Treatment of raw sewage in a temperate climate using a UASB reactor and the hanging sponge cubes process

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Pergamon

War. Sci. Tech. Vol. 36, No. 6-7, pp. 433-440,1997. © 1997 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. 0273-1223/97 $17'00 + 0'00

PH: S0273-1223(97)00552-0

TREATMENT OF RAW SEWAGE IN A TEMPERATE CLIMATE USING A UASB REACTOR AND THE HANGING SPONGE CUBES PROCESS Lalit K. Agrawal*, Yasuhiro Ohashi*, Etsuo Mochida*, Hiroyuki Okui*, Yasuko Ueki*, Hideki Harada** and Akiyoshi Ohashi** * Civil Engineering Technology Department, Technological Division, Tokyu Construction Co. Ltd, 1-16-14 Shibuya, Shif,uya-ku, Tokyo 150, Japan ** Department of Enviornment Systems Engineering, Nagaoka University of Technology (NUT), Nagaoka, Niigata 940-21, Japan ABSTRACT The treatabillty ofraw sewage in a temperate climate (wintertime around 1O-20'C) using an upflow anaerobic sludge blanket (UASB) reactor and the hanging sponge cubes process was evaluated. After being seeded with digested sewage sludge, a 47.1 L UASB reactor was continuously operated for more than 2 years by feeding raw sewage, which had average COD around 300 mg/L (41 % soluble). During summertime at an HRT of 7 h, effluent COD approximately 70 mglL total, 50 mglL soluble and BOD5 20 mglL total, 12 mg/L soluble was obtained. During wintertime also, treatment efficiency and process stability was good. With the hanging sponge cubes process using the effluent of the UASB reactor treating raw sewage, the following results were obtained. The ammonia oxidation rates of 1.9 and 3.5 g NH4-N.m-2.d-1 in a downflow hanging sponge cubes biofilter, under natural air intake only were obtained during wintertime and summertime, respectively. With post-denitrification and an external carbon source, 84% in average N (N03 + N02) was removed with an HRT of less than 1 hour and in the temperature range of 13 to 30'C using an upflow submerged hanging sponge bed bioreactor, under anaerobic conditions. The overall system using a UASB reactor and the hanging sponge cubes process could be quite an attractive treatment alternative. © 1997 IAWQ. Published by Elsevier Science Ltd KEY WORDS Anaerobic treatment; denitrification; hanging sponge cubes process; nitrogen removal; nitrification; post• treatment process; raw sewage; temperate climate; UASB reactor.

INTRODUCTION High-rate anaerobic reactors are becoming increasingly popular for the treatment of various types of wastewater because of their low initial and operational costs, smaller space requirements, high organic removal efficiency and low sludge production, combined with a net energy benefit through the production of biogas. These reactors' operations are based on the immobilization of high concentrations of biomass. Among the various anaerobic reactors developed so far, the UASB reactor (Lettinga, 1980) has been found to be relatively superior because it is simpler and more economical and it neither requires added substratum as in anaerobic filters nor effluent recirculation as in fluidized bed reactors. Also, solid retention times can be maintained at a high level even at a low HRT by the development of a granular'sludge bed (Agrawal, 1997), so that efficient treatment can be carried out at low temperature as well. JWSf 3r':6/7-0 433

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Few studies involve anaerobic treatment of low-concentration wastewaters at low temperature (Switzenbaum, 1978; Grin, 1985; Sanz, 1990; Matsushige, 1990; Man, 1988; Kida, 1993; Rebac, 1995) and a breakthrough with respect to its application with actual sewage in a temperate climate has yet to be found. Moreover, anaerobic pre-treatment followed by effluent-polishing using an aerobic process is a strategy widely used for medium to high concentration wastewaters and the effluent is often discharged into the municipal collection system for further treatment. However, after proper treatment, sewage is discharged directly into natural water bodies. The removal of organics as well as nutrients (nitrogen and phosphorus) from the sewage is critical in order to protect the natural water bodies receiving effluent after final treatment. Insufficient land areas, and high land prices in urban areas often render the use of stabilization ponds unfeasible as a post• treatment process. High investment and operational costs have limited the feasibility of established aerobic processes. The lack of an appropriate post-treatment process to remove residual organics and nutrients remaining after anaerobic treatment may be one reason that anaerobic processes have not been widely adopted for sewage treatment. It is important that such a process be simple, compact, and inexpensive so that the overall system combining anaerobic and post-treatment process remains relatively attractive compared to other available treatment systems. We investigated a UASB reactor for more than 2 years by feeding it raw sewage in a temperate climate (wintertime around 10-20"C). The digested sewage sludge was used to seed the reactor. The results were examined with respect to organics removal, gas and sludge production rate, and sludge retention. We have also developed a hanging sponge cubes process as one option to be used as a post-treatment process. In this process, a large surface area to accommodate microbial growth could be provided in the form of hanging sponge cubes. We investigated a downflow hanging sponge (DHS) cubes biofilter for residual organics removal and nitrification, and an upflow submerged hanging bed (USHB) bioreactor under anaerobic conditions for denitrification. In the DHS biofilter, the waste is trickled through the sponge cubes, which are diagonally linked using nylon string, where it supplies the nutrients for resident microorganisms. All the experiments were conducted under ambient conditions.

MA TERIAL AND METHODS Schematic diagrams of the UASB reactor and hanging sponge cubes process (DHS biofilter and USHB bioreactor) are shown in Fig. 1. The UASB reactor (47.1 L) consists of two parts: a cylindrical column with a conical• shaped bottom, and a gas-liquid-solid (GLS) separator. The reactor column has a height of 1800 mm, and an internal diameter of 155 mm. The GLS separator is fitted with tube settlers and a scum breaker. Ports for obtaining sludge samples are arranged along the length of the reactor, the first one at 100 mm above the base of the column and the others at 200 to 300 mm intervals. The reactor was seeded with digested sewage sludge (equivalent to 514 g volatile solids), which was obtained from a mesophilic sewage sludge digester located in Nagaoka, Japan. The seed sludge had total solids concentration of 33 giL, and volatile solids content of 47%. Biogas was measured with a wet gas meter (Sinagawa, Japan) after passing it through a column of water (acidified with HCI down to a pH of less than 2 and saturated with NaC\). A 24 hour composite sample was used for the analysis of COD and

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BODs. During collection, the sample was stored at 4'C. The DHS biofilter has three falls, each of 2 m high. The effluent of the UASB reactor passes through these falls in series. Each fall comprises 120 sponge cubes (1.5 cm in size) linked diagonally using nylon string. All three falls had been previously inoculated by placing them into activated sludge treating sewage for 48 hours. The USHB bioreactor for denitrification has a liquid volume of 770 mL of which about 28% was occupied by the sponge cubes. The bioreactor was inoculated by placing the cubes into a secondary sedimentation tank of the above sewage treatment plant for 48 hours. The analytical procedures described in the Standard Methods for the Examination ofWater and Wastewater were followed. Pre-treatment of COD samples to remove sulfides was conducted according to Agrawal et at., 1997. Prior to BODs measurements, pre-treated samples were neutralized first. For NH4, N02, and N03 determination, the Hach pack test method (HC-IOOO) was used. The composition of biogas was measured using a gas chromatography (Harada, 1994).

RESULTS 10 •

The performance of the VASB reactor and hanging sponge cubes process (DHS biofilter and USHB bioreactor) in treatment of raw sewage was evaluated under ambient conditions. The DHS biofilter was evaluated for residual organics removal and nitrification under natural air intake only. The VSHB bioreactor under anaerobic conditions was evaluated for denitrification using an external carbon source.

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the fluctuations in influent pH, effluent pH remained very stable throughout the experiment at around 6.75 (Fig. 2C). Influent total COD fluctuated greatly, however, soluble COD was somewhat constant (Fig. 20). The effluent suspended solids (SS) was low and stable except during sludge washout which was because the reactor was almost fully occupied by the sludge bed (Fig. 2E). During the summertime, effluent COO approximately 70 mg/L total, 50 mg/L soluble and BODs 20 mg/L total, 12 mg/L soluble was obtained except during the washout (Fig. 2F, G). During the wintertime also, treatment efficiency and process stability was good. The yield of biogas was 9.5 NL gas/m3 of sewage treated at an average temperature of 12.5·C (January to February), 25.7 NL gas/m 3 at 17.4°C (March to May), and 44.4 NL gas/m 3 at 25.2°C (June to July) (Fig. 2H). The gas contained about 70-75% CH4, 16-20% N2, and 8-10% C02.

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The total amount ofretained sludge on days 0 (seed sludge), 44, 144, 233, 317, 485, and 706 was estimated from the SS and e~o B""~ 800 volatile suspended solids (VSS) concentrations along the reactor :E ~ 400 (Fig.3A). The total amount of the seeded sludge was measured "E~ O+.--...........................,............ """T""...........,......~ at only about 33% after 144 days of reactor operation due to the 0 .... Eo-<.5 0.8 retainment of heavier sludge ingredients only. Thereafter, the o 0.6 amount of retained sludge gradually increased up to the 233rd .~ 0.4 day of operation. The amount after 233 days of operation did not ~ 0.2 ~ O.O+.--...........................,.............................,..................-t vary significantly (549 to 600 g SS) due to occasional sludge 150 300 450 600 750 ~ 0 washout, as explained earlier. The increase in the ratio of VSS to > SS in the sludge bed against time also indicated the enrichment of Time (days) biomass in the reactor (Fig. 3B). The ratio gradually increased to Fig. 3. Sludge retention. (A) total 0.66-0.73 in the sludge bed after over 700 days of operation. amount; and (B) VSS/SS ratio of retained sludge at different Fig. 4 shows the SS and VSS concentrations along the reactor at times an HRT of 7 h. As shown in the figure, the sludge bed occupied most of the height of the reactor. The maximum concentration of 1.8 SS in the sludge bed was at 30 to 70 cm height from the bottom 1.5 of the reactor. At the bottom of the reactor, SS concentration ,....... was lower (18.7 g SS/L). The sludge bed appeared to be in the e 1.2 '-" .... floating mode. Moreover, a part of the upper sludge bed was .J:: OJ) observed to occasionally move upwards, especially during the 0.9 '4) .J:: wintertime, probably due to the occlusion of gas in the bed. It ....0U"" 0.6 was also observed to be descending, probably after liberating the t:'S <1> entrapped gas. The sludge production rate was estimated on the 0.3 et:: basis of washed out and retained sludge over 100 days period. It 0.0 0 5 10 15 20 25 30 35 was found to be equivalerlt to 0.036 kg SS (25 percent ash) per m3 of the sewage treated and was estimated to be only about 25% Cone. (gIL) of the influent SS. Thus, the influent SS, which made up about Fig. 4. Cone. of SS and VSS along 59% of the sewage COD, was effectively retained and the reactor at an HRT of 7 h subsequently biodegraded in the reactor. ~

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Hanging Sponge Cubes Process The DHS biofilter was continuously fed from an overflow tank used to store UASB effluent (Fig. 1). Due to overflow of the settled effluent from the tank COD inside the tank was comparatively higher (COD 100-135 mg/L; 45-65% soluble) than the UASB effluent-COD. The biofilter was operated at a hydraulic loading rate (HLR) of 0.06 m3·m- 2·d- 1 (-30 Lid) during the wintertime period and at an HLR of 0.11 m 3 .m-2.d- 1 (-60 Ud) during the summertime period (Fig. 5). The loading rates are based on the total surface area of the (three fall) sponge cubes. The average ammonia concentration in the influent was 35 mg NH4-N/L. At an HLR of 0.06 m3 .m- 2 .d- l , almost complete nitrification was achieved at around 20·C. As would be expected, the nitrification efficiency decreased as the temperature decreased. However, nitrification was observed below the temperature of lO'C also. The average ammonia oxidation rate during the wintertime was estimated to be 1.9 g NH4- N .m- 2 ·d- l , based on the influent and effluent ammonia concentration. The HLR was increased to

Raw sewage treatment in a temperate climate

0.11 m 3 .m- 2 ·d- 1 when the temperature rose above 20°C. At this loading rate, nearly complete nitrification was achieved when the temperature was above 24°C. The ammonia oxidation rate during the summertime was estimated to be 3.5 g NH4-N.m-2.d-l. Our results were compared to those referred to in a table by ge~en et ai., 1995. It was found that we obtained equal or superior values compared to those obtained by using submerged filters, RBC, and trickling filters.

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Fig. 6 shows the COD, NH4• N, N03 -N and N02 - N . concentration, and pH profiles Time (VASB days) in the DHS biofilter during the wintertime (13°C) at an HLR of Performance of DHS biofilter. (A) ambient temperature; 0.06 m3.m- 2.d- 1 (Fig. 6A) and Fig. 5. (B) influent NH 4-N; and (C) effluent NH 4-N against time during the summertime (25°C) at an HLR of 0.11 m 3·m- 2·d- 1 (Fig. 6B). As shown in the figures, the characteristics of the biofilter during the I wintertime and ~ Q,) summertime were very e~ 2 . ~simtlar. In fall one, N ~ the first two meter ~ e II length, mainly sulfides ~ ~ 4 and COD were ~ "0 removed. As a result, e § III pH increased in fall 6 ~"""--r-"""""T"""'-r--+---+~---tI~....--I one. White material, o 25 50 75100125 6 7 8 0 25 50 75 100125 4 5 6 7 8 possibly sulfur, was observed on the Cone. (mg/L) pH Cone. (mg/L) pH sponge cu bes in the upper part of the fall Fig. 6. Profiles of DHS biofilter. COD, NH4-N, N0 3-N and NOrN one. Nitrification and concentration, and pH during the (A) wintertime (13 oC) at an HLR of COD removal occurred 0.06 m3/m 2/d; and (B) summertime (25 oC) at an HLR of 0.11 m3/m 2/d simultaneously in fall two (2nd two meter length). In fall three (3rd two meter length), mainly nitrification was performed. There was no accumulation of N02 in any fall. As a result of nitrification, pH decreased in fall two and three. During the summertime, effluent NH4-N+N03-N+N02-N concentration was higher than influent NH4-N (Fig. 6A). This was probably because of losses of water due to evaporation as the wastewater was trickled through the hanging sponge cubes. The effluent (from fall three) contained a negligible amount of SS and total COD was only in the range of 10 to 25 mg/L. Moreover, a large part of the COD was non-biodegradable (BODs/COD = 0.1 to 0.2). Rusten et ai., 1995 also referred the effluent filtered COD from a fully nitrifying biofilm reactor as inert.

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The USHB bioreactor was continuously fed with the effluent of the DHS biofilter at various HRT. The HRT was gradually decreased to 0.5 h (Fig. 7A). Sodium acetate was added to the bioreactor as an external carbon source for denitrification. During the experiment, effluent pH was around 9.0 whereas temperature ranged from 13 to 30·C (Fig. 7B). On average 84% N (N0 3 + N02) was removed with an HRT of less than 1 hour (Fig. 7A, C, D). In addition to N03 + N02 removal, some ammonia-nitrogen removal was also observed (Fig. 7C, D). With post-denitrification and an external carbon source, Rusten et al., 1995 reported the total N removal of 80 to 90% at an HRT of less than 3 hours in a moving-bed biofilm reactors. In this study, C:N ratio was between 4 to 6 g COD/g N03-N. This compares favorably to the optimum CN ratio of 4 g COD/g N03-Neq obtained by Rusten et al., 1995 and of 5.0 g COD/g NOx-N obtained by ~egen et al., 1995. Rusten et a/., 1995 used on-line N03-N analyzer that maintained an almost constant CN ratio in the influent.

DISCUSSION UASB Reactor The effluent quality before 233 days of reactor operation was highly inferior when the temperature was below 20·C (Agrawal, 1996). This was due to reduced biomass activity at low temperature and also due to a smaller retained biomass in the reactor (Fig. 3). Results under these conditions showed an effluent soluble COD increase up to 500% (soluble BODs up to 1000%) over the summertime level. After 233 days of operation, effluent soluble COD and BODs increased only about 50% when the temperature decreased below 20·e. Clearly, this result was due to the retention of a highly concentrated, active biomass in the reactor, (Fig. 3 and 4). After 233 days of operation, retained biomass was estimated to be 12.7 kg SS/m 3 reactor volume. However, granules were observed in the -; upper part of the sludge bed 2 years and 4 months HRT (h) 6 > o after the start of the reactor operation. Granules --0% removal were up to 5-6 mm in diameter. Some granules ~ were also observed in the effluent weir. o+---'---.-J~~~~!l!.

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Hanging Sponge Cubes Process The high level of residual organics removal and nitrification by the DHS biofilter was due to a better waste-resident biomass contact and air diffusion. This possibly was achieved as a result of the shape, size, porosity, and arrangement of sponge in the DHS bioreactor. The DASB effluent took in sufficient natural air while trickling through the sponge cubes. Also, most of the organics and all sulfides removal occurred in the first fall of the DHS biofilter. Thus, in the remaining falls, the nitrification process was more efficient. The efficiency of the biofilter was maintained throughout the experiment without squeezing the sponge cubes to expel accumulated sludge out of it. We observed higher developed microorganisms on the surface of the sponge cubes, which consumed a significant amount of sludge. Also, the net growth of biomass after decay in the biofilter should be very low, because

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Raw sewage treatment in a temperate climate

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the biofilter was used to treat low organic wastewater. Thus, very little sludge could be seen on the surface of the cubes, especially of fall two and three. Fig. 8 shows the hanging sponge cubes before (Fig. 8A) and after (Fig. 8B) one year of ?peration of the DHS biofilter. The distribution of influent in a full scale plant may be one of the problem WIth the DHS biofilter in an actual application. We are planning to use a sprinkling and pierced-plate distribution system in which falls are arranged under the holes of the plate to receive the influent.

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We first studied the post-denitrification to evaluate the denitrification capacity of the USHB bioreactor. This is because the post-d~nitrification is independent of the amount of easily biodegradable organic in the influent. The other advantage of the post-denitrification is that the reactor size could be considerably reduced as we also found in our study. In this study, a high level of nitrogen was removed at an HRT as low as 0.5 h. We provided no additional mixing in the reactor. the volume of the sponge cubes was about 28% of the reactor volume. This volume ratio could provide homogeneous water circulation for efficient contact of resident• biomass and wastewater in the submerged biofilm reactor (Lessel, 1991). We would also like to investigate pre-denitrification to utilize residual organics as a carbon source for denitrification. Also, oxidation of H2S, which was contained in the effluent of the UASB reactor, with recycled N03 could be achieved in the pre• denitrification process (Kiihl, 1992; Garuti, 1991). In fact, substantial removal of nitrogenous and organic substances, removal of sulfides, and an increase in the redox potential in the pre-denitrifcation process might have positive effects on the subsequent nitrification process (Collivignarelli, 1990). Thus, by using pre• denitrification, higher ammonia oxidation rate than the present value might be achieved in the DHS biofilter.

CONCLUSIONS The UASB reactor achieved a high level of performance at an HRT of 7 h without a large effect on the effluent quality during periods of low temperature. Moreover, by using the hanging sponge cubes post-treatment process, biological removal of organics and nitrogen could be performed efficiently at high loading rates. Our results clearly showed that the UASB effluent could satisfy regulations in terms of BODs for discharge into rivers even during wintertime after a simple post-treatment, such as one we developed. We suggest that further research on the hanging sponge cubes process, including the pre-denitrification, the distribution system for the biofilter, and the scale up of the hanging sponge cubes process, be carried out on its application for actual wastewater treatment systems.

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ACKNOWLEDGEMENT The authors wish to thank the management committee of the Hinataoka sewage treatment plant (Hiratsuka, Japan), for allowing us to use their plant for this experiment. The authors would also like to acknowledge the support of Yuji Sekiguchi, Izarul Machdar, and Oshio Minoru of NUT.

REFERENCES Agrawal, L. K., Harada, H. and Okui, H. (1997). Treatment of dilute wastewater in a UASB reactor at a moderate temperature: performance aspects. J. Ferment. Bioeng., 83 (2), 179-184. Agrawal, L. K., Mochida, E., Okui, H., Ueki, Y. and Harada, H. (1996). Low temperature treatment of raw sewage in an UASB reactor. Proc. 51th Annual Con! Japan Society of Civil Engineers, 7, 2-3. APHA (1985) Standard Methodsfor the Examination of Water and Wastewater (l6th Ed.). Am. Publ. Hlth. Assoc., Washington, D. C. ~e~en, F. and Gonen~, L E. (1995). Criteria for nitrification and denitrification of high-strength wastes in two uptlow submerged filters. Water Environ. Res., 67, 132-142. Collivignarelli, c., Urbini, G., Farneti, A., Bassetti, A. and Barbaresi, U. (1990). Anaerobic-aerobic treatment of municipal wastewaters with full-scale upflow anaerobic sludge blanket and attached biofilm reactors. Wat. Sci. Tech., 22 (112), 475-482. Garuti, G., Dohanyos, M. and Tilche, A. (1991). Anaerobic-aerobic combined process for the treatment of sewage with nutrient removal: the ananox®process. Sixth Int. Symp. on Anaerobic Digestion, Sao Paulo, Brazil, 371-380. Grin, P., Roersma, R. and Lettinga, G. (1985). Anaerobic treatment of raw domestic sewage in UASB· reactors at temperatures from 9-20°C. Proc. sem.lworkshop: Anaerobic treatment ofsewage. Univ. of Massachusetts, Amherst, USA, 109-124. Harada, H., Uemura, S. and Momonoi, K. (1994). Interaction between sulfate-reducing bacteria and methane-producing bacteria in UASB reactors fed with low strength wastes containing different levels of sulfate. Wat. Res., 28, 355-367. Kida, K., Tanemura, K. and Sonoda, Y. (1993). Evaluation of the anaerobic treatment of sewage below 20'C by novel processes. J. Ferment. Bioeng., 76,510-514. Kiihl, M. and J 0rgensen, B. B. (1992). Microsensor measurements of sulfate reduction and sulfide oxidation in compact microbIal communities of aerobic biofilms. Appl. Environ. Microbiol., 58 (4), 1164-1174. Lettinga, G., Van Velsen, A. F. M., Hobma, S. W. and de Zeeuw, W. (1980). Use of the upflow sludge blanket (USB) reactor concept for biological wastewater treatment, especially for anaerobic treatment.

Biotechnol. and Bioeng., 22,699 -734. Lessel, T. H. (1991). First practical experiences with submerged rope-type bio-film reactors for upgrading and nitrification. Wat. Sci. Tech., 23 (4-6), 825-834. Man, A.W.A. de, Last, A.R.M. van der and Lettinga, G. (1988). The use of EGSB and UASB anaerobic

systems for low strength soluble and complex wastewaters at temperatures ranging from 8 to 30'C. p. 197-209, In Hall, E. R. and Hobson, P.N. (ed.), Proc. 5th Int. Symp. on Anaerobic Digestion, Bologna, Italy. Matsushige, K., Inamori, Y., Mizuochi M., Hosomi, M. and Sudo, R. (1990). The effects of temperature on anaerobic filter treatment for low-strength organic wastewater. Environ. Technol., 11,899-910. Rebac, S., Ruskova, J., Gerbens, S., Lier, 1. 8. van, Starns, A. J. M. and Lettinga, G. (1995). High-rate anaerobic treatment of wastewater under psychrophilic conditions. J. Ferment. Bioeng., 80, 499-506. Rusten, 8., Hem, L. J. and 0degaard, H. (1995). Nitrogen removal from dilute wastewater in cold climate using moving-bed biofilm reactors. Water Environ. Res., 67, 65-74. Sanz, 1. and Polanco-Fdz, F. (1990). Low temperature treatment of municipal sewage in anaerobic fluidized bed reactors. Wat. Res., 24 (4), 463-469. Switzenbaun, M. S. and Jewell, W. J. (1978). Anaerobic attached film expanded bed reactor treatment of dilute organics. Proc. 51 st Annual WPCF Conference, Anaheim, California, 1-164.