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Environmental Management and Health, Vol. 11 No. 2, 2000, pp. 97-117.#MCB University Press, 0956-6163
Received 3 April 1999 Accepted 10 July 1999
Application of anaerobic
fluidized bed reactors in
wastewater treatment: a
review
R. Saravanane and D.V.S. Murthy
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
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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(Heijnenet 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 (Grasiuset 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.
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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 (Heijnenet 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 (Hickeyet 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.
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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 (Gorriset al., 1988; Morgan, 1991; Lianget 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 27-30kg 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 (Hickeyet 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 (Gorriset al., 1989; Sreekrishnanet 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
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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 (Heijhenet 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 micro-carrier pore characteristics on methanogenic fluidized bed performance (Yeeet 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 90m2m±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 (Foxet al., 1990; Balagueret 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.
EMH
wastewater EB 0.063 0.14 0.0004 15-20 Diatomaceous earth 0.21 174-270 0.5-4.4 ± 5-20 7-7.3 1-10 35-77 0.336-0.349± ± 3
Distillery
effluent FB 0.065 1.70 0.006 25 SIRAN 1.5-2 15000 5.9-32.3 ± 55 7.65-8.57 0.46-2.5 82.5-97 0.33 ± ± 53
Glucose FB 0.051 0.76 0.033 36.0 Diatomaceous earth 0.43-0.61 ± 5.9-12 5.3-8.7 ± ± 2.5-3.3 87-100 0.8-1.0 55.8 44.2 32
Hog
waste-water FB 0.1 2.3 0.018 100
Granular activated
carbon 0.4 330-570 0.24-10.4 4.58 35 7.1-7.9± 60-80 0.35 70 30 14
Glucose FB 0.045 0.90.001 ± Sand 0.6 4400-8800 7.5-21 ± 37 7.0 24 80-90 0.47 75 25 58
Vinasse FB 0.115 1.5 0.015 15-70 Granular 0.3-0.45 2500 20-32 6-15 35 6.7-7.0 0-4 70-75 ± ± ± 8
Pozzolana
Acetic accid FB 0.051 0.76 0.002 0.1-1.2 Glass 0.425-0.61 5000 (TOC) 7.5-32 (TOC) 14.5 35 6.75-7.5 ± 99 0.91 ± ± 59
Acetate FB 0.08-0.138 1.1-1.8 0.018 ± Activated carbon 0.18 2550-11070 (TOC) 18.0-401 (TOC) ± 35 7-8 0.6-3.4 97 ± ± ± 67
Ethanol EB 0.05 1.0 0.002 40 ± ± 100-200 7-39 2.5-5.5 30 7.0 0.09-2.1 80-97 ± ± ± 40
wastewater EB 0.4 2.15 0.246 ± Micro-carrier 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
Brewery
wastewater FB 0.16 3m 0.06 40 Sand 0.5 90000 8-14 30 35 6.8-7.4 4.8-33 75 0.35 78-88 12-22 2
Acetate
propionate FB ± ± 0.02 ± Sand 0.1-0.3 2000-3500 58 15-17 37 7.0 1.5 ± ± ± ± 22
Butyrate Tannery
wastewater FB 0.03 1.70 0.001 ±
Granular activated
carbon 0.59-0.84 750-2250 ± ± 35 8.0 1.2-7.6 75 0.22 60-42 40-58 13
Monosodium
glutamatea FB 0.035 1.0 ± ± Activated carbon 0.46-0.59288000-317000 10-31 ± 28-35 7.10 3-12 65 0.25 80.8 19.2 64
Fruit processing
wastewater FB ± ± 0.001 ± Saponite 0.4-0.8 5100 0.0025 ± 35 7-7.6 60-300 97.7-99.2 1.95 ± ± 9
Glucose FB 0.08 1.0 510±3 33 Perlite 0.968 12500 3.27-5.75 8.64 35 6.5-7.0 20-32 85-90 0.0026 ± ± 10
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Biomass and methanogenic activityThe 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.
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A further experimental study (Lauwerset 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±1for uncoated particles and 27 m h±1for 72m 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.
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soehngenii, Methano sarcina mazeiandMethano brevibacter arboriphilus. Theimmunological analysis also showed that the organisms isolated from the butyrate degrading culture could be used as source of inoculum mainly consisting ofMethanothrix 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 Wuet 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.
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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 (Guwyet 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, 5oand 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 5oand 10owere 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 (Rebacet 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.
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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 (Molet 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±3day±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±3day±1for 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 (Andersonet 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±3day±1for 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 13-24kg COD m±3day±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.
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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±3day±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±3day±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 (2-3kg S m±3day±1, acetate (4-6kg C m±3day±1) and nitrate (5kg N m±3day±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(Yodaet 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 CODrkg±1VSS day±1has been calculated using Monod's kinetic model.
The feasibility of anaerobic expanded bed process was experimented (Kato
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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 (Perezet 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±3day±1at temperature of 55oC. At HRT of 0.46 day, the volumetric rate of methane generation was 5.8 m3 of CH4m±3day±1with a methane yield of 0.33 m3CH4kg±1of COD removal. The greatest efficiency of substrate removal was 97 per cent for OLR of 5.9kg of COD m3day±1and HRT of 2.5 days. It has been shown (Collivignarelliet 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; Martinet 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±1TVS day±1with a mean cell residence time of 40 days. The observed methane production rate was 0.22 m3 of CH4kg±1 COD removed, which was constant over the range of F/M ratio applied. Reactor biomass concentrations ranging from 17-22g TVS l±1were 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. Pre-treatment 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.
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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±3day±1.
An innovation (Honget 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 52.5-55.9per cent. Biomass concentration of AFBRs reached a maximum of 91g VS l±1compared to 21g VS l±1achieved 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 (Kadamet 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 (Wheatleyet al., 1997).
Kinetics and modelling in anaerobic fluidized bed reactors
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The substrate consumption kinetics and mass transfer limitations have been predicted (Buffiereet 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; Wuet 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/CO2and 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.
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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 0.09day±1based 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.
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results that biolite could accumulate large amounts of biomass (up to 220mg volatile biofilm solids g±1biolite) 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±3day±1, compared at higher limit to other high rate digesters.
Further research for process improvement should concentrate on biolayer formation, control of biolayer thickness, biogas generation, optimum carrier type and diameter and process instability due to shock loads and inhibitory substances.
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