Upflow Fixed-bed Reactor Research Articles

Natural magnetite as an effective and long-lasting catalyst for CWPO of azole pesticides in a continuous up-flow fixed-bed reactor

The global occurrence of micropollutants in water bodies has raised concerns about potential negative effects on aquatic ecosystems and human health. EU regulations to mitigate such widespread pollution have already been implemented and are expected to become increasingly stringent in the next few years. Catalytic wet peroxide oxidation (CWPO) has proved to be a promising alternative for micropollutant removal from water, but most studies were performed in batch mode, often involving complex, expensive, and hardly recoverable catalysts, that are prone to deactivation. This work aims to demonstrate the feasibility of a fixed-bed reactor (FBR) packed with natural magnetite powder for the removal of a representative mixture of azole pesticides, recently listed in the EU Watch Lists. The performance of the system was evaluated by analyzing the impact of H2O2 dose (3.6–13.4 mg L−1), magnetite load (2–8 g), inlet flow rate (0.25–1 mL min−1), and initial micropollutant concentration (100–1000 µg L−1) over 300 h of continuous operation. Azole pesticide conversion values above 80% were achieved under selected operating conditions (WFe3O4 = 8 g, [H2O2]0 = 6.7 mg L−1, flow rate = 0.5 mL min−1, pH0 = 5, T = 25 °C). Notably, the catalytic system showed a high stability upon 500 h in operation, with limited iron leaching (< 0.1 mg L−1). As a proof of concept, the feasibility of the system was confirmed using a real wastewater treatment plant (WWTP) effluent spiked with the mixture of azole pesticides. These results represent a clear advance for the application of CWPO as a tertiary treatment in WWTPs and open the door for the scale-up of FBR packed with natural magnetite.

Facilely prepared heterogeneous Fenton catalyst Fe(Fe0.69Al0.31)2O4@C for catalytic wet peroxide oxidation of ciprofloxacin in batch and continuous reactor: Reaction intermediates and toxicity evaluation

Herein, by using a simple co-precipitation method Al-doped magnetite spinel nanoparticle encapsulated in the carbon matrix Fe(Fe0.69Al0.31)2O4@C was synthesised and its catalytic activity was examined as a heterogeneous Fenton catalyst towards the catalytic wet peroxide oxidation (CWPO) of antibiotic pollutant 20 ppm ciprofloxacin (CIP) in batch and continuous reactors. Under the optimized conditions, 0.5 g·L-1 Fe(Fe0.69Al0.31)2O4@C in batch reactor mineralized 51 % of CIP was achieved within 180 min at 50 °C and pH 3 by utilizing 1.2 mM or 4S H2O2 (S = stoichiometry; 47 mol H2O2:1 mol CIP) which is significantly less concentration compared to other heterogeneous Fenton catalysts. Impressive long-lasting catalytic activity was demonstrated in the up-flow fixed bed reactor, for 110 h with 44 % TOC removal and less than 1 ppm Fe leaching in the effluent water. Kinetic studies on the rate of decomposition of H2O2 revealed that Fe(Fe0.69Al0.31)2O4@C effectively decomposed the H2O2 like the homogeneous Fenton catalyst. The XPS results confirmed the influence of Al on iron ions in the magnetite structure through a substantial shift in higher binding energy for Fe3+ and introduced a more electropositive character on the Fe3+(δ+) that successively expedites the kinetically slow Fe3+ reduction reaction with H2O2 to produce HOO•. The π electrons of the graphitic carbon facilitate the electron transfer between the carbon matrix and Fe(Fe0.69Al0.31)2O4 nanoparticles to enhance the CIP degradation. The acute toxicity assessment studies validated the non-toxic nature of the effluent water obtained from batch and continuous reactors. A degradation pathway was proposed based on the eleven intermediate products identified using LC-HRMS analysis.

Liquid-Phase Hydrogenation of Phenol in an Advanced Gas–Liquid Concurrent Upflow Fixed-Bed Reactor with Membrane Dispersion

Liquid-Phase Hydrogenation of Phenol in an Advanced Gas–Liquid Concurrent Upflow Fixed-Bed Reactor with Membrane Dispersion

Continuous Heterogeneous Fenton-Type Process for Dye Pollution Abatement Intensified by Hydrodynamic Cavitation

The heterogeneous Fenton-type process was investigated in a continuous upflow fixed-bed reactor (UFBR) of a nano Prussian Blue catalyst supported on γ-alumina beads. Temperature and oxidant concent...

Post-treatment of real municipal wastewater effluents by means of granular activated carbon (GAC) based catalytic processes: A focus on abatement of pharmaceutically active compounds

Pharmaceutically active compounds (PhACs) widely present in urban wastewater effluents pose a threat to ecosystems in the receiving aquatic environment. In this work, efficiency of granular activated carbon (GAC) - based catalytic processes, namely catalytic wet peroxide oxidation (CWPO), peroxymonosulfate oxidation (PMS/GAC) and peroxydisulfate oxidation (PDS/GAC) at ambient temperature and pressure were studied for removal of 22 PhACs (ng L−1 level) that were present in secondary effluents of real urban wastewater. Concentrations of PhACs were measured using Ultra Performance Liquid Chromatography – Triple Quadrupole Mass Spectrometry (UPLC-QqQ-MS/MS). Catalytic experiments were conducted in discontinuous mode using up-flow fixed bed reactors with granular activated carbon (GAC) as a catalyst. The catalyst was characterized by means of N2 adsorption-desorption isotherm, mercury intrusion porosimetry (MIP), elemental analysis, X-ray fluorescence spectroscopy (WDXRF), X-ray diffraction (XRD), thermal gravimetry and differential temperature analyses coupled mass spectrometry (TGA-DTA-MS). Results indicate that the highest efficiency in terms of TOC removal was achieved during CWPO performed at optimal operational conditions (stoichiometric dose of H2O2; TOC removal ~ 82%) followed by PMS/GAC (initial PMS concentration 100 mg L−1; TOC removal ~73.7%) and PDS/GAC (initial PDS concentration 100 mg L−1; TOC removal ~ 67.9%) after 5 min of contact time. Full consumption of oxidants was observed in all cases for CWPO and PDS/GAC at contact times of 2.5 min, while for PMS/GAC it was 1.5 min. In general, for 18 out of 22 target PhACs, very high removal efficiencies (> 92%) were achieved in all tested processes (including adsorption) performed at optimal operational conditions during 5 min of contact time. However, moderate (40 – 70%) and poor (< 40%) removal efficiencies were achieved for salicylic acid, ofloxacin, norfloxacin and ciprofloxacin, which can be possibly attributed to insufficient contact time. Despite high efficiency of all studied processes for PhACs elimination from urban wastewater effluent, CWPO seems to be more promising for continuous operation.

New insights on the removal of diclofenac and ibuprofen by CWPO using a magnetite-based catalyst in an up-flow fixed-bed reactor

This research has been focused on the removal of two anti-inflammatory drugs, diclofenac (DCF) and ibuprofen (IBU), by a continuous catalytic wet peroxide oxidation (CWPO) process using a lab-synthesized nanomagnetic catalyst (Fe3O4/MWCNTs). The central composite rotatable design (CCRD) method was used to study the effect of DCF and IBU concentration (expressed as theoretical oxygen demand (ThOD) between 0 and 52.5 mg L−1) and of the feed stream pH (from 3 to 7) on the removal of total organic carbon (TOC) and the concentration of aromatic compounds (Arm) and total phenolic compounds (TP) by CWPO. It could be observed that DCF was preferably removed from the DCF-IBU aqueous mixture at pH values ranging from 3 to 5. In addition, feed stream pH had a significant effect on the pollutants removal, as well as on TOC, TP and aromatic compounds removal, observing an increasing in the pollutants degradation when feed stream pH decreased from 7 to 3. Quadratic models predicted for response variable, such as TOC, TP and aromatic compounds removal, and their maximum model-predicted removal values were of 90.0, 80.2 and 90.0%, respectively. Finally, as a proof of concept, three environmentally-relevant aqueous matrices, spiked with DCF-IBU mixture, were treated. In this case, relatively high TOC degradation values were found after 20 h reaction time (ca. 57.7, 73.9 and 54.5% in surface water, WWTP effluent and hospital wastewater, respectively). This work deals the first study about DCF-IBU removal in aqueous solution by CWPO, as well as a continuous study using real wastewater that allow to extend the experimental results to a real scenario.

Advanced nitrogen removal in a fixed-bed anaerobic ammonia oxidation reactor following an anoxic/oxic reactor: Nitrogen removal contributions and mechanisms

This study demonstrated the feasibility of anaerobic ammonia oxidation (anammox) served as tertiary nitrogen removal process. An upflow fixed-bed reactor (UFBR) pre-inoculated with anammox bacteria (AnAOB) followed an anoxic/oxic (A/O) reactor treating magnetic-coagulation pretreated municipal wastewater. When bypassing 15% of influent into UFBR, UFBR removed 5.37 mg-TN/L contributing to 23.4% on total TN removal, in which the combination of partial nitritation and partial denitrification with anammox was main nitrogen removal pathway. Relatively low concentrations of NH4+-N and anaerobic environment promoted the growth of ammonia oxidizing archaea (AOA) in the inner-layer of biofilm in UFBR. The cooperation of AOA and ammonia-oxidizing bacteria (AOB) with AnAOB was achieved, with AOA, AOB, and AnAOB abundances of 0.01–0.32%, 0.25–0.44%, and 0.77–2.18% on the biofilm, respectively. Metagenomic analysis found that although AOB was the main NH4+-N oxidizer, archaeal amo gene on biofilm increased threefold during 90 days’ treatment.

Continuous fixed-bed oxidation of metronidazole by the sulfate radical based process over nitric acid treated granular activated carbon

Activation of persulfate in the presence of nitric acid treated granular activated carbon was studied in order to investigate the degradation of metronidazole from aqueous medium in the continuous up-flow fixed-bed reactor. A tubular glass reactor with internal diameter of 2 cm and height of 15 cm was filled with desirable mass of rod shaped activated carbon with diameter of 4 mm and height of 6 mm. The synthetic wastewater containing persulfate and metronidazole was entered into the reactor in up-flow operation. At steady state condition and under the optimum values of operating parameters, about 88 % of influent persulfate was decomposed and decreased to 73 % after 7 h. Under this condition, the metronidazole removal reached to 93 % and gradually decreased to 74 % at the end of process due to the saturation of the catalyst and occupation of available active sites for sulfate radical production. The results indicated promising performance of sulfate radical based advanced oxidation process in continuous fixed-bed reactor for degradation of resistant pollutants from aqueous medium.

Condensation By-Products in Wet Peroxide Oxidation: Fouling or Catalytic Promotion? Part II: Activity, Nature and Stability

The deposition of condensation by-products onto the catalyst surface upon wet peroxide and wet air oxidation processes has usually been associated with catalyst deactivation. However, in Part I of this paper, it was demonstrated that these carbonaceous deposits actually act as catalytic promoters in the oxygen-assisted wet peroxide oxidation (WPO-O2) of phenol. Herein, the intrinsic activity, nature and stability of these species have been investigated. To achieve this goal, an up-flow fixed bed reactor packed with porous Al2O3 spheres was used to facilitate the deposition of the condensation by-products formed in the liquid phase. It was demonstrated that the condensation by-products catalyzed the decomposition of H2O2 and a higher amount of these species leads to a higher degree of oxidation degree The reaction rates, conversion values and intermediates’ distribution were analyzed. The characterization of the carbonaceous deposits on the Al2O3 spheres showed a significant amount of condensation by-products (~6 wt.%) after 650 h of time on stream. They are of aromatic nature and present oxygen functional groups consisting of quinones, phenols, aldehydes, carboxylics and ketones. The initial phenol concentration and H2O2 dose were found to be crucial variables for the generation and consumption of such species, respectively.

Decolorization of textile industry effluent using immobilized consortium cells in upflow fixed bed reactor

Textile dyes are xenobiotic contaminants which pose a potential risk on the ecosystem upon their disposal to the water bodies. This study evaluated the efficiencies of different immobilization matrices for its utilization in a reactor with continuous mode operation for decolorization of textile effluent. An effective consortium of bacteria (Brevibacillus laterosporus) and yeast (Galactomyces geotrichum) were immobilized in different support matrices including calcium alginate, polyvinyl alcohol, stainless steel sponge and polyurethane foam to investigate the decolorization of a model azo dye, Remazol red and textile industry effluent. The microbial consortia immobilized in stainless steel sponge and polyurethane foam exhibited 100% decolorization of 50 mg L−1 Remazol red in 11 and 15 h, respectively; however, calcium alginate and polyvinyl alcohol required considerably more time (20 and 24 h, respectively) for complete decolorization. Among all the matrices, The calcium alginate, stainless steel sponge and polyurethane foam showed >95% decolorization of textile industry effluent within 48 h. The calcium alginate and polyvinyl alcohol exhibited stable performance of decolorization with its repeated use for 5 cycles with >76% decolorization. An upflow fixed bed reactor (total volume- 215 mL) packed with the immobilized cells of consortium onto stainless steel sponge attained ∼90% decolorization of textile industry effluent in continuous operation at 10 mL h−1. The decolorization efficiency of the reactor was well maintained (>90%) when the reactor was used repeatedly for three cycles. The overall results indicated that immobilized mixed consortium cells can be considered as an effective tool for its potential application in removal of xenobiotic textile dyes from the textile industry wastewater with >90% of decolorization efficiency.

Catalytic wet oxidation of high concentration formaldehyde wastewater over Pt/nitrogen-doped activated carbon

High concentration formaldehyde (HCHO) wastewater was treated by catalytic wet oxidation (CWO) using Pt/activated carbons. The activated carbons (ACs) with different surface nitrogen content were prepared by H2O2 pre-oxidation and thermal treatment with melamine. The corresponding Pt/ACs were prepared by excess volume impregnation process and characterized by N2 adsorption/desorption, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The CWO experiments were performed in a co-current upflow fixed bed reactor at 125 °C and 1.0 MPa. The results revealed the positive effect of nitrogen-containing surface groups on the Pt–AC interactions, and resulting in a smaller Pt nanoparticles size and a narrower distribution. Compared with the original Pt/AC, more nitrogen content of Pt/AC showed higher catalytic activity in the mineralization of high concentration formaldehyde (~ 17,000 mg/L) and COD (~ 23,000 mg/L). After 170 h of time on stream, HCHO and COD concentrations of effluent from Pt/AC increased from 482 to 805 mg/L, and from 1895 to 2980 mg/L, respectively, while those from Pt/nitrogen-doped AC just increased from 0.6 to 30.6 mg/L, and from 48 to 345 mg/L even at 350 h of time on stream. Lesser Pt leaching and the extent of agglomeration of Pt NPs were observed over nitrogen-containing Pt/AC. CWO using Pt/nitrogen-doped AC may be applied as a promising treatment for high concentration HCHO wastewater prior to biological system.

Highly dispersed Fe3+-Al2O3 for the Fenton-like oxidation of phenol in a continuous up-flow fixed bed reactor. Enhancing catalyst stability through operating conditions

A highly dispersed Fe3+-Al2O3 catalyst (6 wt% Fe) was used for the catalytic wet hydrogen peroxide oxidation of phenol (1 g/L) in an up-flow fixed bed reactor (UFBR) under continuous operation. To enhance catalytic performance, three simple synthesis strategies were combined: two-stage impregnation of iron citrate, acid washing with CH3COOH and thermal treatment at 900 °C. Solid samples were characterized in depth by several techniques: N2 Physisorption, XRD, SEM–EDAX, TEM, TGA, PZC, TPD of pyridine, XPS and Mössbauer. Peroxidation experiments were performed in an UFBR over a wide range of operating parameters in order to evaluate their influence on phenol mineralization and catalyst stability. Under selected operating condition (T = 90 °C, Wcat = 20 g, QL = 1.2 mL/min and [H2O2]:[Phenol] = 16.8), complete phenol conversion and remarkable TOC reduction of 90% were achieved, with a high H2O2 consumption efficiency (η = 76%) and low Fe leaching (< 3 mg/L). After 70 h of usage at different steady state conditions, the catalyst retained high mineralization levels (XTOC> 70%) but the cumulative iron loss was calculated to be c.a. 20% of the initial Fe loaded in the UFBR. The catalyst was susceptible to leaching due to the accumulation of complexing intermediates such as carboxylic acids. However, acceptable iron leaching values (< 10 mg/L) were achieved when the reactor operating conditions were properly set (55% < XTOC> 80%). The presence of chelating by-products favored also the Fe redistribution inside the catalyst pellets. Nevertheless, catalyst decay in the long-term operation was mainly due to the occurrence and permanence of chelating organic acids. This process was specially promoted by the amphoteric character of the alumina-based catalyst. However, adsorbed species were promptly eliminated by calcination at 500 °C, recovering steady state conversion profiles.

Evaluation of alkalinity spatial distribution in an up-flow fixed bed anaerobic digester.

In this paper, an experimental study upon alkalinity and hydrodynamic behavior in an anaerobic up-flow fixed bed reactor for the treatment of tequila vinasses is presented. Measurements of volatile fatty acids, pH, alkalinity and bicarbonate were obtained at three sampling points in the reactor in the axial axis. Then, the spatial distribution of alkalinity is studied and discussed. Moreover, for further control process purposes, a hydrodynamic model based on the use of two interconnected two-steps reduced AM2 type models is proposed and its parameters are identified using experimental data.

Efficient degradation of organic phosphorus in glyphosate wastewater by catalytic wet oxidation using modified activated carbon as a catalyst

ABSTRACTGlyphosate (PMG) wastewater, which is an organic phosphorus (OP) wastewater containing 200–3000 mg/L PMG, was treated via catalytic wet oxidation (CWO) to degrade PMG to orthophosphate (). The catalysts were activated carbons (ACs) modified by H2O2 oxidation and thermal treatment with ammonia or melamine. The catalysts were characterized using N2 adsorption/desorption, Boehm titration, and X-ray photoelectron spectroscopy. The CWO experiments were performed in a co-current upflow fixed-bed reactor at 110–130°C and under 1.0 MPa. The AC modified by H2O2 and melamine had the highest catalytic activity and had excellent stability in the continuous 55-day test: 100% PMG removal and over 93% OP removal for different samples of real PMG wastewater. More pyrrolic nitrogen, pyridinic nitrogen, and graphitic nitrogen along with quinone oxygen functional groups on the surface of the AC showed higher catalytic activity according to linear fitting results. The identification and quantification of critical reaction intermediaries and the main end products of PMG degradation were possible, and a degradation pathway was proposed.

Kinetics of glyphosate degradation in glyphosate wastewater over nitrogen-doped activated carbon catalyst in an upflow fixed bed reactor

Catalytic wet oxidation (CWO) of glyphosate (PMG) wastewater over a nitrogen-doped activated carbon catalyst was performed in an upflow fixed bed reactor at mild temperature (35–125 °C) and oxygen partial pressure (0.05–0.25 MPa). The catalytic performance was characterized by PMG and organic phosphorus (OP) conversion. All CWO experiments were under kinetic control by choosing suitable average particle size (0.3 mm, 40–80 mesh) and gas flow rate (500 mL min−1). A power law kinetic model was developed to describe PMG and OP conversion, assuming first order kinetics with respect to PMG and OP. This model and its calculated parameters successfully predicted PMG and OP conversion. The obviously different rate constant and activation energy for PMG and OP conversion suggested that removal of the intermediate of aminomethylphosphonic acid is the control reaction for OP conversion. Finally, the competitive oxidation of formaldehyde was insignificant and this model also proved a satisfactory description for treatment of the real PMG wastewaters. OP conversion for the different PMG wastewaters can reach over 90% at mild conditions (125 °C, 0.2 MPa of oxygen partial pressure, 25 min of liquid space time).

English

The objective of this study was to evaluate a combined anaerobic-aerobic upflow fixed-bed reactor with liquid phase recirculation for the removal of nitrogen and organic matter from poultry slaughterhouse wastewater. The reactor performance was evaluated with a hydraulic retention time (HRT) of 11 h and three different recirculation rates (R=0.5; 1 and 2). The highest nitrogen removal efficiency value was obtained with an HRT of 11 h (6.8 h in the anaerobic zone and 4.2 h in the aerobic zone) and a recirculation rate of 2. In this condition, the total nitrogen removal efficiency was 69%, with effluent concentrations of 6 mg NH4+ L-1 and 12 mg NO3- L-1. For all tested conditions, there was good chemical oxygen demand (COD) removal, with efficiency above 95%. The effect of dilution and the favoring of mass transfer caused by the increase in the recirculation rate positively influenced reactor performance. Key words: Anaerobic degradation, nitrification, denitrification, combined reactor.

Removal of nitrate nitrogen and chemical oxygen demand in upflow fixed bed reactor using heterotrophic microorganisms

This communication deals with the removal of contaminants namely nitrate nitrogen (N–NO3−) and chemical oxygen demand (COD) in the form of acetate using an upflow packed bed anaerobic reactor. Effects of various parameters such as nutrient concentration, time, pH and flow rate were studied. The contaminants removal efficiency increased with the increase of initial concentration while the reaction rate of contaminants decreased with the increase of initial pH beyond 7. The effluent flow rate also showed a positive correlation with the contaminant removal rate up to 24 mL/min and beyond that it showed a negative trend. The reaction followed pseudo first order rate and using the same a unified rate equation was developed for both the contaminants such as N–NO3− and COD. The mass transfer rates for the both the contaminants were calculated. Rate equations were also developed using the sorption model, which showed good agreement with the experimental results. Using Monod equation, maximum specific degradation rate as well as half saturation constant for both the contaminants was calculated. Statistical techniques including principal component analyses and multi linear regression analyses were carried out, which substantiated the analysis of the results further.

Biogas desulfurization using autotrophic denitrification process

The aim of this study was to evaluate the performance of an autotrophic denitrification process for desulfurization of biogas produced from a chicken manure digester. A laboratory scale upflow fixed bed reactor (UFBR) was operated for 105 days and fed with sodium sulfide or H2S scrubbed from the biogas and nitrate as electron donor and acceptor, respectively. The S/N ratio (2.5 mol/mol) of the feed solution was kept constant throughout the study. When the UFBR was fed with sodium sulfide solution with an influent pH of 7.7, about 95 % sulfide and 90 % nitrate removal efficiencies were achieved. However, the inlet of the UFBR was clogged several times due to the accumulation of biologically produced elemental sulfur particles and the clogging resulted in operational problems. When the UFBR was fed with the H2S absorbed from the biogas and operated with an influent pH of 8-9, around 98 % sulfide and 97 % nitrate removal efficiencies were obtained. In this way, above 95 % of the H2S in the biogas was removed as elemental sulfur and the reactor effluent was reused as scrubbing liquid without any clogging problem.

Selection and identification of a bacterial community able to degrade and detoxify m-nitrophenol in continuous biofilm reactors

Nitroaromatics are widely used for industrial purposes and constitute a group of compounds of environmental concern because of their persistence and toxic properties. Biological processes used for decontamination of nitroaromatic-polluted sources have then attracted worldwide attention. In the present investigation m-nitrophenol (MNP) biodegradation was studied in batch and continuous reactors. A bacterial community able to degrade the compound was first selected from a polluted freshwater stream and the isolates were identified by the analysis of the 16S rRNA gene sequence. The bacterial community was then used in biodegradation assays. Batch experiments were conducted in a 2L aerobic microfermentor at 28°C and with agitation (200rpm). The influence of abiotic factors in the biodegradation process in batch reactors, such as initial concentration of the compound and initial pH of the medium, was also studied. Continuous degradation of MNP was performed in an aerobic up-flow fixed-bed biofilm reactor. The biodegradation process was evaluated by determining MNP and ammonium concentrations and chemical oxygen demand (COD). Detoxification was assessed by Vibrio fischeri and Pseudokirchneriella subcapitata toxicity tests. Under batch conditions the bacterial community was able to degrade 0.72mM of MNP in 32h, with efficiencies higher than 99.9% and 89.0% of MNP and COD removals respectively and with concomitant release of ammonium. When the initial MNP concentration increased to 1.08 and 1.44mM MNP the biodegradation process was accomplished in 40 and 44h, respectively. No biodegradation of the compound was observed at higher concentrations. The community was also able to degrade 0.72mM of the compound at pH 5, 7 and 9. In the continuous process biodegradation efficiency reached 99.5% and 96.8% of MNP and COD removal respectively. The maximum MNP removal rate was 37.9gm−3day−1. Toxicity was not detected after the biodegradation process.

A novel anaerobic down-flow structured-bed reactor for long-term stable H2energy production from wastewater

BACKGROUND Anaerobic, upflow fixed-bed reactors have been considered suitable for the production of H2 from wastewater because, in contrast to completely mixed reactors, they avoid the washout of H2-producing microorganisms. However, the excessive growth and accumulation of biomass on the bed are unfavorable and cause a drastic drop in biogas production, even though the fermentative process in the liquid medium continues unimpeded. This paper proposes a novel anaerobic fixed-bed reactor configuration intended to control the accumulation of biomass by means of structuring the bed longitudinally and inverting the flow (down-flow). RESULTS Three anaerobic, down-flow structured-bed reactors (ADSBRs) with different support materials for the attachment of microorganisms were operated continuously with and without frequent biomass discharges. The H2 flow was continuously maintained for 120 days, resulting in a volumetric production of 0.6 L H2 L−1 d−1 and a yield of 0.5 mol H2 mol−1cs. Compared with the anaerobic, upflow fixed-bed reactor, the ADSBR increased by 10% the natural removal of biomass induced by the effluent and decreased by 15% the accumulated biomass in the bed. CONCLUSION Hence, this study demonstrates that the ADSBR produces H2 continuously and stably due to the constant renewal of biomass; furthermore, young biomass exhibited more H2-producing microorganisms than either non-producing or H2-consuming microorganisms. © 2015 Society of Chemical Industry