Volumetric Organic Loading Rate Research Articles

Treatment of Petroleum Refinery Wastewater by using UASB Reactors

Petroleum refineries discharged large amount of wastewater -during the refining process- that conta ins hazardous constituents that is hard to degrade. Anaerobic tre atment process is well known as an efficient method to degrade high s trength wastewaters. Up-flow Anaerobic Sludge Blanker (UASB) is a common process used for various wastewater treatments. Two UASB reactors were set up and operated in parallel to ev aluate the treatment efficiency of petroleum refinery wastewater. In thi s study four organic volumetric loading rates were applied (i.e. 0.58, 0.89, 1.21 and 2.34 kg/m 3 ·d), two loads to each reactor. Each load was appli ed for a period of 60 days for the reactor to acclimat ize and reach steady state, and then the second load applied. The chemic al oxygen demand (COD) removals were satisfactory with the removal efficiencies at the loadings applied were 78, 82, 83 and 81 % respectiv ely.

Wastewater treatment in a submerged anaerobic membrane bioreactor

Although most membrane bioreactors are used under aerobic conditions, over the last few years there has been increased interest in their application for anaerobic processes. This paper presents the results obtained when a bench-scale submerged anaerobic membrane bioreactor was used for the treatment of wastewaters generated in the agro-food industry. The reactor was fed with synthetic wastewater consisting of cheese whey and sucrose, and volumetric organic loading rates (OLRs) ranging from 1.5 to 13 kgCOD/(m3*d) were applied. Under the operating conditions studied, the maximum applicable OLR was between 6 and 10 gCOD/(g*L), which fell within the ranges of the high-rate anaerobic wastewater treatment systems, while high concentrations of volatile fatty acids were produced at higher OLR rates. With an OLR of 1.5–10 gCOD/(g*L), the reactor showed 94% COD removal, whereas this value dropped to 33% with the highest applied OLR of 13 gCOD/(g*L). The study therefore confirms that membrane bioreactors can be used for anaerobic wastewater treatment.

English

The dry anaerobic digestion process is an innovative waste-recycling method to treat high-solid-content bio-wastes. This can be done without dilution with water by microbial consortia in an oxygen-free environment to recover potential renewable energy and nutrient-rich fertilizer for sustainable solid waste management. It generally takes place at solid concentrations higher than 10% and enables a higher volumetric organic loading rate, minimal material handling, lower energy requirements for heating, limited environmental consequences and energetically effective performance. The long retention time, poor startup performance, incomplete mixing and the accumulation of volatile fatty acids (VFAs) are considered as the main disadvantages for the solid-state fermentation process. In order to develop feasible dry anaerobic digestion processes, it is important to review the optimization techniques and suggest possible areas where improvements could be made. These include reactor configuration, mixing, solid retention time, feedstocks, organic loading rate, inoculation, co-digestion, pretreatment, percolation, additives and environmental conditions within the digester such as temperature, pH, buffering capacity and VFAs concentration.   Key words: Solid organic wastes, dry anaerobic digestion process, biogas, optimization.

Full scale performance of compost’s leachate treatment by biological anaerobic reactors

A wide range of treatment processes have been applied to leachate treatment with varying success. The aim of this research was to examine the performance of two hybrid biological anaerobic reactors in a new full-scale leachate treatment plant in Rasht composting facility, north of Iran. The studied plant was operated for nearly 12 months and treated up to 50 m(3) day(-1) of fresh compost leachate with a relatively high BOD(5) to COD ratio of around 0.5. The average COD of the raw leachate was measured to be 81 000 mg L(-1). 30% of effective volume of each reactor was filled with clay blocks with a specific area of 87 m(2) m(-3) as the fixed bed. The sequencing of reactors was down flow and up flow, respectively. The maximum COD removal efficiency of 91% was achieved with 4.5 kg COD m(-3 )day(-1) of organic loading rate and 7500 mg L(-1) of volatile suspended solids. There was no significant difference in COD removal efficiency when the reactors were operated with an organic load between 2 and 5 kg COD m(-3 )day(-1). Increasing the volumetric organic loading rate to greater than 6 kg COD m(-3 )day(-1) caused a considerable reduction of COD removal efficiency. The COD removal efficiency decreased to less than 70% when the organic load rose to 10.8 kg COD m(-3 )day(-1). Nevertheless, hybrid anaerobic reactors proved to be a feasible technique for the reduction of organic load from the composting leachate which was the subject of this study.

Anaerobic digestibility of Scenedesmus obliquus and Phaeodactylum tricornutum under mesophilic and thermophilic conditions

Two types of non-axenic algal cultures, one dominated by the freshwater microalgae Scenedesmus obliquus and the other by the marine microalgae Phaeodactylum tricornutum, were cultivated in two types of simple photobioreactor systems . The production rates, expressed on dry matter (DM) basis, were in the order of 0.12 and 0.18 g DM L −1 d −1 for S. obliquus and P. tricornutum respectively. The biogas potential of algal biomass was assessed by performing standardized batch digestion as well as digestion in a hybrid flow-through reactor (combining a sludge blanket and a carrier bed), the latter under mesophilic and thermophilic conditions. Biomethane potential assays revealed the ultimate methane yield ( B 0) of P. tricornutum biomass to be about a factor of 1.5 higher than that of S. obliquus biomass, i.e. 0.36 and 0.24 L CH 4 g −1 volatile solids (VS) added respectively. For S. obliquus biomass, the hybrid flow-through reactor tests operated at volumetric organic loading rate (Bv) of 2.8 gVS L −1 d −1 indicated low conversion efficiencies ranging between 26–31% at a hydraulic retention time (HRT) of 2.2 days for mesophilic and thermophilic conditions respectively. When digesting P. tricornutum at a Bv of 1.9 gVS L −1 d −1 at either mesophilic or thermophilic conditions and at an HRT of 2.2 days, an overall conversion efficiency of about 50% was obtained. This work indicated that the hydrolysis of the algae cells is limiting the anaerobic processing of intensively grown S. obliquus and P. tricornutum biomass.

Effects of the cometabolite/growth substrate ratio on the aerobic degradation of 4-monochlorophenol

Chlorinated phenolic compounds like 4-monochlorophenol (4CP) are characterized by high toxicity even at relatively low concentrations and by strong persistency in both water and soils. Since cometabolism was proved to enhance 4CP biological removal, a conventional Sequencing Batch Reactor (SBR) was used in this study for the cometabolic aerobic degradation of 4CP at different influent concentrations and volumetric organic loading rates (40-50 mg/L and 0.129-0.323 kgCOD-4CP/m3·d, respectively), with sodium acetate (NaAc) as the growth substrate. The effects of different 4CP/NaAc ratios on SBR performances were evaluated in terms of 4CP removal efficiencies and maximum specific removal rates in order to maximize reactor performances: a decrease in NaAc dosage as external growth substrate would lead to definitely lower operating costs. A positive correlation was observed between 4CP maximum specific removal rates and the applied 4CP/NaAc ratios, thus making further reduction in NaAc dosage possible.

Physical-chemical and operational performance of an anaerobic baffled reactor (ABR) treating swine wastewater

Since hog raising concentrates a huge amount of swine manure in small areas, it is considered by the environmental government organizations to be one of the most potentially pollutant activities. Therefore the main objective of this research was to evaluate by operational criteria and removal efficiency, the performance of a Anaerobic Baffled Reactor (ABR), working as a biological pre-treatment of swine culture effluents. The physical-chemical analyses carried out were: total COD, BOD5, total solids (TS), fix (TFS) and volatiles (TVS), temperature, pH, total Kjeldahl nitrogen, phosphorus, total acidity and alkalinity. The ABR unit worked with an average efficiency of 65.2 and 76.2%, respectively, concerning total COD and BOD5, with a hydraulic retention time (HRT) about 15 hours. The results for volumetric organic loading rate (VOLR), organic loading rate (OLR) and hydraulic loading rate (HLR) were: 4.46 kg BOD m-3 day-1; 1.81 kg BOD5 kg TVS-1 day-1 and 1.57 m3 m-3 day-1, respectively. The average efficiency of the whole treatment system for total COD and BOD5 removal were 66.5 and 77.8%, showing an adequate performance in removing the organic matter from swine wastewater.

Performance evaluation and kinetic modeling of the start-up of a UASB reactor treating municipal wastewater at low temperature

A kinetic modeling-based study was carried out to evaluate the start-up performance of a 10-L up-flow anaerobic sludge blanket (UASB) reactor treating municipal wastewater under different organic and hydraulic loading conditions. The reactor was operated for 105 days (around 4 months) below 20 °C and with three different hydraulic retention times of 24, 12 and 5 h. Imposed volumetric organic loading rates (OLR) ranged from 0.57 (±0.05) to 11.78 (±0.85) kg TCOD/m(3)-day. Although relatively high incoming volumetric OLR values were employed to the system, the UASB reactor demonstrated a favorable performance on the anaerobic treatability of municipal wastewater, and no process failure was recorded in the start-up stage. On the basis of experimental results, the modified Stover-Kincannon model was successfully applied to define the start-up kinetics with a very high value of the correlation coefficient (R = 0.9729). Maximum substrate utilization rate constant and saturation constant of the modified Stover-Kincannon model were determined as U (max) = 1.996 g/L-day and K (B) = 1.536 g/L-day, respectively.

A fuzzy-logic-based model to predict biogas and methane production rates in a pilot-scale mesophilic UASB reactor treating molasses wastewater

A MIMO (multiple inputs and multiple outputs) fuzzy-logic-based model was developed to predict biogas and methane production rates in a pilot-scale 90-L mesophilic up-flow anaerobic sludge blanket (UASB) reactor treating molasses wastewater. Five input variables such as volumetric organic loading rate (OLR), volumetric total chemical oxygen demand (TCOD) removal rate ( R V), influent alkalinity, influent pH and effluent pH were fuzzified by the use of an artificial intelligence-based approach. Trapezoidal membership functions with eight levels were conducted for the fuzzy subsets, and a Mamdani-type fuzzy inference system was used to implement a total of 134 rules in the IF-THEN format. The product (prod) and the centre of gravity (COG, centroid) methods were employed as the inference operator and defuzzification methods, respectively. Fuzzy-logic predicted results were compared with the outputs of two exponential non-linear regression models derived in this study. The UASB reactor showed a remarkable performance on the treatment of molasses wastewater, with an average TCOD removal efficiency of 93 (±3)% and an average volumetric TCOD removal rate of 6.87 (±3.93) kg TCOD removed/m 3-day, respectively. Findings of this study clearly indicated that, compared to non-linear regression models, the proposed MIMO fuzzy-logic-based model produced smaller deviations and exhibited a superior predictive performance on forecasting of both biogas and methane production rates with satisfactory determination coefficients over 0.98.

Treatment of municipal wastewater using a contact oxidation filtration separation integrated bioreactor

A new contact oxidation filtration separation integrated bioreactor (CFBR) was used to treat municipal wastewater. The CFBR was made up of a biofilm reactor (the upper part of the CFBR) and a gravitational filtration bed (the lower part of the CFBR). Polyacrylonitrile balls (50mm diameter, 237 m(2)/m(3) specific surface, 90% porosity, and 50.2% packing rate) were filled into the biofilm reactor as biofilm attaching materials and anthracite coal (particle size 1-2mm, packing density 0.947 g/cm(3), non-uniform coefficient (K(80)=d(80)/d(10))<2.0) was placed into the gravitational filtration bed as filter media. At an organic volumetric loading rate of 2.4 kg COD/(m(3)d) and an initial filtration velocity of 5m/h in the CFBR, the average removal efficiencies of COD, ammonia nitrogen, total nitrogen and turbidity were 90.6%, 81.4%, 64.6% and 96.7% respectively, but the treatment process seemed not to be effective in phosphorus removal. The average removal efficiency of total phosphorus was 60.1%. Additionally, the power consumption of the CFBR was less than 0.15 kWh/m(3) of wastewater treated, and less than 1.5 kWh/kg BOD(5) removal.

A direct comparison amongst different technologies (aerobic granular sludge, SBR and MBR) for the treatment of wastewater contaminated by 4-chlorophenol

Environmental concern on chlorinated phenols is rising due to their extreme toxicity even at low concentrations and their persistency in water and soils. Since the high amount of published data often lacks in terms of uniformity, direct comparisons amongst different treatment technologies are very difficult, or even impossible. In this study, granular sludge developed in an acetate-fed Granular sludge Sequencing Batch Reactor (GSBR) was used for the aerobic degradation of low chlorinated 4-chlorophenol (4CP), with readily biodegradable sodium acetate (NaAc) as growth substrate. A conventional Sequencing Batch Reactor (SBR) and a Membrane BioReactor (MBR) were operated in parallel under the same 4CP influent concentrations and/or 4CP volumetric organic loading rates as the GSBR, in order to carry out a direct comparison in terms of 4CP removal efficiencies and specific removal rates, effluent quality, waste sludge production, system simplicity, land area requirement, start-up times, NaAc dosage as growth substrate and maximum applied 4CP volumetric organic loading rate. A decision matrix was built to define the best technology to suit different scenarios: the GSBR was proved to be the most suitable technology when system simplicity, low land area requirement and short start-up times were considered as critical parameters for decision making.

Sludge Accumulation in Two Anaerobic/Facultative Lagoons Treating Swine Manure from Breeding Farms in Oklahoma

Sludge stage and volume were monitored in two lagoons (OK1 and OK2) treating manure from swine breeding farms for a period of 9 years. Solids management differed between the two lagoons. Sludge was left relatively undisturbed in lagoon OK1, whereas solids were removed with each irrigation in lagoon OK2. When left undisturbed, sludge accumulated in a pattern consistent with the complex accumulation model proposed by earlier researchers. Sludge accumulated faster when solids were removed during irrigation compared to the lagoon in which solids were left undisturbed. Accumulation in both disturbed and undisturbed lagoons occurred at a rate less than current ASABE and NRCS standards. Onset of rapid accumulation appears to be related to volumetric organic loading rate rather than separation distance between top of the sludge and the liquid surface. Critical organic loading rates contained in the current standards appear to be adequate to determine the onset of rapid sludge accumulation.

Efeitos do pH, acidez e alcalinidade na microbiota de um reator anaeróbio de manta de lodo (UASB) tratando efluentes de suinocultura

The anaerobic processes used for treating wastewater have been often applied mainly for optimizing treatment systems. Among many of these systems, the UASB is one of the most successfully used. This type of reactor presents a good condition for microorganisms development, and therefore, for organic matter degradation. As a result, the goal of this research was to evaluate the effect of parameters, such as: temperature, pH, acidity and alkalinity on the microorganisms consortia, acclimatized in an UASB reactor, and simultaneously, observing the sludge morphology through a scanning electronic microscopy (SEM), in order to identify the response of the bacteria consortia under this environmental circumstances. The biomass operated under a mesophilic temperature, varying from 19 0 C to 21 0 C. The maximum concentration of volatile acids was 100 mg L -1 , and the volumetric organic loading rate was 59 kgCOD m -3 d -1 . The total alkalinity concentration values were between 2500 and 5550 mgCaCO3 L -1 . The average pH value of the sludge was 7.3. Under these conditions it was observed the development of a well acclimatized granular biomass, composed mainly of filamentous bacteria.

Anaerobic/oxic Treatment of Slurry-type Swine Waste

This paper presents the experimental results in five months operation from a combined anaerobic/oxic system treating swine waste with average concentrations in organic matter and nitrogen of 7,930 mgCOD/L and 671 mgTKN/L, respectively. The system was formed using an upflow anaerobic sludge blanket (UASB) reactor and oxic reactor connected in series with a recycling line of oxic effluents to UASB for its denitrification. The UASB reactor was operated at an organic volumetric loading rate (VLR) of 2.1~4.5 kgTCOD/m3/day and the removal efficiency of TCOD was 66.3~85.4%. The overall removal efficiency of TCOD was more than 99%. The oxic reactor was operated at a nitrogen VLR of 0.10~0.20 kgTKN/ m3/day and the nitrification efficiency was 75%. However, the complete denitrification was observed in the UASB reactor that was due to the optimal temperature and sufficient carbon source. The overall removal rate of TN was about 80%. About 76.2% of the influent COD mass was accountable in a COD mass balance at a level of VLR 3.64 kgCOD/m3/day. The production rate of methane was 0.32 LCH₄/gCODremoved when influent organics, VLR, were recorded by 3.4~4.5 kgCOD/m3/day.

Acetate-fed aerobic granular sludge for the degradation of 4-chlorophenol

Chlorinated phenols are considered a critical environmental problem, due to their extreme toxicity and their widespread use both in industrial and agricultural activities. In this study, aerobic granular sludge was initially developed into an acetate-fed Granulated Sequencing Batch Reactor (GSBR) and then used for the degradation of low chlorinated 4-mono-chlorophenol (4CP), with readily biodegradable sodium acetate (NaAc) as co-substrate. Influent 4CP concentration ranged between 0 and 50 mg/l, with a maximum volumetric organic loading rate of 0.20 kg 4CP/m 3 d (0.32 kg COD-4CP/m 3 d). Differences in granules shape and size were observed with 4CP dosed in the influent at different concentrations, and the effects of such toxic compound on acetate removal were evaluated, with both unacclimated and acclimated biomass. Aerobic granules grown on acetate as carbon source proved to be an interesting solution for the degradation of 4CP, showing good resistance to high 4CP concentrations in the influent even if unacclimated (short term effects). Moreover, the monitoring of intermediate products and the evaluation of chloride release due to 4CP degradation proved that acclimated granular sludge could completely remove 4CP (long term effects), with high specific removal rates.

PERFORMANCE OF A HIGH‐RATE/HIGH‐SHEAR ACTIVATED SLUDGE BIOREACTOR TREATING BIODEGRADABLE WASTEWATER

A high‐rate activated sludge (HRAS) bioreactor with high shear venturi aeration was operated in the laboratory at various organic loading rates to evaluate chemical oxygen demand (COD) removal efficiency and sludge production. Organic loading rates two orders of magnitude higher than conventional activated sludge loading were investigated in the modified HRAS reactor for a period of 41 weeks. Filtered COD removal efficiency varied from 81 to 92 % for organic loading rates of 3 to 85 kg COD m−3 d−1. Observed sludge yield was determined to be 0.10–0.25 g TSS produced g−1 COD removed, under hydraulic and solids retention times (SRTs) approaching less than two hours and two days, respectively. Observed sludge yield actually declined as loading increased and SRT decreased. It was concluded that high‐shear forces created in the reactor due to intense aeration at high volumetric organic loading rates increased substrate utilization rate, improved filtered COD removal efficiency, and kept sludge yields relatively low (despite the very low operating SRTs).

Effect of recycle on treatment of aircraft de-icing fluid in an anaerobic baffled reactors

Aircraft de-icing fluid at 7 000 mg COD/&#8467; was successfully treated in an anaerobic baffled reactor operated with and without recycle at volumetric organic loading rate of between 4 and 11 g COD/&#8467;reactor·d. Reactor recycle was found to improve reactor performance. The anaerobic baffled reactor operated with a 6:1 recycle ratio achieved a minimum hydraulic retention time of 17 h with an acceptable COD removal efficiency of 93% at a volumetric organic loading rate of 9.9 g COD/&#8467;reactor·d. This corresponded to a specific organic loading rate of 0.35 g COD/g VSS·d and specific organic removal rate of 0.32 g CODrem/g VSS·d. Without recycle similar removal efficiency was achieved; however, the loading rates were about 40% less. Due to biomass growth specific organic loading rate was not found to vary significantly through most of the experimental period despite loading rate increases. Hydrodynamically, an anaerobic baffled reactor may be characterised as an in-series continuously stirred tank reactor where the number of continuously stirred tank reactors corresponded to the number of actual compartments. Volatile fatty acid profiles tend to indicate that anaerobic baffled reactor compartmentalisation served to separate acidogenic and methanogenic activities longitudinally through the reactor, with the highest proportion of acidogenic activity in the first compartments. The net accumulated yield within the anaerobic baffled reactor was found to be of 0.007 g VSS/g CODrem when the ABR was operated without recycle and of 0.016 g VSS/g CODrem for the ABR operated with recycle. Water SA Vol.31 (3) 2005: pp.377-384

Effect of feeding strategy on the stability of anaerobic sequencing batch reactor responses to organic loading conditions

The goal of this study was to examine the effect of feeding strategy on the capability for treatment and the stability of an anaerobic sequencing batch reactor (ASBR) under increasing organic loading. The lab-scale ASBR systems were operated at 35 °C using synthetic organic wastewater under both batch and fed-batch operational modes with different feed to cycle time ( F: C) ratios. Experimental studies were conducted over a wide range of volumetric organic loading rates (VOLRs) (1.524 g COD/l/d) by varying the hydraulic retention time (HRT) (1.25, 2.5, and 5 d) and the feed wastewater’s COD (3750–30,000 mg/l). With an F: C ratio greater than or equal to 0.42, the fed-batch mode operation showed higher system efficiency in COD removal, volumetric methane production rate (VMPR), and specific methane production rate (SMPR) as compared to those in the batch mode with identical VOLR and HRT. In the fed-batch mode, the COD removals reached 86–95% with VOLR up to 12 g COD/l/d. The maximums for VMPR of 3.17 l CH 4/l/d and for SMPR of 1.63 g CH 4–COD/g VSS/d were achieved with a VOLR of 12 g COD/l/d at HRTs of 2.5 and 1.25 d, respectively. The fed-batch operation presented a lower concentration of volatile fatty acids (VFAs) than those in the batch operation. A lower concentration of VFAs confirmed the stability and efficiency of the fed-batch mode operation. The specific methanogenic activity (SMA) analysis showed that the VFA-degrading activity of the biomass in the fed-batch mode was higher for acetate and butyrate, and lower for propionate. Determined biomass yield and bacterial decay coefficients in the fed-batch operational mode were 0.05 g VSS/g COD rem and 0.001 d −1, respectively.

Development of empirical models for performance evaluation of UASB reactors treating poultry manure wastewater under different operational conditions

A nonlinear modeling study was carried out to evaluate the performance of UASB reactors treating poultry manure wastewater under different organic and hydraulic loading conditions. Two identical pilot scale up-flow anaerobic sludge blanket (UASB) reactors (15.7 L) were run at mesophilic conditions (30–35 °C) in a temperature-controlled environment with three hydraulic retention times ( θ) of 15.7, 12 and 8.0 days. Imposed volumetric organic loading rates ( L V) ranged from 0.65 to 4.257 kg COD/(m 3 day). The pH of the feed varied between 6.68 and 7.82. The hydraulic loading rates ( L H) were controlled between 0.105 and 0.21 m 3/(m 2 day). The daily biogas production rates ranged between 4.2 and 29.4 L/day. High volumetric COD removal rates ( R V) ranging from 0.546 to 3.779 kg COD removed/(m 3 day) were achieved. On the basis of experimental results, two empirical models having a satisfactory correlation coefficient of about 0.9954 and 0.9416 were developed to predict daily biogas production ( Q g) and effluent COD concentration ( S e), respectively. Findings of this modeling study showed that optimal COD removals ranging from 86.3% to 90.6% were predicted with HRTs of 7.9, 9.5, 11.2, 12.6, 13.7 and 14.3 days, and L V of 1.27, 1.58, 1.78, 1.99, 2.20 and 2.45 kg COD/(m 3 day) for the corresponding influent substrate concentrations ( S i) of 10,000, 15,000, 20,000, 25,000, 30,000 and 35,000 mg/L, respectively.

Production of bio‐hydrogen by mesophilic anaerobic fermentation in an acid‐phase sequencing batch reactor

The pH and hydraulic retention time (HRT) of an anaerobic sequencing batch reactor (ASBR) were varied to optimize the conversion of carbohydrate-rich synthetic wastewater into bio-hydrogen. A full factorial design using evolutionary operation (EVOP) was used to determine the effect of the factors and to find the optimum condition of each factor required for high hydrogen production rate. Experimental results from 20 runs indicate that a maximum hydrogen production rate of 4,460-5,540 mL/L/day under the volumetric organic loading rate (VOLR) of 75 g-COD/L/day obtained at an observed design point of HRT = 8 h and pH = 5.7. The hydrogen production rate was strongly dependent on the HRT, and the effect was statistically significant (P < 0.05). However, no significant effect (P > 0.05) was found for the pH on the hydrogen production rate. When the ASBR conditions were set for a maximum hydrogen production rate, the hydrogen production yield and specific hydrogen production rate were 60-74 mL/g-COD and 330-360 mL/g-VSS/day, respectively. The hydrogen composition was 43-51%, and no methanogenesis was observed. Acetate, propionate, butyrate, valerate, caproate, and ethanol were major liquid intermediate metabolites during runs of this ASBR. The dominant fermentative types were butyrate-acetate or ethanol-acetate, representing the typical anaerobic pathway of Clostridium species. This hydrogen-producing ASBR had a higher hydrogen production rate, compared with that produced using continuous-flow stirred tank reactors (CSTRs). This study suggests that the hydrogen-producing ASBR is a promising bio-system for prolonged and stable hydrogen production.