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Activated Sludge Plants: Dimensioning
Eduardo Cleto Pires
2
History
Seminal paper:
• Ardern, E. and Lockett, W.T. (1914)
Experiments on the oxidation of sewage
without the aid of filters, Journal Society of
Chemical Industries, v.33, p.523
• First experiments were performed in batch reactors
keeping the biological sludge from batch to batch
3
History
Continuous flow reactors soon substituted batch reactors using a settling tank for sludge separation and return.
The process was “discovered” instead of “invented” as Arden & Lockett were investigating sewage treatment using published observations on the effect of oxygen on sewage and almost accidentally observed the effects of sludge recycling.
4
Why the denomination of “activated sludge”?
Ardern & Lockett observed that the sludge improved (faster reaction) after a few batches as if “activated” by the recycling and aeration!
• Thus activated sludge
• Actually, a mixture of microorganisms formed macroscopic
flocs and this diverse and concentrated population of
microorganisms is responsible for the higher efficiency.
5
Definition
Activated Sludge
• is the name given to the mixture of
microorganisms organized as biological flocs
formed by recirculation of the biomass from a
separation device back into the aeration tank.
6
Pros and Cons
Pros• high treatment efficiency• operation control and flexibility• small foot print
Cons• needs precise control and operation• needs frequent laboratory measurement of
many variables• higher costs
7
Activated Sludge Fundamentals
8
Activated sludge process kinetics
9
Substrate utilization rate – U
The substrate utilization rate is the ratio of substrate removal velocity and the mass of microorganisms.
av
dSdtU
X
U - substrate utilization rate [T-1]
S - substrate concentration [ML-3]
Xav - volatile suspended solids at the aeration
tank – MLVSS* [ML-3]
t - time [T]
* - mixed liquor volatile suspended solids
10
Substrate utilization rate – U
Assuming constant U and solving for an interval
equal to the hydraulic detention time (qH):
00 ee
av H av
Q S SS SU or U
X X V
S0 - influent substrate concentrationSe - effluent substrate concentration Q - sewage flowrateV - aeration tank volumeqH - hydraulic detention time (V/Q)
av
dSdtU
X
11
Michaelis-Menten kinetic equation
max
av S
dS U Sdt UX K S
Umax - maximum substrate utilization rateS - substrate concentration at the aeration tankKS - half-velocity constant (substrate concentration when U is equal to Umax
Limiting Substrate Concentration
Su
bst
rate
Util
iza
tion
Ra
te
Ks
Umax
maxU
2
12
Properties of the Michaelis-Menten equation
High substrate concentration: equation approaches zero order kinetics.
Low substrate concentration: equation approaches first order kinetics.
S maxS S K U U
max max maxS S
S S S
U S U S UK S K if k then U kS
K S K K
13
Removal rate of BOD or COD
Apply the Michaelis-Menten or first order kinetics
equation being the COD or the BOD represented by S.
Assuming first order kinetics (S ≡ BOD or COD):
3 1;av
dS LkX S K S k K
dt MT T
For sanitary sewage: K ranges from 0,017 to 0,030 d-1.
14
Mass balance around the aeration tank
Mass balance assuming first order kinetics:
0 e av e
Influent mass of Effluent mass of Removed mass ofsubstrate per substrate per substrate per
unit time unit time unit time
QS QS kX S V
V Se Xav
Aeration TankQ S0 Q Se
15
Mass balance around the aeration tank
0 0e e
e av e
S S S S
kS X K S
This equation is used to estimate the needed hydraulic detention time to reach the desired effluent concentration
V=
QSimplifying and remembering that
where K = kXav
16
Mass balance considering recirculation
0
0
r e r e av e
removalinfluent effluent
e av e
QS Q S Q Q S kX S V
simplifying
QS QS kX S V
• This is the same expression obtained without
sludge return (recirculation).• Thus the recirculation of the sludge has no effect on
the mass balance (it is an internal process!).
17
Mass balance considering recirculation
If recirculation has no effect on mass
balance why is it used?
• To keep the biomass in the aeration tank!
• To uncouple the biomass retention time
from the hydraulic retention time.
18
Mass balance considering recirculation
Then, how could we avoid or reduce
sludge recirculation?
• Using attached biomass (biomass carrier).
• suspended growth x attached growth
19
Food to Microorganisms ratio (F/M)
This ratio is also known as food-mass
ratio.
Mass refers to the amount of
microorganisms measured as the
concentration as volatile suspended
solids
20
Food / Microorganisms ratio (F/M)
It is the ratio of the available food [ F ]
(substrate) in the aeration tank and the
quantity of microorganisms that will feed on
this substrate (MLVSS) [ M ].
0
av
QSF
M X V
21
Food / Microorganisms ratio (F/M)
Usual values (NBR 12209:2011)• high rate
• F/M from 0,70 to 1,10 kgBOD/kgMLVSS
• conventional • F/M from 0,20 to 0,70 kgBOD/kgMLVSS
• extended aeration• F/M 0,15 kgBOD/kgMLVSS
22
Solids retention time – θc
Average time that a particle remains in aeration.
Also known as:• sludge age• mean cell residence time
Numerically it is equal to the mass of
suspended solids at the aeration tank and
the mass of wasted solids (excess sludge).
23
Solids retention time – θc
effluent biomass
(sludge) - carriedby the effluent wastewater
biomass (sludge ) inthe aeration ta
wasted biomass(
avc
u uv e ev
slud )
nk
ge
X V
Q X Q X
Qe - effluent flowrate (withdraw at the secondary clarifier)Qu - sludge flowrate (withdraw at the secondary clarifier)Xav - volatile suspended concentration solids at the aeration tankXuv - volatile suspended solids concentration in the wasted sludgeXev - volatile suspended solids concentration in the treated wastewater
24
Solids retention time – θc
Expected values
• Conventional processes
• 4 to 15 days
• θc < 4 d: floc is not dense enough to settle
• θc > 15 d: floc is too small and does not settle
25
Solids retention time – θc
Controls nitrification:
• high qc favors nitrification
Used as a control parameter:
• estimation of the sludge volume to be wasted
26
Synthesis and auto-oxidation
Synthesis• a fraction of the organic matter is synthesized
into new cells• the mass of microorganisms increases
Endogenous respiration or auto-oxidation• a fraction of the cells decays
• the mass of viable microorganisms decreases
27
Synthesis and auto-oxidation
Degradation of the organic matter
Organic Matter
CO2, H2O, N2, P End products
CO2, H2O, NH3, P Non-biodegradable
end products
New cells
Energy Synthesis
Endogenous respiration
28
Synthesis and auto-oxidation
Synthesis fraction• biomass yield Y
a S
substrate utilizationactive microorganismsrategrowth rate
dX dSY
dt dt
29
Synthesis and auto-oxidation
Decay fraction (endogenous respiration)• endogenous decay coefficient- kd
a endogenousd av
concentration ofactive microorganismsactive microorganisms
decay rate
dXk X
dt
30
Synthesis and auto-oxidation
Balance of cell mass at the aeration tank:
aa endogenousav S
Synthesis Decay
avd av
dXdXdX
dt dt dt
dX dSY k X
dt dt
Y - 0,40 to 0,50 mgSSV/mgBODremoved This is the produced sludge!
kd - 0,05 to 0,10 (mgSSV/d)/mgSSV
31
Relating θc, Y, kd and U
Mass balance around the WWTP:
0av
Influent volatilesolidsVolatile solids
mass at the WWTP
ee ev u uv d av
Volatile solids mass in thetreated effluent and in the resulting mass balance be
wasted sludge
dXV QX
dt
dSQ X Q X Y k X V
dt
tween the volatile
synthesized and decaied solids
32
Relating θc, Y, kd and U
e e ev u uvd
av av
dS Q X Q XYk
X dt X V
avc
u uv e ev
X V
Q X Q X
Assuming steady state and neglecting the influent active volatile solids:
but
then1 e
dc av
dSYk
X dt
Full equation:
0av
e ev u uv
ed av
dXV QX Q X Q X
dtdS
Y k X Vdt
33
Relating θc, Y, kd and U
0
0 0
1 1
1 1
ed
c c av
c e c eav
d c av d c
Q S SYU k Y
X V
Y S S YQ S SX V
t k X k
one develops other relations:Rearranging 1 ed
c av
dSYk
X dt
34
Sludge production
Net sludge yield - ∆X:
0 e d av
decayedproduced
X Y S S Q k X V
The sludge yield is also expressed as:
0obs eX Y S S Q
35
Sludge production
The net yielded sludge is the sludge that
needs to be wasted.
• this sludge is digested in anaerobic reactors (large
plants) prior to discharge or, in small plants, mixed
with lime for chemical stabilization and discarded.
36
Relation between Y and Yobs
Equating both equations for ∆X:
0
1
avobs d
e
obsd c
X VY Y k
S S Q
YY
k
or
37
Sludge recirculation
Keeps a high and constant sludge
concentration at the aeration tank.
Inoculation of the aeration tank speeding
the stabilization of the organic matter.
38
Sludge recirculation
Recommended recirculation rates:
• MLVSS < 3500 mg/L 25%
• 3500 < MLVSS < 4500 mg/L 50%
• MLVSS > 4500 mg/L 100%
39
Oxygen requirements
Oxygen is consumed• to provide energy for synthesis of new cells• endogenous respiration
Injection of oxygen (air) provides• mixing in the aeration tank, keeping the flocs
suspended• stripping (removal) of volatile compounds,
formed as metabolites or existing in the polluted water
40
Oxygen requirements
2 avO S XM a M b M
2
av
O 2 2
S 0 e
X av
2
M = required mass of O [kgO /d]
M = removed BOD [(S -S )Q] [kgBOD/d]
M = mass of volatile solids in the aeration tank [kg]= X Volume
a = fraction of the removed matter used for synthesis [kgO /kgBO removed
2
D ]
b = endogenous respiration oxygen consumption coefficient [kgO /kgMLVSS ]
2 0O e avM a S S Q b X V
Required mass of oxygen:
Thus
41
Oxygen requirements – design criteria
Approximate values for a' and b’
• a’ ≈ 0,52
• b’ ≈ 0,12 d-1
Minimum oxygen concentration at the
aeration tank
• 1,5 a 2 mgO2/L
42
Choosing an aeration system
Consider
• shape of the aeration tank
• mixing requirements
• cost
• operation
43
Aeration systems
Conventional• air is injected into the liquid phase in the
aeration tank and oxygen from the air transfers to the water• diffused air• mechanical mixing• mixed systems (diffused air + mechanical mixing)• conventional systems are useful for biomass
concentration up to 4.500 mg/l
44
Aeration systems
Pure oxygen• oxygen is injected directly into the liquid phase
• oxygen concentrator
• liquid oxygen tank
• pure oxygen requires specific equipment for injection
• high biomass concentration, up to 8.000 mg/l
• low hydraulic retention time: 3 hr!
Surface aeration
45
46
Surface aerationMaintenance: many units to keep running
Bubbling aeration
47
48
Fine bubble diffusers
49
Distribution of diffusers in an aeration tank
Distribution of diffusers in an aeration tank
50
Micro holes (micro pore) pipe diffuser
51
52
Dimensioning of the aeration system
Diffused air
• air pressure needs to surpass
•water column (static pressure)
•pipeline pressure drop
•pressure drop at the diffusers
53
Dimensioning of the aeration system
Diffused air • blowers (compressors)
• centrifugal blowers• flowrate above 30 m3/min and pressure
from 5 to 7 MWC• positive displacement blowers
• flowrate below 30 m3/min and pressure above 6 MWC
54
Power requirement
P - blower power [kW]Mair - required air mass [kg/s]Qair - air flow rate [m3/s]R - gas constant for air [8,314 kJ/kmol.K]8,41 - air constant [kg/kmol] – adjustment of unitsT0 - inlet absolute temperature of the air [K]pe - absolute pressure at the blower inlet [atm]ps - absolute pressure at the blower outlet [atm]h - blower efficiency [0,70 to 0,80]
0,283
0 18,41
air s
e
M RT pP
p
1,2
air air air
3air
M Q
kg/m
55
Recommendations
first estimate of pressure drop
• 1.2 to 1.5 water column
blowers should be capable do deliver 1.5
times the required air flow
always have a spare blower
56
Dimensioning of surface aerators
Surface aerators
• Required power is estimated using manufacturer’s data
• oxygen transfer rate as a function of power: standard
conditions (sea level at 20°C and tap water)
• the standard condition values are corrected to field
conditions: altitude, temperature, sewage characteristics
• Besides oxygen transfer it is necessary to consider
mixing (suspension of the solid phase)
57
Correcting to field conditions
200 1.029.02
TSW LC CN N
N - oxygen mass transferred under field conditionsN0 - oxygen mass transferred under standard conditionsCSW - oxygen saturation concentration in the aeration tank at temperature T
- assumed as 95% of the tap water saturation concentrationCL - oxygen concentration in the saturation tank
- usually the minimum value is 2.0 mgO2/la - correction coefficient to take into account industrial wastewater mixed
with the sanitary sewage- usual values are in the range 0.8 to 0.9
NOTE: some constant values may differ depending on the author or country standards.
58
Secondary clarifiers
The quality of the secondary clarifier is fundamental to assure the operation of activated sludge plants.
Dimensioning depends on sludge settleability and standard design procedures and dimensions.
Dimensions used should be the ones that provide the highest safety factors.
59
Nitrogen removal with denitrification
Clarifier
Recirculation
Anoxic tankBOD removalDenitrification
Aeration tankBOD removalNitrification
Influent
Effluent
Wastedsludge
60
Nitrogen removal with denitrification
Oxidation Ditch
61
References
Tchobanoglous, G.; Burton, F.L. and Stensel, H.D. Wastewater Engineering Treatement and Reuse (Metcalf & Eddy). McGraw Hill, 4th. ed., 2003 (Chap. 7 and 8)