Outline Introduction Case Studies Anoxic Zone Mixing Channel Mixing Theory and Criteria Mixing Theory Design Criteria Recommendations
Where Do We Mix at WWTPs? Introduction Flow Equalization Tanks Headworks Wet Wells Flow Distribution Boxes Aeration Tanks & Channels Anoxic and Anaerobic Reactors Final Effluent (Re aeration) Disinfection tanks Sludge Digesters Processes with Chemical Addition Mixing is a typical process in wastewater treatment
Why Do We Mix? Introduction Keep solids in suspension Enhance mass transfer and microbial kinetics Improve chemical reaction with pollutants Provide aeration / odor control Improve effective residence time, decrease short circuiting Mixing is an important process in wastewater treatment.
How Do We Mix? Introduction Draft Tubes By Kocamemi There are different solutions/equipment for mixing.
Why Talk About Mixing? Introduction Mixing exists in most plants Mixing consumes a lot of energy Design criteria vary There are a lot of over designed/operated mixers Process performance requires adequate mixing Sustainability depends on everybody s effort.
Why after Primary Treatment? Introduction Heavy/high density solids have been removed; Following typical criteria might waste energy: opportunities to improve Types of mixing: Rapid mixing Continuous mixing
Outline Introduction Case Studies Anoxic Zone Mixing Channel Mixing Theory and Criteria Mixing Theory Design Criteria Recommendations
Anoxic Zone Mixing - Background Case Study 1 Plant wet weather peak flow 63 MGD Upstream grit removal and primary treatment with EDIs Aeration tank volume: 5.4 MG with two trains Four anoxic zones, volume is 44,800 ft 3 for each zone, dimensions: 94 ft (L) X 30 ft (W) X 15.9 ft (D) Expected MLSS concentrations: 2,000 to 5,500 mg/l How to provide best anoxic mixing design?
Anoxic Zone Mixing Mixer Type Case Study 1 Coarse bubble diffusers were excluded due to concerns of oxygen introduction Submersible mixers: ABS and Flygt Pictures courtesy of ABS and Flygt/Xylem
Anoxic Zone Mixing Mixer Type Case Study 1 Invent HYPERCLASSIC Mixer Pictures courtesy of Invent Environmental Technologies Inc Had higher cost to install at specific site.
Anoxic Zone Mixing Mixer Type Case Study 1 Floating Mixers: Siemens and AquaDDM Picture from Aqua-Aerobic Systems website Better vertical mixing to create the concentration gradient for bioselectors.
Anoxic Zone Mixing Design Case Study 1 Design Iterations 1 Reference MetCalf & Eddy Recommendation Mixer Size (Hp) Mixing Zone Volume (1,000 cubic feet) Mixing Energy Input (Hp/1,000 cubic feet) Total Power Draw (Hp) Annual Energy Cost* 20 (17 Hp) 22.4 0.75 136 $53,300 2 WEF Recommendation 10 22.4 0.45 80 $31,400 3 Manufacturer's Design Criteria 5 22.4 0.22 40 $15,700 Similar Projects by 4 3 22.4 0.13 24 $9,400 Others * Plant average electricity cost is $0.06/kwh; electricity price inflation is not considered. 50-70% reduction in annual energy cost from step 2
Case Study 1 Anoxic Zone Mixing 3Hp or 5 Hp? VFD was excluded from design Two speed motor is not available Easy change out of mixer impellers Manufacturer physical lab and CFD modeling were not conclusive Fort Collins Drake Facility in Colorado: 15 Hp/MG, or 0.11 Hp/1,000 cubic feet Big mixers could cause tank bottom concrete deterioration over time. Decision: To conduct full scale testing at one half of the anoxic zones during construction.
Case Study 1 Anoxic Zone Mixing Another Idea Pulsed Large Bubble Mixing EPA 832-R-10-005, 2010 EnviroMix / BioMix
Channel Mixing Background Case Study 2 Mixed liquor channel with MLSS = 2,000 5,500 mg/l Channel volume: 33,480 ft 3 Channel Dimensions: 944 ft (L) X 15 ft (W) X 6 ft (D)
Channel Mixing Alternatives Case Study 2 1) Main aeration blowers with a throttling valve pressure drop of 3 to 4 psi 2) Mechanical mixers too many units 3) New dedicated blowers capital cost 4) Existing PD blowers and coarse bubble diffusers 5) Existing PD blowers and retractable fine bubble diffusers
Case Study 2 Channel Mixing Air Requirement Design Options Reference Design Criteria (scfm/ft 2 ) Channel Bottom Area (ft 2 ) Air Demand (scfm) Blower Power Draw (Hp) Annual Energy Cost* 1 Typical Manufacturer Recommendations 0.12 14,166 1700 60 $23,530 2 Lower Limit of Mixing Requirements 0.06 14,166 850 30 $11,760 * Plant average electricity cost is $0.06/kwh; electricity price inflation is not considered.
Outline Introduction Case Studies Anoxic Zone Mixing Channel Mixing Theory and Criteria Mixing Theory Design Criteria Recommendations
Theory and Criteria Mixing Theory Velocity Gradient A measurement of mixing effectiveness G is an average number not homogeneous Mixing creates circulation (velocity) and shear Given V and µ, P needs to increase to increase G
Theory and Criteria Mixing Theory Velocity Gradient Radial flow impeller in a baffled tank
Theory and Criteria Mixing Theory Velocity Gradient Axial flow impeller in a baffled tank
Theory and Criteria Mixing Theory Velocity Gradient Picture courtesy of Flow Science CFD Modeling of a mixing tank 23
Mixing Theory Velocity Gradient Picture by Maxon, Fowler and Kehn, Water World CFD Modeling of a mixing tank 24
Mixing Theory Power Number Steffe, Rheological methods in food process engineering Different mechanical impellers has different power draw and mixing effect. 25
Mixing Design Considerations Theory and Criteria For mixing for solids suspension, consider the following factors: Tank Dimensions Baffles The impeller: material, shape Settling velocity of solids: specific gravity, size, etc. Solids concentration Temperature Difficult to quantity; a highly empirical process.
Theory and Criteria Mixing Theory Power Number Different mechanical impellers have different power draws and mixing effects.
Design Criteria Theory and Criteria Anoxic Zone Mixing Requirements: MetCalf & Eddy: 0.75 to 1.50 Hp / 1,000 ft 3 WEF MOP8: 0.45 to 0.75 Hp / 1,000 ft 3 Manufacturer's Criteria: 0.22 Hp / 1,000 ft 3
Design Criteria Theory and Criteria Summarized by Sam Jeyanayagam, 2007 Modeling Wastewater Aeration Systems to Discover Energy Savings Opportunities. By Steven A. Bolles
Why Less Power Is Possible? Theory and Criteria Better understanding and better design (e.g., big blade low speed mixers) Lower density of solids to suspend/mix Better tank/channel geometry Baffle design Bulk flow velocity to be considered!
Why Less Power Is Possible? For anoxic and anaerobic reactors, By Jared Wray, P.E., and Daniel Alper, KSB, 2012
Outline Introduction Case Studies Anoxic Zone Mixing Channel Mixing Theory and Criteria Mixing Theory Design Criteria Recommendations
Recommendations Design and operate beyond published design criteria How to improve mixing efficiency: Low speed big diameter mixers Conduct physical models Conduct CFD Modeling Install Baffles Install VFD or two speed motors
Recommendations Improvement Process: Compare current mixing power input with design criteria and other facilities Identify and analyze possible improvement alternatives Calculate Capital Improvement Costs Calculate Energy Savings Determine Payback Period Verify feasibility Design and implement the improvement
Summary Mixing is typical, important, and energy consuming Mixing design needs careful analysis, rather than just following design criteria Mixing energy saving opportunities might be available after primary treatment Significant savings can be achieved by careful design and operation