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Enhanced sludge granulation in upow anaerobic sludge

blanket (UASB) reactors by aluminum chloride

H.Q. Yu a,*, H.H.P. Fang a, J.H. Tay b

a Centre for Environmental Engineering Research, Department of Civil Engineering, The University of Hong Kong, Hong Kongb School of Civil & Structural Engineering, Nanyang Technological University, Singapore

Abstract

Two upow anaerobic sludge blanket (UASB) reactors were concurrently operated for 146 days to examine the

eects of aluminum chloride (AlCl3) on the sludge granulation process during start-up. Sludge granulation (dened as

that over 10% of granules were larger than 2.0 mm) was achieved in the control reactor (R1) in approximate three

months. Introduction of Al3 at a concentration of 300 mg/l reduced the sludge granulation time by approximate one

month. Throughout the experiment the AlCl3-added reactor (R2) had a higher biomass concentration, e.g., 13.8 g-

MLVSS/l versus 12.8 g-MLVSS/l on Day 146. Granules became visible earlier in R2 compared with R1 (35 days versus

65 days). The average size of granules from R2 was larger than that from R1. The results demonstrated that AlCl 3enhanced the sludge granulation process in the UASB reactors. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Aluminum chloride; Granular sludge; Start-up; UASB

1. Introduction

The upow anaerobic sludge blanket (UASB) reactor

has been used increasingly in recent years to treat a

variety of industrial wastes and municipal wastes

(Lettinga et al., 1993; Fang et al., 1995). The UASB

process involves the anaerobic degradation of organic

wastes using a biomass which is not attached to a sup-

port medium but which aggregates, under favorableconditions, to produce particles with good settlement

characteristics. These particles are known as granules,

and their formation, commonly termed granulation,

generally enhances the eciency of the process, pro-

ducing high biomass retention times. Microbial granu-

lation involves dierent tropic bacterial groups, and

physico-chemical and microbiological interactions.

Many factors contribute, in one form or another, to the

granulation process (Schmidt and Ahring, 1996).

Granulation may be initiated by bacterial adsorption

and adhesion to inert matters, inorganic precipitates

and/or to each other through physico-chemical interac-

tions and syntrophic relationships (Fang et al., 1995;

Schmidt and Ahring, 1996). These substances serve as

initial precursors (carriers or nuclei) for new bacterial

growth. These loosely adhered bacterial aggregates are

strengthened by extracellular polymers secreted by bac-

teria and form rmly attached initial granules (Kosaricand Blaszczyk, 1990; Schmidt and Ahring, 1996). It has

been shown that divalent metal ions, such as Ca2 and

Fe2, enhance the granulation (Mahoney et al., 1987;

Schmidt and Ahring, 1993; Shen et al., 1993). Divalent

ions were reported to play an important role in micro-

bial aggregation. It was found that extracellular poly-

mers prefer to bind multi-valent metals due to the

formation of stable complexes (Rudd et al., 1984; Ma-

honey et al., 1987).

Aluminum chloride (AlCl3) is widely used as an in-

organic coagulant in water and wastewater treatment. It

is conceivable that this coagulant might also enhance the

granulation process through physico-chemical functions

Chemosphere 44 (2001) 3136

*

Corresponding author. Fax: +852-2559-5337.E-mail address: [email protected] (H.Q. Yu).

0045-6535/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 4 5 - 6 5 3 5 ( 0 0 ) 0 0 3 8 1 - 7


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as do calcium and iron salts. However, no study has

been reported that Al3 could benet for granulation.

Therefore, the aim of this study was to investigate the

eects of AlCl3 on the sludge granulation process during

start-up.

2. Materials and methods

Experiments were performed in parallel in two

identical reactors for 146 days. The empty bed volume of

each reactor was 7.3 l with an internal diameter of 100

mm and a liquid height of 930 mm. The two reactors

were housed in a temperature control room maintained

at 35C. One reactor (R1) without adding AlCl3 was

served as control, while AlCl3 was added to another

reactor (R2) to give an inuent 300 mg/l of Al 3.

Soluble synthetic wastewater was used as feed to thereactors. The inuent chemical oxygen demand (COD)

was kept constant at 4000 mg/l throughout the study.

Table 1 shows the detailed composition of the synthetic

waste. The organic COD in wastewater was provided by

peptone, glucose and meat extract, whereas several nu-

trients and trace elements, such as nitrogen, phosphorus,

sulfur, calcium, iron and magnesium were also added.

The ratio of COD:N:P was approximately 200:4:1. The

buer capacity was maintained by the addition of so-

dium bicarbonate.

The seeding inoculum was the sludge taken from the

anaerobic digesters of a local sewage treatment plant.The raw sludge was screened through a 0.2 mm sieve to

remove the big debris and bers before seeding. The

sieved sludge had 23.6 g/l of suspended solids (SS), 17.0

g/l of volatile suspended solids (VSS), and a sludge

volume index (SVI) of 43 ml/g. The specic methano-

genic activity (SMA) was 0:26 g CH4 COD=gVSSd). Each reactor was seeded with 3.5 l of inoculum

with biomass content of 8.5 g-VSS/l.

Biomass were periodically taken from the sampling

points arranged along the columns to determine the

mixed liquid VSS concentrations. Gas production was

recorded by wet gas meters (Shinagawa W-NK-0.5) and

gas composition was analyzed by Gas Chromatography

(HP 5890A) with a 2 m long and 3 mm-ID packed

column (Haye-Sep Q, 80/100 mesh) and thermal con-

ductivity detector with a temperature of 2000C. Helium

was used as the carrier gas in the gas chromatographyoperation with a owrate of 30 ml/min.

For estimating the size distribution of the sludge

particles taken from the bottom sampling point, solid

samples were classied into six fractions using labora-

tory sieves (Endecotts Ltd., London, England) with

various openings (0.2, 0.6, 1.0, 2.0, 4.0 mm). The sludge

particles were rst placed in the sieve with the biggest

opening (4.0 mm). The particles were gently submerged

in water and shaken to let the smaller particles pass

through this sieve. The particles passing through were

collected in a container. The smaller particles collected

in the above sieve were then placed in the next sieve(opening of 2.0 mm) and the above procedures were

repeated until all of the ve sieves were used. The sludge,

taken from the bottom sampling point, was measured

for SMA. The SMA test was conducted in a 250 ml

Kimax ask at 35 1C under anaerobic conditions asdescribed previously (Fang et al., 1994). The analyses of

COD and mixed liquor VSS were performed according

to the standard methods (APHA, AWWA and WEF,

1992).

3. Results and discussion

The initial organic loading rate (OLR) for both re-

actors was set at 2.0 g-COD/l/dbased on the total liquid

volume of 7.3 l and a hydraulic retention time (HRT) of

48 h. The loading rate was then increased in steps to 2.7,

4.0, 5.3 and 8.0 g-COD/l/dby reducing the HRT to 36,

24, 18, and 12 h, correspondingly.

The COD removal eciencies of the two reactors are

illustrated in Fig. 1. Initially the COD removal e-

ciencies were low, but the R2 had a slightly higher re-

moval than R1. With the progress of the experiment, the

COD removal eciencies of the two reactors generally

Table 1

Composition of the synthetic wastewater of 4000 mg-COD/l

Constituents mg/l

Bacteriological peptone 800

Glucose 2720

Meat extract (Lab-Lemco powder) 560

Sodium bicarbonate, NaHCO3 15002500

Calcium chloride, CaCl2 2H2O 38Magnesium sulfate, MgSO4 7H2O 42Ammonium chloride, NH4Cl 160

Ferrous sulfate, FeSO4 7H2O 32Potassium dihydrogen ortho-phosphate,

KH2PO4

80

Fig. 1. COD removal eciency.

32 H.Q. Yu et al. / Chemosphere 44 (2001) 3136


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kept increasing, but the eciency of R2 increased faster

than that of R1. However, after reaching an OLR of 5.3

g-COD/l/d, R1 and R2 achieved similar levels of COD

removal.

Table 2 shows the change in MLVSS concentration

in the two reactors. The overall biomass increase was0.030 g-VSS/dfor R1 and 0.034 g-VSS/dfor R2. The

initial decrease in MLVSS concentrations was due to the

washout of biomass. The MLVSS concentration reached

a minimum in R1 on Day 30. Throughout the study, R1

had a lower MLVSS concentration compared with R2,

although the dierence was insignicant at the nal

stages.

Because of inferior settleability, more biomass was

washed out from R1, resulting in a higher VSS con-

centration in the euent of R1 than that of R2 at the

initial stages (Fig. 2). The lower euent VSS concen-

tration from R2 was likely resulting from with the im-provement of biomass settleability through addition of

AlCl3.

The proles of biomass concentration at dierent

stages are shown in Table 3. Initially, the biomass was

loose and expanded easily. With the proceeding of

granulation, the biomass was progressively stratied

with the granules settled in the lower part of the reaction

zone. When granules were formed increasingly in the

reaction zone, a dense sludge bed and a thin sludge

blanket were formed with a clear interface betweenthem.

As shown in Fig. 3, the biogas production increased

at a rapid rate initially in the two reactors (Days 125).

At the later stages (after Day 25), biogas production

increased with the increase in OLR. An interesting

phenomenon was observed regarding the methane per-

centages (Fig. 4). Initially, the methane percentages of

the two reactors were low and increased considerably in

a short period. On Day 50, the average methane per-

centages were approximately 72% for R1 and 73% for

R2 with no respect to earlier or later granule initiation

Table 2

Biomass MLVSS concentrations (g/l) in each reactor

Reactor Operatinal time (d)

0 20 30 60 90 120 146

R1 8.5 7.0 6.1 7.3 9.5 11.8 12.9

R2 8.5 6.7 7.3 8.9 11.1 12.6 13.4

Fig. 2. Euent VSS level.

Fig. 3. Biogas production rate.

Fig. 4. Methane percentage.

Table 3

Proles of biomass concentration (g/l)

Height

(cm)

R1 R2

Day 30 Day 60 Day 90 Day 146 Day 30 Day 60 Day 90 Day 146

2 18.6 24.7 42.9 57.5 22.5 39.6 47.4 59.6

12 18.2 23.0 41.1 55.1 22.4 35.4 45.2 56.9

32 18.0 22.2 37.4 48.5 19.3 31.6 32.8 50.4

52 5.8 10.8 11.8 32.6 9.1 9.8 17.6 29.3

72 1.9 1.1 0.9 0.8 1.2 0.8 1.2 0.5

82 0.4 0.3 0.3 0.2 0.2 0.2 0.1 0.1

H.Q. Yu et al. / Chemosphere 44 (2001) 3136 33


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and values of OLR. This may suggest that the methane

percentages had no direct correlation with the formation

of granules.

After 30 days of operation, small granules with di-

ameters of 0.200.6 mm became visible at the bottom of

R2. These initial granules grew rapidly; after 30 moredays of operation large granules with diameters over 2.0

mm were formed. The rapid granulation was mainly due

to increasing biological growth under the high OLR (4.0

g-COD/l/d). Physical stratication to the sludge blanket

under high OLR and gas production also contributed to

the rapid granulation. After about 30 days of rapid

growth, the growth rate of granules was reduced, indi-

cating that a mature and stable granulation had been

developed. The reactor R1 had a slower granulation

process compared with R2.

The granule size distributions of the two reactors are

illustrated in Fig. 5. In R1, no granules were found untilDay 65. Sixty-ve days into start-up, approximately 45%

of the sludge samples were in the range of 0.20.6 mm.

On Day 146, approximately 60% of the granules from

R1 were in the range of 1.04.0 mm and only 8% of the

sample measured above 4.0 mm. The granule size dis-

tribution for R1 was signicantly dierent from that for

R2 on any given day. For R2, 12% of the samples

measured above 2.0 mm on Day 60. By Day 146, ap-

proximately 11% of granules in R2 had a diameter over

4.0 mm. Larger granule sizes in R2 may be due to the

presence of AlCl3 as a binding factor. This result implies

that the presence of AlCl3 had promoted granule for-

mation by allowing aggregates to form earlier and to

achieve a larger size.For the purpose of comparison, the ``time needed to

accomplish granulation'' in this study is dened as when

over 10% of granules were larger than 2.0 mm in the

reactor. Accordingly, the granulation was achieved in

R1 within three months, while granulation was achieved

in R2 after two months of operation. R2 had a higher

biomass concentration and had visible granules earlier

compared with R1. The average size of granules in R2

was larger at any given stages (Fig. 5). These results

clearly indicate that 300 mg=l Al3 improved the bio-mass retention and achieved a fast granulation process.

In R2, more bacteria were maintained (Table 2), and theCOD digestion rate was also higher at the initial stages

compared with R1. Therefore, AlCl3 showed a positive

eect on the COD digestion rate at the initial stages. The

positive eect of Al3 in granulation are likely to be

explained by the assumption that Al3 lowers the n

potential (Schmidt and Ahring, 1996). In addition, the

Al3 can bridge between negatively charged groups on

cell surfaces, which is important in adhesion phenomena

(Alibhai and Forster, 1986; Quarmby and Forster,

1995).

Table 4 shows the SMA of the granular sludges on

Days 30, 60, 90 and 146, using acetate as substrate. The

SMA of the seed sludge was 0:26 g CH4 COD=gVSS=d. The methanogenic activity of granules increasedsteadily with increasing OLR. After OLR was increased

to 5.3 g-COD/l/d, the SMA value of R2 dropped slightly,

while the SMA value of R1 continued to increase.

Compared with the biomass from R1, the lower SMA

values of the biomass in R2 might be attributed to the

presence of larger granules, in which mass transfer was

reduced. Mass transport inside the granules is consid-

ered to be solely by diusion (Pavlostathis and Giraldo-

Gomez, 1991). The resistance to substrate diusion in-

side granules increases proportionally with physical

granular size, making the substrate less available to thegranule core and eventually resulting in substrate de-

ciency or depletion (Alphenaar et al., 1993).

The dry mass percentage of the granules from R2 was

21%, much higher than the corresponding value (10%)

Fig. 5. Size distributions (by weight) of granules taken from the

bottom sampling points of (a) the control; (b) the reactor added

with Al3 A: d< 0:2 mm; B: 0:2 < d< 0:6 mm; C:

0:6 < d< 1:0 mm; D: 1:0 < d< 2:0; E: 2:0 < d< 4:0 mm; F:d> 4:0 mm).

Table 4

SMA (g CH4 COD=g VSS=d) using acetate as substratefor the biomass in two reactors

Reactor Operating time (d)

0 30 60 90 146

R1 0.26 0.70 1.04 1.28 1.32

R2 0.26 1.02 1.24 1.13 1.10



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of the granules from R1. This indicates that the presence

of aluminum increased the dry mass of the granules

mainly by increasing the concentration of minerals in the

granules. The increased mineral content was very likely

the result of more aluminum precipitates trapped in the

granules. Aluminum accumulation within granulesmostly resulted from the interaction of aluminum ions

with exopolysaccharide polymers because aluminum

concentration in bacterial cells is very low. The addi-

tional inorganic portion in the granules from R2 might

also be a reason to lower down the SMA values as

shown in Table 4.

It should be noticed that the dierence of sludge

granulation process among the two reactors was mainly

found only at the initial stages (Days 160). At the nal

stage (Days 130146), the control reactor behaved very

similarly to the Al3-added reactor; the former had a

little less biomass concentration but a slightly higherspecic methanogenic activity than the latter, resulting

in very similar COD removals and euent VSS levels in

the two reactors. This suggests that the addition of AlCl3played an important role only in the initial stage of

granulation and its eect was diminished after the de-

velopment of mature granules.

The scanning electric microscopic observation of

sludges revealed that the sludges from R1 had a very

similar composition and morphology to those from R2.

The granules from both reactors were predominantly

composed ofMethanosarcina-like species.

Comparison between the present study and the

studies regarding the eect of Ca2 and Fe2 on sludgegranulation shows that, Al3 had a positive eect very

similar to Ca2 and Fe2 (Mahoney et al., 1987; Schmidt

and Ahring, 1993; Shen et al., 1993). All these ions

promoted granule formation by allowing aggregates to

form earlier and to achieve a larger size, and resulted in

a faster granulation process and a shortened start-up

period for UASB reactors. Besides, the addition of these

ions did not lead to in a dierence in predominant mi-

croorganisms. These suggest that all the three ions en-

hanced the sludge granulation process through the same

physico-chemical functions. However, when Ca2 is

added, mineral precipitates such as CaCO3 andCa5OHPO43 are formed and there is a risk of cemen-tation of the UASB reactors (van Langerak et al., 1998).

The same case is true for the formation of FeS when

Fe2 is added to UASB reactors. On the other hand,

there is no such a risk when Al3 is used. Therefore, Al3

might be a better option than Ca2 and Fe2 for en-

hancing the sludge granulation in UASB reactors.

4. Conclusions

In this study sludge granulation was achieved in the

control reactor in approximately 3 months. Introduc-

tion of Al3 at a concentration of 300 mg/l reduced the

sludge granulation time by approximately 1 month.

The Al3-added reactor had a higher biomass concen-

tration throughout experiment. It had visible granules

earlier (35 days versus 65 days) and larger granules

compared with R1. The results demonstrated thatAlCl3 enhanced sludge granulation process. However,

its eect diminished as granulation process became

mature.

Acknowledgements

The authors wish to thank the Hong Kong Research

Grants Council for the partial support of this study and

one author (HQY) also wishes to thank The University

of Hong Kong for the Research Fellowship.

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