (Jan.1934).

2. Standard Methods for the Examinationof Water and Wastewater. 12thEd., Amer. Pub. Health Assn., NewYork, N. Y. (1965).

3. Donaldson, W., Some Notes on the Op-eration of Sewage Treatment Works.Sew. Works Jour., 4, 1, 48 (Jan.1932).

4. Vesilind, P. A., The Influence of Stir-ring in the Thickening of BiologicalSludge. Ph.D. thesis, University ofNorth Carolina, Chapel Hill, N. C.(19G8).

5. Dick, R. I., and Ewing, B. B., The Rheology of Activated Sludge. TinsJournal, 39, 4, 543 (Apr. 1907).

C. Roberts, E. J., ThickeningArt or Sci-ence? Mining Eng., 1, 61 (1949).

7. Vesilind, P. A., Discussion of Evalua-tion of Activated Sludge ThickeningTheories by R. I. Dick and B. B.Ewing. ' ' Jour. San Eng. Div., Proc.Amer. Sor. Civil Engr., 94, SA1, 185(1968).

8. Dick, R. I., and Ewing, B. B., Evalua-tion of Activated Sludge Thicken-ing Theories. Jour. San. Eng. Div.,Proc. Amer. Soc. Civil Engr., 93, SA4,9 (1967).

9. Rudolfs, W., and Lacy, I. O., Settlingand Compacting of Activated Sludge.Sew. Works Jour., 6, 4, 647 (July1934).

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APPENDIX I

THE EFFECT OF POLYMER FLOCCULATIONON THE SETTLING BEHAVIOR

OF ACTIVATED SLUDGE

byGeorge A. Farnsworth

andRichard I . Dick

Prepubl icat ion Manuscript

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THE EFFECT OF THE DEGREE OF FLOCCULATIONON THE BASIC SETTLING BEHAVIOR OF ACTIVATED SLUDGE

George A. Fransworth and Richard I. Dick

The efficiency, mode of operation, and cost of treatment by theactivated sludge process depend to a considerable degree on the efficiencyof the final settling operation. It follows that if close control andeconomical design of the process are to be achieved, settling tank designpractices should be based on a firm conception of the fundamental factorsgoverning the settling and thickening of activated sludge. The more prac-ticable aspects of final settling tank design and the influence of thedesign on the overall activated sludge process have been considered else-where (Dick, 1970b). The work reported here (Farnsworth, I967) was partof a more fundamental investigation of the settling properties of acti-vated sludge.

ACTIVATED SLUDGE SEDIMENTATIONSedimentation of a concentrated ideal suspension comprised of par-

ticles of uniform size and shape has been described by Kynch (1952). Hismodel has been shown to provide an accurate description of the sedimenta-tion of suspensions such as glass beads (Shannon and Tory, 1965) and sandgrains (Dick and Ewing, 1967) ' n which fluid drag and gravity are the onlyforces acting on the particles. However, considerable deviations fromKynch's theory have been found with flocculant suspensions such as clay(Gaudin and Fuerstena, 1962) and activated sludge (Dick and Ewing, 1967)

George A. Farnsworth is Public Works Officer, Centerville Beach NavalFacility, Ferndale, California, and Richard I. Dick is Professor of CivilEngineering at the University of Illinois at Urbana.

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Dick and Ewing (1967) showed that existence of a continuous structure withinactivated sludge even at normal mixed liquor suspended solids concentrationscaused a dependence of settling velocity upon sludge depth. Experimentally,they found the relationship between sludge depth, D, and settling velocity,v, to be

where R is the intercept on the ordinate axis of a plot of D/v as a functionof D and S is the slope of the curve. The form of the relationship has beenconfirmed by Vesilind (1968) and others. When R in equation (l) is zero oras D approaches infinity,

v - vu - I (2)where v is the ultimate settling velocity or the velocity at which theinterface would subside if it received no support from underlying sludge.Hence, the reciprocal of S is a measure of the settling velocity of sludgeif it conformed to Kynch's theory, and R, called the retardation factor,is a measure of the deviation of settling characteristics from those of anideal suspension. The retardation factor has been found (Dick and Ewing,1967) to vary with suspended solids concentration, e, according to therelations

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