Anaerobic wastewater treatment is a very old method of water purification. Anaerobic processes have been employed for the treatment of highly concentrated domestic and industrial wastewater for well over a century (McCarty, 1981). To be more specific, anaerobic processes are widely used in industries such as breweries and beverages, distilleries and fermentation, chemical, pulp and paper and food wastewater treatment(Frankin, 2001). Typically, there two steps involved in anaerobic treatment of wastewater, acid formation and methane formation (Demirel & Yenigun, 2002).
The most widespread used anaerobic wastewater treatment system worldwide is the upflow anaerobic sludge blanket (UASB) reactor (McCarthy, 2001), which was developed by Dr. Gatze Lettinga and colleagues in the late 1970's at the Wageningen University (The Netherlands). As for the expanded granular sludge bed (EGSB), it is actually a modification of UASB reactor (Kato, Field, Versteeg, & Lettinga, 1994). The use of effluent recirculation with taller reactor (or a high height/diameter ratio) and a granular biomass rather than fixing to a carrier, created the expanded granular sludge bed (EGSB), in which a high superficial velocity is also applied to make the granular biomass expanded(Seghezzo L. , Zeeman, van Lier, Hamelers, & Lettinga, 1998; Frankin, 2001).
1.2 History of process (EGSB) development
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Not until 1984, several full-scale expanded granular sludge bed reactors have been applied. Before that, expanded granular sludge bed reactor was only found to be tested on the laboratory/pilot-scale and not on full scale, which was contrary to the other reactor types, where numerous full-scale reactors are in operation (Heijnen, Mulder, Enger, & Hoeks, 1989).
Franklin (2001) reported that UASB is most predominant process built in plant worldwide before 1997. As increasingly gaining popularity, 50% of the plants built during 1997-2000 are based on EGSB type process. The number of plants categorized per type of process is shown in Table 1. This change is also illustrated in Figure 2, where it is apparent that the traditional UASB system is gradually being replaced by ESGB type systems. This is due to the effectiveness and competitive advantages of the EGSB system.
Table : Process used in different country from 1990 to 2000(Frankin, 2001)
Number of plants
% average over database
% average over 1990-1996
% average 1997-2000
Low rate (lagoon/contact)
Total in database
Figure : Total number of UASB plants (open bar) and EGSB plants (ï¬lled bar) (Frankin, 2001)
2. REACTOR CHARACTERISTICS
Expanded bed refers to such kind of a bed in which the biomass grows on the surface of the small particles (typically diameters in the range of 0.2-2.0 mm), and therefore could cause an expansion of the bed. Other factors such as gas hold-up within the bed, and/or formation of a loose-packed bed by particle rearrangement could as well lead to the bed expansion. In an expanded bed reactor, the particles seem to be in steady state, however, the bed can be seen to expand gradually during a period of days (Cooper, 1981). Figure 1 shows a typical used expanded bed reactor in water treatment processes (Crittenden, 2005). Air, as an optional factor, determines whether the reactor is aerobic or anaerobic.
Figure : Expanded-Bed Up flow Reactor (Crittenden, 2005)
Generally, an expanded bioreactor, which shares a similar operation mode with a fluidized bed, is a tube with filled with fluidized suspended particles serving as a support for the biomass immobilization. The dimensions usually range from 10 to 500 m3 with a ratio of height to diameter of 1-5. Figure 2 is a schematic of anaerobic expanded bed process with recycle. Untreated influent flows upward of the reactor through the bed that is formed by the microbial film. The microbial film is developed and maintained on the support materials which are generally made up with inert small particles. Typically, these particles are small enough so that they can be suspended by the upward liquid flow. Part of the treated effluent will be recycled back into the inlet through an external loop and mixed with the influent for its dilution. This could decrease the substrate concentration and adjust pH. Residual treated effluent is then discharged as well as biogas generated during the aerobic or anaerobic process. It should be noticed that the support medium particles should not either sink or outflow (Jordening, 2005)(Jogdand, 2010).
Figure : Schematic of Anaerobic Expanded Bed Process (Jogdand, 2010)
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However, when applied to biological processes, an expanded bed has several different aspects compared to a fluidized bed reactor. Firstly, the velocity required to maintain a delicate attached living microbial film is lower. Secondly, the separation, retardation and bio-coagulation of the fine suspended particles are also different. Additionally, an expanded bed reactor can achieve larger biomass concentration and less microbial film shearing than a fluidized bed could. These differences make the expanded bed reactor a better bioreactor than a fluidized bed reactor (Jogdand, 2010).
The operation performance of an expanded bed reactor depends on support materials much more than other types of reactor do for the reason that fluidization characteristics depend on density and diameter of the support. Some of the materials that have been used in expanded bed systems include sand, sepiolite, granular activated carbon (GAC), pumice, biomass granules and biolite, etc. Generally, these materials have diameters much less than 1 mm. The small particles can increase the surface area for colonization, decrease the superficial upflow velocity and diffusion limitation. In addition, since microorganisms in pores can be protected against the shear force, therefore, porous materials can prevent the biomass gradients that usually happen when non-porous materials are used (Jordening, 2005). Tsuno and Kawamura (Tsuno & Kawamura, 2009) developed an expanded bed anaerobic reactor using the porous material GAC as support material. Figure 3 was the schematic diagram of the experimental apparatus.
Figure : Schematic Diagram of the Experimental Apparatus (Tsuno & Kawamura, Development of an expanded-bed GAC reactor for anaerobic treatment of terephthalate-containing wastewater, 2009)
The reactor was made of a thick Plexiglas tube with a diameter of 10 cm and a volume of 10 L. GAC was placed in the reactor as support material. Wastewater was circulated from top of the reactor to back to the bottom to achieve an expansion of the medium by 25% together with the influent wastewater. Feed water flow rate was controlled by the flow meter. The water temperature in the reactor was maintained at 30 â„ƒ using a constant temperature circulator. The gas produced was collected in a gasbag at the top of the reactor. In their research, GAC showed good adsorption ability of the contaminants in the wastewater. Its adsorption ability contributed greatly to the performance stability, especially when the system just started as well as the loading rate just increased (Tsuno & Kawamura, 2009).
Therefore, for an anaerobic expanded bed reactor, components such as bed settling zone which generally is a tube, feed water pump, effluent recycle system, flow meter as well as temperature controller are generally required. For an aerobic reactor, an additional aeration provided either directly or externally to the reactor (Cooper, 1981) would also be required in order to provide enough oxygen to maintain an aerobic biomass growing environment.
Compared with other conventional processes, both anaerobic and aerobic expanded bed processes can produce higher biomass concentrations. Because of the higher reaction rate inside the expanded bed reactor, a relatively smaller reactor volume, longer solids retention time and a low requirement for flow velocity can be obtained. All of these advantages could eventually lead to a lower investment. Furthermore, an anaerobic process could operate more efficiently than an aerobic process due to the very dense anaerobic microbial film, higher substrate concentrations and longer solid retention time in an anaerobic expanded bed reactor. Besides, effluent from an expanded bed reactor is usually of high clarity and has a much lower solids concentration than do other types of reactors (Cooper, 1981)(Jordening, 2005).
One concern regarding the anaerobic expanded bed reactor is that biological denitrification could happen in such a bio-reactor system because of the existence of nitrogen sources (Cooper, 1981). Denitrification, a biological process during which the reduction of nitrate to nitric oxide, nitrous oxide, and last nitrogen gas happens (Tchobanoglous, 2003), could leave nitrogen in the oxygen demanding form. Under this circumstance, it is necessary to consider a subsequent treatment process to remove nitrogen gas or converse it into ammonia (Cooper, 1981).
Since suspended solids can cause reactor clogging, especially at the entrance region of the reactor, and further influence the reactor performance, therefore, it is of great importance to control the solids concentration in the influent wastewater. Effective pre-treatment of wastewater during the coagulation, flocculation and sedimentation process would be necessary. Beside, application of certain kinds of device, such as a valve at the bottom of the reactor could also remove particles such as sand and other materials that could cause the clogging (Jordening, 2005).
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Another concern regarding the expanded bed reactor is the achievement of the uniform dispersion of support particles in the settling zone in compliance with minimizing material loss. In order to keep the support materials from getting lost, increasing the bed height may be applied. However, the ratio of height to diameter should be maintained during a proper range because the axial concentration gradients could increase with the height of the reactor. Furthermore, increase of reactor diameter could increase the difficulties to uniformly expand the materials simultaneously. Therefore, a balance between the two considerations should be found (Jordening, 2005).
Despite the many concerns associated with anaerobic expanded bed reactors, they are still more promising than aerobic processes. Therefore many technologies regarding the anaerobic treatment have been proposed and developed. Among them, the upflow anaerobic sludge bed (UASB) and expanded granular sludge bed (EGSB) have experienced a rapid development over the past years. An UASB reactor is an extremely simple reactor but allows for a high rate anaerobic treatment of wastewater. Wastewater first goes through the evenly spaced inlets at the bottom into the tank, then passes upwards the sludge bed where anaerobic microbial reactions happened (Saleh, April 2003). At top of the reactor, a Gas-Liquid-Solids Separator (GSS) is used to separate solids from the effluent as well as to emit gas out. The typical up flow velocity is 0.5-1.0 m/h and the height to depth is 0.2-0.5. An UASB has a capacity of 10-15 kg/m3/d to treat high-strength organic wastewater. However, one drawback associated with UASB is that solids in the influent have the potential to be accumulated in the UASB and cause latent effect to the effluent quality. Under this circumstance, the EGSB reactor is developed to deal with the problem (Lim S. , 2006).
An EGSB reactor is designed with a faster upward flow rate, typically higher than 6 m/h, allowing the wastewater to pass through the sludge bed. Together with the high velocity, a ratio of height to width ranging from 4 to 5 provides the microorganisms in the sludge bed more chances to react with the wastewater. This enables the reactor to treat high-strength organic wastewater at a loading rate of about 30 kg/m3/d which is at least twice of the capacity of an UASB(Lim S. , 2006). The utilization of tall reactors, or installation of an effluent recycle, or both can be applied to increase the upflow velocity. These factors make the main structure of an EGSB different from that of an UASB. Besides, an EGSB reactor can also be used to treat low-strength soluble wastewater and wastewater that contain non-biodegradable suspended solids (Saleh, April 2003), especially at low to mid temperature (Lim S. , 2006).
3. APPLICATION OF EXPANDED BED REACTOR
Expanded bed system is one of the most advanced technologies for anaerobic treatment of wastewater (Woodard, 2006). The bed works in the same principle as fluidized bed. The only difference is in the size of particle that will be fluidized. Generally sand, gravel or plastics are fluidized which are bigger in size in case of an expanded bed reactor (than in fluidized bed)(Thakur, 2006).
Expanded bed is commonly used for treating industrial waste. Full scale expanded bed systems are basically used in nitrification of industrial waste. Organic waste coming from sugar and distillery, pulp and paper, slaughter house, textile and dairy food industry are being treated using anaerobic expanded bed reactors for a long time(Rajeshwari, Balakrishnan, Kansal, Lata, & Kishore, 2000). Industrial waste is considered as high strength wastewater while municipal and domestic sewage wastewater is considered as a very dilute type wastewater.
Dairy industries use anaerobic treatment system for treating the waste they produce. In the test of removing milk fat using an EGSB reactor, it was seen that most of the fat get adsorbed on the granules of the bed and later gets decomposed. A feasibility test conducted based on the results of a survey among cheese producers in New York city revealed that efficient treatment of cheese whey is possible by using anaerobic treatment in an expanded bed reactor(Switzenbaum & Danskin, Anaerobic Expanded Bed Treatment of Whey, 1982). Attached film expanded bed reactor was also found suitable for efficient removal of low strength organic waste(less than 600mg/L) at a lower temperature and higher loading rate(Switzenbaum & Jewel, 1980).
Anaerobic digestion of long-chain fatty acids by using UASB and expanded granular sludge bed reactors was investigated in early 90's (Rinzema, 1993). The inoculums in this experiment were from potato processing wastewater and sugar-beet refinery wastewater, respectively. Concentrated stock solutions of sodium caprate and sodium laurate were used as the target pollutants. From the results obtained, the conventional UASB reactors cannot achieve efficient contact between the substrate and all the biomass when lipids contribute 50% or more to the COD of the waste water. Whereas, EGSB reactors did perform well in mixing and contact between substrate and biomass. They can accommodate loading up to 30 kg COD m-3/day with COD remove efficiency of 83%-91% and can be operated at HRT of 2 h without any problems.
In another study, the anaerobic degradation of a mixture of maleic, oxalic, fumaric, acetic and formic acids was studied by using EGSB reactor (Dinsdale, HawkesF.R., & Hawkes, 2000). As the wet-air oxidation and advanced oxidation processes do not usually give 100% oxidation of the refractory compounds, a significant level of short chain organic compounds will remain in the effluent. Served as a post-treatment process, the results suggested the total COD removal of 98% was observed at 6 h HRT and an organic loading rate of 10 kg CODm-3d-1. For each acids and WAO simulated effluent, the performance of the EGSB reactor is given in Table 1. To summarize, all of the major short-chain organic acids produced by AOPs can be successfully treated by anaerobic degradation.
Table : The treatment efficiency of the EGSB to single feed and simulated WAO effluent (22)
The large scale EGSB plant in Germany in the field of food processing industry was described in literature for several sectors such as potato, brewery, distillery, etc. (Austermann-Huan.U., 1999). The wastewater (amount of 1700 m3/d) passed through a screen and a fat separator into a balancing tank, which also operated as an acidification tank. The EGSB, as a methane reactor, had a height of 14 m with a water volume of 750 m3. With an average filtered COD of 3500 mg/L in the influent, the removal efficiency was 70%-85%, resulting in of the biogas production of about 80% methane content.
Treatment of palm oil mill effluent (POME) by EGSB reactor revealed that ,in start-up stage, the COD removal efficiency in the reactor reached more than 85% after only 2-d operation, which indicates EGSB reactor has a great potential to treat POME(Zhang Y. , 2008). Operated continuously at 35Â°C for 514 d, the total COD removal rate at steady state was invariably in the range of 89% and 96%, while, the biogas production was comparatively low. Some operation problems such as scum formation, sludge ï¬‚otation, blockage of pipeline, corrosion and blockage of gas meter were also mentioned in this paper.
In ammonium oxidation in EGSB, COD removal was more than 80% for a high organic influent concentration (500mg/L)(Jianlong & Jing, 2005).
The EGSB reactor was found capable of performing with a high capacity of denitrifying nitrite as soon as it was added, indicating that a great amount of denitrifiers were present in the inoculums granular sludge(Zhang D. , 2001). The results suggested of an analysis of nitrite containing wastewater showed that the efficiencies of COD removal varied from 92% to 97% at volumetric loading rates up to 6.56 g-1 COD L-1 d-1. Nitrite was most completely denitrified and resulted in more than 97% nitrite removal at nitrite volumetric loading rates up to 0.9 g NO2-1-N L-1 d-1 in the EGSB reactor. These results revealed that nitrite containing water could be anaerobically treated in an EGSB reactor.
Treatment of slaughterhouse wastewater by using EGSB reactor under 350C was studied (Nunez, 1999). The reactor has an internal diameter of 0.044 m and it is 1.4 m high and the total volume is 2.7 L. The superficial upflow velocity is approximately 8 m/h. The result showed during 152-278 days, the reactor achieved the maximum COD removal to 90.6% and methane production rate from COD to 80.9% at 7 h HRT. For TSS removal, it is indicated that the TSS washout is mainly formed by sludge and partially degraded solids. The accumulation of fats in the sludge was not significant.
A new branch of research is focusing on the use of EGSB reactor in psychrophilic anaerobic waste treatment at a low temperature. At such a low temperature biological and chemical activities become slower and so energy requirement will be more (Lettinga, Rebec, & Zeeman, Challenge of Psychrophilic Anaerobic Wastewater Treatment, 2001). Treatment of Brewery waste water under psychrophilic condition ended up with results contradicting to such statement. It was seen that in Brewery waste treatment, both mesophilic and psychrophilic conditions endow with similar COD removal efficiencies (Connaughton, Collins, & O'Flaherty, 2006b). Biodegradation of phenol containing wastewater at psychrophilic condition (9.5-15 in an EGSB reactor was conducted (Scully, Collins, & O'Flaherty, 2006). It was indicated that phenol can be successfully removed from the wastewater at a low temperature (below 10 and at applied organic loading rate up to 2 kg m-3d-1. From 9.5-15, the COD removal efficiency of 90% was observed. The biogas yields of 3.31 CH4 g-1 COD d-1 were recorded, which indicated that anaerobic biological treatment of phenol at mesophilic and psychrophilic conditions was economically feasible compared to other similar studies.
For low-temperature biological treatment of toluene-containing wastewater, by employing EGSB reactor, a high removal rate was observed. The reactor was operated at a hydraulic retention time of 24 h at applied organic loading rates of 0.71-1.43 kg COD m-3 d-1 and toluene concentrations of 5-104 mg/L. Additionally, toxicity assays were conducted to investigate the activity and toluene toxicity thresholds of key trophic groups, respectively, within the seed and reactor biomass samples. During a 630 d trial, the COD and toluene removal efficiencies of 70-90% and 55-99% were observed, respectively. The toxicity profiles suggested an increase of the toxicity level occurred over the trial period relative to the seed sludge (Enright, Collins, & O'Flaherty, 2007).
Expanded bed is rarely used in treating municipal waste. A few plants are working on nitrification and de nitrification of municipal waste. The prime concern for this rareness is scale up factors and proprietary constraints. Such a plant is economically unattractive as well.
P. Castilla et al. (2009) employed the EGSB reactor to study the biodegradation of a mixture of municipal wastewater and organic garbage leachate. The EGSB reactor of 2.8 L was inoculated with 840 mL of granular sludge (46.5 gVSS/L) equivalent to 39.0 gVSS and operated at 6 and 4 h HRT at an upï¬‚ow superï¬cial velocity of 12.6 and 10.2 m/h, respectively. The influent characteristics, in g/L: 12323.2 COD soluble; 8.58 TS; 17.8 acetic acid; 4.2 butyric acid; 0.5 valeric acid and 0.3 propionic acid; 0.45 NH4; 21.4 total protein; pH of 5.09, were diluted with raw municipal wastewater that contained, in mg/L: 22391 SCOD; 120 TS; 25 NH4; pH of 8.2. Both wastes were combined in a ratio 1:100 to form a mixture of 658.6225.4 mg/L SCOD with a mean pH of 7.6, and was fed to all reactors. The results suggested for a longer HRT (6 h), the COD removal reached 845% in the steady stage. Furthermore, short HRT (4 h) had no adverse effects on the reactor performance and EGSB reactor is a suitable option for treatment of highly concentrated municipal wastewater.
P. Castilla et al. (2009) employed the EGSB reactor to study the biodegradation of a mixture of municipal wastewater and organic garbage leachate. The EGSB reactor of 2.8 L was inoculated with 840 mL of granular sludge (46.5 gVSS/L) equivalent to 39.0 gVSS and operated at 6 and 4 h HRT at an upï¬‚ow superï¬cial velocity of 12.6 and 10.2 m/h, respectively. The influent characteristics, in g/L: 12323.2 COD soluble; 8.58 TS; 17.8 acetic acid; 4.2 butyric acid; 0.5 valeric acid and 0.3 propionic acid; 0.45 NH4; 21.4 total protein; pH of 5.09, were diluted with raw municipal wastewater that contained, in mg/L: 22391 SCOD; 120 TS; 25 NH4; pH of 8.2. Both wastes were combined in a ratio 1:100 to form a mixture of 658.6225.4 mg/L SCOD with a mean pH of 7.6, and was fed to all reactors. The results suggested for a longer HRT (6 h), the COD removal reached 845% in the steady stage. Furthermore, short HRT (4 h) had no adverse effects on the reactor performance and EGSB reactor is a suitable option for treatment of highly concentrated municipal wastewater (Castilla, et al., 2009).
Membrane coupled EGSB can treat municipal wastewater at a lower temperature. A study showed that COD removal efficiency of an EGSB was same as a up flow reactor for such prescribed treatment condition. But after some time during the treatment a cake of solid can form on the membrane which can severely reduce the efficiency (Lim S. , 2006). Low strength synthetic waste was treated with expanded bed reactor and the results were compared with the removal efficiency obtained for domestic sewage. It was seen that higher removal efficiency of waste can be achieved for synthetic waste (Rocky & Froster, 1985).
3.1 Application in textile
Textile mills produce a large amount of waste water and waste dyes. The colored material or dyes or their breakdown can produce toxic chemicals which is extremely undesirable for aquatic life. Textile dyes are generally difficult to treat in an activated sludge plant because of high organic load. Anaerobic treatment can provide decolonization of textile dyes and can handle high organic loads associated with the wastewater.
Typically a textile process may include sizing, desizing, scouring, bleaching, dying, printing and finishing and effluent can be produced from each step and needs to be treated.
Cotton yarn and fabric finishing industries produce a large amount of low organic loaded warm wastewater(Athanasopoulos, 1992). As the aerobic biodegradability is very low for these wastes, expanded bed reactor was employed to operate an anaerobic operation of textile waste treatment. It was found that, biodegradation of wastewater is very low and pH is the limiting factor for the degradation process. A pilot plant study for this purpose was operated with a 4m high reactor with the capacity of 2000L. The temperature of the reactor was maintained within 350CÂ±10C.The daily control of the reactor was maintained by controlling temperature, feed and recycle flow rate, pH, alkalinity, biogas production and composition, COD etc. During the operation the COD loading increased from 0.10kg/m3 to 0.63kg/m3 in a day. In the anaerobic degradation process, COD removal varied from 50%-70% and the production of biogas was 0.06-0.35l/g COD removed.70-80% methane was produced during the removal. At a COD concentration higher than 0.68kg/m3, biogas production stopped although COD removal continued to be 35% (approx.). Volatile fatty acid concentration was low all time but decreased with an increase in COD loading. The pH of the reactor had to be within 7.0 to 7.5.At a pH higher than this, the biogas production gradually diminished. The short retention time can allow significant COD removal not because of biodegradation but for absorption of organics on biomass that takes place even after the biogas production comes to an end. The results were found to be co-operating with the results obtained for an up flow filter.
Terepthalate is one of the main organic constituents that come out from a terepthalate process. Terepthalic acid is an important ingredient in the production of textile fibers. Generally the wastewater containing terepthalate undergoes aerobic biological treatment. From the previous studies it was known that anaerobic biodegradation of terepthalate is inhibited if the concentration of effluent is more than 150 mg/L (Tsuno & Kawamura, 2009). An anaerobic granular activated carbon expanded bed was found to be successful in removing organics and forming methane in wastewater treatment plant. An expanded bed with GAC was developed to treat wastewater both physically (by adsorption on GAC) and biologically (biodegradation by microorganism growth). Adsorption of organic on GAC occurs in the beginning of the removal process. Freundlich isotherm can explain the nature of adsorption. The reactor was found to be stable and successful in removing organics up to a COD loading rate of 2.9-4 kg COD /m3 when the loading was gradually increased. Simultaneously terepthalate loading rate was recorded to be 0.75-1 kg COD/m3.So,the removal was more than 90%. 90% of methane was produced in the system as well. An increase in terepthalate concentration in influent (85 kg/m3 to 430kg/m3) can eventually reduce and stop the production of methane. The critical value of loading for acceptable methane production was found to be 150mg/L. Above this value the concentration becomes too much and inhibits the biodegradation by inactivating the microbes. So, expanded bed GAC can be a satisfactory choice of anaerobic degradation of terepthalate in case of moderate strength of wastewater.
A laboratory scale expanded bed sludge bed reactor was used to treat polyamide waste in a 2 L reactor. The reactor was immunized with 16g VSS/l granular sludge and organic COD/m3.d while the up flow velocity was 10m/h. Good removal of dye was observed in the experiment. Especially azo dye degradation was reasonable. The optimum condition of treatment was not achieved in this work. Generally dye molecules are not readily degradable and require a co-substrate. The aerobic degradability of the wastewater was assessed with respirometry and was found negative. It was also found that anaerobic treatment does not help to improve the biodegradation in a later aerobic process rather it can be way of improvement. The dye wastewater was not adequate enough to supply co-substrate in the treatment process and the optimization analysis was not possible to make with respect of co-substrate and hydraulic retention time (HRT).
An acid reactor followed by an EGSB for methanation can offer an outstanding result compared to the situation of only using EGSB for textile dye and color removal. This was found in a research where performance of a two phase reactor was compared with that of a single phase reactor in terms of color, dye and COD removal (Bhattacharyya & Singh, 2010). The two phase reactor gives a better performance than a single phase reactor for high wastewater loading rate. For HRT of 7.5 hrs, two phase reactor can afford 90% COD removal while single phase reactor can only afford this when HRT is 15 hrs. At an HRT of 9hrs only 50% COD removal was found possible in a single phase reactor. Color and dye removal results also follow the same roadmap. Single phase reactor can remove 90% of influent color and dye with HRT of 18hrs while two phase reactor system can remove 80% of the influent color and dye within 7.5 hrs. Higher methane recovery was reported for single phase reactor system.
Table : Experimental condition of synthetic wastewater and actual wastewater treated in an anaerobic expanded bed reactor (Tsuno & Kawamura, 2009)
Table : Removal of dye, color and TCOD in the two phases and single phase reactor system (Bhattacharyya & Singh, 2010)
Figure : Variation of biogas production over time in a methane reactor of (a) two phase system and (b) single phase system
3.2 Application in the field of fermentation process water
A plant with two anaerobic 2-stage units was built in Delft in the Netherlands for purification of their fermentation process waste-waters. The unit is made up of 2 identical reactors, the acidification reactor Rl and the methanation reactor R2. Instead of making the granular sludge expanded in the anaerobic reactor, carriers are used and higher flow rate is applied to make the particles fluidized in the reactor. The reactor configuration is given in Figure 6. For the Delft plant, the reactor diameter is 4.6 m and the expanded bed height is 13 m. The expanded bed volume is 215 m3 and the total reactor height is 21 m. As can be seen, the total reactor height is much larger than the bed height because there is a 3-phase separator situated on the top of the reactor, together with a gas buffer D and liquid buffer C. The 3- phase separator is also able to separate the particles that flow out the reactor. The gas pressure in the buffer is maintained at 0.3 - 0.5 bar, which enables the transport of the biogas to the boiler house without a gas compressor. Because of this integrated design, the reactor is completely sealed and safe with respect to gas emissions, especially without emission of H2S containing biogas (up to 2% H2S in Delft). This is a favourable design in terms of odour control and safety regulation.
Figure 6: 2-stage anaerobic treatment system
The waste-water is fed into the system via pump 1 and enters the reactor together with recycled water, from pump 2, through the liquid distributor. In addition, this distributor is also capable of handling waste-water which is high in sediment without clogging. The Gist-brocades system does not contain a mechanical device for biolayer control. It is believed that gas/liquid turbulence would cause sufficient biolayer shear force. In the reactor, the recycle ratio is low () and hence the energy consumption of pumping is low as well (). Sand of 0.1-0.3 mm in diameter is used as carrier at a liquid superficial velocity of 8-20 m/h.
The reactor start-up in Delft took place in 1984, feeding yeast waste-water. Reactor R2 was inoculated with 1 m3 of sand containing methanogenic bio-layers. The performance of the anaerobic process is summarized in Table 5.
Table : The operation parameters for full-scale anaerobic system
Gist-brocades Delft 1984
COD efficiency (%)
Water flow (m3/h)
COD conversion (kg/m3d)
Biomass concentration (kg VSS/m3)
Fatty acids effluent (mg/L)
Specifically, 4-5 months after start-up an average COD-conversion capacity of 20 (kg COD) m-3day-1 was achieved. The COD remove efficiency was up to 70%, while the fatty acids concentration in the effluent is less than 100 mg/L. The methanogenic activity is high, being 0.5 (kg COD) (kg VSS)-1day-l for the acidification stage and 2.5 (kg COD) (kg VSS)-1 day-1 for the methanation stage. The reactor system can handle widely fluctuating waste-waters. Even a highly toxic peak of NO3- or large pH (3-10) fluctuation caused no harm to the treatment process.
Table : Summery of various applications of expanded bed reactor
Field of application