INTRODUCTION

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 al., 2009).

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.

Fig. 1:	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.

Fig. 2:	Comparison of COD (%) reduction of hybrid reactor and AFBR

Fig. 3:	Variation of pressure drop and COD (%) reduction verses different bed height

Fig. 4:	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.

Fig. 5:	COD reduction versus time with different hydraulic retention time (HRT)

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.

CONCLUSION

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.