Oxygen Transfer

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<ul><li><p>5-Oxygen Transfer_F12.doc </p><p> 1 </p><p>Activated Sludge - Types of Processes and Modifications </p><p> 1 Conventional </p><p>Influent Effluent</p><p>PF Aeration Tank </p><p>HRT = 8 - 15 hrs </p><p>O2 supply</p><p>Return sludge Waste sludge</p><p>O2 demand</p><p>Tank length</p><p>2 Tapered Aeration</p><p>Influent Effluent</p><p>PF Aeration Tank </p><p>Return sludge Waste sludge O2 supply</p><p>Tank length O2 demand</p><p>3 Step Aeration</p><p>Influent</p><p>Effluent</p><p>PF Aeration Tank </p><p>O2 supply</p><p>Return sludge Waste sludge</p><p>O2 demand</p><p>Tank length</p><p>4 Completely Mixed</p><p>Influent Completely Mixed Aeration Tank Effluent</p><p>O2 supply</p><p>Return sludge Waste sludge</p><p>O2 demand</p><p>Tank length</p><p>Alternate waste </p><p>sludge drawn </p><p>off point</p><p>Alternate waste </p><p>sludge drawn </p><p>off point</p><p>Alternate waste </p><p>sludge drawn </p><p>off point</p><p>Alternate waste </p><p>sludge drawn </p><p>off point</p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 2 </p><p>5 Contact Stabilization</p><p>Effluent</p><p> Influent</p><p>HRT = 3 - 6 hrs</p><p>Return sludge Waste sludge</p><p>6 Kraus Process</p><p>Waste sludge</p><p>Effluent</p><p> Influent Aeration Tank</p><p>Reaeration Tank</p><p>HRT = 24 hrs</p><p>(Nitrification) Digested supernatant</p><p>Digested sludge</p><p>Alternate waste </p><p>sludge drawn </p><p>off point</p><p>Alternate waste </p><p>sludge drawn </p><p>off point</p><p>HRT = 20 -</p><p>90 min</p><p>Aeration alone can account for half of the operation costs at a typical treatment plant (p. 8, </p><p>Logan, 2008) </p><p>Gas Transfer theory </p><p>1. General Gas Transfer Equation </p><p>The rate of oxygen transfer </p><p> L sdC</p><p>K a C Cdt</p><p>where Cs = oxygen concentration in the liquid at saturation, mg/L </p><p> = f (T, dissolved solids) </p><p> C = oxygen concentration in the liquid at time, t </p><p> KLa = oxygen transfer rate coefficient, hr-1</p><p> = f (T, types of diffuser, depth of aerator, types of mixer, tank geometry) </p><p> Cs - C = dissolved oxygen deficit, D, mg/L </p><p>C &gt; Cs</p><p>C &lt; Cs</p><p>t</p><p>CsC</p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 3 </p><p>2. Two-film Theory (Lewis and Whitman, 1923) </p><p>Cl = concentration of gas in bulk liquid </p><p>Csl = concentration of gas in surface liquid </p><p>Cg = concentration of gas in bulk gas </p><p>Csg = concentration of gas in surface gas </p><p>Gas film control Liquid film control Cg</p><p>-for very soluble gas - for not very soluble gas</p><p>e.g., NH3 e.g., O2</p><p> = Csl = Csg = Cg</p><p>Cg</p><p>Cl</p><p>Csg = Csl = Cl</p><p>Cl</p><p>=</p><p>Air - turbulent (well </p><p>mixed body of air)</p><p>Liquid film - laminar molecular </p><p>layer</p><p>Liquid - turbulent (well mixed </p><p>body of water)</p><p>Gas film - laminar molecular layer - </p><p>stagnant mass of air (molecular </p><p>diffusion) </p><p>Cl</p><p>Cg</p><p>Csl</p><p>Csg</p><p>microlayer </p><p>resistance (60 </p><p>um)</p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 4 </p><p>3. Diffusion </p><p>- Gas diffusion (molecular diffusion) through a liquid film </p><p>- Ficks first law of diffusion </p><p> VC CJ DA</p><p>t x</p><p>L3 M L</p><p>2 M M </p><p>------- = ----- L2 ------ = ---- </p><p>T L3 T L</p><p>3 L T </p><p>where D = molecular diffusion coefficient, L2 T</p><p>-1 </p><p> A = surface area, L2 </p><p> x = liquid film thickness, L </p><p>Since 2 1</p><p>l sC CC C</p><p>x x x x</p><p>V = film volume between the gas and liquid interface </p><p>Assuming V= constant, </p><p>2 1 2 1</p><p>l s s l s lC C C C C C CJ V DA DA DAt x x x x x</p><p> s lC D</p><p>V A C Ct x</p><p> L s lC</p><p>V K A C Ct</p><p>where KL = D/x = oxygen transfer rate, LT-1</p><p>divided by V yields </p><p> L s lC A</p><p>K C Ct V</p><p>Let a = A/V </p><p> L s lC</p><p>K a C Ct</p><p> L 1 </p><p> --- --- </p><p> T L </p><p>The rate of O2 transfer is </p><p>controlled by a liquid film </p><p> Cg</p><p>x1</p><p> Cs</p><p>x2</p><p>Cl</p><p>x </p><p>x</p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 5 </p><p>where Cs Cl = concentration gradient, major driving force KLa = oxygen transfer rate coefficient, T</p><p>-1, hr</p><p>-1 </p><p>KLa depends on types of gas and liquid (film thickness), increased by mixing intensity, waves </p><p>a depends on surface area, A, increased by finer bubbles </p><p>Bubbles </p><p>In general, the rate of oxygen transfer increases with: </p><p>a) decreasing bubble size (larger contact area) b) longer contact time c) added turbulence </p><p>- Gas transfer increases with area A </p><p>- A/V increases by producing fine bubbles and/or breaking the surface </p><p>Bubble diameter, mm Bubble diameter, mm</p><p>10</p><p>Bu</p><p>bb</p><p>le r</p><p>isin</p><p>g v</p><p>elo</p><p>city</p><p>KL</p><p>2 10 2</p><p>From two observations, </p><p> optimum size = 2 mm </p><p> &lt; 2 mm clogging problem in diffuser heads (bacterial slime), more maintenance </p><p>&gt; 2 mm, tends to lose KL, O2 transfer rate </p><p>Smaller bubble size gives slower </p><p>velocity, thus more contact time. </p><p>Smaller bubble size gives smaller KL, less </p><p>- turbulence, less surface breaking </p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 6 </p><p>Evaluation of KLa value </p><p> L sdC</p><p>K a C Cdt</p><p> 01</p><p>o</p><p>C t</p><p>LC</p><p>s</p><p>dC K a dtC C</p><p> 0( 1) lno</p><p>C t</p><p>s LCC C K a t </p><p> 0lno</p><p>C t</p><p>s LCC C K a t </p><p> ln lns s o LC C C C K a t </p><p> ln lns s o LC C C C K a t </p><p>ln s Ls o</p><p>C CK a t</p><p>C C</p><p>LK a ts</p><p>s o</p><p>C Ce</p><p>C C</p><p>Intercept =</p><p>Slope = -KLa</p><p>t</p><p> ln sC C</p><p> ln s oC C</p><p>Slope = -KLa</p><p>t</p><p>ln s</p><p>s o</p><p>C C</p><p>C C</p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 7 </p><p>Oxygen Transfer and Oxygen Requirements </p><p>1. Importance </p><p> 2</p><p>m</p><p>O</p><p>DO</p><p>K DO</p><p> a. The rate of aerobic microbial metabolism is independent of the DO concentration above a </p><p>critical (minimum) value. </p><p> b. Below the critical value, the rate is reduced by the limitation of oxygen required for </p><p>respiration. </p><p> c. Critical DO concentrations reported in the literature for activated-sludge system range </p><p>from 0.2 to 2.0 mg/L. </p><p> - For conventional and high-rate aeration basin = 0.5 mg/L </p><p> - A typical DO for activated sludge operation would be 2.0 mg/L (W. C. King, PE. Exam, </p><p>p. 230) </p><p>2. Oxygen Transfer Models </p><p> Cell membrane</p><p>Liquid film</p><p>CO2</p><p>DO</p><p>(Rate of O2 transfer) Rate of O2 utilization</p><p>Microbial cell</p><p>Bubble</p><p> dC/dt = KLa (Cs - C) dC/dt = r </p><p> Figure x.x. Schematic diagram of oxygen transfer in activated sludge. </p><p> - Oxygen is dissolved in solution and then extracted from solution by the biological cells. </p><p> At steady state, [the rate of oxygen transfer] = [the rate of oxygen utilization] </p><p>m</p><p>m/2</p><p>Critical DO cinc (0.2 - 2 mg/L)</p><p>KO2 DO (mg/L)</p><p>20.2 0.5</p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 8 </p><p>In clean water </p><p>The rate of oxygen transfer </p><p> L sdC</p><p>K a C Cdt</p><p> where Cs - C = dissolved oxygen deficit, mg/L </p><p> Kla = oxygen transfer rate coefficient, hr-1</p><p> = f (T, types of diffuser, depth of aerator, types of mixer, tank geometry) </p><p> Cs = oxygen concentration in the liquid at saturation, mg/L </p><p> C = oxygen concentration in the liquid at time, t </p><p> KLa depends on temperature, types of diffuser/ mixer, depth of aerator, </p><p> tank geometry </p><p>Effect of temperature on KLa - vant Hoff-Arrhenius relationship </p><p> 20, ,20</p><p>T</p><p>La T La CK K </p><p> 6-61 (ME, p. 286) </p><p> where K La, T = oxygen mass-transfer coefficient at temperature T, s-1</p><p> K La, 20C = oxygen mass-transfer coefficient at 20 C, s-1</p><p> Range of value = 1.015 - 1.040 Typical value = 1.024 </p><p>In general, the rate of oxygen transfer increases with: </p><p> a ) decreasing bubble size </p><p> b ) longer contact time </p><p> c ) added turbulence </p><p>In wastewater </p><p> The rate of oxygen transfer from air bubble to wastewater in an aeration tank: </p><p> L sdC</p><p>K a C Cdt</p><p>where dC/dt = rate of oxygen transfer, mg/L/hr </p><p> = alpha factor or coefficient (oxygen transfer coefficient) of the wastewater </p><p> = beta factor or coefficient (oxygen saturation coefficient) of the wastewater KLa = oxygen transfer rate coefficient, hr</p><p>-1 </p><p> Cs = oxygen concentration at saturation, mg/L </p><p> C = oxygen concentration in the liquid at time t, mg/L </p><p> Cs - C = dissolved oxygen deficit in wastewater, mg/L </p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 9 </p><p>The alpha () factor </p><p>- The alpha () factor is the oxygen transfer coefficient factor for waste (see Table 5-32, 4th ME 447) </p><p> - is defined as the ratio of the oxygen transfer coefficient in water to that in clean water: </p><p> (KLa) in wastewater </p><p> = --------------------------- (KLa) in clean water </p><p>2) The factor is influenced by many conditions related to: </p><p> a ) the characteristics of the wastewater (temp, soluble BOD, SS conc) </p><p> b ) the aeration equipment (types of aerators, mixing intensity, tank configuration) </p><p>3. The magnitude can even change between the influent and effluent ends of plug-flow aeration </p><p>tank. </p><p> Viessman &amp; Metcalf &amp; </p><p> Hammer Eddy </p><p> VH ME (p. 429) King (PE Exam, p. 230) </p><p>_____________________________________________________________________________ </p><p>For municipal wastewater 0.7 - 0.9 (0.4-1.1) .3 - 1.2 </p><p> 0.82 </p><p> Fine-bubble diffusers as low as 0.4 0.4 - 0.8 </p><p> Mechanical aerator as high as 1.1 0.6 - 1.2 </p><p>____________________________________________________________________________ </p><p>The beta () factor </p><p>The beta () factor is the salinity-surface tension correction factor (4th ME 447) </p><p>- is defined as the ratio of the DO saturation concentration in the wastewater to that in clean </p><p>water: </p><p> Cs in wastewater </p><p> = ------------------------ Cs in clean water </p><p>The value is influenced by the wastewater constituents (at constant temperature) including: a ) dissolved solids, salts </p><p> b ) dissolved organics </p><p> c ) dissolved gases </p><p>For municipal wastewater = 0.7 - 0.98, commonly 0.95 (ME, 429) = 0.95 (King, PE. Exam, p. 230) </p><p> = 0.9, seldom less than 0.8 (VH) </p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 10 </p><p>Dissolved Oxygen Utilization Rate, r (mg O2/L-hr) </p><p>The rate of DO utilization by microorganisms in an activated-sludge system can be determined </p><p>by placing a sample of mixed liquor in a closed container and measuring the dissolved-oxygen </p><p>depletion with respect to time. </p><p>(a) (b)</p><p>MLVSS</p><p>- O2 is used for cell synthesis and </p><p>respiration. </p><p> I </p><p>1) The oxygen utilization rate, r, is the slope of the resultant curve. </p><p>2) The oxygen utilization rate, r, depends on the microorganisms ability to metabolize waste </p><p>organics based on such factor as: </p><p> a ) F/M ratio </p><p> b ) mixing conditions </p><p> c ) temperature </p><p>3) A general range for the oxygen utilization rate r in the mixed liquor of conventional and completely mix (high-rate) activated-sludge systems is: </p><p> r = 20 - 100 mg/Lhr. (Typical range: 20 - 80 mg/Lhr) </p><p>O2 probe</p><p>MLVSS</p><p>Slope = r = oxygen utilization rate</p><p>mgO2/L-hr</p><p>(b) End of plant</p><p>(a)</p><p> Front of plant</p><p>Time (hr)</p><p>DO (mg/L) </p><p>remaining</p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 11 </p><p>Air and Oxygen </p><p>- At sea level and at 20C, dry air has a density of ~1.2 kg/m3 varying with pressure and </p><p>temperature. </p><p>(SI units) </p><p>a) The density of air is 1.205 kg/m3 at 20C and 1 atm. </p><p>b) O2 content in air is 23% (w/w); i.e., 0.23 kg O2/kg air </p><p>2 2</p><p>3 3</p><p>0.23 1.205 0.27715kg O kg air kg O</p><p>kg air m air m air </p><p> 1 m3 of air contains 0.27715 kg of O2 under the standard conditions (T=20C, p=1 atm). </p><p>(US customary units) </p><p> a) Air density = 0.075 lb/ft3 </p><p> b) O2 content in air is 23% (w/w); i.e., 0.23 lb O2/lb air </p><p>2 2</p><p>3 3</p><p>0.075 0.23 0.0173</p><p>1</p><p>lb air lb O lb O</p><p>ft air lb air ft air</p><p>1 ft3 of air contains 0.0173 - 0.0174 lb of O2 </p><p>Power Requirement </p><p>1) Purposes of Aeration </p><p> a ) Provide oxygen </p><p> - to satisfy microbial oxygen demand, r </p><p> b ) Provide mixing </p><p> - Mixing requirements range from 0.75 to 1.50 HP per 1000 ft3 of tank volume (King, </p><p>PE Exam, p. 230). </p><p>2) The aerator power required depends on: </p><p> a ) Type of activated-sludge process </p><p> b ) BOD loading </p><p> c ) Oxygen transfer efficiency of the aerator equipment. </p><p>3) Aerator performance </p><p> - Aeration systems are compared on the basis of mass of gaseous oxygen transferred to </p><p>dissolved oxygen per unit of energy expended: </p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 12 </p><p>Oxygen Transfer Rate </p><p>2 2 2 2lb of O lbs O kg of O kg O or horsepower-hr HP-hr killowatt-hr kW-hr</p><p>Oxygen Transfer Efficiency, OTE (%) </p><p>2</p><p>2</p><p>mass of O dissolved (transferred) in waterOTE(%) = </p><p>mass of gaseous O applied </p><p>The values specified for efficiency are based on operation in clean water with zero DO </p><p>concentration and standard conditions (20C, 1 atm). </p><p>Example Oxygen Transfer Efficiency (%) Use SI units </p><p>62.43 m3 of air is required (need to be applied) per kg of BOD applied to an aeration </p><p>tank. The aerator is capable of transferring 1.7 kg of O2 (dissolved) per kg BOD applied. What </p><p>is the oxygen transfer efficiency (OTE)? </p><p>Assumptions: </p><p> The density of air at 20C and 1 atm is 1.205 kg/m3. Since air contains 23% O2 (w/w), </p><p>(0.23 kg O2/kg air)(1.205 kg air/m3) = 0.27715 kg O2/ m</p><p>3 of air </p><p>1 m3 of air contains 0.27715 kg of O2 under the standard conditions (T = 20C, p = 1 atm). </p><p> Mass of O2 dissolved (transferred) in water </p><p>Oxygen transfer efficiency (OTE) = ----------------------------------------------------- </p><p> Mass of gaseous O2 applied </p><p>O2 transferred (dissolved) = 1.7 kg of O2 / kg BOD applied. </p><p> 62.428 m3 of air 0.27715 kg O2 17.30 kg of O2 </p><p>O2 applied = ------------------------- ---------------------- = ------------------------- </p><p> kg of BOD applied m3 of air kg of BOD applied </p><p> 1.7 kg of O2 / kg BOD applied </p><p>OTE (%) = ------------------------------------------------ (100) = 9.8 % </p><p> 17.30 lb of O2/ kg of BOD applied </p><p>Unit conversion: 1 kg = 2.2046 lb, 1 ft3 = 0.028317 m</p><p>3 </p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 13 </p><p>Example Oxygen Transfer Efficiency (%) US customary units </p><p>1000 ft3 of air is required (need to be applied) per lb of BOD applied. The aerator is </p><p>capable of transferring 1.7 lb of O2 (dissolved) per lb BOD applied. What is the oxygen transfer </p><p>efficiency (OTE)? Note: 1 ft3 of air under the standard conditions (T = 20C, p = 1 atm) </p><p>contains 0.0174 lb of O2. </p><p>(Solution) </p><p> Assumptions: Air contains 23% O2 (w/w) and air density is 0.0174 lb/ ft3 </p><p> Mass of O2 dissolved (transferred) in water </p><p>Oxygen transfer efficiency (OTE) = ----------------------------------------------------- </p><p> Mass of gaseous O2 applied </p><p>O2 transferred (dissolved) = 1.7 lb of O2 / lb BOD applied. </p><p> 1000 ft3 of air 0.0174 lb of O2 17.4 lb of O2 </p><p>O2 applied = ------------------------- ----------------------- = ------------------------- </p><p> lb of BOD applied ft3 of air lb of BOD applied </p><p> 1.7 lb of O2 / lb BOD applied </p><p>OTE (%) = ------------------------------------------------ (100) = 9.8 % </p><p> 17.4 lb of O2 / lb of BOD applied </p><p>Oxygen Transfer Rate </p><p> Cell membrane</p><p>Liquid film</p><p>CO2</p><p>DO</p><p>(Rate of O2 transfer) Rate of O2 utilization</p><p>Microbial cell</p><p>Bubble</p><p> dC/dt = KLa (Cs - C) dC/dt = r r = oxygen utilization rate </p><p>a. Under steady-state conditions of oxygen transfer in an activated-sludge system, the rate of oxygen transfer to dissolved oxygen is equal to the rate of oxygen utilization: </p><p> Change in DO in wastewater = O2 transfer rate - O2 utilization rate </p><p> L sdC</p><p>K a C C rdt</p><p> (1) </p><p> where r = oxygen utilization rate by microorganisms in activated sludge, mg/L </p></li><li><p>5-Oxygen Transfer_F12.doc </p><p> 14 </p><p>At steady state, dc/dt =0 </p><p>L</p><p>s</p><p>rK a</p><p>C C</p><p> (2) </p><p>For clean water (at standard test conditions, 20C) </p><p> L sdC</p><p>K a C C rdt</p><p> (1) </p><p>At steady state, dC/dt = 0 and let r = ro </p><p> o L sr K a C C (2) </p><p>The DO deficit is maximum when C = 0, thus </p><p> or oo L s Ls</p><p>rr K a C K a</p><p>C this KLa is the smallest KLa value. </p><p>When the test is conducted under standard conditions at T = 20C, </p><p>,20</p><p>,20</p><p>o...</p></li></ul>

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