Study guide
3.3 Flocculation & Jar Testing
California Water/Wastewater Operator Exam
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1 Chapter 1: California Water Regulatory Framework & Certification Ladders
1.1 State Water Board & Regional Boards Structure 1.2 Drinking Water Certification: T1-T5 & D1-D5 Requirements 1.3 Wastewater Certification: Grades I-V & OIT Requirements 1.4 Safe Drinking Water Standards, MCLs & Emerging Contaminants 1.5 California Waterworks Standards (Title 22, Chapter 16) 1.6 Public Water System Types, Permits & the Sanitary Survey
2 Chapter 2: California Water Rights, Title 22 & Environmental Policy
2.1 California Title 22 Recycled Water Standards 2.2 California Environmental Quality Act (CEQA) & Proposition 218 2.3 Water Rights & California Water Resources
3 Chapter 3: Drinking Water Source Quality & Coagulation/Flocculation
3.1 Source Water Quality & Watershed Management 3.2 Coagulation Chemistry & Flash Mixing 3.3 Flocculation & Jar Testing 3.4 Wells, Aquifers & Groundwater Hydraulics 3.5 Raw Water Storage, Clearwells & Reservoir Management
4 Chapter 4: Clarification & Granular Media Filtration
4.1 Sedimentation & Advanced Clarification 4.2 Granular Media Filtration Mechanics & Media Types 4.3 Filter Operation, Backwash Cycles & SWTR Compliance
5 Chapter 5: Disinfection, Oxidation & Advanced Water Treatment
5.1 Chlorination Chemistry & Breakpoint Curves 5.2 Disinfection Kinetics & CT Calculations 5.3 Alternative Disinfection: Ozone, UV & Chlorine Dioxide 5.4 Advanced Treatment: Membranes, AOP & PFAS Adsorption 5.5 Chloramination & Nitrification Control 5.6 Iron & Manganese Removal 5.7 Fluoridation Practice & Compliance 5.8 Corrosion Control Treatment & Water Stability 5.9 Taste & Odor Control and Algal Toxin Management
6 Chapter 6: Wastewater Preliminary & Primary Treatment
6.1 Preliminary Treatment: Screening & Grit Removal 6.2 Primary Clarification Operations 6.3 Primary Sludge Characteristics & Pumping 6.4 Stabilization Ponds, Lagoons & Small-Plant Processes
7 Chapter 7: Secondary Biological Treatment & Activated Sludge
7.1 Activated Sludge Process Fundamentals 7.2 Process Control Parameters: F/M, MCRT & SVI 7.3 Activated Sludge Microbiology & Troubleshooting 7.4 Fixed-Film & Hybrid Biological Processes
8 Chapter 8: Biological Nutrient Removal, Water Reuse & Solids Handling
8.1 Biological Nitrogen Removal: Nitrification & Denitrification 8.2 Biological Phosphorus Removal (EBPR) & Chemical Precipitation 8.3 Anaerobic & Aerobic Digestion 8.4 Sludge Thickening, Dewatering & Biosolids Disposal 8.5 Wastewater Disinfection: Chlorination, Dechlorination & UV
9 Chapter 9: Distribution Systems, Collection Networks & Hydraulics
9.1 Hydraulics, Pressure & Flow Dynamics 9.2 Centrifugal Pumps, Motors & VFDs 9.3 Distribution Network Infrastructure & Cross-Connection Control 9.4 Wastewater Collection Systems, SSMP & FOG Management
10 Chapter 10: Laboratory Analytics, Safety & Water Math
10.1 Water & Wastewater Laboratory Methods 10.2 Laboratory Quality Assurance & Regulatory Reporting 10.3 Chemical Safety, Hazardous Gases & Confined Space 10.4 Essential Water & Wastewater Math for Operators
11 Chapter 11: Federal & California Drinking Water Rules
11.1 The Revised Total Coliform Rule & Assessments 11.2 The Lead and Copper Rule and Service Line Replacement 11.3 Surface Water Treatment Rules & Log Removal Credit 11.4 Stage 1 & Stage 2 Disinfection Byproduct Rules 11.5 Secondary MCLs, Notification Levels & Response Levels 11.6 Public Notification, Consumer Confidence Reports & Recordkeeping
12 Chapter 12: Distribution System Design, Demand & Hydraulics
12.1 System Layout, Pressure Zones & Distribution Maps 12.2 Water Demand, Peaking Factors & Fire Flow 12.3 Distribution Storage: Types, Sizing & Turnover 12.4 Head Loss, C-Factor & Distribution Hydraulic Calculations 12.5 Water Hammer, Surge Control & Air Management
13 Chapter 13: Distribution Equipment Operation, Maintenance & Inspection
13.1 Valves: Types, Operation & Exercising Programs 13.2 Fire Hydrants: Types, Testing & Maintenance 13.3 Water Meters: Types, Accuracy & Meter Management 13.4 Instrumentation, In-Line Sensors, Telemetry & SCADA 13.5 Electrical Systems, Motors & Standby Generators 13.6 External Corrosion, Cathodic Protection & Asset Inspection
14 Chapter 14: Water Mains, Piping & Field Construction
14.1 Pipe Materials & Pipe Selection 14.2 Joints, Fittings, Thrust Restraint & Bedding 14.3 Main Installation, Tapping & Service Line Installation 14.4 Leak Detection, Water Loss Auditing & Main Repair 14.5 Main Cleaning & Maintenance: Flushing, Pigging & Lining 14.6 Excavation, Trench Safety & Utility Locating
15 Chapter 15: Disinfecting Distribution Facilities & Maintaining Water Quality
15.1 Water Main Disinfection (AWWA C651) 15.2 Disinfecting Storage Reservoirs (C652) & Wells (C654) 15.3 Maintaining Distribution Water Quality: Residual, Biofilm & Discolored Water 15.4 Cross-Connection Control Program Administration & Device Testing
16 Chapter 16: Plant Equipment, Chemical Feed & Maintenance
16.1 Chemical Feed Systems: Metering Pumps, Dry Feeders & Calibration 16.2 Gas Chlorination Equipment & Hypochlorite Systems 16.3 Blowers, Compressors & Aeration Equipment 16.4 Flow Measurement Devices: Meters, Weirs & Flumes 16.5 Preventive Maintenance, CMMS & Asset Management
17 Chapter 17: Utility Management, Administration & Emergency Response
17.1 Planning, Organizing, Directing & Controlling a Utility Work Group 17.2 Budgets, Water Rates & Financial Management 17.3 Operator Training, Staffing & Safety Program Development 17.4 Energy Management & Optimization 17.5 Emergency Response Planning, Risk & Resilience & Security 17.6 Water Conservation Planning & Drought Response
18 Chapter 18: Applied Operator Mathematics Workshop
18.1 The Exam Conversion Sheet, Units & Dimensional Analysis 18.2 Distribution & Hydraulic Math Workshop 18.3 Treatment & Wastewater Process Math Workshop
3.3 Flocculation & Jar Testing
Key Takeaways Explain This Topic AI
Flocculation promotes gentle, controlled particle collisions to aggregate microfloc into dense, settleable macrofloc without exceeding the hydrodynamic shear threshold that tears flocs apart.
The Camp-Stein velocity gradient (G) for flocculation is maintained between 10 and 70 s^-1, yielding a dimensionless Camp number (Gt) of 20,000 to 100,000 over 20 to 45 minutes of hydraulic retention time.
Tapered flocculation progressively steps down velocity gradients across 3–4 compartments (e.g., Stage 1 G=50–70 s^-1, Stage 2 G=30–50 s^-1, Stage 3 G=10–25 s^-1) to protect expanding floc structures from shearing.
Coagulant aids—including cationic, anionic, and non-ionic polymers, bentonite clay, and micro-sand ballasted systems (Actiflo)—strengthen floc bridges and dramatically increase clarifier settling rates.
Standard Jar Testing replicates rapid mix (100 rpm for 1 min), slow mix (30 rpm for 20 min), and settling (10–15 min) to evaluate settled turbidity, floc sizing, and D/DBP precursor (TOC/UV254) removal.
Last updated: August 2026
- Flocculation & Jar Testing
- Flocculation Kinetics & Particle Agglomeration Principles
Following rapid destabilization in the flash mix basin, raw water enters the flocculation basin . Flocculation is the physical process of providing gentle, continuous mixing to promote interparticle collisions, causing destabilized sub-micron pin-flocs to aggregate into large, visible, rapid-settling macroflocs (0.1 mm to 3.0 mm in diameter).
Particle collision kinetics occur via two distinct physical regimes:
Perikinetic Flocculation : Particle collisions driven solely by thermal Brownian motion . This mechanism operates on particles smaller than 1 μ m 1\,\mu\text{m} 1 μ m and is complete within seconds after flash mixing.
Orthokinetic Flocculation : Particle collisions induced by fluid velocity gradients (fluid shear) and bulk turbulence created by mechanical impellers or hydraulic baffles. Orthokinetic flocculation dominates the growth of macrofloc ( > 1 μ m >1\,\mu\text{m} > 1 μ m ).
The Hydrodynamic Shear Threshold
Flocculation design represents a delicate balance between collision frequency and floc shear breakup : Rate of Agglomeration ∝ G ⋅ N 1 ⋅ N 2 \text{Rate of Agglomeration} \propto G \cdot N_1 \cdot N_2 Rate of Agglomeration ∝ G ⋅ N 1 ⋅ N 2 Rate of Floc Breakup ∝ G 1.5 ⋅ d p \text{Rate of Floc Breakup} \propto G^{1.5} \cdot d_p Rate of Floc Breakup ∝ G 1.5 ⋅ d p
Where G G G is the velocity gradient, N N N is particle concentration, and d p d_p d p is floc diameter. If mixing energy ( G G G ) is too low, collision frequency drops, leaving un-agglomerated pin-floc that carries over onto filters. If G G G is too high, hydrodynamic shear forces exceed the internal tensile strength of the floc, tearing macroflocs into non-settleable fragments that will not reform.
Camp-Stein Velocity Gradient ( G G G ) & Dimensionless Camp Number ( G t Gt Gt )
In water treatment plant design, mixing intensity and energy input in flocculation basins are governed by the Camp-Stein equations : G = P μ V G = \sqrt{\frac{P}{\mu V}} G = μ V P G t = G × t Gt = G \times t Gt = G × t
Where:
G = Velocity gradient ( s − 1 ) G = \text{Velocity gradient } (s^{-1}) G = Velocity gradient ( s − 1 )
P = Power dissipated into fluid ( ft-lb/s or Watts ) P = \text{Power dissipated into fluid } (\text{ft-lb/s or Watts}) P = Power dissipated into fluid ( ft-lb/s or Watts )
μ = Absolute dynamic viscosity of water ( l b ⋅ s / f t 2 or P a ⋅ s ) \mu = \text{Absolute dynamic viscosity of water } (lb\cdot s/ft^2 \text{ or } Pa\cdot s) μ = Absolute dynamic viscosity of water ( l b ⋅ s / f t 2 or P a ⋅ s )
V = Basin volume ( f t 3 or m 3 ) V = \text{Basin volume } (ft^3 \text{ or } m^3) V = Basin volume ( f t 3 or m 3 )
t = Hydraulic Retention Time (HRT) in seconds ( minutes × 60 ) t = \text{Hydraulic Retention Time (HRT) in seconds } (\text{minutes} \times 60) t = Hydraulic Retention Time (HRT) in seconds ( minutes × 60 )
G t = Dimensionless Camp Number (total mixing energy parameter) Gt = \text{Dimensionless Camp Number (total mixing energy parameter)} Gt = Dimensionless Camp Number (total mixing energy parameter) [Flocculation Design Window]
- Gt < 20,000 20,000 - 100,000 Gt > 100,000
- ◄───────────────► ◄───────────────────────────────────────────────► ◄──────────────►
- Under-Mixing OPTIMAL MACROFLOC AGGLOMERATION & SETTLING Floc Shearing
- (Pin-Floc Escape) Stage 1: G=50-70 | Stage 2: G=30-50 | Stage 3: G=10-25 (Destruction)
- Standard Flocculation Design Criteria
- Velocity Gradient ( G G G ) : 10 to 70 s − 1 10\text{ to }70\,s^{-1} 10 to 70 s − 1
- Total Hydraulic Retention Time ( t t t ) : 20 to 45 minutes 20\text{ to }45\,\text{minutes} 20 to 45 minutes
- Dimensionless Camp Number ( G t Gt Gt ) : 20 , 000 to 100 , 000 20,000\text{ to }100,000 20 , 000 to 100 , 000
Impact of Water Temperature on Viscosity: As water temperature approaches freezing ( 0 ∘ C 0^\circ\text{C} 0 ∘ C to 4 ∘ C 4^\circ\text{C} 4 ∘ C ), the dynamic viscosity of water ( μ \mu μ ) increases by nearly 80 % 80\% 80% compared to summer temperatures ( 20 ∘ C 20^\circ\text{C} 2 0 ∘ C ). Because G = P / ( μ V ) G = \sqrt{P / (\mu V)} G = P / ( μ V ) , an increase in viscosity dampens the velocity gradient for a given motor power output, requiring operators to increase variable-speed mixer RPM in winter to maintain target G G G -values.
Tapered Flocculation Architecture
Modern conventional treatment plants utilize Tapered Flocculation , dividing the flocculation basin into three or four compartmentalized stages connected in series: Raw Water + Coagulant
- │
- ▼
- ┌─────────────────┬─────────────────┬─────────────────┐
- │ STAGE 1 │ STAGE 2 │ STAGE 3 │
- │ G = 50–70 s⁻¹ │ G = 30–50 s⁻¹ │ G = 10–25 s⁻¹ │
- │ Rapid Collision │ Macrofloc Growth│ Gentle Maturation│ ──► To Clarifier / Settling
- │ Pin-Floc Form │ Interparticle │ Zero Floc Shear │
- │ │ Bridging │ │
- └─────────────────┴─────────────────┴─────────────────┘
- HRT: 10 min HRT: 10 min HRT: 10 min
- ◄──────────────── Total HRT: 30–45 min ─────────────►
- Stage-by-Stage Operating Goals
Stage 1 ( G = 50 to 70 s − 1 G = 50\text{ to }70\,s^{-1} G = 50 to 70 s − 1 ) : High mixing energy drives rapid initial collisions between destabilized micro-particles, rapidly forming visible pin-floc.
Stage 2 ( G = 30 to 50 s − 1 G = 30\text{ to }50\,s^{-1} G = 30 to 50 s − 1 ) : Intermediate mixing energy allows pin-flocs to aggregate into larger macroflocs while preventing shear.
Stage 3 ( G = 10 to 25 s − 1 G = 10\text{ to }25\,s^{-1} G = 10 to 25 s − 1 ) : Gentle stirring provides final floc maturation and densification without exceeding the fragile shear threshold of large macroflocs.
Compartmental Baffling: Each stage is separated by slotted redwood or fiberglass baffle walls to eliminate hydraulic short-circuiting and dead zones, ensuring all water receives the full design retention time.
- Flocculator Mechanical & Hydraulic Configurations
- 1. Mechanical Paddle Wheel Flocculators
Horizontal Reel Paddle Wheels : Large rotating shafts oriented perpendicular or parallel to flow with paddle blades. Paddle tip speed is strictly limited to 0.5 to 3.0 ft/s 0.5\text{ to }3.0\,\text{ft/s} 0.5 to 3.0 ft/s ( 0.15 to 0.9 m/s 0.15\text{ to }0.9\,\text{m/s} 0.15 to 0.9 m/s ) to eliminate localized shear at the blade edges.
Vertical Turbine / Hydrofoil Impellers : Modern axial-flow impellers mounted on vertical shafts driven by Variable Frequency Drives (VFDs); offer superior energy efficiency and uniform velocity distribution.
2. Hydraulic Baffled Basins (Non-Mechanical)
Around-the-End Baffles : Horizontal serpentine channels where water turns 180 degrees around walls. Ideal for shallow basins.
Over-and-Under Baffles : Vertical serpentine channels where water flows over weirs and under bottom baffles. Ideal for deep basins.
Head Loss Calculation : Velocity gradient is generated by hydraulic head loss ( h L h_L h L ) across baffles: G = ρ g h L μ t G = \sqrt{\frac{\rho g h_L}{\mu t}} G = μ t ρ g h L .
Coagulant Aids, Polymers & Ballasted Flocculation ┌────────────────────────────────────────────────────────────────────────┐
- │ POLYMERS & COAGULANT AIDS │
- ├──────────────────────────┬─────────────────────────────────────────────┤
- │ Cationic Polymers │ High positive charge density, low MW. Acts │
- │ │ as primary coagulant aid; reduces alum dose │
- ├──────────────────────────┼─────────────────────────────────────────────┤
- │ Anionic Polymers │ Negatively charged, very high MW. Bridges │
- │ │ metal hydroxide flocs with multivalent ions │
- ├──────────────────────────┼─────────────────────────────────────────────┤
- │ Non-Ionic Polymers │ Polyacrylamides (neutral). Forms physical │
- │ │ interparticle bridges; tough, dense flocs │
- ├──────────────────────────┼─────────────────────────────────────────────┤
- │ Bentonite Clay │ High-density clay added to crystal-clear │
- │ │ raw water (<2 NTU) to provide floc nuclei │
- ├──────────────────────────┼─────────────────────────────────────────────┤
- │ Ballasted Micro-Sand │ Actiflo system: 100 µm silica sand + polymer│
- │ (High-Rate Flocculation) │ Settling velocity increases 10x-20x │
- └──────────────────────────┴─────────────────────────────────────────────┘
- Polymer Classifications
Cationic Polymers : Low-to-medium molecular weight, high positive charge density. Used as primary coagulant aids to reduce primary metal coagulant dosages by 30–50%.
Anionic Polymers : Extremely high molecular weight with negative charges along the polymer chain. Used as flocculant aids to bridge metal hydroxide flocs in the presence of divalent cations ( C a 2 + , M g 2 + Ca^{2+}, Mg^{2+} C a 2 + , M g 2 + ).
Non-Ionic Polymers : Neutral polyacrylamides that form long physical bridges, producing extremely tough, shear-resistant flocs suitable for direct filtration or centrifuge dewatering.
Ballasted High-Rate Flocculation (Actiflo)
In ballasted flocculation , fine silica micro-sand ( 100 to 150 μ m 100\text{ to }150\,\mu\text{m} 100 to 150 μ m ) and an anionic polymer are injected into the flocculator alongside the primary coagulant. The micro-sand serves as a heavy, dense seed onto which floc binds. Settling velocities increase from 0.5 – 1.0 gpm/ft 2 0.5\text{--}1.0\,\text{gpm/ft}^2 0.5 – 1.0 gpm/ft 2 (conventional) to 15 – 35 gpm/ft 2 15\text{--}35\,\text{gpm/ft}^2 15 – 35 gpm/ft 2 , reducing plant footprint by over 85 % 85\% 85% . Hydrocyclones separate and recycle the micro-sand.
Standard Jar Testing Operating Procedure (SOP)
The Jar Test is the essential bench-scale laboratory procedure used by water treatment operators to simulate plant flash mixing, flocculation, and sedimentation to optimize chemical dosages. Jar 1 (0 mg/L) Jar 2 (10 mg/L) Jar 3 (20 mg/L) Jar 4 (30 mg/L) Jar 5 (40 mg/L) Jar 6 (50 mg/L)
- ┌────────────┐ ┌────────────┐ ┌────────────┐ ┌────────────┐ ┌────────────┐ ┌────────────┐
- │ [Paddle] │ │ [Paddle] │ │ [Paddle] │ │ [Paddle] │ │ [Paddle] │ │ [Paddle] │
- │ Raw Water │ │ Alum Dose │ │ Alum Dose │ │ Alum Dose │ │ Alum Dose │ │ Alum Dose │
- └────────────┘ └────────────┘ └────────────┘ └────────────┘ └────────────┘ └────────────┘
- ◄─── 1 min Rapid Mix (100 rpm) ──► ◄─── 20 min Tapered Mix (50/30/15 rpm) ──► ◄─── 15 min Settling ──►
- Step-by-Step SOP
Sample Collection & Characterization : Collect a fresh, representative raw water sample. Record initial temperature, pH, turbidity, total alkalinity, and UV254 absorbance.
Setup Apparatus : Fill six 2-liter square B-Ker jars (or round 1-liter beakers) with exactly 2.0 L of raw water. Mount onto the gang-stirrer apparatus.
Chemical Preparation & Dosing : Prepare fresh stock solutions ( 1 mL of 10 g/L solution in 1 L = 10 mg/L 1\,\text{mL of } 10\,\text{g/L solution in } 1\,\text{L} = 10\,\text{mg/L} 1 mL of 10 g/L solution in 1 L = 10 mg/L ). Add varying coagulant doses across jars (e.g., 0, 10, 20, 30, 40, 50 mg/L).
Rapid Mix Phase : Operate stirrers at 100 rpm 100\,\text{rpm} 100 rpm ( G ≈ 300 – 400 s − 1 G \approx 300\text{--}400\,s^{-1} G ≈ 300 – 400 s − 1 ) for exactly 1.0 minute 1.0\,\text{minute} 1.0 minute while injecting chemicals.
Tapered Flocculation Phase : Step down paddle speeds to simulate plant basins:
- 50 rpm 50\,\text{rpm} 50 rpm for 5 minutes ( G ≈ 50 s − 1 G \approx 50\,s^{-1} G ≈ 50 s − 1 )
- 30 rpm 30\,\text{rpm} 30 rpm for 10 minutes ( G ≈ 30 s − 1 G \approx 30\,s^{-1} G ≈ 30 s − 1 )
- 15 rpm 15\,\text{rpm} 15 rpm for 5 minutes ( G ≈ 15 s − 1 G \approx 15\,s^{-1} G ≈ 15 s − 1 )
Visual Observations : Record the time to initial pin-floc formation (seconds) and evaluate floc size using a standard index (Pinpoint, Small, Medium, Large, Heavy).
Settling Phase : Raise paddles; allow quiescent settling for 10 to 15 minutes 10\text{ to }15\,\text{minutes} 10 to 15 minutes . Observe settling velocity.
Supernatant Analysis : Draw settled water from the sample ports ( 10 cm 10\,\text{cm} 10 cm below liquid surface). Measure and plot Settled Turbidity (NTU) , final pH, residual alkalinity, and filtered UV254 absorbance.
Enhanced Coagulation for D/DBP Rule Compliance
Under the EPA/SWRCB Stage 1 Disinfectants and Disinfection Byproducts (D/DBP) Rule , utilities treating surface water must achieve specified percentage removals of Total Organic Carbon (TOC) prior to chlorination to minimize Trihalomethane (TTHM) and Haloacetic Acid (HAA5) formation.
- Raw Water TOC (mg/L) Raw Alkalinity: 0 – 60 mg/L Raw Alkalinity: >60 – 120 mg/L Raw Alkalinity: >120 mg/L
- >2.0 to 4.0 35.0% TOC Removal 25.0% TOC Removal 15.0% TOC Removal
- >4.0 to 8.0 45.0% TOC Removal 35.0% TOC Removal 25.0% TOC Removal
- >8.0 50.0% TOC Removal 40.0% TOC Removal 30.0% TOC Removal
Optimizing Enhanced Coagulation: To achieve mandatory TOC removal in low-alkalinity waters, operators use jar testing to identify the "point of diminishing returns" (PODR)—adding coagulant and acid (sulfuric acid H 2 S O 4 H_2SO_4 H 2 S O 4 ) to depress coagulation pH to 5.5 to 6.3 , where humic and fulvic acids precipitate most efficiently.
- Operational Scenarios & Troubleshooting
- Scenario 1: Cold Water Flocculation Failure
Situation: In January, raw water temperature drops to 3°C. Settled water turbidity doubles from 0.8 NTU to 2.4 NTU, and un-agglomerated pinpoint flocs pass directly through the sedimentation basin onto the filters. Diagnostic: High water viscosity dampens particle motion, and cold water slows the chemical hydrolysis kinetics of alum. Operator Action:
Perform a jar test with refrigerated raw water to simulate 3°C basin temperatures.
Evaluate switching from standard alum to Polyaluminum Chloride (PAC) , which is pre-hydrolyzed and insensitive to cold water temperatures.
Add a low dose ( 0.05 to 0.10 mg/L 0.05\text{ to }0.10\,\text{mg/L} 0.05 to 0.10 mg/L ) of non-ionic polymer flocculant aid in Stage 2 flocculation to bridge pin-flocs into robust macroflocs.
Scenario 2: Severe Floc Shearing from Excessive Mixing
Situation: A new operator increases the Stage 3 flocculator paddle VFD from 20% to 80% to "improve mixing." Settled water turbidity immediately surges, and filters rapidly blind with head loss. Diagnostic: Excessive G G G -values in Stage 3 ( G > 80 s − 1 G > 80\,s^{-1} G > 80 s − 1 ) exceeded the floc shear threshold, shattering fragile macroflocs into microscopic sheared fragments that cannot settle. Operator Action: Immediately throttle Stage 3 mixer speeds back to G = 10 – 20 s − 1 G = 10\text{--}20\,s^{-1} G = 10 – 20 s − 1 .