Wastewater Purification: Aerobic Granulation in Sequencing Batch Reactors - Chapter 8 doc

Moreover, the unbalanced microbial growth inside aerobic granules has been sup-posed to be due to the mass diffusion limitation in aerobic granules, which would further lead to a heterog

and Oxygen in Aerobic Granules

Yong Li, Zhi-Wu Wang, and Yu Liu

CONTENTS

8.1 Introduction 131

8.2 Size-Dependent Kinetic Behaviors of Aerobic Granules 132

8.3 Description of Diffusion Resistance in Aerobic Granules 133

8.4 Simulation of Mass Transfer in Aerobic Granules 135

8.4.1 Model Development 136

8.4.2 Substrate Profile in Aerobic Granules with Different Radiuses 139

8.4.3 Oxygen Profiles in Aerobic Granules with Different Radiuses 139

8.4.4 Diffusion Profiles of Substrate in Aerobic Granules at Different Bulk Substrate Concentrations 139

8.4.5 Diffusion Profiles of Dissolved Oxygen in Aerobic Granules at Different Substrate Concentrations 141

8.4.6 Prediction of Bulk Substrate Concentration in an Aerobic Granules Reactor 143

8.5 Conclusions 144

Symbols 145

References 146

8.1 INTRODUCTION

Up-to-date, intensive research has been dedicated to the effect of various operating

parameters on aerobic granulation in sequencing batch reactors (SBRs) (chapters 1

to7) However, very limited information is currently available about the diffusion

behaviors of substances inside aerobic granules Tay et al (2002) found that a model

dye was only able to penetrate 800 μm beneath the surface of aerobic granules, while

Jang et al (2003) reported a penetration depth of 700 μm for dissolved oxygen from

the surface of aerobic granules Meanwhile, oxygen diffusion limitation in nitrifying

aerobic granules was detected by microelectrode (Wilen, Gapes, and Keller 2004)

Moreover, the unbalanced microbial growth inside aerobic granules has been

sup-posed to be due to the mass diffusion limitation in aerobic granules, which would

further lead to a heterogeneous internal structure (see chapter 11) These indicate

that without a proper control of the mass diffusion, the structural stability of aerobic

granules might not be sustainable

In view of the importance of substrate and oxygen in microbial culture, this

chapter attempts to offer insights into the diffusion behaviors of substrate and

oxy-gen in aerobic granules The factors that determine their respective diffusion in

aero-bic granules are also discussed, with special focus on the reactor operation as well as

aerobic granule characteristics

8.2 SIZE-DEPENDENT KINETIC BEHAVIORS OF

AEROBIC GRANULES

Mass diffusion limitation often occurs in attached microbial communities, such as

biofilms, and it suppresses microorganisms from fully accessing the substrate and

oxygen in bulk solution As a result, the microbial activity is lowered by deficiency

of the energy source To inspect the existence of mass diffusion limitation in aerobic

granules, Y Q Liu, Liu, and Tay (2005) determined the specific COD removal rates

and specific growth rates of aerobic granules with different sizes It appears from

figure 8.1 that the specific COD removal rate decreased markedly with the increase

in the size of aerobic granules, indicating that the microbial activity inside aerobic

granules is inhibited as the granule size increases Moreover, the specific growth rate

of aerobic granules was inversely dependent on the granule size (figure 8.2) A plot of

the specific growth rate against the specific substrate utilization rate further reveals

that the slow substrate utilization by large-sized aerobic granules results in a low

specific growth rate (figure 8.3) Such a relationship between the specific growth and

substrate utilization rates is consistent with the prediction by microbial growth theory

(Metcalf and Eddy 2003) Consequently, the kinetic behaviors of aerobic granules

depicted by the specific growth and substrate utilization rates are size dependent















FIGURE 8.1 Granule size-dependent specific chemical oxygen demand (COD) removal

rate (Data from Liu, Y Q., Liu, Y., and Tay, J H., 2005 Lett Appl Microbiol 40:

312–315.)

8.3 DESCRIPTION OF DIFFUSION RESISTANCE IN

AEROBIC GRANULES

It appears from figures 8.1 and 8.2 that the observed kinetic behaviors of aerobic

granules with different sizes is ultimately the result of diffusion limitation of

sub-strate or oxygen in aerobic granules In order to look into this, Y Q Liu, Liu, and Tay

(2005) introduced the concept of an effectiveness factor (I), which can be calculated

as follows:

H  rate with diffusion limitation rate withoutt diffusion limitation (8.1)







 

FIGURE 8.2 Granule size-dependent specific biomass growth rate of aerobic granules (Data

from Liu, Y Q., Liu, Y., and Tay, J H 2005 Lett Appl Microbiol 40: 312–315.)

0.015 0.020 0.025 0.030 0.035 0.040 0.045

FIGURE 8.3 Specific growth rate of aerobic granules versus specific substrate utilization

rate; data from figures 8.1 and 8.2.

In a case where diffusion limitation is negligible,I approaches unity, while I

is close to zero if the diffusion limitation becomes significant as compared to the

reaction Figure 8.4 shows that theI tended to drop quickly with the increase of the

granule radius This would result from the presence of mass diffusion limitation in

large aerobic granules

The Thiele modulus is a measurement of the ratio of granule surface reaction rate

to the mass diffusion rate If the Monod kinetics for microbial reaction is applied, Y

Q Liu, Liu, and Tay (2005) proposed the following modified Thiele modulus (K) by

introducing dimensionless concentrations:

F R X M

o m

X S o es

The Thiele modulus combines the individual effect of specific growth rate Mm,

granule radius R, initial biomass X 0 , substrate concentrations S 0 , and diffusivity of

substrate D esin granules Obviously, a high G value means high surface reaction

rate and low diffusion rate, and vice versa A quasi-linear relationship of the Thiele

modulus to the granule sizes is shown infigure 8.5 This seems to indicate that the

mass diffusion dominates the surface reaction and becomes limiting in large aerobic

granules Under similar cultivation conditions of granules, Tay et al (2003) also

reported that small granules with radius of approximately 300 μm consisted entirely

of live biomass As can be seen in figure 8.5, a very low Thiele modulus was found in

aerobic granules with a radius of 300 μm, that is, the microbial reaction is dominant

over diffusion resistance in granules, and maintained the live cells throughout the

entire aerobic granules In contrast, Tay et al (2002) detected an anaerobic layer at

a depth of 800 to 900 μm from the surface of aerobic granules by the fluorescence









FIGURE 8.4 Effectiveness factor versus (I) the radius of aerobic granule (Data from

Liu, Y Q., Liu, Y., and Tay, J H 2005 Lett Appl Microbiol 40: 312–315.)

in situ hybridization (FISH) method This suggests that the oxygen diffusion became

the rate limiting factor over microbial reaction, which is evidenced by a large Thiele

modulus, as shown in figure 8.5

Picioreanu, van Loosdrecht, and Heijnen (1998) reported that a porous,

mushroom-like, loose biofilm structure was observed at a low mass transfer rate,

while the biofilm structure was compact and smooth at increased mass transfer rates

In general, the radius of aerobic granules is bigger than the thickness of biofilm, thus

the effect of mass transfer resistance on the structure of aerobic granules is

remark-able It appears from equation 8.2 that mass transfer resistance in aerobic granules

is closely related to the radius of the aerobic granules, the maximum specific growth

rate, initial biomass, and substrate concentrations and diffusivity of the substrate

So far, it has been reported that selection of slow-growing bacteria and increase of

shear rate favors the formation of compact and stable aerobic granules (Tay, Liu, and

Liu 2001; Y Liu, Yang, and Tay 2004) Obviously, slow-growing bacteria have small

specific growth rates, leading to small-sized granules According to equation 8.2,

a small value of the Thiele modulus can be expected in this case, that is, the mass

transfer resistance in this kind of aerobic granule would be lowered Similarly,

small aerobic granules can be cultivated at relatively high shear force, and this in

turn results in a low value of the Thiele modulus (equation 8.2) In fact, the Thiele

modulus and the effectiveness factor have been widely applied in biofilm research

for decades These two parameters also can provide useful information for

quantita-tively understating the mass transfer resistance in aerobic granules

8.4 SIMULATION OF MASS TRANSFER IN AEROBIC GRANULES

Mass transfer limitation has been observed in aerobic granules The modeling

of the substrate diffusion in biofilms has been well studied, for example, a

one-dimensional model for biofilms has been proposed by Wanner and Gujer (1986), and

Radius of Aerobic Granule

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

FIGURE 8.5 Thiele modulus versus radius of aerobic granule (Data from Liu, Y Q.,

Liu, Y., and Tay, J H 2005 Lett Appl Microbiol 40: 312–315.)

a three-dimensional model reflecting the heterogeneous structures of biofilms was

also established by Picioreanu, van Loosdrecht, and Heijnen (1998) Based on the

study of biofilm, Li and Liu (2005) investigated the description of diffusion process

in aerobic granules

8.4.1 M ODEL D EVELOPMENT

Mature aerobic granules have an equilibrium or stable size when growth and detachment

forces are balanced (Y Liu and Tay 2002) In the development of the one-dimensional

model for aerobic granules, Li and Liu (2005) made the following assumptions:

1 An aerobic granule is isotropic in physical, chemical, and biological

prop-erties, such as density and diffusion coefficient

2 An aerobic granule is ideally spherical

3 No nitrification and anaerobic degradation happen in the process

4 Aerobic granules respond to the change of bulk substrate concentration so

quickly that the response time can be ignored

According to Bailey and Ollis (1986), the mass balance equations between the two

layers whose radiuses are, respectively, r and r + dr can be expressed as follows:

ds dr

s

2 2

2

¥

§

´

¶

in whichO is the substrate conversion rate, s is the substrate concentration, and D sis

the diffusion coefficient In the approach by Li and Liu (2005), the substrate

conver-sion rate was given by the Monod-type equation:

N R M

x

x s s

Y

s

/

in whichSx is the biomass density, Y x/sis the biomass yield, andN and Nmax
are the

specific growth rate and the maximum specific growth rate, respectively Substituting

equation 8.4 into equation 8.3 leads to the following expression:

ds dr

s

s

s x

x s

2 2

2

¥

§

´

¶

µ  Mmax R

/

(8.5)

It was assumed that the derivative at the center of the granule is zero and the

substrate concentration at the surface of the granule equals the bulk solution (Li and

Liu 2005), that is:

ds

0

s S

Perez, Picioreanu, and van Loosdrecht (2005) applied equation 8.5 to the study of

spherical biofilms, and analytically solved this equation by assuming that the growth

rate is zero-order or first-order As noted by Li and Liu (2005), such a simplified

treatment of equation 8.5 leads to the inaccuracy of the prediction, and also limits

the application of this equation to a very narrow range As equation 8.5 is a

non-homogenous equation, a numerical method to solve it completely was developed

without any assumption on it (Li and Liu 2005) This method is based on the finite

difference method (FDM) (Hoffman 2001) The radius is thus divided into n grids,

that is:

r

r r

s

s

i i i i i

¥

§¦

´

¶µ

2

2

Mmax ii

s i x

x s

R

/

(8.8)

This numerical scheme is applied to all situations without simplifying

assump-tions and therefore the accuracy is increased The program was written in Matlab™

language and run under Matlab 7.0 which allows an easy visualization of the

simu-lated data (Li and Liu 2005) It should be emphasized that equation 8.8 can also be

applied to oxygen if the set of parameters for the substrate is replaced with the set of

parameters for dissolved oxygen

After the substrate concentration is determined, the substrate utilization rate (O1)

of a single aerobic granule can be calculated as:

0

1

4

Y x s ° x  s r dr

R

/

(8.9)

Summing up the substrate utilization rate of all the aerobic granules gives the

total substrate utilization rateOall:

x s x R

i

m

i

¤ 1 °  4 2

0

1 /

(8.10)

in which m is the number of aerobic granules in the reactor and R iis the radius of

the granule being calculated (Li and Liu 2005) According to Li and Liu (2005), the

average radius R of aerobic granules can be expressed as follows:

R

i

m

¤

1

1

(8.11)

Substituting equation 8.11 into equation 8.10 yields:

x s x

R

m

4 2

0

(8.12)

After deformation, equation 8.12 becomes:

P

all x

x s

R

Y

°

4 3

4

4 3

2 3 0

in which 4P RR3 x mis equal to the total biomass in the reactor Thus, equation 8.13

can be simplified to:

P

all

x s

R

XV Y

°

4

4 3

2 3 0

(8.14)

in whichV is the reactor volume At time dt, the change of substrate concentration in

the reactor can be described as:

dS

X Y

bulk all

x s

R

°

P

4

4 3

2 3 0

(8.15)

In a time period fromT 0 to T, the change in the substrate concentration is given

by equation 8.16:

Y

bulk

x s

R

T

T

§

¦

´

¶ µ

°

°

M P

4

4 3

2 3 0

0

d

At an initial substrate concentration,S bulk 0, the bulk substrate concentration at

any time t can be calculated as:

Y

bulk bulk bulk

bulk

x s

0

0

2

4

4 3

$

M

R

T

T

3 0

0 °

´

¶

As discussed earlier, equation 8.17 cannot be solved analytically, and the method

based on the finite difference principle is thus applied to solve this equation (Li and

Liu 2005) At each time step, the state is considered pseudo-static, which means the

bulk substrate concentration is constant at each time step Then change of the bulk

substrate concentration is a process of mapping, that is, the substrate concentration

is determined by the previous time step This model is valid for different substrates

so long as the parameters are replaced with the specific substrate parameters In this

study, equation 8.8 was applied to organic substrate and dissolved oxygen under

vari-ous operation conditions

8.4.2 S UBSTRATE P ROFILE IN A EROBIC G RANULES WITH D IFFERENT R ADIUSES

Li and Liu (2005) simulated the substrate profiles in bioparticles with a mean size

of 0.1 to 1.0 mm, and the initial acetate concentration and granule biomass

concen-tration were kept at 465 mg COD L–1and 6250 mg L–1volatile solids, respectively,

while the initial DO concentration was controlled at 8.3 mg L–1 Model predictions

show that the acetate in terms of COD is able to penetrate to the center of all sizes of

aerobic granules, and the microbial utilization of COD was able to proceed

through-COD profiles in figure 8.6c to e show the platforms beneath the depths of 0.23, 0.11,

and 0.07 mm from the surface of aerobic granules with radiuses of 0.5, 0.75, and

1.0 mm, respectively As assumed, there should be no autotrophic bacteria in aerobic

granule, thus these COD platforms in figure 8.6c to e seem to imply that bacteria at

those platform depths of the aerobic granule should have ceased normal metabolic

activity Furthermore, the noticeable level of COD present at those granule centers

points to another possibility accounting for the lowered microbial activity, that is,

dissolved oxygen could be a limiting factor at those depths (figure 8.6c to e)

8.4.3 O XYGEN P ROFILES IN A EROBIC G RANULES WITH D IFFERENT R ADIUSES

Oxygen profiles in aerobic granules with radiuses of 0.1 to 1.0 mm are simulated

(figure 8.7) Similar to the COD profiles in figure 8.6, oxygen can diffuse into the

entire aerobic granules with radius of 0.1 and 0.4 mm (figure 8.7a and b) For aerobic

granules with a radius bigger than 0.5 mm, prominent oxygen diffusion limitation

turns out Oxygen is only able to penetrate to 0.27, 0.64, and 0.1 mm from the

sur-face of aerobic granules with radiuses of 0.5, 0.75, and 1.0 mm, and the remaining

depth in the aerobic granule is deficient in dissolved oxygen (DO) (figure 8.7c, d,

and e) Zero DO depths inside the aerobic granules are in good agreement with those

COD platforms (figure 8.6c to e), and microbial activity at the depth of those COD

platforms was seriously limited by the availability of DO

The DO-reachable depths shown in figure 8.7 appear to be inversely related to

the size of the aerobic granule This suggests that larger aerobic granules will be

subjected to an even more severe diffusion limitation of DO and further cause a drop

in microbial activity It can thus be concluded that, in large aerobic granules, acetate

or organic substrate would not be a limiting factor, and the whole microbial process

would be dominated by the availability of DO inside the aerobic granule

8.4.4 D IFFUSION P ROFILES OF S UBSTRATE IN A EROBIC G RANULES AT

D IFFERENT B ULK S UBSTRATE C ONCENTRATIONS

The results presented in figures 8.6 and 8.7 were obtained at a fixed bulk COD

con-centration of 465 mg L–1, the effect of bulk COD concentration on the mass diffusion

in aerobic granule is not taken into account In order to clarify this point, the COD

out aerobic granules with the radius of 0.1 to 0.4 (figure 8.6aand b) In contrast, the

profiles in an aerobic granule with radius of 0.5 mm were simulated at different bulk

COD concentrations of 100, 200, and 300 mg L–1, respectively (Li and Liu 2005)

Figure 8.8shows soluble COD can penetrate through the entire aerobic granule at

a bulk COD concentration of 300 mg L–1or above, while COD becomes a limiting

factor and sharply drops to nil at depths of 0.25 and 0.1 mm at the bulk COD

con-centrations of 100 and 200 mg L–1, respectively This suggests that COD availability

may also be a limiting factor for microbial growth at low concentrations, and this is

strongly dependent on the level of external substrate concentration in bulk solution

Radius (mm)

0 200

400

600

800

1000

0 200

400

600

800

1000

0 200 400 600 800 1000

0 200 400 600 800 1000

0 200 400 600 800 1000

Radius (mm)

Radius (mm) (c)

Radius (mm) (d)

Radius (mm) (e)

FIGURE 8.6 Substrate profiles in aerobic granules with radiuses of 0.10 (a), 0.40 (b),

0.50 (c), 0.75 (d), and 1.00 mm (e) (Data from Li, Y and Liu, Y 2005 Biochem Eng J

27: 45–52.)