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7/28/2019 1 Promembrane2 - Tom Arnot
1/28
Department of Chemical Engineering
Dr Tom ArnotProMembrane International Conference.
Promotion and Focussing of Current Research Activities of Membrane Technology in Water Treatment in the
Mediterranean Region ,Sfax, Tunisia, 5 th 6 th May 2008.
Design & Optimisation of AerobicMembrane Bioreactors.
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Submerged Membrane Bioreactor
Screenedinfluent
Treated &disinfectedpermeate
WasteSludge
In
Out
BiologicalProcess Area
FiltrationProcess Area
Air in
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
MBR Optimisation Strategy
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Kinetics & Stoichiometry:
( ) d S k S K S
+= max
S X
Y S X =
)/(
O X Y O X
=)/(
Kinetics:
Yields:Biomass produced per substrate consumed.
Biomass produced per oxygen consumed.
= specific growth rate h -1
max = maximum specific growth rate h -1
K S = overall substrate affinity constant mg l-1
k d = overall biomass death rate h -1
S = substrate concentration in the reactor mg l -1
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
( ) d S k
S K S
+= max
Time on the x-axis can be seen as analogous to the sludgeage or residence time, C, in a water treatment bioreactor.
Growth phase: Logarithmic Declining Stationary Endogenous
Cell state: Dispersed FlocculatingConventional
High rate Extended Aerobic digestion
Sludge
Substrate
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Membrane Bioreactor Modelling:
Q f , S f Q p , S, O
Q w, S, X w, O
S, X, V, ODesign terms:
Q = flow (m3
h-1
)V = volume (m 3)Q p/Qw = flux ratio
Concentrations (all in mg l -1):S = substrate (BOD)X = biomassO = dissolved oxygen
Feed stream
Permeate stream
Waste stream
The key unknowns for design& construction are , C andhence V , and membrane flux,and hence required area, A .
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Mass balance concept:
Mass inthrough the
system boundary
Massaccumulated
within thesystem
=
Mass outthrough the
system boundary
-
Massgenerated
within thesystem
+
Massconsumed
within thesystem
-
)/( S X
w p
f f
Y X
S V Q
S V
QS V
Q
dt dS
=
= 0
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Substrate (BOD) balance:
)/( S X
w p f
f
Y X
S V Q
S V
QS V
Q
dt dS
=
but as Q f = Q p + Q w this becomes: ( ) )/( S X f f
Y X
S S V
Q
dt dS =
.
At steady state 0=dt dS
, so: ( ))/( S X
f f
Y X
S S V
Q =,
rearranging gives: S S Y X f S X
)/(
=
where V Q f =
1and = hydraulic residence time (h),
or )/(
S X f Y
X S S
=
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Substrate (BOD) balance summary:
S S Y X f S X
)/(
=
V Q f =
1
)/(
S X f Y
X S S
=
Q f , S f Q p , S, O
Q w, S, X w, O
S, X, V, O
These equations link together water quality (feed & treated), biomassconcentration and growth rate,stoichiometry, and reactor volume.
or
7/28/2019 1 Promembrane2 - Tom Arnot
10/28
Department of Chemical Engineering
Mass balance concept:
Mass inthrough the
system boundary
Massaccumulated
within thesystem
=
Mass outthrough the
system boundary
-
Massgenerated
within thesystem
+
Massconsumed
within thesystem
-
ww X
V Q X
dt dX =
= 0
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Biomass balance:
ww X
V Q X
dt dX = (assume X f & X p = 0)
but at steady state 0=
dt dX
, and X = X w if the reactor is well
mixed, so:
X V Q
X w= , ie V Q
w= and as V Qw
C
=
1, we get
C
1=, where
C = cell (biomass, X) residence time (h).Combining this with the kinetic equation we get:
( )d
S C
k
S K
S +
== max1 , so if we define S as a target value for water
quality we can calculate , and hence C. We can therefore usesludge wasting to control the biology and water quality.
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Biomass balance summary:Q f , S f Q
p , S, O
Q w, S, X w, OS, X, V, O X V
Q X w=
V Q
w=V Qw
C
=
1( ) d S C
k S K
S +
== max1
So if we define S as a target value for water quality wecan calculate , and hence C. We can therefore usesludge wasting to control the biology and water quality.
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Biomass Yields are Lower in MBRs
- C
05000
10000
150002000025000300003500040000
1 10 100 1000 10000Sludge Age DaysHRT = 2.7 h,Y = 0.4, k = 0.07 d -1, kd = 0.06 d
-1
B i o m a s s
( m g
/ l )
00.050.1
0.150.20.250.30.350.4
O b s e r v e
d Y i e l d
Biomass
Yield
= HRT = 6 h, Y (X/S) = 0.4, max = 0.35 h -1, k d = 0.0025 h -1
Typical for activated sludge
- C
Typicalfor MBR
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
X V
S Q
X QV
S
X
S
m f f f
f
f f
ratio =
==
The Food to Micro-organism ratio:
MBR +
Typicalactivated
sludge
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Mass balance concept:
Mass inthrough the
system boundary
Massaccumulated
within thesystem
=
Mass outthrough the
system boundary
-
Massgeneratedwithin the
system
+
Massconsumedwithin the
system
-
= 0
( ) V OQV OQY X OOak dt dOw p
O X L =
)/(
*
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Oxygen balance: ( ) V OQ
V
OQ
Y X
OOak dt dO w p
O X L
=)/(
* ,
but as Q f = Q p + Q w this becomes: ( ) V OQ
Y X
OOak dt dO f
O X L
=)/(
* .
At steady state 0=
dt dO
, so: ( ) V OQ
Y X
OOak f
O X L
+=)/(
* , and solving for O
gives: ( )( )1
)/(
)/(*
=
ak Y X Y Oak O
LO X
O X L
, or ( )OOY Y O X ak
O X
O X L
=*
)/(
)/(
.
We can now check to see whether the system is able to supply enoughoxygen. k La = volumetric mass transfer coefficient (typically 70 h -1),O* = saturated oxygen concentration (approx 10 mg l -1 @ 1 bar & 15 C)
B. Assume that there is no useful oxygen in the feed, ie O f = 0.
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Oxygen balance summary:
V Q f =
1
Q f , S f Q p , S, O
Q w, S, X w, O
S, X, V, O
or ( )1
)/(
)/(*
=
ak Y
X Y Oak
O LO X
O X L
( )OOY Y O X
ak O X
O X
L
=
*)/(
)/(
7/28/2019 1 Promembrane2 - Tom Arnot
18/28
Department of Chemical Engineering
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Fouling Control with
Immersed Membranes
Use of membraneswith a lower cost,under conditions of low trans-membrane(TMP) pressure andlimited flux.
Use three-phase (gas/ liquid / biomass)flow, permeate
backwash, & fluxrelaxation, to controlfouling.
Filtration
J l i m
i t e
d ,
T M P
l o w a n
d
c o
n s
t a n
t a
l o n g m e m
b r a n e
filteringlayer membrane
Transfer enhanced bythree-phase flow
BackwashingPeriodic backwashing
gives cake destabilization
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Effects of gassing rate on flow:
Increasing energy consumption
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Membrane modelling / sizing:
( ) ( )n J J J J k dt dJ = 2*
A
Q J p=, (A = membrane area m 2)
n = fouling mechanism index (0, 1, 1.5, 2 - after Hermia ) a function of TMP, particle size, flux, back flushfrequency, flux relaxation, and gassing rate.J * = critical flux a function of TMP, back flushfrequency, flux relaxation, particle size, biomassconcentration (X), and gassing rate.k
J= fouling rate a function of TMP, flux, flux
relaxation, back flush frequency, particle size, biomassconcentration (X), and gassing rate.
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Membrane fouling mechanismsFrom Hermias analysis the value of n varies with different membranefouling mechanisms:
(a) n = 2.0 for complete blocking,
(b) n = 1.5 for standard blocking,
(c) n = 1.0 for incomplete pore blocking (intermediate fouling),
(d) n = 0 for cake filtration.
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Department of Chemical Engineering
However it is not always so simple!
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Critical membrane flux versus f/m ratio
0
5
10
15
20
25
0 0.05 0.1 0.15 0.2
f/m Ratio (kg kg -1 d -1)
C
r i t i c a
l F l u x
( l m
- 2 h - 1 )
Gassing rate, u g = 88 mm s -1; biomass, X = 17,420 mg l -1
J*How does this move with varying
gassing rates and biomass values?
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Residual membrane fouling rate ( k J ) versusinverse gassing rate ( u g) for various fluxes(J = 20, 24 & 28 l m -2 h-1), and a fixed
biomass ( X = 17,420 mg l -1).
Increased aerationcan be used to
achieve higher fluxesfor less TMP at thesame biomassconcentration.
J
g
eudt
TMP d 3893.0684.0)( =
We can link fouling rate to flux
and gassing rate:
How generic?
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
Q f and Sf are characteristics of the feed stream, and hence known.
k La is a function of aeration this may be designed for.max , K S and k d depend on kinetics, Y (X/S) and Y (X/O) fromstoichiometry.
S is a target for the treated water quality select an appropriate value.The key unknowns for design & construction are therefore , C , Vand A (i.e. J *).
Design - steady state summary:
S S Y X f S X
)/(
=
V Q f =
1
V Q w
C
== 1
( ) d S k
S K S +=
max
( )1
)/(
)/(*
=
ak Y X Y Oak O
LO X
O X L** J QQ
J Q A w f p ==
7/28/2019 1 Promembrane2 - Tom Arnot
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Department of Chemical Engineering
MBRs are optimised on the basis of cost:Capital costs (usually amortised at 6% over 20 years): Treatment tank volume Membrane installation and pumps Aeration (blowers / compressors) Off gas treatment (filtering, scrubbing etc)
Operating costs: Aeration (blower / compressor operation) Off gas treatment (not always necessary) Sludge disposal (increasingly important) Membrane replacement (becoming less important)
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Department of Chemical Engineering
Acknowledgements
Research colleagues: Prof John Howell, Dr Robert Field, Dr Hwee Chuan Chua,
Dr Miaw-Ching Sim, Dr Wenjun Liu, George Skouteris,Kerry-Anne Young
Previous and current funding:
UK EPSRC + 7 water utility companies, 1999-2002. EU OLAPS Project, 1999-2003. UK EPSRC, 2000-2003. UK MOD, 2003-2005. EU PURATREAT Project, 2006-2009, www.puratreat.com.