Early warning indicators for mesophilic anaerobic digestion of corn stalk: a combined experimental and simulation approach

Boosting methane production and raw waste activated sludge treatment in a microbial electrolysis cell-anaerobic digestion (MEC-AD) system: The effect of organic loading rate

This study deals with the effect of different organic loading rates (OLRs) on the organic load removal and the productivity of methane, in a microbial electrolysis cell-anaerobic digestion (MEC-AD) system treating raw waste activated sludge (WAS). For comparison, two identical reactors, a control (AD) and a MEC-AD reactor were constructed. Both reactors operated for 131 days, during which different Organic Loading Rates (OLRs) were examined; 1.1, 1.7 and 2.9 gCOD/(L*d). The results showed that the MEC-AD reactor performed better, relative to the AD reactor, at high OLRs. Specifically, the additional total particulate carbon and Kjeldahl nitrogen removal reached 12% and 13%, respectively, at an OLR of 1.7 gCOD/(L*d), while they reached 19% and 13%, respectively, at an OLR of 2.9 gCOD/(L*d). Moreover, the biogas production and the methane content increased by 30% and 6%, respectively, at an OLR of 1.7 gCOD/(L*d) and by 32% and 5%, respectively, at an OLR of 2.9 gCOD/(L*d). The electrochemical measurements indicated that the power output increased from 5 to 30 mW/m2 when the OLR increased from 1.1 to 2.9 gCOD/(L*d). Overall, the results showed that the MEC-AD accelerated and enhanced the WAS treatment, boosting methane productivity.

Screening of early warning indicators for full-scale dry anaerobic digestion of household kitchen waste

Process monitoring is an essential measure to achieve efficient and stable performance in anaerobic digestion, thus requiring identification of effective early warning indicators. However, the application of early warning indicators to full-scale dry anaerobic engineering biogas plant still remains elusive. This study evaluated the effectiveness of common early warning indicators (including CH₄, CO₂, H₂S, volatile fatty acids (VFAs), alkalinity (ALK), total ammonia concentration (TAN) and free ammonia concentration (FAN)) in monitoring the instability of anaerobic digestion process at a practical engineering plant. The results showed that the individual indicators could not provide a sufficient early warning time before the digester fell into failure collapse. In comparison, the coupling indicators (the ratio of CH₄/CO₂, CH₄/pH, and CH₄/H₂S) had sensitive response to perturbation, which could regard as a potential early warning indicator, with the early warning time of 6, 7 and 10 days, respectively. Moreover, the VFA/ALK could be used as auxiliary indicators due to the limitation of complex detection methods. In addition, the result also indicated that the application of some warning indicators needs to be further verified, when transferring the result of laboratory scale to the practice application scenarios. This study provides insight into the stable operation of dry anaerobic engineering.

Microbial Biogas Production from Pork Gelatine

This research describes the results of the anaerobic digestion of gelatine as a potential hydrogen source with heat-shocked inoculum. The concentrations of applied gelatine were of VSS (volatile suspended solids) ranging from 10 g VSS/L to 30 g VSS/L. The initial process pH was 5.5, and, depending on the concentration, reached pH values from 7.5 to 7.8 after 55 days. Although the inoculum was heat-shocked in 30 g VSS/L of collagen, the process that occurred was hydrogenotrophic anaerobic digestion. In gelatine concentrations below 30 g VSS/L, hydrogen production was dominant only during the first 5 days of the experiments. Then, there was a change from dark fermentation to hydrogenotrophic methane production. The optimal hydrogen and methane yields resulted from the concentrations of 10 g VSS/L (7.65 mL ± 0.01 mL H2/g VSS and 3.49 ± 0.01 L CH4/g VSS). Additionally, 10 g VSS/L had the lowest accumulated emission of hydrogen sulphide (10.3 ± 0.01 mL of H2S), while 30 g VSS/L (0.440 ± 0.01mL H2S/g VSS) produced the lowest yield. After a lag time, the hydrogen production and hydrogen sulphide grew with a specific ratio, depending on the concentration. The hydrogen sulphide emission and sulphur added analysis proved that hydrogen sulphide originating from biogas created by bacteria remains longer than that from a substrate.

Thermodynamics of volatile fatty acid degradation during anaerobic digestion under organic overload stress: The potential to better identify process stability

Anaerobic digestion (AD) operating under organic overload stress usually increases the potential for process instability, leading to significant economic and ecological consequences. Volatile fatty acids (VFAs) accumulation is regularly considered a major factor during AD and their degradation is subject to thermodynamic constraints. To date, no study has systematically investigated the mechanisms of VFA degradation on process stability from the perspective of thermodynamics. Hence, increased substrate-to-inoculum ratio was applied in this study to simulate organic overload stress using batch tests with Hybrid Pennisetum. As a result, VFAs accumulation increased, accompanied by decreased methane yield, slower methane production kinetics and even severe process instability. Metagenomic analysis demonstrated that the accumulated propionate and butyrate were degraded by methyl-malonyl-CoA and the β-oxidation pathway while syntrophic acetate oxidation was preferred during acetate degradation. The deviation of stability parameters to varying degrees from the recommended threshold values was observed. However, a subsequent thermodynamic analysis revealed that moderate organic overload stress merely retarded the syntrophic oxidation of propionate, butyrate, and acetate. As a result, the methanogenic activity decreased, and the lag phase of AD was extended, but no adverse thermodynamic effects actually occurred. Changes in the Gibbs free energy for syntrophic propionate and acetate oxidation have the potential to better identify process stability. This study provided novel insights into the underlying thermodynamic mechanisms of VFA degradation and may have important implications for improving the current diagnostic mode for AD process stability.

Model-based mid-infrared spectroscopy for on-line monitoring of volatile fatty acids in the anaerobic digester

The performance of anaerobic digestion is significantly governed by the concentration of volatile fatty acids (VFAs). Though the titration and near-infrared spectroscopy have been used to measure the VFAs in the digester, there is still lack of the establishment of on-line monitoring of VFAs in practical application. An effective quantification method based on mid-infrared (MIR) spectroscopy was developed, and used to measure the concentrations of VFAs in the anaerobic bioreactor nondestructively in parallel. The wavelet denoising (WD) spectra were used as the spectral preprocessing option. Compared with other pretreatment methods, the established calibration model built by WD spectra showed satisfactory results. Further, the model was verified using high performance liquid chromatography (HPLC), and predictions were made using real reactor effluent samples. Based on this theoretical work, a set of equipment for the in-situ online monitoring of VFAs was designed, which has high feasibility and effectively solves the problems with the current VFAs online monitoring process. These results provide a new solution for on-line monitoring of the anaerobic digestion, and have great potential for practical application.

Evaluation of the biogas potential of a lignocellulosic residue

Anaerobic digestion (AD) or methanization is a biological process that allows the treatment of organic wastes and the production of renewable energy as biogas. This research describes assessment and optimization of dates pedicels rich in organic components under the conditions of mesophiles in co-digestion in association with the cattle manure and sludge of the wastewater treatment plant (WWTP). To realize this objectives, we investigated the impact of substrate load (date pedicel) on the co-digestion anaerobic of date pedicels and the sludge in wastewater treatment plants. The different parameters of the digester stability such as the pH, VFA, CAT, ammoniacal nitrogen, and the volume of biogas produced were followed. The results show that by increasing the mass of the pedicels and therefore the percentage of the substrate in the reactor, from 0.5 to 1% (W/V), the volume of produced biogas increases by five times. However, an increase in the percentage of the substrate introduced into the digester from 1 to 2% resulted in a slight decrease in the volume of biogas produced. The use of fresh cattle manure as inoculum shown a significant improvement (40%) in the production of biogas, which can be attributed to the nature of the inoculum. Indeed, cattle manure fresh is more adapted for the fermentation of lignocellulosic materials unlike sludge of wastewater treatment plants which may contain toxic substances that limit their use in AD.

Recent developments in monitoring technology for anaerobic digesters: A focus on bio-electrochemical systems

With the increasing popularity of waste to energy conversion, demand for large-scale operation of anaerobic digestors has emerged in the market. However, the process instabilities in anaerobic digestors limit the expansion of facilities to high loading rates. The irregularities in the process can be addressed directly by altering the feedstock characteristics provided an on-hand, robust, and sensitive monitoring device is available. In this context, the bioelectrochemical system has emerged as an excellent tool for monitoring and optimizing the anaerobic process within the reactor. This article reviews the gradual evolution in techniques and approaches for monitoring of anaerobic digestion (AD) process. An analysis of the recently popular biosensing techniques has been done with a focus on the bioelectrochemical monitoring system and its operation mode. A brief attempt to highlight the current challenges in the field of bioelectrochemical process monitoring for AD has also been made, which can be supportive of future research.

Towards upscaling the valorization of wheat straw residues: alkaline pretreatment using sodium hydroxide, enzymatic hydrolysis and biogas production

Lignocellulosic biomass is considered as a recalcitrant substrate for anaerobic digestion due to its complex nature that limits its biological degradation. Therefore, suitable preprocessing for the improvement of the performance of conventional anaerobic digestion remains a challenge in the development of anaerobic digestion technology. The physical and chemical characteristics of wheat straw (WS), as a representative lignocellulosic biomass, have a significant impact on the anaerobic digestion process in terms of quantity and quality of the produced biogas. This study aimed at investigating the enzymatic saccharification and detoxification of straw prior to anaerobic digestion with the final objective of enhancing the performance of conventional anaerobic systems of recalcitrant fractions of agricultural waste. The experimental activity was performed in lab and pilot scale treating WS. Alkaline delignification of straw using sodium hydroxide (NaOH) was studied prior to enzymatic hydrolysis for the production of easily biodegradable sugars. After defining the optimum conditions for the pretreatment scheme, the anaerobic digestability of the effluents produced was measured. Finally, the final liquid effluents were fed to a pilot scale anaerobic digester of 0.5 m³ volume, applying an increasing organic loading rate (OLR) regime (in terms of chemical oxygen demand (COD) from 0.2 to 15 kg COD/m³/day). The optimum conditions for the delignification and enzymatic hydrolysis of WS were defined as 0.5 M NaOH at 50 °C for 3-5 h and 15 μL Cellic CTec2/g pretreated straw at 50 °C. It was proven that the resulting liquid effluents could be fed to an anaerobic digester in the ratio that they are produced with satisfactory COD removal efficiencies (over 70%) for OLRs up to 10 kg COD/m³/day. This value is correspondent to a hydraulic retention time of around 7.5 days, much lower than the respective one for untreated straw (over 12 days).

Moving average convergence and divergence indexes based online intelligent expert diagnosis system for anaerobic wastewater treatment process

Anaerobic wastewater treatment process is efficient but unstable due to various disturbances, such as refractory organics and influent organic overloading. Therefore, sensitive and accurate status diagnosis is important for reasonable control to improve the stability of anaerobic process. In this study, an online intelligent expert diagnosis system for anaerobic process was established based on moving average convergence and divergence (MACD) indexes of gas- and liquid-phase parameters, combined with online monitoring system and expert diagnosis database. The effect of this diagnosis system was verified through refractory organics and organic overloading shock experiments. Results showed that this diagnosis system could make rapid, accurate and comprehensive diagnosis, predictions and early-warning. MACD algorithm could enhance pattern recognition capacity of status parameters, overcome the lagging of anaerobic process and filter irregular noisy fluctuations of status parameters. MACD index of H₂ partial pressure is suitable as sensitive early-warning indicator in the initial shock stage.

Anaerobic co-digestion of corn stover and chicken manure using continuous stirred tank reactor: The effect of biochar addition and urea pretreatment

The performance of biochar mediated anaerobic co-digestion (co-AD) of corn stover (CS) and chicken manure (CM) using continuous stirred tank reactor (CSTR) was studied. Results showed that urea pretreated CS (UPCS) and biochar addition in anaerobic digestion (AD) system can improve co-AD. The effect of urea pretreatment is similar to that of biochar addition, and their synergistic effect was apparent under medium and high OLR conditions. When the OLR was 4.2 and 6.3 g VS/L/d, the biochar mediated UPCS/CM co-AD operated stably with the VMP of 2.160 and 1.616 L/L/d, and VMP of the biochar mediated UPCS /CM were 32.8%-89.6% and 27.8%-96.4% higher than other reactors, respectively. The results reveal that urea pretreatment and biochar addition promoted AD process through strengthening the buffer capacity system established by ammonia nitrogen and volatile fatty acids and improving the degradation of lignocellulosic biomass.

Existing Empirical Kinetic Models in Biochemical Methane Potential (BMP) Testing, Their Selection and Numerical Solution

Biochemical Methane Potential (BMP) tests are a crucial part of feasibility studies to estimate energy recovery opportunities from organic wastes and wastewater. Despite the large number of publications dedicated to BMP testing and numerous attempts to standardize procedures, there is no “one size fits all” mathematical model to describe biomethane formation kinetic precisely. Importantly, the kinetics models are utilized for treatability estimation and modeling processes for the purpose of scale-up. A numerical computation approach is a widely used method to determine model coefficients, as a replacement for the previously used linearization approach. However, it requires more information for each model and some range of coefficients to iterate through. This study considers existing empirical models used to describe biomethane formation process in BMP testing, clarifies model nomenclature, presents equations usable for numerical computation of kinetic parameters as piece-wise defined functions, defines the limits for model coefficients, and collects and analyzes criteria to evaluate and compare model goodness of fit.

Data-driven fault detection methods for detecting small-magnitude faults in anaerobic digestion process

Early detection of small-magnitude faults in anaerobic digestion (AD) processes is a mandatory step for preventing serious consequence in the future. Since volatile fatty acids (VFA) accumulation is widely suggested as a process health indicator, a VFA soft-sensor was developed based on support vector machine (SVM) and used for generating the residuals by comparing real and predicted VFA. The estimated residual signal was applied to univariate statistical control charts such as cumulative sum (CUSUM) and square prediction error (SPE) to detect the faults. A principal component analysis (PCA) model was also developed for comparison with the aforementioned approach. The proposed framework showed excellent performance for detecting small-magnitude faults in the state parameters of AD processes.

Detection of early imbalances in semi-continuous anaerobic co-digestion process based on instantaneous biogas production rate

The aim of this study was to investigate the use of biogas production rate kinetics for the monitoring of anaerobic co-digestion. Recent extensive studies of degradation pathways showed that acetoclastic methanogenesis is not always the main pathway. Hydrogenotrophic methanogenesis and syntrophic acetate oxidation can also dominate, mostly for operating conditions with high concentrations of ammonia or volatile fatty acids … These conditions are also known to cause instability in the digester's operation especially in co-digestion due to substrate variability. Therefore, co-digestion experiments were conducted with several co-substrates using a continuously stirred 35-L tank reactor. Degradation pathways and their potential shifts were identified by monitoring variations in biogas production rate kinetics using a principal component analysis model. The shifts in the degradation pathways were used to monitor the process. These shift points were found to provide early warnings of instabilities in the anaerobic co-digestion process.