Continuous Anaerobic Digestion Process Research Articles

Towards a sustainable energy future: Evaluating Arundo donax L. in continuous anaerobic digestion for biogas production

AbstractIn response to the EU's REPowerEU initiative (COM (2022) 108) which encourages an increase in biogas production by 20% in member states by 2030 to boost energy independence, it has become essential to identify sustainable alternatives to traditional feedstocks for biogas production, especially in the EU Member states where there is still high dependence on corn silage as the main raw material in biogas plants. While corn silage, predominantly used in the European biogas plants today, serves primarily for the livestock sector, alternative sources need to be explored. Therefore, this study aimed to evaluate the potential of Arundo donax, a perennial energy crop, as an alternative feedstock in a continuous anaerobic process. The biogas yield and its quality, characterized by CH4, CO2, H2S and O2 content, were determined during a continuous process with A. donax, compared with two mixed feedstocks of A. donax and corn silage over a 5‐month period in a continuous anaerobic digestion process. The results revealed that A. donax exhibits a biogas yield and methane content comparable to corn silage, indicating its potential as a viable and sustainable alternative feedstock for biogas production.

A hybrid approach of anaerobic digestion model no. 1 and machine learning to model and optimize continuous anaerobic digestion processes

Anaerobic digestion is a promising approach to dispose of biodegradable waste and wastewater, generating biogas as an alternative energy resource. This work proposed a so-called M-CADM1 for continuous anaerobic digestion simulation, which combined the machine learning and anaerobic digestion model No.1 (ADM1). The detailed reaction path and intermediate products in different stages of anaerobic digestion are specified in ADM1. The kinetic parameters were modified by machine learning. The characteristics (elemental composition) of feedstocks were used to predict kinetic parameters. A total of 75 biomass samples were used to establish for machine learning models. Five element contents (C, H, O, N, S), feedstock feed rate, and anaerobic digestion temperature were used as the input. The kinetic parameters were set as output. The sensitivities of 17 kinetic parameters were evaluated. 7 kinetic parameters with the highest sensitivities were selected as ADM1 model inputs by sensitivity analysis. The R2 and RMSE were used as the index to evaluated the accuracy of machine learning model. The best R2 and RMSE reached 0.84 and 0.196. The TIC was used as the index to evaluated the accuracy of M-CADM1. By comparing the simulated value with the experimental value, the accuracy of the overall M-CADM1 expressed by TIC of kitchen waste was 0.036. The organic acid content and pH in the reactor were considered as indicators to study the accuracy and stability of the M-CADM1. Trends in organic acids, free ammonia or hydrogen inhibition, and pH were consistent with experimental continuous anaerobic digestion results.

Investigation into the effects of different recycled magnetic additives on anaerobic co-digestion of sludge and straw

The addition of magnetic additives has been found to enhance the anaerobic co-digestion of sludge and straw. Due to strong magnetic properties, these additives can be easily separated from the digestate, allowing for recycling during the continuous anaerobic digestion process. This study aimed to investigate the impact of different recycled magnetic additives (specifically, magnetic biochar and Fe3O4) on the anaerobic co-digestion of sewage sludge with corn straw. The results showed that the addition of recycled Fe3O4 with an initial amount of 10 g/kg resulted in the highest cumulative methane yield of 295.70 mL/g VSadded, which increased by 79.81% as compared to the control. The results suggest that magnetic biochar with higher carbon content exerts a more noticeable impact on shortening the digestion lag phase time, as indicated by kinetic analysis. With the addition of recycled Fe3O4, the Methanosaeta and Methanosarcina enhanced significantly. Especially Methanosarcina, which was increased from 3.85% to 6.28%, resulting in a 2.43% increase. Furthermore, the predominant aceticlastic pathway of digestion process was consolidated, and the enriched Methanosarcina broadened the pathways of methanogenesis. This research provides a potential utilization strategy of recycling Fe3O4 for accelerating the anaerobic degradation of urban and rural areas solid waste.

Investigating the Evolution of Structural Characteristics of Humic Acid Generated during the Continuous Anaerobic Digestion and Its Potential for Chromium Adsorption and Reduction

Humic acid (HA), as an important by-product, has been demonstrated to affect anaerobic digestion performance and subsequent land application of digestate via the batch anaerobic digestion process. However, the knowledge about the evolution of structure and function of HA during continuous anaerobic digestion (AD) is still unclear. Therefore, the current study examined the structural changes in HA produced during the continuous AD process and its metal-adsorption-reduction abilities. The results of three-dimensional fluorescence spectroscopy showed a general upsurge in humic-like components’ abundance (70–77%), with an increase in humification index (2.56–3.43). Likewise, the content of HA increased from 4.8 g L−1 to 6.9 g L−1 in the continuous AD process. The evolution of C-H, O-H, C=O, C=C, and C-O functional groups of HA was observed via the 2D COS FTIR analysis. Moreover, the concurrent dynamics of functional groups contributed to the higher adsorption (255.2 mg g−1) of Cr (VI) and reduction (60.3 mg g−1) of Cr (VI) to Cr (III) after 168 days of the continuous AD process. The findings of the current study not only advanced understanding of the evolution of HA during continuous anaerobic digestion and its metal remediation potential but also support further research toward developing an eco-friendly and innovative strategy for the remediation of heavy metals contaminated soils employing anaerobic digestate as an auxiliary agent.

Multivariable Robust Regulation of Alkalinities in Continuous Anaerobic Digestion Processes: Experimental Validation

A multivariable adaptive feedback control for highly uncertain continuous anaerobic digestion processes is proposed to regulate the volatile fatty acids (VFA) concentration, the strong ions concentrations, and the total and intermediate alkalinities. The multivariable control scheme includes a Luenberger observer to estimate both the unmeasured variables (i.e., VFA) and unknown microbial growth kinetics. The control approach is designed using an exponential Lyapunov function to resemble the typical exponential biological growth of the involved microbial consortia. Taking into account physicochemical equilibrium, alkalinities are represented as a function of the state variables. As a result, the control problem becomes a regulation problem on alkalinities, and in turn, a tracking control problem on the state variables, with two manipulated variables—the dilution rate and the feed rate of a strong alkali solution—while the state variables’ set-points are given as a function of pH. The implementation of this multivariable control scheme was experimentally tested and validated in a 0.982 m3 pilot plant treating agro-industrial wastewater and demonstrated to be robust in the face of unknown microbial growth kinetics. Results showed the potential for practical application and optimization of industrial digesters.

On-line optimization of biomethane production in continuous AD processes via model-based ESC approach.

This paper is aimed at designing a class of model based extremum-seeking feedback controllers (ESC) for the on-line optimization of biomethane production rate in continuous anaerobic digestion (AD) processes. The ESC scheme is based on the modelling error compensation approach coupled with a first-order gradient estimator. The feedback control law is able to keep the concentration of volatile fatty acids (VFAs) near the unknown optimal setpoint while the methane production rate is maximized. Unlike other ESC algorithms applied to AD processes, the proposed control scheme includes a continuous uncertain estimator and avoids excessive oscillations in the control action. Numerical simulations compare the performance of the proposed ESC controller with traditional perturbation-based and sliding mode based approaches.

Software Sensor for Online Estimation of the VFA’s Concentration in Anaerobic Digestion Processes via a High-Order Sliding Mode Observer

This paper deals with the design of a software sensor based on a high-order sliding mode observer (HOSMO) for online estimation of the concentration of volatile fatty acids (VFA’s) in a class of continuous anaerobic digestion processes (AD). Taking into account the limited availability for online monitoring of AD processes, in this contribution it is assumed that only the methane outflow rate is available for online measurement. Considering a simple two-dimensional AD model, it is shown that the VFA concentration can be detectable from the online measurement of the methane outflow rate. The estimation method is based on a local coordinate transformation that converts the original model into an observable system. In the observable form, a supertwisting algorithm is proposed in order to provide robustness under perturbation and uncertainty conditions. The observer convergence is analyzed by using Lyapunov stability techniques. Numerical simulations illustrate the effectiveness of the proposed estimation sch...

Developing an anaerobic digester with external Zeolite filled column for enhancing methane production from swine manure – A feasibility study

ABSTRACTDevelopment of digesters with an external zeolite column facilitates the convenient removal of the zeolite with TAN, without disturbing the continuous anaerobic digestion process. A digester with an inside zeolite bed (In-Zeo) and digester without adding zeolite (No-Zeo) were employed to compare the process performance with digester with external zeolite column (EX-Zeo). The cumulative, CH4 yields were 5% and 15% greater in the EX-Zeo, and the In-Zeo digesters respectively compared to the No-Zeo digesters. Also, the % VS reduction was 49%, 55% and 41%, respectively in the Ex-Zeo, In-Zeo and No-Zeo digesters. The results indicated that treatment with 7% zeolite during anaerobic digestion has the potential to improve biodegradation of swine manure. The addition of zeolite appeared to reduce TAN from the digestate, thereby enhancing the CH4 yield. Zeolite could be used either internally or externally to enhance CH4 production through anaerobic digestion of swine manure.

Renewable energy from thermophilic anaerobic digestion of winery residue: Preliminary evidence from batch and continuous lab-scale trials

The winemaking process generates many by-products besides wastewater, mainly grape marcs, grape stalks, and wine lees. Anaerobic digestion is particularly suitable to treat winery waste because of its high content of nutrient-rich organic matter and for its noticeable energetic potential. To date, only results from mesophilic tests have been extensively reported. In this study, potential methane production and kinetic constants were determined by batch trials under thermophilic conditions and compared with mesophilic values already reported in literature. Grape marcs and wine lees appeared to be the most promising substrates with an estimated potential of 0.34 and 0.37 Nm3CH4/kgVSfed, respectively, while grape stalks generated only 0.13 Nm3CH4/kgVSfed. In order to assess the feasibility of a continuous anaerobic digestion process, a lab-scale semi-continuous reactor was constructed. Because of the consumption of buffer capacity, the biological process was difficult to control. On the other hand, biogas was produced when working with a hydraulic retention time of 40 d and with previously fermented grape marcs; a specific biogas production of 0.29 Nm3/kgVSfed was observed. The results of the continuous tests were used to calculate the potential energy recovery from grape marcs produced in Italy (808 thousands of tons per year) in terms of heat and electricity; about 245 GWh of heat and 201 GWh of electricity per annum could be generated in Italian scenario.

Dynamic characterization of an anaerobic digester during the start-up phase by pH time-series analysis

In this paper, an alternative methodology for the online monitoring of key variables during the start-up phase of continuous anaerobic digestion processes is presented. The proposal is based on the application of fractal analysis to pH time-series obtained from a lab-scale fixed-bed reactor (FBR) used in the tequila vinasses treatment. Results showed that fractal parameters exhibit a dynamic behavior closely related to that of key process variables such that, chemical oxygen demand (COD), volatile fatty acids (VFA) and biogas production, whose on-line monitoring is difficult and expensive in practice. Thus, the industrial implementation of the here proposed ideas is quite encouraging, since it is shown that fractal analysis of conventional pH signals can be used as an inexpensive and easy to implement tool for anaerobic digesters diagnosis, which could drastically reduce the time and problems currently associated to the start-up phase.

Influent Fractionation for Modeling Continuous Anaerobic Digestion Processes.

The first dynamic model developed to describe anaerobic digestion processes dates back to 1969. Since then, considerable improvements in identifying the underlying biochemical processes and associated microorganisms have been achieved. These have led to an increasing complexity of both model structure and the standard set of stoichiometric and kinetic parameters. Literature has always paid attention to kinetic parameter estimation, as this determines model accuracy with respect to predicting the dynamic behavior of biogas systems. As sufficient computing power is easily available nowadays, sophisticated linear and nonlinear parameter estimation techniques are applied to evaluate parameter uncertainty. However, the uncertainty of influent fractionation in these parameter optimization procedures is generally neglected. As anaerobic digestion systems are currently increasingly used to convert a broad variety of organic biomass to methane, the lack of generally accepted guidelines for input characterization adapted to the simulation model's characteristics is a considerable limitation of model application to these substrates. Directly after the introduction of the standardized Anaerobic Digestion Model No. 1 (ADM1), several publications pointed out that the model's requirement of a detailed influent characterization can hardly be fulfilled. The main shortcoming of the model application was addressed in the reliable and practical identification of the model's input state variables for particulate and soluble carbohydrates, proteins and lipids, as well as for the inerts. Several authors derived biomass characterization procedures, most of them dedicated to a particular substrate, and some of them being of general nature, but none of these approaches have resulted in a practical standard protocol so far. This review provides an overview of existing approaches that improve substrate influent characterization to be used for state of the art anaerobic digestion models.

Anaerobic digestion of Chinese cabbage waste silage with swine manure for biogas production: batch and continuous study

The aim of this study was to investigate the potential for anaerobic co-digestion of Chinese cabbage waste silage (CCWS) with swine manure (SM). Batch and continuous experiments were carried out under mesophilic anaerobic conditions (36–38°C). The batch test evaluated the effect of CCWS co-digestion with SM (SM: CCWS=100:0; 25:75; 33:67; 0:100, % volatile solids (VS) basis). The continuous test evaluated the performance of a single stage completely stirred tank reactor with SM alone and with a mixture of SM and CCWS. Batch test results showed no significant difference in biogas yield up to 25–33% of CCWS; however, biogas yield was significantly decreased when CCWS contents in feed increased to 67% and 100%. When testing continuous digestion, the biogas yield at organic loading rate (OLR) of 2.0 g VSL−1 d−1 increased by 17% with a mixture of SM and CCWS (SM:CCWS=75:25) (423 mL g−1 VS) than with SM alone (361 mL g−1 VS). The continuous anaerobic digestion process (biogas production, pH, total volatile fatty acids (TVFA) and TVFA/total alkalinity ratios) was stable when co-digesting SM and CCWS (75:25) at OLR of 2.0 g VSL−1 d−1 and hydraulic retention time of 20 days under mesophilic conditions.

Robust Regulation of Alkalinity in Highly Uncertain Continuous Anaerobic Digestion Processes

In this paper, a multiple-input multiple-output (MIMO) model-based robust control scheme is proposed for the indirect control of both total alkalinity and the ratio (intermediate alkalinity)/(total alkalinity) by regulating volatile fatty acid concentrations and strong ions concentration, while guaranteeing the so-called operational stability in anaerobic digestion (AD) processes. The proposed MIMO regulator is an adaptive controller derived from an AD model which incorporates the physicochemical equilibrium of the system as well as the use of a robust interval observer to estimate key process bounds that are used in the computation of the control efforts. Numerical simulations were carried out for a number of operating conditions under the most uncertain scenarios. Results showed that the proposed multivariable control law is able to recover the system stability around a pre-determined set point in the face of parameter uncertainty and load disturbances.

Application of low-strength ultrasonication to the continuous anaerobic digestion processes: UASBr and dry digester

In this study, the positive effects of low-strength ultrasonication (LS-ultrasonication) on the anaerobic digestion (AD) performance were investigated by continuously operating an upflow anaerobic sludge blanket reactor (UASBr) and a dry digester. In the ultrasonicated UASBr system (1 s per min, 0.05 W/mL), ultrasonication enhanced the CH₄ production by 38% and 19% in an ambient and a mesophilic condition, respectively. In addition, a different sludge yield and a changed electron flow were observed after ultrasonication. In the ultrasonicated dry digestion system (2 s per 30 s, 0.0025 W/mL), a 40% increase in the production of CH₄ was observed after lowering the total solid content of the reactor from 12% to 10%, implying that a high solid content diminished the ultrasonic stimulation effect. Moreover, the ultrasonication strength itself appeared to be a more crucial factor than the ultrasonication density during the application of LS-ultrasonication in the AD system.

A continuous-flow approach for the development of an anaerobic consortium capable of an effective biomethanization of a mechanically sorted organic fraction of municipal solid waste as the sole substrate

An effective mesophilic continuous anaerobic digestion process fed only with a mechanically sorted organic fraction of municipal solid waste (MS-OFMSW) was developed. During a preliminary 3-month experimental phase, the microbial consortium was acclimated toward MS-OFMSW by initially filling the reactor with cattle manure and then continuously feeding it with MS-OFMSW. The Hydraulic Retention Time (HRT) and Organic Loading Rate (OLR) were 23 days and 2.5 g/L/day, respectively. After 4 weeks, the reactor reached stationary performances (84% COD removal yield, 0.15 L CH 4 /g CODremoved methane production yield). The acclimated consortium was then employed in a second run in which the reactor was operated under steady state conditions at the previous HRT and OLR for 73 days. The COD removal and the methane production yield increased up to 87% and 0.25 L CH 4 /g CODremoved, respectively. The capability of the acclimated consortium to biomethanize MS-OFMSW was further studied via batch digestion experiments, carried out by inoculating the target waste with reactor effluents collected at the beginning of first run and at the end of the first and second run. The best normalized methane production (0.39 L CH 4 /g initial COD) was obtained with the inoculum collected at the end of the second run. Molecular analysis of the microbial community occurring in the reactor during the two sequential runs indicated that the progressive improvement of the process performances was closely related to the selection and enrichment of specific hydrolytic and acidogenic bacteria in the reactor.

Development of an effective continuous anaerobic digestion process for the biomethanization of a mechanically-sorted organic fraction of municipal solid waste

Development of an effective continuous anaerobic digestion process for the biomethanization of a mechanically-sorted organic fraction of municipal solid waste

Observer-based input estimation in continuous anaerobic wastewater treatment processes

In this paper a "virtually controlled observer" (VCO) is proposed to estimate simultaneously the influent substrate concentration and unmeasured state variables in continuous anaerobic digestion processes. The hypothetical (unmeasured) influent substrate concentration is updated by a feedback control, which is regulated by the estimation error of a measured output. The proposed approach is firstly illustrated upon a simple continuous bioprocess. Then, the performance of the observer is tested via numerical simulations in a continuous anaerobic digestion model. Results showed that it is possible to estimate both, the influent substrate concentration and unmeasured states in the digester in the face of parametric uncertainties.

Robust Control of Volatile Fatty Acids in Anaerobic Digestion Processes

This paper is focused on the experimental implementation of a robust control scheme for the regulation of volatile fatty acids (VFA) in continuous anaerobic digestion processes. The robust scheme is made of an output feedback control, and an extended Luenberger observer is used to estimate the uncertain terms of the process (i.e., influent concentration and process kinetics). The control scheme is implemented in a pilot plant up-flow fixed-bed reactor that is treating industrial wine distillery wastewater. The performance of the robust scheme is tested over a period of 36 days, under different set-point values and several uncertain scenarios, including model mismatch, badly known parameters, and load disturbances. Experimental results show that the VFA concentration can be effectively regulated over a wide range of operating conditions. In addition, it is shown that the control scheme has a structure that improves its performance in the presence of noisy measurements and control input saturations.

Estudio cinético del proceso de digestión anaerobia de aguas de lavado de aceitunas de almazara en reactores de mezcla completa con microorganismos inmovilizados

A kinetic study of the continuous anaerobic digestion process of wastewaters from the washing of olives prior to the oil production process was carried out in a completely mixed reactor with sepiolite immobilised biomass at 35° C. Four influent COD concentrations, 4,5, 3,5, 2,5 y 1,5 g COD/I were used as substrate operating for a hydraulic retention time range of 4,5 to 1,25 days. The Michaelis-Menten model has been applied, obtaining the values of the maximum substrate utilization rate, k ( (g COD/g VSS day), and the kinetic constant of the process, K s (g COD/I), for each influent substrate concentration studied. The values of these parameters significantly decreased with decreasing concentration of influent wastewater, showing the highest values for the greatest influent substrate concentration used as feed. The microbial yield coefficient, Y x , and sludge decay rate coefficient, k d , were also determined to be 0,14 g VSS/g COD and 0,003 days -1 respectively.

Anaerobic treatment of wastewater produced in the manufacture of cellulosic pulp from wheat straw

Abstract A study of the continuous anaerobic digestion process of wastewater, produced in the manufacture of cellulosic pulp from wheat straw, was carried out. An influent wastewater with a COD of 45 g.l‐1 was used and hydraulic retention times ranging between 29 and 8 days were tested. For a hydraulic retention time of 8 days a start‐up of acidification was observed in the reactor. The biodegradability of the wastewater ranged between 50% and 70% depending on the average COD of the medium. The inhibitory characteristics of this wastewater was shown through methanogenic activity tests. Lastly, a kinetic model of overall parameters was employed to correlate the results obtained.