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Transcript of 304-Notes 8 BioTreat 2013
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ENVE 304
UNIT OPERATIONS AND
PROCESSES OF WASTEWATERTREATMENT
Biological Oxidation / Growth Kinetics / Reactor types /
Mass Balances
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• Types and characteristics of
wastewaters
• Physical unit operations
• Biological oxidation
• Biological wastewater treatment
processes
• Handling & disposal of 2
TOPICS
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1.Types and characteristics of wastewaters
–Wastewater management, Environmental laws and
regulations, Types of wastewaters, Physical, chemicaland biological characteristics of WW, Main WWTP units
1. Physical unit operations
–Flow measurement, Screening , Coarse solid reduction,Grit removal, Equalization, Sedimentation, Flotation,Oxygen transfer, Aeration
1. Biological Oxidation 3
TOPICS
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Biological Oxidation
• Classification of microorganisms
• Oxidation equations – Aerobic, anaerobic, nitrification, denitrification – Required oxygen amounts
• Growth kinetics• Reaction orders• Reactor types (completely-mixed, plug-flow, etc)• Main equations and derivations in flow through
systems, in systems with recycle, operatingparameters, SVI definition
4
Learning Objectives:
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Bacterial Growth
Biomass producedcanbe measured:
VSS, Particulate
COD, Protein
content, DNA, ATP, Turbidity
measurements,
Bacterial cell
count
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0
0.03
0.06
0.09
0.12
µ ,
1 / h r
0.15
0.18
0 300 600 900 1200
So
, mg COD/L
1500
µm /2
µm
Ks
S*
Monod
Haldane
µ *
6
Bacterial Growth
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Monod equation (substrate-limited growth, no inhibition)
Haldane equation (inhibition)
7
)(S K
S
s
m
+=
µ µ
)
/
(2
i s
m
K S S K
S
++
=µ
µ K i = inhibition constant,
mg/L
µ = specific growth rate, mg new cells / mg cells.d (1/T)µm = maximum specific growth rate, mg new cells / mg cells.d
(1/T)S = concentration of growth-limiting substrate, mg/LK s = half-velocity constant, substrate concentration at one-half
the maximum specific utilization rate, mg/L
Bacterial Growth
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First order
Zero order
8
Haldane eq.
}Beyond this point, the processcannot grow and will fail.
Monod eq.
s
m
K
S µ µ =Ks >> S, then,
m µ µ =Ks << S, then,
)/21
(*
i s
m
K K
S
+= µ µ
i s K K S /* =
Bacterial Growth
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Rate of utilization of soluble substrates
kY m = µ Y
k m µ =
)( S K Y
XS r
s
m
su +−=
µ
S K
kXS r
s
su+
−=
rsu = substrate utilization rate, mg/L.d
k = maximum specific substrate utilization rate, mg substrate/mgcells.d
µm = maximum specific growth rate, mg new cells/mg cells.dY = true s nthesis ield coefficient m VSS m bsCOD
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Other rate expression for the utilization of solublesubstrates
k r su −=
kS r su −=
kXS r su −=
o
suS
S kX r −=
Rate of utilization of soluble substrates
a e o u a on o so u e
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a e o u za on o so u esubstrate
11
kY m = µ Y
k m µ =
)( S K Y
XS r
s
m
su +−=
µ
S K
kXS r
s
su+
−=
rsu = substrate utilization rate, mg/L.d
k = maximum specific substrate utilization rate, mg substrate/mgcells.d
µm = maximum specific growth rate, mg new cells/mg cells.dY = true s nthesis ield coefficient m VSS m bsCOD
Rate of biomass growth with soluble
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Rate of biomass growth with solublesubstrates
12
X k Yr r d su g −−=
X k S K
kXS Y r d
s
g −+
=)(
rg = net biomass production rate, mg VSS/L.d
Y = true (synthesis) yield coefficient, mg VSS /mg bsCODX = biomass concentration, mg/Lkd = endogenous decay coefficient, mg VSS/mg VSS.d
NET GROWTH RATE
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Typical kinetic coefficients for the activated–
sludge process for the removal of organicmatter from domestic wastewater (200C)
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Total VSS And Active Biomass
X k f r d d Xd )(=
V QX X k f X k Yr r iOd d d suVSS X T /)( ,, ++−−=
VSS X d suact X T r X k Yr F ,, /)( −−=
rxd = rate of production of
cell debris, mg VSS/L.d
f d = fraction of biomass thatremains as cell debris(0.1-0.15 mg VSS/mgVSS)
rxT, VSS = total VSS production rate,
mg/L.dQ = influent flowrate, L/d
XO,i = influent nbVSS concentration,
mg/LV= volume of the reactor, L
Active fractionof biomass
Rate of celldebrisproduction
nbVSS intheinfluent
Netbiomass
growthrate
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su g bio
r r Y /−=
Net biomass yield: to estimate amount of active microorganisms
Observed yield: to estimate amount of sludge produced
suVSS X obs r r Y T
/,−=
(Considers decay of m/o)
Considers decay of m/o, includes VSScontent due to cell debris and influent
nbVSS
Biomass yield =g biomass produced
g substrateconsumed
Yield Coefficient
Synthesis (True) yield: to estimate amount of biomass produced
during cell synthesis relative to the amountof substrate degraded.
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Rate of oxygen uptake
g suO r r r 42.1−−=
rO = oxygen uptake rate, mg O2/L.d
rsu = substrate utilization rate, mg bsCOD/L.d
1.42 = COD of the cell tissue (C5H7NO2), mg bsCOD/mg VSSr = net rate of biomass growth, mg VSS/L.d
READING ASSIGNMENT: CHP 7, ESTIMATION OF BIOMASS YIELDFROM BIOENERGETICS
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Growth kinetics for nitrifiers
17
dn
n
nmn k
N K
N −
+= )(
µ µ Assuming excess
DO is available.
dn
On
nmn k
DO K
DO
N K
N −
++= ))((
µ µ
To account forthe effects of DO
µn = Specific growth rate of nitrifying bacteria, mg new cells / mg cells.d
(1/T)µnm = Maximum specific growth rate of nitrifying bacteria, mg new cells /
mg cells.d (1/T)kdn = endogenous decay coefficient for nitrifying organisms, mg VSS/mg
VSS.dK n = half-velocity constant, mg/L
N = Nitrogen concentration, mg/L
The rate of substrate utilization for
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The rate of substrate utilization indicating a slowerutilization rate in the anoxic zone
S K
kXS r
s su+
−=η
To indicate the effect of oxygen (which inhibits nitratereduction by repressing nitrate reduction enzyme):
η ))()(('
'
3,
3
3 DO K
K NO K
NOS K
kXS r O
O
NO s s
su+++
−=
The rate of substrate utilization fordenitrifiers
ƞ = Fraction of denitrifying bacteria in the biomass, mg VSS / mg VSSK S,NO3 = half-velocity coefficient for nitrate limited reaction, mg/L
K’O = DO inhibition coefficient for nitrate limited reaction, mg/L
E l
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Example
An industrial wastewater with a biodegradable solubleCOD content of 300 mg/L and nonbiodegradable VSS of 50mg/L is treated in an activated sludge process. Influentflowrate is 1000 m3/d. The biomass concentration in the
aeration basin of 105 m3
is 2000 mg/L. The biodegradablesoluble COD removal efficiency is 95%.
k = 5 d-1, K s = 40 mg/L,
Y = 0.40 g VSS/g bsCOD,k
d
= 0.10 g VSS/g VSS.d
Cell debris per dry weight = 0.10
Determine; Net biomass yieldObserved yield
VSS production rate & activebiomass fraction
R t T
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Reactor Types
Batch reactors
Complete-mix reactorsPlug-flow reactors
Plug-flow with axial dispersion
Ideal flow reactors
Non-ideal flow
reactors
Materials-balance equation in a
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Materials-balance equation in asystem boundary
Accumulation ratewithin thesystemboundary
=
Inflow rate
to thesystemboundary
-
Generation
rate withinthe systemboundary
+
Outflow
rate fromthesystemboundary
V r QC QCoV dt
dC c+−=
R t i d d
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Rate expressions and orders
k r c =
kC r c =
2kC r c =
C K
kC r c +=
Zero-order reaction
Second- order reaction
Saturation (mixed-order) reaction
Differential Method
First-order reaction
n
c kC r =
Bi l i l T t t P
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Biological Treatment Processes
23
SUSPENDED GROWTH
PROCESSES
ATTACHED GROWTHPROCESSES
Modeling Biological Treatment
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SUSPENDED GROWTH PROCESSES
ATTACHED GROWTH (BIOFILM) PROCESSES
The microorganisms responsible for treatment aremaintained in liquid suspension by appropriate mixing.
• Activated-sludge process(es)• Aerated lagoons
• Aerobic digestion
• Anaerobic contact processes• Anaerobic digestion
The microorganisms responsible for treatment areattached to an inert packing material (rock, gravel,sand, wide range of plastics, synthetic materials etc.).
• Trickling filters• Rotating biological contactors
• Packed-bed reactors, fluidized-bed reactors, expanded-bed reactors
Modeling Biological TreatmentProcesses
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COMPLETE-MIX REACTORS
MODELING
SUSPENDED GROWTH PROCESSES
Complete-mix reactors
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Complete-mix reactors
V r QC QCoV dt
dC c+−=
Systemboundary
Accumulation rate = inflow rate - outflow rate +generation rate
Assumptions : well-mixed, constant volume, C and T are uniform
τ k
C
QV k
C C
+=
+=
1)/(1
00
Under steady-state conditionsand first-order reaction (r
c=
-kC)
Q, C0 Q, C
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MODELING
SUSPENDED GROWTH PROCESSES
COMPLETE-MIX
ACTIVATED-SLUDGE PROCESS
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Complete-mix reactors
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Biomass Mass
Balance:
[ ] V r X Q X QQQX V dt
dX g Rwewo ++−−= )(
rg = net rate of biomass production, g VSS/ m3.
d
Accumulation rate = inflow rate - outflow rate + net growthrate
Assuming; X 0 = 0 (can be neglected) and steady-state conditions
prevail;
V r X Q X QQ g Rwew =+− )(
d u s Rwew k
X
r Y
VX
X Q X QQ−−=
+− )(
1/SRT
µ =SRT
1
SRT= Solids retention
Complete mix reactors
Complete-mix reactors
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Rwew X Q X QQ
VX
+− )(SRT =
If no clarifier following the aeration basin,thus, no recyle; R=0, Qw=0,
τ ==QX
VX
QX
VX
e
SRT =
τ SRT == theoretical hydraulic detention time, V/Q, dτ
Biomass MassBalance:
SRT= Solids retention time, d
Complete mix reactors
Complete-mix reactors
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-rsu/X = U = Specific substrate utilization rat
g BOD or COD/g VSS .d
d
u s k X
r Y
SRT −−=
1
S0= influent soluble substrate concentration,
g BOD or bsCOD/m3
S= effluent soluble substrate concentration,g BOD or bsCOD/m3
X
S S
VX
S S QU
X
r su
τ
−=
−== 00 )(
d k YU SRT
−=1
d
s
k S K
kS Y
SRT −
+=
1
[ ]
1)(
1
−−
+=
d
d s
k Yk SRT
SRT k K S
Biomass Mass
Balance:
Complete mix reactors
Complete-mix reactors
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SludgeQw, XR,S
Effluent
(Q-Qw), Xe,S
Influent
Q, X0,
S0
Return activated sludge
QR, XR, S
S, X,
V
ClarifierAeration tank
Substrate Mass Balance:
V r QS QS V
dt
dS suo +−=
Assuming; steady-state conditions
))((0S K
kXS
Q
V S S
s +=−
+
−=
SRT k
S S Y SRT X
d 1
)( 0
τ [ ]
1)(
1
−−
+=
d
d s
k Yk SRT
SRT k K S
Complete mix reactors
Complete-mix reactors
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Mixed liquor VSS concentration:
Inert material mass balance =
X0,i = nbVSS concentration in influent, g/m3
Xi = nbVSS concentration in aeration tank, g/m3
rx,i = rate of nbVSS production from cell debris,g/m3.d
rx,i
= X k f d d )(
SRT X k f SRT X X d d ii )()/(,0 += τ
X T = X + Xi
X T= total MLVSS concentration in aeration tank, g
VSS/m3
X= biomass concentration, g VSS/m3
Xi = inert VSS; nonbiodegradable VSS (nbVSS), g
VSS/m3
V r SRT V X QX V dt
dX
i xii
i
,,0 /+−=
At steady-stateconditions
τ τ
SRT X SRT X k f
SRT k
S S Y SRT X i
d d
d
T 00 )(
1
)(++
+
−=
Complete mix reactors
Complete-mix reactors
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Mixed liquor VSS concentration:
X T = X + Xi
X T= total MLVSS concentration in aeration tank, g
VSS/m3
X= biomass concentration, g VSS/m3
Xi = inert VSS; nonbiodegradable VSS (nbVSS), g
VSS/m3
If both BOD removal and nitrification;
τ τ τ
SRT X
SRT k
NOY SRT SRT X k f
SRT k
S S Y SRT X i
dn
xn
d d d
T
00
1
)()(
1
)(+
+++
+
−=
Ox = concentration of NH4-N in the influent that is nitrified
dn = endogenous decay coefficient for nitrifying organisms, g VSS/gVSS.d
p
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Complete-mix reactors
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TSS produced daily :
90.080.0 −=TSS
VSS (typical biomass ratio; 0.85)
)(85.085.085.000, VSS TSS Q D
C B A P TSS X T −++++=
= net waste activated sludge produced daily, measured in terms of TSS, gTSS
0= influent wastewater TSS concentration, g/m3
VSS0 = influent wastewater VSS concentration, g/m3
Mass of MLVSS (XVSS) V = (Px,VSS) SRT
Mass of MLSS (X TSS) V = (Px,TSS) SRT
TSS X T P ,
p
Complete-mix reactors
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Observed Yield:
)(1)(
1 0
,0
S S
X
SRT k
SRT Y k f
SRT k
Y Y
i
d
d d
d
obs−
++
++
=
nbVSS ininfluent
Heterotrophic biomass
Celldebris
)(11)(1 0
,0
S S
X
SRT k
Y
SRT k
SRT Y
k f SRT k
Y
Y
i
dn
n
d d d
d obs
−++++++=
Nitrifying
bacteria
biomass
p
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Oxidized nitrogen:
In separate nitrification or
In combined BOD removal and nitrification
Q(NOx) = Q(TKN0) – QNe – 0.12 Px,bio
Ne = effluent NH4-N
0.12 = N content in C5H7NO2
Oxygen Requirements:
Oxygen used = bCOD removed – COD of waste sludge
RO = Q(S0-S) - 1.42 Px,bio + 4.33Q (NOx)
RO = oxygen required, kg/d
Px,bio = biomass as VSS wasted per day, kg/d
Design & Operating Parameters
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SRT = the most critical parameter of activated-sludgedesign.
It represents; the average period of time during whichthe sludge has remained in the system
r BOD removal= 3-5 d (dependent on mixed-liquor temperature)
at 18-250C= SRT ~ 3 d (to limit nitrification), even 1 d
d
s
k S K
kS Y
SRT −
+=
1SRT affects treatmentprocess performance,aeration tank volume,
sludge production, oxygenrequirements.
Solids retention time (SRT):
Design & Operating Parameters
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Process performance and stability
Washout; S = S0 SRT = SRTmin
d
sk S K
kS
Y SRT −+= 0
0
min
1
If S0 >> K s;
Safety factor (SF) = SRTdes/ SRTmin
d md k k Yk SRT
−≈−≈ µ min
1
(SF= 2-20)
Design & Operating Parameters
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Food to microorganisms (F/M) ratio:
X
S
VX
QS
biomassmicrobial total
rate substrateapplied total
M
F
τ 00 ===
Specific substrate utilization rate:
100
)/( E M F U = E = BOD or bsCOD process removal efficiency, (S0-S)*100/S0
X
S S U
τ
−= 0
d k YU SRT
−=1
d k E M F
Y SRT
−=100
)/(1
(g BOD or bsCOD/ gVSS.d)
F/M = 1.0 g BOD or bsCOD/ g VSS.d for high-rate systemsF/M = 0.04 g BOD or bsCOD / g VSS.d for extended-aeration
Design & Operating Parameters
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Volumetric organic loading rate:
)/10( 3
0
kg g V
S Q Lorg =
Lorg = volumetric organic loading rate, kg BOD/m3.d
Q= influent wastewater flowrate, m3/dV= aeration tank volume, m3
S0 = influent BOD concentration, g/m3
Usually, Lorg is 0.3-3.0 kg BOD or COD /m3.d (applied to the aeration ta
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Mixed-liquor settling characteristics Should be considered while designing secondary
clarifier;Sludge volume index (SVI) : is the volume of 1 g of sludge after 30 min of settling. (suspended solids interms of MLSS)
g
mL
Lmg solids suspended
g mg LmL sludgeof volume settled
SVI == /,
)/10)(/,( 3
SVI <100 good settling sludge (desired)
SVI > 150 filamentous growth, poor settling
1)/(100
100
−=
SVI P R
w
With SVI; R required tomaintain a fixed MLSSin the aeration tank canbe determined; Pw = MLSS as expressed as percentage
(eq. 0.3% ~ 3000 mg/L)
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SludgeQw, XR,S
Effluent
(Q-Qw), Xe,S
Influent
Q, X0,
S0
QR, XR, S
S, X,
V
ClarifierAeration tank
Assuming; steady-stateconditions
SECONDARY CLARIFIER
0 = X(Q + QR) - QRXR - XRQW - QeXe
(Q-Qw
=Qe
)
Mass balance around the clarifier:
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SECONDARY CLARIFIER
[ ] X X
SRT XV XQQ
R
R−
−=
)/(
Q
O R
Recycle ratio = R =
X X
X R
Q
O
R
R
−==Mass balance around the aeration tank:
Mass balance around the clarifier:
1)/(
)/(1
−−=
X X
SRT R
R
τ
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SECONDARY CLARIFIER
A
X QQSLR R )( +
=
Solids loading rate (SLR):
A= clarifier surface area, m2
ypical return sludge pumping rate: 50 - 75% of Qave
esign average capacity = 100 - 150% of the design flowrat
Return sludge concentration, XR = 4000 – 12000 mg/L
EXAMPLE
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Design a complete-mix activated sludge process to treatprimary effluent of 10000 m3/d by both BOD removal andnitrification.
Influent BOD = 150 g/m3 (bCOD/BOD=1.6)
Effluent BOD = 2 g/m3
Influent TKN = 35 mg/L,
Effluent NH4-N = 0.5 mg/L
NH4-Nin / TKNin = 0.65
SRT = 15 d
Theoretical detention time = 8 h Yn= 0.12
K s = 40 mg/L,
Y = 0.40 g VSS/g bCOD,kd = 0.08 g VSS/g VSS.d
kdn = 0.06 g VSS/g VSS.d =
Determine
• Oxygen demand in theaeration tank
• Oxygen uptake rate• Oxygen required fornitrification
• Tank biomass concentration
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PLUG-FLOW
ACTIVATED-SLUDGE PROCESS
MODELING SUSPENDED GROWTH PROCESSES
Plug-flow reactors
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Under steady-state conditions and nth-order reaction (r c = -kCn )
V r QC QC V t
C c x x x ∆+−=∆∂
∂∆+||
x x+ x∆
V ∆
= change in average concentration with
time, (mg/L.s)C = constituent concentration, mg/L
= differential volume element, L
r c = reaction rate for constituent C, mg/L.s
t
C
∂
∂
cr x
C
A
Q
t
C +
∆
∆−=
∂
∂
τ k Q
V k
Q
ALk dx
Q
Ak
C
dC L
n
C
C −=−=−=−= ∫ ∫ 00
L
Q, CQ, Co
τ k Q
V k
C
dC n
C
C −=−=∫
0
cr x
C
A
Q
t
C +
∂
∂−=
∂
∂
Non-ideal Plug-flow reactors:ith i l
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with axial
dispersion
)2/exp()1()2/exp()1(
)2/1exp(422
0 d aad aa
d a
C
C
−−−+=
C= effluent concentration, mg/LC0 = influent concentration, mg/L
a = (1+4kτd)1/2
d = dispersion factor = D/uL
D=coefficient of axial dispersion,m2/s
u=fluid velocity, m/sL= characteristic length, m
As d goes to infinity, complete-mix, if d=0 then plug-
flow.
Wehner andWilhelm equation
To check if the flow is ideal or not
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READING ASSIGNMENT:Residence time distribution (RTD) CurvesChp 4-4 (Metcalf & Eddy, 4th Ed.)
Tracer study analyses
Plug-flow reactor
Pulse dose Step input
Kinetic model of the plug-flowreactor
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reactor
Two simplifying assumptions:
- Microorganisms concentration in influent =microorganisms concentration in effluent
applies only if; SRT / Ʈ >5
then; average microorganism concentration: X
- Rate of substrate utilization through the tank;S K
S X k r
s
su+
−=
After integration, and substitution of equation for X
d
i s
k S S K S S
S S k Y
SRT −
++−
−=
)/ln()1()(
)(1
0
0
α )1(
0
α
α
+
+=
S S Si
Si= influent concentration to reactor after dilution with recycle flow
α = recycle ratio