2/22/2011. Presentation Outline. Overview of Wastewater Aeration. Basic Equation. Some Acronyms. dc dt. dc dt TDS BP T C L

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1 Presentation Outline Overview of Wastewater Aeration Randal W. Samstag, P.E. Carollo Engineers 1. Some definitions 2. Types of aeration devices 3. What affects aeration efficiency? 4. How is aeration efficiency measured? 5. What affects process efficiency? 6. Case study: Kinetics and aeration Some Acronyms Transfer Efficiency SOTE, SAE d AD / AT eblower e motor Site Conditions/ Water Quality TDS BP T D. O. Biological Process SRT YNet Ncontent, VSS f NO3 recycled Basic Equation dc K dt dc dt a K L * C C L a( C * L C L Rate of oxygen transfer (ppd) ) Overall mass transfer coefficient (1/day) Equilibrium DO concentration (mg/l) Oxygen concentration in the liquid (mg/l) 1

2 Oxygen Transfer Rate Equation Standard Conditions OTE ( C C T 20 * 20 f * 20 CL ) SOTE OTE f = In-place oxygen transfer efficiency (%) SOTE = Standard oxygen transfer efficiency (% under Standard Conditions) Parameter U.S. Practice European Practice Type water Tap Tap Water temperature 20 O C 20 O C C L 0 mg/l 0 mg/l Barometric pressure 1 atm 1 atm Air flow 20 O C 0 O C 36% R.H. 0% R.H. γ = lb air/ft 3 ρ = kg air/m 3 = lb O 2 /ft 3 = 300 g O 2 /m 3 Standard Aeration Efficiency (SAE) The rate of oxygen transfer (standard conditions) per unit power input, which may be based on either delivered power (DP) or wire power (WP) Standard Aeration Efficiency (SAE) In this presentation I will always use SAE as the rate of oxygen transfer per unit of electrical (wire) power! Units: pounds per hour per horsepower (pph/hp) Mueller, et al., Aeration: Principles and Practice (2002) 2

3 Types of Aeration Devices 1. Surface mechanical 2. Diffused aeration 3. Hybrid 4. Cascade Surface Mechanical Aeration Devices 1. High speed propeller 2. Low speed mixer / aerator 3. Horizontal rotors 4. Discs Surface Mechanical Aeration Devices 1. High speed propeller 2. Low speed mixer / aerator 3. Horizontal rotors 4. Discs Transfer oxygen by transporting the water into the air (or high purity oxygen). High Speed Propeller Aerators 1. Direct drive coupled propeller aerators 2. Usually floating (not fixed mounted) 3. Used for lagoons and open-topped tanks 4. Standard Aeration Efficiency (SAE): 1.8 to 2.5 pph/hp (Mueller et 3

4 Low Speed Mechanical Conventional 1. Pitched-bladed turbine with lower mixing impeller for deeper applications 2. Fixed-mounted 3. Descendents of early 1920s designs 4. Used for air and HPO 5. SAE 2.5 to 3.0 Low Speed Mechanical (Newer Design) 1. Flat bladed mixer/aerator 2. Top mounted (no bottom impeller) 3. Recent design for HPO upgrades 4. SAE 3.2 to 3.5 pph/hp (Carollo Test) Rotor Aerators Disc Aerators 1. Horizontal mixing impellors 2. Used commonly in oxidation ditches 3. Developed in US and Europe in 1930s 4. SAE 2.5 to Horizontal mixing impellors 2. Used commonly in oxidation ditches 3. Often used for simultaneous nitrification denitrification (SND) 4. SAE 2.0 to 3.0 4

5 RBC Disc Aerators Diffused Aeration 1. Rotating biological contactors (RBS) 2. Mechanical drive or air sparged 3. Low aeration efficiency 1. Coarse bubble a. Orifices b. Tray type c. Static tube 2. Medium bubble a. Membrane tubes 3. Fine bubble a. Porous discs or domes b. Membrane discs c. Membrane panels Diffused Aeration 1. Coarse bubble a. Orifices b. Tray type c. Static tube 2. Medium bubble a. Membrane tubes 3. Fine bubble a. Porous discs or domes b. Membrane discs c. Membrane panels Diffused aeration transfers oxygen by passing air through the water. Coarse Bubble Diffused Aeration 1. Orifice diffuser 2. Plastic materials 3. Higher alpha than fine bubble 4. SOTE: 12-15% 5. SAE :

6 Coarse Bubble Diffused Aeration 1. Tray type diffuser 2. Stainless steel 3. Higher alpha than fine bubble 4. SOTE: 12-15% 5. SAE : Coarse Bubble Diffused Aeration 1. Static tube aerator 2. Air lift pumping action 3. Typically used in lagoons 4. SOTE: 0.6 to 1.4 %/ft 5. SAE: 1.8 to 3 Medium Bubble Membrane Tubes 1. Medium bubble tubes 2. Various materials: EPDM, PVC, ceramic, other plastics 3. SOTE: %/ft 4. SAE: 5.0 to 7 Fine Bubble Aeration 1. Fine bubble porous discs or domes 2. Oldest type of aeration diffuser 3. Materials: ceramics or porous plastics 4. Requires acid cleaning 5. SOTE: %/ft 6. SAE: 5.9 to 9 6

7 Fine Bubble Aeration Fine Bubble Aeration 1. Fine bubble membrane discs 2. Introduced in 1960s 3. Materials: Earlier, poly vinyl chloride (PVC); Now, ethylene-propylene dimers (EPDM) 4. Cleaned by hosing 5. SOTE: %/ft 6. SAE: 5.9 to 9 1. Fine bubble membrane panels 2. Developed by Messner in Germany 3. Higher pressure loss than discs 4. SOTE: %/ft 5. SAE: 5 to 11 Hybrid Aerators Hybrid Aerators 1. Submerged turbines 2. Open bladed turbine with sparger ring of air below 3. Problems with unbalanced forces 4. SAE: 1.75 to Aspirator Aerators 2. Propeller drives air down a shaft to the propeller p hub 3. Mounted on floats 4. Low efficiency 5. SAE: 0.6 to 1.5 7

8 Hybrid Aerators Hybrid Aerators 1. Aspirator Aerators 2. Submersible pump draws down a shaft to the impeller 3. Mounted on basin floor 4. Used in aerated grit chambers 5. Low efficiency 6. SAE: 0.6 to Jet Aeration 2. Combines liquid pumping with gas entrainment 3. Piping i and jet materials: polypropylene, fiberglass, or stainless steel 4. SAE: pph/hp (Yunt) Cascade Aerators 1. Cascade Aerators 2. Pump water over trays to encourage droplet formation 3. Used for simple water treatment (iron removal) 4. Trickling Filters are cascade aerators 5. Efficiency not well documented Comparative Aeration Efficiency Device SOTE (%/ft) SAE (pph/ehp) Diffused Air Fine Bubble Membrane Panels Fine Bubble Porous Disc or Dome Fine Bubble Membrane Disc Medium Bubble Tube Coarse Bubble Tray Mechanical Aeration Low Speed Aerators Horizontal Rotors Discs High Speed Propeller Hybrid Aeration Jet Aeration Submerged Turbines Aspirator Aerators Cascade??? 8

9 A Note on Fine Bubble Diffused Aeration 1. OTE depends on a number of factors a. Solids residence time (SRT) of the MLSS b. Diffuser density (AD/AT) c. Diffuser air rate (cfm/diffuser) d. Diffuser distribution in the basin 2. Under some circumstances (high air rate, fine bubble diffusers not uniform on basin floor) coarse bubble diffusers can be nearly as efficient as fine bubble diffusers How is Efficiency Measured? 1. SOTE: Clean Water Transfer Test 2. OTE f : Off gas testing SOTE: Clean Water Transfer Test 1. ASCE Oxygen Transfer Committee Procedure 2. De-oxygenate basin with sulfite 3. Re-aerate 4. Measure DO and shaft and motor power during reaeration OTE f : Off gas Testing 1. For measurement of alphas for diffused aeration 2. Measure concentration of oxygen in a hood over the mixed liquor 9

10 Filtered Carbonaceous BOD Reactor 1 Reactor 2 Reactor 3 Reactor 4 Reactor 5 Reactor 6 Reactor 7 Reactor /22/2011 Why is tank configuration important? Monod Kinetics A Quick Look at Treatment Process Efficiency The aeration tank configuration, whether plug flow or completely mixed, makes a difference in the efficiency of treatment maxs ( K S max s out out Rate of growth (1/day) S out = BOD concentration (mg/l) ) The rate of growth / uptake of BOD is proportional to the initial concentration Maximum rate of growth (1/day) K s = Half-saturation coefficient (mg/l) Multiple Pass (Plug Flow) With Fine Bubble Aeration Sample Point 2 Plug Flow Tank Residence Time Distribution (RTD) Soluble BOD Profile Four Pass Fine Bubble Tank n = 53 Two Pass Fine Bubble Tank n = 18 Sample Point Soluble BOD Profile C / Co CONC. (mg/l) / o 10

11 Filtered Carbonaceous BOD Reactor 1 Reactor 2 Reactor 3 Reactor 4 Reactor 5 Reactor 6 Reactor 7 Reactor 8 2/22/2011 Oxidation Ditch Tanks (Completely Mixed) Completely Mixed Tanks RTD Curve Soluble BOD Profile Sample Point 1 Two Ditches in Series n = 1.44 Single Oxidation Ditch n = 1.00 Soluble BOD Profile C / Co CONC. (mg/l) Sample Point /o Conclusion? Plug flow tanks have a higher concentration in the front of the reactor -> Higher reaction rates in systems with Monod kinetics Case study: Comparison of upgrade choices for an oxidation ditch for higher energy efficiency and enhanced nutrient removal 11

12 Classic Oxidation Ditch with Rotor Aerators Influent Reactor 1 - Brush Reactor 2 Reactor 3 Reactor 4 Reactor 8 Reactor 7 Waste Sludge Reactor 6 Reactor 5 - Brush Effluent Consider a Number of Alternatives 1. Localized fine bubble aeration and banana blade mixers 2. Plug flow upgrade with floor coverage fine bubble aeration 3. Anaerobic selector plug flow upgrade with fine bubble aeration 4. Anaerobic selector plug flow upgrade with low DO simultaneous nitrification denitrification (SND) 5. Modified Ludzak-Ettinger (MLE) upgrade with fine bubble aeration Localized Fine Bubble Upgrade with Mixer Plug Flow with Floor Coverage Fine Bubble Aeration Influent Reactor 1 Reactor 2 Reactor 3 Reactor 4 Reactor 1 Reactor 2 Reactor 3 Reactor 4 Influent Reactor 5 Reactor 8 Reactor 7 Reactor 6 Reactor 5 Reactor 8 Reactor 7 Reactor 6 Effluent Effluent Waste Sludge Waste Sludge 12

13 Anaerobic Selector Plug Flow Upgrade with Fine Bubble Aeration Reactor 1 Reactor 2 Reactor 3 Reactor 4 Anaerobic Selector Plug Flow Low DO Simultaneous Nitrification Denitrification (SND) Influent Reactor 1 Reactor 2 Reactor 3 Reactor 4 Reactor 8 Reactor 7 Reactor 6 Reactor 5 Influent Reactor 8 Reactor 7 Reactor 6 Reactor 5 Effluent Effluent Waste Sludge Waste Sludge MLE with Fine Bubble Aeration Comparison Influent Reactor 1 Reactor 2 Reactor 3 Reactor 4 1. Oxygen demand 2. Power Reactor 8 Reactor 7 Reactor 6 Reactor 5 3. Effluent Quality Effluent a. Ammonia b. Total inorganic nitrogen (TIN) c. Total phosphorus (Total P) Waste Sludge 4. Phosphorus accumulating organisms (PAO) 13

14 Oxygen Consumption Power Consumption Oxygen Demand, ppd Oxygen Demand, ppd Power Consumption, bhp Power Consumption, bhp Ox Ditch Mixed Plug Flow Plug Flow AS AS SND MLE Plug 50 Flow 0 Ox Ditch Mixed Plug Flow Plug Flow AS AS SND MLE Plug Flow Effluent Ammonia Effluent TIN Effluent Ammonia, mg/l Effluent Ammonia, mg/l Effluent TIN, mg/l Effluent TIN, mg/l Ox Ditch Mixed Plug Flow Plug Flow AS AS SND MLE Plug Flow 0 Ox Ditch Mixed Plug Flow Plug Flow AS AS SND MLE Plug Flow 14

15 Effluent Total P PAO Percentage 6 Effluent Total P, mg/l Effluent Total P, mg/l PAO, % PAO, % % 25.00% % % % 0 Ox Ditch Mixed Plug Flow Plug Flow AS AS SND MLE Plug Flow 5.00% 0.00% Ox Ditch Mixed Plug Flow Plug Flow AS AS SND MLE Plug Flow Summary Comparison Final Conclusions Description Ox Ditch Mixed Plug Flow Plug Flow AS AS SND MLE Plug Flow Oxygen Demand, ppd Power Consumption, bhp Effluent Ammonia, mg/l Effluent TIN, mg/l Effluent Total P, mg/l PAO, % 0.03% 0.03% 0.03% 18.39% 27.27% 18.39% 1. There is a wide variety of aeration devices for wastewater aeration 2. Generally, full-floor coverage, fine-bubble aeration is the most energy efficient 3. Plug flow tanks have a higher process efficiency than completely mixed tanks 4. Anaerobic selector and MLE configurations can achieve TIN and P removal and good settleability 15

16 Acknowledgements Michael K. Stenstrom Rod Reardon Mueller et al. King County Olympus Terrace Sewer District Thank you! Questions? Randal W. Samstag Carollo Engineers rd Avenue, Suite 1600 Seattle, WA