Disposal of large quantum of biodegradable waste devoid of adequate treatment
results in momentous ecological pollution. Anaerobic digestion offers prospective
energy savings and is a more stable process for intermediate and high strength
organic sewages. In such process, apart from treating the wastewater, the methane
generated from the anaerobic system can be recovered. Energy conservation in
industrial processes became a foremost apprehension and those processes rapidly
emerged as an acceptable alternative. This led to the advances of a range of
reactor designs apposite for the treatment of low, medium and high strength
waste. The distillery effluent is one of the most complex and strongest organic
industrial effluents, having intensely high COD and BOD values. Distillery industries
have turned out to be a foremost source of pollution, as 88% of its raw materials
are transformed into waste and discharged into the water bodies, causing water
pollution. A 30,000 L day-1 (LPD) distillery on and average generates
about 4.5 lakh liters of waste per day. According to current scenario the wastewater
treatment facility should necessitate less space, less sludge production associated
with disposal facilities, optimal energy constraint and probable yield of biogas,
for all these requirement it is necessary to focus on the new investigation,
from the precedent and by such means, it is well identified that the anaerobic
process is competitive enough to meet over the needs. Conventional digesters
such as sludge digester and anaerobic Continuous Stirred Tank Reactors (CSTR)
have been used in India for many decades in sewage treatment plants for stabilization
of the activated sludge and sewage solids.
The fluidized bed expertise presents a series of benefits compared to other
kinds of anaerobic processes, like high organic loading rates and short hydraulic
retention times (Kunii and Levenspial, 2005). In fluidized
bed reactor, fluid flows upward through a bed of particle with velocity as much
as necessary to fluidize the medium, the microorganism grows as biofilm on a
support material such as activated carbon, sand, polymeric material etc., the
nutrient necessary for microbial growth are transformed from the surface of
biofilm to the inner region via diffusion. The fluidized bed offers high reactor
bio-mass hold-up and a long mean cell residence time. The large biofilm-liquid
interfacial area, high interfacial velocities and good mass transfer characteristics
are the main advantages of this type of bioreactors. To prevail over the restrictions
of fluidized bed such as uncontrolled growth of fixed biomass, which changes
the hydrodynamics of particle a hybrid reactor has been developed. The hybrid
reactor is an improved version of the direct fluidization system and merges
the aspects of the direct and inverse fluid bed reactors. This reactor gives
strong resistance to large variations in loading rate. The inverse fluidized
bed is introduced for the fact that the particle density is lower than fluid.
The application of the so-called inverse fluidized bed, in which low density
particles (matrix particle density smaller than that of liquid) are fluidized
by a downhill of liquid or an uphill of gas all the way through a bed let the
management of biomass loading in the reactor (Sokol et
Immobilization of biocatalysts has received increasing interest nowadays, which
offers a hopeful possibility for the improvement of the effectiveness of bioprocess.
Compared with the free cell, the immobilized cell has several advantages which
include the increase in degradation rate due to modified biomass loading along
with the advantage of carrying out the process at high dilution rate with efficient
and enhanced control than previous. In addition it provides a way for catalytic
stability of biocatalysts (Wang and Wu, 2004).
The hybrid reactor design consists of direct fluidized bed in the internal
region and inverse fluidized bed in annular region amid anaerobic environment.
Direct fluidization is attained by pumping the waste water from the bottom of
the reactor (high density ceramic particles were used as support particles).
The inverse fluidization is achieved by the downhill current of the running
water (less bulk plastic support particles were used) (Lakshmi
et al., 2000). The down flow arrangement enables the over coated
particles to be recovered in the underneath of the bed. The aim of this work
was to investigate the performance of a hybrid anaerobic lab scale reactor applied
to the treatment of distillery waste waters. The novel reactor setup increases
the active reactor area and decreases the operating cost by utilizing the energy
supplied for the direct fluidization to the inverse.
MATERIALS AND METHODS
Wastewater characterization: Distillery industry wastewater was dark brown in color with alcoholic noxious whiff. The pH, TSS ( Total Suspended Solids), COD (Chemical Oxygen Demand), BOD(Biochemical oxygen Demand) and volatile acids were found to be 8.5-9, 4400, 34000, 12000 and 3185 mg L-1, respectively.
Experimental set-up: The reactor column made up of polyacrylic material
consisted of a raiser as an inner direct fluidization core of height 1 m and
diameter 50 mm. Outer down-comer case of inverse fluidization is of height 0.6
m and diameter 150 mm. It also consists of a disengaging section of height 0.6
m and diameter 200 m as shown in the Fig. 1. Gradual expansion
of the effluent to the inner core for direct fluidization is done by conical
diffuser with cone angle 10 degree.
|| Experimental setup of the reactor
Thus, the pressure loss due to sudden expansion is minimized. The effluent
is fed into the raiser of the column by means of mono-block pump (0.5 HP motor)
and the liquid flow rate is monitored by using a rotameter (range 0-500 L h-1).
The pressure head variation is measured using a U-tube manometer. Carbon tetra
chloride mixed with iodine pellets were used for indicating the pressure head.
The treated effluent is recycled through a vent which is located underneath
the column. The inlet and outlet pressure head were correspondingly measured
using the manometer. For the direct fluidization ceramic support particles of
2 mm diameter and bulk density of 1416 kg m-3 were used and for inverse
fluidization spherical plastic beads of 4 mm diameter and bulk density of 618
kg m-3 were used
SAMPLE COLLECTION AND PRESERVATION
The effluent was collected from the oxidation pond in Pondicherry municipal wastewater treatment facility and stored at 19°C in a refrigerator to prevent biodegradation. The collected sample was from the effluent and was settled for an hour. The supernatant of the sample which was free of suspended solids was used for determination of effluent COD.
Inoculation and biofilm formation: Inoculums were made ready in 1:1
ratio of distillery wastewater and a mixture of sewage sludge and cow dung.
The above solution was maintained in anaerobic condition for minimum 24 h at
room temperature and ceramic and plastic beads were added in separate flasks
and incubated in shaker flask for 24 h for immobilization of the microbes over
the beads. The culture was kept for a period of 15-20 days for the growth of
biomass over the support particles. Since, cow dung is used for inoculation
a mixed culture of micro-organism was obtained (Garrett
et al., 2008).
Experimental procedure: Micro organisms were coated on the support particles
by incubating it along with the culture media for few days. The organic and
inorganic materials present in the waste water acts as the substrate for the
metabolic activity of the microorganism. The organisms were incubated for a
time interval of 30 to 40 days for the growth of biomass. The microorganisms
coated on the surface of support particle are then packed in the fluidizing
column for 80 mm height. The support particle acts as the solid phase while
the waste water is used as the liquid phase. The effluent is pumped from the
bottom of the fluidizing column and the liquid flow rate was controlled with
the help of a rotameter and a flow control valve. By sparging nitrogen in the
reactor anaerobic condition was achieved. The inverse fluidization was achieved
by the down flow current of the liquid. The parameters which influence the reduction
rate of COD in the effluent are temperature, pH, concentration of the microbes
and age of immobilization. There are other parameters like liquid superficial
velocity upon which the growth of the biofilm depends.
Sampling and analytical methods: The samples for the hybrid reactor
and Anaerobic Fluidized Bed Bioreactor (AFBR) system were analyzed for pH, 5-day
Biological Oxygen Demand (BOD5), COD, TSS and VSS (Volatile Suspended Solids).
The inoculated reactor was monitored by measuring gas production by water displacement
method (Perez et al., 2001). The methane quantity
was measured by GC.
RESULT AND DISCUSSION
A comparative study of hybrid anaerobic reactor with equal volume of anaerobic
fluidized bed reactor for the treatment of distillery wastewater shown in the
Fig. 2, maximum degradation was seen in hybrid reactor. The
hydrodynamic characteristics of hybrid reactor were experimentally studied.
Pressure drop was found to be a hinge than other aspects. In case of fluidization
by single phase the pressure drop across the bed increases with the flow rate.
Once the fluidization starts the pressure drop across the bed remains constant,
whereas the bed height increases with the flow rate of the fluidizing medium
(Garcia-Calderon et al., 1998). The use of low
density particles increase the throughput capacity of the system, although in
such cases, high pressure drops are encountered compared to high density particles
(Buffiere and Moletta, 1999). The experiments with the
different initial bed heights of 4, 5, 7 and 9 (cm) in both direct and inverse
bed were performed. The variation of reduction COD with different initial bed
height is shown in Fig. 3. It was inferred that the percentage
reduction in COD increases with increase in bed height. The increase in COD
reduction with bed height was due to increase in volume of biomass support particles
which lead to increase in biomass concentration for the degradation of wastewater
(Souza et al., 2004). Increase in bed height
in turn increases the pressure drop in the column.
|| Comparison of COD (%) reduction of hybrid reactor and AFBR
|| Variation of pressure drop and COD (%) reduction verses different
|| Methane gas production versus time with different hydraulic
retention time (HRT)
From the Fig. 3 total pressure drop increased from 324.2
N m-2 for 4 cm equal bed height in both inverse and direct in the
column to 621 N m-2 for 9 cm column. Hence, optimum bed height of
5 cm was chosen for the pilot plant operation. Main factor responsible for degradation
is the biomass that attached to the support particles, although some degradation
may occur due to suspended active cells when the reactor was operated in a mixing
regime. The comparative study had proved that the novel reactor has high efficiency
in treating the organic loads than the normal fluidized bed reactor. Also, the
time taken for degradation was less. And hence, the novel reactor can be used
to treat various other industrial wastes and the efficiencies may be articulated
and the novel reactor may be scaled up for industrial application. For distillery
effluent a maximum COD removal efficiency of 91% was observed which was relatively
higher than the normal fluidized bed reactors. Fig. 4 shows
the methane production resulting from the HYBRID reactors with different HRT.
The Fig. 5 represents the % COD removal at various HRT, it
was observed that the COD reduction decreased with HRT. This trend was consistent
with previous reports (Fang and Yu, 2000). An increase
in the HRT would result in a decrease in the wastewater linear velocity through
the support, improving the mass transfer from the liquid to the biofilm and
therefore, favoring the process performance.
|| COD reduction versus time with different hydraulic retention
This observation was in agreement with previous reports (Gavala
et al., 1999). When the HRT increased from 0.5 to 2 h, the values
of percentage COD removal increased from 82.74 to 95.29%. It is clear that for
the lower HRTs (0.5 and 1 h) the COD removal efficiencies and especially the
methane productions are inferior to those obtained with the higher HRTs (2 h).The
results obtained in this work also show a similar tendency to what was reported
by Sambo et al. (1995). The value of methane
production increased in 2.91 L day-1 when the HRT increased from
0.5 to 2 h. The biogas produced in the Hybrid Reactor contained 57-60% of methane,
35-40% of carbon dioxide and 1% of traces.
Comparative study has proved that the novel reactor has high efficiency in treating the organic loads than the normal fluidized bed reactor. Also, the time taken for degradation is less. The hybrid pilot plant treating distillery factory effluents had shown high process stability, decreases the operating cost by utilizing the energy supplied for the direct fluidization to the inverse. And hence, the novel reactor can be used to treat various other industrial wastes and the efficiencies may be articulated and the novel reactor may be scaled up for industrial applications.