CFD for Wastewater Case Studies: Sedimentation, Mixing and Disinfection

CFD for Wastewater Case Studies: Sedimentation, Mixing and Disinfection Randal Samstag Civil and Sanitary Engineer Ed Wicklein Carollo Engineers Joel Ducoste North Carolina State University Stephen Saunders Ibis Group CFD

Case Study: Activated Sludge Clarifier Radial flow clarifier Questions: Optimum Depth? Optimum Inlet? Optimum Feedwell? Optimum Effluent Zone?

Case Study: Activated Sludge Clarifier Existing Condition CFD

Case Study: Activated Sludge Clarifier Alternative Inlet Configurations

Case Study: Activated Sludge Clarifier Alternative Velocity Vector Plans

Case Study: Activated Sludge Clarifier Alternative Velocity Plans

Case Study: Activated Sludge Clarifier Alternative Velocity Profiles

Case Study: Activated Sludge Clarifier Comparison Solids Profiles

Case Study: Mixing Jet mixing and aeration in a sequencing batch reactor (SBR) * 415,350 mixed tetrahedral cells 2,108,308 nodes Inlet flow into jet nozzles Outlet flow to pump suction Air added as second phase Solids transport, settling, and density impact modeled by UDF Mesh projected onto model surfaces. * Samstag, Wicklein, et al. (2012)

Case Study: Mixing Velocity profiles for pumped mixing and aeration Simulated Pumped Mixing Profile Simulated Aeration Profile

Case Study: Mixing Comparison of pumped mix velocity profiles for increasing jet velocities Existing (2.5 m/sec Jet) 3.5 m/sec Jet 3.0 m/sec Jet 4.0 m/sec Jet

Case Study: Mixing Comparison of solids profiles for increasing jet velocities Existing (2.5 m/sec Jet) 3.5 m/sec Jet 3.0 m/sec Jet 4.0 m/sec Jet

Case Study: Mixing Comparison of power levels for different mixing devices. Type of Mixer Reference Basis of Test Samstag et al. 2.5 m/sec jet CFD Jet Aeration (2012) Samstag et al. 3.25 m/sec jet CFD Jet Aeration (2012) Samstag et al. 3.5 m/sec jet CFD Jet Aeration (2012) Samstag et al. 4.0 m/sec jet CFD Jet Aeration (2012) Samstag and Vertical Hydrofoil II Field test Wicklein (2014) Vertical Samstag and Field test Hyperboloid I Wicklein (2014) Horizontal Wicklein et al. CFD Simulation propeller (2013) Randall and Randall Submersible Mixer Field test (2010) Horizontal Wicklein et al. CFD Simulation propeller (2014) CoV Equivalent 10% CoV Power Level (W/m3) 23.00% 17.71 19.00% 24.7 7.00% 14.49 5.00% 15.4 3.31% 0.5 3.39% 1.8 5.40% 4.5 8.32% 5.3 13.80% 7.6

Case Study: Mixing Comparison of density-coupled and neutral density simulations Density-coupled Solids transport model calculates the local solids concentration based on flow regime. The influence of the local solids concentration on the local density is then iteratively calculated. This approach was verified by the field solids profile test data.

Case Study: Mixing Comparison of density-coupled and neutral density simulations Neutral Density Solids transport model calculates the local solids concentration based on flow regime. Influence of the local solids concentration on the local density was turned off. This approach over-predicted measured solids mixing.

Case Studies: Disinfection Dye, disinfectant, and microorganism transport in a chemical disinfection system Validation of disinfection in a ultraviolet disinfection system

System Hydraulic Efficiency The hydraulic efficiency of a contact tank system is measured in CFD simulations with a tracer. Using a time dependent CFD model, tracer is introduced at the point of disinfectant injection and its concentration is monitored at the model outlet. Data gathered at the model outlet are used to plot a residence time distribution (RTD) curve. The slope and inflection points of the RTD curve are indicative of the hydraulic performance of the contact system. Data points T10, TTDT and T90 are inputs used for accepted evaluation protocol. Streaklines colored by elapsed time normalized against TTDT T10: time at which tracer concentration has reached 10% of the target value Residence Time Dstribution 1 TTDT: Theoretical Detention time ( Voltank/Qeffluent ) BF: Baffle Factor T10/TTDT Values near 1.0 indicate good plug flow. Values lower than 0.3 indicate some short circuiting is taking place. MI: Morrill Index - T90/T10 Values greater than 5.0 indicate some flow is getting hung up in recirculation zones. normalized concentration 0.9 T90: time at which tracer concentration has reached 90 % of the target value T90 0.8 0.7 0.6 TTDT 0.5 0.4 0.3 0.2 0.1 T10 0 0 0.5 t/ttdt 1 1.5

Disinfectant Decay The efficacy of the contact system is dependent on the time the active microorganisms in the effluent are exposed to disinfectant at sufficient concentration to neutralize them. Disinfectants currently in use like sodium hypochlorite or peracetic acid begin to decay immediately upon introduction to the effluent stream. Their levels of concentration are modeled by UDF based on published data. Contours of disinfectant concentration normalized against target concentration level. Contour level 1.0 denotes the target concentration.

Neutralization of Microorganisms The rates at which microorganisms are neutralized are specific to the disinfectant in use and the target microorganism. For example, different coliforms will have differing sensitivities to the disinfectant in use. Their neutralization is simulated using UDF s based on published data. Log(N/N0) (bio. organisms) Contours of microorganism population. N is the local population and N0 is the population in the effluent prior to exposure to the disinfectant.

UV Reactor Validation and the use of CFD In an effort to quantify the UV reactor performance, UV reactors must be validated at different flow rates, water quality conditions, and lamp power settings Reason: to determine if the target reduction equivalent dose (RED) is being met To ensure that the performance can be monitored with a UV sensor. Validation tests are the only means of confirming UV reactor performance since UV does not leave a measurable disinfectant residual

Validation Validation tests can be performed: Onsite at the WTP May not be practical or can be cost prohibitive for large WTPs. Offsite at designated validation facilities Tests are performed under conditions that mimic the range of possible conditions seen at the intended WTP Tests are performed assuming a worst case hydraulic condition, which is assumed by placing an elbow upstream from the reactor inlet

Validation Objectives Investigate the impact of an upstream elbow in a pilotscale UV reactor using biodosimetry measurements. Perform numerical simulations of the pilot-scale reactor to evaluate model performance. Perform simulations of a full-scale reactor with alternative upstream elbow configurations using a validated model.

Validation Objectives Investigate the impact of an upstream elbow in a pilotscale UV reactor using biodosimetry measurements. Perform numerical simulations of the pilot-scale reactor to evaluate model performance. Perform simulations of a full-scale reactor with alternative upstream elbow configurations using a validated model.

Methods: Microbial UV Response Kinetics The first order UV response kinetics was used to represent MS2 and B. subtilis. -1 Y=-0.065x-0.583 log(n0/nd) 0 B. subtilis R^2 = 0.99 1 2 3 4 5 0 10 0 20 30 40 50 Fluence, mj/cm2 Log Inactivation (N/No) -0.5 MS2 sample Linear (MS2 sample) -1-1.5-2 -2.5-3 -3.5 MS2 y = -0.0549x - 0.1192 R2 = 0.9934-4 -4.5-5 0 10 20 30 40 50 60 Fluence (mj/cm2) 70 80 90 100 60 70 80

Methods: Reactors Investigated Reactor A Without Upstream Elbow With Upstream Elbow

Results: Impact of Influent Elbow on Microbial Log Inactivation Reactor A, 1 lamp LP without elbow UVT (%) Flow CFD Model B subtilis MS2 (gpm) B subtilis MS2 92 84 7.6 2.61 1.88 Experimental B subtilis MS2 2.95 0.26 2.06 0.28 Reactor A, 1 lamp LP with elbow UVT (%) Flow CFD Model B subtilis MS2 (gpm) B subtilis MS2 92 84 7.6 2.06 Experimental B subtilis MS2 1.43 2.13 0.10 1.16 0.38 The experimental and numerical results showed that the presence of an upstream elbow led to the reduction in the log inactivation of MS2 and B. subtilis The reduction in log inactivation was caused by a shift in the low fluence peak of the fluence distribution due to the presence of the elbow CFD numerical modeling results displayed reasonable agreement with the experimental biodosimetry data.

Results: Impact of Influent Elbow on Fluence Distribution for pilot-scale reactor No Elbow Elbow 0.14 Particle number fraction 0.12 0.1 0.08 0.06 0.04 0.02 0 0 25 50 75 100 125 Fluence (mj/sq cm) 150 175 200

Methods: Reactors Investigated Reactor B Straight pipe Alt Elbow A Alt Elbow B

Results: Impact of Alternative Influent Piping Configuration on Full-Scale reactor performance UVT (%) 86.5 Flow rate (MGD) 3 Reactor B: 4 lamps, MP Original Straight Alternative A Design pipe elbow 1.03 1.20 1.16 Alternative B elbow 1.40 l The straight pipe configuration provides a 17 % improvement to the original elbow and 3.5 % improvement over alternative A elbow configuration. l Alternative B elbow configuration was found to improve the log inactivation by almost 40 % compared to the original configuration. l Alternative B elbow configuration further displayed a 17 % improvement over the straight pipe configuration

Results: Eulerian representation of microbial transport in Reactor B Original configuration Straight pipe configuration

Results: Eulerian representation of microbial transport in Reactor B Alternative A elbow configuration Alternative B elbow configuration

Results: Velocity vector profile in Reactor B Straight pipe configuration Alternative A elbow configuration

Results: Velocity vector profile in Reactor B Alternative B elbow configuration: Side View Alternative B elbow configuration: End View

Results: Impact of Alternative Influent Piping Configuration on Full-Scale reactor performance The Microbial transport and velocity vector analyses for Reactor B revealed: There may be more reactor short circuiting with the initial elbow, the straight pipe, and alternative A elbow configurations than with alternative B elbow configuration The improved performance with alternative B elbow was due to the change in direction of the core fluid in the elbow away from the outlet. The combined impact of a recirculation zone with the centrifugal and pressure forces acting on the fluid in the initial and alternative A elbow configurations causes a greater fraction of the microorganisms to move towards the outlet

Results: Impact of Alternative Influent Piping Configuration on Full-Scale reactor performance The Microbial transport and velocity vector analyses for Reactor B revealed: There may be more reactor short circuiting with the initial elbow, the straight pipe, and alternative A elbow configurations than with alternative B elbow configuration The improved performance with alternative B elbow was due to the change in direction of the core fluid in the elbow away from the outlet. The combined impact of a recirculation zone with the centrifugal and pressure forces acting on the fluid in the initial and alternative A elbow configurations causes a greater fraction of the microorganisms to move towards the outlet

Thank you! Questions? Randal Samstag randal.samstag@rsamstag.com Joel Ducoste jducoste@ncsu.edu Stephen Saunders ibisgroup@bellsouth.net