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Application of anaerobic
fluidized bed reactors in
wastewater treatment: a
review
R. Saravanane and D.V.S. Murthy
Anaerobic
fluidized bed
reactors
97
Received 3 April 1999
Accepted 10 July 1999
Indian Institute of Technology (IIT) Madras, India
Keywords Waste, Water industry, Reactors
Abstract During the past ten years, anaerobic process has become a popular technology for
treating concentrated effluents. Research and development programmes led by both engineers
and microbiologists have resulted in a better understanding of the microbiology of anaerobic
reactions and reactor design for anaerobic processes. Considerable progress has been achieved in
the development of high rate anaerobic reactors with several configurations for treating
concentrated industrial effluents. In this review, attention is paid to highlighting the conceptual
and full scale developments of anaerobic fluidized bed reactors, in respect of process performance,
design concepts, start-up of the reactor, stability of the system with respect to various operating
parameters, reactor configurations, comparison with competing reactor designs for concentrated
industrial effluents and kinetics and modelling of reactor systems.
Introduction
Anaerobic technology for the treatment of wastes and wastewater has been
known since the beginning of the twentieth century. The initial applications
were for sewage sludge stabilization. Although anaerobic treatment process is
inherently advantageous (no oxygen consumption, low sludge yield and CH4
production) the process has not been successfully implemented owing to
classical disadvantages like low sludge activity, low reactor capacity,
unsuitability of the process and inhibitory effects. However, with the
advancement in microbiology and environmental biotechnology, these
disadvantages have been overcome during the past decade. In addition, a
number of reactor configurations have been developed leading to high biomass
concentrations such as upflow anaerobic sludge blanket (UASB) reactor,
anaerobic contact filter, down flow stationary fixed film and fluidized bed
systems.
In India, UASB and filter reactors were successfully implemented for the
treatment of concentrated industrial effluents and fluidized bed systems were
at laboratory and pilot scale study. In turn, developed countries have
implemented a few full-scale fluidized bed reactors in industrial effluent
treatment plants.
The objective of this paper is to review and critically analyse the process
parameters of anaerobic fluidized bed reactor with a view to their possible
application in industrial effluent treatment. This review may motivate
researchers and engineers to apply and develop new configurations, enabling a
Environmental Management and
Health, Vol. 11 No. 2, 2000,
pp. 97-117. # MCB University
Press, 0956-6163
EMH
11,2
98
better understanding of the mechanisms governing the efficiency of
anaerobically treated industrial effluent plants.
Conceptual outline of the anaerobic fluidized bed process
The performance of biological fluidized bed reactors was first recognized for
aerobic carbon oxidation and denitrification. Research in the historical
development of fluidized bed technology has been reported by several
groups(Heijnen et al., 1989). In a fluidized bed reactor, fine carrier particles are
used for the microbial film development. These particles with entrapped
biofilm are fluidized by high upflow fluid velocities generated by a combination
of the influent and recirculated effluents. A reactor is said to be an expanded
bed when the resulting expansion is up to 30 per cent and fluidized bed
(Grasius et al., 1997) when it is more than 30 per cent. A conceptual outline of a
fluidized bed reactor is shown for illustration (Figure 1).
Advantages and disadvantages of anaerobic fluidized bed process
The fluidized bed process claims various potential advantages over other high
rate anaerobic reactors such as upflow anaerobic sludge blanket (UASB)
reactor, filter reactors and downflow stationary fixed film reactor (DSFF).
Figure 1.
Fluidized bed reactor
These are: high sludge activity, high treatment efficiency, no clogging of
reactors, no problems of sludge retention, least chance for organic shock loads
and gas hold up as well as small area requirements.
Some of the reviews (Heijnen et al., 1989, Schugerl, 1989) demonstrated that
the majority of these disadvantages have been overcome in recent years, which
were:
.
Problems due to long start-up times due to biolayer formation on the
carrier.
.
Difficulties due to control of biolayer thickness.
.
High-energy consumption due to very high liquid recirculation ratio.
.
High investment cost for liquid distribution to obtain uniform
fluidization especially in a large-scale plant.
Anaerobic fluidized bed process characteristics
The performance evaluation of any anaerobic treatment process necessitates a
detailed study of start-up of process, biomass and biolayer formation, microbial
population dynamics, process stability with respect to shock loads and
inhibition, types of wastewater which can be treated and various reactor
configurations.
Start-up of anaerobic fluidized bed process
The start-up of anaerobic fluidized bed process is initiated by the development
of biomass and subsequent attachment to carriers. A review (Hickey et al.,1991)
on the start-up of high rate anaerobic treatment systems reported the
development of biofilms due to the influencing parameters such as liquid flux
rate, scale of the reactor, gas flux and organic loading rate. Shear at both macro
and micro scales were found to influence the biofilm thickness. The macro-scale
effects were characterised by liquid flux rate and Reynolds number, whereas
microscale effects by abrasion from particle-particle interaction, gas flux rate,
reactor height and bed height and influence of micro-regions of higher shear
induced by the distribution network. The biofilm thickness was found to vary
directly with respect to the rate of organic loading.
The experiments (Heppner et al., 1992; Hsu and Shiek, 1993; Shieh and Hsu,
1996) conducted respectively on anaerobic fluidized bed reactors with
propionate and acetic acid as substrates revealed the start-up performance.
After 24 days of batch operation with propionate as the medium, the reactor
was operated for eight months with organic loading rate of 8.5kg COD m±3
day±1, yielding a maximum gas production of 4.5m 3 m±3 day±1 at a removal
efficiency of 94-99 per cent. While Methanocorpusculum sp. and Methano
spirillum sp. adhered to the sand in the early phase of the start-up, the
consortium in the mature biofilm consisted of Desulfobulbus sp., Methanothrix
soehngenii and species of at least three different genera of hydrogenotrophic
methanogens. The reactors with acetic acid as substrate showed excellent total
organic carbon (TOC) removal (i.e. greater than 97 per cent at a feed
Anaerobic
fluidized bed
reactors
99
EMH
11,2
100
concentration of 5000mg TOC l-1) and stable methane production, i.e. 0.901
CH4 g-1 of TOC removal, at the early stages of start-up process. The strategy
based on maximum substrate loading controlled by reactor pH significantly
shortens the start-up regime. In this case, the reactor attained steady state
conditions approximately 140 days after start-up. On the other hand, a start-up
time of 200 days was required when a strategy based on maximum substrate
utilization was adopted. The biomass loss in anaerobic fluidized bed reactor
was found independent with respect to start-up of the reactor. The
experimental studies (Gorris et al., 1988; Morgan, 1991; Liang et al., 1993; Araki
and Harada, 1994) conducted on brewery, synthetic wastewater and ice-cream
wastewater evaluated the efficiency of treatment and changing microbial
activities, leading to methanogenic biofilm formation during start-up of reactor.
The COD removal efficiency reached 85 per cent at volumetric loading of 2730kg COD m±3 day±1 with methane content up to 72 per cent in the biogas.
Upflow velocities (4-25 mh±1) caused a prominent difference in the pattern of
initial biofilm formation. Microbial activities with respect to acetate production,
hydrogen utilizing and acetate utilizing methanogenesis increased up to 3-4
times as that of suspended growth sludge. A comparative start-up performance
of anaerobic reactors including fluidized and expanded bed, was reported
(Balaguer et al., 1997) for high strength wastewater at 37oC and for different
support materials.
Inoculation
A number of different inoculum sources have been used to seed fluidized beds
for low and high strength wastewater, primarily screened sludge. Supernatant
from municipal or animal manure digesters have been investigated (Hickey et
al., 1991) for starch-based food processing waste, chemical waste and soft drink
bottling waste.
The influence of seeding conditions (Ehlinger et al., 1989) on the initial
biofilm development during the start-up of the reactor fed with synthetic
protein wastewater was also reported. The efficiency of the reactor and the
composition of biofilm changed with respect to composition and pH of the
inoculum. The seeding and start-up periods were found to be critical phases
resulting in physiological stress on the biomass. Adjustment of pH of seeding
from 7 to 8.5 increased efficiency of the process. The most active biofilm was
found at the bottom of the reactor, necessitating further research on the
uniform distribution of active biofilm in the entire volume of the reactor. It has
been shown to be possible (Gorris et al., 1989; Sreekrishnan et al., 1991; Zellner
et al., 1994) to degrade organic substrate by an anaerobic mixed culture
resulting in rapid oxidation and biofilm development during steady state
operation.
Carrier type and conditioning
A relative performance efficiency of anaerobic fluidized bed using various
carriers was reported by Hickey et al. (1991). Sand was used as a medium for
starch-based food processing wastes, chemical wastes, brewery waste, bakery
waste and paper mill foul condensate. Zeolite, sand and activated carbon were
used for the treatment of sewage resulting in percentage removal of COD from
27-60.
There were indications that the carrier particle diameter had some influence.
It has been found (Heijhen et al. 1989) that the start-up using sand of 0.35mm
diameter was much faster than that using sand of 0.75mm in diameter. The
reason was thought to be the lower liquid shear with smaller particles.
The experimental results from the investigation on the effects of microcarrier pore characteristics on methanogenic fluidized bed performance (Yee et
al., 1992) revealed that under similar start-up conditions, porous micro-carriers
obtained from diatomaceous earth were capable of reducing the start-up times
by more than 50 per cent as compared to sand. More than 90 per cent of total
reactor cell mass was immobilized on porous micro-carriers as opposed to 80
per cent on sand.
Consequently, porous micro-carriers were found to be conducive for better
proliferation of slow-growing methanogenic bacterial consortia. The
experimental data clearly indicated that surface area, total pore volume and
mean pore diameter should be used concomitantly to obtain better insight into
the cell retention capacity of a given porous micro-carrier.
It was evident from a study (Breitenbucher et al., 1990) that open-pore
sintered glass material (SIRAN) was an attractive support media for the
treatment of organically highly polluted wastewater. Microorganisms
attachment and biofilm formation was accelerated by the large surface area (up
to 90m2 m±3) of the carrier. Granules and beads have been proven in fluidized
bed reactor system, maintaining COD loading rates of 206kg m±3 day±1 by
treating evaporator condensates from pulp industry. Very short start-up
periods were followed by a stable operation. The high biomass concentration
was protected against washout and remains on the large inside surface area of
the SIRAN carriers. This resulted in maximum degradation rates at low
retention times and allows the construction of small and compact reactor
systems for the treatment of high strength industrial wastewater.
The fluidized and expanded bed anaerobic reactors showed (Fox et al., 1990;
Balaguer et al., 1997) relative efficiencies of different carriers viz., low-density
anthracite, granular activated carbon, sepiolite, pumice and sand for the
treatment of synthetic wastewater and vinasse. At steady state, granular
activated carbon (GAC) reactor retained 3.75-10 times the attached biomass
retained on the other media tested and GAC reactor accumulated biomass at a
faster rate during start-up. Shear losses reflected the biomass accumulation
with two sand sizes and anthracite media having shear loss coefficients 6-20
times greater than that of the GAC medium. Hence it was concluded that GAC
medium proved to be efficient for starting up of process, and sepiolite and
pumice for low energy consumption in a fluidized bed.
A summary of carriers studied for various industrial and synthetic effluents
is shown in Table I.
Anaerobic
fluidized bed
reactors
101
Type of
waste/
effluent
Municipal
wastewater
Distillery
effluent
Glucose
Hog wastewater
Glucose
Malting
wastewater
Yeast
wastewater
Sucrose
Vinasse
Acetic accid
Acetate
Ethanol
Yeast
wastewater
Acetic acid
Municipal
wastewater
Olive mill
wastewater
Butyrate
Brewery
wastewater
Brewery
wastewater
Acetate
propionate
Butyrate
Tannery
wastewater
Monosodium
glutamatea
Fruit
processing
wastewater
Glucose
Carrier
Type
Diameter
D (m)
Height
H (m)
Volume
(m3)
VFB/vR
(%)
EB
0.063
0.14
0.0004
15-20
FB
FB
0.065
0.051
1.70
0.76
0.006
0.033
25
36.0
FB
FB
0.1
0.045
2.3
0.9
0.018
0.001
100
±
EB
0.2
7.5
0.225
±
FB
FB
FB
FB
FB
EB
±
0.056
0.115
±
0.75
1.5
0.011
0.002
0.015
25
25
15-70
0.051
0.08-0.138
0.05
0.76
1.1-1.8
1.0
0.002
0.018
0.002
FB
FB
0.045
0.051
0.90
0.76
FB
0.05
FB
FB
0.1
±
EB
FB
FB
Operating conditions
COD
loading
rate
Vsup
(Kgm±3day±1)
(mh±1)
T (o/c)
PH
(h)
COD
removal
(%)
5-20
7-7.3
1-10
35-77
0.336-0.349
±
±
3
±
5.3-8.7
55
±
7.65-8.57
±
0.46-2.5
2.5-3.3
82.5-97
87-100
0.33
0.8-1.0
±
55.8
±
44.2
53
32
0.24-10.4
7.5-21
4.58
±
35
37
7.1-7.9
7.0
±
24
60-80
80-90
0.35
0.47
70
75
30
25
14
58
1436
8.8-14.6
6.0
13-20
6.5-7.5
2.4
66-72
0.3-0.6
±
±
55
27-33
25
20-32
33
10
6-15
37
35
35
6.9-7.1
7.0
6.7-7.0
8.7-10.2
4.8-72
0-4
75
80
70-75
68
70
±
32
30
±
24
54
8
7.5-32 (TOC)
18.0-401 (TOC)
7-39
14.5
±
2.5-5.5
35
35
30
6.75-7.5
7-8
7.0
±
0.6-3.4
0.09-2.1
99
97
80-97
±
±
±
0.91
±
±
±
±
±
±
±
±
59
67
40
37
35
7.0
6.75-7.5
240
4
47
97
0.35
0.9
±
±
±
±
61
30
Feed
Concn.
(mg1±1)
Diatomaceous earth
0.21
174-270
0.5-4.4
±
SIRAN
Diatomaceous earth
Granular activated
carbon
Sand
1.5-2
0.43-0.61
15000
±
5.9-32.3
5.9-12
0.4
0.6
330-570
4400-8800
±
±
0.3
0.3-0.45
±
CH4/biogas
composition
Biogas
rate
(m3kg±
1COD) (%)
Diameter
d (mm)
Type
EMH
11,2
102
Table I.
Studies of anaerobic
fluidized bed treatment
on the laboratory/pilot
scale
Reactor
0.1-1.2
±
40
Glass
Biolite
Granular
Pozzolana
Glass
Activated carbon
±
0.425-0.61
0.18
±
12000
2500
2500
5000 (TOC)
2550-11070 (TOC)
100-200
0.001
0.001
±
35
Sand
Glass beads R-633
0.6
0.0065
16000
5000 (TOC)
19.2
6 (TOC)
±
21
0.55
0.002
75
Sand
0.6
±
0.003
0.003
±
15
Saponite
Sand
0.4
2.15
0.246
±
Micro-carrier
0.16
3m
0.06
40
Sand
±
±
0.02
±
Sand
FB
0.03
1.70
0.001
±
FB
0.035
1.0
±
FB
FB
±
0.08
±
1.0
0.001
510±3
Methane
(CH4)
Others
(CO2+H2S)
(%)
Ref.
nos
0.2
1600
4-24
50
35
7.0
5-68
75
0.365
±
±
11
0.5
0.7-0.8
1950
5000
4.2
8-10
±
±
35
37
6.6-7.3
7.0
96-840
12
92
97
0.25
±
±
84
±
16
50
43
0.075-0.1
11300-29600
5-13
±
30-35
7.4-7.8
24-1944
94-98
0.33
75.2-80.8
19.2-24.8
70
0.5
90000
8-14
30
35
6.8-7.4
4.8-33
75
0.35
78-88
12-22
2
0.1-0.3
2000-3500
58
15-17
37
7.0
1.5
±
±
±
±
22
Granular activated
carbon
0.59-0.84
750-2250
±
±
35
8.0
1.2-7.6
75
0.22
60-42
40-58
13
±
Activated carbon
0.46-0.59
288000-317000
10-31
±
28-35
7.10
3-12
65
0.25
80.8
19.2
64
±
33
Saponite
Perlite
0.4-0.8
0.968
5100
12500
0.0025
3.27-5.75
±
8.64
35
35
7-7.6
6.5-7.0
60-300
20-32
97.7-99.2
85-90
1.95
0.0026
±
±
±
±
9
10
Notes: FB = fluidized bed; EB = expanded bed; VFB = volume of fluidized bed; T = temperature; Vsup - superficial velocity; = hydraulic retention time;
a
= fermentation wastewater
Biomass and methanogenic activity
The biomass development and attachment were considered to be vital factors
in determining the performance efficiency of fluidized bed reactors.
Biofilm development and biomass formation
The identification and characterization of methanogenic flora were considered
to be the first step in assessing the biomass activity. The composition and
distribution of methanogenic flora in a fluidized sand bed biofilm ANITRON
reactor were studied (Kobayashi et al., 1988) by immunological methods, as
well as by phase-contrast and scanning electron microscopy. Experimental
observations revealed the presence of Methanothrix and Methanosarcinae
along with reference organisms viz. Methanobacterium formicicum and
Methanosarcina barkeri, prominent at the top of the reactor where turbulence
and shear were low. The other methanogens detected were scarce. They were
not major acetate utilizers and hence probably play a small role in methane
production in the reactors. However, the role and variations of these minority
populations of methanogens remain to be determined.
An experimental study on the effect of operating variables on biofilm
formation (Sreekrishnan et al., 1991) demonstrated that initial circulation of
inoculum through the sand bed followed by continuous substrate feeding
resulted only in floc formation at low dilution rates (0.1 to 0.2 h±1). However,
biofilm development to a small extent took place at higher dilution rate (0.6 h±1).
It was also observed that higher dilution rates enhanced the biofilm formation. It
has been shown that the sludge resulting from the breakdown of glucose would
have 64 per cent acidifying bacteria and 36 per cent methanogenic bacteria.
Under identical operating conditions, inoculum having higher methane
producing capacity (thereby higher number of methanogenic bacteria) resulted
in a large number of film-covered particles with more uniform film thickness
and stable operation of the bioreactor. An experimental study (Morgan et al.,
1996) conducted using ice-cream waste water revealed that all microbial aspects
of sludge, including the number of methanogens, non-methanogens, acidogen,
the enzymatic activity and the structure of biomass remained similar in the four
reactors (UASB, upflow filter, anaerobic fluidized bed and contact process)
during start-up. It has been shown (Gorris et al., 1988; 1989) to be possible that
biofilm development in anaerobic fluidized bed reactor could be influenced by
wastewater composition and type of inoculum. The influence of volatile fatty
acid composition of wastewater on biofilm development was studied. Scanning
electron microscopic examination revealed that immobilization of bacteria on
the sand initially occurred in crevices and that thereupon the structure of sand
particles became colonized. At steady state, microbial population showed
different species of hydrogenotrophic and acetotrophic types. In the cases of
inocula studied for biofilm development, they indicated no difference in the
bacterial composition of biofilms formed during start-up and steady state
conditions.
Anaerobic
fluidized bed
reactors
103
EMH
11,2
104
A further experimental study (Lauwers et al., 1990) conducted with sand as
carrier and synthetic wastewater as feed, revealed the early phase of biofilm
development during start-up of anaerobic fluidized bed process. The results
indicated that facultative anaerobic bacteria were abundantly present in the
outermost biofilm layers of mature sludge granules equivalent to main primary
colonizers of the sand. Microscopic examination of biofilm indicated the
presence of Methanothrix species in primary colonizers.
The effect of biomass accumulation and biomass loss on liquid superficial
velocities, substrate utilization rate and efficiency of anaerobic fluidized bed
process has been evaluated based on experimental results of Calderon et al.
(1998) and Shieh and Hsu (1996). Experimental results obtained for perlite
particles as carrier showed intrinsic relationship between terminal velocities
and bed expansion of up flow and down flow fluidized beds. Terminal
velocities of particles at different biofilm thicknesses calculated from
experimental bed expansion data, were found to be much smaller than those
obtained when Cd (drag coefficient) was determined from the standard drag
curve. This difference was explained by the fact that free rising particles do not
obey Newton's law for free settling. Terminal settling velocities were observed
to be 65 m h±1 for uncoated particles and 27 m h±1 for 72 m biofilm particles.
Wide range of terminal velocities explained the fact that particles are not
homogeneous in density due to irregular shape. The phenomena of biomass
loss during reactor start-up and steady state operation of an anaerobic fluidized
bed reactor using porous media particles fed with acetic acid were evaluated.
The biomass loss rate during reactor start-up was found to be correlated to
both substrate utilization and biogas production. However, the amount of
biomass lost did not impede the progress of reactor start-up and a rapid
building of attached biomass in the reactor was attainable.
Microbial population dynamics in biolayers
The biomass activity in a fluidized bed reactor was considered as a dynamic
system involving an equilibrium between growth and biofilm shearing. As
mentioned earlier, experimental studies revealed the biofilm development and
biomass formation by methanogenic species surviving under various
environmental conditions such as pH, temperature, substrate, turbulence, shear
and wastewater composition.
A kinetic study (Kuba et al., 1990) conducted on fluidized bed reactor with
synthetic zeolite as support media and acetic, propionic and butyric acids as
substrate showed a good treatment efficiency upto a volumetric loading rate of
4kg COD m±3 day±1. The changes of volatile fatty acids concentrations with
time were clearly expressed with the Monod's growth model in both batch
experiments using attached biomass in the bed and detached biomass from the
support media.
The biofilm development and population dynamics have been identified
(Woolersheim et al., 1989; Zellner et al., 1991; Araki and Harada, 1994) in terms
of syntrophic consortium consisting of syntrophosporasp., Methanothrix
soehngenii, Methano sarcina mazei and Methano brevibacter arboriphilus. The
immunological analysis also showed that the organisms isolated from the
butyrate degrading culture could be used as source of inoculum mainly
consisting of Methanothrix soehngenil opfikon, Methano bacterium formicicum
MF and Methano spirillum hungatei JF1. The biofilm thickness for different
upflow velocities (4-25 m h±1) was evaluated to assess methanogenic activity.
The dynamics and reaction kinetics with respect to distributed fraction of
methanogens have been demonstrated by Wu et al. (1998). A new kinetic model
was developed by combining removal efficiencies of conventional and tapered
fluidized bed reactors (CFB, TFBs) were in good agreement with the
experimental results. The biofilm thickness in TFBs was thicker than that in
CFB, resulting in performance enhancement with TFBs. The simulated results
from the kinetic model showed that methanogenesis was the rate limiting step
of degradation of simple organic compound (sucrose) and the COD
concentration in the effluent was mainly contributed by the intermediates of
volatile fatty acids. The distributed fractions of acidogens and methanogens
experimentally found to be 0.4 and 0.6 respectively.
Process performance, stability and inhibition
The process stability of an anaerobic degradation process could be sustained
due to balanced magnitude of COD concentration, waste water flow rate, pH
and temperature during steady state operation. It was quite obvious from a
laboratory study (Denac and Dunn, 1998) that packed bed and fluidized bed
anaerobic reactors operated with molasses and whey feeds showed variations
in performance due to measured variables such as COD loading rate, pH,
temperature, rate of organic acid production, gas composition and rates. The
performance of degradation with respect to molasses as feed indicated that the
rate of degradation in packed bed and fluidized bed reactors were found to be
the same in the strongly substrate limited range and further increase in
fluidized bed was observed with a maximum, at concentration of 3 g COD l±1.
This was probably due to the fact that at higher reaction rates (higher reactor
concentrations), the diffusion limitations in the packed bed reactor started to
play a role in the overall process.
The process performance and inhibition on butyrate degradation in a
fluidized bed reactor were experimentally evaluated (Labib et al., 1992) with
respect to increased concentrations of hydrogen and acetate. The COD removal
efficiency was found to be greater than 97 per cent with a specific loading rate
of 8-10g of COD g±1 of volatile solid per day and a volumetric loading rate of
10g of COD±1 day±1. Most of the time the gas phase hydrogen partial pressure
in the reactor was between 50 and 100 m g l±1. When formate was fed to the
reactor, H2± utilizing methanogens had a very high capacity to produce
methane (15ml min ±1 or 19.4g of COD l±1 day±1). At steady state during
butyrate as feed, methane production from hydrogen was 1.5ml min±1 or 1.9g of
COD l±1 day±1, the hydrogen seemed to have a large and under-utilized capacity
Anaerobic
fluidized bed
reactors
105
EMH
11,2
106
at steady state. Butyrate oxidation can be inhibited by increase in the butyrate
concentration, with complete inhibition when the butyrate concentration was
greater than 5000mg of COD l±1.
The performance of anaerobic fluidized bed treatment on waste activated
sludge (Huang et al., 1989) has given better insight on the degree of sludge
solubilization. At 35oC, anaerobic fluidized bed digesters were able to provide
an adequate degree of biological sludge stabilization within a hydraulic
retention time of 1-10 days, depending on the extent of initial sludge
solubilization.
Biogas hydrogen content as a control parameter was evaluated (Guwy et al.,
1997) with respect to operational conditions of an anaerobic fluidized bed
reactor fed with synthetic baker's yeast wastewater. Step overloads produced a
sharp peak in biogas hydrogen level measured on line, e.g. an increase of
loading rate from 40-63kg COD m±3 day±1, increased hydrogen concentration
from 290-640mg l±1 within three hours. However, switching from an older,
partly acidified batch to a fresh batch of feed at constant COD gave a marked
increase in the biogas hydrogen content from 200-800mg l±1.
It has been shown (Wu and Huang, 1995) to be possible that the performance
of anaerobic fluidized bed reactor could be enhanced by changing geometrical
configuration to a tapered form. Bead shaped activated carbon was used with
synthetic wastewater of acetate concentration, 2550mg l±1. Tapered angles
tested for the study included 2.5o, 5o and 10o. The substrate removal efficiencies
of tapered fluidized beds were found to be significantly higher than those of
conventional fluidized bed reactors verified through kinetic model and
experimental study. The experimental substrate removal efficiencies
corresponding to tapered angles of 5o and 10o were found to be same and higher
than at 2.5o. Therefore, it was concluded from the study that the performance of
the conventional fluidized bed reactor could be significantly enhanced by
changing the geometrical configuration to a tapered form.
The feasibility of an expanded granular sludge bed (EGSB) system for the
treatment of malting wastewater under psychrophilic conditions was
investigated (Rebac et al., 1997) in a temperature range from 13-20oC. The COD
of malting wastewater used in the study was estimated to 1436mg l±1. During
the reactor operation at 16oC, the COD removal efficiency was found to be 56
per cent at organic loading rate of 8.8kg COD m-3 day±1 and at HRT of 2.4
hours. At 20oC, removal efficiency attained 72 per cent at OLR of 14.6 COD m±3
day±1 corresponding to HRT of 1.5 hours. These findings indicated that
anaerobic treatment of low strength wastewater at low temperature might
become a feasible option.
The metallic pathway for anaerobic degradation of chlorophenol and
nitrophenol (Tseng et al., 1994a, b; Khodadoust et al., 1997) as inhibitor has
been identified and quantitatively evaluated for fluidized bed process. The
removal efficiency of pentachlorophenol reached 99.9 per cent. The
experimental results concluded that chloride or nitrogen at para-position was
found to be more toxic than ortho and meta positions. The response of fluidized
bed process in terms of stability was studied for shock loadings (organic,
hydraulic, pH and temperature) (Borja and Banks, 1995) and toxic shocks on
carriers (Mol et al., 1993) using ice-cream and brewery wastewater.
Wastewater treatment in anaerobic fluidized bed reactors
A summary of results of laboratory and pilot scale studies on anaerobic
fluidized bed reactors are summarised in tabular form (Table I). A remarkable
variation in COD loading of 10-50kg COD m±3 day±1, temperature of 10-35oC
and COD removal of 70 per cent-98 per cent were reported for industrial
wastewater in general.
An anaerobic fluidized bed treatment using brewery wastes carried out in a
laboratory scale (Ozturk et al., 1989) resulted in a COD removal efficiency of
greater than 75 per cent at OLR of 9.5kg COD m±3 day±1 for a period of 82 days
from start-up. A COD to methane conversion of 87 per cent was achieved. The
observed methane yield reached a maximum value of 0.34-0.35m3 CH4 kg±1
COD removed. Experimental results have suggested that the COD removal
efficiency varied as a function of COD loading and neither the bed COD nor
HRT alone significantly affected the performance of the reactor. It was
observed that the distribution of the biomass hold-up near the top of the reactor
might have reached concentrations of greater than 20,000 mg l±1.
A pilot scale study (Anderson et al., 1990) conducted on brewery wastewater
as feed showed a COD removal efficiency of more than 75 per cent at an organic
loading rate of 8.9kg COD m±3 day±1 for less than 82 days from start-up. About
340 litres of methane was produced per kg of COD removed. This represented
87 per cent recovery of energy value from the waste treated. The steady-state
biomass hold-up in the AFBR was strongly dependent on the COD loading
applied. The Monod's kinetic parameters were determined using steady state
operating data and compared to experimental results.
An expanded micro-carrier bed process was used (Yoda et al., 1991) to
evaluate the COD removal efficiency of brewery's yeast processing wastewater
in which a COD removal efficiency of 97-99 per cent was achieved at OLR of 1324kg COD m±3 day±1. It was found that cobal and nickel were insufficient in the
yeast processing wastewater and were added externally to enhance the growth
of methanogenic bacteria during start-up of the reactor. It was clearly
demonstrated in a full-scale installation that the microcarrier bed process could
provide a reliable and predictable way to cultivate granular sludge necessary
for efficient anaerobic treatment.
A pilot scale performance of upflow anaerobic sludge blanket reactor
(UASB) and fluidized bed reactor (FBR) has been compared (Iza et al., 1990) for
the treatment of sugar beet wastewater. The COD removal efficiency was found
to be greater than 90 per cent in both cases and the UASB reactor was able to
achieve efficiencies greater than 98 per cent with a better quality effluent (COD
less than 100 mg l±1). Biomass washed out from both reactors was easily
settleable and could be separated with a simple lagoon. Foam formation
especially in FBR, made separation more difficult. The FBR was operated even
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at hydraulic retention times shorter than two hours, whereas UASB was
limited to 12 hours, the FBR was shown to achieve OLR greater than 30kg COD
m±3 day±1.
The feasibility of treatment of monosodium glutamate fermentation
wastewater was evaluated (Tseng and Lin, 1990) in terms of removal efficiency
and methane content in the biogas. A BOD removal efficiency of 90 per cent
was attained with a methane content of 80.8 per cent and OLR of 10.1-31.1kg
COD m±3 day±1.
Laboratory studies (Gommers et al., 1988a, b) carried out using a
denitrifying fluidized bed reactor revealed the possibility of simultaneous
oxidation of sulphide and acetate during start-up and steady state. Sulphide (23kg S m±3 day±1, acetate (4-6kg C m±3 day±1) and nitrate (5kg N m±3 day±1) were
effectively removed. The oxidation of elemental sulphur, an intermediate of
sulphide oxidation to sulphate was the rate-limiting step in both the activity
measurements and the reactor.
The anaerobic expanded micro-carrier bed process has been shown(Yoda et
al., 1989) to be feasible for the cultivation of granular sludge similar to that
formed in the UASB process.
Matsumoto and Noike (1991) and Converti et al. (1993) have demonstrated
the start-up and steady state performance of a fluidized bed process in terms of
substrate composition and organic loading rate by varying the influent flow
rate to the reactor. Under operating conditions of COD loading 5.8-108kg m±3
day±1, hydraulic retention times of 0.45-8h, the equilibrium biomass hold-ups
increased with increasing COD loading and varied from 15,000-32,000mg VSS
l±1. Yeast extract or glucose with mixed acid substrate showed better removal
efficiencies than acetic acid alone as substrate. Degradation efficiency and
methane production rate were shown to be substantially affected by an
increase in organic loading rate from 4-24kg COD m±3 day±1. Hence an
operational value in terms of maximum theoretical specific degradation rate,
1.76kg CODr kg±1 VSS day±1 has been calculated using Monod's kinetic model.
The feasibility of anaerobic expanded bed process was experimented (Kato
et al., 1994; Alderman 1998) for the treatment of low strength wastewater
including municipal wastewater up to a COD concentration of 700mg l±1.
Sludge hold-up was found to be feasible within a narrow range of operational
conditions in an expanded granular sludge bed (EGSB) reactor. Within optimal
hydraulic loading range of 2.5-5.5m h ±1, there was no problem of piston
formation nor sludge washout as long as OLR was below 7kg CODm±3 day±1.
However, in case of higher OLR requirement, up to 12kg COD m±3 day±1,
special attention should be given to the sludge washout. It has been concluded
that use of a gas-liquid-solid (GLS) device or an improved type of immobilized
biomass for high hydraulic loads would satisfy the constraint in the
applicability of EGSB reactors. As a pre-treatment option for small organically
overloaded wastewater treatment facilities, the anaerobic expanded bed reactor
(AEBR) can compete with other conventional remedial methods when the
degree of overloading exceeds approximately 1.3-1.75 times the design loading,
with the cost effectiveness of the AFBR unit increasing as the degree of organic
over loading increases.
Fluidized bed treatment for municipal wastewater has been established
(Sanz and Polanco, 1990) at low temperature with BOD load as the operating
parameter.
Similarly another attempt has been made (Perez et al., 1997) to explore the
feasibility of pre-treatment of concentrated wastewater such as wine distillery
and vinasses in thermophilic range. Experimentally, it was confirmed that
anaerobic fluidized bed systems could achieve greater than 82.5 per cent of
COD reduction at a COD loading of 32.3kg COD m±3 day±1 at temperature of
55oC. At HRT of 0.46 day, the volumetric rate of methane generation was 5.8 m3
of CH4 m±3 day±1 with a methane yield of 0.33 m3 CH4 kg±1 of COD removal.
The greatest efficiency of substrate removal was 97 per cent for OLR of 5.9kg
of COD m3 day±1 and HRT of 2.5 days. It has been shown (Collivignarelli et al.,
1991) to be possible to remove organic and nutrient substances from municipal
wastewater at affordable cost. UASB, aerobic fixed bed-reactor and anoxic
fluidized bed reactor were compared for technical and economical feasibility. It
was concluded that economical advantages in terms of operating cost for
various reactors have to be optimized with respect to plant engineering factors.
The application of anaerobic fluidized bed technology has been shown (Chen
et al., 1998; Martin et al., 1993) to achieve considerable removal efficiency in the
treatment of tannery wastewater and fermented olive mill wastewater. More
than 75 per cent of COD reduction was achieved for tannery wastewater up to
an F/M ratio at 0.4g COD g±1 TVS day±1 with a mean cell residence time of 40
days. The observed methane production rate was 0.22 m3 of CH4 kg±1 COD
removed, which was constant over the range of F/M ratio applied. Reactor
biomass concentrations ranging from 17-22g TVS l±1 were achieved for tannery
wastewater and the COD removal efficiency for olive mill wastewater was
evaluated to 92 per cent at HRT of 4-35 days. The results of experiments were
fitted to Michaelis-Menten equation to describe substrate uptake pattern. Pretreatment of olive mill wastewater was found to increase the rate of substrate
uptake by a factor of 3.2 when compared to untreated olive wastewater.
Recent experimental studies by Sreekrishnan et al. (1996) and Chen et al.
(1997) have proved the technical feasibility of anaerobic fluidized bed reactors
for treating yeast industry wastewater and hog wastewater. Experimental
results of yeast industry wastewater treatment revealed that COD reduction
was restricted by the acidification step of the overall reaction which in turn
attributed to methanogenic step of overall process. Separating the acidification
and methanation stages improved the performance but not to any considerable
extent. Anaerobic fluidized bed treatment of hog wastewater resulted in an
efficiency of filtered BOD5 to greater than 80 per cent and unfiltered BOD5 to
more than 60 per cent at volumetric loadings as high as 10.4kg COD m±3 day±1.
Large quantity of alkalinity present in hog wastewater supplemented the
alkalinity requirement for effective digestion. The specific methane yield was
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found to vary from 43-71 per cent of theoretical value and decreased with
increasing loading. However, the biogas produced had high methane content
(more than 70 per cent) at organic loading rate less than 4kg COD m±3 day±1.
An innovation (Hong et al., 1997) on the performance of anaerobic reactors
revealed the possibility of removal of total organic carbon at high efficiency
under pseudo-steady-state operation. Comparative evaluation of two anaerobic
fluidized bed reactors, a packed bed anaerobic reactor, and suspended growth
anaerobic reactor fed with glucose as main carbon source, demonstrated that
anaerobic fluidized bed reactors were less affected by interruptions and
adverse operating conditions than others. The immobilized media
(diatomaceous earth and activated carbon) provided in AFBRs were found to
have better performance than others due to high cell retention ability with
specific biogas production up to 1.71g±1 of TOC against 1.31g±1 of TOC for
packed and suspended growth reactors. High TOC removal efficiencies were
achievable under pseudo state operation. A consistent methane content in
biogas was observed to be 52.5-55.9 per cent. Biomass concentration of AFBRs
reached a maximum of 91g VS l±1 compared to 21g VS l ±1 achieved in packed
bed reactor. Extremely high biomass concentrations in AFBRs were possible
due to the high specific surface area available.
In recent years, the possibility of treating propellant wastewater containing
nitrotoluene and diaminotoluene has been attempted (Maloney et al., 1995;
1998) successfully using granular activated carbon as carrier.
Combined treatment of wastewater using co-digestion concept has been tried
(Kadam et al., 1998) for the treatment of distillery and pharmaceutical effluents
using upflow fluidized bed reactor.
Reactor configurations and full-scale anaerobic treatment systems
Several new reactor configurations were shown to be possible, one or two stage
configurations EGSB (expanded granular sludge blanket) reactor and
anaerobic/aerobic treatment using fluidized bed and airlift suspension reactors,
for treatment of concentrated effluent. Based on the excellent laboratory pilot
performance of anaerobic fluidized bed process, the construction of full-scale
reactors began in the early 1980s. The Ecolotrol HY-FLO reactor, Dorr-oliver
Anytron process and Gist-brocades are some of the examples of full-scale
reactors located in Europe.
The application of immobilization in fluidized bed process has been
investigated (Heijnen et al., 1990) to compare the relative performance of
fluidized bed process with conventional high rate digesters.
An updated list of distribution of anaerobic reactors including fluidized/
expanded bed, countrywide in Europe was compiled in a European Committee
(EC) survey report (Wheatley et al., 1997).
Kinetics and modelling in anaerobic fluidized bed reactors
Several researchers have attempted to evaluate the performance of anaerobic
fluidized process in terms of reaction kinetics and mass transfer limitations.
The substrate consumption kinetics and mass transfer limitations have been
predicted (Buffiere et al., 1995a; Motta and Cascante, 1996) based on dynamic
status of methanogenic biofilm. The experimental data were classified
according to the value of influent flow rate, and a set of data corresponding to
flow rates between 35.44 and 41.83 ml min±1 was selected for model testing.
The test resulted in a reasonable agreement between the zero-order kinetic
model (with complete substrate penetration) and the experimental data. Mass
transfer limitation on single and multisubstrate were explained based on
reaction scheme of molecular diffusion process. Effectiveness factor
calculations were performed in steady state for each bacterial group taken into
account in the process. Biofilm size and thickness were separately compared for
acidogenic and methanogenic phases. This type of comprehensive modelling
may also be extended to complex systems, involving several substrates and
group of organisms.
A few studies (Kuba et al., 1990; Martin et al., 1993; Labib et al., 1993;
Buffiere et al., 1995b; Wu et al., 1998) carried out using fluidized bed process
demonstrated that the attached biomass, methanogenesis, organic loading rate
and residence times could affect the kinetics of anaerobic fluidized bed process.
A model with anaerobic butyrate-degrading consortia was developed using
Monod's kinetics for H2/CO2 and acetate as substrates. The Monod's model was
modified for butyrate oxidation to incorporate inhibition by acetate and
hydrogen and the effect of a thermodynamic driving force. The bacterial
growth yield was made dependent on the concentrations of reactants and
products. The stability of butyrate-degrading consortia was observed to be
more sensitive to acetate loadings than to hydrogen. High acetate
concentrations resulted in loss of butyrate oxidizing biomass and acid
accumulation, leading to a pH drop and subsequent process failure. High solid
retention times substantially overload and reduce alkalinity requirements for
control of pH. The hydraulic residence time and organic loading rate were
experimentally observed to affect the anaerobic digestion of wine vinasse
subjected to fluidized bed process. The regions of high removal, thus illustrate
that when organic loading was too small, a small part of biomass was inactive.
Specific activity values justified better activity of biomass for a biofilm of small
area and thickness.
The effect of solid retention time, substrate utilization and gas production on
the kinetics of anaerobic fluidized bed process was evaluated by Ray et al.
(1989) using waste activated sludge as feed. In the anaerobic digestion of waste
activated sludge, the kinetics of process was modelled by assuming that the
initial gas-production rate was proportional to the soluble biodegradable
substrate. The model confirmed that the rate-limiting step in the digestion
process was the conversion of a particulate biomass into a soluble substrate
rather than the conversion of soluble organic to acetate or acetate to methane.
A practical approach was introduced (Furumai et al., 1991) to use active
biomass concentration and surface coverage of biofilm as indices for predicting
the kinetic behaviour of anaerobic fluidized bed process. Experimental
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observation on synthetic wastewater revealed that the overall growth yield of
acetogenic and methanogenic bacterial groups reached a value of 0.05 (COD
cell/COD substrate) based on COD conversion in steady state condition. Active
biomass was estimated using maximum substrate consumption rate and
overall rate constant of biomass loss due to decay and attrition was estimated
to be 0.09 day±1 based on the active biomass.
A recent innovation from experimental studies (Csikor et al., 1994; Huang
and Wu, 1996) have shown widened horizon for hydrodynamic behaviour of
biomass, investigated respectively for fluidization of biofilm-coated particle
and dissipation of specific energy rate from biofilms in anaerobic fluidized bed
reactors. As the terminal settling Reynolds number was found to be
independent of characteristics of particles, a new approach was developed to
describe the fluidization of biofilm coated particle. The model was based on two
new parameters : the expansion coefficient and the specific occupied particle
volume at zero flow, which are readily determinable and characteristic
parameters of the fluidized particles, being independent of reactor size, shape,
liquid velocity and quantity of carrier particles. The model was found to be
suitable for modelling bed porosity or biomass concentration as a function of
biofilm thickness and upflow liquid velocity. It was concluded from the study
that there could be an optimal biofilm thickness above which not only could the
diffusion limitation increase, but the overall biomass concentration decreases at
a given liquid velocity. The experimental results showed that dissipation rate
varied with operating flow rate and expansion characteristics, found to the
inversely proportional to thickness of biofilm. It was concluded from the
observations that dissipation rates could be a very powerful tool for studying
the erosion effect at the biofilm surface and steady state biofilm thickness
distribution in conventional and tapered fluidized-bed bioreactors.
A further extention of kinetic behaviour (Borja and Banks, 1994; Prakash
and Kennedy, 1996) was also been verified using light carriers such as saponite
and biolite. An anaerobic bioreactor (1.251 volume) operating at 35oC,
containing saponite-immobilized biomass, worked satisfactorily over hydraulic
retention times from 2.5-12.5 days, and removed 97 per cent of initial COD.
Guiot's kinetic model was used to determine the macro-energetic parameters of
the system and reported a yield coefficient of 0.09g VSS g±1, COD and a specific
rate of substrate uptake for cell maintenance of 0.035g COD g±1 VSS day±1. The
experimental results also showed the rate of substrate uptake correlated with
the concentration of biodegradable substrate through an equation of MichaelisMenten type. The biolite used as carrier in anaerobic fluidized bed process
showed rapid start-up and good steady state performance with favourable
density, size and surface properties. Both maximum efficiency profile (MEP)
and maximum load profile (MLP) start-up techniques led to the development of
healthy anaerobic biofilm in a period of five weeks, capable of treating more
than 90 per cent of the influent waste at a loading rate of 2.5kg COD m±3 day±1
and hydraulic retention time of one day. It was evident from experimental
results that biolite could accumulate large amounts of biomass (up to 220mg
volatile biofilm solids g±1 biolite) resulting in high removal efficiencies at high
organic loading rates.
Conclusion
A summary of results of laboratory and pilot scale studies extracted from
extensive literature survey are presented in tabular form (Table I). From the
extensive literature survey, it is clear that anaerobic fluidized bed process has
become an established means for the treatment of concentrated industrial
effluents. A successful implementation of full-scale systems has proven the
technical feasibility of this relatively recent process. From the critical review of
experimental results, it is evident that the treatment efficiency can go up to 98
per cent with COD loading rate of 50kg COD m±3 day±1, compared at higher
limit to
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Application of anaerobic
fluidized bed reactors in
wastewater treatment: a
review
R. Saravanane and D.V.S. Murthy
Anaerobic
fluidized bed
reactors
97
Received 3 April 1999
Accepted 10 July 1999
Indian Institute of Technology (IIT) Madras, India
Keywords Waste, Water industry, Reactors
Abstract During the past ten years, anaerobic process has become a popular technology for
treating concentrated effluents. Research and development programmes led by both engineers
and microbiologists have resulted in a better understanding of the microbiology of anaerobic
reactions and reactor design for anaerobic processes. Considerable progress has been achieved in
the development of high rate anaerobic reactors with several configurations for treating
concentrated industrial effluents. In this review, attention is paid to highlighting the conceptual
and full scale developments of anaerobic fluidized bed reactors, in respect of process performance,
design concepts, start-up of the reactor, stability of the system with respect to various operating
parameters, reactor configurations, comparison with competing reactor designs for concentrated
industrial effluents and kinetics and modelling of reactor systems.
Introduction
Anaerobic technology for the treatment of wastes and wastewater has been
known since the beginning of the twentieth century. The initial applications
were for sewage sludge stabilization. Although anaerobic treatment process is
inherently advantageous (no oxygen consumption, low sludge yield and CH4
production) the process has not been successfully implemented owing to
classical disadvantages like low sludge activity, low reactor capacity,
unsuitability of the process and inhibitory effects. However, with the
advancement in microbiology and environmental biotechnology, these
disadvantages have been overcome during the past decade. In addition, a
number of reactor configurations have been developed leading to high biomass
concentrations such as upflow anaerobic sludge blanket (UASB) reactor,
anaerobic contact filter, down flow stationary fixed film and fluidized bed
systems.
In India, UASB and filter reactors were successfully implemented for the
treatment of concentrated industrial effluents and fluidized bed systems were
at laboratory and pilot scale study. In turn, developed countries have
implemented a few full-scale fluidized bed reactors in industrial effluent
treatment plants.
The objective of this paper is to review and critically analyse the process
parameters of anaerobic fluidized bed reactor with a view to their possible
application in industrial effluent treatment. This review may motivate
researchers and engineers to apply and develop new configurations, enabling a
Environmental Management and
Health, Vol. 11 No. 2, 2000,
pp. 97-117. # MCB University
Press, 0956-6163
EMH
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better understanding of the mechanisms governing the efficiency of
anaerobically treated industrial effluent plants.
Conceptual outline of the anaerobic fluidized bed process
The performance of biological fluidized bed reactors was first recognized for
aerobic carbon oxidation and denitrification. Research in the historical
development of fluidized bed technology has been reported by several
groups(Heijnen et al., 1989). In a fluidized bed reactor, fine carrier particles are
used for the microbial film development. These particles with entrapped
biofilm are fluidized by high upflow fluid velocities generated by a combination
of the influent and recirculated effluents. A reactor is said to be an expanded
bed when the resulting expansion is up to 30 per cent and fluidized bed
(Grasius et al., 1997) when it is more than 30 per cent. A conceptual outline of a
fluidized bed reactor is shown for illustration (Figure 1).
Advantages and disadvantages of anaerobic fluidized bed process
The fluidized bed process claims various potential advantages over other high
rate anaerobic reactors such as upflow anaerobic sludge blanket (UASB)
reactor, filter reactors and downflow stationary fixed film reactor (DSFF).
Figure 1.
Fluidized bed reactor
These are: high sludge activity, high treatment efficiency, no clogging of
reactors, no problems of sludge retention, least chance for organic shock loads
and gas hold up as well as small area requirements.
Some of the reviews (Heijnen et al., 1989, Schugerl, 1989) demonstrated that
the majority of these disadvantages have been overcome in recent years, which
were:
.
Problems due to long start-up times due to biolayer formation on the
carrier.
.
Difficulties due to control of biolayer thickness.
.
High-energy consumption due to very high liquid recirculation ratio.
.
High investment cost for liquid distribution to obtain uniform
fluidization especially in a large-scale plant.
Anaerobic fluidized bed process characteristics
The performance evaluation of any anaerobic treatment process necessitates a
detailed study of start-up of process, biomass and biolayer formation, microbial
population dynamics, process stability with respect to shock loads and
inhibition, types of wastewater which can be treated and various reactor
configurations.
Start-up of anaerobic fluidized bed process
The start-up of anaerobic fluidized bed process is initiated by the development
of biomass and subsequent attachment to carriers. A review (Hickey et al.,1991)
on the start-up of high rate anaerobic treatment systems reported the
development of biofilms due to the influencing parameters such as liquid flux
rate, scale of the reactor, gas flux and organic loading rate. Shear at both macro
and micro scales were found to influence the biofilm thickness. The macro-scale
effects were characterised by liquid flux rate and Reynolds number, whereas
microscale effects by abrasion from particle-particle interaction, gas flux rate,
reactor height and bed height and influence of micro-regions of higher shear
induced by the distribution network. The biofilm thickness was found to vary
directly with respect to the rate of organic loading.
The experiments (Heppner et al., 1992; Hsu and Shiek, 1993; Shieh and Hsu,
1996) conducted respectively on anaerobic fluidized bed reactors with
propionate and acetic acid as substrates revealed the start-up performance.
After 24 days of batch operation with propionate as the medium, the reactor
was operated for eight months with organic loading rate of 8.5kg COD m±3
day±1, yielding a maximum gas production of 4.5m 3 m±3 day±1 at a removal
efficiency of 94-99 per cent. While Methanocorpusculum sp. and Methano
spirillum sp. adhered to the sand in the early phase of the start-up, the
consortium in the mature biofilm consisted of Desulfobulbus sp., Methanothrix
soehngenii and species of at least three different genera of hydrogenotrophic
methanogens. The reactors with acetic acid as substrate showed excellent total
organic carbon (TOC) removal (i.e. greater than 97 per cent at a feed
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concentration of 5000mg TOC l-1) and stable methane production, i.e. 0.901
CH4 g-1 of TOC removal, at the early stages of start-up process. The strategy
based on maximum substrate loading controlled by reactor pH significantly
shortens the start-up regime. In this case, the reactor attained steady state
conditions approximately 140 days after start-up. On the other hand, a start-up
time of 200 days was required when a strategy based on maximum substrate
utilization was adopted. The biomass loss in anaerobic fluidized bed reactor
was found independent with respect to start-up of the reactor. The
experimental studies (Gorris et al., 1988; Morgan, 1991; Liang et al., 1993; Araki
and Harada, 1994) conducted on brewery, synthetic wastewater and ice-cream
wastewater evaluated the efficiency of treatment and changing microbial
activities, leading to methanogenic biofilm formation during start-up of reactor.
The COD removal efficiency reached 85 per cent at volumetric loading of 2730kg COD m±3 day±1 with methane content up to 72 per cent in the biogas.
Upflow velocities (4-25 mh±1) caused a prominent difference in the pattern of
initial biofilm formation. Microbial activities with respect to acetate production,
hydrogen utilizing and acetate utilizing methanogenesis increased up to 3-4
times as that of suspended growth sludge. A comparative start-up performance
of anaerobic reactors including fluidized and expanded bed, was reported
(Balaguer et al., 1997) for high strength wastewater at 37oC and for different
support materials.
Inoculation
A number of different inoculum sources have been used to seed fluidized beds
for low and high strength wastewater, primarily screened sludge. Supernatant
from municipal or animal manure digesters have been investigated (Hickey et
al., 1991) for starch-based food processing waste, chemical waste and soft drink
bottling waste.
The influence of seeding conditions (Ehlinger et al., 1989) on the initial
biofilm development during the start-up of the reactor fed with synthetic
protein wastewater was also reported. The efficiency of the reactor and the
composition of biofilm changed with respect to composition and pH of the
inoculum. The seeding and start-up periods were found to be critical phases
resulting in physiological stress on the biomass. Adjustment of pH of seeding
from 7 to 8.5 increased efficiency of the process. The most active biofilm was
found at the bottom of the reactor, necessitating further research on the
uniform distribution of active biofilm in the entire volume of the reactor. It has
been shown to be possible (Gorris et al., 1989; Sreekrishnan et al., 1991; Zellner
et al., 1994) to degrade organic substrate by an anaerobic mixed culture
resulting in rapid oxidation and biofilm development during steady state
operation.
Carrier type and conditioning
A relative performance efficiency of anaerobic fluidized bed using various
carriers was reported by Hickey et al. (1991). Sand was used as a medium for
starch-based food processing wastes, chemical wastes, brewery waste, bakery
waste and paper mill foul condensate. Zeolite, sand and activated carbon were
used for the treatment of sewage resulting in percentage removal of COD from
27-60.
There were indications that the carrier particle diameter had some influence.
It has been found (Heijhen et al. 1989) that the start-up using sand of 0.35mm
diameter was much faster than that using sand of 0.75mm in diameter. The
reason was thought to be the lower liquid shear with smaller particles.
The experimental results from the investigation on the effects of microcarrier pore characteristics on methanogenic fluidized bed performance (Yee et
al., 1992) revealed that under similar start-up conditions, porous micro-carriers
obtained from diatomaceous earth were capable of reducing the start-up times
by more than 50 per cent as compared to sand. More than 90 per cent of total
reactor cell mass was immobilized on porous micro-carriers as opposed to 80
per cent on sand.
Consequently, porous micro-carriers were found to be conducive for better
proliferation of slow-growing methanogenic bacterial consortia. The
experimental data clearly indicated that surface area, total pore volume and
mean pore diameter should be used concomitantly to obtain better insight into
the cell retention capacity of a given porous micro-carrier.
It was evident from a study (Breitenbucher et al., 1990) that open-pore
sintered glass material (SIRAN) was an attractive support media for the
treatment of organically highly polluted wastewater. Microorganisms
attachment and biofilm formation was accelerated by the large surface area (up
to 90m2 m±3) of the carrier. Granules and beads have been proven in fluidized
bed reactor system, maintaining COD loading rates of 206kg m±3 day±1 by
treating evaporator condensates from pulp industry. Very short start-up
periods were followed by a stable operation. The high biomass concentration
was protected against washout and remains on the large inside surface area of
the SIRAN carriers. This resulted in maximum degradation rates at low
retention times and allows the construction of small and compact reactor
systems for the treatment of high strength industrial wastewater.
The fluidized and expanded bed anaerobic reactors showed (Fox et al., 1990;
Balaguer et al., 1997) relative efficiencies of different carriers viz., low-density
anthracite, granular activated carbon, sepiolite, pumice and sand for the
treatment of synthetic wastewater and vinasse. At steady state, granular
activated carbon (GAC) reactor retained 3.75-10 times the attached biomass
retained on the other media tested and GAC reactor accumulated biomass at a
faster rate during start-up. Shear losses reflected the biomass accumulation
with two sand sizes and anthracite media having shear loss coefficients 6-20
times greater than that of the GAC medium. Hence it was concluded that GAC
medium proved to be efficient for starting up of process, and sepiolite and
pumice for low energy consumption in a fluidized bed.
A summary of carriers studied for various industrial and synthetic effluents
is shown in Table I.
Anaerobic
fluidized bed
reactors
101
Type of
waste/
effluent
Municipal
wastewater
Distillery
effluent
Glucose
Hog wastewater
Glucose
Malting
wastewater
Yeast
wastewater
Sucrose
Vinasse
Acetic accid
Acetate
Ethanol
Yeast
wastewater
Acetic acid
Municipal
wastewater
Olive mill
wastewater
Butyrate
Brewery
wastewater
Brewery
wastewater
Acetate
propionate
Butyrate
Tannery
wastewater
Monosodium
glutamatea
Fruit
processing
wastewater
Glucose
Carrier
Type
Diameter
D (m)
Height
H (m)
Volume
(m3)
VFB/vR
(%)
EB
0.063
0.14
0.0004
15-20
FB
FB
0.065
0.051
1.70
0.76
0.006
0.033
25
36.0
FB
FB
0.1
0.045
2.3
0.9
0.018
0.001
100
±
EB
0.2
7.5
0.225
±
FB
FB
FB
FB
FB
EB
±
0.056
0.115
±
0.75
1.5
0.011
0.002
0.015
25
25
15-70
0.051
0.08-0.138
0.05
0.76
1.1-1.8
1.0
0.002
0.018
0.002
FB
FB
0.045
0.051
0.90
0.76
FB
0.05
FB
FB
0.1
±
EB
FB
FB
Operating conditions
COD
loading
rate
Vsup
(Kgm±3day±1)
(mh±1)
T (o/c)
PH
(h)
COD
removal
(%)
5-20
7-7.3
1-10
35-77
0.336-0.349
±
±
3
±
5.3-8.7
55
±
7.65-8.57
±
0.46-2.5
2.5-3.3
82.5-97
87-100
0.33
0.8-1.0
±
55.8
±
44.2
53
32
0.24-10.4
7.5-21
4.58
±
35
37
7.1-7.9
7.0
±
24
60-80
80-90
0.35
0.47
70
75
30
25
14
58
1436
8.8-14.6
6.0
13-20
6.5-7.5
2.4
66-72
0.3-0.6
±
±
55
27-33
25
20-32
33
10
6-15
37
35
35
6.9-7.1
7.0
6.7-7.0
8.7-10.2
4.8-72
0-4
75
80
70-75
68
70
±
32
30
±
24
54
8
7.5-32 (TOC)
18.0-401 (TOC)
7-39
14.5
±
2.5-5.5
35
35
30
6.75-7.5
7-8
7.0
±
0.6-3.4
0.09-2.1
99
97
80-97
±
±
±
0.91
±
±
±
±
±
±
±
±
59
67
40
37
35
7.0
6.75-7.5
240
4
47
97
0.35
0.9
±
±
±
±
61
30
Feed
Concn.
(mg1±1)
Diatomaceous earth
0.21
174-270
0.5-4.4
±
SIRAN
Diatomaceous earth
Granular activated
carbon
Sand
1.5-2
0.43-0.61
15000
±
5.9-32.3
5.9-12
0.4
0.6
330-570
4400-8800
±
±
0.3
0.3-0.45
±
CH4/biogas
composition
Biogas
rate
(m3kg±
1COD) (%)
Diameter
d (mm)
Type
EMH
11,2
102
Table I.
Studies of anaerobic
fluidized bed treatment
on the laboratory/pilot
scale
Reactor
0.1-1.2
±
40
Glass
Biolite
Granular
Pozzolana
Glass
Activated carbon
±
0.425-0.61
0.18
±
12000
2500
2500
5000 (TOC)
2550-11070 (TOC)
100-200
0.001
0.001
±
35
Sand
Glass beads R-633
0.6
0.0065
16000
5000 (TOC)
19.2
6 (TOC)
±
21
0.55
0.002
75
Sand
0.6
±
0.003
0.003
±
15
Saponite
Sand
0.4
2.15
0.246
±
Micro-carrier
0.16
3m
0.06
40
Sand
±
±
0.02
±
Sand
FB
0.03
1.70
0.001
±
FB
0.035
1.0
±
FB
FB
±
0.08
±
1.0
0.001
510±3
Methane
(CH4)
Others
(CO2+H2S)
(%)
Ref.
nos
0.2
1600
4-24
50
35
7.0
5-68
75
0.365
±
±
11
0.5
0.7-0.8
1950
5000
4.2
8-10
±
±
35
37
6.6-7.3
7.0
96-840
12
92
97
0.25
±
±
84
±
16
50
43
0.075-0.1
11300-29600
5-13
±
30-35
7.4-7.8
24-1944
94-98
0.33
75.2-80.8
19.2-24.8
70
0.5
90000
8-14
30
35
6.8-7.4
4.8-33
75
0.35
78-88
12-22
2
0.1-0.3
2000-3500
58
15-17
37
7.0
1.5
±
±
±
±
22
Granular activated
carbon
0.59-0.84
750-2250
±
±
35
8.0
1.2-7.6
75
0.22
60-42
40-58
13
±
Activated carbon
0.46-0.59
288000-317000
10-31
±
28-35
7.10
3-12
65
0.25
80.8
19.2
64
±
33
Saponite
Perlite
0.4-0.8
0.968
5100
12500
0.0025
3.27-5.75
±
8.64
35
35
7-7.6
6.5-7.0
60-300
20-32
97.7-99.2
85-90
1.95
0.0026
±
±
±
±
9
10
Notes: FB = fluidized bed; EB = expanded bed; VFB = volume of fluidized bed; T = temperature; Vsup - superficial velocity; = hydraulic retention time;
a
= fermentation wastewater
Biomass and methanogenic activity
The biomass development and attachment were considered to be vital factors
in determining the performance efficiency of fluidized bed reactors.
Biofilm development and biomass formation
The identification and characterization of methanogenic flora were considered
to be the first step in assessing the biomass activity. The composition and
distribution of methanogenic flora in a fluidized sand bed biofilm ANITRON
reactor were studied (Kobayashi et al., 1988) by immunological methods, as
well as by phase-contrast and scanning electron microscopy. Experimental
observations revealed the presence of Methanothrix and Methanosarcinae
along with reference organisms viz. Methanobacterium formicicum and
Methanosarcina barkeri, prominent at the top of the reactor where turbulence
and shear were low. The other methanogens detected were scarce. They were
not major acetate utilizers and hence probably play a small role in methane
production in the reactors. However, the role and variations of these minority
populations of methanogens remain to be determined.
An experimental study on the effect of operating variables on biofilm
formation (Sreekrishnan et al., 1991) demonstrated that initial circulation of
inoculum through the sand bed followed by continuous substrate feeding
resulted only in floc formation at low dilution rates (0.1 to 0.2 h±1). However,
biofilm development to a small extent took place at higher dilution rate (0.6 h±1).
It was also observed that higher dilution rates enhanced the biofilm formation. It
has been shown that the sludge resulting from the breakdown of glucose would
have 64 per cent acidifying bacteria and 36 per cent methanogenic bacteria.
Under identical operating conditions, inoculum having higher methane
producing capacity (thereby higher number of methanogenic bacteria) resulted
in a large number of film-covered particles with more uniform film thickness
and stable operation of the bioreactor. An experimental study (Morgan et al.,
1996) conducted using ice-cream waste water revealed that all microbial aspects
of sludge, including the number of methanogens, non-methanogens, acidogen,
the enzymatic activity and the structure of biomass remained similar in the four
reactors (UASB, upflow filter, anaerobic fluidized bed and contact process)
during start-up. It has been shown (Gorris et al., 1988; 1989) to be possible that
biofilm development in anaerobic fluidized bed reactor could be influenced by
wastewater composition and type of inoculum. The influence of volatile fatty
acid composition of wastewater on biofilm development was studied. Scanning
electron microscopic examination revealed that immobilization of bacteria on
the sand initially occurred in crevices and that thereupon the structure of sand
particles became colonized. At steady state, microbial population showed
different species of hydrogenotrophic and acetotrophic types. In the cases of
inocula studied for biofilm development, they indicated no difference in the
bacterial composition of biofilms formed during start-up and steady state
conditions.
Anaerobic
fluidized bed
reactors
103
EMH
11,2
104
A further experimental study (Lauwers et al., 1990) conducted with sand as
carrier and synthetic wastewater as feed, revealed the early phase of biofilm
development during start-up of anaerobic fluidized bed process. The results
indicated that facultative anaerobic bacteria were abundantly present in the
outermost biofilm layers of mature sludge granules equivalent to main primary
colonizers of the sand. Microscopic examination of biofilm indicated the
presence of Methanothrix species in primary colonizers.
The effect of biomass accumulation and biomass loss on liquid superficial
velocities, substrate utilization rate and efficiency of anaerobic fluidized bed
process has been evaluated based on experimental results of Calderon et al.
(1998) and Shieh and Hsu (1996). Experimental results obtained for perlite
particles as carrier showed intrinsic relationship between terminal velocities
and bed expansion of up flow and down flow fluidized beds. Terminal
velocities of particles at different biofilm thicknesses calculated from
experimental bed expansion data, were found to be much smaller than those
obtained when Cd (drag coefficient) was determined from the standard drag
curve. This difference was explained by the fact that free rising particles do not
obey Newton's law for free settling. Terminal settling velocities were observed
to be 65 m h±1 for uncoated particles and 27 m h±1 for 72 m biofilm particles.
Wide range of terminal velocities explained the fact that particles are not
homogeneous in density due to irregular shape. The phenomena of biomass
loss during reactor start-up and steady state operation of an anaerobic fluidized
bed reactor using porous media particles fed with acetic acid were evaluated.
The biomass loss rate during reactor start-up was found to be correlated to
both substrate utilization and biogas production. However, the amount of
biomass lost did not impede the progress of reactor start-up and a rapid
building of attached biomass in the reactor was attainable.
Microbial population dynamics in biolayers
The biomass activity in a fluidized bed reactor was considered as a dynamic
system involving an equilibrium between growth and biofilm shearing. As
mentioned earlier, experimental studies revealed the biofilm development and
biomass formation by methanogenic species surviving under various
environmental conditions such as pH, temperature, substrate, turbulence, shear
and wastewater composition.
A kinetic study (Kuba et al., 1990) conducted on fluidized bed reactor with
synthetic zeolite as support media and acetic, propionic and butyric acids as
substrate showed a good treatment efficiency upto a volumetric loading rate of
4kg COD m±3 day±1. The changes of volatile fatty acids concentrations with
time were clearly expressed with the Monod's growth model in both batch
experiments using attached biomass in the bed and detached biomass from the
support media.
The biofilm development and population dynamics have been identified
(Woolersheim et al., 1989; Zellner et al., 1991; Araki and Harada, 1994) in terms
of syntrophic consortium consisting of syntrophosporasp., Methanothrix
soehngenii, Methano sarcina mazei and Methano brevibacter arboriphilus. The
immunological analysis also showed that the organisms isolated from the
butyrate degrading culture could be used as source of inoculum mainly
consisting of Methanothrix soehngenil opfikon, Methano bacterium formicicum
MF and Methano spirillum hungatei JF1. The biofilm thickness for different
upflow velocities (4-25 m h±1) was evaluated to assess methanogenic activity.
The dynamics and reaction kinetics with respect to distributed fraction of
methanogens have been demonstrated by Wu et al. (1998). A new kinetic model
was developed by combining removal efficiencies of conventional and tapered
fluidized bed reactors (CFB, TFBs) were in good agreement with the
experimental results. The biofilm thickness in TFBs was thicker than that in
CFB, resulting in performance enhancement with TFBs. The simulated results
from the kinetic model showed that methanogenesis was the rate limiting step
of degradation of simple organic compound (sucrose) and the COD
concentration in the effluent was mainly contributed by the intermediates of
volatile fatty acids. The distributed fractions of acidogens and methanogens
experimentally found to be 0.4 and 0.6 respectively.
Process performance, stability and inhibition
The process stability of an anaerobic degradation process could be sustained
due to balanced magnitude of COD concentration, waste water flow rate, pH
and temperature during steady state operation. It was quite obvious from a
laboratory study (Denac and Dunn, 1998) that packed bed and fluidized bed
anaerobic reactors operated with molasses and whey feeds showed variations
in performance due to measured variables such as COD loading rate, pH,
temperature, rate of organic acid production, gas composition and rates. The
performance of degradation with respect to molasses as feed indicated that the
rate of degradation in packed bed and fluidized bed reactors were found to be
the same in the strongly substrate limited range and further increase in
fluidized bed was observed with a maximum, at concentration of 3 g COD l±1.
This was probably due to the fact that at higher reaction rates (higher reactor
concentrations), the diffusion limitations in the packed bed reactor started to
play a role in the overall process.
The process performance and inhibition on butyrate degradation in a
fluidized bed reactor were experimentally evaluated (Labib et al., 1992) with
respect to increased concentrations of hydrogen and acetate. The COD removal
efficiency was found to be greater than 97 per cent with a specific loading rate
of 8-10g of COD g±1 of volatile solid per day and a volumetric loading rate of
10g of COD±1 day±1. Most of the time the gas phase hydrogen partial pressure
in the reactor was between 50 and 100 m g l±1. When formate was fed to the
reactor, H2± utilizing methanogens had a very high capacity to produce
methane (15ml min ±1 or 19.4g of COD l±1 day±1). At steady state during
butyrate as feed, methane production from hydrogen was 1.5ml min±1 or 1.9g of
COD l±1 day±1, the hydrogen seemed to have a large and under-utilized capacity
Anaerobic
fluidized bed
reactors
105
EMH
11,2
106
at steady state. Butyrate oxidation can be inhibited by increase in the butyrate
concentration, with complete inhibition when the butyrate concentration was
greater than 5000mg of COD l±1.
The performance of anaerobic fluidized bed treatment on waste activated
sludge (Huang et al., 1989) has given better insight on the degree of sludge
solubilization. At 35oC, anaerobic fluidized bed digesters were able to provide
an adequate degree of biological sludge stabilization within a hydraulic
retention time of 1-10 days, depending on the extent of initial sludge
solubilization.
Biogas hydrogen content as a control parameter was evaluated (Guwy et al.,
1997) with respect to operational conditions of an anaerobic fluidized bed
reactor fed with synthetic baker's yeast wastewater. Step overloads produced a
sharp peak in biogas hydrogen level measured on line, e.g. an increase of
loading rate from 40-63kg COD m±3 day±1, increased hydrogen concentration
from 290-640mg l±1 within three hours. However, switching from an older,
partly acidified batch to a fresh batch of feed at constant COD gave a marked
increase in the biogas hydrogen content from 200-800mg l±1.
It has been shown (Wu and Huang, 1995) to be possible that the performance
of anaerobic fluidized bed reactor could be enhanced by changing geometrical
configuration to a tapered form. Bead shaped activated carbon was used with
synthetic wastewater of acetate concentration, 2550mg l±1. Tapered angles
tested for the study included 2.5o, 5o and 10o. The substrate removal efficiencies
of tapered fluidized beds were found to be significantly higher than those of
conventional fluidized bed reactors verified through kinetic model and
experimental study. The experimental substrate removal efficiencies
corresponding to tapered angles of 5o and 10o were found to be same and higher
than at 2.5o. Therefore, it was concluded from the study that the performance of
the conventional fluidized bed reactor could be significantly enhanced by
changing the geometrical configuration to a tapered form.
The feasibility of an expanded granular sludge bed (EGSB) system for the
treatment of malting wastewater under psychrophilic conditions was
investigated (Rebac et al., 1997) in a temperature range from 13-20oC. The COD
of malting wastewater used in the study was estimated to 1436mg l±1. During
the reactor operation at 16oC, the COD removal efficiency was found to be 56
per cent at organic loading rate of 8.8kg COD m-3 day±1 and at HRT of 2.4
hours. At 20oC, removal efficiency attained 72 per cent at OLR of 14.6 COD m±3
day±1 corresponding to HRT of 1.5 hours. These findings indicated that
anaerobic treatment of low strength wastewater at low temperature might
become a feasible option.
The metallic pathway for anaerobic degradation of chlorophenol and
nitrophenol (Tseng et al., 1994a, b; Khodadoust et al., 1997) as inhibitor has
been identified and quantitatively evaluated for fluidized bed process. The
removal efficiency of pentachlorophenol reached 99.9 per cent. The
experimental results concluded that chloride or nitrogen at para-position was
found to be more toxic than ortho and meta positions. The response of fluidized
bed process in terms of stability was studied for shock loadings (organic,
hydraulic, pH and temperature) (Borja and Banks, 1995) and toxic shocks on
carriers (Mol et al., 1993) using ice-cream and brewery wastewater.
Wastewater treatment in anaerobic fluidized bed reactors
A summary of results of laboratory and pilot scale studies on anaerobic
fluidized bed reactors are summarised in tabular form (Table I). A remarkable
variation in COD loading of 10-50kg COD m±3 day±1, temperature of 10-35oC
and COD removal of 70 per cent-98 per cent were reported for industrial
wastewater in general.
An anaerobic fluidized bed treatment using brewery wastes carried out in a
laboratory scale (Ozturk et al., 1989) resulted in a COD removal efficiency of
greater than 75 per cent at OLR of 9.5kg COD m±3 day±1 for a period of 82 days
from start-up. A COD to methane conversion of 87 per cent was achieved. The
observed methane yield reached a maximum value of 0.34-0.35m3 CH4 kg±1
COD removed. Experimental results have suggested that the COD removal
efficiency varied as a function of COD loading and neither the bed COD nor
HRT alone significantly affected the performance of the reactor. It was
observed that the distribution of the biomass hold-up near the top of the reactor
might have reached concentrations of greater than 20,000 mg l±1.
A pilot scale study (Anderson et al., 1990) conducted on brewery wastewater
as feed showed a COD removal efficiency of more than 75 per cent at an organic
loading rate of 8.9kg COD m±3 day±1 for less than 82 days from start-up. About
340 litres of methane was produced per kg of COD removed. This represented
87 per cent recovery of energy value from the waste treated. The steady-state
biomass hold-up in the AFBR was strongly dependent on the COD loading
applied. The Monod's kinetic parameters were determined using steady state
operating data and compared to experimental results.
An expanded micro-carrier bed process was used (Yoda et al., 1991) to
evaluate the COD removal efficiency of brewery's yeast processing wastewater
in which a COD removal efficiency of 97-99 per cent was achieved at OLR of 1324kg COD m±3 day±1. It was found that cobal and nickel were insufficient in the
yeast processing wastewater and were added externally to enhance the growth
of methanogenic bacteria during start-up of the reactor. It was clearly
demonstrated in a full-scale installation that the microcarrier bed process could
provide a reliable and predictable way to cultivate granular sludge necessary
for efficient anaerobic treatment.
A pilot scale performance of upflow anaerobic sludge blanket reactor
(UASB) and fluidized bed reactor (FBR) has been compared (Iza et al., 1990) for
the treatment of sugar beet wastewater. The COD removal efficiency was found
to be greater than 90 per cent in both cases and the UASB reactor was able to
achieve efficiencies greater than 98 per cent with a better quality effluent (COD
less than 100 mg l±1). Biomass washed out from both reactors was easily
settleable and could be separated with a simple lagoon. Foam formation
especially in FBR, made separation more difficult. The FBR was operated even
Anaerobic
fluidized bed
reactors
107
EMH
11,2
108
at hydraulic retention times shorter than two hours, whereas UASB was
limited to 12 hours, the FBR was shown to achieve OLR greater than 30kg COD
m±3 day±1.
The feasibility of treatment of monosodium glutamate fermentation
wastewater was evaluated (Tseng and Lin, 1990) in terms of removal efficiency
and methane content in the biogas. A BOD removal efficiency of 90 per cent
was attained with a methane content of 80.8 per cent and OLR of 10.1-31.1kg
COD m±3 day±1.
Laboratory studies (Gommers et al., 1988a, b) carried out using a
denitrifying fluidized bed reactor revealed the possibility of simultaneous
oxidation of sulphide and acetate during start-up and steady state. Sulphide (23kg S m±3 day±1, acetate (4-6kg C m±3 day±1) and nitrate (5kg N m±3 day±1) were
effectively removed. The oxidation of elemental sulphur, an intermediate of
sulphide oxidation to sulphate was the rate-limiting step in both the activity
measurements and the reactor.
The anaerobic expanded micro-carrier bed process has been shown(Yoda et
al., 1989) to be feasible for the cultivation of granular sludge similar to that
formed in the UASB process.
Matsumoto and Noike (1991) and Converti et al. (1993) have demonstrated
the start-up and steady state performance of a fluidized bed process in terms of
substrate composition and organic loading rate by varying the influent flow
rate to the reactor. Under operating conditions of COD loading 5.8-108kg m±3
day±1, hydraulic retention times of 0.45-8h, the equilibrium biomass hold-ups
increased with increasing COD loading and varied from 15,000-32,000mg VSS
l±1. Yeast extract or glucose with mixed acid substrate showed better removal
efficiencies than acetic acid alone as substrate. Degradation efficiency and
methane production rate were shown to be substantially affected by an
increase in organic loading rate from 4-24kg COD m±3 day±1. Hence an
operational value in terms of maximum theoretical specific degradation rate,
1.76kg CODr kg±1 VSS day±1 has been calculated using Monod's kinetic model.
The feasibility of anaerobic expanded bed process was experimented (Kato
et al., 1994; Alderman 1998) for the treatment of low strength wastewater
including municipal wastewater up to a COD concentration of 700mg l±1.
Sludge hold-up was found to be feasible within a narrow range of operational
conditions in an expanded granular sludge bed (EGSB) reactor. Within optimal
hydraulic loading range of 2.5-5.5m h ±1, there was no problem of piston
formation nor sludge washout as long as OLR was below 7kg CODm±3 day±1.
However, in case of higher OLR requirement, up to 12kg COD m±3 day±1,
special attention should be given to the sludge washout. It has been concluded
that use of a gas-liquid-solid (GLS) device or an improved type of immobilized
biomass for high hydraulic loads would satisfy the constraint in the
applicability of EGSB reactors. As a pre-treatment option for small organically
overloaded wastewater treatment facilities, the anaerobic expanded bed reactor
(AEBR) can compete with other conventional remedial methods when the
degree of overloading exceeds approximately 1.3-1.75 times the design loading,
with the cost effectiveness of the AFBR unit increasing as the degree of organic
over loading increases.
Fluidized bed treatment for municipal wastewater has been established
(Sanz and Polanco, 1990) at low temperature with BOD load as the operating
parameter.
Similarly another attempt has been made (Perez et al., 1997) to explore the
feasibility of pre-treatment of concentrated wastewater such as wine distillery
and vinasses in thermophilic range. Experimentally, it was confirmed that
anaerobic fluidized bed systems could achieve greater than 82.5 per cent of
COD reduction at a COD loading of 32.3kg COD m±3 day±1 at temperature of
55oC. At HRT of 0.46 day, the volumetric rate of methane generation was 5.8 m3
of CH4 m±3 day±1 with a methane yield of 0.33 m3 CH4 kg±1 of COD removal.
The greatest efficiency of substrate removal was 97 per cent for OLR of 5.9kg
of COD m3 day±1 and HRT of 2.5 days. It has been shown (Collivignarelli et al.,
1991) to be possible to remove organic and nutrient substances from municipal
wastewater at affordable cost. UASB, aerobic fixed bed-reactor and anoxic
fluidized bed reactor were compared for technical and economical feasibility. It
was concluded that economical advantages in terms of operating cost for
various reactors have to be optimized with respect to plant engineering factors.
The application of anaerobic fluidized bed technology has been shown (Chen
et al., 1998; Martin et al., 1993) to achieve considerable removal efficiency in the
treatment of tannery wastewater and fermented olive mill wastewater. More
than 75 per cent of COD reduction was achieved for tannery wastewater up to
an F/M ratio at 0.4g COD g±1 TVS day±1 with a mean cell residence time of 40
days. The observed methane production rate was 0.22 m3 of CH4 kg±1 COD
removed, which was constant over the range of F/M ratio applied. Reactor
biomass concentrations ranging from 17-22g TVS l±1 were achieved for tannery
wastewater and the COD removal efficiency for olive mill wastewater was
evaluated to 92 per cent at HRT of 4-35 days. The results of experiments were
fitted to Michaelis-Menten equation to describe substrate uptake pattern. Pretreatment of olive mill wastewater was found to increase the rate of substrate
uptake by a factor of 3.2 when compared to untreated olive wastewater.
Recent experimental studies by Sreekrishnan et al. (1996) and Chen et al.
(1997) have proved the technical feasibility of anaerobic fluidized bed reactors
for treating yeast industry wastewater and hog wastewater. Experimental
results of yeast industry wastewater treatment revealed that COD reduction
was restricted by the acidification step of the overall reaction which in turn
attributed to methanogenic step of overall process. Separating the acidification
and methanation stages improved the performance but not to any considerable
extent. Anaerobic fluidized bed treatment of hog wastewater resulted in an
efficiency of filtered BOD5 to greater than 80 per cent and unfiltered BOD5 to
more than 60 per cent at volumetric loadings as high as 10.4kg COD m±3 day±1.
Large quantity of alkalinity present in hog wastewater supplemented the
alkalinity requirement for effective digestion. The specific methane yield was
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found to vary from 43-71 per cent of theoretical value and decreased with
increasing loading. However, the biogas produced had high methane content
(more than 70 per cent) at organic loading rate less than 4kg COD m±3 day±1.
An innovation (Hong et al., 1997) on the performance of anaerobic reactors
revealed the possibility of removal of total organic carbon at high efficiency
under pseudo-steady-state operation. Comparative evaluation of two anaerobic
fluidized bed reactors, a packed bed anaerobic reactor, and suspended growth
anaerobic reactor fed with glucose as main carbon source, demonstrated that
anaerobic fluidized bed reactors were less affected by interruptions and
adverse operating conditions than others. The immobilized media
(diatomaceous earth and activated carbon) provided in AFBRs were found to
have better performance than others due to high cell retention ability with
specific biogas production up to 1.71g±1 of TOC against 1.31g±1 of TOC for
packed and suspended growth reactors. High TOC removal efficiencies were
achievable under pseudo state operation. A consistent methane content in
biogas was observed to be 52.5-55.9 per cent. Biomass concentration of AFBRs
reached a maximum of 91g VS l±1 compared to 21g VS l ±1 achieved in packed
bed reactor. Extremely high biomass concentrations in AFBRs were possible
due to the high specific surface area available.
In recent years, the possibility of treating propellant wastewater containing
nitrotoluene and diaminotoluene has been attempted (Maloney et al., 1995;
1998) successfully using granular activated carbon as carrier.
Combined treatment of wastewater using co-digestion concept has been tried
(Kadam et al., 1998) for the treatment of distillery and pharmaceutical effluents
using upflow fluidized bed reactor.
Reactor configurations and full-scale anaerobic treatment systems
Several new reactor configurations were shown to be possible, one or two stage
configurations EGSB (expanded granular sludge blanket) reactor and
anaerobic/aerobic treatment using fluidized bed and airlift suspension reactors,
for treatment of concentrated effluent. Based on the excellent laboratory pilot
performance of anaerobic fluidized bed process, the construction of full-scale
reactors began in the early 1980s. The Ecolotrol HY-FLO reactor, Dorr-oliver
Anytron process and Gist-brocades are some of the examples of full-scale
reactors located in Europe.
The application of immobilization in fluidized bed process has been
investigated (Heijnen et al., 1990) to compare the relative performance of
fluidized bed process with conventional high rate digesters.
An updated list of distribution of anaerobic reactors including fluidized/
expanded bed, countrywide in Europe was compiled in a European Committee
(EC) survey report (Wheatley et al., 1997).
Kinetics and modelling in anaerobic fluidized bed reactors
Several researchers have attempted to evaluate the performance of anaerobic
fluidized process in terms of reaction kinetics and mass transfer limitations.
The substrate consumption kinetics and mass transfer limitations have been
predicted (Buffiere et al., 1995a; Motta and Cascante, 1996) based on dynamic
status of methanogenic biofilm. The experimental data were classified
according to the value of influent flow rate, and a set of data corresponding to
flow rates between 35.44 and 41.83 ml min±1 was selected for model testing.
The test resulted in a reasonable agreement between the zero-order kinetic
model (with complete substrate penetration) and the experimental data. Mass
transfer limitation on single and multisubstrate were explained based on
reaction scheme of molecular diffusion process. Effectiveness factor
calculations were performed in steady state for each bacterial group taken into
account in the process. Biofilm size and thickness were separately compared for
acidogenic and methanogenic phases. This type of comprehensive modelling
may also be extended to complex systems, involving several substrates and
group of organisms.
A few studies (Kuba et al., 1990; Martin et al., 1993; Labib et al., 1993;
Buffiere et al., 1995b; Wu et al., 1998) carried out using fluidized bed process
demonstrated that the attached biomass, methanogenesis, organic loading rate
and residence times could affect the kinetics of anaerobic fluidized bed process.
A model with anaerobic butyrate-degrading consortia was developed using
Monod's kinetics for H2/CO2 and acetate as substrates. The Monod's model was
modified for butyrate oxidation to incorporate inhibition by acetate and
hydrogen and the effect of a thermodynamic driving force. The bacterial
growth yield was made dependent on the concentrations of reactants and
products. The stability of butyrate-degrading consortia was observed to be
more sensitive to acetate loadings than to hydrogen. High acetate
concentrations resulted in loss of butyrate oxidizing biomass and acid
accumulation, leading to a pH drop and subsequent process failure. High solid
retention times substantially overload and reduce alkalinity requirements for
control of pH. The hydraulic residence time and organic loading rate were
experimentally observed to affect the anaerobic digestion of wine vinasse
subjected to fluidized bed process. The regions of high removal, thus illustrate
that when organic loading was too small, a small part of biomass was inactive.
Specific activity values justified better activity of biomass for a biofilm of small
area and thickness.
The effect of solid retention time, substrate utilization and gas production on
the kinetics of anaerobic fluidized bed process was evaluated by Ray et al.
(1989) using waste activated sludge as feed. In the anaerobic digestion of waste
activated sludge, the kinetics of process was modelled by assuming that the
initial gas-production rate was proportional to the soluble biodegradable
substrate. The model confirmed that the rate-limiting step in the digestion
process was the conversion of a particulate biomass into a soluble substrate
rather than the conversion of soluble organic to acetate or acetate to methane.
A practical approach was introduced (Furumai et al., 1991) to use active
biomass concentration and surface coverage of biofilm as indices for predicting
the kinetic behaviour of anaerobic fluidized bed process. Experimental
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observation on synthetic wastewater revealed that the overall growth yield of
acetogenic and methanogenic bacterial groups reached a value of 0.05 (COD
cell/COD substrate) based on COD conversion in steady state condition. Active
biomass was estimated using maximum substrate consumption rate and
overall rate constant of biomass loss due to decay and attrition was estimated
to be 0.09 day±1 based on the active biomass.
A recent innovation from experimental studies (Csikor et al., 1994; Huang
and Wu, 1996) have shown widened horizon for hydrodynamic behaviour of
biomass, investigated respectively for fluidization of biofilm-coated particle
and dissipation of specific energy rate from biofilms in anaerobic fluidized bed
reactors. As the terminal settling Reynolds number was found to be
independent of characteristics of particles, a new approach was developed to
describe the fluidization of biofilm coated particle. The model was based on two
new parameters : the expansion coefficient and the specific occupied particle
volume at zero flow, which are readily determinable and characteristic
parameters of the fluidized particles, being independent of reactor size, shape,
liquid velocity and quantity of carrier particles. The model was found to be
suitable for modelling bed porosity or biomass concentration as a function of
biofilm thickness and upflow liquid velocity. It was concluded from the study
that there could be an optimal biofilm thickness above which not only could the
diffusion limitation increase, but the overall biomass concentration decreases at
a given liquid velocity. The experimental results showed that dissipation rate
varied with operating flow rate and expansion characteristics, found to the
inversely proportional to thickness of biofilm. It was concluded from the
observations that dissipation rates could be a very powerful tool for studying
the erosion effect at the biofilm surface and steady state biofilm thickness
distribution in conventional and tapered fluidized-bed bioreactors.
A further extention of kinetic behaviour (Borja and Banks, 1994; Prakash
and Kennedy, 1996) was also been verified using light carriers such as saponite
and biolite. An anaerobic bioreactor (1.251 volume) operating at 35oC,
containing saponite-immobilized biomass, worked satisfactorily over hydraulic
retention times from 2.5-12.5 days, and removed 97 per cent of initial COD.
Guiot's kinetic model was used to determine the macro-energetic parameters of
the system and reported a yield coefficient of 0.09g VSS g±1, COD and a specific
rate of substrate uptake for cell maintenance of 0.035g COD g±1 VSS day±1. The
experimental results also showed the rate of substrate uptake correlated with
the concentration of biodegradable substrate through an equation of MichaelisMenten type. The biolite used as carrier in anaerobic fluidized bed process
showed rapid start-up and good steady state performance with favourable
density, size and surface properties. Both maximum efficiency profile (MEP)
and maximum load profile (MLP) start-up techniques led to the development of
healthy anaerobic biofilm in a period of five weeks, capable of treating more
than 90 per cent of the influent waste at a loading rate of 2.5kg COD m±3 day±1
and hydraulic retention time of one day. It was evident from experimental
results that biolite could accumulate large amounts of biomass (up to 220mg
volatile biofilm solids g±1 biolite) resulting in high removal efficiencies at high
organic loading rates.
Conclusion
A summary of results of laboratory and pilot scale studies extracted from
extensive literature survey are presented in tabular form (Table I). From the
extensive literature survey, it is clear that anaerobic fluidized bed process has
become an established means for the treatment of concentrated industrial
effluents. A successful implementation of full-scale systems has proven the
technical feasibility of this relatively recent process. From the critical review of
experimental results, it is evident that the treatment efficiency can go up to 98
per cent with COD loading rate of 50kg COD m±3 day±1, compared at higher
limit to