& j0S LI BRA FLY OF THE U N IVERSITY Of ILLI NOI5 6ZS nesc ENGINEERING COW - ROOM The person charging this material is re- sponsible for its return on or before the Latest Date stamped below. Theft, mutilation, and underlining of books are reasons for disciplinary action and may result in dismissal from the University. University of Illinois Library SEP 2 6 1968 . - L161— O-1096 Digitized by the Internet Archive in 2013 http://archive.org/details/effectofinducedf42farn CIVIL ENGINEERING STUDIES SANITARY ENGINEERING SERIES NO. 42 7/ THE EFFECT OF INDUCED FLOCCULATION ON THE SETTLING AND THICKENING BEHAVIOR OF ACTIVATED SLUDGE By GEORGE ALLEN FARNSWORTH Supported By U. S. Public Health Service Traineeship EH 66-616 and U. S. Dept. of Interior Research Project WP 01011-01 DEPARTMENT OF CIVIL ENGINEERING UNIVERSITY OF ILLINOIS URBANA, ILLINOIS THE EFFECT OF INDUCED FLOCCULATION ON THE SETTLING AND THICKENING BEHAVIOR OF ACTIVATED SLUDGE by George Allen Farnsworth Special Problem Submitted in Partial Fulfillment for the Requirements of the Master of Science Degree in Sanitary Engineering Department of Civil Engineering University of Illinois Urbana , 111 inois August. 1967 if t^y^^ < ~,„ " mm ABSTRACT It has been shown by Dick (1965) that the settli n g velocity of activated sludge deviates from the velocity predicted by the Kynch analysis which is present'y the basis of the criteria fo>- evaluating the mass loading capabilities of a settling tank. The extent to which the settling velocity of activated sludge fails to conform to the velocity predicted by the Kynch analysis is a function of the sludge depth and retardation factor. Since the Kynch analysis fails to consider the flocculent nature of activated sludge, this study was undertaken to determine the effect of the flocculent nature of activated sludge on its deviation from ideal settling behavior,. An attempt was made to correlate the degree of artificially induced floc- culation with the retardation factor, structural support force, zone set- tling velocity, and consolidation rate constant of activated sludge. The results show that the study retardation factor and structural support force were not significantly affected by the polymer flocculent used in the study. The zone settling velocity of the sludge was found to be affected by the polymer at large initial sludge depths. The consoli- dation rate constant of the sludge was found to be decreased by f loccu lat ion. The results of the rate constant determinations also showed that a direct relationship between the product of sludge concentration and depth versus rate constant existed for activated sludge. This is in direct con- flict with other investigations which have predicted an inverse relationship. Finally the results repeatedly indicate that due to the present lack of design criteria and methods for operational activated sludge set- tling basins, laboratory settling tests conducted to collect design data must closely approximate expected settling tank conditions if valid design data are to be obtained. 1 1 1 ACKNOWLEDGMENTS The author wishes to express his sincere thanks to Dr, Richard I. Dick, Assistant Professor of Sanitary Engineering (whose previous work, as will be seen, formed the basis for this study), for his interest and assist- ance in every facet of this work Dr„ Dick gave freely of his tine and knowledge and encou r aged the author during the inevitable low moments of his study. It is hoped for the benefit of those students to follow that they will gain as much from an association with Dr.. Dick as this author has, The author also wishes to thank his laboratory assistants, Sajal Chakrabarti and Ali Rasool javaher i , for their cooperation and assistance in performing much of the lengthv and tediojs experimentation required for this study. Thanks are also in order to the United States Navy for rot in- ducting the author until after the completion of his laboratory work. This investigation was supported in. part by Traineeship No, EH 66-616 from the u\ S» Public Health Service. IV TABLE OF CONTENTS Page ACKNOWLEDGMENTS iii LIST OF TABLES vi LIST OF .FIGURES vii L INTRODUCTION 1 II . STUDY OBJECTIVES AND THEIR THEORETICAL JUSTIFICATION k ZONE SETTLING VELOCITY k STRUCTURE 1 1 SLUDGE COMPACTION ik SUMMARY OF OBJECTIVES 16 III. MATERIALS AND METHODS 18 INTRODUCTION J 8 SLUDGE SETTLING BEHAVIOR 18 PRELIMINARY DETERMINATIONS 22 Flocculent Dose 22 Optimum Fill Time 26 Ter.pcral Variations in Flocculated Sludge Settl ing Velocity 29 IV, RESULTS AND DISCUSSION 32 DATA ANAL/SIS 32 Retardation Factor 32 Settling Velocity 32 Structural Support Force *+0 Sludge Compaction hi Page DISCUSSION OF THE RESULTS kl Retardat ior Factor kl Set t 3 i rg Velocity k$ Structural Support Force 48 Sludge Compaction ^9 V. CONCLUSIONS 50 REFERENCES 51 APPENDIX A 53 VI LIST OF TABLES Number Page 1 CALCULATED CONSTANTS IN EXPRESSION RELATING RETARDATION FACTOR TO CONCENTRATION k$ VI 1 LIST OF FIGURES Figure p age 1 TYPICAL SETTLING VELOCITY VERSUS DEPTH CURVES FOR A NON-FLOCCULATED ACTIVATED SLUDGE 8 2 TYPICAL D/V VERSUS D CURVES FOR A NON-FLOCCULATED ACTIVATED SLUDGE 8 3 TYPICAL SETTLING CURVE OF INTERFACE HEIGHT VERSUS TIME 20 U TURBIDITY VERSUS pH 23 5 SUPERNATANT TURBIDITY VERSUS PURIFLOC C31 DOSE 23 6 DEPTH VERSUS PERCENT OF MEAN SOLIDS CONCENTRATION 28 7 DEPTH VERSUS PERCENT OF MEAN SOLIDS CONCENTRATION 28 8 SETTLING VELOCITY VERSUS TIME TO FILL 3»50 ft. 30 9 TIME OF FLOCCULATION VERSUS TIME TO FILL 3*50 ft 30 10 SETTLING VELOCITY OF ARTIFICIALLY FLOCCULATED SLUDGE VERSUS TIME AFTER ADDITION OF FLOCCULENT 31 11 SETTLING VELOCITY VERSUS DEPTH FOR NON-FLOCCULATED ACTIVATED SLUDGE 33 12 SETTLING VELOCITY VERSUS DEPTH FOR ARTIFICIALLY FLOCCULATED ACTIVATED SLUDGE 33 13 SETTLING VELOCITY VERSUS DEPTH FOR ARTIFICIALLY FLOCCULATED ACTIVATED SLUDGE 3^ }k D/V VERSUS D CURVES FOR NON-FLOCCULATED ACTIVATED SLUDGE 3^ 15 D/V VERSUS D CURVES FOR ARTIFICIALLY FLOCCULATED ACTIVATED SLUDGE 35 16 D/V VERSUS D CURVES FOR ARTIFICIALLY FLOCCULATED ACTIVATED SLUDGE 35 17 RETARDATION FACTOR VERSUS SOLIDS CONCENTRATION 36 18 ULTIMATE SETTLING VELOCITY VERSUS SOLIDS CONCENTRATION 37 VI 11 Figure Page 19 SETTLING VELOCITY AT AN INITIAL SLUDGE DEPTH OF 5.00 ft VERSUS SOLIDS CONCENTRATION 37 20 SETTLING VELOCITY AT AN INITIAL SLUDGE DEPTH OF 2o50 ft VERSUS SOLIDS CONCENTRATION 38 2! VOLUME INDEX VERSUS SLUDGE SOLIDS CONCENTRATION 38 22 LAMINAR STRUCTURAL SUPPORT FORCE (RELATIVE FORCE UNITS) VERSUS SOLIDS CONCENTRATION k] 23 CONSOLIDATION RATE CONSTANT VERSUS SOLIDS CON- CENTRATION x INITIAL DEPTH, (CD) 43 2k CONSOLIDATION RATE CONSTANT VERSUS DEPTH AT COMPACTION POINT kk I. INTRODUCTION Since the conception of the activated sludge process, over a half- century ago, much effort has been directed toward studying the fundamental biological mechanisms of the process and optimizing the design of the oxidation systern^ In this enthusiasm for developing the biological oxidation phase of the activated sludge process the final settling operation, an integral and often limiting phase of the process, has been generally overlooked., Conceptual models of the settling behavior of idealized suspensions nave been developed and have been verified from observations of the settling of approximately ideal suspensions* From these theoretical models, the current settling tank design practices have evolved under the assumption that activated sludge suspensions approach the ideal The validity of this characterization of activated sludge as an ideal Suspension has been challenged recently by Dick ( 1 965 ) who has shown that the area of a final settling tank as determined from actual batch settling tests on activated sludge is 125 percent to 285 percent greater than the tank area calculated from prevailing theory. Dick (1965) also found that the settling behavior of a sand sus- pension, an idea! suspension, conformed to the behavior predicted by the ideal settling theories while the settling behavior of activated sludge did not. It seems, then, that activated sludge is not an ideal suspension , a n d that methods for designing activated sludge settling tanks must be based upon the fundamental properties of activated sludge. The effect of sludge sett leabi I ity and settling design on the overall efficiency of the activated sludge process may be seen when considering the primary goal of the entire process. The effluent from the primary treatment processes contains a relatively high concentration of nonsett leabl e BOD. The oxidation phase of the process must convert this nonsett 1 eable BOD into a settleable fo r m This end is achieved by seeding the influent waste with microorganisms which proceed to oxidize the nonsett 1 eabl e BOD to carbon dioxide and water in order to derive energy to maintain reproduction. As the bacteria grow in numbers, the nonsett leabl e BOD concentration is reduced, The bacteria ultimately agglomerate into large communities called floe which are of sufficient size and density to settle under quiescent conditions. Thus the nonsett leabl e BOD is partially oxidized and partially converted to a settleable form which must be removed if a significant reduction in BOD is to be achieved. The final settling tank performs this function by providing optimum settling conditions under which the bacteria settle Gut of the waste water If no settling occurs, the oxidation process will have served on;y to convert the waste BOD from one form to another with limited overall removal . A portion of the settled BOD (sludge) must be returned to + . he aeration tank to seed the influent waste, and any excess sludge must receive further treatment. In either case a sludge handling system, including pumps and piping, is required to transport the sludge. Since the sludge is a sus- pension of settled solids in water, the sludge handling system size and cr.st is a function of the concentration of solids per unit vo'ume of water. An example given by Dick ( 1 965) shows that an increase in solids concentration from i percent to 4 percent in the return sludge reduces the volume of the return sludge to be transported by k3 percent and the required aeration tank volume by 14 percent. Thus the final settling tank not only must promote maximum remeval of solids from the waste water by settling, but must also thicken the settled solids to a desirable consistency The efficiency of the final thickener also directly affects the operation of the aeration unit- For example, suppose an aeration tank designed for a mixed liquor suspended solids concentration (MLSS) of 2500 mg/1 and a return sludge concentration of 10,000 mg/1 receives from the settling tank a return sludge at 7500 mg/1 with no change in the return sludge flow rate. The f ood- to-microorganisrns ratio (F/M) in the aeration tank will increase to k/$ of its design value,. If this incease is suffi- cient to shift the aeration tank culture into the log growth phase, the tendency of the organisms to agglomerate into settleabie f'cc may be reduced, (Both Tenney, 1965 and McKinney, 1956 have observed decreased flocculation in the early bacterial growth phases) In summary, the efficiency, operation, and cost of the activated sludge process are to a considerable degree dependent upon the efficiency of the final settling operation. Current settling tank design practices based upon ideally conceived theories which assume an unvarying suspension, have proved useful in approximating settling tank size and configuration in that settling tanks can be designed which will give solids removals of 90 percent or better if a settleable sludge is produced in the aeration tank., Present methods, however, are so restricted by the assumption of an unvarying sludge that inevitable variations in aeration tank performance which ad- versely affect sludge set t I eabil i ty cannot be effectively offset by the settling tank design. If close control and optimum economic design of the activated sludge process are to be achieved, settling tank design practices must be based upon a firm conception of the fundamental laws governing the settling and thickening of activated sludge, The present study of the effects of polymer induced flocculation on activated sludge settling was undertaken as a step toward this end. II. STUDY OBJECTIVES AND THEIR THEORETICAL JUSTIFICATION ZONE SETTLING VELOCITY Tie "ideal suspens ions" described in the current settling models are assumed to be composed of particles of uniform size and shape which are assumed to be uniformly distributed throughout any horizontal pla^e ir a settling column, Particle continuity equations and the assumptior of f^uid drag and gravity as being the only forces acting on the settling particles form tie foundation for derivation of the ideal settling models. Investigations into the app' icabi 1 ity of tne^e "-cdels to va 'i._.i suspension nave produced conflicting results* Workers have concluded tnat the models are completely, partially or slightly characteristic of natural settling depending upon the type of suspension studied-, The degree of ap- plicability of the ideal settling models seems to depend upon how nearly the suspension approaches the ideal. Sand grams which na^e only negligible attraction between particles seem to approach ideal settling behavior Dick, 1965)- Clay particle suspensions nave been shown to deviate from idea! settling behavior (Gaudin and Fuerstenau, (962). Further deviation was noted for flocculent clay particles by Michaels and Bojger (1962). Ma-ci^i (1962) found that the zone settling velocity of a given concentration of activated sludge could be increased by stirring. This is in direct :onf < t with Kynch ' s settling Model (typically idealized) which gwe; the zone settling velocity of a suspension as a furction of solids concentration d'.ie and takes no account of interpart ic 1 e forces (Kynch, 1952) . Comparison of the results of the above investigations tc the degree of attraction or adhesion between particles within the observed suspension indicates that while mutual particle attraction may be neg- ligible in an idealized suspension, it may become a prime variable in the settling of nonideal suspensions- This certainly suggests that a model which neglects an important characteristic cf a nonideal suspension must be modified or redeveloped before it can be realistically applied towards describing the settling behavior of that suspension. When considering the nature of activated sludge suspensions, a -umber of factors are evident which are neglected in the ideal settling model So Activated sludge solids are never of uniform size or shape. The sludge organisms tend to bind together to form large communities called floe, the size and shape of which are neither uniform nor constant over any given time, The degree to which fiocculation ^the agglomeration e»f individual organisms) occurs is greatly influenced by the environment in which the organisms live. Changes in fluid turbulence, substrate, available oxygen and metabolic activity, for example, are all capable of hindrance and/or stimulation of floe formation and are factors to be evaluated before applying a model to activated sludge settling The ability of sludge organisms to flocculate alsc presents the possibility that a continuous structure throughout the s-dge mass with at least plastic resistance to deformation might be formed. Recent papers on tne mechanism of biological fiocculation give further reason to be'ie^e that such a structure may exist during settling. Tenney and Stumrr ' 96M and Tenney (1965) propose that the agglomeration of organisms is caused by polymer bridging between ceils. T he polymer, they feel, is excreted under ail stages of growth by the organism in the form of capsular material which surrounds the cell wall More recently, Oabtree, et al_. (1966) state that some organisms without capsules flocculate readily while others rich in capsular material do not flocculate at alK Crabtree, e_t a]_, found that the addition of glucose tc carbon starved cells initiated synthesis of PHB, pol y-beta-hydroxybutyric acid, by the cells, and that synthesis and accumu- lation of PHB within the cells a'ways preceded f loccul at ion „ Conversely, floe formation was prevented by metabolic blocking of PHB synthesis, and def locculat ion of cells followed endogenous dissimilation of PHBo Finally PHB-rich cells, isolated native PHB, and purified PHB all demest r dted adhesive properties, Crabtree, et_ aj_. proposed that "floes may originate from ere or more polymer-rich eel's of the Zoog'ea species" which they found tc produce large amounts of PHB under proper metabolic conditions Initiation of 'apid polymer synthesis causes the cells to divide 1 ^ccmpl etei y leaving r ne new cells attached to eacn other., This incomplete division may be repeated continuously under proper nutritional conditions and ^ay result in "irregular lateral 'y linked cells." Under turbulent conditions in 'ne aeration tank, collision of the cell chains occurs The ceil chains m=y at first adhere due to mechanical entangl emert , but as polymer acc^^'ares within the cell clumps the chains may become "firmly linked by specific ester bonds which behave as ester linked beta-hydroxybuty r ic acid " This aggregation process might well be repeated until the flee grr w tc a^ equilibrium size determined by the shear rate in the turbulent liquid. Under quiescent settling conditions the floe inevitably contact each other due to local turbulence and the high volumetric concentration of the floe. Since the floe are saturated with "sticky" PHB. this sub- stance could conceivably cause the floe to adhere to each other. As the floe density increases with further settling, a continuous structure may be forrred throughout the sludge mass, Dick ('965) anticipated the existence of a continuous sludge structure in his study to determine the actual extent to which activated sludge deviates from the settling behavior predicted by the idealized settling models and to develop a realistic model of activated sludge settling- Dick proposed that the initial depth of a suspension in a settling column should influence the initial settling velocity of the solid- iiquid interface if the solid particles in the upper regions of the sus- pension receive support from the underlying particles. The deeper suspension would settle faster than a shallow suspension since the greater weight of overlying particles would cause the underlying support to fail at a greater rate. After determining the zone settling velocities of different activated sludges at various initial settling column depths, Dick found that "In every case, an increase in depth of the suspension was accompanied by an increase in settling velocity As mignt be expected the depth dependence for a given sludge was more pronounced at higher concentrations," In attempting to quantitize the degree of depth dependence of settling velocity, Dick developed an equation relating the settling veloc- ity of a giver sludge concentration to depth and f ne support exerted or the sludge interface by the underlying solids. This equation was developed from the general expression for the curves obtained when initial depth divided by settling velocity, D/V, was plotted against initial depth, D, for various solids concentrations of a single sludge, The values of D and V were obtained from curves fitted to a plot of the measured values of D and V which were determined from laboratory settling tests. Figures 1 and 2 are typical plots of V versus D and D/V versus D, respectively. o 2 0) ! I r i r — i — 0.15 3330 mg// 3830 0.10 4480 5485 0.05 1 1 1 1 1 1 Depth, ft FIG. I . TYPICAL SETTLING VELOCITY VERSUS DEPTH CURVES FOR A NON- FLOCCULATED ACTIVATED SLUDGE . 80 c E > o Depth, ft FIG. 2. TYPICAL D/V VERSUS D CURVES FOR A NON -FLOCCULATED ACTIVATED SLUDGE . The general equation of the curves in Figure 2 is y = mx + b. Substituting the experimental variables into the general equation gives: D/V = . SD + R, 1 where D = initial depth V = initial settling velocity S = slope R = intercept or retardation factor. Solving Equation 1 for initial settling velocity yields; M = SD + R Note that, with a given sludge, R is a constant for a given sclids concentra- tion* As R approaches zero, the settling velocity, Equation 2, is independent of the initial depth, therefore, R may be taken as some measure of the depth dependence of V, Consequently, from Dick's hypothesis that vertical support exists in a column of settling sludge, R, termed the retardation factor, was interpreted by Dick "to be a measure of the support which the sludge at the interface receives from the sludge below it." Rearranging Equation 2 to the form. indicates that as depth becomes very large, the quantity R/D becomes neg- ligible, and V approaches 1/S° At an infinite depth V = 1/S. k 10 Recalling that S represents the curve slopes in Figure 2, S becomes S - *& - I/V. 5 Therefore, Equation k becomes v u = l/S - -± = V, 6 where V , the ultimate settling velocity, is the value of the initial u settling velocity at an infinite depth- Dick interpreted the ultimate settling velocity "to represent the zone settling velocity at which the interface would subside if it received no support from underlying sludge,," It should be made clear at this point that Dick does not present Equation 2 as a model of activated sludge settling behavior. This equation was used merely as a means of determining the actual extent to which acti- vated sludge settling deviates from idealized settling behavior and as a means of verifying his hypothesis of the existence of particle Support within the sludge mass. The retardation factor, indicative of the mechanical support forces present in the subsiding sludge mass, was found to be related exponentially tc the sludge solids concentration by the equation, D hic "7 R = ge , 7 where R = retardation factor e = natural log base g, h = constants for a given sludge c = gravimetric solids concentration, 11 The constants g and h, from Dick's data seem to be related somehow to the bio^ogica^ cnar acterist ics of the sludge. Bulking sludges gave very high values of g and h compared to those of nonbulking sludges* Furthermore, a series of settling tests on sand suspensions having solids solid volumetric concentrations equivalent to those of the activated sludges tested yielded retardation factors of Zero f c r all concentrations. This result also reinforces the supposition that g and h are dependent upon biological factors. Since R. was zero for all finite values of c for the sand suspensions, the constant g also had to be zero. Activated sludge, a highly flocculent Suspension, yielded a finite g value in all cases, while the sand suspension, with little attraction between particles, yielded g values of zero in all cases. It may be, then, that the constant, g, may be indicative of the degree of adhesion between sludge scsid particles If it is true that g is a measure of the degree of attraction between the particles in a suspension, then stimulation or hindrance of the flocculation of an activated sludge should yield a corresponding change in the g a»~d R values of the sludge Under this hypothesis, a study was urdertaken to deter- mine the correlation between the g and R values of a given sludge and the degree of flocculation of its siudge solids. It was hcped that the results would give further insight into the importance cf sludge flocculation char- acteristics in activated sludge settling behavior. STRUCTURE The data collected from this study were also employed in an attempt to further define the nature of the interpart ic :1 e support in terms of the flocculent properties of the sludge. This analysis was made possible by the model cf activated sludge settling developed by Dick (1965)- 12 Dick's analysis of Equation 2 led him to the hypothesis that a distinct sludge structure which deforms plastically during sett sing is formed within the sludge mass, and that this structure, if present, should cause the sludge to exhibit a yield strength. (The biological factors which give clues as to the possible means of formation of this structure are discussed in the last section. ) An investigation into the rheological properties of activated sludge showed that the sludge did indeed possess a yield strength which varied exponentially with the solids concentration of the sludge. The yield strength was thought to be a measure of the "structural support force" exerted on the subsiding sludge-water interface by the sludge mass during sett 1 i ng . Dick's model of sludge settling behavior included this structural support force. The model was developed by assuming the settling s'udge mass to be in dynamic equilibrium and summing the forces exerted on tne sludge during settling. Since water must escape upwa r d through the s'udge mass during settling, two models were developed, one for laminar flow and one for turbulent flow, The models developed by Dick (1965' ^ r e shewn be'ow. v ■ v - iqo laminar flow 8 V = V " V turbulent flow, where V = settling velocity c = sludge solids concentration F = structural support force D = initial sludge depth K. through K. = constants reflecting fluid and sludge characteristics. 13 The predictions of Equations 8 and 9 were compared to observed settling behavior, and although defects in the models were noted, reasonable agreement between mathematical and empirical data was obtained. Note that each model reduces to V = f(c) as F is very small and/or D is very large. Activated sludge then should conform to ideal behavior in the absence of structura 1 support and/or at infinite initial suspension depths. Equations 2, 8 and 9 can be manipulated to show the relationship between F , c, D, S, and P {Dick, '965) The expressions for F as a function of these variables for each flow regime are: F = K77— — ^r turbulent flow st bD + 3* r DR F 1 = Kcr, or> laminar flow 11 si SD + 2R In these equations K is a constant for a given sludge. The exact physical nature of the sludge structure remains unde- fined. Dick saw the failure of this structure during settling as an extremely complex event. The escape or squeeze of water from voids within the sludge mass exerts a drag force on the sludge mass and may be the major resistance to settling during its initial phases, However, as the s'udge settles and the local solids concentration increases, the importance of the resistance to settling by water squeeze may be diminished by the increased resistance to compaction offered by the physical structure of the s'udge particle arrangement . The deformation of this structure seems To be akin to secondary consolidation of clay soils, while no mathematical treatment of this phenomenon exists at present (Leonards, 1962), it is known that secondary consolidation occurs as a result of the time dependent destruction;, rearrange- ment and reformation of bc n ds between solid particles under stress, with the ultimate aim of Nature be; r q T naT of reforming these bonds at the lowest possible stress levels. Since the constants present in his models account for fluid forceSj Dick interpreted the structural support force., F , to be indicative of the plas'ic resistance offered by the sludge structure to this rearrangement of sludge pa "tides during compaction. Since Dick related the structural >.ecc r orce mathematically to arables which were to be measured during this study, it was thought that some cor re i at ion could be obtained between the relative magnitudes of F s calculated from the measured variables and the flocculation characteristics of the sludge under study. SLUDGE COMPACTION The structural deformation of the sludge during + ne r :'ial erases of the compaction process is a plastic or creep flow phenomenon. The sludge structure deforms plastically under all loads a nd does not undergo an instan- taneous failure as would be the case in the failure of a rigid structure. However s after long periods of compaction a rigid structure may be formed if the solids reach a packing arrangement in w^j:- " ^e solids are in actual physical contact with their neighbors. Achievi-c such an arrangement may reqjire the displacement of rigid water films bound to each organism (Huekelekian and Wiesburg , 1956). The sludge volume changes which occur as a result of the secondary consolidation process and of the displacement of bound water are highly time dependent and rnay s therefore, occur only after long consolidation times. Knowledge of those mechanisms which yield sludge volume changes only after long fre pe-iods is essential to the development of fundamental concepts c f sludge settling behavior.. However, these mechanisms may be of only academic interest w^e- :onsidering their importance in development of settling T -~* desigr methods. Generally., the thickening portion of the final settling tank -.•st concentrate the settled sludge in the least possible time» At present, T ~>e determination of the volume requirements of a settling tank hi will achieve this goal is based upon a sludge consolidation rate ~ept developed by Roberts ( 1 9^+9) ■» Roberts hypothesized that the rate of escape of water f rc~ the sludge mass was dependent upon * ^e /c lume of water remaining ;n the mass. The equation developed from this hypothesis was: f - -* O - U .„ ^e--e D = dilution factor, weight water/weight solids T = time since start, of sludge compaction K = rate constant D = ultimate dilution factor at T equals infinity [ntegrating Equation 12 gives: °KT = 1 n D - D )' where = dilution factor at; T equals zero (compaction point, o D = dilution factor at some time ;) T. 16 Assuming that the sludge solids are uniformly distributed, the dilution factor may be approximated by the sludge interface height (Eckenfelder and Mel binge.- 1957)- Using this approximation, Equation 13 becomes (H t - HJ " KT " '" (H - H ) • ,k The rate constant, K, or rate of sludge compaction may be determined from settling test data by determining the slope (K.) of a semi-log plot of H - H to versus T. The values of H and H^ may be taken from plots of interface height in a settling column versus time. Roberts' (19^+9) method provided a convenient method of evaluating the effects of the flocculent characteristics of activated sludge on the con- solidation rate. It was thought that a change in degree of f locculation would affect the escape of water through the sludge mass and would 3 therefore, produce a corresponding change in the sludge consolidation rate constants. In achieving the prime objectives of this study? it was also hoped that the reproducibility of the work of Behn (1957) and Yoshioka, e^ aj^. (1955) who found Roberts' rate constants to be inversely proportional to depth at which the sludge enters compaction, and to both initial suspension dep'h and solids concentration respectively could be evaluated. SUMMARY OF OBJECTIVES The objectives of this study and their theoretical justification nave been outlined in the preceding sections. "!"he overall objective was to determine the effects, if any, of a change in the degree of f locculation of a given sludge on the sett leabi 1 i ty and compactabi 1 i ty of the sludge. This overall objective was thought best achieved by designing experimental 17 procedures to allow achievement of a series of more specific aims which were tc determine the effect of induced f loccu lation on: 1 . the g and R values, 2o the zone settling velocity, 3. the structural support force, F , and k. consolidation by determining the effect on the rate constants of a given sludgeo It was also thought that achieving the above-listed objectives would give further insight into the actual benefits to be derived from polymer in- duced sludge flocculation as a treatment plant operation aid. 18 III. MATERIALS AND METHODS INTRODUCTION Batch settling tests to determine sludge settling properties were run on sludge samples taken from a local activated sludge waste treatment plant. The degree of flocculation on the sludges tested was varied by dosing the sludges with varying amounts of a commercial pol yelectrolyte. Data from the settling tests were compared to determine changes in sludge settling properties with variation of the amount of pclyelectrolyte used. SLUDGE SETTLING BEHAVIOR The sludge observed in this study was taken from the Urban.a-Champa ign Sanitary District main treatment plant and transported to an aerated storage tank in the laboratory. The sludge was then aerated for 2k hours prior to testing in order to bring the sludge to room temperature (20°C) and to mini- mize temporal variations in the sludge settling velocity. This was found to be a necessary precaution by Dick (1965) who observed that while significant changes in sludge settling velocity occurred during the first 10 to 20 hours after removal of the sludge from the treatment plant, such changes became minimal after 2k hours. After this stabilization period, settling tests to determine the settling behavior of a volume of sludge from the laboratory storage tank were conducted in an apparatus similar to that used by Dick (1965) . Each of four plexiglass columns, 3-5 in. in diameter, were filled at a predetermined rate with activated sludge. The sludge was pumped from a 10 gal reservoir through a valved port in each column base. Upon filling the columns to varying initial depths, the sludge was allowed to settle and the height of each sludge 19 interface above each column base was plotted as a function of time over ar half-hcur period. Figure 3 is a typical settling curve. At the completion of the four settling tests, the valved ports were opened to allow the sludge to return by gravity flow to the reservoir. Care was taken to prevent the deposition of solids in the piping system and columns after the settling tests had been completed. This procedure was repeated twice for each sludge solids concentration so that settling data for six different initial sludge depths and two 3-5 ft control depths were obtained. SoHds concentrations were changed by diluting the sludge in the reservoir with a predetermined volume of supernatant liquor. Following dilution, the airflow rate through the reservoir was increased for a short time to obtain complete mixing of the supernatant liquor. After mixing the reservoir contents, five 15 ml sludge samples were taken from various res- ervoir depths. The solids concentration of these samples was determined by the Gooch crucible technique (Dick, 1965). The average of the five determin- ations was considered to be a good approximation of the solids content of the reservoir sludge. Before repeating the settling test procedure, the distribution system and columns were flushed with tapwater to remove any deposited solids from the previous tests. Settling tests were run on a total of four differ- ent solids concentrations on the sludge. (A settling test series on four sludge dilutions required 6 to 8 hours.) Following the series of observations made on the raw sludge : , the testing procedure was repeated to obtain settling data on an artificially f Sacculated sludge," The reservoir was filled with a known volume of raw An "artificially flocculated sludge" is a sludge which has been dosed with Pu r ifloc C31. "Raw sludge" denotes a sludge which has not been artificially flocculated. 20 0) X From this series of tests, a plot of turbidity versus pH was obtained (Figure k) . Figure 5 indicates that for a sludge at pH = 7^, the optimum dosage range was 5°6 mg/g to I 1.0 mg/g (10 to 20 mg/l). In obtaining this curve no sludge pH adjustment was necessary^ The pH of the test sludge was 7°! and the pH of the flocculent was 7-0. Figure k shows the effect of pH on turbidity f° r a constant flocculent dose of 5-7 mg/g (10 mg/l)* Note that in contrast to turbidity changes in Figure 5 little change in turbidity occurred over the test pH range ir. Figure k. It. was noted during the test run that the sludge possessed a high buffering capacity* From these results it was concluded that no pH control would be requi r ed during the settling tests if the sludge to be tested had a pH within the range of 6.8 to 7-5- During the first flocculation test series it was found that a wide range of flocculent doses did not significantly affect sludge pH , As a precautionary measure the sludge pH was checked during the flocculation of the sludge prior to settling. The pH of the sludge did not change throughout the entire settling test period* Since the study objectives required observation of changes of sludge settling behavior with changes in the degree of sludge flocculation, two flocculent doses within the range of mg/l to 5 6 mg/ 1 were used. This range covered the descending portion of the curve in Figure 5", there- fore, incremental flocculent doses in this range produced changes in the degree of flocculation of the sludge. It. was assumed for this study that supernatant turbidity could be used as a measure of the degree of flocculation of the sludge by Purifloc C3 1 • This parameter has been found to be sensitive to changes in flocculent 25 dose by Black and Chen (1965)- In assuming supernatant turbidity to be a measure of the degree of sludge f Jocculat ion, it was inherently assumed that the degree of supernatant clarity was directly related to the degree of f Iccculat ion. Since the degree of flocculation relates to the degree to which the sludge solids agglomerate into settleable floe which, in n j influences the clarity of the supernatant, it was thought that the assumption was reasonably valid. It must be admitted, however, that use of the turbidity measurement was primarily an expediency dictated by the lack at present of a better measure of the degree of flocculation. One other measure of flocculation suggested by Tenney (1965) was evaluated and found to be too insensitive for use in this study, Tenney has used the percent dispersion of microorganisms in the supernatant as a measure of f loecu lation. Measurement of this parameter requires determining the total COD of the sludge, the COD of a filtered sludge sample, and the COD of the supernatant after one-half hour settling. The percent dispersion of the microorganisms in the supernatant for a given fiocculent dose is then calculated by % dispersion = supernatant COD - soluble COD mixed liquor COD- solub'e COD x 100. 14 Determination of this parameter required much time and the result was ar insensitive measure of supernatant clarity. Dte tc the insensitivity of this parameter and the time involved in its determination, turbidity was chosen as a measure of flocculation for this study. The concern for pH in this study was due to its influence on the properties of both the sludge bacteria and the polymer. The polymer used, Purifloc C3U was a long-chain organic molecule. The diffusion character- istics and the extent of coiling of the polymer chain are directly affected 26 by pH The surface charge of the bacteria is also altered by the pH of its water environment. Optimum Fill Time Dick (1965) developed a settling column filling procedure which gave uniform solids distributions for all initial sludge depths. This pro- cedure, with slight modification, was used in determining the optimum column fill rate for the settling tests conducted in this study. Figure 3 shows a typical settling curve. The parameter, t,, , called the time of f 1 occul at ion, is indicative of the time required after the pump is shut off for turbulence to subside and for the sludge solids to ref locculate . After this flocculation time has elapsed, the soMds begin to settle. For reproducible settling results, the column filling rate must create optimum turbulence in the column to maintain uniform solids distribution during filling Determination of the optimum fill time for this study was ac- complished by trial and error. A column was filled to 3*5 ft with a sludge of known solids content; the time to fill to this depth was noted. The settling cur*/e for the sludge at this depth was then obtained. The time of flocculation for this fill time was determined as shown in Figure 3 Fol- lowing this determination, the column was refilled to 3-5 ft at the same fill rate, and sludge samples were taken at one foot column intervals im- mediately after stopping the pump„ (The contents of the sampling taps in the column were displaced prior to sampling.) The solids concentration of each of the samples was determined by the Gooch crucible technique (Dick, 1965) 27 The columns were drained after sampling and again refilled to 3 5 ft at the same fill rate. At the end of the predetermined time of f loccu lat ion, solids samples were again taken at one foot intervals* This procedure was repeated several times, the results being that an average solids concentration at one foot, column intervals was obtained at both t = and t = tr for a given fill time The results were plotted as column height versus percent of mean solids concentration, (The mean solids concentration was determined by averaging the solids content of several samples taken from the sludge reser- voir.) The criteria for accepting or rejecting a fill time was that the solids profile at t = and t = t r should fall within 5 percent of the mean solids concentration. The above procedure was repeated using different fill times until an acceptable fill time was found, Figures 6 and 7 summarize the results obtained from the above procedure, A fill time of 1.00 min was chosen for use in the settling tests. This fill time (Figure 6) gave solids distributions which came closest to meeting the given criteria- Note that the solids distributions for the dilute sludge (Figure 7) were very erratic even though the fill times were quite close to one minute- As a result of these findings, the lowest so" ids con- centration used in the settling tests was kept well above 1370 mg/1 . The solids concentration used in determining the optimum fill time fell about mid-range of the settling test solids concentrations used. During the settling tests, the pump rate was adjusted for various depths so that aH depths greater than or equa' to 3-5 ft would be filled in cne minute- The fill time for depths less than 35 ft was a fraction of one Tii n ute determined by the depth desired divided by 3-5 ft- 28 s t f = 1.4 min. Fill At 1.0 min. V s = 0.096 ft/min SS = 4300 mq/JL t = 1.3 min. 90 100 Percent FIG. 6 DEPTH VERSUS PERCENT OF MEAN SOLIDS CONCENTRATION. FIG. 7 t = o I l I l I i A r \ \ t f = 0.9 min. Fill At 1.05 min. V $ = 0.264 ft/min. SS= 1370 mg// t = 0.90min. 80 no 120 90 100 Percent DEPTH VERSUS PERCENT OF MEAN SOLIDS CONCENTRATION. 29 Plots of average settling velocity and time of flocculation versus fill time were obtained from the data given in the determinations. Figure 9 follows the expected relationship between tr and fill time. The lower the fill times (high flow rates) cause greater turbulence than the higher fill times (low flow rates) and would be expected to yield higher flocculation times. The same follows for Figure 8; the extreme fill times allow stratification of solids to occur and would be expected to affect sett 1 ing veloc ity. Temporal Variations in Flocculated Sludge Settling Velocity A sludge was flocculated with k.2 mg/g Purifloc C31> and settling tests were run over a 10 hour period to determine whether or not temporal changes in settling velocity occurred. Figure 10 shows that little change occurred over the 10 hour period. The control settling tests run during the tests on flocculated sludge also indicated that no change occurred. 30 0.120 0.090 0.40 0.60 0.80 1.00 1.20 0.40 Time To Fill 3.50ft, min. FIG. 8 SETTLING VEUDCITY VERSUS TIME TO FILL 3.50 ft. 1.60 c E I 80i— I 60 1.40 — 1.20 — 1.00 0.80 0.40 60 RG. 9 0.60 0.80 1.00 120 1.40 Time To Fill 3.50 ft, min. TIME OF FLOCCULATION VERSUS TIME TO FILL 3.50ft. 31 o.io 008 c I Z 0.06 o o 0) > C 0.04 a> CO 02 Purifloc C 31 At 4.2 mg/g SS At 3250 mg/X -O 12 3 4 5 6 Time, hrs FIG. 10 SETTLING VELOCITY OF ARTIFICIALLY FLOCC- ULATED SLUDGE VERSUS TIME AFTER ADDITION OF FLOCCULENT 32 IV. RESULTS AND DISCUSSION DATA ANALYSIS The settling curves obtained in this study were analysed to deter- mine the effects of artificial flocculation by Purifloc C31 on the settling properties of activated sludge. The procedure used in analysing the data is out 1 i ned bel ow. Retardation Factor Figures 11, 12, and 13 show the zone settling velocities obtained from the laboratory settling curves plotted as a function of initial depth. A family of curves for various solids concentrations of each sludge was fitted visually to the data* Values of zone settling velocity at various depths 1 were obtained from these fitted curves and were manipulated to obtain family plots of D/V versus D for various concentrations of each sludge as shown in Figures 14, 15, and 16= The D/V versus D plots showed that a finite value of R, the retardation factor, existed for all sludge concert rat ions studied and that Dick's expression of settling velocity as a function of depth and retardation (Equation 2) fitted the curves obtained. Since Dick l s analysis was verified by the data, the retardation factors were plotted as a function of solids concentration for each s'iudge on semi-log paper (Figure 17) to determine the values of g and h for each sludge. Sett 1 inq Ve loc ity In determining the effect of flocculation on sludge zone settling velocity, the ultimate settling velocity and the settling velocity at two finite depths were plotted as a function of solids concentration for each sludge (Figures 18, 19> and 20). The ultimate settling velocities for various 33 0.15 0.10 o o c 0.05 — — r~ T — T" ! - 1 — " 1 — Purifloc C3I at mg/g — 3330 mg// a 3830 4480 A 5485 1 L A, 1 1 1 1 1 FIG. II Depth, ft SETTLING VELOCITY VERSUS DEPTH FOR NON- FLOCCULATED ACTIVATED SLUDGE . 0.20 0,15 o o > c «) en 0.10 0.05 Purifloc C3I at 0.97 mg/g 4465 mg// 4680 5130 6145 FIG. 12 Depth, ft SETTLING VELOCITY VERSUS DEPTH FOR ARTIFICIALLY FLOCCULATED ACTIVATED SLUDGE. 34 0.15 010 o o c 0) CO 0.05 FIG. 13 Depth, ft SETTLING VELOCITY VERSUS DEPTH FOR ARTIFICIALLY FLOCCULATED ACTIVATED SLUDGE . > Q 12 3 4 5 6 7 Depth, ft FIG. 14 D/V VERSUS D CURVES FOR NON -FLOCCULATED ACTIVATED SLUDGE. 35 160 Purifloc C3I At 0.97 mg/g 6145 mg/Jl 5180 4680 4465 2 3 4 5 6 7 Depth, ft FIG. 15 D/V VERSUS D CURVES FOR ARTIFICIALLY FLOCCULATED ACTIVATED SLUDGE. 6050 mg// Purifloc C3I At 243 mg/g Depth, ft FIG. 16 D/V VERSUS D CURVES FOR ARTIFICIALLY FLOCCULATED ACTIVATED SLUDGE. 36 500 - -O 300 -O- mg/g Purifloc C 31 ■-*- 0.97 mg/g Purifloc C3I -D— 2.43 mg/g Purifloc C 31 100 70 50 30 o c o o T3 & >° 2000 4000 Solids Concentration, mg/JL 6000 FIG. 17 RETARDATION FACTOR VERSUS SOLIDS CONCENTRATION. 37 20 15 — c o 010 > C o m o 05 0) o E mg/g Punfloc C 31 A 97 mg/g Punfloc C 3 a 2 43 mg/g Punfloc C 3 FIG. 18 mg/g 0.97 mg/g 243 mg/g 6000 2000 4000 Solids Concentration, mg/j£ ULTIMATE SETTLING VELOCITY VERSUS SOLIDS CONCENTRATION. c 'E O o in Q° 0.15 10 U — 005 > c mg/g Punfloc C 31 A 97 mg/g Punfloc C3I — a 2 43 mg/g Punfloc C3I — mg/g- ' \^ ~ 97 mg/g ! 2 43 mg/g -^A x. 1 1 a> (/) oi 2000 4000 6000 Solids Concentration, mg/>£ FIG. 19 SETTLING VELOCITY AT AN INITIAL SLUDGE DEPTH OF 5.00ft VERSUS SOLIDS CONCENTRATION. 38 .£ 0.15 o in n Q° 0.10 — < O O ~ 0.05 > c 0) mg/g Purifloc C 31 a 97 mg/g Purifloc C 31 -a 2.43 mg/g Purifloc C 31 \~\ s- 0.97 mg/g mg/g- , I 2.43 mg/g — -_J ^v 1 1 2000 4000 6000 Solids Concentration , mgA? FIG. 20 SETTUNG VELOCITY AT AN INITIAL SLUDGE DEPTH OF 2.50ft VERSUS S0UDS CON- CENTRATION. 0.15 _U> CD C a> E 5 0.10 0.05 O A ■□ 2.4: mg/g Purifloc C 31 - 0.97 mg/g Purifloc C 31 mg/g Purifloc C 31 2000 Solids 6000 FIG. 21 4000 Concentration, mg/X VOLUME INDEX VERSUS SLUDGE SOLIDS CONCENTRATION. 39 solids concentrations were obtained from the slopes of the D/V versus D curves. [For example, Figure )k shows that the slope of the D/V versus D curve for raw sludge at 3830 mg/1 was 8.8 min/ft. From Equation 6, the corresponding zone settling velocity in the absence of retardation and/or at infinite sludge depths (the ultimate settling velocity) was 1/8 8 ft/ min or 0>'25 ft/min ] The zone settling velocities for various solids concentrations were taken from the fitted curves shown in Figures 11, } 2, and 13 at depths of 5 00 ft and 2 50 ft. To aid in the settling velocity analysis> a relative measure of the half hour settled sludge volume was developed Si n ce Dick (1965) has found that, the settling velocity of activated sludge is influenced by the column diameter at column diameters smaller than 3-5 in., it was thought that the standard sludge volume index, which is determined from half hour settling tests in 1 liter graduated cylinders (.$. t an da r d Me t h od s , ;965)> would not be representative of the half hour settled sludge volumes obtained in the 3»5 in. settling columns. Therefore, the volume parameter used in the analysis was arbitrarily defined as the volume occupied by the sludge in the 3 5 in. columns after 30 min settling from an initial depth of 3 5 ft. This parameter was termed the volume index This volume index was calculated directly from the laboratory settling curves as fc31ows^ (H_ )(3 l^d 2 /4)'28 32) (c)(3^d 2 A)(H o )f28 32)' where H_ = sludge interface height after 30 min settling from a>~ initial sludge depth of 3 5 ft, ft d = column diameter, ft kO c = initial solids concentration, g/1 H = initial sludge depth, ft. An arbitrary value of 3=5 ft was selected and used consistently for all VI determinations, VI = volume index, 1 /g Equation 15 may be reduced to; VI = hi cH A plot of the volume indices as a function of initial sludge solids concen- tration was obtained for each sludge (Figure 21) „ The volume index data were compared to other data obtained in this study to show, as will be seen later, that an arbit rar il y def ined vol ume i ndex is in itself of little use in settling tank design practiceo [No attempt was made to correlate VI with the standard sludge volume index (SVI) described in Standard Methods (1965); therefore, the VI values obtained in this study could not be compared to the standard SVI's obtained in other invest igat ions j Structural Support Force Figure 22 shows the relative values of the laminar structural support force at two initial sludge depths. These values were calculated from Equation 11 and were plotted as a function of solids concentration for both the raw sludge and the sludge dosed with 2»43 mg/1 Purifloc C3 1 - The parameters used in calculating the "laminar support forces were obtained from the D/V versus D curves shown in Figures 14 and 15- Relative values were thought to be sufficient as only relative changes in the support with changes in the degree of flocculation were of interest; therefore, the constant K, which is related to the weight of sludge solids was not calculated k] O mg/g Purifloc C 31 □ 2.43 mg/g Purifloc C 31 6000 5000 ■£? 4000 c 3 0) u £ ® 3000 o 0) QC I? 2000 — 1000 — 3000 4000 5000 6000 7000 3000 4000 5000 6000 7000 Solids Concentration, mg/X FIG. 22 LAMINAR STRUCTURAL SUPPORT FORCE (RELATIVE FORCE UNITS) VERSUS SOLIDS CONCENTRATION. kl Support force values for the sludges were calculated, and the results indicated that little change in F had occurred during f 1 occul at ion. Only the two sludges which differed most in their degrees of flocculation (0 mg/g and 2.^3 mg/g) are shown in Figure 22> Since both the turbulent and laminar equations for structural support force (Equations 10 and 11) gave similarly poor results when F and F . were calculated from the experimental data, only one support force expression (Equation 11, laminar flow) is represented in Figure 22* Sludge Compaction The consolidation rate constant K was calculated for each labor- atory settling test by the method outlined in Chapter II, To evaluate the reproducibility of the conclusions of Behn (1957) and Yoshioka et_ aj_. ('955), the k values for each sludge were plotted as a function of the product of initial sludge depth and solids concentration, CD, (after Yoshioka et al .) and as a function of the depth at which the settling so' ids entered com- paction (after Behn) In each plot only the data for the mg/1 and 2.^3 mg/1 Purifloc C3 1 doses were shown in order to eliminate unnecessary con- jestion on the graphs. The 0.97 mg/1 Purifloc C31 s'udge rate constants gave the same results when plotted as did the other two sludges, thus the 0-97 mg/1 dose was excluded for the sake of clarity. The plots described above are shown in Figures 23 and 2k. DISCUSSION OF THE RESULTS Retardation Factor Figure 17 shows the curves fitted by the least squares technique to the data obtained from the D/V versus D plots (Figures \k and 16). The ^3 0.08 c I c o w c o o o a: o o CO c o o 0.07 0.06 0.05 0.04 0.03 0.02 0.01 1 1 o mg/g Purifloc C3I A 2.43 mg/g Purifloc C3I 5000 10000 15000 20000 Solids Concentration x Initial Depth, mg. ft /I 25000 FIG. 23. CONSOLIDATION RATE CONSTANT VERSUS SOLIDS CONCENTRATION x INITIAL DEPTH . (CD) . kk c E c o u> c o o d> o QC a ■o o c o o u.ou 1 mg/g Purifloc — 1 1 C3I A 2.43 mg/g Purifloc C3I A 0.70 0.60 • // - A // 0.50 A // i 0.40 • // 1 If A 0.30 f 0.20 ' / - / 0.10 f° v o J A o o I 1 1 FIG. 24 12 3 4 Depth At Compaction Point , ft CONSOLIDATION RATE CONSTANT VERSUS DEPTH AT COMPACTION POINT . ^5 curves indicate that the flocculent doses used in this study did not have a significant effect upon the sludge retardation factor. No ordered variation in the slopes of the curves is evident, and the reliability of the fitted curves is obviously low due to the limited number of data points. (The number of data points obtained was limited by the excessive time required to run the three settling test series, about 36 hours. Longer run times would have increased the possibility of temporal variations in the sludge*) The g and h values of each sludge as calculated from the fitted curves in Figure 17 3^^ shown in Table I There seems to be no correlation between either g or h and the flocculent doses used in this study. TABLE I CALCULATED CONSTANTS IN EXPRESSION RELATING RETARDATION FACTOR TO CONCENTRATION Purifloc C31 Dose 3. h mg/g 0. 1956 0.000847 0.97 mg/g . 3 1 ^0 0.000728 2.43 mg/g 0.1185 0.000967 Settl inq Vel ocity Figures 18, 19> and 20 show that flocculation had a significant effect on zone settling velocity at infinite depths and a lesser effect at lower initial sludge depths. Note that the curves representing the two artificially flocculated sludges are nearly identical in each of the plots. This result indicates that increasing the Purifloc C3 1 dose from 0.97 mg/g to 2.43 mg/g did not yield a significant increase in the degree of sludge flocculation. Figure 5 shows that the degree of flocculation should have increased with an increase in flocculent dose; however, the mixing procedure kG used in the settling tests which was not as closely controlled as the pro- cedure used in obtaining the flocculation curve (Figure 5) may have limited the degree of flocculation obtainable during the settling tests. Recall that the dosed sludge was mixed in the aerated reservoir and then pumped into the settling columns (Chapter III) at the start of the settling tests, Since a constant fill time was used in the tests, the time of flocculation was nearly constant for all tests. This ref locculation time may have limited the degree to which the sludge flocculated regardless of the Purifloc C31 dose. The slopes of the curves in Figures 18, 19> and 20 show that the effect of flocculation on sludge settling velocity for a given sludge solids concentration decreased at the lower initial sludge depths. At infinite initial sludge depths no underlying support force exists to hinder the set- tling velocities of the sludge particles. Thus at large initial sludge depths flocculation, which causes sludge particles to agglomerate and in- crease in mass, would cause the substantial increase in sludge settling velocity shown in Figure 18. At lower initial sludge depths the under- lying support force becomes more pronounced and thus causes the sludge to settle at less than ultimate settling velocities Due to this support force and due to the fact that flocculation did not seem to affect sludge retardation significantly, it would follow that flocculation would have a decreased effect on settling velocity at low initial depths. The results shown in Figures 18, 19> and 20 are significant, in that they show that the benefits to be obtained from plant scale floccula- tion must be evaluated at actual settling tank depths and solids concen- trations if accurate information is to be obtained. The figures show that hi f locculat ion tests or. an activated sludge of a given solids concentration would give varying results depending upon the sludge depth used to conduct the tests. Note that at low solids concentrations (3000-5000 mg/1) settling tests on flocculated sludge at great initial depths would show greater im- provement in settling velocity than would be found if low initial depths were used. At solids concentrations greater than 5250 mg/1 tests would also show varying degrees of decrease in settling velocity by flocculation again depending on the test sludge depth used. All this indicates that laboratory sca'e settling tests may give results which would not be in- dicative of sludge settling behavior in an actual settling tank. The type of plot shown in Figures 18, *9> and 20 might prove useful in determining the optimum economic design of a settling tank to be used for settling flocculated sludges. A set of curves for various doses of flocculent could be developed from settlirg tests on various probable sludge solids concentrations and tank, depths Benefits and costs for various solids and tank depths cou'd then be calculated The savings in settling tank surface area (construction cost) for a given solids concentration and tank depth over the tank area required for a raw sludge at the same solids concentration and depth would constitute the benefit. The cost would be the construction cost, plus the cost of flocculation- Costs and benefits could be calculated for other solids concentrations and tank depths. The net benefits or cost-benefit ratio could be maximized depending upon the economic criteria and the optimum settling tank design chosen. It must be kept in mind that if an analysis of the above nature is to be reliable, the laboratory curves must be derived from tests which closely approximate actual settling tank conditions. k8 Figure 21, the plot of volume index as a function of solids con- centration, emphasized the limitations of any arbitrarily defined volume index to be used in settling tank design,, The plot shows that the floc- culated sludges occupied a smaller volume after 30 min settling than the raw sludge; however, Figure 21 alone does not indicate whether this improve- ment in VI was due to an increased settling velocity or to an increased compaction rate or a combination of the two. Since the flocculated sludges generally settled at a greater rate and consolidated at a lessee rate than the raw sludges, the VI seems to have been a crude measure of sludge settling velocityo If it had been assumed without qualification that the VI was a measure of consolidation rate and if the VI had beer used in designing a settling tank, an unrealistic design would have been obtained (it will be shown later that the flocculated sludge generally compacted at a slower rate than the raw sludge.) Thus without further information volume index measurements may be misleading when used as dosing paramete-s Structural Support F orce In computing the values of F . and F the constant in Equatu^s s' st 10 and 11 was assumed to be the same for both sludges since the volume of Purifloc 031 used could not have changed appreciably the effective weight of sludge solids* (The constant was defined by Dick ( 1 965) as the "effective weight of sludge solids in a cross sectional area of settling basin per unit depth and concentration J 1 ) The relative values of F . and F were s 1 s t computed for all three sludges tested for initial sludge depths of 5 00 ft and 250 ft. The results showed that the Purifloc C3 1 doses had little effect on both laminar and turbulent support forces. T he data shown in ^9 Figu r es 22Aa r "d22B are for the laminar model only. The turbulent model gave similarly poor correlation between F and Purifloc C3 1 dose, thus these re- sults were not shown here. Sludge Compaction The consolidation rate data were widely scattered which caused difficulty in fitting reliable curves to the data (Figures 23 and 2^+) . The curves shown, therefore, are merely indicative of the trend of the data. Figures 23 and 2k indicate that flocculation generally decreased the sludge consolidation rate for a given solids concentration and sludge depth* Note also that, the detrimental effect of flocculation increases with High CD (Figure 23) and D (Figure 2k) values. These results are further proof of c the necessity for evaluation of flocculated sludge behavior at concentrations and depths at which the flocculent is to be used in plant scale settling. One object of this study was to reproduce the concl sic^s of Be 1 --- (1957) and Yoshioka et ak (1955)- Behn (1957) predicted an irverse re- lationship between the consolidation rate and the dept^ at >/vhich the sr i ids entered compaction. Yoshioka et a_[» (1955) suggested an inverse relationship between consolidation rate and both initial sludge depth and solids concen- tration. While the reliability of the curves shown in Figures 23 and 2k may be questioned., the trend of the data is obvious. The data show that K varies directly with CD and compaction point depth. 50 V, CONCLUSIONS 1. No correlation between sludge g and h values and the flocculent doses used in this study was found. 2 Flocculation significantly affects sludge zone settling velocity at large depths and has less effect on settling velocity at lower finite depths due to the existence of a support force at the lower depths. 3. Settling velocity-polymer concentration curves might prove useful in econcmic evaluation of settling tanks to be designed for artificially flocculated sludge- 4. The evaluation of benefits to be obtained from flocculation of activated sludge must be done at sludge depths and solids concentrations found in actual plant operation if realistic information is to be obtained. 5, Any arbitrarily defined volume index in itself has limited use in set- tling tank design. 6, No change in structural support force with flocculation was found. 7 - The consolidation rate constant of the activated sludge obse-ved in this study was decreased by flocculation. This decrease was minimal at low CD values and more pronounced at high CD values 8 Consolidation rate data indicate that the effect of flocculation on con- solidation rate of a given sludge must be evaluated for actual plant sludge depths and solids concentrations. 9- A direct relationship between the sludge consolidation rate constant and sludge depth times solids concentration (CD) was found. 10, A direct relationship was found to exist between the consolidation rate and the compaction point depth of the sludge 51 REFERENCES 1. Behn, V. Co 1957- Settling Behavior of Waste Suspensions. J, San it . Engo Pi v. Am. Soc. Civil Engrs . , 83 : SA5 , 1 -20. 2 Black, A. P., and Chen, C. 1 965 E lectrophoretic Studies of Coagulation and Flocculation of River Sediment Suspensions with Aluminum Sulfate. J_. Am. Water Works Assn ., , 57,35^-362. 3. Crabtree, K„, Boyle, W., McCoy, E. , and Rohlich, G. A. 1966. A Mechanism of Floe Formation by Zooglea Raruigera. J. Water Pollution Control Fed ., J8: 1969-1980. 4. Dick, R. I. 1965 Applicability of Prevailing Gravity Thicken ing Theories to Activated Sludge. Civil Engineering Studies, Sanitary Engineering Series No. 31- University of Illinois, Urbana, Illinois, 214 p.. 5 Eckenfelder, W. W., Jr., and Melbinger, N. 1957- Settling and Compaction Characteristics of Biological Sludges, I. General Considerations. Sewage and Ind Wastes, 29. 1 1 14- 1 1 22 . 6 Gaudin, A. M., and Fuerstenan 3 M. C. 1962 Experimental and Mathematical Model of Thickening, Trans. Soc Mining E^. , 223.122-129 7- Heukelekian, H. , and Weisburg, E. 1956, Bo-jnd Water and Activated Sludge Bulking. Sewage and Ind . Wastes, 23: 558-57*+° 8 Kynch, G. J. 1952. A Theory of Sedimentation. Trans . Faraday Soc . , 48: 166-176. 9. Leonards, G. A. 1962. Engineering Properties cf Soils. Fc^rdat ion Enqr. , (Leonards, G. A., Ed.) McGraw-Hill New York, 66-241. 10. Mancini, J. L 1962 Gravity Clarifier and Thickener Design. Prcc 17th Ind. Waste Corf. , Purdue Univ., 267-277 11. McKinney, R. E. 1956 Biological Flocculation. Biological T re a t _n-e n *■_ .; f Sewag e and Industrial Wastes, Vol. I. (McCabe, B J., and Eckenfesde-. W. W , Jr., Eds.) Reirlhold, New York, 88-100.. 12. Michaels, A. S., and Bolger, J. C. 1962 Settling Rates and Sediment Volumes of Flocculated Kaolin Suspensions. Ind. Eng. Chem. Fundamentals, 1:24-33- 13 Roberts., E. J. 1949. Thickening - Art or Science? Mining Eng. , J_;6l-64. 1 4 . Standard Methods for th e Examination of Water and Was tewater. 1 960 . 11th Ed,, Am. Pub, Health Assn., New York, 629 p. 52 5» Tenney, Mo W» 1965- Chemical Enhancement of Biological Waste Treat- mer t. Ph»D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass, 16 Tenney, Mo Wo, and Stumm, W. 1965° Chemical Flocculation of Micro- organisms in Biological Waste Treatment J. Water Pollution Control Fed., j$£: 1370-1388. 17. Yoshioka, N., Hotta, Y., and Tanaka, S. 1955- Batch Settling of Homo- geneous Flocculated Sluries. Chem. Eng . (Tokyo) , 19»6l9-626. 53 APPENDIX A SYMBOLS Symbol Quant i ty Dimensions 3 c gravimetric suspended solids concen- F/L t ration D depth L D dilution factor d imersion less D ultimate dilution factor at T equals dimensionless infinity 00 D dilution factor at T equals zero dimensionless o D dilution factor at some time,T dimensionless d column diameter L e natural log base dimers ionl ess F structural support force F F . structural support force in laminar F model F structural support force in turbulent F model f function dimens ion I ess g constant relating retardation factor T to an exponential function of concen- tration 3 h slope of plot of logarithm of retardation L /F factor vs. concentration H depth of column of sludge at some time, t L H depth of column of sludge at t equals zero L H^ depth of column of sljdge at t equals L inf ini ty H, n sludge interface height after 30 minutes L settling from an initial depth of 3 <■ 5 ft 5k Symbol K r K 2' K R S V V u VI Quantity constants reflecting fluid and sledge characteristics rate constant in compression zone retardation factor slope of D/V vs. D curve time since start of sludge compaction time of flocculation sett ling velocity ultimate settling velocity volume index Dimensions T T T/L T T L/T L/T 3 LVF