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COLLEGE OF ENGINEERING Chemical, Biological & Environmental Engineering Optimization of Secondary Clarifier Draft Tube Configuration for the City of Corvallis Stephanie Rich, Shumin Lu, Jon Curry Wastewater Treatment Future Work Acknowledgments Draft Tube Configurations Experimental Metrics RAS Concentration • Measured using centrifuge • Sample taken from feed well • Optimally 10,000-17,000 mg/L Settlometer • Settleometer used to determine Sludge Volume Index (SVI) • Optimal SVI <100 mL/g Sludge Blanket Height • Measured using Sludge Judge • Another evaluation of sludge settling quality • Optimally less than 3 ft to show good compression settling Pressure Head • Differential gauge to measure the change in water height between feed well and outside clarifier • Typically between 4-8 inches at the Corvallis Wastewater Reclamation Facility • Include friction loss due to piping to improve current model • Darcy-Weisbach Equation for friction loss A “stress test” is when a process is pushed to the point of failure. We used this test to determine a relationship between pressure head and influent flow rate as show below. Thank you to the Corvallis Wastewater Reclamation Facility Operations Department: Stan Miller, Gene Freel, Matt Mead, Jim Green, James Hughes, Les Wiensz, and Mark Lankford, Utilities Division Manager Tom Hubbard, and Dr. Philip Harding. vs 4-12 MGD 12-18 MGD Biological processes use organisms to degrade pollutants in municipal wastewater. These processes include an aeration basin for waste degradation, and a secondary clarifier to settle out sludge and concentrate it for further use. RAS: Return Activated Sludge from secondary clarifier to the aeration basin To improve secondary clarifier performance by predicting optimal draft tube configurations using experimental data and theoretical modeling. Model Improvement: Stress Test Conclusions 0 10 20 30 40 0 5 10 15 20 ∆H (in) Inflow (MGD) Theoretical Experimental Pressure head predicted as a function of open draft tube area and RAS pump rate vs experimentally measured values. The model is only accurate for low flows, and needs to be further improved for influent flow rates above 12 MGD. • RAS concentrations were increased with proposed high flow configuration • Sludge flow can be modeled as turbulent pipe flow • Clarifier performance depends on many process parameters in addition to draft tube configuration Secondary Clarifier Modeling Profile view of secondary clarifier with 4 draft tubes on each side. Sludge is collected from the bottom of the clarifier and sent through draft tubes in the feed well. 0.0 0.5 1.0 1.5 2.0 RAS Flow (MGD) Plant Inflow (MGD) Q(RAS,in) Q(RAS,out) 4 9 12 6 18 24 Figure of inflow and outflow predictions based on theoretical assumptions shown above. The model is accurate for low flows, but not for high flows. Feed Well Sludge Blanket Center box contains 8 draft tubes. A1 A2 A4 A3 B1 B2 B3 B4 Assumptions: • Potential energy from pressure head is equal to kinetic energy driving the sludge to flow in the draft tubes • Two clarifiers have identical performance • Sludge density = 1400 kg/m 3 • Friction loss due to piping is negligible Equations: • , = , • , = (∆, ) • , = ∗ RAS concentration increased after the draft tube configuration was changed according to high flow conditions. Average influent flow rates ranged from 6-25 MGD during this period. Higher RAS concentration indicates a successful configuration due to process improvement. Open Area Sludge Inlet Sludge inlet from bottom of clarifier leading into feed well via sludge pipes. A single draft tube taken out from the feed well for demonstration of open area. Channel of wastewater flow going to disinfection basin. High flow rates were observably more turbulent—pushing the clarifier closer to its maximum limit. Objective 0 5 10 15 20 25 30 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 1/1 1/11 1/21 1/31 2/10 2/20 3/2 3/12 3/22 4/1 4/11 4/21 Plant Flow (MGD) RAS Concentration (mg/L) Date Before Our Configuration After Our Configuration Plant Inflow (MGD) ℎ = 2 2 ∆ =ℎ + ∆ Head Loss (ΔH), Sludge Velocity (v), Open Area of Draft Tubes (A open ), Pipe Lengths (L pipe ), Friction factors (f f ). Sludge Viscosities (µ), Reynold’s Numbers (turbulent flow), Pipe material (PVC), Pump Rate (%), Influent Flows (Q in ) Parameters Influencing Clarifier Performance:
