Wastewater Purification: Aerobic Granulation in Sequencing Batch Reactors - Chapter 5 potx

5.2 THE ROLE OF SBR VOLUME EXCHANGE RATIO IN AEROBIC GRANULATION According to figure 5.1, the mixed liquor volume exchange ratio, or volume exchange ratio for short, is the volume of eff

Discharge Time in Aerobic Granulation

Zhi-Wu Wang and Yu Liu

CONTENTS

5.1 Introduction 69

5.2 The Role of SBR Volume Exchange Ratio in Aerobic Granulation 70

5.3 Effect of Volume Exchange Ratio on Aerobic Granulation 71

5.4 Effect of Volume Exchange Ratio on Sludge Settleability 73

5.5 Effect of Volume Exchange Ratio on Production of Extracellular Polysaccharides 75

5.6 Effect of Volume Exchange Ratio on Calcium Accumulation in Aerobic Granules 75

5.7 Volume Exchange Ratio Is a Selection Pressure for Aerobic Granulation 76

5.8 Effect of Discharge Time on Formation of Aerobic Granules 78

5.9 Effect of Discharge Time on Settleability of Bioparticles 79

5.10 Effect of Discharge Time on Cell Surface Hydrophobicity 82

5.11 Effect of Discharge Time on Production of Extracellular Polysaccharides 82

5.12 Conclusions 83

References 84

5.1 INTRODUCTION

It appears from the preceding chapters, among all the operation parameters that have

been discussed so far, only settling time can serve as an effective selection pressure

for aerobic granulation However, a basic question to be addressed is if there are

still other parameters that can also play the roles of selection pressure in aerobic

granulation other than the identified settling time The answer to such a question is

essential for developing the design and operation strategy for rapid and stable aerobic

granulation in both small- and large-scale sequencing batch reactors (SBRs) This

chapter looks into two other potential candidate parameters that may act as selection

pressures in aerobic granulation in SBR, namely SBR volume exchange ratio and

discharge time

5.2 THE ROLE OF SBR VOLUME EXCHANGE RATIO IN

AEROBIC GRANULATION

According to figure 5.1, the mixed liquor volume exchange ratio, or volume exchange

ratio for short, is the volume of effluent that is withdrawn after a preset settling time

divided by the total working volume of a column SBR:

Volume exchange ratio ›

›

R (

, (

2

in which r is the radius of a column SBR, and H is the working height of the column

SBR This equation clearly shows that the volume exchange ratio is proportionally

related to L To look into the potential role of volume exchange ratio in aerobic

granulation, Wang, Liu, and Tay (2006) designed and ran four identical column

SBRs at different volume exchange ratios of 20% to 80% (figure 5.1), while the other

operating conditions were all maintained at the same levels

P

Feeding pump

80%

Air

60% 40% 20%

Discharging pump

P

P

P

Substrate

4°C P

FIGURE 5.1 Schematics of four SBRs operated at the respective volume exchange ratios of

80%, 60%, 40%, and 20% (From Wang, Z.-W 2007 Ph.D thesis, Nanyang Technological

University, Singapore With permission.)

5.3 EFFECT OF VOLUME EXCHANGE RATIO ON

AEROBIC GRANULATION

Wang, Liu, and Tay (2006) investigated the effect of volume exchange ratio on the

formation of acetate-fed aerobic granules The evolution of sludge morphology in the

course of SBR operation at different volume exchange ratios is shown in figure 5.2

Morphologies of aerobic granules formed in four reactors appeared to be closely

corre-lated with the applied volume exchange ratio; that is, only 8 days after reactor startup,

FIGURE 5.2 Morphologies of sludge cultivated at different volume exchange ratios in the

course of aerobic granulation in SBRs; scale bar: 6 mm.

aerobic granules first appeared in the SBR operated at the highest volume exchange

ratio of 80%, while aerobic granules were subsequently observed at the volume

exchange ratios of 60%, 40%, and 20%, respectively, 6, 12, and 20 days later It can

be seen infigure 5.2that the larger and more spherical aerobic granules were formed

at the higher volume exchange ratio of 80%, whereas bioflocs were cultivated and

became predominant in the SBR operated at the lower volume exchange ratio of 20%

It is apparent from figure 5.2 that only a mixture of bioflocs and aerobic granules

was cultivated at small volume exchange ratios Analysis of the fraction of aerobic

granules formed in each SBR reveals that nearly a pure aerobic granular sludge blanket

was indeed developed at the volume exchange ratio of 80% (figure 5.3) In contrast,

almost no aerobic granulation was found at the volume exchange of 20%, indicating

a failed granulation (figure 5.3) It is thus reasonable to consider that the SBR volume

exchange ratio can play an essential role in aerobic granulation, and a high SBR volume

exchange ratio facilitates rapid and successful aerobic granulation in SBR

As presented in the preceding chapters, aerobic granules can be simply

distin-guished from bioflocs by their large particle size The mean size of aerobic

gran-ules cultivated at different volume exchange ratios are presented infigure 5.4 The

size of the aerobic granules tended to increase with the increase in the SBR volume

exchange ratio, for example, the size of aerobic granules developed at the volume

exchange ratio of 20% was smaller than 1 mm, whereas aerobic granules as large as

about 3.8 mm were obtained at the volume exchange ratio of 80%

In the operation of nitrogen-removal SBRs, Kim et al (2004) also manipulated

the SBR discharge height so as to impose on microorganisms two slightly different

selection pressures in terms of minimum settling velocity of 0.6 and 0.7 m h–1 Even

such a marginal difference in the minimum settling velocity could also result in

distinct morphologies of cultivated sludge For example, large bioparticles of 1.0 to

2.0 mm were harvested at the (V s)minof 0.7 m h–1, while only small bioparticles of 0.1

to 0.5 mm were cultivated at the (V s)minof 0.6 m h–1(Kim et al 2004) Microscopic

observation further revealed that the high volume exchange ratio SBR favored the

Volume Exchange Ratio (%)

0 20 40 60 80 100

20 40 60 80 100

FIGURE 5.3 Fraction of aerobic granules in four SBRs run at volume exchange ratios of 20%

to 80% (Data from Wang, Z.-W., Liu, Y., and Tay, J.-H 2006 Chemosphere 62: 767–771.)

cultivation of spherical granular sludge (figure 5.5b), while only bioflocs instead

of granular sludge were developed in the SBR run at the low volume exchange

(figure 5.5a) Similar to the findings by Wang, Liu, and Tay (2006), the time for

formation of aerobic granules and to reach steady state was significantly shortened

at a high volume exchange ratio (Kim et al 2004)

5.4 EFFECT OF VOLUME EXCHANGE RATIO ON

SLUDGE SETTLEABILITY

In the field of biological wastewater treatment, sludge volume index (SVI) has been

used commonly as a good indicator of microbial sludge settleability Figure 5.6

Volume Exchange Ratio (%)

0 20 40 60 80 100

0 1 2 3

FIGURE 5.4 Comparison of mean size of aerobic granules developed at volume exchange

62: 767–771.)

2 µm

FIGURE 5.5 Morphology of steady-state granules obtained at different minimum settling

Sci Technol 50: 157–162 With permission.)

shows comparison of the settleability of sludge cultivated at different volume

exchange ratios in SBRs It can be seen that the sludge SVI was inversely correlated

to the volume exchange ratio, that is, a sludge with excellent settleability would be

developed at a high volume exchange ratio For example, the settleability of sludge

cultivated at the volume exchange ratio of 80% is almost three times superior to

that harvested at the volume exchange ratio of 20% Kim et al (2004) also reported

similar results showing that high volume exchange ratio SBR corresponding to a

high (V s)min could promote the development of sludge with excellent settleability,

indicated by a low SVI of 50 mL g–1(figure 5.7)

FIGURE 5.6 Sludge volume index (SVI) versus volume exchange ratios in SBRs (Data

from Wang, Z.-W., Liu, Y., and Tay, J.-H 2006 Chemosphere 62: 767–771.)

FIGURE 5.7 Sludge volume index (SVI) versus minimum settling velocities (Vs)min

deter-mined from the volume exchange ratios (Data from Kim, S M et al 2004 Water Sci Technol

50:157–162.)

Volume Exchange Ratio (%)

–1 )

20 40 60 80 100

(V s ) min (m h –1 )

–1 )

40 50 60 70 80 90

5.5 EFFECT OF VOLUME EXCHANGE RATIO ON PRODUCTION OF

EXTRACELLULAR POLYSACCHARIDES

Extracellular polysaccharides (PS) are a kind of bioglue that interconnects

individ-ual cells into the three-dimensional structure of attached-growth microorganisms

(seechapter 10) A high applied volume exchange ratio was found to stimulate cells

to produce more PS (figure 5.8) As discussed in chapter 10, PS indeed is not an

essential cell component under normal living conditions, and its production is only

necessary when microbial cells are subjected to stressful conditions Figure 5.8

seems to indicate that the high SBR volume exchange ratio can impose a pressure on

microbial sludge, leading to an enhanced production of PS

5.6 EFFECT OF VOLUME EXCHANGE RATIO ON

CALCIUM ACCUMULATION IN AEROBIC GRANULES

Calcium ion was accumulated significantly in aerobic granules developed at high

volume exchange ratio, for example, the calcium content in granules cultivated at

the volume exchange ratio of 80% was almost three times higher than that obtained

at the volume exchange ratio of 20% (figure 5.9) Figure 5.10 shows further that

the mean size of the aerobic granules tended to increase with the calcium content,

while an inverse trend was found for SVI According to Stokes law, the increase in

particle size will improve the settling ability of particles, and this in turn results in a

lowered SVI (figure 5.10) The improved settleability of bioparticles can effectively

prevent them from being washed out of the SBR at a high volume exchange ratio

(figure 5.9) Thus, it is most likely that the selective accumulation of calcium would

be a defensive strategy of microbial aggregates to resist the hydraulic discharge from

the reactor through the calcium-promoted increases in their size and settleability in

terms of SVI (figure 5.10) In fact, it is generally believed that calcium may facilitate

Volume Exchange Ratio (%)

–1 SS)

0.0 0.1 0.2 0.3

FIGURE 5.8 Extracellular polysaccharide production at different volume exchange ratios.

(Data from Wang, Z.-W., Liu, Y., and Tay, J.-H 2006 Chemosphere 62: 767–771.)

anaerobic granulation (Schmidt and Ahring 1996; Yu, Tay, and Fang 2001), while

evidence also shows that the removal of calcium from the anaerobic granule matrix

results in lowered strength of upflow anaerobic sludge blanket (UASB) granules

(Pereboom 1997) Consequently, a certain amount of calcium in biogranules would

improve their long-term stability

5.7 VOLUME EXCHANGE RATIO IS A SELECTION PRESSURE FOR

AEROBIC GRANULATION

It appears fromchapter 4that the settling time of SBR can serve as a selection

pres-sure for aerobic granulation, for example, at a short settling time, bioparticles with

poor settleability would be washed out according to the minimum settling velocity:

T

S S

in which t s is settling time and L is the traveling distance of the bioparticles above

the discharge port, which is proportionally correlated to the volume exchange ratio

of SBR (figure 5.11)

At a designed settling time and discharge height, bioparticles with a settling

velocity less than (V s)min are washed out of the reactor, while those with a settling

velocity greater than (V s)minare retained (figure 5.11) It is obvious that the selection

pressure in terms of minimum settling velocity (V s)minis not only a function of settling

time (t), but also depends on the discharge height (L), which can be translated to the

volume exchange ratio as given in equation 5.2 This means that the volume exchange

ratio can be another essential selection pressure for successful aerobic granulation

Volume Exchange Ratio (%)

0.00 0.05 0.10 0.15 0.20 0.25

FIGURE 5.9 Calcium content of sludge cultivated at different volume exchange ratios in

SBRs (Data from Wang, Z.-W., Liu, Y., and Tay, J.-H 2006 Chemosphere 62: 767–771.)

Mean Bioparticle Size (mm)

15 30 45 60 75

Calcium Content (g g–1SS)

0.5 1.0 1.5 2.0 2.5 3.0 3.5

FIGURE 5.10 Correlations among size of bioparticles (O), SVI (/), and calcium content.

(Data from Wang, Z.-W., Liu, Y., and Tay, J.-H 2006 Chemosphere 62: 767–771.)

L

FIGURE 5.11 Schematic diagram of a column SBR.

According to equation 5.2, the minimum settling velocity is the function of settling

time and discharge height or volume exchange ratio for an SBR with a given

diam-eter By controlling (V s)min, bioparticles can be effectively selected according to their

respective settleability This means that selection of bioparticles indeed can be realized

by manipulating settling time and volume exchange ratio To examine the collective

effects of SBR volume exchange ratio and settling velocity on aerobic granulation,

figure 5.12 shows the correlation of the fractions of aerobic granules in SBRs to (V s)min

calculated from various settling times (seechapter 6) and volume exchange ratios As

expected, the degree of aerobic granulation in an SBR is determined by (V s)min

It appears from figure 5.12 that at a (V s)minless than 4 m h–1, only a partial aerobic

granulation can be achieved in SBR, and the growth of suspended sludge seems to be

promoted in this case The typical settling velocity of conventional activated sludge is

generally less than 5 m h–1(Giokas et al 2003) This implies that for an SBR operated

at a (V s)min below the settling velocity of conventional activated sludge, suspended

sludge cannot be effectively withdrawn In this case, suspended sludge will easily

out compete aerobic granules, which will lead to the instability and even failure of

aerobic granular sludge SBRs Now it is clear that suspended sludge will take over the

entire reactor at low (V s)min, as shown in figure 5.12 To achieve rapid and enhanced

aerobic granulation in SBRs, the minimum settling velocity (V s)minmust be controlled

at a level higher than the settling velocity of suspended sludge (see chapter 6)

5.8 EFFECT OF DISCHARGE TIME ON FORMATION OF

AEROBIC GRANULES

As illustrated infigure 5.13, discharge time of SBR (t d) is defined as the time preset

to withdraw the volume of the mixed liquor above the discharge port of the SBR, and

(Vs)min (m h –1 )

0 20 40 60 80 100

FIGURE 5.12 Fraction of aerobic granules versus (Vs)min, obtained from studies of volume

exchange ratio (D) and settling time ($) (Data on volume exchange ratio from Wang, Z.-W.,

Liu, Y., and Tay, J.-H 2006 Chemosphere 62: 767–771; data on settling time from Qin, L.,

Liu, Y., and Tay, J H 2004 Biochem Eng J 21: 47–52.)

it can be expressed as the ratio of the discharge volume of SBR (V e)to the discharge

flow rate of the SBR (Q e):

1

D D D

Wang (2007) studied the potential effect of discharge time on aerobic granulation

in SBRs For this purpose, four identical SBRs were operated at different discharge

times of 5 to 20 minutes, while all other operating conditions were kept at the same

levels.Figure 5.14shows the morphologies of bioparticles developed at the various

discharge times It can be seen that smooth, round aerobic granules were successfully

cultivated at a short discharge time of 5 minutes, and only floc-like bioparticles were

observed in the SBR operated at the longest discharge time of 20 minutes Moreover,

figure 5.15shows that the mean size of the bioparticles was inversely related to the

applied discharge time This seems to indicate that a prolonged discharge time would

delay or prevent the formation of aerobic granules in SBR even though both settling

time and volume exchange ratio are properly controlled

As discussed inchapter 4, the fraction of aerobic granules over the whole sludge

blanket in an SBR represents the degree of aerobic granulation that can be achieved

under given operating conditions Figure 5.16 shows that the fraction of aerobic

granules decreased as the applied discharge time was prolonged from 5 to 20 minutes,

for example, in the SBR run at the discharge time of 20 minutes, almost no aerobic

granules were formed Similar to settling time and volume exchange ratio, the

observed failure of aerobic granulation at the long discharge time may imply that this

parameter could also serve as a kind of selection pressure for aerobic granulation

5.9 EFFECT OF DISCHARGE TIME ON

SETTLEABILITY OF BIOPARTICLES

As presented in the preceding chapters, settleability of bioparticles can be

evalu-ated by a simple parameter, namely the SVI.Figure 5.17shows a comparison of the

Qd

FIGURE 5.13 Illustration of the discharge time.