Continuous Anaerobic Reactor Research Articles

Enrichment of specific electro-active microorganisms and enhancement of methane production by adding granular activated carbon in anaerobic reactors

Direct interspecies electron transfer (DIET) via conductive materials can provide significant benefits to anaerobic methane formation in terms of production amount and rate. Although granular activated carbon (GAC) demonstrated its applicability in facilitating DIET in methanogenesis, DIET in continuous flow anaerobic reactors has not been verified. Here, evidences of DIET via GAC were explored. The reactor supplemented with GAC showed 1.8-fold higher methane production rate than that without GAC (35.7 versus 20.1±7.1mL-CH4/d). Around 34% of methane formation was attributed to the biomass attached to GAC. Pyrosequencing of 16S rRNA gene demonstrated the enrichment of exoelectrogens (e.g. Geobacter) and hydrogenotrophic methanogens (e.g. Methanospirillum and Methanolinea) from the biomass attached to GAC. Furthermore, anodic and cathodic currents generation was observed in an electrochemical cell containing GAC biomass. Taken together, GAC supplementation created an environment for enriching the microorganisms involved in DIET, which increased the methane production rate.

Response of a continuous anaerobic digester to temperature transitions: A critical range for restructuring the microbial community structure and function

Temperature is a crucial factor that significantly influences the microbial activity and so the methanation performance of an anaerobic digestion (AD) process. Therefore, how to control the operating temperature for optimal activity of the microbes involved is a key to stable AD. This study examined the response of a continuous anaerobic reactor to a series of temperature shifts over a wide range of 35–65 °C using a dairy-processing byproduct as model wastewater. During the long-term experiment for approximately 16 months, the reactor was subjected to stepwise temperature increases by 5 °C at a fixed HRT of 15 days. The reactor showed stable performance within the temperature range of 35–45 °C, with the methane production rate and yield being maximum at 45 °C (18% and 26% greater, respectively, than at 35 °C). However, the subsequent increase to 50 °C induced a sudden performance deterioration with a complete cessation of methane recovery, indicating that the temperature range between 45 °C and 50 °C had a critical impact on the transition of the reactor's methanogenic activity from mesophilic to thermophilic. This serious process perturbation was associated with a severe restructuring of the reactor microbial community structure, particularly of methanogens, quantitatively as well as qualitatively. Once restored by interrupted feeding for about two months, the reactor maintained fairly stable performance under thermophilic conditions until it was upset again at 65 °C. Interestingly, in contrast to most previous reports, hydrogenotrophs largely dominated the methanogen community at mesophilic temperatures while acetotrophs emerged as a major group at thermophilic temperature. This implies that the primary methanogenesis route of the reactor shifted from hydrogen- to acetate-utilizing pathways with the temperature shifts from mesophilic to thermophilic temperatures. Our observations suggest that a mesophilic digester may not need to be cooled at up to 45 °C in case of undesired temperature rise, for example, by excessive self-heating, which offers a possibility to reduce operating costs.

Inoculation of paperboard mill sludge versus mixed culture bacteria for hydrogen production from paperboard mill wastewater.

A comparative evaluation of paperboard mill sludge (PMS) versus mixed culture bacteria (MCB) as inoculum for hydrogen production from paperboard mill wastewater (PMW) was investigated. The experiments were conducted at different initial cultivation pHs, inoculums to substrate ratios (ISRs gVS/gCOD), and hydraulic retention times (HRTs). The peak hydrogen yield (HY) of 5.29 ± 0.16 and 1.22 ± 0.11 mmol/gCODinitial was occurred at pH = 5 for MCB and PMS, respectively. At pH of 5, the HY and COD removal achieved the highest values of 2.26 ± 0.14 mmol/gCODinitial and 86 ± 1.6% at ISR = 6 for MCB, and 2.38 ± 0.25 mmol/gCODinitial and 60.4 ± 2.5% at ISRs = 3 for PMS. The maximum hydrogen production rate was 93.75 ± 8.9 mmol/day at HRT = 9.6 h from continuous upflow anaerobic reactor inoculated with MCB. Meanwhile, the 16S ribosomal RNA (rRNA) gene fragments indicated a dominance of a novel hydrogen-producing bacterium of Stenotrophomonas maltophilia for PMS microbial community. On the other hand, Escherichia fergusonii and Enterobacter hormaechei were the predominant species for MCB.

Improving biogas production from microalgae by enzymatic pretreatment

In this study, enzymatic pretreatment of microalgal biomass was investigated under different conditions and evaluated using biochemical methane potential (BMP) tests. Cellulase, glucohydrolase and an enzyme mix composed of cellulase, glucohydrolase and xylanase were selected based on the microalgae cell wall composition (cellulose, hemicellulose, pectin and glycoprotein). All of them increased organic matter solubilisation, obtaining high values already after 6h of pretreatment with an enzyme dose of 1% for cellulase and the enzyme mix. BMP tests with pretreated microalgae showed a methane yield increase of 8 and 15% for cellulase and the enzyme mix, respectively. Prospective research should evaluate enzymatic pretreatments in continuous anaerobic reactors so as to estimate the energy balance and economic cost of the process.

Effect of bioreduced graphene oxide on anaerobic biotransformation of nitrobenzene in an anaerobic reactor

Bioreduced graphene oxide (BRGO) has been proven to be capable of accelerating nitrobenzene (NB) biotransformation by anaerobic sludge (AS). To realize its application, in the present study, two continuous anaerobic reactors (R1 with BRGO/AS composite; R2 with AS) were employed to treat NB-containing wastewater during a long-term run. Compared with R2, the start-up time of R1 was shortened from 70 to 45 d and R1 exhibited better removal efficiency (87% of R1 and 74% of R2 with an influent NB concentration of 200 mg L−1 at hydraulic retention time = 24 h, NaCl = 3%). Moreover, R1 exhibited better stability with over 81% NB removal efficiency within 90 d. Further study demonstrated that the presence of BRGO facilitated microorganisms to secrete extracellular polymeric substances, resulting in the higher electrochemical and dehydrogenase activities of R1 compared with those of R2.

Hydrogen production from cassava processing wastewater in an anaerobic fixed bed reactor with bamboo as a support material

Attempting to associate waste treatment to the production of clean and renewable energy, this research sought to evaluate the biological production of hydrogen using wastewater from the cassava starch treatment industry, generated during the processes of extraction and purification of starch. This experiment was carried out in a continuous anaerobic reactor with a working volume of 3L, with bamboo stems as the support medium. The system was operated at a temperature of 36°C, an initial pH of 6.0 and under variations of organic load. The highest rate of hydrogen production, of 1.1 L.d-1.L-1, was obtained with application of an organic loading rate of 35 g.L-1.d-1, in terms of total sugar content and hydraulic retention time of 3h, with a prevalence of butyric and acetic acids as final products of the fermentation process. Low C/N ratios contributed to the excessive growth of the biomass, causing a reduction of up to 35% in hydrogen production, low percentages of H2 and high concentrations of CO2in the biogas.

Comparative study on response of thiocyanate shock load on continuous and fed batch anaerobic-anoxic-aerobic sequential moving bed reactors

A comparative study on response of a toxic compound thiocyanate (SCN－) was carried out in continuous and fed batch moving bed reactor systems. Both systems had three sequential anaerobic, anoxic and aerobic reactors and operated at same hydraulic retention time. Feed SCNwas first increased from 600 mg/L to 1,000 mg/L for 3 days (shock 1) and then from 600 to 1,200 mg/L for 3 days (shock 2). In anaerobic continuous reactor, increase of effluent COD (chemical oxygen demand) due to shock load was only 2%, whereas in fed batch reactor it was 14%. In anoxic fed batch reactor recovery was partial in terms of SCN-, phenol, COD and NO₃--N and NO₂--N removals and in continuous reactor complete recovery was possible. In both systems, inhibition was more significant on aerobic reactors than anaerobic and anoxic reactors. In aerobic reactors ammonia removal efficiency deteriorated and damage was irreversible. Present study showed that fed batch reactors showed higher substrate removal efficiency than continuous reactors during regular operation, but are more susceptible to toxic feed shock load and in nitrifying reactor damage was irreversible.

Microbial-chemical indicator for anaerobic digester performance assessment in full-scale wastewater treatment plants for biogas production

Anaerobic digestion was introduced into wastewater treatment plants several years ago, but anaerobic digestion performance has not yet been achieved. The variability of the microbial community in digesters is poorly understood, and despite the crucial role of anaerobic digestion reactors, the microbial equilibrium that yields the best performance in these reactors has only recently been hypothesised. In this study, two full-scale continuous anaerobic reactors, placed in Torino’s main wastewater treatment plant in northern Italy, were followed to develop a summary indicator for measuring anaerobic digestion performance. A total of 100 sludge samples were collected. The samples were characterised chemically and physically, and microbial groups were quantified by qRT-PCR. A chemical biological performance index strictly correlated to specific biogas production (rho=0.739, p<0.01) is proposed. This approach will produce new management tools for anaerobic digestion in wastewater treatment plants.

Study of novel pure culture HBCD-1, effectively degrading Hexabromocyclododecane, isolated from an anaerobic reactor

In this study, two pure strains, named HBCD-1 and HBCD-2, were isolated from a continuous anaerobic reactor over 300-days acclimation, which processed high capability of biodegrading Hexabromocyclododecane. Both of the two strains degraded HBCD diastereomers in different extents, especially strain HBCD-1, which interestingly degraded α-HBCD effectively. All of the degrading results were well fitted with the first-order kinetics model. By morphological observation and 16S rRNA gene sequence analysis, the strain HBCD-1 showed highest similarity with Achromobacter sp. Under the optimal culturing conditions of 30°C, pH 7 and the initial HBCD concentration of 500μg/L, the biodegradation rate of HBCD-1 reached 90% after 8days treatment. Moreover, during the biodegradation process by HBCD-1 strain, the concentration of bromide ion was lower than the theoretical value. Finally, 4 metabolites were determined by liquid chromatography–tandem mass spectrometry (LC–MS/MS), as well as a biodegradation pathway was proposed.

Influence of ferric oxyhydroxide addition on biomethanation of waste activated sludge in a continuous reactor.

This study investigated the potential of enhancing the activity of iron-reducing bacteria (IRBs) to increase the biomethanation rate of waste activate sludge (WAS). The effects of biostimulation by ferric oxyhydroxide (Phase 2) and bioaugmentation with an enriched IRB consortium (Phase 3) were examined in a continuous anaerobic reactor treating WAS. Compared to the control operation (Phase 1), significant rises in methane yield (10.8-59.4%) and production rate (24.5-52.9%) were demonstrated by the biostimulation and bioaugmentation treatments. Visible structural changes were observed in bacterial community with the phases while not in archaeal community. Acinetobacter- and Spirochaetales-related populations were likely the major players driving anaerobic iron respiration and thus leading to enhanced biomethanation performance, in Phases 2 and 3, respectively. Our results suggest an interesting new potential for enhancing biomethanation of WAS.

Long-term anaerobic digestion of microalgae grown in HRAP for wastewater treatment. Effect of microwave pretreatment

This paper describes the anaerobic digestion of microalgal biomass from high rate ponds in continuous anaerobic reactors for biogas production. With hydraulic retention times (HRT) of 15 and 20 days, the volatile solids (VS) removal did not exceed 30%, and the methane production rate ranged between 0.12 and 0.14 L CH4/L day. To improve process performance, microwave irradiation at 900 W for 3 min (specific energy ∼70,000 kJ/kg VS) was applied as a pretreatment step. The VS removal increased to 40 and 45% at 15 and 20 days HRT, respectively. Consequently, the methane production rate increased to 0.16 and 0.20 L CH4/L day at 15 and 20 days HRT, respectively. Microscopic analysis confirmed cell wall damage, although generally without lysis, after irradiating microalgal biomass. However, the energy consumption was much higher than the extra energy production of the process. Indeed, microalgal biomass should not only be thickened but also dewatered if microwave irradiation was to be applied as a pretreatment to anaerobic digestion for biogas production.

Performance Evaluation of Two Anaerobic Reactors for Removing Sulphate from Industrial Effluents

Reactors hydrodynamic condition and the choice of substrate are important factors for the optimization and implementation of this sulphate-reducing biological process. This study evaluated two continuous anaerobic reactors, UASB and fluidized bed. The maximum removal achieved by the UASB reactor when operated without recirculation was 65% for a substrate load applied to 3.55 kg/m3 d. When the mass transfer conditions of the reactor were improved by recirculating the biomass, the sulphate removal efficiency increased to 89%, representing a removal rate of 1.94 kg SO42-/m-3 .d-1. Glycerol was used with the carbon and electron source for SRB in the fluidized bed reactor and was compared to the performance with lactate. For the same sulphate load applied, a removal efficiency of 70%-90% was observed, resulting in a residual concentration average of 254 mg/L sulphate. Glycerol, which is a by-product from biodiesel production, is a potential choice as substrate for sulphate reduction.

Effect of Substrate Concentration on Hydrogen Production from Fermentation of Sugar Wastewater

The biological hydrogen production by the sugar wastewater is an effective way to achieve the reclamation. In this paper, the effect of substrate concentration on the hydrogen production is discussed through employing the self-made continuous flow anaerobic fermentation hydrogen production reactor, taking the sludge in urban sewage treatment plant as the inoculated sludge and the simulated sugar wastewater as the substrate. The experimental results show that the best hydrogen production effect can be obtained when the temperature is (37±1) °C, HRT is 7h, the water alkalinity is around 530mg/L and the substrate concentration is 5000mg/L, namely the organic load is 60kgCOD/(m3·d). The volumes of gas production and hydrogen production both reach the maximum. The average values are respectively 36.2L/d and 21.8L/d. The obtained hydrogen production rate is 0.93kgCOD/(m3·d). During the whole process, the proportion of volatile acid composition remains stable, which is the butyric acid-type fermentation. When the concentration of COD is increased to 6000-8000mg/L, the ability of hydrogen production of system will be significantly dropped due to the increase of pH of system. The hydrogen production performance can be restored through artificially and timely lowering the water alkalinity. However, the hydrogen production rate will be decreased compared to the previous situation.

Methane production from the microalga Scenedesmus sp. AMDD in a continuous anaerobic reactor

Methane production from the microalga Scenedesmus sp. AMDD digested in a continuously operated anaerobic reactor was studied under a range of conditions. Preliminary anaerobic toxicity assays revealed that methane yields and glucose consumption were inhibited by algae/sludge ratios in excess of 0.1 (g/g). Continuous digestions of microalgae at two hydraulic retention times (16 and 58days) and two influent concentrations of algal feed (11 and 20gTVSL−1) were evaluated. Based on the amount of degraded algal biomass, a methane yield of 0.4LCH4gTVS‐1 was estimated. However, at a hydraulic retention time of 16days only 52–53% of the algal biomass was degraded, primarily due to the limited hydrolysis of the algal cellular material. An increase of retention time to 58days did not improve the degradation efficiency. Anaerobic decomposition of microalgae biomass appeared to affect composition of the microbial consortium in the digester, promoting the growth of sulfate-reducing bacteria leading to high levels of H2S in the biogas.

State Estimation by Artificial Neural Networks in a Continuous Bioreactor

A based neural networks state observer to estimate biomass, substrate and methane in a continuous anaerobic reactor is introduced in this paper. The observer is designed from a recurrent high order neural network with a hyperbolic tangent as activation function and an extended Kalman filter as learning algorithm. The observer structure is validated via simulations and using experimental data obtained from an anaerobic continuous stirred tank at lab scale. This prototype is used to treat real slaughterhouse wastewater and it is operated in continuous mode. The obtained results show that the proposed observer is able to reproduce adequately the biomethane production and the substrate (related to chemical oxygen demand) in the methanogenesis stage; besides, methanogenic bacteria are also well estimated but some modifications are required in order to reach better results.

Long-term hydrolytic capacity evaluation of a thermophilic anaerobic digester treating sewage sludge

This study presents an evaluation of the hydrolytic activity of a continuous thermophilic anaerobic reactor in long-term operation. The hydrolytic coefficient was estimated by fitting a three-reaction model of the anaerobic digestion process with experimental data obtained from a pilot thermophilic digester operated for about 2 years. The model fitting and the cross-validation indicate that this model can represent the behavior of the system in a proper way; moreover, the results show a variation of the hydrolytic capacity of the system throughout the evaluation period. The increase in the hydrolytic coefficient is in agreement with the increase in the organic load applied to the reactor, which shows the capacity of the continuous reactor to select populations according to the input conditions of the system.

Treatability of landfill leachate by combined upflow anaerobic sludge blanket reactor and aerated lagoon

Continuous upflow anaerobic sludge blanket reactor performs more favorably at the higher organic loading rate than other anaerobic treatment. The treatment of municipal landfill leachate of Shiraz’s city investigated using continuous flow anaerobic reactor and subsequently aerated lagoon. Landfill leachate has chemical oxygen demand of 45,000–90,000 mg/L and ammonia nitrogen at 1,000–2,500 and heavy metals that can impact biological treatments. Capacity of anaerobic and aerobic reactors is 10 and 20 L that operated at detention time of 2 and 4 days, respectively. Organic loading rate of upflow anaerobic sludge blanket is between 0.5–20 g chemical oxygen demand/L/day. Chemical oxygen demand removal efficiencies are between 57–87, 35–70 and 66–94% in the anaerobic, aerobic and whole system, respectively. As the entry, leachate organic loading rate increased from 1 to 20 g/L/day, the chemical oxygen demand removal efficiency reached a maximum of 71% and 84% in the anaerobic reactor and whole system, respectively, at high organic loading rate. Ammonium removal efficiency was about 54% after the aerobic stage.

Correlations between molecular and operational parameters in continuous lab-scale anaerobic reactors

In this study, the microbial community characteristics in continuous lab-scale anaerobic reactors were correlated to reactor functionality using the microbial resource management (MRM) approach. Two molecular techniques, denaturing gradient gel electrophoresis (DGGE) and terminal-restriction fragment length polymorphism (T-RFLP), were applied to analyze the bacterial and archaeal communities, and the results obtained have been compared. Clustering analyses showed a similar discrimination of samples with DGGE and T-RFLP data, with a clear separation between the meso- and thermophilic communities. Both techniques indicate that bacterial and mesophilic communities were richer and more even than archaeal and thermophilic communities, respectively. Remarkably, the community composition was highly dynamic for both Bacteria and Archaea, with a rate of change between 30% and 75% per 18 days, also in stable performing periods. A hypothesis to explain the latter in the context of the converging metabolism in anaerobic processes is proposed. Finally, a more even and diverse bacterial community was found to be statistically representative for a well-functioning reactor as evidenced by a low Ripley index and high biogas production.

Application of Kernel Partial Least Square to Predict Biological Hydrogen Production by Enterobacter aerogenes

Hydrogen production by dark fermentation, where hydrogenase enzymes catalyze the oxidation or evolution of molecular hydrogen from two protons (H + ) and electrons, is an economic and environmentally friendly technology for producing clean energy. However, the long-term operations of a continuous anaerobic reactor for fermentative hydrogen production were frequently unstable. In this study, a kernel partial least squares (KPLS) algorithm is employed to develop an online estimation of the key process variables in a biological hydrogen production process by Enterobacter aerogenes in minimal time and with minimal cost. The KPLS approach is potentially very efficient for predicting key quality variables of nonlinear processes by mapping an original input space into a high-dimensional feature space. The proposed kernel-based algorithm could effectively capture the nonlinear relationship in the process variables and show far better performance in the prediction of the key process variable compared with the conventional linear PLS and other nonlinear PLS methods.

Ultrasound pre-treatment for anaerobic digestion improvement

Prior research indicates that ultrasounds can be used in batch reactors as pre-treatment before anaerobic digestion, but the specific energy required at laboratory-scale is too high. This work evaluates both the continuous ultrasound device performance (efficiency and solubilisation) and the operation of anaerobic digesters continuously fed with sonicated sludge, and presents energy balance considerations. The results of sludge solubilisation after the sonication treatment indicate that, applying identical specific energy, it is better to increase the power than the residence time. Working with secondary sludge, batch biodegradability tests show that by applying 30 kWh/m3 of sludge, it is possible to increase biogas production by 42%. Data from continuous pilot-scale anaerobic reactors (V=100 L) indicate that operating with a conventional HRT=20 d, a reactor fed with pre-treated sludge increases the volatile solids removal and the biogas production by 25 and 37% respectively. Operating with HRT=15 d, the removal efficiency is similar to the obtained with a reactor fed with non-hydrolysed sludge at HTR=20 d, although the specific biogas productivity per volume of reactor is higher for the pretreated sludge. Regarding the energy balance, although for laboratory-scale devices it is negative, full-scale suppliers state a net generation of 3-10 kW per kW of energy used.