M EASU R I NG SED I M ENTATI ON-FLOCCU LAT IO N EFFICIENCIES HENRY C. BRAMER AND R I C H A R D D. HOAK Mellon Institute, Pittsburgh, Pa.
A procedure for estimating the degree of flocculation occurring in a sedimentation basin permits evaluation of coagulant aids under field conditions despite variations in the suspensions being treated. Basin performance is related to an empirical laboratory determination and to basin performance as predicted by equations which apply to the case of discrete particle sedimentation. The laboratory procedure can also be used to determine the potential worth of coagulant aids without reference to any specific sedimentation basin. The application of the procedure is demonstrated by data obtained in laboratory model basins using kaolin clay suspensions and in field trials of a coagulant aid in a clarifier treating blast furnace gas-washer water.
HE
authors have described the case of discrete particle
Tsedimentation in terms of an empirical laboratory determination (1-4). The “sedimentation index” (SI) measures the performance of a settling basin and is equivalent to the time, in minutes, required to achieve a similar degree of separation in a l-liter Griffin beaker under quiescent settling conditions when flocculation does not occur. Present research is directed toward a similar quantification of the sedimentationflocculation process-Le., sedimentation with particle growth. Of particular interest is the case in which coagulant aids may be used in present equipment to improve sedimentation efficiency. I n general, separate flash mixing and flocculation mixing are not provided. A useful technique for the evaluation of coagulant aids and the prediction of basin performance has emerged in the course of this investigation; it is presented here in itself and as helpful in defining the nature of the problem.
A settling rate curve for use in the sedimentation index determination is constructed by allowing samples of a suspension to settle quiescently in I-liter Griffin beakers for periods of time from 1 to 60 minutes. At the end of a settling period, a 250-ml. portion is withdrawn from the midpoint of the beaker and analyzed for suspended solids. A plot of the suspended solids concentrations us. the settling time is the settling rate curve desired. Values of concentration us. the logarithm of the settling time usually plot as a straight line, allowing reasonable extrapolations. If the suspension settles as discrete particles, the effluent concentration of a basin can be estimated from this curve as the concentration corresponding to the calculated value of sedimentation index. Values of sedimentation index for rectangular basins may be calculated from the following set of equations ( 3 ) . These equations summarize the design procedure for most simple basins of reasonable size; an actual design should not be attempted without study of all of the referenced articles.
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316
l & E C PROCESS D E S I G N A N D DEVELOPMENT
where SI = sedimentation index, min. (SI), = SI corrected for depth effects L = pit length, ft. W = pit width, ft. D = pit depth, ft. Q = flow rate, c.f.m. m = hydraulic radius = WD/2D bV, ft. = indicator function 8 = critical particle diameter, ft. d = solids density, lb./cu. ft. Pa = liquid density, lb./cu. ft. PL = liquid viscosity, lb./ft. min. I.( = gravitational constant, ft./min./min. g U = superficial liquid velocity, ft./min.
+
laboratory Experiments
Settling rates were determined for kaolin suspensions in 1liter beakers, using a Phipps & Bird multiple stirrer, under two conditions : during quiescent settling following an initial mix a t 100 r.p.m. for 5 minutes, and during quiescent settling after the initial rapid mix followed by a flocculation mix at 20 r.p.m. for 10 minutes. The latter set of conditions was shown to result in nearly optimum flocculation and subsequent sedimentation in several sets of experiments in which the flocculation mix was from 5 to 30 minutes at speeds of 10 to 40 r.p.m. Portions were withdrawn after settling times of 1, 5, and 10 minutes from replicate initial suspensions of 1000, 500, and 100 p.p.m. Typical settling rate curves are shown in Figure 1. The differences between the two sets of curves show the tendency of this material to flocculate and suggest a method for the quantitative demonstration of the potential effectiveness of coagulant aids. The settling rate curves obtained are shown in Figures 2, 3, and 4. The dashed portions of the curves are extrapolated values. The differences in separation efficiency, due only to differences in initial concentrations, are striking and demonstrate how very important this parameter can be. I t was assumed that a settling basin handling such suspensions would show effluent concentrations between the two curves for various values of sedimentation index calculated for the basin. If no flocculation occurred in the basin, the upper curve would describe the performance; the lower curve would presumably describe the operation if optimum flocculation were achieved. Two small model settling basins were operated in the laboratory with kaolin suspensions. Model A was 13.5 inches wide, 6.75 inches deep, and 26.0 inches long; model B was 24.0 inches wide, 10.5 inches deep, and 24.0 inches long. Model A has a single inlet above the water surface; model B has three
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Kaolin suspensions at 500 p.p.m. VOL. 5
NO. 3
JULY 1966
317
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I I E C PROCESS D E S I G N AND DEVELOPMENT
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Clarifier evaluation
Normal operation
Table 1. Flow Rate,
G.P.M .
Calculated SI Values for Model Basins Sedimentation Index, SI Basin A Basin B
3 7 11
1.02 0.39 0.22
1.15 0.44 0.27
Table II. Parameters of Model Basins Flow Rate, G.P.M. 3 7 11
Basin A Ret. time, min. Re 3.42 1.46 0.93
3343 7799 12260
Basin B Ret. time, min. 8.73 3.74 2.38
Re 2040 4766 7492
parallel inlets submerged a t the water depth. Each model was operated a t 3, 7 , and 11 gallons per minute with suspensions of 1000, 500, and 100 p.p.m. of kaolin. Calculated SI values for the models ranged from 0.22 to 1.15 minutes as functions of flow rates and basin geometry, as shown in Table I. The results are shown graphically in Figures 5, 6, and 7 . The effluent concentrations actually measured in the model basins are plotted us. the calculated SI values for the flow rates used in the experiments. With initial concentrations of 1000 p.p.m., the results were about as expected; the performances of the basins were similar, as predicted by the calculated SI values, and some tendency toward flocculation was noted. At the lower initial concentrations, however, the results were not a t all as predicted, the actual performances being very much better than anticipated. The performance of basin A was substantially better than of basin B, despite the lower retention times and higher superficial Reynolds (Re) numbers in the former, as shown in Table 11. VOL. 5
NO. 3
JULY 1966
319
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9. Clarifier evaluation Polyelectrolyte added
I t is clear from these data that the mathematical description of sedimentation-flocculation efficiencies is not to be a simple one. Initial concentration and the degree of turbulence through the basin are the variables which appear to be of most importance. Basin inlet conditions may also be very important. Field Evaluations
This general procedure was used to evaluate the effectiveness of a polyelectrolyte in improving the removal of blast furnace flue dust in a 45-foot diameter clarifier a t a steel mill. Samples were taken during a day of normal operation and during a day in which the polyele,ctrolyte was being added in the inlet flume to the clarifier. The calculated SI value for this clarifier was 10.0.
Table 111.
Settling Time, Min.
3 5 10 15 20 40
Influent Settling Rate Determinations Solids in Susjension, P.P.M. W i t h Polyelectrolyte hToormal Operation SI Floc. sz Floc.
...
171
...
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Table IV.
Influent Effluent
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Clarifier Operation Solids Concentration, P.P.M. n’ormal With operation polyelectrolyte
18,236 138
21,445 79
l & E C PROCESS D E S I G N A N D DEVELOPMENT
Settling rates were determined on influent samples according to the conventional SI procedure and also following a 10minute flocculation mix a t 20 r.p.m. on a Phipps & Bird multiple stirrer. Total suspended solids were determined on both influent and effluent streams. Average analyses are given in Tables I11 and IV. Settling rate curves for normal operation and with the polyelectrolyte added are given in Figures 8 and 9, respectively. Without the added polyelectrolyte, suspended solids after 10 minutes’ quiescent settling are reduced from 95 p.p.m. to 50 p.p.m. with optimum flocculation mixing; with the polyelectrolyte, to 60 p.p.m. from 180 p.p.m. with optimum flocculation mixing The effluent concentrations measured indicate no flocculation under normal operating conditions, but nearly optimum flocculation with the added polyelectrolyte. The method allows the evaluation of the coagulant aid on its own merit and measurement of the effect in the clarifier despite variations in the influent suspension.
literature Cited
(1) Bramer, H. C., Hoak, R. D., IND.ENG.CHEWPROCESS DESIGN DEVELOP. 1, 185 (1962). (2) Zbid., 3, 46 (1964). (3) Hoak, R. D., Bramer, H. C., “Scale Pit Design,” Chicago Regional Technical Meeting, - American Iron and Steel Institute, Sept. 25, 1963. (4) Traina, V. P., Bramer, H. C., “Theoretical Design of Sedimentation Basins and Field Evaluation.” Ohio Water Pollution Control Conference, June 18, 1964. RECEIVED for review September 27, 1965 ACCEPTEDFebruary 24, 1966 Contribution of the Water Resources Research Project sustained at Mellon Institute since 1938 by the American Iron and Steel Institute.
