Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge

Transcription

1 41 [Japanese Journal of Water Treatment Biology Vol.34 No ] Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge MASAFUMI TATEDA1, YOUNGCHUL KIM2 and WESLEY O. PIPES School of Environmental Science, Engineering & Policy, Drexel University/ 31st and Chestnut Street, Philadelphia, Pennsylvania 19104, U.S.A. Currently, Department of Environmental Engineering, Osaka University,/ 2-1 Yamadaoka, Suita, Osaka , Japan. 2 Currently, Department of Environmental Engineering, Hanseo University/ Seosan, Chungnam , Korea Abstract Key words: activated sludge process, transition rate constant, sludge volume index, settling characteristics, modeling INTRODUCTION The activated sludge process, which was first operated in Salford, England in 1914, has become the most widely used form of biological wastewater treatment 1). An activated sludge process has four essential elements, viz. 1) aeration tanks, 2) settling tanks, 3) return sludge facilities and 4) waste sludge facilities. Proper functioning of an activated sludge process requires an adequate degree of thickening in the settling tank for both sludge wasting and returning enough sludge to maitain the desired concentration of suspended solids in the aeration tank. Proper thickening of activated sludge in the lower part of the settling tanks results in a high return sludge suspended solids concentration (RSSS). Most of the underflow

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3 Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge 43 of a bulking activated sludge from zone settling to compression, 2) to find how various suspended solids concentrations influence the transition and 3) to determine how the SVI influences the transition. The long range goal is to develop a method to improve the use of data from batch settling tests for predicting the results of continuous thickening operations. MATERIALS AND METHODS

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5 Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge 45 Table 1 DELCORA operaional data for days when the SBSS was estimated.

6 46 Japanese J.Wat. Tret. Biol. Vol.34 No.1 Table 2 The settling data of the June 12th sample SB

7 Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge 47 Fig. 1 (A) Plot of settling data from 6/12/92 Sample ; Si=2.070g/l, SVI = 145ml/g, (B) Graph illustrating the Thomas Method for the 6/12/92 sample.

8 48 Japanese J.Wat. Tret. Biol. Vol.34 No.1 Fig. 2 (A) Plot of settling data from 8/18/92 Sample ; Si = 2.453g/l, SVI = 155ml/g, (B) Graph illustrating the Thomas Method for the 8/18/92 sample.

9 Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge 49 Table 3 The settling data of the August 18th sample curve is virtually a perfectly fit to the original setting curve (Fig. 3 -A). For the samples which have a lag time, there is a disagreement between the original curve and the calculated curve during the lag period and the zone settling period. The calculated settling curve is an idealization, i. e. the original settling curve without the lag period (Fig. 3-B). These two curves meet when the transition between zone settling and compression starts and then the two curves follow the same trace. For settling curves with a lag period there is a considerable difference between the original curve and the curve calculated for the zone settling velocity using k. The rate is illustrated in Fig. 4. The zone settling velocities or the initial settling velocities (ISVs) are 1.87cm/min and 1.33cm/min for the original curve and calculated curve, respectively. When the MLSS is 2,000mg/l, the estimation of solids flux from the calculated curve will be 38,298kg/m2/day which is a 30percent underestimation from the one calculated from the observed curve (53,858kg/m2 /day). Rate Constants for Transition The rate constants of the samples which do not have a lag period describe the entire settling curve in each case. On the other hand, the rate constants of the samples which have a lag period describe the settling curves from the period of transition. This analysis, therefore, is intended to describe the transition from zone settling to compression and does not describe either zone settling or compression. The estimated rate constants for all of 166 settling curves were plotted as a function of

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11 Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge 51 Fig. 4 the reciprocal initial suspended solids concentration (Si-1) and are presented in Fig. 5. In Fig. 5 there is a trend for k to increase with an increase in Si-1 but the data are widely scattered. If it is assumed that the transition rate constant is small when the value of SVI is high and vice versa, the dispersion of plots on the graph can be attributed to the differences of the range of SVI. When the range of SVI is classified into several groups, the correlation between kt and Si can be described by the following equation: kt=a+(b)(si-1). (5) In equation (5), k t represents the rate constant for the transition in min-1, Si represents the initial suspended solids concentration in gram per liter and values of a and b are obtained by applying a linear regression. The results of the relationship for each SVI ranges are presented in Tables 4 and 5. These tables are based on data from 166 settling tests with Si ranges from 1.315g/l to 4.290g/l and SVI ranges from 81 ml/g to 204ml/g.

12 52 Japanese J.Wat. Tret. Biol. Vol.34 No.1 Fig. 5 Rate constant vs. the reciprocal initial suspended solids concentration. Fig. 6

13 Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge 53 Table 4 Values of coefficients a and b for equation (5) with respect to SVI Effect of SVI on the Rate Constant The kt is strongly influenced by the SVI. Fig.7 shows that the relationship between the slope (b) (Table 4) and the mean SVI value (Table 5) for each linear regression in Fig. 6. When the Si is constant, the kt value decreases with increase in SVI. The k t value in Fig. 6 will be, for instance, when the Si is 1,540mg/l (Si-1=0.65l/g). Prediction of Rate Constants By using the chart in Fig. 6, rate constants were predicted for each samples and the result was shown in Fig. 8. The simple linear regression is kt(predicted)= ( )(kt Fig. 7 The slope of regression vs. Mean SVI.

14 54 Japanese J.Wat. Tret. Biol. Vol.34 No.1 Table 5 Values of sample sizes, mean SVI and standard deviations for each group (observed))r2= (6) Equation (6) manifests a robustness of a prediction of a k t value by using the developed chart. It is possible to estimate the rate constant using data from only one sludge settling test. The primary SUMMARY objective of this study was to characterize transition from zone settling to compression of an activated sludge from a full scale plant by using the first order equation. This batch settling tests were conducted on samples from one wastewater treatment plant over a period of fourteen months. By classifying SVI values into six groups of range, relationships between the rate Fig. 8 The predicted rate constants vs. the calculated rate constants.

15 Modeling the Transition from Zone Settling to Compression of a Bulking Activated Sludge 55 constants and two parameters (Si and SVI) were found and the model for prediction of the rate constant for transition was developed. If the initial suspended solids concentration and the sludge volume index are known, the rate of subsidence during transition can be estimated. The MLSS and SVI are easily measured in the laboratory and this is done routinely for many activated processes. The transition rate constant, k t, may be helpful for operation and design of settling tanks in an activated sludge process. REFERENCES 1) Alleman, J. E. and Prakasam, T. B. S.: Reflections on seven decades of activated sludge history, Journal of WPCF, 55, (1983). 2) Coulson, J. M. and Richardson, J. F.: Chemical Engineering, Pergamon Press, London (1954). 3) Bhargava, D. S. and Rajagopal, K.: Differentiation between transition zone and compression in zone settling, Water Research, 27, (1993). 4) American Public Health Association: Standard Methods for the Examination of Water and Wastewater., American Public Health Association, Washington, DC. (1991). 5) Kim, Y. C.: Solids storage function of activated sludge settling tanks during hydraulic overload, Drexel University, Philadelphia, Pennsylvania, U.S.A. (1995). 6) Weber, J. W. J.: Physicochemical Process for water quality control, Wiley- Interscience, New York (1972). 7) Thomas, H. A. J.: Graphical determination of BOD curve constants, Water & Sewage Works, 97, (1950). (Submitted ) (Accepted )