Heavy metals interact with the microbial community and affect biogas production in anaerobic digestion: A review.

The production of biogas from anaerobic digestion (AD) of residual agro-food biomasses represents an opportunity for alternative production of energy from renewable sources, according to the European Union legislation on renewable energy. This review provides an overview of the various aspects involved in this process with a focus on the best process conditions to be used for AD-based biogas production from residual agro-food biomasses. After a schematic description of the AD phases, the biogas plants with advanced technologies were described, pointing out the strengths and the weaknesses of the different digester technologies and indicating the main parameters and operating conditions to be monitored. Subsequently, a brief analysis of the factors affecting methane yield from manure AD was conducted and the AD of fruit and vegetables waste was examined. Particular attention was given to studies on co-digestion and pre-treatments as strategies to improve biogas yield. Finally, the selection of specific microorganisms and the genetic manipulation of anaerobic bacteria to speed up the AD process was illustrated. The open challenges concern the achievement of the highest renewable energy yields reusing agro-food waste with the lowest environmental impact and an increment of competitiveness of the agricultural sector in the perspective of a circular economy.

Recent Updates on the Use of Agro-Food Waste for Biogas Production

There is a lack of literature reporting the measurement and prediction of biochemical methane potential (BMP) of vegetable crop residues (VCRs) and similarly, the kinetic assessment on the anaerobic digestion process of VCR is rarely investigated. In this paper, the BMP tests of five different vegetable (snap bean, capsicum, cucumber, eggplant, and tomato) crop residues were conducted at feed to inoculum ratio (F/I) of 2.0 under mesophilic (36 ± 1 °C) conditions. A series of single-variable and multiple-variable regression models were built based on organic components (hemicellulose, cellulose, lignin, total fat, total sugar, and crude protein) for BMP prediction. Three kinetic models, including the first-order kinetic model, the Chen and Hashimoto model, and the modified Gompertz model, were used to simulate the methane yield results of VCR and obtain valuable model parameters simultaneously. As a result, the BMPs and volatile solids (VS) degradation degree of different VCRs were respectively in the range of 94.2–146.8 mL g−1 VS and 40.4–49.9%; the regression prediction models with variables lignin (R2 = 0.704, p = 0.076), variables crude protein and lignin (R2 = 0.976, p = 0.048), and variables total fat, hemicellulose, and lignin (R2 = 0.999, p = 0.027) showed the best performance on BMP prediction among the single-factor, two-factor, and three-factor models, respectively. In addition, compared to the other two kinetic models, the modified Gompertz model could be excellently fitted (R2 = 0.986–0.998) to the results of BMP experiment, verification deviations within 0.3%.

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Evaluation of Biochemical Methane Potential and Kinetics on the Anaerobic Digestion of Vegetable Crop Residues

Occurrence, effect, and fate of residual microplastics in anaerobic digestion of waste activated sludge: A state-of-the-art review.

To describe anaerobic fermentation, many mathematical models have been suggested. A commonly accepted hypothesis in microbial growth is the speed of cellular reproduction, which is proportional to the concentration of cells at that instant. The constant of proportionality between the speed of growth and cell concentration is called cell growth rate, μ. In many occasions, the cell growth rate is considered constant. This leads to conclude that the concentration of cells versus time presents an exponential function. The consideration of this equation provides a good adjustment in the beginning of central phase of the anaerobic fermentation process. However, it moves away from the measurements when there is a limited reproduction due to lack of nutrients and competition between the cells in the environment. This produces a sigmoidal variation in concentration. To find a suitable fit function for all phases of the process, Gompertz proposes a model that considers the cell growth rate as variable. In this chapter, the Gompertz model, kinetic models, transference, and cone models are evaluated. Different adaptations to fit the variables to the obtained values in the experiments have been reviewed.

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Review of Mathematical Models for the Anaerobic Digestion Process

The anaerobic co-digestion of biomass waste, a promising process of reusing resources, is capable of improving methane production. However, the characteristics and composition of fermenting raw material negatively influence the efficiency of methane production. Optimization experiments were systematically performed in this study through anaerobic co-digestion with urea-ammoniated rice straw (UARS) and food waste (FW) as co-substrates. Anaerobic co-digestion of UARS and FW in biogas production under mesophilic conditions (35 °C) was investigated in a 1 L enclosed triangular flask with a total organic load of 6 g volatile solids (VS)/L. The optimal mixing ratio of UARS to FW was close to 1:3, and the methane yield increasing by 8.83% compared with the sole substrate. Furthermore, based on the optimization ratio, supplementation of cobalt (Co) and nickel (Ni) on co-digestion were significantly superior to that of a single element. Additionally, kinetic analysis indicated that trace element remarkably facilitated the reaction rate of co-digestion. Noteworthy, the addition of Co, Ni, and the combination of Co and Ni achieved very significant (p < 0.01) improvement of 6.45, 8.36, and 13.65%. Meanwhile, Ni was substantially promoted the removal rate of VS, enhanced the operational stability of co-digestion and increased the methane content significantly.

Optimization of mixing ratio of ammoniated rice straw and food waste co-digestion and impact of trace element supplementation on biogas production

Predictive Model of Methane Production Based on the Compositional Features of Spent Edible Mushroom Substrate

For the purpose of enhancing the biogas production and operation stability of system at low temperature, a two-phase anaerobic fermentation facility for rural household energy generation was proposed. In this facility, the quantity of the fermented materials could be balanced by controlling the hydraulic retention time, which based on pH of the acidification tank and the fermentation tank. In addition, a portion of the biogas generated could be used to heat the acidification tank and the fermentation tank. Results shows that the optimal daily production rate of biogas for 1 m 3 fermentation liquid was about 1.47 L/L d -1 at a mixing ratio of cow: swine: chicken manure was 3: 1: 0.5. The production rate obtained in this study was more than four times higher than that from traditional single-phase processes (0.35 L/L d -1 ). About 5.43 m 3 biogas can be produced daily per household with an average CH 4 content of 76.8%. The two-phase process developed in this work will also reduce environmental pollution and increase energy production efficiencies.

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Yong Sun

Papers

Optimization of mixing ratio of ammoniated rice straw and food waste co-digestion and impact of trace element supplementation on biogas production

Predictive Model of Methane Production Based on the Compositional Features of Spent Edible Mushroom Substrate

Enhancement of Biogas Production in Two-Phase Anaerobic Fermentation System for Lower-Temperature Applications