Fermentative hydrogen production and bacterial community structure in high-rate anaerobic bioreactors containing silicone-immobilized and self-flocculated sludge

A novel continuously stirred anaerobic bioreactor (CSABR) seeded with silicone-immobilized sludge was developed for high-rate fermentative H2 production using sucrose as the limiting substrate. The CSABR system was operated at a hydraulic retention time (HRT) of 0.5-6 h and an influent sucrose concentration of 10-40 g COD/L. With a high feeding sucrose concentration (i.e., 30-40 g COD/L) and a short HRT (0.5 h), the CSABR reactor produced H2 more efficiently with the highest volumetric rate (VH2) of 15 L/h/L (i.e., 14.7 mol/d/L) and an optimal yield of ca. 3.5 mol H2/mol sucrose. The maximum VH2 value obtained from this work is much higher than any other VH2 values ever documented. Formation of self-flocculated granular sludge occurred during operation at a short HRT. The granule formation is thought to play a pivotal role in the dramatic enhancement of H2 production rate, because it led to more efficient biomass retention. A high biomass concentration of up to 35.4 g VSS/L was achieved even though the reactor was operated at an extremely low HRT (i.e., 0.5 h). In addition to gaining high biomass concentrations, formation of granular sludge also triggered a transition in bacterial community structure, resulting in a nearly twofold increase in the specific H2 production rate. According to denatured-gradient-gel-electrophoresis analysis, operations at a progressively decreasing HRT resulted in a decrease in bacterial population diversity. The culture with the best H2 production performance (at HRT = 0.5 h and sucrose concentration = 30 g COD/L) was eventually dominated by a presumably excellent H2-producing bacterial species identified as Clostridium pasteurianum.     

Anaerobic Hydrogen Production from Sucrose Using an Acid‐Enriched Sewage Sludge Microflora

A novel and high‐rate anaerobic sequencing bath reactor (ASBR) process was used to evaluate the hydrogen productivity of an acid‐enriched sewage sludge microflora at a temperature of 35 °C. In this ASBR process a 4 h cycle, including feed, reaction, settle, and decant steps, was repeatedly performed in a 5 L reactor. The sucrose substrate concentration was 20 g COD/L; the hydraulic retention time (HRT) was maintained at 12–120 h at the initial period and thereafter at 4–12 h. The reaction/settle period ratio, which is the most important parameter for ASBR operation was 1.7. The experimental results indicated that the hydrogenic activity of the sludge microflora was HRT‐dependent and that proper pH control was necessary for a stable operation of the bioreactor. The peak hydrogenic activity value was attained at an HRT of 8 h and an organic loading rate of 80 kg COD/m³ × day. Each mole of sucrose in the reactor produced 2.8 mol of hydrogen and each gram of biomass produced 39 mmol of hydrogen per day. An overly‐short HRT might deteriorate the hydrogen productivity. The concentration ratios of butyric acid to’acetic acid, as well as volatile fatty acid and soluble microbial products to alkalinity can be used as monitoring indicators for the hydrogenic bioreactor.     

Operation strategies for biohydrogen production with a high-rate anaerobic granular sludge bed bioreactor

Long-term operation for biohydrogen production with an efficient carrier-induced granular sludge bed (CIGSB) bioreactor had encountered problems with poor biomass retention at a low hydraulic retention (HRT) as well as poor mass-transfer efficiency at a high HRT or under a prolonged operation period. This work was undertaken to develop strategies enabling better biomass retention and mass-transfer efficiency of the CIGSB reactors. Supplementation of calcium ion was found to enhance mechanical strength of the granular sludge. Addition of 5.4–27.2 mg/l of Ca²⁺ also led to an over three-fold increase in biomass concentration and a nearly five-fold increase in the H₂ production rate (up to 5.1 l H₂/h/l). Two reflux strategies were utilized to enhance the mass-transfer efficiency of the CIGSB system. The liquid reflux (LR) strategy enhanced the H₂ production rate by 2.2-fold at an optimal liquid upflow velocity of 1.09 m/h, which also gave a maximal biomass concentration of ca. 22 g VSS/l. Similar optimal H₂ production rate was also obtained with the gas reflux (GR) strategy at a rate of 1.0–1.49 m/h, whereas the biomass concentration decreased to 2–7 g VSS/l and thereby the specific H₂ production rate was higher than that with LR. The operation strategies applied in this work were effective to allow stable and efficient H₂ production for nearly 100 days.     

Rapid formation of hydrogen-producing granules in an anaerobic continuous stirred tank reactor induced by acid incubation

A novel approach to rapidly initiate granulation of hydrogen-producing sludge was developed in an anaerobic continuous stirred tank reactor at 37 degrees C. To induce microbial granulation, the acclimated culture was subject to an acid incubation for 24 h by shifting the culture pH from 5.5 to 2.0. The culture was resumed to pH 5.5 after the incubation and the reactor was operated at hydraulic retention times (HRTs) of 12, 6, 2, 1, and 0.5 h in sequence. Microbial aggregation took place immediately with the initiation of acid incubation and granules were developed at 114 h. No granule was observed in the absence of acid incubation in the control test. Changing the culture pH resulted in improvement in surface physicochemical properties of the culture favoring microbial granulation. The zeta potential increased from -11.6 to -3.5 mV, hydrophobicity in terms of contact angle improved from 31 degrees to 43 degrees and extracellular proteins/polysaccharides ratio increased from 0.2 to 0.5-0.8. Formation of granular sludge facilitated biomass retention of up to 32.2 g-VSS/L and enhanced hydrogen production. The hydrogen production rate and hydrogen yield increased with the reduction in HRT at an influent glucose concentration of 10 g/L once steady granular sludge layer was formed, achieving the respective peaks of 3.20 L/L x h and 1.81 mol-H(2)/mol-glucose at 0.5 h HRT. The experimental results suggested that acid incubation was able to initiate the rapid formation of hydrogen-producing granules by regulating the surface characteristics of microbial aggregates in a well-mixed reactor, which enhanced the hydrogen production.     

Acclimation Strategy of a Biohydrogen Producing Population in a Continuous‐Flow Reactor with Carbohydrate Fermentation

Poor startup of biological hydrogen production systems can cause an ineffective hydrogen production rate and poor biomass growth at a high hydraulic retention time (HRT), or cause a prolonged period of acclimation. In this paper a new startup strategy was developed in order to improve the enrichment of the hydrogen‐producing population and the efficiency of hydrogen production. A continuously‐stirred tank reactor (CSTR) and molasses were used to evaluate the hydrogen productivity of the sewage sludge microflora at a temperature of 35 °C. The experimental results indicated that the feed to microorganism ratio (F/M ratio) was a key parameter for the enrichment of hydrogen producing sludge in a continuous‐flow reactor. When the initial biomass was inoculated with 6.24 g of volatile suspended solids (VSS)/L, an HRT of 6 h, an initial organic loading rate (OLR) of 7.0 kg chemical oxygen demand (COD)/(m³ × d) and an feed to microorganism ratio (F/M) ratio of about 2–3 g COD/(g of volatile suspended solids (VSS) per day) were maintained during startup. Under these conditions, a hydrogen producing population at an equilibrium state could be established within 30 days. The main liquid fermentation products were acetate and ethanol. Biogas was composed of H₂ and CO₂. The hydrogen content in the biogas amounted to 47.5 %. The average hydrogen yield was 2.01 mol/mol hexose consumed. It was also observed that a special hydrogen producing population was formed when this startup strategy was used. It is supposed that the population may have had some special metabolic pathways to produce hydrogen along with ethanol as the main fermentation products.     

实验室模拟高负荷SPAC厌氧反应器运行

采用模拟废水, 对新型高负荷螺旋式自循环(Spiral automatic circulation, SPAC)厌氧反应器的运行性能进行了实验室模拟研究。结果表明: 在30oC, 水力停留时间(HRT)为12 h, 进水COD浓度从8000 mg/L升至20 000 mg/L的条件下, 反应器的COD去除率为91.1%~95.7%, 平均去除率为93.6％。在进水浓度为20 000 mg/L, HRT由5.95 h缩短至1.57 h的工况下, COD去除率从96.0%降低至78.7%, 反应器达到最高容积负荷率306 g COD/(L·d), 最大容积COD去除率240 g/(L·d), 最高容积产气率131 L/(L·d)。该反应器对基质浓度的连续提升具有良好的适应能力。进水COD浓度由8000 mg/L提升至20 000 mg/L时, 出水COD浓度一直处在较低水平(平均为852?mg/L), 容积COD去除率和容积产气率分别提高162%和119%。该反应器对HRT的连续缩短也有良好的适应能力。HRT由5.95 h缩短至1.57 h时,反应器容积COD去除率和容积产气率分别升高191%和195%。     

Bioreactors configured with distributors and carriers enhance the performance of continuous dark hydrogen fermentation

Anaerobic granular sludge bed (AnGSB) bioreactors were supplemented with activated carbon carriers and combined with distributors (e.g., acrylic resin board, stainless steel net and plastic net) installed at different locations to investigate the effect of distributor/carrier on biohydrogen production efficiency. The results show that plastic net stimulated the substrate/microorganisms contact and sludge granulation, thereby leading to a much better H₂ production performance when compared with those obtained from traditional CSTR. The highest H₂ production rate (7.89 L/h/L) and yield (3.42 mol H₂/mol sucrose) were obtained when two pieces of plastic nets were installed at both 4 cm and 16 cm from the bottom of AnGSB without carrier addition and the bioreactor was operated at a HRT of 0.5 h. For the AnGSB installed with two pieces of plastic net distributors, addition of carriers led to significant improvement on the H₂ production efficiency at a longer HRT (1–4 h) when compared with the carrier-absent system.     

Hydrogen production with immobilized sewage sludge in three-phase fluidized-bed bioreactors

Municipal sewage sludge was immobilized with a modified alginate gel entrapment method, and the immobilized cells were used to produce hydrogen gas in a three-phase fluidized bed. The hydrogen-producing fluidized beds were operated at different liquid velocity (U(0)) and hydraulic retention time (HRT). The results show that in response to operating liquid velocities, the fluidized-bed system had three flow regimes, namely, plug flow, slug flow, and free bubbling. Pressure fluctuation analysis was used to analyze the hydrodynamic properties in this three-phase fluidized bed when it was under a steady-state production of biogas. With a steady-state biogas production rate (U(g)) of 0.196 mL/s/L, a transition state occurred at a liquid velocity (U(0)) of 0.85 cm/s. As U(0) < 0.85 cm/s, the system was basically a nonhomogeneous fluidized bed, whereas the bed became homogeneous when U(0) was higher than 0.85 cm/s. The fluidized bed can be stably carried out at high loading rates (HRT as low as 2 h). Hydrogen fermentation results show that the maximal hydrogen production rate was 0.93 L/h/L and the best yield (Y(H)2(/sucrose)) was 2.67 mol H(2)/mol sucrose.     

Influence of substrate concentration on the stability and yield of continuous biohydrogen production

The effect of substrate concentration (sucrose) on the stability and yield of a continuous fermentative process producing hydrogen was studied. High substrate concentrations are attractive from an energy standpoint as they would minimise the energy required for heating. The reactor was a CSTR; temperature was maintained at 35 degrees C; pH was controlled between 5.2 and 5.3, and the hydraulic retention time (HRT) was 12 h. Online measurements were taken for ORP, pH, temperature, %CO2, gas output and %H2, and data logged using a MatLAB data acquisition toolbox. Steady-state operation was obtained at 10, 20 and 40 g/L of sucrose in the influent, but a subsequent step change to 50 g/L was unsustainable. The hydrogen content ranged between 50% and 60%. The yield of hydrogen decreased as the substrate concentration increased from 1.7 +/- 0.2 mol/mol hexose added at 10 g/L, to 0.8 +/- 0.1 mol/mol at 50 g/L. Sparging with nitrogen improved the hydrogen yield by at least 35% at 40 g/L and at least 33% at 50 g/L sucrose. Sparging also enabled steady-state operation at 50 g/L sucrose. Addition of an extra 4 g/L of n-butyric acid to the reactor operating at 40 g/L sucrose increased the butyrate concentration from 9,830 to 18,900 mg/L, immediately stopping gas production and initiating the production of propionate, whilst the addition of 2 g/L taking the butyrate concentration to 12,200 mg/L did not do so. It was shown that operation at 50 g/L sucrose in a CSTR in butyrate fermentation is possible.     

The inhibitory effects and removal of dieldrin in continuous upflow anaerobic sludge blanket reactors

The inhibitory effects and removal efficiency of dieldrin (DLD) in anaerobic reactors were investigated. Anaerobic toxicity assay (ATA) experiments conducted in batch reactors revealed that 30 mg/l DLD had inhibitory effects on the unacclimated mixed anaerobic cultures. Continuous reactor experiments performed in a lab-scale two-stage upflow anaerobic sludge blanket (UASB) reactor system which was fed with ethanol as the sole carbon source, indicated that anaerobic granular cultures could be successfully acclimated to DLD. Chemical oxygen demand (COD) removal efficiencies were 88-92% for the two-stage system. The influent DLD concentration of 10 mg/l was removed by 44-86% and 86-94% in the second stage and overall UASB system, respectively. Biosorption of DLD on granular anaerobic biomass was found to be a significant mechanism for DLD removal in the UASB system. The maximum DLD loading rate and minimum HRT achievable for the first stage UASB reactor were 0.5 mg/lday (76 microg DLD/g VSS.day) and 10 h, respectively, which resulted in the overall COD removal efficiency of 85%.     

Application of polyethylene glycol immobilized Clostridium sp. LS2 for continuous hydrogen production from palm oil mill effluent in upflow anaerobic sludge blanket reactor

A novel polyethylene glycol (PEG) gel was fabricated and used as a carrier to immobilize Clostridium sp. LS2 for continuous hydrogen production in an upflow anaerobic sludge blanket (UASB) reactor. Palm oil mill effluent (POME) was used as the substrate carbon source. The optimal amount of PEG-immobilized cells for anaerobic hydrogen production was 12% (w/v) in the UASB reactor. The UASB reactor containing immobilized cells was operated at varying hydraulic retention times (HRT) that ranged from 24 to 6 h at 3.3 g chemical oxygen demand (COD)/L/h organic loading rate (OLR), or at OLRs that ranged from 1.6 to 6.6 at 12 h HRT. The best volumetric hydrogen production rate of 336 mL H₂/L/h (or 15.0 mmol/L/h) with a hydrogen yield of 0.35 L H₂/g COD_removed was obtained at a HRT of 12 h and an OLR of 5.0 g COD/L/h. The average hydrogen content of biogas and COD reduction were 52% and 62%, respectively. The major soluble metabolites during hydrogen fermentation were butyric acid followed by acetic acid. It is concluded that the PEG-immobilized cell system developed in this work has great potential for continuous hydrogen production from real wastewater (POME) using the UASB reactor.     

Performance characteristics of a two-stage dark fermentative system producing hydrogen and methane continuously

The performance of a mesophilic two-stage system generating hydrogen and methane continuously from sucrose (10-30 g/L) was investigated. A hydrogen-generating CSTR followed by an upflow anaerobic filter were both inoculated with anaerobically digested sewage sludge, and ORP, pH, gas output, %H(2), %CH(4) and %CO(2) monitored. pH was controlled with NaOH, KOH or Ca(OH)(2). Using NaOH as alkali with 10 g/L sucrose, yields of 1.62 +/- 0.2 mol H(2)/mol hexose added and 323 mL CH(4)/gCOD added to the hydrogen and methane reactors respectively were achieved. The overall chemical oxygen demand (COD) reduction was 92.6% with 0.90 +/- 0.1 g/L sodium and 316 +/- 40 mg/L residual acetate in the methane reactor. Operation at 20 g/L sucrose and NaOH as alkali led to impaired volatile fatty acid (VFA) degradation in the methane reactor with 2.23 +/- 0.2 g/L sodium, 1,885 mg/L residual acetate, a hydrogen yield of 1.47 +/- 0.1 mol/mol hexose added, a methane yield of 294 mL/gCOD added and an overall COD reduction of 83%. Using Ca(OH)(2) as alkali with 20 g/L sucrose gave a hydrogen yield of 1.29 +/- 0.3 mol/mol hexose added, a methane yield of 337 mL/gCOD added and improved the overall COD reduction to 91% with residual acetate concentrations of 522 +/- 87 mg/L. Operation at 30 g/L sucrose with Ca(OH)(2) gave poorer overall COD reduction (68%), a hydrogen yield of 1.47 +/- 0.2 mol/mol hexose added, a methane yield of 138 mL/gCOD added and residual acetate 7,343 +/- 715 mg/L. It was shown that sodium toxicity and overloading are important issues for successful anaerobic digestion of effluent from biohydrogen reactors in high rate systems.     

High rate biological treatment of sulfate-rich wastewater in an acetate-fed EGSB reactor

An expanded granular sludge bed reactor, inoculated with acclimated sulfidogenic granular sludge, was operated at 33 °C and fed with acetic acid as COD source and sulfate as electron acceptor. The bioreactor had a sulfate conversion efficiency of 80–90% at a high sulfate loading rate of 10.4 g SO₄ ^2--S/l.d after only 60 days of start-up. This was achieved by implementing a dual operational strategy. Firstly acetic acid was dosed near stoichiometry (COD over sulfur ratio = 2.0 to 2.2) which allowed almost complete sulfate removal. Secondly the pH in the bioreactor was kept slightly alkaline (7.9 ± 0.1) which limited the concentration of the inhibitory undissociated hydrogen sulfide H₂S (pK_a = 7). This allowed the acetotrophic sulfate reducing bacteria to predominate throughout the long term experiment. The limitations of the EGSB technology with respect to the sulfate conversion rate appeared to be related to the biomass wash-out and granule deterioration occurring at superficial upflow velocities above 10 m/h. Increasing the recirculation flow caused a drop in the sulfate reduction rate and efficiency, an increase of the suspended sludge fraction and a considerable loss of biomass into the effluent, yielding bare mainly inorganic granules. Elemental analysis revealed that a considerable amount of the granular sludge dry matter at the end of the experiment, at an upflow velocity of 20 m/h, consisted of calcium (32%), mainly in the form of carbonate deposits, while organic matter only represented 7%.     

A new process for the continuous production of succinic acid from glucose at high yield, titer, and productivity

A novel three stages continuous fermentation process for the bioproduction of succinic acid at high concentration, productivity and yield using A. succiniciproducens was developed. This process combined an integrated membrane-bioreactor-electrodialysis system. An energetic characterization of A. succiniciproducens during anaerobic cultured in a cell recycle bioreactor was done first. The very low value of Y(ATP) obtained suggests that an ATP dependent mechanism of succinate export is present in A. succiniciproducens. Under the best culture conditions, biomass concentration and succinate volumetric productivity reach values of 42 g/L and 14.8 g/L.h. These values are respectively 28 and 20 times higher compared to batch cultures done in our laboratory. To limit end-products inhibition on growth, a mono-polar electrodialysis pilot was secondly coupled to the cell recycle bioreactor. This system allowed to continuously remove succinate and acetate from the permeate and recycle an organic acids depleted solution in the reactor. The integrated membrane-bioreactor-electrodialysis process produced a five times concentrated succinate solution (83 g/L) compared to the cell recycle reactor system, at a high average succinate yield of 1.35 mol/mol and a slightly lower volumetric productivity of 10.4 g/L.h. The process combined maximal production yield to high productivity and titer and could be economically viable for the development of a biological route for succinic acid production.     

Start‐up of Anaerobic Hydrogen Producing Reactors Seeded with Sewage Sludge

The procedure for starting‐up continuously stirred tank reactors (CSTR) for acclimating anaerobic hydrogen‐producing microorganisms with sewage sludge was investigated. Initially, feeding with glucose and sucrose as well as mixing were carried out in semicontinuous mode; hydraulic retention time (HRT) was in an order of 20, 15, 10, 5, 2.5 and 2 days. When the pH declined to its lowest value (pH 5.18), it was adjusted to 6.7 using sodium hydroxide (1 N). At the same time, the semi‐continuous operation was changed to a continuous one. Finally, the pH was continuously regulated at approximately 6.7. The results indicate that this procedure can be used to cultivate seed sludge for hydrogen production from sewage sludge resulting in a large hydrogen production in less than 60 days. When the substrate was glucose, a hydrogen yield of 1.63 mol H₂/mol glucose and a specific hydrogen production rate of 321 mmol H₂/g VSS day at an HRT of 13.3 h was achieved. When the substrate was sucrose with the same HTR, a hydrogen yield of 4.45 mol H2/mol sucrose and a specific hydrogen production rate of 707 mmol H₂/g VSS day was obtained.     

Quantitative analysis of a high-rate hydrogen-producing microbial community in anaerobic agitated granular sludge bed bioreactors using glucose as substrate

Fermentative H₂ production microbial structure in an agitated granular sludge bed bioreactor was analyzed using fluorescence in situ hybridization (FISH) and polymerase chain reaction-denatured gradient gel electrophoresis (PCR-DGGE). This hydrogen-producing system was operated at four different hydraulic retention times (HRTs) of 4, 2, 1, and 0.5 h and with an influent glucose concentration of 20 g chemical oxygen demand/l. According to the PCR-DGGE analysis, bacterial community structures were mainly composed of Clostridium sp. (possibly Clostridium pasteurianum), Klebsiella oxytoca, and Streptococcus sp. Significant increase of Clostridium/total cell ratio (68%) was observed when the reactor was operated under higher influent flow rate. The existence of Streptococcus sp. in the reactor became more important when operated under a short HRT as indicated by the ratio of Streptococcus probe-positive cells to Clostridium probe-positive cells changing from 21% (HRT 4 h) to 38% (HRT 0.5 h). FISH images suggested that Streptococcus cells probably acted as seeds for self-flocculated granule formation. Furthermore, combining the inspections with hydrogen production under different HRTs and their corresponding FISH analysis indicated that K. oxytoca did not directly contribute to H₂ production but possibly played a role in consuming O₂ to create an anaerobic environment for the hydrogen-producing Clostridium.     

Biological hydrogen production by immobilized cells of Clostridium tyrobutyricum JM1 isolated from a food waste treatment process

A fermentative hydrogen-producing bacterium, Clostridium tyrobutyricum JM1, was isolated from a food waste treating process using 16S rRNA gene sequencing and amplified ribosomal DNA restriction analysis (ARDRA). A fixed-bed bioreactor packed with polyurethane foam as support matrix for the growth of the isolate was operated at different hydraulic retention time (HRT) to evaluate its performance for hydrogen production. The reactor achieved the maximal hydrogen production rate of 7.2 l H(2)l(-1)d(-1) at 2h HRT, where hydrogen content in biogas was 50.0%, and substrate conversion efficiency was 97.4%. The maximum hydrogen yield was 223 ml (g-hexose)(-1) with an influent glucose concentration of 5 g l(-1). Therefore, the immobilized reactor using C. tyrobutyricum JM1 was an effective and stable system for continuous hydrogen production.     

Long-term impact of dissolved O(2) on the activity of anaerobic granules

The impact of influent dissolved O(2) on the characteristics of anaerobic granular sludge was investigated at various dissolved O(2) concentrations (0.5-8.1 ppm) in 1- and 5-L laboratory-scale upflow anaerobic sludge bed (UASB)-like anaerobic/aerobic coupled reactors with a synthetic wastewater (carbon sources containing 75% sucrose and 25% acetate). The rate of dissolved O(2) supplied to the coupled reactor was as high as 0.40 g O(2)/L(rx).d, and the anaerobic/aerobic coupled reactors maintained excellent methanogenic performances at a COD loading rate of 3 g COD/L(rx).d even after the reactors had been operated with dissolved O(2) for 3 months. The activities of granular sludge on various substrates (glucose, propionate, and hydrogen) were not impaired, and acetate activity was even improved over a short term. However, after 3 months of operation, slight declines on the acetoclastic activities of granules were observed in the coupled reactor receiving the recirculated fluid containing 8.1 ppm dissolved O(2).Methane yield in the anaerobic control reactor and anaerobic/aerobic coupled reactors revealed that a significant aerobic elimination (up to 30%) of substrate occurred in the coupled reactors, as expected. The presence of dissolved O(2) in the recirculated fluid resulted in the development of fluffy biolayers on the granule surface, which imposed a negative impact on the settleability of granular sludge and caused a slightly higher sludge washout. This research shows that the anaerobic/aerobic coupled reactor can be successfully operated under O(2)-limited conditions and is an ideal engineered ecosystem integrating oxic and anaerobic niches. (c) 1996 John Wiley & Sons, Inc.     

Feasibility of expanded granular sludge bed reactors for the anaerobic treatment of low-strength soluble wastewaters

The application of the expanded granular sludge bed (EGSB) reactor for the anaerobic treatment of low-strength soluble wastewaters using ethanol as a model substrate was investigated in laboratory-scale reactors at 30oC. Chemical oxygen demand (COD) removal efficiency was above 80% at organic loading rates up to12 g COD/L . d with influent concentrations as low as 100 to 200 mg COD/L. These results demonstrate the suitability of the EGBS reactor for the anaerobic treatment of low-strength wastewaters. The high treatment performance can be attributed to the intense mixing regime obtained by high hydraulic and organic loads. Good mixing of the bulk liquid phase for the substrate-biomass contact and adequate expansion of the substrate-biomass contact and adequate expansion of the sludge bed for the degassing were obtained when the liquid upflow velocity (V(up)) was greater than 2.5 m/h. Under such conditions, an extremely low apparent K(s) value for acetoclastic methanogenesis of 9.8 mg COD/L was observed. The presence of dissolved oxygen in the wastewater had no detrimental effect on the treatment performance. Sludge piston flotation from pockets of biogas accumulating under the sludge bed occurred at V(up) lower than 2.5 m/h due to poor bed expansion. This problem is expected only in small diameter laboratory-scale reactors. A. more important restriction of the EGSB reactor was the sludge washout occurring at V(up) higher than 5.5 m/h and which was intensified at organic loads higher than 7 g COD/L. d due to buoyancy forces from the gas production. To achieve an equilibrium between the mixing intensity and the sludge hold-up, the operation should be limited to an organic loading rate of 7 g COD/L d. and to a liquid up-flow velocity between 2.5 and 5.5 m/h (c) 1994 John Wiley & Sons, Inc.     

Biohydrogen production in a three-phase fluidized bed bioreactor using sewage sludge immobilized by ethylene–vinyl acetate copolymer

Ethylene–vinyl acetate (EVA) copolymer was used to immobilize H₂-producing sewage sludge for H₂ production in a three-phase fluidized bed reactor (FBR). The FBR with an immobilized cell packing ratio of 10% (v/v) and a liquid recycle rate of 5 l/min (23% bed expansion) was optimal for dark H₂ fermentation. The performance of the FBR reactor fed with sucrose-based synthetic medium was examined under various sucrose concentration (C_so) and hydraulic retention time (HRT). The best volumetric H₂ production rate of 1.80 ± 0.02 H₂ l/h/l occurred at C_so = 40 g COD/l and 2 h HRT, while the optimal H₂ yield (4.26 ± 0.04 mol H₂/mol sucrose) was obtained at C_so = 20 g COD/l and 6 h HRT. The H₂ content in the biogas was stably maintained at 40% or above. The primary soluble metabolites were butyric acid and acetic acid, as both products together accounted for 74–83% of total soluble microbial products formed during dark H₂ fermentation.