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Aeration and Stripping

Aeration and Stripping. CE 547. They are unit operations that depend on the flow of masses between phases, when difference in concentration exists between those phases ( that is called mass transfer) Aeration is needed to:

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Aeration and Stripping

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  1. Aeration and Stripping CE 547

  2. They are unit operations that depend on the flow of masses between phases, when difference in concentration exists between those phases ( that is called mass transfer) Aeration is needed to: • Provide oxygen to be used by microorganisms in activated sludge processes • Remove iron and manganese Stripping is needed to: • Remove CO2 during softening because CO2 reacts with lime which makes the treatment cost very high • Remove H2S from water • Remove benzene, carbon tetrachloride, and vinyl chloride

  3. Aeration is performed through: • Diffusers • Surface aerators • Turbine aerator with an air sparger Stripping is performed through: • Nozzles • Perforated plates • Stripping towers or packed towers

  4. Interface Mass Transfer and Gas and Liquid Boundary Layers • The Figure shows formation of boundary layers in both operations • Two phases; liquid phase and gas phase • Interface is the touching surface of both phases ( thickness = 0) • In each case, there are two boundary layers (films) formed • Mass transfer passes through the boundary layer • In aeration, the direction of flow of mass transfer is from gas phase to liquid phase because the concentration in the gas phase is larger than that in liquid phase • In stripping, the direction of flow of mass transfer is from liquid phase to gas phase because the concentration in liquid phase is larger than that in gas phase. So, the liquid phase is stripped

  5. Mathematics of Mass Transfer yi and xi are equal and because the thickness of the interface is equal to zero, then yi and xi are at equilibrium with respect to each other. Consider the aeration (absorption ) process in the Figure

  6. If [y] is the concentration in the bulk gas phase then, The driving force towards the interfacial boundary = [y] – [yi] and ky = gas-film coefficient of mass transfer [y] = bulk concentration in the gas phase

  7. on the other hand, kx = liquid-film coefficient of mass transfer [x] = bulk concentration in the liquid phase Thus

  8. Since xi and yi are difficult to determine, x* and y* will be used. x* and y* are values when x and y are to attain equilibrium. So, This does not represent the true driving force

  9. To cover for that: Kx and Ky are the overall mass transfer coefficients for liquid and gas phases, while kx and ky are the individual mass transfer coefficients

  10. Introduce a parameter (a) to Kx and Ky, where (a) is the area of interfacial area per unit bulk volume of the mass transfer destination medium Kxa and Kya have the dimensions of per time (1 / t)

  11. Mechanics of Aeration • Oxygen is necessary to microorganisms in activated sludge (aeration tanks) • Equilibrium concentration of oxygen or saturation concentration at certain pressure and temperature is [Cos] which corresponds to [x*] • If oxygen concentration at any time is [C] which corresponds to [x] • The driving force for mass transfer is [Cos] – [C] • The rate of oxygen increase is • In aeration tanks, Kxa is written as KLa

  12. Equipment Specifications • Aeration equipment are reported at standard conditions (in tap or distilled water) • 20 C • 1 atmospheric pressure • 0 mg/l of dissolved oxygen • Under such conditions, the equation becomes the standard oxygen rate (SOR) (KLa)20 = the KLa at standard conditions [Cos,20,sp] = the saturation DO at 20 C and standard pressure

  13. If testing was not done at standard conditions, then use •  = temperature correction factor ( 1.024 in wastewater) • T = temperature in degrees Celsius at testing condition • Pressure effect is assumed to be negligible • The ability of an equipment to transfer oxygen is

  14. (KLa)w and [Cos,w] are KLa and [Cos] of wastewater • The equation represents the actual oxygenation rate (AOR) • (KLa) can be found from: • From the above equations:

  15. Study Examples 1, 2, and 3

  16. Determination of Aeration Parameters • In the lab AOR can be determined • SOR must be calculated to match with values given by manufacturers • To do that  and  must be determined

  17. Determination of  • Simple Procedure • Need 1 liter jar • Fill it half with sample • Shake vigorously to saturate air or oxygen • Measure dissolved oxygen concentration = [Cos, w] • Use Table 1 to get [Cos] at the temperature of the experiment • Calculate 

  18. If the test was not conducted at standard pressure (1 atm), the [Cos] obtained from the Table must be corrected. Also pressure should be corrected. • Pb barometric pressure at altitude z (N/m2) • Pb0 = barometric pressure at z = 0 = 101325 N/m2 • B is the temperature lapse rate = 0.0065 K/m • T0 = temperature in K at z = 0 • g = 9.81 m/s2 • R = is the gas constant for air = 289.9 N . m/kg . K

  19. [Cos] varies directly with pressure • If the test was conducted at barometric pressure, then • [Cos,sp] = [Cos] from the Table at standard pressure of • Ps = Pb0 = 760 mm Hg

  20. Pressure Corresponding to [Cos] The pressure depends on the type of aerator • Barometric pressure for: • For apron, cascade and surface aerators • For spray and plate towers • Average depth of submergence • For submerged aerators such as bubble-diffusion and turbine type of aerators

  21. If submergence depth is Zd, the average pressure (P) is •  = specific weight of water or mixed liquor • [Cos] then becomes

  22. Study Examples 4, 5, 6, and 7

  23. Determination of  Integrate from t = 0 to t = t and C = 0 to C = C To get:

  24. Since [Cos] is constant, then one pair of values of t and C is needed to determine KLa

  25. In case where there are several pairs (from an experiment): • Solution 1 • Draw a straight line relationship between t and ln([Cos]-[C]/[Cos]) • Slope of the line = KLa • Solution 2 • Average t’s and ln([Cos]-[C]/[Cos]) to get single pair • Use this pair to solve for KLa

  26. When averaging • Or • KLa can be corrected to (KLa)20, which is one of the factors needed to calculate α

  27. Now, the other factor [(KLa)w, 20] should be determined • Since microorganisms in wastewater need oxygen for respiration, then the following equation: • Can be written as • Where r = respiration rate

  28. The equation can be re-written as: • (K’La)w is the apparent overall mass transfer, it compasses • (KLa)w the true overall mass transfer • r • In a similar way, we can obtain the following equation

  29. Solve for (K’La)w

  30. If n values of [C] is available, then • Can be re-written as: • Solve for (KLa)w; r is constant

  31. Study Example 8

  32. Calculation of Actual Oxygen Requirement (AOR) • V = volume of the reactor • Q0 = inflow to the reactor • Qw = outflow of wasted sludge • S0 = influent CBOD (Carbon BOD) • S = outgoing CBOD (in the effluent) • Xu = concentration of organisms in wasted sludge • N0 = nitrogen concentration in influent • N = nitrogen concentration in effluent

  33. Study Example 9

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