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Performance of Dual-Media Expanded Bed Bioreactor.

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Dev. Chem. Eng. Mineral Process. 13(5/6), pp. 645-654, 2005.
Performance of Dual-Media Expanded Bed
Bioreactor
R. Abdul-Rahman*, N. Zainol and A. Abu-Bakar
Department of Chemical and Process Engineering, Universiti
Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia
Adsorption and biological treatment are two possible approaches to remove chloroorganic and organic compounds. Granular activated carbon (GAC) biofilm reactors
combine these two features, the adsorptive capacity and irregular shape of GAC
particles providing niches for bacterial colonisation protected fiom high jluid forces,
while the variety offitnctional groups on the surface enhance the attachment of
microorganisms. The biofilm process is compact and offers reactions in both aerobic
and anoxic states. Studies on removal of nitrogen constituents by a biofilm process
were carried out using a dual-media expanded bed bioreactor, with GAC and plastic
media as support media. The plastic media also acts as a filter for the effluent.
Experiments were carried out at F:M of about 0.45 and hydraulic residence times
(HRT) of 48, 24 and 12 hours. Bed expansion was maintained at 20-30% by
recirculation flow. Aerobic condition was maintained at dissolved oxygen (DO) of
about 2 mg/l throughout the bed. Chemical oxygen in demand (COD) in feed was
1000 mg/L while the total-N was 100 mg/L. Analysis showed that the process is able
to maintain very stable conditions, achieving substantial COD removal of about 85%
and total-N removal of about 80%. Biofilm biomass measurements showed an
increasefiom 400 mg/l at HRT of 48 hours to 10,100 mg/l at HRT 12 hours, showing
that much higher biomass concentrations may be contained in a biofilm process as
compared to a conventional suspended biomass process. Bioreactors contain their
own ecosystems, the nature of the community and the state of microorganisms define
the kinetics and determine reactor performance. Growth kinetic parameters obtained
are Ye = 0.3421 mg/mg, pn = 0.2252 day-’, KH= 319.364 mgA and bH = 0.046 day-’.
The denitrification kinetic parameters obtained are YHD = 0.9409 mg/mg,
/JHD = 0.1612 day-’, KHD = 24.6253 mg/l and bHD = 0.0248 day-’. These parameters
enable prediction of required reactor sizes and operational parameters. The plastic
media has greatly improved effluent clarification by 98% as compared to singlemedia (GAC) only reactor.
Keywords: Biofilm; GAC; plastic media; expanded bed; growth kinetics;
&nitrification kinetics.
* Authorfor correspondence (rakmi@vlsi.eng.ukm.my).
645
R. Abdul-Rahman, N. Zainol and A. Abu-Bakar
Introduction
Biofilm systems such as trickling filters, rotating biological contactors (RBC), airlift
suspension reactors (ASR), turbulent bed, fixed bed, moving bed, fluidised bed,
recycled bed and upflow anaerobic-sludge blanket have been widely studied for
wastewater treatment. These biofilm systems are used for (biological oxygen demand)
BOD oxidation, nitrification, denitrification and methanogenesis (Cao and Alaerts,
1995). Biofilm processes are able to retain relatively high biomass concentrations
resulting in short hydraulic retention times (HRT, often less than 20 hours), better
performance stability and higher volumetric removal rates, thus reducing reactor size,
area requirement and capital costs (Cao and Alaerts, 1995; Turan, 2000). In recent
years, the potential of biofilm reactors to remedy toxic liquid effluents, especially
wastewater containing chlorinated organics, is gaining recognition. Biofilm reactors
have active biomass even at very low concentration of target organic chemicals,
rendering the reactor more efficient for removing the trace toxic compounds in
wastewaters. In addition, biofilms are found to be less sensitive to the presence of
toxic and inhibitory materials, and more resistant to shock loading than dispersed
growth systems (Caldeira et al., 1999). Important features of this process which make
it cost effective for industrial wastewater treatment are: low operating and
maintenance cost, energy recovery in the form of methane gas, low production of
excess sludge, and odourless operation (Annachhatre, 1996).
A variety of support media have been employed for biofilm development such as
sand, granulated activated carbon (GAC), plastics, and various types of clays. Surface
characteristics such as unit surface area, porosity and surface roughness have been
found to influence biofilm formation during startup. Of all factors, surface roughness
seem to be an important media characteristic influencing the biofilm formation. It
has been observed that initial microbial attachment invariably occurs in the crevices
of support media where attached biomass is well protected from fluid shear, and
thereupon the support surfaces become colonised. Media with higher surface
toughness have a higher number of such well-protected sites. GAC is one of the
media which always provides superior biofilm attachment and growth. This has been
attributed to its higher surface roughness, and its ability to adsorb organic compounds
which serve as a food source for bacteria (Annachhatre, 1996).
In expanded bed bioreactor, most of the biomass is attached as films to small-sized
inert media. The biomass covered support particles are fluidised through a high
vertical velocity of the incoming waste achieved through a high degree of recycle
(Hickey et al., 1991). Various support materials, such as sand, PVC and granular
activated carbon, has been used in expanded bed bioreactors. Size and density of the
media determine the economics and stability of the operation. Smaller particles
provide larger surface area for biofilm attachment. In addition, lighter particles can be
fluidised at lower upflow velocities, thereby reducing the recycle rate. The purpose of
this study was to evaluate the performance of a dual-media expanded bed bioreactor.
The biomass concentration and kinetics parameters were investigated.
Denitrification Kinetics
Limitation of NO3-N in a denitrification system is similar to limitation of oxygen in
an aerobic system. Combining the Monod equation for NO3-N, and the biomass and
646
Peformance of Dual-Media Expanded Bed Bioreactor
N03-N balances gives the kinetic parameters: c(m, KHD,bHD and YHD.To obtain these
parameters, the Monod equation for the electron acceptor can be expressed as:
where subscripts 'i' and 'e' denote mfluent and effluent respectively.
vx
The biomass residence time for denitrification, Ox, is given by: 8, = Qixe
With:
1
p=-+b,
8,
Substituting for p in the above equation for denitrification and rearranging:
K m ; L
- 8,
NPHD PHD 1+ 8,b,
Then KHDand PHD values can be obtained by plotting
1 + 8,b,
1
versus -.
N
Mass balance for NO3-N gives:
dN
dt
1
Substituting p = -+ b,
At steady state, -= 0, therefore: p =
QiYmWi -Ne)
vx
in the above equation and rearranging:
8,
1
YHD%
+--bm
- Q i W i -Ne)
vx
YHD
This equation can be used to obtain the parameters bHD and YHDby plotting
Qi
(Ni
- Ne)
vx
versus
-.1
8,
Materials Used and Experimental Methods
(0 Reactor
An expanded-bed column reactor (height 150 cm, diameter 10 cm) was employed.
Activated carbon particles (diameter 0.80-1.50 mm) and plastic media were used as
support media. An aerator maintained DO at 2-2.5 mg/L; and a recycle pump included
for hydrodynamic and expansion purposes. Bed expansion was maintained at 20-30%.
64 7
R. Abdul-Rahman, N. Zainol and A. Abu-Bakar
(ii) Wastewater
To ensure wastewater composition consistency, simulated wastewater fed glucose as
the C source, and the salt medium as given in Table 1. Bicarbonate buffer of pH 7
maintained the system at pH of 7-8.
Table 1. Simulated WastewaterSalt, Buffer and Nutrients.
I Content
I Concentration (mgh')
Salt solution
NH,NO,
I Various concentrations
57.20
I
FeC136H20
MnC124H20
NaHCO,
(iii) Reactor operation
The reactor was continuously fed in the upflow mode by means of adjustable
peristaltic pumps. Wastewater was fed at 12 Wday. Initially, HRT was 48 hours,
giving F:M of 0.38 to 0.88. After 109 days, HRT was changed to 24 hours, giving
F:M of 0.30 to 0.78. At day 300, the HRT was further reduced to 12 hours, giving
F:M of 0.21 to 0.55. COD was analysed using a spectrophotometer and methods as in
the Spectrophotometric Instrument Manual. Biofilm biomass on activated carbon was
measured by digesting in sodium hydroxide.
Results and Discussion
(i) COD removal
At HRT of 48 hours, the COD removal (see Figure 1) increased slowly to 70%.
Aeration helps substrates and oxygen to enter the biofilm and prevents accumulation
of toxics. As biofilm stabilises, its efficiency increases. After 20 months, the COD
removal varies from 75 to 85%. In addition, the results of this study were compared
with other findings as shown in Table 2. Show and Tay (1999) used an anaerobic
filter system with Raschigs ring as the support media; Kim et al. (2002) used a
fluidised bed bioreactor with polyurethane as support media. The effluent COD
concentration in our study was about 150 mg/l, and the removal efficiency was
significantly higher than in the other studies.
(ii) Nitrogen constituent removal
Denitrification occurs in anoxic regions in inner biofilm layers as shown by decreases
in Nitrate-N and Total-N, and compared to influent levels in Figure 2. Initially,
nitrogen removal increased to 73%, but the percentage decreased to 44% when the
HRT was reduced to 24 hours. The percentage nitrogen removal increased above 65%
as the biofilm stabilises. Once stabilised, the variation in HRT will not affect the
nitrogen removal. Even when HRT decreased to 12 hours after 300 days, nitrogen
removal increased to 80%. Table 3 compares our Total-N removal with other studies.
648
Per$ormance of Dual-Media Expanded Bed Bioreactor
1200
CODi
HRT:Ph
1000
b
800
I
i
2'60~ '
0
I
I
I
1
I
I
I
I
I
,
I
I
I
I
I
I
I
I
100
200
300
400
500
600
700
900
800
1000
days
Figure 1. Concentration of COD in feed (CODi) and effluent (CODe).
Table 2. Comparison of COD removal.
Study
System
Support media
Reactor
COD
HRT
COD
volume
influent
(h)
removal
0)
(mg/l)
12
1000
12.0
85.0
PA)
Expanded bed
GAC and
bioreactor
plastic media
Show and
Anaerobic
Raschig rings
15
10000
15.0
78.0
Tay (1999)
filter
Kim et al.
Fluidisedbed
Polyurethane
1420
870
9.5
68.8
(2002)
bioreactor
This study
649
R. Abdul-Rahman. N.Zainol and A. Abu-Bakar
120
,
I HRT48b
100
sE
I
I
1
I
HRT-24bf
I
HRT-12 b
I
+
80 -
7
- 60-
itrate-Ni
#
I-
40
tal-Ne
20 -
Nitrate-Ne
I
I
0
I
0
100
200
I
I
'
[ 300
I
I
I
400j
I
I
500
600
days
700
800
Figure 2. Concentration of nitrogen in influent (0 and effluent (e).
Study
System
Support media
This study
GAC and plastic
media
(1997)
Expanded Bed
Bioreactor
Activated Sludge
and Rotating
Biological
Contactor (RBC)
Anaerobic and
Aerobic Biofilm
Sison et al.
Fixed Bed
Chuang et
al. (1997)
Takai et al.
Reactor
volume
(I)
12.0
129.8
Mesh-like contact
media
(chlorinated
vinyl) and stringlike contact media
(vinylidene
chloride)
GAC
28.0
Basalt particles
3.0
4.0
( 1996)
Van
Benthum
et al.
(1998)
650
Biofilm Airlift
Suspension and
Anoxic
Chemostat
900
1000
Pe$onnance of Dual-Media Expanded Bed Bioreactor
(iii) Growth and denitrijkation kinetics
The kinetic parameters for growth and denitrification were calculated from graphs in
Figure 3 by applying the Monod kinetic models. The values of the parameters,
specific growth rate (p) and half saturation coeficient (K), are very dependent on the
organisms and substrate employed. Readily biodegradable substrates are characterised
by high values of p and low values of K, whereas slowly biodegradable substrates
have low p and high K values. The kinetic parameters obtained from this study are
listed in Table 4 and Table 5 . The half saturation coefficient for N03-N (KHD)as the
terminal electron acceptor is lower than KH. Therefore, substrate for growth is slowly
biodegradable compared to denitrification. The parameter Y is influenced both by the
substrate degraded and the microorganism performing the degradation, and is a
reflection of the energy available in the substrate. In this study the Y value for
denitrification (YHD) is higher than for growth (YH), proving that substrates for
denitrification (N03-N) have more energy to degrade the culture than substrates for
growth (COD).The decay coefficient for denitrification (bHD) was found to be lower
than those for growth (bH) because it has much lower values (Bryers, 1985).
18 1
1
GnphforGrmthKintie p d t n
16
-
Gnph for Gmwth Wit pramten
14.
p 12-
y 14182~
+ 4.447
I-
f 10-
6
8-
!i
6-
:
u,= 0.2252
Ib = 319.364
" 1
0
OM
0.1
0.15
02
OMn
ODW
o m
OW8
ISe
15RT
25
Graph for D e n i t S i h n Kinai: P u d m
lM=
0.16U
Khd = 34.6153
003
0035
O D ~ ow6
I@RT
om
OD55
0.01
0.03
0.05
1INe
0.07
0.09
Figure 3. Graphsfor kinetic parameters.
65 I
R. Abdul-Rahman. N.Zainol and A. Abu-Bakar
Table 4. Comparison of growth kinetic parameters.
I Reference
Growth Parameters
Process and
C source
PH
(day3
Expanded bed
bioreactor
Bioreactor
(Phenol)
0.2252
3 19.364
0.3421
1.440
13525
0.380
1.128
4558
0.390
2.900
25
0.490
This study
0.0459
Garcia et al.
( 1997)
a. Aspergillus
terreus
b. Geotrichum
candidum
I
Activated sludge
t
Activated sludge
Entrapped
microbial cell
Methanol)
Activated sludge
0.630
+P=-
( 1995)
I
Table 5. Comparison of denitrijication kinetic parameters.
652
Orhon et al.
I
0.1100
I Avcioglu et al.
I
Peformance of Dual-Media Expanded Bed Bioreactor
Conclusions
Expanded-bed biofilm processes have the advantages of holding a higher biomass
concentration than suspended biomass systems, having smaller pressure drops than
fixed-bed biofilm systems, and no bed-clogging problems. Analysis showed that the
process is able to maintain a very stable condition, achieving substantial COD
removal of about 85% and total-N removal of about 80%. The kinetic parameters for
growth and denitrification were found to be within the same range as those found by
other researchers, even though their processes were not exactly of the same type. The
inclusion of plastic media (dual-media reactor) has greatly improved effluent
clarification by nearly 98% when compared to a single-media only (with GAC)
reactor.
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654
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