PERFORMANCE CHARACTERISTICS OF THE STATIC GRANULAR BED REACTOR

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1 PERFORMANCE CHARACTERISTICS OF THE STATIC GRANULAR BED REACTOR Kristin F. Mach and Timothy G. Ellis Iowa State University, Department of Civil and Construction Engineering, Ames, Iowa ABSTRACT The Static Granular Bed Reactor (SGBR) is a new and innovative process for the treatment of low to medium strength wastewater. The SGBR was simple to operate and produced a high quality effluent with low chemical oxygen demand (COD), low volatile fatty acids (VFA), and low suspended solids concentrations. Two SGBRs were operated at ambient conditions (22 ± 2 C) for a period of eighteen months. The reactors had exceptional organic removal at all HRTs. Methane production was near theoretical production based on COD conversion. Granules were observed to increase in size and had significantly different morphology from the original seed granules during the course of the study. INTRODUCTION An innovative new anaerobic process has been developed for medium to low strength wastewater. The Static Granular Bed Reactor s (SGBR) simple construction, easy operation, and exceptional removal characteristics make it ideal for both industrial and municipal sites. The reason behind the exceptional performance characteristics of the SGBR is its high microbial density achieved by seeding the reactor with active anaerobic granules such as those from an operating upflow anaerobic sludge blanket (UASB) reactor, anaerobic migrating blanket reactor (AMBR), or anaerobic sequencing batch reactor (ASBR). The formation of granules has been observed in many studies. Hulshoff Pol et al. (1983) found that most granules need an inert support structure to form upon, giving the organisms a building block. Others have noted that organisms adhere to other organisms forming the structure base (MacLeod et al., 1990 and Guiot et al., 1992). Usually additional selection pressure is needed to force the organisms together, such as the velocity force from an upflow reactor (Guiot et al., 1992). However there is a selective mechanism process that determines which microorganism clusters will stay and those to be washed out in the wastage process. By combining the two ideas, an optimum granule environment may be to encourage microbes to adhere by providing an external selection pressure and to use a wastewater low in solids which may induce a natural selection process. Sufficient nutrient and energy sources for organism development is an important consideration in granule formation. Organic loading may also influence the forming of granule structures. Hulshoff Pol et al. (1983) observed granule formation at loading

2 conditions above 0.6 kg COD kg VSS -1 day -1. However, lower loadings were expected to be sufficient to support the metabolism of established granules in this study. Granule formation is not limited to anaerobic organisms (Lettinga et al., 1997). However most research has focused on anaerobic granules. The internal structure of the granule may vary depending on the type of substrate being degraded. Some granules contain layers created by different species of organisms working in a symbiotic relationship. Non-carbohydrate feed sources have been shown to produce homogenous, non-layered granules (Fang et al., 1994). Sucrose, brewery, and other wastewaters produce a visibly organized and layered structure because of the methanogenic conversion steps involved. In most anaerobic granules, the outer layer usually breaks down complex substrates into volatile fatty acids which then are broken down to acetate and methane deeper into the granule (Fang et al., 1994, 1995). A wide consortia of organisms can be found on the surface of the granule (Morgan et al., 1991). It has also been observed that filametous Methanothrix may be critical in the granulation process (Fang et al., 1995) possibly to hold the organisms together. Because of the array of organisms in close proximity and their interaction with each other, granules seem to have a high conversion rate and are easily adaptable to different substrates. Several studies have shown the diverse microbial communities within the granules. Because of the diversity, the granules are suitable for low strength wastewater, requiring shorter acclimation periods. The ideal treatment process would be where the wastewater could contact as much of the surface of the granules as possible. The Static Granular Bed Reactor creates that situation by using a fixed bed of active granules in a downflow configuration. Typically, anaerobic systems are capable of operating effectively over a wide range of organic loadings. A typical range for high rate systems is 3.2 to 32 kg/m 3 d -1 (Speece, 1996). Good organic removal was achieved in an anaerobic fluidized bed reactor treating distillery wastewater was loaded at 15 g COD/L (Perez et. al. 1998). At an HRT of 1.2 and 0.65 days, the anaerobic fluidized bed reactor removed 83 and 75% of the total COD respectively. Yan and Tay (1996) examined an UASB treating brewery wastewater with a strength of 2000 mg COD/L. The UASB attained 89% soluble COD removal, but required supplemented alkalinity concentrations of 1200 mg CaCO 3 /L for optimal gas production. Harris and Dague (1993) operated two anaerobic filters, one at thermophilic (55 C) temperatures and one at mesophilic (35 C) temperatures. The thermophilic filter obtained peak total COD removals of 88% at a 48 hour HRT at 2.75 g COD/L day -1. At higher loadings, the COD removal decreased. The mesophilic reactor was not able to handle high organic loadings, but at the lower OLR the filter was capable of 90% COD removal. The sensitivity of anaerobic systems to temperature changes is thought to be due to the methanogens. An anaerobic sequencing batch reactor (ASBR) was operated at temperatures ranging from 5-25 C. At psycrophilic temperatures methanogenic activity

3 was decreased when compared to the activity at 25 C, but was still stable (Banik et. al., 1997). Methanogenic activity decreases at lower temperatures and may cause a build up of volatile acids. Some systems, however, are able to produce effluent with low VFA concentrations. An Expanded Granular Sludge Bed (EGSB) was operated at OLRs up to 16.8 kg COD m -3 d -1 at HRTs ranging from hours. Operating at 20 C, the reactor removed 70% of COD and also maintained volatile acid concentrations in the effluent less than 150 mg COD/L (Rebac et. al. 1997). Several studies have been performed with anaerobic systems treating low strength wastewater. Ndon and Dague (1997) examined the performance of an ASBR at different HRTs and operating temperatures. Operating at 35 C, with a substrate concentration of 1000 mg COD/L and HRTs of 24 and 12 hours, the ASBR achieved soluble COD removal of 93 and 81%, respectively. Collins et. al. (1998) treated primary clarifier effluent with an expanded granular bed reactor (EGBR) and achieved greater than 90% COD removal at 20 C. Similar to the SGBR, the system also had very low VFA concentrations. Orozco (1996) achieved an optimum of 92% COD removal using an anaerobic plug flow reactor operated at 13 to 17 C with synthetic wastewater. In contrast to the UASB and the EGBR, the SGBR is a bed of anaerobic granules operated in a downflow mode without flow recirculation. The resulting high granule density optimizes the contact between the microorganisms on the granule surface and the wastewater. The simplicity of the SGBR operation may offer a significant advantage over other systems which require recirculation pumping, elaborate solids/liquid/gas separation devices temperature controllers, or sophisticated underdrain and backwashing systems. The purpose of this study was to evaluate the performance of the SGBR under various HRT conditions with a constant feed strength of 1000 mg COD/L at ambient temperatures (22 ± 2 C). In addition, the granules were observed for changes in size and morphology. MATERIALS AND METHODS Two laboratory-scale SGBRs were observed for approximately 18 months during this study. Both were constructed of 0.25 inch plexiglas with a working volume of 1 liter. The difference between the two SGBRs was the diameter of the reactors. SGBR 1 had an inner diameter of 4 inches and SGBR 2 had an inner diameter of 2.5 inches. Anaerobic granules were obtained from an operating UASB at Heilemans s Brewery in LaCrosse, Wisconsin. Various HRTs were examined with a feed concentration of 1 g COD/L. The SGBRs were operated at ambient temperatures (22 ± 2 C). A synthetic wastewater consisting of non-fat dry milk amended with sodium bicarbonate and micronutrients (See Table 1) was used for the study. The reactor was fed several times per hour to attain the desired hydraulic residence time (HRT). Feed was stored at 4 C until pumped into the reactor. A Masterflex peristaltic pump was used to transport the feed to the reactor. Effluent was refrigerated at 4 C until analyzed.

4 The reactors were started at a 36 hour HRT to allow the granules to acclimate to the substrate. Once steady state was obtained, the HRT was lowered. Steady state for this study was defined as consistent gas production, gas composition, and chemical oxygen demand (COD) removal were obtained. Steady state conditions were usually achieved within two months of operation at the new condition. Set up and operation of the reactor required minimal effort. In addition to the reactor chamber, a single feed pump was needed. Granule acclimation to the NFDM feed source appeared to occur rapidly. The SGBR was operated in a downflow manner and the effluent flowed out by gravity. A T connector was used to control the liquid level in the reactor (See Figure 1). Only minor maintenance was needed to maintain the SGBR. Occasionally, the feed was temporarily reversed to dislodge any granules which clogged the underdrain system. A separate connection was installed to prevent the feed from contaminating the effluent tubing. Chemical oxygen demand (COD), volatile fatty acids (VFA), alkalinity, ph, biological oxygen demand (BOD 5 ), and ammonia were tested following procedures in Standard Methods for the Examination of Water and Wastewater (Eaton et. al. 1995). Gas composition was analyzed using a Gow Mac gas chromatograph. Gas composition was measured every other week. COD, VFA, and alkalinity were tested weekly. ph was measured daily and BOD 5 was measured once each HRT period. Effluent samples were collected from daily composite samples, and the feed was sampled directly from the feed container. RESULTS AND DISCUSSION The SGBRs had excellent performance in several areas with respect to COD removals, volatile acid concentrations, and low suspended solid concentrations (See Tables 3 and 4 Operating characteristics of the SGBR. and Analytical results of the SGBR. ). As the HRT was lowered, both reactors maintained total COD removal of 95% or greater (Figure 2 COD removal for the SGBRs. ). BOD 5 results confirmed the high organic removal with results of 95% reduction or greater. In addition to excellent organic removal, the reactors maintained low VFA concentrations in the effluent. The VFA concentration, as tested weekly using method 5560 C in Standard Methods, averaged 11.5 mg/l as acetic acid. However, gas chromatagraph (GC) analysis suggested the titration method may have over estimated the VFA concentration. Acetic, propionic, butyric, and valeric acid concentrations were measured at or below the GC detection limit of 1 mg/l. Collins et. al. (1998) obtained similar VFA results (20 mg/l, by Standard Methods) treating primary clarifier effluent in a EGBR. Both the SGBR and the EGBR were operated at or near 20 C. The low VFA concentration in the effluent was an indication of effective methanogenisis. The COD balance for the SGBRs confirmed the conversion of COD to methane (Table 3 Operating characteristics of the SGBR. ).

5 Figure 3 shows the average daily gas production for the reactors and vertical lines indicate the HRT condition (Figure 3 Gas production in the SGBRs. ). Gas production increased over time due to the higher organic loading rate, and the methane content was 80% or greater for the length of the experiment. By comparison, two anaerobic filters operated in series reported lower methane concentrations (Howerton and Young, 1987). The lead reactor produced 63-66% methane and the second reactor produced 69-75% methane. The lower methane production corresponds with lower COD removal. In the SGBR, the high methane content can be attributed to high COD removal based on the COD balance. Effluent suspended solids (SS) concentrations were lower than 30 mg/l for all operating conditions, except one in SGBR 1 during the initial start up. The effluent consistently mets the NPDES permit requirement (30 mg/l and 30 mg/l BOD 5 ) for some surface water discharges. Few anaerobic systems are capable of such low effluent suspended solids and BOD 5 concentrations. The SGBR may be one of the few anaerobic systems capable of providing effluent suitable surface discharge. Because of the high quality effluent, the discharge from treatment could be easily disposed. The effluent could be released into a receiving stream with no adverse effects due to the low COD and SS concentrations provided that nutrient removal is not required. Stream discharge would help both municipal and industrial sites lower treatment and processing costs. In addition, the effluent could be used as rinse water or other non potable uses. The increased availability of non potable water could cut down on water cost for the industry. Overall the implementation of the SGBR may be profitable for industrial and municipal facilities. The SGBR had a unique configuration with minimal equipment needed for operation. It required little upkeep and therefore minimal training for operation. The infrequent clogging problems were quickly resolved by reversing the feed direction or bubbling offgas through the granule bed. Start up of the reactor was observed to be very short. Within days the lab SGBRs were producing methane and removing organics. In addition to simple operation, the SGBR occupied a small volume compared to other processes. This was be attributed to the high biomass concentration of the granules (i.e. low food to microorganism ratio, F:M). The compact reactor size would be useful when adding the SGBR to an existing facility with limited land area. The smaller reactor volume required results in lower capital costs as well. Another advantage of the SGBR was its bed of active granules. Anaerobic filters use packing material or some other support structure for the attachment of biomass. Howerton and Young (1987) operated two anaerobic filters in series at 30 C. At a system organic loading rate (OLR) of 4 kg m -3 d -1, a peak efficiency of 98% COD removal was achieved for the two filters. The effluent from the second filter contained less than 90 mg COD/L and suspended solids less than 90 mg/l. One disadvantage with this set-up is limiting the growth of the biomass particles to space available on the support media. Once the media is full the biomass must grow on top of itself. The

6 SGBR is not limiting in the growth of biomass. Because of the granule bed, the organisms can grow, unrestricted, in any direction. Granule growth was observed in both SGBRs. Scanning electron microscopy showed a diverse microbial population in the granules. Several observations were made from the SEM pictures. Granules from the operating SGBRs show significant morphology changes from granules obtained from original UASB source (Figure 4 Scanning electron micrographs of original seed granule and granule from SGBR 1 and SGBR 2. ). In addition to morphology changes, spirochetes and diatoms were easily identified from the SEM results. Several diatoms similar to the one in Figure 5 (Figure 5 Diatom found inside granule. ) were observed, which were likely left over from diatomaceous earth filtration in the brewery from which the granules were obtained. No distinct layers were observed, but the dilute wastewater may not create a large gradient causing layering (Figure 6 Sliced granule showing no apparent layers. ). Banik et. al. (1997) observed that ASBR granules grown at 25 C showed no apparent layering. Particle size analysis was performed on the granules using microscopic photography. An increase in volume occupied by the granules in the reactor was observed and size analysis confirmed the change in granule structure. At the start of the experiment, the majority (60%) of the granules ranged in size from 0.7 to 1 mm in diameter. Eight months later, 89% of the granules measured greater than 1.0 mm in diameter. The net increase in biomass growth as evidenced by an increase in granule size benefited the reactor performance by further decreasing the F:M ratio. The SGBR s high quality effluent may be attributed to the high solids retention time (SRT) and low F:M ratio. The SGBR had a SRT of approximately 500 days at a 6 hour HRT. An ASBR treating low strength wastewater reported an SRT of 50 days at a 6 hour HRT and a temperature of 20 C (Dague et. al. 1998). During longer HRTs, the SGBR s SRT should increase creating an even higher quality effluent which is evident in the data. CONCLUSIONS The development of the SGBR was a significant accomplishment for anaerobic treatment of low to medium strength wastewaters. The COD and BOD 5 results confirmed the high organic removal, and the COD balance confirmed that the majority of the COD was converted to methane. The effluent produced was low in suspended solids and VFAs. Actual VFA concentrations may have been even lower than reported due to a possible over-estimation of the Standard Methods titration method. Operation of the SGBR was simple since no extra equipment such as mixers, sophisticated gas/solids/liquid separators, and heat exchangers were not required. The downflow configuration retained biomass since granules were not washed out. In addition, recycle pumping was not required as it is in an upflow reactor. Granules become buoyant during gas production and may be washed out in an upflow configuration.

7 The SGBR produced high quality effluent making it ideal for on-site treatment and possible stream discharge. Effluent from the SGBR meets NDPES permitting requirements for BOD 5 and SS levels (30 mg/l for both conditions). The effluent may be used as a non-potable water source for many industrial uses. Overall the SGBR is an innovative new treatment process which may have many useful applications. ACKNOWLEDGEMENTS This work was funded by a grant from the U.S.D.A. through the Iowa Biotechnology Byproducts Consortium. REFERENCES Angenent, L.T. (1998) Development of a New High-Rate Anaerobic Process for the Treatment of Industrial and Domestic Wastewater: The Anaerobic Migrating Blanket Reactor (AMBR). Ph.D. Dissertation. Iowa State University, Ames, Iowa. Alibhai, K.R.K. and Forster, C.F. (1986) An Examination of the Granulation Process in UASB Reactors. Environmental Technology Letters, 7, Banik, G.C.; Ellis, T.G.; Dague, R.R. (1997) Structure and Menthanogenic Activity of Granules from an ASBR Treating Dilute Wastewater at Low Temperatures. Water Sci. Tech., 36, Collins, A.G.; Theis, T.L.; Kilambi, S.; He, L.; Pavlostathis, S.G. (1998) Anaerobic Treatment of Low-Strength Domestic Wastewater Using an Anaerobic Expanded bed Reactor. J. Environ. Eng., Dague, R.R.; Banik, G.C.; Ellis, T.G. (1998) ASBR Treatment of Dilute Wastewater at Psychrophilic Temperatures. Water Environment Research, 70, Eaton, A.D.; Clesceri, L.S.; Greenberg, A.E. (1995) Standard Methods for Examination of Waster and Wastewater, 19 th ed. American Public Health Association, Washington, DC. Fang, H.H.P; Chui, H.K.; Li, Y.Y. (1995) Microstuctural Analysis of UASB Granules Treating Brewery Wastewater. Wat. Sci. Technol., 31, Fang, H.H.P.; Chui, H.K.; Li, Y.Y. (1994) Microbial Structure and Activity of UASB Granules Treating Different Wastewater. Proc. of the Seventh International Symposium on Anaerobic Digestion. IAWQ, Capetown, South Africa Guiot, S.R.; Pauss, A.; Costerton, J.W. (1992). A Structured Model of the Anaerobic Granule Consortium. Wat. Sci. Technol., 25, Han, Y.; Sung, S.; Dague, R.R. (1997) Temperature Phased Anaerobic Digestion of Wastewater Sludges. Wat. Sci. Tech., 36,

8 Harris, W.L. and Dague, R.R. (1993). Comparative Performance of Anaerobic Filters at Mesophilic and Thermophilic Temperatures. Water Environment Research, 65, Howerton, D.E. and Young, J.C. (1987). Two-Stage Cyclic Operation of Anaerobic Filters. Journal WPCF., 59, Hulshoff Pol, L.W.; Lettinga G., Velzeboer, G.T.M; de Zeeuw, W.J. (1983). Granulation in UASB reactors. Wat. Sci. Technol., 15, Lettinga, G.; Hulshoff Pol, L.W.; Zeeman, G.; Field, J.; van Lier, J.B.; van Buuren, J.C.L.; Janssen, A.J.H.; Lens, P. (1997) Anaerobic Treatment in Sustainable Environmental Production Concepts. Proc. of the Eighth International Symposium on Anaerobic Digestion. IAWQ, Sendai, Japan, Vol. 2, MacLeod, F.A.; Guiot, S.R.; Costerton, J.W. (1990) Layered Structure of Bacterial Aggregates Produced in an Upflow Anaerobic Sludge Bed and Filter Reactor. Appl. Environ. Microbiol., 56, Morgan, J.W.; Evison, L.M.; Forster, C.F. (1991). The Internal Architecture of Anaerobic Sludge Granules. J. Chem. Tech. Biotechnol., 50, Ndon, U.J. and Dague, R.R. (1997) Effects of Temperature and Hydraulic Retention Time on Anaerobic Sequencing Batch Reactor Treatment of Low-Strength Wastewater. Wat. Res., 31, Orozco, A. (1997) Pilot and Full-Scale Anaerobic Treatment of Low-Strength Wastewaters at Sub-Optimal Temperature with a Hybrid Plug Flow Reactor. Proc. Of the Eighth International Symposium on Anaerobic Digestion. IAWQ, Sendai, Japan, Vol. 2, Perez, M.; Romero, L.I.; Sales, D. (1998) Comparative Performance of High Rate Anaerobic Thermophilic Technologies Treating Industrial Wastewater. Wat. Res., 32, Rebac, S.; van Lier, J.B.; Janssen, M.G.J.; Dekkers, F.; Swinkels, K.T.M.; Lettinga, G. (1997) High-Rate Anaerobic Treatment of Malting Waste Water in a Pilot-Scale EGSB System Under Psychrophilic Conditions. J. Chem. Tech. Biotechnol., 68, Speece, R.E. (1996) Anaerobic Biotechnology for Industrial Wastewaters. Archae Press, Nashville, TN. Yan, Y.G. and Tay, J.H. (1996). Brewery Wastewater Treatment in UASB Reactor at Ambient Temperature. J. Environ. Eng.,

9 Table 1. Feed composition. Component Nonfat dry milk Sodium bicarbonate Trace element solution Amount added per liter of feed 1.03 g 0.34 g 0.1 ml

10 Table 2. Trace element solution. Compound FeCl 2 4H 2 O ZnCl 2 NiCl 2 6H 2 O CoCl 2 6H 2 O MnCl 2 4H 2 O Concentration 35.6 g/l 2.08 g/l 4.05 g/l 4.04 g/l 3.61 g/l

11 Table 3. Operating characteristics of the SGBR. HRT (hours) Length of operation (days) OLR (kg/m 3 d -1 ) CH 4 produced (L/day) 1 Theoretical CH 4 (L/day) 1 % of theoretical TSS (g/l) VSS (g/l) SGBR SGBR at standard temperature and pressure

12 Table 4. Analytical results of the SGBR. SGBR 1 SGBR 2 HRT Feed Conc. (mg COD/L) Eff. Conc. (mg COD/L) ph eff. Alk (mg/l) VFA (mg/l) as acetic acid 2 % CH 4 1 % CO 2 1 % BOD rem. H 2 S (ppm) 3 NH 4 (mg/l) normalized to 0% N 2 2 as measured by titration. GC analysis showed NDL for acetic, proprionic, n-butyric acid 3 in gas phase

13 Figure 1. Schematic diagram of the SGBR. Biogas Feed Pump Refrigerated Feed Bottle Open to air Sulfide Scrubber Gas Meter Liquid Level Bed of Anaerobic Granules Stainless steel mesh underdrain Refrigerated Effluent Bottle

14 Figure 2. COD removal for the SGBRs Removal, % SGBR SGBR 2 Total COD Soluble COD

15 Figure 3. Gas Production in the SGBRs. Gas Production, L/day at STP hr. 16 hr. 12 hr. 8 hr. 36 hr. 24 hr. 16 hr. 12 hr. 8 hr. 6 hr. 5 hr. SGBR 1 SGBR Length of Operation, days 5 hr.

16 WEFTEC 2000 Figure 4. Scanning electron micrographs of original seed granule (top) and granule from SBGR 1 (middle) and SGBR 2 (bottom).

17 WEFTEC 2000 Figure 5. Diatom found inside granule.

18 Figure 6. Sliced granule showing no apparent layers.