Field and CFD Analysis of Jet Aeration and Mixing Randal W. Samstag 1 *, Edward A. Wicklein 1, Roderick D. Reardon 2, Robert J. Leetch 3, R. (Robbie) M. Parks 3, and Colin D. Groff 3 1 Carollo Engineers, Seattle, Washington. 2 Carollo Engineers, Winter Park, Florida. 3 JEA, Jacksonville, Florida. * To whom correspondence should be directed: rsamstag@carollo.com. ABSTRACT Field tests and three-dimensional computational fluid dynamics (CFD) modeling were conducted to evaluate effectiveness of mixing in a jet aeration system for a sequencing batch reactor (SBR). The SBR is part of the Blacks Ford Regional Water Reclamation Facility (BFRWRF) of the JEA utility in Jacksonville, Florida. Solids measurements were taken with the SBR tanks operating in both aerated and un-aerated pumped mixing modes. A density-coupled CFD model incorporating solids settling and transport as user-defined functions (UDF) was calibrated to the field solids profile data and used to identify the capacity of the system to maintain solids suspension with different power levels in both mixing modes. The modeling indicated that while the existing system was adequate to ensure complete mixing during aerated periods, the existing pumped mixing power level of approximately 7.7 W/m 3 (39 horsepower per million gallons hp/mg) would need to be increased to at least 30.8 W/m 3 (156 hp/mg) to maintain a conventional mixing criterion during un-aerated mixing. As a follow up to the mixing evaluation, simulations were prepared in which density coupling of the solids transport model to the fluid momentum was turned off in the UDF. Results from these experiments indicated that the neutral density simulation (no density coupling) seriously over-predicted the degree of mixing seen in the calibrated density-coupled model. Since neutral density CFD evaluations for mixing are a common practice in the industry, this finding is significant. KEYWORDS: Mixing, solids profile, CFD, sequencing batch reactor, density-coupled, simulations INTRODUCTION One of the recommendations of a previous study for the Blacks Ford Regional Water Reclamation Facility (BFRWRF) by Carollo Engineers (2010) was to perform a computational fluid dynamic (CFD) evaluation of the jet aeration mixing system for the sequencing batch reactor (SBR) system at the BFRWRF. The BFRWRF includes influent screening, influent equalization, SBR treatment in four SBR tanks, effluent equalization, cloth media filtration, and ultraviolet (UV) disinfection. Figure 1 presents an aerial photo of the plant site. The SBR tanks at the BFWRF incorporate pumped jet mixing and jet aeration. This type of mixing / aeration system relies on recirculation of the contents of the SBR tank through a manifold with a series of high velocity discharge nozzles at the bottom of the SBR tank. The system is aerated by mixing the pumped flow with atmospheric air pressurized by blowers.
The BFRWRF discharges to a reclaimed water distribution system or to the Blacks Ford Wetland Treatment System. Effluent quality requirements for wetland discharge include a limit on total nitrogen compounds of 3 mg/l. To achieve this high degree of nitrogen removal, the SBR tanks operate in cyclic aerated and un-aerated modes. During the aerated mode, ammonia in the influent wastewater is converted to oxides of nitrogen. Removal of oxidized nitrogen by biological denitrification occurs during un-aerated pumped mix cycles. It had been thought that inadequate mixing in the SBR system during un-aerated pumped mix cycles might be inhibiting the ability of the BFRWRF to remove nitrogen. The following paper presents a summary of field tests and CFD investigations undertaken to evaluate pumped and air mixing in the SBR system at the BFRWRF. MATERIALS AND METHODS Field tests were conducted at the BFRWRF to establish solids concentration profiles under normal operating conditions. Solids concentration measurements were taken at multiple depths at two locations at the edge of operating SBR tank. Solids measurements were calibrated to laboratory samples of mixed liquor suspended solids using an Insite Instrumentation Group Model 3150 portable suspended solids analyzer. Simulations were conducted using FLUENT, Release 13.0 provided by ANSYS, Inc. The geometric model and computational mesh were developed in ANSYS GAMBIT, Version 2.4.6. User defined functions (UDF) were used to model solids settling and transport and to implement density coupling with the fluid flow (See Wicklein and Samstag, 2009.) Aeration was simulated using the standard multi-phase input facilities of FLUENT. BACKGROUND SBR Data Each SBR tank at the BFRWRF is 25.9 m (85 foot - ft) in diameter and has a volume of 3,407 m 3 (0.9 MG). Each tank includes a jet aeration system with three jet headers and three mixing pumps. The main jet header in each tank contains 24 discharge nozzles and receives a flow of 23,950 m 3 /d (4,394 gallons per minute - gpm) from a 30 kw (40 hp) end suction centrifugal pump mounted outside each tank. In addition, each tank includes two supplemental jet headers, each with six (6) discharge nozzles, which receive flow from individual 5,900 m 3 /d (1,080 gpm) submersible 5.6 kw (7.5 hp) pumps.
SBR Tanks Figure 1. Blacks Ford Regional Water Reclamation Facility Plant Site. Table 1 presents hydraulic data for the BFRWRF SBR tanks pumped mix system. In addition to the mixing pumps, each tank system includes a 93 kw (125 hp) multistage centrifugal blower with a capacity of 1,530 3,300 m 3 /hr (900 1900 cubic foot per minute - cfm). The jet aeration system discharges into the SBR tank through 76 mm (3-inch in) diameter nozzles. The air mixing system incorporates a smaller, inner nozzle, 38 mm (1-1/2 in) in diameter. Pump mixing power was estimated by applying a multiplier of 1.25 to the velocity head through the 38 mm diameter inner nozzles. This produces the approximate total dynamic head (TDH) of 6.5 m (21 ft) on the mixing pumps and a total estimated power consumption for the system of approximately 26 kw (35 hp).
Table 1. Hydraulic Characteristics of Existing SBR Pump Mix System. Description Value Main Header Pump Flow (m 3 /day) 23,949 Number of Nozzles per Header 24 Outer Nozzles Nozzle Diameter (mm) 76.2 Jet Velocity (m/sec) 2.53 Inner Nozzles Nozzle Diameter (mm) 38 Jet Velocity (m/sec) 10.1 Supplemental Headers Number of Headers 2 Pump Flow, Each (m 3 /day) 5,886 Number of Nozzles per Header 6 Outer Nozzles Nozzle Diameter (mm) 76.200 Jet Velocity (m/sec) 2.49 Inner Nozzles Nozzle Diameter (mm) 38.1 Jet Velocity (m/sec) 9.96 Total Pump Flow (m 3 /day) 35,722 Average Velocity (m/sec) 2.52 Inner Nozzle Total Pumping Head Power (kw) 26.2 Motor horsepower rating (kw) 41
Geometric model Figure 2 presents an illustration of the three-dimensional geometric model prepared for the BFRWRF SBR tanks for CFD analysis. The model shows the three jet headers, the main header pump intake, the auxiliary header pumps and intakes, and the effluent decanters. The effluent decanters were not required for the flow simulation, but were included to simulate the fluid environment. Figure 3 illustrates the polyhedral computational mesh implemented in the meshing software projected onto model surfaces. Figure 2. SBR Tank Geometric Model.
Figure 3. SBR Tank Computational Mesh. RESULTS Field test results Field tests were conducted at the BFRWRF to establish solids concentration profiles under normal operating conditions. Solids concentration measurements were taken at multiple depths at two locations at the edge of the operating SBR tank No. 3 using a calibrated optical solids measurement probe. Testing took place on July 8, 2011. Sample locations were: 1) near to the wall at one location adjacent to the main platform for Tank Number 3 between the effluent decanters and 2) adjacent to the access ladder for one auxiliary header submersible pump. Sample locations are shown schematically in Figure 4.
Figure 4. Field Test Sample Locations. Mixed liquor suspended solids (MLSS) measurements were made: 1) during mixed cycles with both air and pumping operational and 2) during pumping-only mix cycles. The measured MLSS concentration for the day of the test was approximately 2,400 mg/l and the sludge volume index (SVI) was approximately 100 ml/g. Figure 5 presents results of sampling during a period when both the pumped mix system and the aeration blower were operational. The data indicate a fully mixed condition with a slight accumulation of floatable solids. Solids concentrations were in the range of 2,320 to 2,380 mg/l for the four measurement locations below the water surface and 2,550 mg/l for the measurement at the water surface.
Depth (m) Air On Main Platform Legend 0.0 2550 Concentration (mg/l) 1.5 2380 < 1000 650 3.0 2350 1000 < C < 1500 1250 4.6 2370 1500 < C < 2000 1800 6.1 2320 2000 < C < 2500 2450 2500 < C < 3000 2650 > 3000 3400 Figure 5. Field Test Results with Air On. Figure 6 presents results from sampling at the location of the main platform during a period with the mixing pump in operation and with the aeration blower off. A series of four measurements were made at the main platform location at different times following shutdown of tank mixing pumps: 1) immediately after shutdown, 2) 25 minutes after shutdown, 3) 66 minutes after shutdown, and 4) 83 minutes after shutdown. The data indicate that at this location, concentrations less than 1,000 mg/l were present at the top 1.5 m level of the tank after 83 minutes of mixing and reached a maximum of 2,950 mg/l at the tank bottom after 25 minutes of mixing. At later times, the measured bottom concentrations were approximately 2,600 mg/l. Figure 7 shows results of measurements adjacent to the location of the auxiliary header submersible pump access ladder. The access location in the test tank was on the opposite side of the tank from that shown in the figure. The figure is based on the tank model, which was based on manufacturer s installation drawings. After 42 minutes of mixing, the solids concentration at the 1.5 m level layer was 1,380 mg/l and the concentration at the bottom was measured at 2,200 mg/l. After 75 minutes of pumped mixing, the top-level concentration was 650 mg/l and the bottom concentration was 2,450 mg/l. Conclusions from the field test measurements include the following: 1) Distinct separation takes place during pumped mix cycles under the current configuration with average MLSS concentrations of 2,400 mg/l separating to less than 1,000 mg/l in the top layer and nearly 3,000 mg/l in the bottom layer. 2) Measurements indicate the dynamic mixing condition in the tanks. Solids concentrations at the bottom of the tank varied from 2,290 mg/l at the beginning of the pumped mix cycle at the main platform location to 2,950 mg/l after 25 minutes but reduced to 2,650 mg/l after 66 minutes and 2,540 mg/l after 83 minutes.
Depth (m) Main Platform 0 minutes Legend 1.5 2230 Concentration (mg/l) 3.0 2210 < 1000 650 4.6 2100 1000 < C < 1500 1250 6.1 2070 1500 < C < 2000 1800 6.4 2290 2000 < C < 2500 2450 2500 < C < 3000 2650 Depth (m) Main Platform 25 minutes > 3000 3400 1.5 2150 3.0 2270 `` 4.6 2390 6.1 2950 Depth (m) Main Platform 66 minutes 1.5 2450 3.0 2350 4.6 2550 6.1 2640 Depth (m) Main Platform 83 minutes 0.0 650 1.5 2510 3.0 2590 4.6 2690 6.1 2540 Figure 6. Main Platform Field Pump Mix Test Results Solids Concentration. Depth (m) Pump Ladder 42 minutes Legend 1.5 1380 Concentration (mg/l) 3.0 1920 < 1000 650 4.6 2180 1000 < C < 1500 1250 6.1 2200 1500 < C < 2000 1800 2000 < C < 2500 2450 Depth (m) Pump Ladder 75 minutes 2500 < C < 3000 2650 0.0 650 > 3000 3400 1.5 1590 `` 3.0 2450 4.6 2610 6.1 2450 Figure 7. Pump Ladder Field Pump Mix Test Results Solids Concentration.
Initial model simulations Using the model geometry described in the previous section, CFD simulations were initially configured to approximate current conditions of operation of the pumped mixing and mixing / aeration system. The model was configured with fixed influent boundary conditions for velocity through the inlet nozzles and outlet boundary limitations on flow to the pump inlets. Two thirds of the total inlet flow was constrained to exit the tank through the main pump outlet and one sixth of the total flow through each of the auxiliary header outlets. As shown in Table 1, existing pump capacities deliver inlet port velocities of approximately 2.5 m/sec (8.3 feet per second - fps). Figure 8 presents a graphic illustration of velocity profiles at three cross sections through the SBR tank at locations centered on inlet nozzles in the pump headers. The velocity is high near the nozzle, but the velocities rapidly diminish as the fluid jet is dissipated into the main tank contents. Velocities are less than 0.13 m/sec (0.43 fps) over a majority of the tank. Figure 9 illustrates the velocity profile centered on one of the nozzles. The model was configured with custom UDF to simulate suspended solids settling and transport as a scalar within the tank. These UDF also simulate the effect of density changes resulting from the solids concentration profile on fluid momentum. Simulations were initiated with an assumed uniform concentration of 2,400 mg/l mixed liquor suspended solids (MLSS) throughout the tank. Settling velocities were calculated based on an assumed value for the sludge volume index (SVI) of 100 ml/g, using the revised Daigger equation (Daigger 1995). These values were used to approximate conditions at the BFRWRF during the field-testing. Simulations were paused to estimate the solids concentration profile at different times after initiation of a pumped mixing cycle. Figure 10 presents the estimated solids concentration profile following different times after the start of the pumped mix cycle: 25, 44, and 66 minutes. The figure shows a small layer of relatively clear water at the top of the tank that is predicted after 25 minutes of mixing with the clear layer increasing to approximately one-eighth of the tank depth after 66 minutes. Corresponding solids concentration buildup in the bottom of the tank is predicted to concentrations in excess of 3,500 mg/l, even after only 25 minutes of mixing. Comparison of solids concentration CFD results with the results from the field test gives a reasonable qualitative agreement, although the CFD model results indicate modestly greater solids separation than seen in the field. This could be due to several factors: 1) slight overprediction of settling velocities or 2) slight inaccuracies in the field measurements.
Figure 8. Velocity Profiles for the Three Pump Headers for Existing Conditions.
Figure 9. Velocity Profile with Existing Configuration Near to Inlet Nozzle.
Figure 10. Estimated Concentration Profiles at Different Mixing Times (SVI 100 ml/g). As a check on the influence of the first factor, a series of simulations were conducted with a slower settling velocity, calculated from the Daigger relation for a higher SVI, 150 ml/g. This series of simulations is shown in Figure 11. The results here are closer to the field test results. The value for SVI in the equipment manufacturer s original design calculations is listed as 200 (Fluidyne 2002). This assumption of a relatively high SVI would be conservative with respect to solids settling time in the reactor, but is not conservative with respect to mixing efficiency. We did not check the adequacy of the manufacturer s settling time calculations as part of the current work.
Figure 11. Estimated Concentration Profiles at Different Mixing Times (SVI 150 ml/g). Figure 12 presents an illustration of the predicted velocity profile under conditions of pumped mixing with aeration operating at its maximum rate of approximately 102 m 3 /hr (60 cfm) per nozzle, or a total of 3,670 m 3 /hr (2,160 cfm). Figure 13 presents the predicted solids concentration profile under these conditions of pumped mixing with air. These simulations were developed using the Carollo UDF for solids setting and density coupling and the FLUENT mixture model for simulation of air and water mixing. Under the conditions modeled, the aeration represented a 70 percent volumetric ratio at the nozzle for an effective nozzle velocity of the mixture of 9.3 m/sec (30.5 fps). The simulations indicate that mixing with air is effective to produce essentially complete mixing of solids in the tank. This result agrees with the field tests.
Figure 12. Estimated Velocity Profiles with Full Aeration.
Figure 13. Estimated Concentration Profiles with Full Aeration. DISCUSSION CFD Evaluation Based on the initial model simulations, a series of tests were conducted for different operating conditions using the calibrated model. Since both the field tests and the model simulations indicated that the pumped air system produced fully mixed conditions, the model mixing evaluation focused on different mixing flows during the pumped mix-only cycle. A series of mixing jet velocities were simulated. The simulations were conducted using progressively increasing pumped mixing flows from the existing total pumping rate of 35,700 m 3 /day (6,550 gpm) to approximately 56,800 m 3 /day (10,400 gpm). As shown in Table 2, this increase in flow of approximately 160 percent through the same header manifolds would result in approximately a four-fold increase in pump power consumption.
Table 2. Comparison of Estimated Power Requirements for Different Pump Rates. Total Pumping Rate (m 3 /d) Average Port Velocity (m/sec) Estimated Total Power Requirement (kw) Power Level (W/m 3 ) 35,728 2.52 26 7.7 42,521 3.00 44 13.0 49,717 3.50 71 20.7 56,765 4.00 105 30.8 Figure 14 presents estimated velocity profiles for the different pumping rates outlined in Table 2 for an asumed SVI of 150 ml/g. There is a modest increase in the relative portion of the tank with tank velocity as high as 0.6 m/sec (2 fps) as the nozzle velocity is increased to 4.0 m/sec (13 fps), but these zones of relatively high velocity are concentrated in the bottom and center of the tank under the influence of the main jet manifold. Figure 15 presents comparable velocity profiles for an assumed SVI value of 100 ml/g. Figure 16 presents the predicted effect of increasing jet velocity on tank solids distribution using a settling velocity for a SVI of 100 ml/g. As shown in the figure, a significant difference is observed as jet velocity is increased. Increasing the jet velocity from 2.5 m/sec (8.3 fps) to 3.5 m/sec (11.5 fps) reduces deposition of solids on the tank floor and eliminates clear water at the top of the tank. Increasing the jet velocity to 4.0 m/sec (13 fps) does not produce significantly better solids mixing in the simulations than is the case with 3.5 m/sec jet velocity. Figure 17 shows corresponding results assuming a settling velocity calculated for a SVI of 150 ml/g. Here it is seen that increasing jet velocity from the existing rate of 2.5 m/sec (8.3 fps) to 3.0 m/sec (9.8 fps) is sufficient to produce reasonably good mixing. Increasing the jet velocity above this rate to 3.5 m/sec (11.5 fps) eliminates the formation of clear water at the top of the tank after 25 minutes of mixing, but still results in accumulation of some solids at the bottom of the tank. Increasing the jet velocity to 4.0 m/sec (13 fps) is required to eliminate most significant solids deposition. Table 3 presents a quantitative assessment of mixing at different jet velocities based on the CFD evaluation. The table shows the average suspended solids concentration in each of seven layers of the tank from the top to the bottom, assuming a SVI of 150 ml/g after 25 minutes of mixing. The overall tank averages vary slightly from the assumed initial concentration of 2,400 mg/l. because the layer values were estimated from the different tank layers at a single point at a side location of the tank. The maximum deviation of either the minimum or the maximum layer concentration from the average concentration in the tank is shown a the bottom of the table. Engineering specifications often use a criterion for good mixing that this deviation should be less than 10%. Based on the CFD modeling, the 4.0 m/sec (13 fps) jet velocity would be the only jet velocity to meet this criterion.
Figure 14. Estimated Velocity Profiles with Increasing Jet Velocity (150 SVI).
. Figure 15. Estimated Velocity Profiles with Increasing Jet Velocity (100 SVI).
Figure 16. Solids Concentration Profiles with Increasing Jet Velocity (100 SVI).
Figure 17. Solids Concentration Profiles with Increasing Jet Velocity (150 SVI)
Table 3. Average TSS Concentrations by Layer from CFD modeling (150 ml/g SVI). Average TSS Concentration (mg/l) Layer 2.5 m/sec 3.0 m/sec 3.5 m/sec 4.0 m/sec Top 1,208 1,404 2,102 2,155 2 2,385 2,331 2,280 2,285 3 2,519 2,374 2,322 2,308 4 2,538 2,422 2,448 2,387 5 2,554 2,518 2,526 2,443 6 2,604 2,620 2,511 2,456 Bottom 3,008 2,806 2,559 2,500 Average 2,402 2,353 2,392 2,362 Max Deviation From Average (%) 50% 40% 12% 9% Table 4 presents a different measure of mixing. This table illustrates the cumulative mass of the tank below the tank average concentration, which would be the tank condition if it were completely mixed. This may be taken as an estimate of the reduction in denitrification rate below that which would have occurred if the tank were completely mixed. Since nitrate and soluble carbon would be completely mixed at the end of an aeration cycle and are not settleable, this criterion measures the amount of the tank mass that is removed from access to nitrate and carbon, compared to a completely mixed condition. There is no commonly used rule for selection of the degree of mass deviation that is acceptable. If a level of 5 percent were chosen, then the required jet velocity would be 3.5 m/sec (11.5 fps).
Table 4 Deviation of Mass Distribution from a Completely Mixed Condition SBR Mixing Evaluation Deviation of Mass in Indicated Layer from a Completely Mixed Condition (% of total) Layer 2.5 m/sec 3.0 m/sec 3.5 m/secs 4.0 m/sec 1 7.1% 5.8% 1.7% 1.3% 2 0.1% 0.1% 0.7% 0.5% 3 0.0% 0.0% 0.4% 0.3% 4 0.0% 0.0% 0.0% 0.0% 5 0.0% 0.0% 0.0% 0.0% 6 0.0% 0.0% 0.0% 0.0% 7 0.0% 0.0% 0.0% 0.0% Sum 7.2% 5.9% 2.8% 2.0% Mixing Criteria Evaluation Table 5 presents a comparison of different mixing criteria and their associated power to volume (or unit power) ratios. The table shows the four levels of jet velocity tested in the CFD modeling of BFRWRF, the mixing criterion that this velocity could meet, and the power level required for this level of mixing in watts per cubic meter (W/m 3 ). Power levels varied from 7.7 W/m 3 (39 hp/mg) for the existing system with a jet velocity of 2.53 m/sec (8.3 fps) to 30.8 W/m 3 (156 hp/mg) for the system with the jet velocity increased to 4.0 m/sec (13 fps). The table also includes a limited number of data for other mixing systems. One system that has demonstrated relatively effective mixing at a very low power level is a large, horizontal-shaft banana blade mixer in a typical oxidation ditch racetrack configuration. This installation has seen relatively little MLSS separation during anoxic mixing cycles with a power level of 3.7 kw (5.0 hp) operating in an oxidation ditch tank with a total volume of approximately 3,800 m 3 (1.0 million gallons). No other test data are available from this system. Another system for which the senior author has witnessed a factory test is a surface mixer designed for aeration at high speed, but mixing at low speed. This installation met a specified criterion of measured water velocity greater than 0.6 m/sec (2 fps) near to the bottom of a 4.6 m (15-ft) deep tank. The power level for this test was approximately 7.6 W/m 3 (39 hp/mg). Two other systems for which test data are available (Oton, et al., 2009) are shown in the table. Vertical shaft turbine mixers (called hydrofoil mixers in the referenced paper) were tested in 2008 at the Blue Plains WWTP against hyperboloid mixers. Solids measurements were taken in
the tank profile with each type of mixer operating. The conventional vertical shaft mixer could achieve no better than 28 percent deviation of min / max solids concentration from the tank average at a unit power level of 7.6 W/m 3 (39 hp/mg). The hyperboloid mixer achieved better mixing (less than 11 percent deviation) at approximately one-half the unit power level. A typical rule of thumb for wastewater mixing in the United States would be 40 hp/mg or 7.9 W/m 3. This is approximately the same as the power level of the existing jet mixers at BFRWRF which could meet a mixing performance criterion of only less than 50 percent deviation from tank average based on the results of the present study. We are not aware of data for alternate mixing systems comparable to the CFD results presented in this research. Mixing systems have often been simulated in CFD, but to our knowledge, the effect of mixing system power levels on computed solids concentration profiles has not been published for other mixing systems. If a mixing criterion of less than 5 percent total mass deviation is selected for future evaluation of BFRWRF SBR mixing, then it is seen that a jet velocity of 3.5 m/sec (11.5 fps) would be necessary. This would require a pumping level of approximately 50,000 m 3 /day (9,000 gpm) and a power level of approximately 20.7 W/m 3 (105 hp/mg). Based on the data from the alternate systems shown in Table 5, it is likely that an alternate system of mixing would be more power efficient than any jet mixing system tested in the CFD modeling.
Table 5. Comparison of Required Volumetric Power Input Based on Different Mixing Criteria. Power Level Type of Mixer Test Location Basis of Test Mix Criterion (W/m 3 ) Jet Aeration JEA Blacks Ford SBR Existing power level < 50% Min / Max Deviation 7.7 Jet Aeration JEA Blacks Ford SBR 3.0 m/sec jet velocity < 40% Min / Max Deviation 13.0 Jet Aeration JEA Blacks Ford SBR 3.5 m/sec jet velocity < 5% Mass Deviation 20.7 Jet Aeration JEA Blacks Ford SBR 4.0 m/sec jet velocity < 10% Min / Max Deviation 30.8 Large Propeller in Racetrack Mukilteo, WA Operating Little MLSS separation ~1 Surface Mixing Impeller Manufacturer's site Carollo witnessed test 0.6 m/sec (2 fps) bottom velocity 7.6 Hydrofoil mixer Blue Plains WWTP Field tests < 30% Min / Max Deviation 7.6 Hyperboloid mixer Blue Plains WWTP Field tests < 11% Min / Max Deviation 4.0 Comparison with Neutral Density Simulations Figure 18 presents a comparison of solids profiles for the existing system after 25 minutes of pumped mixing with a simulation of the same condition but with the density coupling in the UDF turned off (neutral density simulation.) The neutral density simulations (incorporating solids settling and transport, but not density coupling) over-predicted the level of mixing found in the field-calibrated, density-coupled CFD tests. The effect of density gradients in activated sludge mixing should not be ignored. Simulations of mixing assuming neutral density (clear water) are an inadequate basis for design.
Figure 18. Comparison of Solids Profiles from Density-coupled and Neutral Density Models SUMMARY AND CONCLUSION A series of three-dimensional CFD model simulations incorporating solids settling and density coupling were performed to evaluate mixing conditions in the jet aeration SBR tanks at the BFRWRF. These were compared to the results of solids measurements conducted using an optical solids probe. Mixing with air was also measured in the field and simulated. Conclusions from the study are as follows: 1) A rough qualitative calibration of the mixing model was achieved to the results of field tests. The field tests showed significant solids separation during pumped mixing cycles. The CFD model indicated slightly greater solids separation than the field test using the recorded field value of SVI of approximately 100 ml/g. Using a solids settling velocity calculated from the Daigger equation for a SVI of 150 ml/g produced simulations that were closer to the field results than simulations based on a SVI of 100 ml/g. 2) A series of simulations were conducted with increasing pumped mix flow, resulting in jet velocities up to 4.0 m/sec (13 fps), compared to the existing pumped mixing jet velocity of 2.5 m/sec (8.3 fps). Increasing the jet velocity to 3.0 m/sec (9.8 fps) reduced solids separation in the tank, but did not eliminate it. With an assumed SVI of 150 ml/g clear water at the top of the tank was eliminated. Increasing the jet velocity to 3.5 m/sec (11.5 fps) eliminated all clear water separation and produced relatively well-mixed conditions in the tank but did not eliminate solids settlement entirely. Increasing the jet velocity to 4.0 m/sec (13 fps) produced relatively complete mixing. 3) Increasing the jet velocity from the existing 2.5 m/sec (8.3 fps) to 3.0 (9.9 fps) would require an approximately 70 percent increase in pump power from the existing power level of 7.7 W/m 3 (39 hp/mg), while increasing jet velocity to 3.5 m/sec (11.5 fps) would require approximately 270 percent increase in pump power. Increasing the jet velocity to 4.0 m/sec (13 fps) would require a four-fold increase in pump power to 30.8 W/m 3 (156 hp/mg). 4) To maintain a maximum deviation of solids concentration of less than 10 percent from average, a jet velocity of 4.0 m/sec (13 fps) would be required for BFRWRF. To maintain a maximum mass deviation of 5 percent from average, a jet velocity of 3.5 (11.5 fps) would be
required. Improving the mixing system will provide more efficient denitrification within a given anoxic cycle time and may aid in a better settling sludge. Furthermore, more efficient denitrification might allow anoxic cycle times to be reduced, and consequently the aeration cycle times to be lengthened, thereby providing better nitrification during cold weather conditions. Improved mixing could also result in further optimization of supplemental carbon dose. 5) It is likely that an alternate system of mixing, like a horizontal submersed propeller, a floating mixer, or a top-mounted impeller (especially with a hyperboloid shape), would be more power efficient for anoxic mixing for this application than any level of jet mixing investigated in this research. 6) Both the field test and the model simulations indicated that mixing with pumped flow plus air is adequate to produce fully mixed conditions in the tank. An increase in aeration rate would not produce better mixing. 7) Simulations were conducted using the calibrated model for typical mixing conditions using a UDF with the density couple turned off. This neutral density simulation is the typical practice for evaluation of tank mixing today. These simulations showed the profound influence of the density couple in achieving an adequate calibration of the model. Simulations without the density couple (neutral density) significantly overestimated the degree of solids mixing measured in field tests and predicted by the calibrated model with solids coupling included. 8) The protocol used in this investigation including field testing and CFD evaluations incorporating solids settling and density couple is recommended for evaluation of other types of mixing devices to establish a better database of comparative mixer efficiencies. Neutral density simulations should not be used for evaluation of mixing for activated sludge applications. ACKNOWLEDGEMENTS REFERENCES Carollo Engineers (2010) Blacks Ford Water Reclamation Facility, Plant Reliability Evaluation. Daigger, G.T. (1995) Development of Refined Clarifier Operating Diagrams Using Updated Settling Characteristics Database. Water Environment Research, 67, 95. Oton, S. et. al. (2009) The Fine Line Between Thorough Mixing and Energy Consumption. WEF Nutrient Removal Conference Proceedings, 2009. Edward A. Wicklein and Randal W. Samstag (2009) Comparing Commercial and Transport CFD Models for Secondary Sedimentation. Proceedings of the 82nd Annual WEFTEC Conference; Orlando, Florida.