1 Promembrane2 - Tom Arnot

7/28/2019 1 Promembrane2 - Tom Arnot

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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.


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Submerged Membrane Bioreactor

Screenedinfluent

Treated &disinfectedpermeate

WasteSludge

In

Out

BiologicalProcess Area

FiltrationProcess Area

Air in


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MBR Optimisation Strategy


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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


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( ) 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


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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 .


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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


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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

=


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Substrate (BOD) balance summary:

S S Y X f S X

)/(

=

V Q f =

1

)/(

S X f Y

X S S

=


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


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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


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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.


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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.


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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


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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


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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 =

)/(

*


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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.


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Oxygen balance summary:

V Q f =

1


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

=

*)/(

)/(


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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


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Effects of gassing rate on flow:

Increasing energy consumption


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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.


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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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However it is not always so simple!


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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?


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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?


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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 ==


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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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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.

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1 Promembrane2 - Tom Arnot

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