Two-stage Anaerobic Digestion Process Research Articles

Co-digestion of filter cake, biogas effluent, and anaerobic sludge for hydrogen and methane production: Optimizing energy recovery through two-stage anaerobic digestion

This study assesses a two-stage anaerobic digestion process designed to efficiently recover energy through hydrogen and methane production by co-digesting filter cake (FC), biogas effluent (BE), and anaerobic sludge (AS) from the sugar and ethanol industry. The optimal proportions of FC, BE, and AS were determined in batch fermentation experiments using Design Expert software, which identified a suitable ratio of 31.05:28.95:0.00 (g VS/L), respectively. The results indicated that hydrogen production in the first stage could occur solely through the hydrogenic bacteria present in the BE and FC mixture, without the need for AS as an inoculum. This optimized FC:BE ratio was then applied in a semi-continuous fermentation system, achieving a hydrogen production rate of 193.6 mL H2/L.d and a hydrogen yield of 9.8 mL H2/g VS at an optimal hydraulic retention time (HRT) of 3 d. In the subsequent second stage, the effluent from the hydrogen reactor was used for methane production. This stage achieved a methane production rate of 422.0 mL CH4/L·d and a methane yield of 140.2 mL CH4/g VS, with an HRT of 20 d. Overall, the two-stage process exhibited an impressive energy output, peaking at 5.17 kJ/g VS. This suggests the potential for an annual electricity generation of 194,655 MWh and an estimated reduction of 85,668 tCO2eq/year in greenhouse gas emissions. This study highlights the efficiency of the two-stage anaerobic digestion process for harnessing energy through the co-digestion of FC and BE, without the need for additional AS inoculum. The findings demonstrate the potential for sustainable energy recovery from industrial waste streams while mitigating environmental impacts.

A volatile fatty acids adaptive observer-based hierarchical optimal controller design to maximum gas production of two-stage anaerobic digestion process

Volatile fatty acids (VFA), which influence the stability and gas production, are important state variables in anaerobic digestion (AD) process. Three main components of VFA can indicate the metabolic state of AD internal environment and be considered as a better basis for AD monitoring strategies. A VFA adaptive observer (VFA-AO) is proposed to estimate unknown gas production kinetic parameters and unmeasurable states simultaneously, including the concentration of three main components of VFA. Otherwise, using the estimated information, a VFA-AO-based hierarchical optimal controller (VFA-AO-based HOC) is designed to maximize gas production rates of AD. The proposed controller is compared with two existing control algorithms. Simulation results show that, despite the concentration of pollutants in the inlet fluctuates, VFA-AO achieves a reliable states and kinetic parameters estimation, and VFA-AO-based HOC maximizes gas production.

Enhancing bio-hydrogen and bio-methane production of concentrated latex wastewater (CLW) by Co-digesting with palm oil mill effluent (POME): Batch and continuous performance test and ADM-1 modeling

This study aimed at investigating the biohydrogen and biomethane potential of co-digestion from palm oil mill effluent (POME) and concentrated latex wastewater (CLW) in a two-stage anaerobic digestion (AD) process under thermophilic (55 ± 3 °C) and at an ambient temperature (30 ± 3 °C) conditions, respectively. The batch experiments of POME:CLW mixing ratios of 100:0, 70:30, 50:50, 30:70, and 0:100 was investigated with the initial loadings at 10 g-VS/L. The highest hydrogen yield of 115.57 mLH2/g-VS was obtained from the POME: CLW mixing ratio of 100:0 with 29.0 of C/N ratio. While, the highest subsequent methane production yield of 558.01 mLCH4/g-VS was achieved from hydrogen effluent from POME:CLW mixing ratio of 70:30 0 with 21.8 of C/N ratio. This mixing ratio revealed the highest synergisms of about 9.21% and received maximum total energy of 19.70 kJ/g-VS. Additionally, continuous hydrogen and methane production were subsequently performed in a series of continuous stirred tank reactor (CSTR) and up-flow anaerobic sludge blanket reactor (UASB) to treat the co-substate. The results indicated that the highest hydrogen yield of POME:CLW mixing ratio at 70:30 of 95.45 mL-H2/g-VS was generated at 7-day HRT, while methane production was obtained from HRT 15 days with a yield of 204.52 mL-CH4/g-VS. Thus, the study indicated that biogas production yield of CLW could be enhanced by co-digesting with POME. In addition, the two-stage AD model under anaerobic digestion model no. 1 (ADM-1) framework was established, 9.10% and 2.43% of error fitting of hydrogen and methane gas between model simulation data and experimental data were found. Hence, this research work presents a novel approach for optimization and feasibility for co-digestion of POME with CLW to generate mixed gaseous biofuel potentially.

The application of Anaerobic Digestion Model No. 1 for the optimization of biogas production from maize silage, pig manure, cattle manure, and digestate in a full-scale biogas plant

The main aim of this study was to evaluate the applicability of a mathematical model based on Anaerobic Digestion Model No. 1 (ADM1) for optimizing biogas production during a two-stage anaerobic digestion process in a full-scale biogas plant. Biogas production was analyzed in two variants. Variant I involved two decision variables: the proportion of the gaseous phase in the total volume of each digester within a preset limit of 0.05 to 0.95. Four decision variables were adopted in variant II: total digester volume during the first and second stage of anaerobic digestion, and the proportion of the gaseous phase in each digester. Digester volume was set at 10–1000 m3 for the primary digester, and at 300–5000 m3 for the secondary digester. The proportion of the gaseous phase was also set within the range of 0.05–0.95. Optimal digester volume (131 m3 for the primary digester and 5000 m3 for the secondary digester) and the optimal proportions of the gaseous phase in total digester volume (0.35 and 0.05, respectively) were generated by the modified ADM1xp model. The modeled parameters predicted a 67% increase in methane yields in a two-stage anaerobic digestion process (acidogenesis and methanogenesis).

Trajectory tracking of two-stage anaerobic digestion process: A predictive control with guaranteed performance and saturated input, based on ultra-local model

To address the robust output feedback tracking problem for total outflow rate of two-stage anaerobic digestion (AD) process with input saturation, a novel performance guaranteed ultra-local model (ULM)-based predictive control (PG-ULMPC) is proposed. First, model of the two-stage AD process is considered which presents the characteristics of the production rate of hydrogen and methane. Then, a new control strategy is developed, which relies only on input/output data and does not require any model knowledge of two-stage AD process. Based on ULM, the designed PG-ULMPC strategy consists of time-delay estimation (TDE), prescribed performance function (PPF) as well as predictive control using feedback correction (FC). The ULM is utilized to approximate the complex model of two-stage AD process in a short sliding time window and simplify controller design. The TDE is employed to estimate the lumped disturbance. The PPF guarantees that the tracking error is limited within the performance bounds whose convergence time and precision can be artificially preset. The predictive control using FC stabilizes the closed-loop system and provides saturated control input. To demonstrate the PG-ULMPC performances, numerical simulations of two-stage AD process in presence of unknown parameter disturbances with nonlinear Proportional–Integral (PI) controller, with ULMPC and with PG-ULMPC are realized by using Matlab/Simulink. The obtained results illustrate the better performance of the proposed PG-ULMPC strategy. Moreover, under the measurement noise, by using filtered derivative technique, the proposed method can still achieve satisfactory performance and has little control chattering.

Anaerobic co-digestion of agro-industrial wastes using anaerobic sequencing batch reactor for bio-energy recovery: Focus on process performance and stability of the methanogenic step

Anaerobic co-digestion of tannery wastewater (TWW) with dairy wastewater (DWW) using a two-phase anaerobic sequencing batch reactor (ASBR) was evaluated. For this purpose, laboratory-scale anaerobic digesters, one for acidogenesis and the other for methanogenesis, with 0.4 and 0.6 L of working volume, respectively, operated in semi-continuous mode, were used. The results showed that a methanogenic reactor treating the effluent acidogenic reactor (50 % TWW and 50 % DWW) exhibits the best process performances in terms of daily biogas (64.7 mL/day), methane production (461 mL/day), specific methane yield (892 mL CH4/g COD), methane content (71.2 %), and total chemical oxygen demand (TCOD) removal efficiency (85.6 %) when operated at a hydraulic retention time (HRT) of 15 days. This indicated that the addition of DWW improves the anaerobic co-digestion process, increasing biogas production (64.7 mL/day) by about 41 % as compared with anaerobic co-digestion of 100 % TWW (27.4 mL/day). The process stability of the methanogenic stage of the two-stage anaerobic digestion process was also evaluated, and the obtained result showed suitable pH (6.70), no total volatile fatty acid (TVFA) accumulation (VFA/alkalinity: 0.363), alkalinity (3350 mg CaCO3/L), and ammonia (231 mg/L) within the optimum operating range. Furthermore, high overall performance efficiency in terms of TCOD reduction (88.8 %) in the two-phase (acidogenesis and methanogenesis) anaerobic digester of 50:50 (TWW:DWW) was obtained when operated at a HRT of 18 days (3 days for acidogenesis and 15 days for methanogenesis). Anaerobic co-digestion of TWW with DWW generally resulted in increased process stabilities and performances compared to mono-digestion of TWW.

Critical Assessment of Hydrogen and Methane Production from 1G and 2G Sugarcane Processing Wastes Using One-Stage and Two-Stage Anaerobic Digestion

Sugarcane is a lignocellulosic crop which is used to produce sugar in sugarcane processing industries. Globally, sugarcane processing industries generate solid and liquid wastes amounting to more than 279 million tons per annum and by-products; namely, trash, bagasse, mill mud, and molasses. The valorisation of waste and by-products has recently increased and is playing a significant role in achieving policies and goals associated with circular bioeconomy and sustainable development. For the valorisation of sugarcane processing industry waste and by-products, a number of technologies are well established and in use, while other innovative technologies are still ongoing through research and development with promising futures. These by-products obtained from sugarcane processing industries can be converted into biofuels like hydrogen and methane via anaerobic digestion. Molasses belongs to the first-generation (1G) waste, while trash, bagasse, and mill mud belong to second-generation (2G) waste. Various studies have been carried out in converting both first- and second-generation sugarcane processing industry wastes into renewable energy, exploiting anaerobic digestion (AD) and dark fermentation (DF). This review emphasises the various factors affecting the AD and DF of 1G and 2G sugarcane processing industry wastes. It also critically addresses the feasibility and challenges of operating a two-stage anaerobic digestion process for hydrogen and methane production from these wastes.

Hydrogen and Methane Production in a Two-Stage Thermophilic Anaerobic Digestion System by Co-Digestion of Kitchen Waste and Municipal Sewage Sludge with a High Solid Content

The thermophilic anaerobic fermentation of mixed materials for simultaneously producing biohydrogen and methane has become a research hotspot. In this study, thermally treated kitchen waste (KW) and municipal sewage sludge (MSS) were used as substrates for a thermophilic two-stage anaerobic digestion process. The effects on gas production performance were investigated in both the hydrogen-producing stage and methane-producing stage under different organic load rates (OLRs). The production rates, volume, removal rates, and energy yields of volatile solids (VS) were optimal under higher OLRs for both the hydrogen (OLR = 12.5 g VS/L·day) and methane stages (OLR = 5 g VS/L·day). These conditions also provided the optimal average removal rate and total energy yield of VS for the entire anaerobic fermentation system (65.4% and 30.9 kJ/g VS, respectively). Overall, the results demonstrate the superior efficiency of VS removal and hydrogen and methane production in a two-stage thermophilic anaerobic co-digestion of KW and MSS with a high solid content under proper material mix ratio and higher OLRs. This study provides the basis for material mass ratio, mixtures' pretreatment, and operation control for future pilot tests.

Application of Doehlert design in optimizing the solid-state hydrogenogenic stage augmented with biomass fly ash in a two-stage biohythane production process

This study aimed to optimize the solid-state hydrogenogenic stage supplemented with biomass fly ash in a two-stage anaerobic digestion (AD) process for biohythane production from the organic fraction of municipal solid waste (OFMSW). Doehlert’s experimental design was used to obtain the optimal set of two investigated variables, namely total solids (TS) content and biomass fly ash dosage in the defined ranges of 0–20 g/L and 20–40%, respectively. Applying the optimal conditions of TS content (29.1%) and fly ash dosage (19.2 g/L) in the first stage led not only to a total H2 yield of 95 mL/gVSadded, which was very close to the maximum H2 yield predicted by the developed model (97 mL/gVSadded), but also to a high CH4 yield of 400 mL/gVSadded (76% of the theoretical CH4 yield). Moreover, the biohythane obtained from the optimized two-stage process met the standards of a biohythane fuel with an H2 content of 19% v/v.

Enhanced hydrogen and methane production from date fruit wastes using semi continuous two-stage anaerobic digestion process with increasing organic loading rates

Biological conversion of date wastes under continuous conditions is a challenge due to their high content of biodegradable organic matter. In this study, hydrogen (H2) and methane (CH4) production in a two-stage process to increase the energy recovery from date pulp wastes (DPW) was investigated. In the first stage, the effect of increasing organic leading rate (OLR) on dark fermentation (DF) performances was evaluated in a semi continuously stirred tank reactors (SCSTR) without nutrients addition. The highest volumetric H2 production rate (VHPR) and specific H2 yield (SHY) of 0.76 L/L.d and 81 LH2/kg volatile solid (VS), respectively were achieved at high OLR of 11.74 gVS/L.d. In the second stage, the DF effluent was used as substrate to feed the SCSTR of anaerobic digestion (AD). The highest volumetric methane production rate (VMPR) of 2.92 L/L.d was achieved with a high OLR (4.25 gVS/L.d) and short hydraulic retention time (HRT, 8 days), producing specific methane yield (SMY) of 530 LCH4/kgVS. Thus, the two-stage process can be an effective strategy for high rate renewable energy recovery (13 kJ/gVSinlet) and organic waste processing.

Biogas production of food waste with in-situ sulfide control under high organic loading in two-stage anaerobic digestion process: Strategy and response of microbial community

A two-stage anaerobic digestion process utilizing food waste was investigated in this study, without any additive and co-digestion. Solid content, temperature and pH value were key controlling factors for hydrolysis, which results the optimized food waste hydrolysate with COD/VSfood waste of 2.67. Efficient biogas production was maintained in long-term operation (>150 d) without any additive, and methane production yields up to 699.7 mL·gVS-1·d-1 was achieved under organic loading rate (OLR) of 31.0 gVS·d-1. Methane production can be recovered (70.4 %) after temperature shock within 30 days. This study confirmed the possibility to establish two-stage food waste anaerobic digestion system under high organic load. pH, OLR, and temperature are key factors to maintain stable biogas production, while pH control was performed as a in situ sulfide control technology (75.8 % sulfide reduction). This study provides practical strategies for food waste utilization and decreasing carbon footprint.

Comparative Study of Methane Production in a One-Stage vs. Two-Stage Anaerobic Digestion Process from Raw Tomato Plant Waste

An anaerobic digestion process performed in two stages has the advantages of the production of hydrogen in addition to methane, and of further degradation of the substrate over the conventional process. The effectiveness of the implementation of this system for the treatment of lignocellulosic waste has been demonstrated. In 2020, more than 180 million tons of organic waste were generated worldwide from tomato crop production, posing a serious environmental risk. In the present investigation, methane production was compared in a two-stage system versus one-stage system from non-pretreated tomato plant residues. For this, different temperature (37 and 55 °C) and initial pH (5.5 and 6.5) conditions were evaluated during hydrogenesis and a constant temperature (37 °C, without pH adjustment) during methanogenesis. At the same time, a one-stage treatment (37 °C, without pH adjustment) was run for comparison purposes. The two-stage treatment in which the highest production of hydrogen, 12.4 mL/g VS, and methane, 252.3 mL/g VS, was observed occurred under the conditions of pH 6.5 and at 37 °C. However, this energy production was statistically similar (p < 0.5) to the one-stage treatment (365.4 mL CH4/g VS). Furthermore, there were also no significant differences in the removal of volatile solids between the different treatments.

THEORETICAL COMPARATIVE STUDIES OF THE BIOGAZ YIELDS FROM ONE-STAGE AND TWO-STAGE ANAEROBIC DIGESTION PROCESSES

This paper deals with the theoretical comparison of biogas and energy yields of one-stage anaerobic digestion processes (OSAD) with biomethane production and two-stage anaerobic digestion processes (TSAD) producing biohydrogen and biomethane. The comparative study of the biogas yield from OSAD and TSAD systems is performed on the base on mathematical models obtained in our previous papers. The possible maximal yields of biohydrogen and biomethane are calculated by the static characteristics and extremum points of both systems. Simulation results suggest that in comparison to OSAD, the increase in biogas (biohydrogen and biomethgane) production of TSAD can reach to 75.18%. The energy produced from two-stage anaerobic digestion processes is 1.32 to 1.486 times greater than those from one-stage anaerobic digestion processes (depending of the inlet organics concentrations), which means TSAD is a better choice considering biogas and energy production.

Environmental assessment of a two-stage high pressure anaerobic digestion process and biological upgrading as alternative processes for biomethane production

Biomethane plays a key role in achieving decarbonization and sustainable development goals. According to the objectives that arise, choosing the most suitable production system allows optimization of production, thereby reducing CO2 emissions. In this study, three biomethane production scenario life cycle assessments were compared to determine which would maintain the lowest CO2 emissions. Conventional anaerobic digestion and an innovative process called two-stage high pressure anaerobic digestion were considered. These methods were combined with two upgrading processes: water scrubbing and biological upgrading. Cattle manure and sugar beets were used as substrates for the process. Emissions were 805.6 gCO2eq/m3CH4 for the traditional biogas production process combined with water scrubbing and 450.3 gCO2eq/m3CH4 for the two-stage anaerobic digestion process combined with biological upgrading. Furthermore, the analysis demonstrated that these values would be reduced by 29.5 % and 48.0 % if electrical energy were produced using only renewable energy sources.

Nonlinear Optimization-Based Robust Control Approach for a Two-Stage Anaerobic Digestion Process

A two-stage anaerobic digestion (AD) process has been applied to improve the efficiency of methane production from various organic materials. However, the performance of traditional process controllers may be limited by differences in the rate of biochemical reactions, process uncertainties, and the consequences of interconnection between the two bioreactors. In this work, a nonlinear optimization-based control strategy that applies an analytical model predictive control (AMPC) scheme with an adaptive optimal set-point is proposed for the control of the two-stage AD system. The objectives of the proposed control system are to stabilize the system under uncertain operating conditions and maximize biomethane production. The optimal set-points for the controller are adapted in real-time operation, and then the control system is performed to manipulate the controlled output to the optimal trajectories. Compensators and nonlinear state observers are applied to handle the process/model mismatch and estimate unmeasured variables. The proposed control system is applied to the process with disturbances, fluctuations of inlet stream concentrations, and changes in the bacterial growth rate, and the control performance is investigated. Simulation results show that the developed control scheme automatically adjusts the optimal set-points and provides adequate control actions to maintain the maximum rate of methane production. The results of this investigation demonstrate that the control strategy promotes different biochemical reactions, avoids the inhibition effect, and handles the mutual effects between acidogenic and methanogenic bioreactors for methane production effectively.

Two-stage anaerobic digestion of food waste: Enhanced bioenergy production rate by steering lactate-type fermentation during hydrolysis-acidogenesis

This study proposed a lactate-based two-stage anaerobic digestion (AD) process to enhance bioenergy production rate from food waste (FW) and investigated the effect of inoculum addition and culture pH on hydrolysis-acidogenesis and further methanization. A series of batch fermentations were performed with an enriched lactate-producing consortium and without inoculum addition under controlled (5.7) and uncontrolled pH (initial 6.7) conditions. The interplay between the studied factors dictated the fate of lactate, particularly if it is produced and accumulated in the fermentation broth or is consumed by butyrogenic bacteria. Only the self-fermentation of FW with uncontrolled pH resulted in lactate accumulation (0.2 g/g volatile solid (VS) fed) with limited off-gas production (0.32 NL/L) and VS losses (≈16%). Such lactate-rich broth was successfully digested through biochemical methane potential tests, resulting in a maximum bioenergy production rate of 2891 MJ/ton-VS fed per day, which was two-fold higher compared to that achieved by one-stage AD.

EXTREMUM SEEKING BASED COMPOSED RECURSIVE MODEL FREE CONTROL OF TWO-STAGE ANAEROBIC DIGESTION PROCESS

In this paper, a new structure of extremum seeking algorithm is applied to the two-stage anaerobic digestion process to maximize the outflow rate of both hydrogen and methane. The model of the two-stage AD process is presented, which provides the characteristics of the total gas production rate. Based on the original Extremum Seeking Control (ESC), a novel Composed Recursive Model Free Controller (CRMFC) is added for maximum tracking for the gas production in the bioreactors. The proposed controller comprises a recursive model free stabilization term and a recursive time delay compensation term. Standard ESC, Newton-based ESC and Kalman filter (KF) based ESC are respectively combined with the new model-free controller to verify the proposed structure. Numerical simulations illustrate the performance of the proposed controller.

Enhancing pathogen inactivation in pig manure by introducing thermophilic and hyperthermophilic hygienization in a two-stage anaerobic digestion process

Conventional mesophilic anaerobic digestion (AD) is widely used to treat animal manure, but pathogen inactivation remains a significant challenge. Thermophilic and hyperthermophilic hydrolysis pretreatment was thus introduced as a hygienization step in a two-stage anaerobic digestion process. Results from 100-day experiments showed culturable Escherichia coli (E. coli) reduction was up to 6.9 log10 through a hygienization step, but RT-qPCR tests showed much less reduction in viable E. coli. The difference between viable and culturable cells revealed the complexity in pathogen inactivation. High-throughput sequencing indicated that the second stage in the two-stage AD further reduced the relative abundance of pathogens, including Enterococcus, Streptococcus, and Acinetobacter, while Clostridium_sensu_stricto still exhibited high relative abundance. Thermophilic hygienization induced a 6.7% increment in methane production, while hyperthermophilic pretreatment showed minimal effect on methane production. Focusing on the energy recovery and environmental safety point of view, the introduction of an integrated system incorporating the thermophilic and two-stage anaerobic process is recommended.

Comparison Study of Dynamic Models for One-stage and Two-stage Anaerobic Digestion Processes

Compared to traditional one-stage anaerobic digestion processes (OSAD) with biomethane production, this paper focuses on the study of biogas yield of the two-stage anaerobic digestion processes (TSAD), which are able to produce simultaneously biohydrogen and biomethane from lignocellusic materials. The one-stage and two-stage systems for anaerobic digestion are obtained based on our previous papers. Additionally, the static characteristics and extremum points of both systems are also investigated. The tracking of maximum points is realized by fuzzy-PID controller. Simulation results suggest that in comparison to OSAD, the increase in biogas production of TSAD can reach to 75.18% by control action, which means TSAD is a better choice considering biogas production.

Life cycle assessment of biogas production from mixtures of vinasse and concentrated latex wastewater

Environmental impacts of biogas production were evaluated by a life cycle assessment (LCA) of mixtures of vinasse and concentrated latex wastewater (CLWW) as: 1) single-stage and two-stage anaerobic digestion with different ratios and initial pH; 2) LCA of thermal energy from biogas. Results showed that almost all two-stage anaerobic digestion processes produced higher amounts of biogas and methane than the single-stage. Environmental impact was assessed for 1 MJ of thermal energy production. Two-stage anaerobic digestion produced less environmental impact in terms of global warming potential (GWP), acidification (AD) and energy use (EU) than single-stage at 64.82, 64.86 and 64.72%, respectively. Anaerobic digestion significantly contributed to environmental impact using sodium hydroxide solution for pretreatment.