Aerobic digestion of starch wastewater in a fluidized bed
bioreactor with low density biomass support
M. Rajasimman∗, C. Karthikeyan
Department of Chemical Engineering, Annamalai University, Annamalai Nagar 608002, Tamil Nadu, India
Received 8 January 2006; received in revised form 23 June 2006; accepted 29 August 2006
Available online 3 September 2006
A solid–liquid–gas, multiphase, fluidized bed bioreactor with low density particles was used in this study to treat the high organic content starch
industry wastewater. The characteristics of starch wastewater were studied. It shows high organic content and acidic nature. The performance of
a three phase fluidized bed bioreactor with low density biomass support was studied under various average initial substrate concentrations, by
varying COD values (2250, 4475, 6730 and 8910mg/L) and for various hydraulic retention times (8, 16, 24, 32 and 40h) based on COD removal
efficiency. The optimum bed height for the maximum COD reduction was found to be 80cm. Experiments were carried out in the bioreactor at
an optimized bed height, after the formation of biofilm on the surface of low-density particles (density=870kg/m3). Mixed culture obtained from
the sludge, taken from starch industry effluent treatment plant, was used as the source for microorganisms. From the results it was observed that
increase in initial substrate concentration leads to decrease in COD reduction and COD reduction increases with increase in hydraulic retention
time. The optimum COD removal of 93.8% occurs at an initial substrate concentration of 2250mg/L and for the hydraulic retention time of 24h.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Fluidized bed; Starch wastewater; Low density particles; Three phase
During the last few years the application of fluidization in
the field of biotechnology has increased considerably . The
main application of fluidization principle is in the field of envi-
ronmental biotechnology. Fluidized bed bioreactor has several
advantages over other conventional reactors for the treatment
of wastewater. The limitation of the fluidized bed reactor in
wastewater treatment is the biofilm thickness. There is a prob-
lem of increase in biofilm thickness when the microorganisms
in the biofilm multiply. This limits diffusion of oxygen and/or
the organic substrate to the deeper layers of the biofilm. Star-
vation of the microorganisms at the base of the biofilm causes
used for biological aerobic wastewater treatment .
∗Corresponding author. Tel.: +91 9842565098.
E-mail address: raja email@example.com (M. Rajasimman).
transfer aspects of this kind of reactor [6,7]. This bioreactor is
successfully used for ferrous iron oxidation by Thiobacillus fer-
Cassava is one of the world’s most important staple food
crops. The industrial uses of starch and starch products are
numerous. In the international trade cassava enjoys a good
position as raw material for compound animal feed, mostly in
two basic forms of processing, namely cassava chips and cas-
sava pellets. The wastewater generated from cassava industries
are highly organic and acidic in nature. Several authors have
reported the physical methods, chemical methods of treatment
and anaerobic digestion of starch industry wastewater [13,14].
Only a few research works [15,16] are available on degrada-
tion of starch effluent by aerobic microbes and hence this work
focuses on the treatment of the starch industry wastewater by
aerobic microorganisms in a fluidized bed bioreactor with low
density biomass support. The objective of this study is to char-
acterize the starch wastewater and to treat it in this reactor by
varying the hydraulic retention time and initial substrate con-
0304-3894/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
centration. Experiments were also conducted to find out the
optimum bed height and to study the behavior of air holdup.
2. Materials and methods
2.1. Experimental setup
The schematic diagram of the fluidized bed bioreactor is
shown in Fig. 1. The column was made of Perspex had the
dimensions of 0.092m internal diameter, 1.7m height with a
conical bottom and a wall thickness of 3mm. The air was intro-
duced by means of a sparger, located just below the supporting
mesh, which helps in uniform mixing. Airflow rate is measured
by a rotameter and a valve is used to control the flow rate. The
reactor is monitored for pH and it is maintained by the addi-
tion of acid or alkali. The low density biomass support particles
were made of polypropylene of density 870kg/m3and irregular
in shape with more surface area (390m2/m3). It requires a low
gas velocity for being expanded.
2.2. Reactor inoculation and startup
Hydrodynamic studies were carried out to find out the air
holdup for various air flow rate and bed heights. The reactor
was filled with the supporting material to give 80cm initial bed
height from the supporting mesh. Rajasimman and Karthikeyan
 studied the effect of bed height on COD removal and found
that a bed height of 80cm was found to be optimum for starch
starch wastewater without any minerals. The inoculums were
Fig. 1. Schematic diagram of fluidized bed bioreactor: (1) storage tank; (2)
(7) fluidizing section; (8) discharge valve.
Characteristics of starch industry wastewater
Chemical oxygen demand (mg/L)
Biological oxygen demand (mg/L)
Total solids (mg/L)
Volatile solids (mg/L)
Total dissolved solids (mg/L)
Total suspended solids (mg/L)
Volatile suspended solids (mg/L)
prepared from the sludge taken from the starch industry effluent
treatment plant. It was introduced into the reactor along with
the biomass support particles and substrate, to start the growth
of microorganisms on the surface of supporting materials. Air
was supplied at a rate of 48.64cm3/s, which is sufficient for
biomass growth, and pH is maintained between 5.9 and 6.1. It
was found that a pH value of 6 was found to be the optimum for
The set up was left for 20 days with aeration in order to enhance
microbial film formation on the low density support material.
the reactor is withdrawn leaving the biomass-laden particles.
Then the substrate was pumped into the reactor and air was
supplied at the same rate.
2.3. Experimental procedure
The characteristics of starch wastewater are given in Table 1.
All the analysis were made according to the procedures given in
APHA . In this study, co current mode of operation was car-
the reactor. Experiments were carried out in a semi continuous
mode, i.e., air was supplied continuously and liquid was intro-
duced into the reactor in batch mode during the startup of the
reactor. After attaining constant biomass loading, the wastewa-
ter was pumped into the reactor. Air was supplied with a flow
The performance of the reactor was studied by measuring COD
values at regular intervals of time. Then the flow rate of air was
varied and COD reduction was observed for all the flow rates.
The above procedure is repeated for various initial concentra-
tions of the starch industry wastewater (2250, 4475, 6730 and
8910mg/L) and for various hydraulic retention time i.e. for 8,
16, 24, 32 and 40h.
3. Results and discussion
3.1. Wastewater characterization
Table 1 shows the characteristics of starch industry wastewa-
high organic content and pH value of 4.5 indicates acidic nature
of the wastewater. The acidic nature is due to release of acid
from roots and also use of acids during final stage operations.
The ratio of BOD to COD was 0.67, which indicates that starch
Fig. 2. Effect of air velocity on air holdup for various initial bed heights.
wastewater is biologically degradable. From Table 1 it was also
observed that the wastewater has higher solid content.
3.2. Air holdup and bed height optimization
The hydrodynamics studies are important in designing the
reactor for application of wastewater treatment. From Fig. 2,
it was inferred that the air holdup increases with increase in
air flow rate and decreases after reaches the critical velocity.
Air holdup also increases with increase in bed height and then
decreases after reaches a bed height of 80cm. These results are
well matched with the findings of Rajasimman and Karthikeyan
The variation of COD reduction with initial bed height was
shown in Fig. 3. It was inferred that the reduction in COD
increases with increase in bed height and then decreases after
reaches an optimum bed height of 80cm. The increase in COD
reduction with bed height was due to increase in volume of
centration for the degradation of wastewater. Further increase in
tion in air holdup. This observation is consistent with the results
obtained by Ocheing et al.  in which it was observed that
the particle loading affects phase mixing and affects the mass
transfer characteristics. Hence all the experiments were carried
out at a fixed bed height of 80cm, which was found to be the
optimum for the maximum degradation of starch industry efflu-
Fig. 3. Effect of initial bed height on COD reduction.
Fig. 5. Effect of initial substrate concentration on %COD reduction-HRT of
ent . These findings are well supported by Ochieng et al.
 and Sokol .
3.3. Initial substrate concentration
The results obtained for various initial concentration and
hydraulic retention times were shown in Figs. 4–8. Fig. 4 shows
the percentage of COD reductions for the hydraulic retention
time of 8h at various initial concentration of the starch wastew-
ater. The maximum COD removal of 72.8% was observed at
an initial concentration of 2250mg/L and a minimum of 63.6%
for 8910mg/L for the air flow rate 69.77cm3/s. It was inferred
that as the initial concentration of the substrate increases, the
percentage reduction of COD decreases and at low substrate
concentration the degradation occurs at a faster rate than at
Fig. 6. Effect of initial substrate concentration on %COD reduction-HRT of
Fig. 7. Effect of initial substrate concentration on %COD reduction-HRT of
higher concentrations. This may be due to presence of low level
of organics present in the wastewater and the lesser resistance
offered by them. With increase in concentration of effluent, the
quantity of pollutants and organics to be treated were found to
increase which in turn was found to inhibit the degradation rate.
It was also observed that as the flow rate increases the COD
reduction increases and then decreases after reaches a velocity
called critical velocity. This may be due to, the air holdup starts
decreasing after the critical velocity and also the retention time
of gas was also found to be low. This is in agreement with the
findings of Sokol . Figs. 5–8 show the percentage reduction
for various hydraulic retention time of 16, 24, 32 and 40h. The
same trend was observed for the treatment at various hydraulic
3.4. Hydraulic retention time
Hydraulic retention time is an important parameter in the
the effect of hydraulic retention time on the percentage reduc-
tion of COD. From Fig. 9, it was observed that the degradation
efficiency increases with increase in HRT for all the initial sub-
strate concentrations. The maximum removal of 95.6% occurs
at 40h for the initial concentration of 2250mg/L and for the
initial concentration of 8910mg/L, a reduction of 89.4% was
observed. However a removal efficiency of 93.8% was achieved
COD values. Hence the optimum value was found to be 93.8%
which occurs at a HRT of 24h.
Fig. 8. Effect of initial substrate concentration on %COD reduction-HRT of
Fig. 9. Influence of HRT on %COD reduction.
The experiments were conducted in a fluidized bed reactor
with low-density biomass support at various HRT and for dif-
ferent initial substrate concentrations. From the present study, it
was found that this bioreactor with low-density particles could
this work the following conclusions were drawn:
• The characteristics of starch wastewater show high organic
content and acidic nature. It also has high solid content.
• From the hydrodynamic studies it was observed that the
air holdup increases with increase in air velocity and then
decreases after reaching the critical velocity. Similarly air
holdup increases with increase in initial bed height and
decreases after critical bed height.
• Reduction of COD is depends on the initial substrate concen-
tration, hydraulic retention time and air flow rate for a fixed
• The percentage reduction of COD increases with increase in
HRT but decreases with increase in initial substrate concen-
tration. An optimum COD removal of 93.8% occurs at an
initial substrate concentration of 2250mg/L for the airflow
The authors wish to express their gratitude for the support
Nagar, India in carrying out the research work in Environmental
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