This preview shows page 1. Sign up to view the full content.
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. ...
View
Full
Document
This note was uploaded on 09/13/2011 for the course CIVL 242 taught by Professor Gh during the Fall '09 term at HKUST.
 Fall '09
 GH

Click to edit the document details