T10 - Tutorial 10 Tutorial 10 So ([email protected]) Kin...

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Unformatted text preview: Tutorial 10 Tutorial 10 So ([email protected]) Kin ([email protected]) Activated Sludge Process Activated Sludge Process Activated Sludge Process Activated Sludge Process • F/M ratio (mg/mg/day) – How many mg of food load to 1 mg of biomass per day QS F /M = XV o • Sludge age (day) – Mean cell­residence time – Sludge age, solid retention time, sludge retention time (SRT) – Retention time of sludge or “generation time” of sludge XV θc = Qw X r F/M ratio F/M ratio • F/M ratio can only regulated by sludge concentration (MLSS) X • It is related to another important design parameter: Sludge­loading rate – 1 kg sludge can remove how many Kg of BOD5 Q( S − S ) L= XV o e s – Se: effluent BOD5 concentration • Normal range: 0.2­0.6 Kg BOD5/Kg SS/day • Usually Ls = 0.9 F/M Another Loading rate: Lv Another Loading rate: L • If MLSS is kept constant – To determine the volume of aeration tank Q( S − S ) L= V o e V – Normal range: 0.3­0.8 Kg BOD5/m3/day Why we need to take sludge out? Why we need to take sludge out? Catabolism (oxidation) CO2+H2O+etc. ? Kg COD 1 Kg BOD O2 Catabolism Energy ?Kg BOD New cells Anabolism (cell synthesis) BOD + O2 + bacteria CO2 +H2O + new cells + other byproducts Why need to take sludge out? Why need to take sludge out? Growth ­ Decay Net growth Wasted • Growth rate > Decay rate • To maintain MLSS at a normal level, we need to waste some sludge • Such wasted sludge is called excess sludge Sludge Growth Kinetics Sludge Growth Kinetics Growth Removed amount of BOD (mg/day) dX V = Q0 ( S 0 − S e )Y dt growth BOD removal rate (mg/day) • Unit: mg MLSS/day • Yield coeff (Y) mg MLSS/ mg BOD removed – Can be MLSS/BOD, MLSS/COD, MLVSS/BOD, MLVSS/ COD, depends on unit used. Sludge Growth Kinetics Sludge Growth Kinetics Decay • Kd XV (kg MLSS/day) – Kd: endogenous decay constant (1/d) (1/d) Sludge Growth Kinetics Sludge Growth Kinetics Q0 ( S 0 − S e )Y − K d XV = Qw X r Q0 (S0 − Se ) Qw X r Y − Kd = XV XV 1 μ − Kd = θc Sludge Growth Kinetics Sludge Growth Kinetics Q0 (S0 − Se ) μ= Y XV Specific growth rate (1/d) • μ ≥ Kd • μ increase, θc decrease decrease – Sludge growth faster, therefore we need to waste more Sludge sludge sludge age decrease • μ decrease, θc increase increase – Sludge growth slower, therefore we need to waste less Sludge sludge sludge age increase Calculation Example of F/M and θ c A 30,000­m3 aeration tank receives the following sewage flow for treatment. Its BOD removal efficiency is 95%. Under standard operation conditions, the Y, Kd and Xr are found to be 0.65 Kg MLSS/Kg BOD5, 0.06 1/day, and 10,000 mg/L, respectively. In order to keep the MLSS at 2000 mg/L, what are the optimal values of both sludge age and F/M that should be adopted in the operation? And each day how many Kg of excess sludge is needed to be wasted? And what is the daily volume of excess sludge needed to be withdrawn? Solution Given Qo =100,000 c.m.d., and So=250 mg/L Q0S0 100,000 × 250 F/M = = = 0.42 XV 2,000 × 30,000 Q0 (S0 − Se ) 100,000 × 250 × 0.95 μ= Y= 0.65 = 0.26 / day XV 2,000 × 30,000 1 = µ − K = 0.26 − 0.06 = 0.2 / day θ d c Thus, θ c =5 days. Both F/M and sludge age are fine for the operation. XV 2,000 × 30,000 Qw X r = = = 12,000Kg / d θc 5 × 1,000 Mass in dry wt (Replace 1/5 of dry mass) 12,000( Kg / d ) Qw = = 1,200m 3 / d 10( Kg / m 3 ) Solids content: 1% in excess sludge Actual volume of excess sludge (1.2% of Qo) Sludge Volumn Index (SVI) Sludge Volumn Index (SVI) • SVI definition: sludge volume (V, in mL) of 1.0 g sludge (in dry wt) after an adequate settling time of 30 min. mL SV SVI = X g/L Sludge Volumn Index (SVI) Sludge Volumn Index (SVI) 1 1,000(mg / g) 106 (g / mL) = = (mg / L) = X r 1(mL) SVI SVI SVI × 1,000(L) • SVI is an important parameter in monitoring the sttleability of sludge in the secondary clarifier • Should keep between 80 to 110 • Too low: sludge too old and being mineralized • Too high: sludge bulking, causes poor settling of sludge. ...
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This note was uploaded on 09/13/2011 for the course CIVL 242 taught by Professor Gh during the Fall '09 term at HKUST.

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