THE ROLE OF ACTIVATED SLUDGE SOLIDS IN AN ACTIFLO SYSTEM Chen-An Lien and Andrew P. Kruzic ABSTRACT Civil and Environmental Engineering Department, University of Texas at Arlington Arlington, Texas 76011, USA This research was conducted using jar tests to investigate the impact of adding activated sludge solids to the Actiflo process, a high-rate clarification (HRC) system, to increase BOD removal in HRC systems treating raw wastewater during periods of high wet-weather flows (WWF). A study of dosages and impacts of ferric sulfate, polymer, microsand and activated sludge on performance of the Actiflo process was investigated. Experiments were designed to determine the dosages of activated sludge and other chemicals necessary to achieve an effluent BOD 5 level of 30mg/L in this physical-chemical process with an influent BOD 5 concentration of 100 mg/l. With a 490 mg/l dose of activated sludge and 15 minutes mixing time, the effluent BOD removal efficiency was enhanced by approximately 30% using lower chemical dosages in jar tests. KEYWORDS Actiflo, high rate clarification, activated sludge. INTRODUCTION The Actiflo process is a ballasted flocculation and high-rate settling process that utilizes microsand to increase floc settling rates (see Figure 1). Because the Actiflo process is a physical-chemical process it is well adapted to rapid startup and automatic operation required for treating WWF, however it has limited capacity for removing soluble BOD. It was hypothesized that activated sludge addition would result in significantly higher levels of soluble BOD removal due to absorption into bacterial cells. (Nielsen et al., 2004) An additional hypothesis was that the biomass solids would also act as a kind of biosorbent/biocoagulant that enmeshes influent particles, possibly allowing for lower coagulant doses. (Desjardins et al., 2002; Huang and Li, 2000; Jacobsen and Hong, 2002; Sauvignet, 2003; Plum et al., 1998; Lmasuen et al., 2004; Sawey et al., 1999) The jar tests were performed in a manner to simulate the addition of return activated sludge to the headworks of the Village Creek Wastewater Treatment Plant in Fort Worth, Texas. There is 6748
approximately 15 minutes of contact time in the pipe from the headworks to the Actiflo process (see Figures 2 and 3). The Actiflo system at the Village Creek WWTP was designed to meet a National Pollutant Discharge Elimination System (NPDES) permit that requires that the effluent BOD 5 from the HRC be less than 45 mg/l at all times. (Sawey et al., 1999) A specific goal of this study was to determine whether activated sludge addition could lower the total effluent BOD 5 and produce an effluent that meets a specific effluent BOD 5 goal (25~35 mg/l BOD 5 ) with a high degree of confidence for influent BOD 5 values as high as 100 mg/l. Fig. 1 Parallel Treatment Train with Conventional Treatment and the Actiflo Process (Sawey et al., 1999) 6749
Fig. 2 Modified Parallel Treatment Train with RAS Addition to the Actiflo Process (Sawey et al., 1999) Sludge Addition Point Activated Sludge Pre-aeration Facility Effluent: w/ activated sludge addition: 25~40 mg/l w/o activated sludge addition:40~60mg/l (BOD 5 ) Wasted Sludge Processing Influent: BOD 90~150 mg/l (WWF simulation) Coagulant Fe2(SO4)3 300 rpm 30 Sec Polymer Sludge Hydrocyclone Microsand Lamella Plate Settler Raw wastewater Additional Sludge Mixing 180 rpm 15 min (time expected in pipe) Injection Mixing 300 rpm 3 min Flocculation Tank 100 rpm 2 min Settling tank 3 min P Microsand/sludge Mixture P Sludge Pre-aeration Recycled Activated Sludge from Secondary clarifier Fig. 3 Process Flow Diagram of the Actiflo Treatment Process with Additional Activated Sludge Process 6750
MATERIAL AND METHODS Samples were collected from the bar screen influent and from the final filtration effluent at the Village Creek Wastewater Treatment Plant. Raw wastewater was diluted with filtration effluent to simulate wet weather peak flows. Activated sludge samples were collected from the secondary clarifiers and aerated at least one hour to assure the biomass was active. This pre-aeration process was used in the laboratory experiments to compensate for the lack of aeration during transport to the laboratory and preparation for testing. It may not be necessary to aerate activated sludge in actual operation. Jar tests were performed using procedures typically used to simulate the Actiflo process except when activated sludge was added. In those jar tests the sludge solids were added to and mixed with the wastewater for 15 minutes at 180 rpm as a first step. The influent BOD 5 was approximately 100 mg/l. The concentration of returned activated sludge collected from secondary clarifier was approximately 4900 mg/l during the testing period. For a 10% sludge dose, it can be assumed that 490 mg/l of activated sludge solids would be added into the process. This study was divided into four sets of experiments. The first two sets focused on turbidity removal only, while in the last two sets of experiments, COD & BOD removal with and without activated sludge were determined to define the role of activated sludge solids in the Actiflo process. In the Section I and II tests, it was hypothesized that turbidity removal rate could be used to estimate effluent quality. The optimum chemical dosages of ferric sulfate, polymer and microsand were selected in the Section I based only on effluent turbidity. In the Section II tests, the turbidity removal rate was also used to determine the effect of sludge addition on chemical requirements and process performance. BOD and COD tests were used in Section III and IV to clarify the impacts and role of sludge addition in the Actiflo system. In the Section III tests, different doses of ferric sulfate, polymer, and sludge were used with a fixed dose of microsand (see Table 1). BOD, COD and turbidity were measured in the Section III tests, but the optimum chemical and sludge doses were determined using the BOD and COD results. The Section III tests lead to the conclusion that adding activated sludge in the Actiflo system helps to lower effluent BOD and decrease chemical requirements for treating WWFs. However, only four of the Section III tests were performed with no sludge addition and these were performed with only one set of coagulant doses. The specific objective of the Section IV tests was to more clearly demonstrate the impact of activated sludge addition on both BOD removal and required coagulant doses (see Table 2). 6751
Experimental data from 232 jar tests were analyzed using analysis of variance (ANOVA) in STATISTICA. The chemical and sludge dosages were set as independent factors and turbidity removal percentage, and effluent COD and effluent BOD were set as dependent variables. The main effects of each test and the interaction between factors under incomplete/complete statistical designs were observed. The multiple trials of the experiment were performed and the data were pooled for analysis. (Greenberg et al., 1992) Table 1 Simulation Matrix of Actiflo Treatment Process (Section III) Experimental Step Sludge Dose or Chemical Dosages Mixing Speed Sludge Pre-aeration Duration 60 minutes Sludge Dose 0% 5.0% 7.5% 10.0% 180 rpm 15 minutes Coagulant (Ferric Sulfate), mg/l 52.5 70.0 87.5 105.0 300 rpm 30 seconds Ballast Agent 3.0 300 rpm Microsand, g/l 3 minutes Polymer, mg/l 0.50 0.75 1.00 300 rpm Flocculation 100 rpm 2 minutes Settling 0 rpm 3 minutes Table 2 Simulation Matrix of the Actiflo Treatment Process (Section IV) Experimental Step Sludge Dose or Chemical Dosages Mixing Speed Duration Sludge Pre-aeration 60 minutes Sludge Dose 0% 10.0% 180 rpm 15 minutes Coagulant (Ferric Sulfate), mg/l 70.0 105.0 300 rpm 30 seconds Ballast Agent 3.0 300 rpm Microsand, g/l 3 minutes Polymer, mg/l 0.50 0.75 1.00 300 rpm Flocculation 100 rpm 2 minutes Settling 0 rpm 3 minutes RESULTS AND DISCUSSION This study provided a better understanding of the turbidity and BOD removal in the Actiflo process using activated sludge, ferric sulfate, polymer and microsand. The results of the section experiments are: 6752
Section I) Turbidity removal was found to be sensitive to the dosages of ferric sulfate and polymer but not microsand. As expected, turbidity removal efficiency increases with increasing ferric sulfate and polymer doses. However, microsand dose over a range of 2~5 g/l, did not provide a significant influence on turbidity removal. The optimum coagulant doses to achieve approximately 90% removal of turbidity in the Section I without sludge addition were: ferric sulfate = 105 mg/l, polymer = 1.0 mg/l and microsand = 3.0 g/l. Section II) In the section II tests, sludge addition did not significantly improve the turbidity removal rate. Although activated sludge addition in the Actiflo process may increase the initial turbidity, results from Section II tests show that good effluent quality, defined as turbidity removal efficiencies over 90%, does not require higher chemical doses when activated sludge is added compared to no sludge addition. However the sludge addition did not result in significantly higher turbidity removal. Section III) In the Section III tests, BOD and COD removal were found to be sensitive to sludge and ferric sulfate doses but not polymer doses (see Figures 4 and 5). As seen in the figures, sludge addition to the Actiflo process can significantly lower effluent BOD and COD compared to no sludge addition. For 10% v/v activated sludge addition, approximately 490mg/L of activated sludge solids, the BOD 5 was decreased from approximately 100 mg/l to 33 mg/l. Section IV) The Section IV tests confirmed the Section III results in that sludge addition resulted in significantly lower effluent BOD and COD (see Figure 6). For a ferric sulfate dose of 105mg/L, the effluent BOD was decreased from 49.1 mg/l to 33.4 mg/l (see Figure 7). When the ferric sulfate was 70 mg/l, the effluent BOD was lowered from 52.4 mg/l to 34.2 mg/l. Based on the result that there was only a small difference in effluent BOD at the two ferric sulfate doses with sludge addition it appears that a lower ferric sulfate dose can be used when activated sludge is added. A similar result was observed when effluent BOD is compared with polymer dose (see Figure 8). Sludge addition allows for a reduced polymer dose, perhaps down to 0.5 mg/l. The effluent COD results in Section IV test directly mirrored the BOD results (see Figures 9, 10 and 11). Our comparison of BOD and COD test results were so similar that it appears that the quicker COD test can be substituted for the BOD for process control purposes. It appears from our results that the activated sludge dose can be varied to achieve a desired effluent BOD with the added advantage that the higher costs associated with sludge addition may be partially offset by savings in chemical coagulants. (see Table 3) 6753
Table 3 Section Test Comparison in Section IV of Simulation Actiflo Process Ferric Sulfate (mg/l) Polymer (mg/l) Microsand (g/l) Recycled Activated sludge (%) Enhanced BOD Removal Efficiency (%) IV Conventional Actiflo Process Enhanced Actiflo Process with additional Sludge Chemical Reduction (%) 105.0 1.0 3.0 0.0 70.0 0.75 3.0 10.0 32% 33.3 25% Fig. 4 Section III Sludge Dose and Effluent BOD One-Way ANOVA Effect Plot Polymer: 1.0 mg/l Ferric Sulfate: 105 mg/l Microsand:3 mg/l Non-sludge addition (Baseline Condition) Polymer: 0.5~1.0 mg/l Ferric Sulfate: 52.5~105 mg/l Microsand:3 mg/l Sludge addition % 7.5 10.0 6754
Fig. 5 Section III Sludge Dose and Effluent COD One-Way ANOVA Effect Plot Polymer: 1.0 mg/l Ferric Sulfate: 105 mg/l Microsand:3 mg/l Non-sludge addition (Baseline Condition) Polymer: 0.5~1.0 mg/l Ferric Sulfate: 52.5~105 mg/l Microsand:3 mg/l Sludge addition 0.0 5.0 7.5 10.0 % Fig. 6 Sludge Dose and Effluent BOD One-Way ANOVA Effect Plot in Section IV Test --- With and Without Activated Sludge Addition 6755
Fig. 7 Ferric Sulfate and Effluent BOD One-Way ANOVA Effect Plot in Section IV Test Fig. 8 Polymer and Effluent BOD One-Way ANOVA Effect Plot in Section IV Test 6756
Fig. 9 Sludge Dose and Effluent COD One-Way ANOVA Effect Plot in Section IV Test --- With and Without Activated Sludge Addition Fig. 10 Ferric Sulfate and Effluent COD One-Way ANOVA Effect Plot in Section IV Test 6757
Fig. 11 Polymer Dose and Effluent BOD One-Way ANOVA Effect Plot in Section IV Test CONCLUSIONS The specific conclusions of this study are: 1. In the normal Actiflo process turbidity removal is sensitive to the ferric sulfate and polymer doses. Turbidity removal efficiency increases with increasing ferric sulfate and polymer over the dose range used in this study. Microsand doses over a range of 2 to 5 g/l do not significantly influence turbidity removal. 2. BOD removal in an enhanced Actiflo process (with activated sludge addition) is sensitive to activated sludge and ferric sulfate doses. BOD removal increases with increasing ferric sulfate and activates sludge over the dose range used in this study. In the enhanced Actiflo process polymer doses over a range of 0.5 to 1.0 mg/l do not significantly influence BOD removal. 3. Turbidity removal efficiency does not relate well to BOD removal in the normal and enhanced Actiflo process, most likely because the mechanisms responsible for colloidal solids removal do not remove soluble BOD. 6758
4. Addition of return activated sludge in the Actiflo process (enhanced Actiflo ) can significantly increase BOD removal compared to the normal Actiflo process. The likely mechanism for the improved performance is soluble BOD uptake by the activated sludge microorganisms. 5. The addition of activated sludge to the Actiflo process can significantly reduce the required doses of coagulants while achieving better BOD removal. REFERENCES: Desjardins, C., Koudjonou, B., and Desjardins, R. (2002). Laboratory Study of Ballasted Flocculation. Water Research, Vol. 36, No. 3, 2002, 744-754. Greenberg, A. E., Clesceri, L.S., and Eaton, A.D. (1992). Standard Methods for the Examination of Water and Wastewater, 18th Edition. Huang, J-C., and Li, L. (2000). An Innovative Approach to Maximize Primary Treatment Performance. Water Science and Technology, Vol. 42, No. 12, 2000, 209-222. Jacobsen, J., and Hong, S-N. (2002) Microsand Ballasted Flocculation and Clarification for the High Rate Treatment of Strom Waters and Sewer Overflows. Watershed 2002, Water Environment Federation, Feb. 2002. Lmasuen, E., Judd, S., and Sauvignet, P. (2004). High Rate Clarification of Municipal Wastewaters: a Brief Appraisal. Journal of Chemical Technology & Biotechnology, Vol. 79, No. 8, Aug. 2004, 914-917. Nielsen, P. H., Thomsen, T. R., and Nielsen, J. L. (2004). Bacterial Composition of Activated Sludge Importance for Floc and Sludge Properties. Water Science and Technology, Vol. 49, No. 10, 2004, 51-58. Plum, V., Dahl, C. P., Bentsen, L., Petersen, C. R., Napstjert, L., and Thomsen, N. B. (1998). The Actiflo Method. Water Science and Technology, Vol. 37, No. 1, 1998, 269-275. Sauvignet, Philippe. (2003). Sand-Ballasted Flocculation Technology. Quarry Management, Vol. 30, No. 11, Nov. 2003, 17-18. Sawey, R. W., Gerrity, D., and West, R. (1999). Implementing Alternative Wet Weather Treatment Technology. Apr. 1999, Camp Dresser & McKee, Fort Worth, Texas. 6759