Mathematical Modeling and Simulation of Anaerobic Digestion of Solid Waste

Methane liquid-gas phase distribution during anaerobic sludge digestion: A thermodynamic approach

Anaerobic digestion is certainly one of the options that can help solve the dilemma of energy demand, waste management and climate crisis mitigation mainly. Under ideal conditions, it is expected that all biomethane will be transferred from the liquid to the gaseous phase, ensuring maximum recovery. However, for concentrated wastewater or complex organic waste blends composed of functional groups with different sizes, the molecular interactions become important since the system is not only biphasic. Since the formation and transfer of a gas is related to the equilibrium condition, a thermodynamic approach could help to estimate the degree of variation of biomethane between the liquid and gaseous phases. Therefore, this investigation aimed to obtain the distribution between liquid and gaseous phases of the biomethane produced in the sewage sludge anaerobic digestion considering the substrate as a non-ideal solution. The nonlinear differential equations of the ADM1 were integrated with Aspen Plus® to verify the equilibrium conditions and the model was calibrated with data obtained through experiments conducted in a lab scale sequence batch reactor (SBR) fed with synthetic substrate (1500 mgCOD.L^-1) and inoculated with flocculent sludge (500 mgSSV.L^-1) from a full-scale UASB reactor. Considering the thermodynamic approach, the percentage of dissolved biomethane remains below the 2.97% percentage estimated by using the ADM1. It is possible to state that the principle of ideality is maintained in the system. On the other hand, CO₂ phase distribution was considerably affected by the species defined in the equilibrium reactions. Therefore, it can be stated that Henry's Law simplification in ADM1 model is valid to represent the phenomenon investigated.

Modelling of autogenerative high-pressure anaerobic digestion in a batch reactor for the production of pressurised biogas

Background

Pressurised anaerobic digestion allows the production of biogas with a high content of methane and, at the same time, avoid the energy costs for the biogas upgrading and injection into the distribution grid. The technology carries potential, but the research faces practical constraints by a.o. the capital investment needed in high-pressure reactors and sensors and associated sampling limitations. In this work, the kinetic model of an autogenerative high-pressure anaerobic digestion of acetate, as the representative compound of the aceticlastic methanogenesis route, in batch configuration, is proposed to predict the dynamic performance of pressurised digesters and support future experimental work. The modelling of autogenerative high-pressure anaerobic digestion in batch configuration, which is not extensively studied and simulated in the present literature, was developed, calibrated, and validated by using experimental results available from the literature.

Results

Under high-pressure conditions, the assessment of the Monod maximum specific uptake rate, the half-saturation constant and the first-order decay rate was carried out, and the values of 5.9 kg COD kg COD⁻¹ d⁻¹, 0.05 kg COD m⁻³ and 0.02 d⁻¹ were determined, respectively. By using the predicted values, excellent fittings of the final pressure, the CH₄ molar fraction and the specific methanogenic yield calculation were obtained. Likewise, the variation in the gas–liquid mass transfer coefficient by several orders of magnitude showed negligible effects on the model predictive values in terms of methane molar fraction of the produced biogas, while the final pressure seemed to be slightly influenced.

Conclusions

The proposed model allowed to estimate the Monod maximum specific uptake rate for acetate, the half-saturation rate for acetate and the first-order decay rate constant, which were comparable with literature values reported for well-studied methanogens under anaerobic digestion at atmospheric pressure. The methane molar fraction and the final pressure predicted by the model showed different responses towards the variation of the gas–liquid mass transfer coefficient since the former seemed not to be affected by the variation of the gas–liquid mass transfer coefficient; in contrast, the final pressure seemed to be slightly influenced. The proposed approach may also allow to potentially identify the methanogens species able to be predominant at high pressure.

An integrated anaerobic digestion and microbial electrolysis system for the enhancement of methane production from organic waste: Fundamentals, innovative design and scale-up deliberation

In the foreseeable future, renewable energy generation from electromethanogenesis to be more cost-effective energy. Electromethanogenesis system is a recent and efficient CO₂ to methane technology to upgrade biogas to 100% methane for power generation. And this can be attained through by integrating anaerobic digestion with microbial electrolysis system. Microbial electrolysis system can able to support carbon reduction on cathode and oxidation on anode by CO₂ capture thereby provides more CH₄ production from an integrated anaerobic digestion system. Scale-up the recent advance technique of microbial electrolysis system in the anaerobic digestion process for 100% methane production for power generation is need of the hour. The overall objective of this review is to facilitate the recent technology of microbial electrolysis system in the anaerobic digestion process. At first, the function of electromethanogenesis system and innovative integrated design method are outlined. Secondly, different external parameters such as applied voltage, operating temperature, pH etc are examined for the significance on process optimization. Eventually, electrode selections, electrode spacing, surface chemistry and surface area are critically reviewed for the scale-up considerations of integration process.

Recovery of Household Waste by Generation of Biogas as Energy and Compost as Bio-Fertilizer—A Review

Nowadays, organic waste and especially household waste represents a significant global issue due to population growth. The anaerobic digestion (AD) process is an essential operation contributing powerfully to the valorization of organic waste including food waste in terms of renewable energy generation (biogas) and the rich-nutrient residue that can be utilized as bio-fertilizer. Thus, this process (AD) allows for good recovery of household waste by generating biogas and compost. However, the AD operation has been affected by several key factors. In this paper, we aim to involve different critical parameters influencing the AD process, including temperature, pH, organic loading rate (OLR), carbon to nitrogen ratio (C/N), and total solid content (TS(%)). Further, the paper highlights the inhibition caused by the excessive accumulation of volatile fatty acids (VFAs) and ammoniac, which exhibits the positive effects of co-digestion, pretreatment methods, and mixing techniques for maintaining process stability and enhancing biogas production. We analyze some current mathematical models explored in the literature, such as distinct generic, non-structural, combined, and kinetic first-order models. Finally, the study discusses challenges, provides some possible solutions, and a future perspective that promises to be a highly useful resource for researchers working in the field of household waste recovery for the generation of biogas.

Development of a Mathematical Model and Validation for Methane Production Using Cow Dung as Substrate in the Underground Biogas Digester

South Africa is highly dependent on conventional fuels from non-renewable energy sources such as coal and fossil fuels. The methane from biogas is a renewable energy source for generating electricity using a combined heat and power plant. Therefore, the aim of the study is to develop and validate a mathematical model for predicting methane production in an underground biogas digester. The developed model was able to predict the production of methane gas as separate entity differing from other models. A total of 286 datasets were used as a trained dataset for the model development, and 144 datasets served as test data for the validation of the model, making a total of 430 measured datasets of all the predictors. The determination coefficient (R2) and the p-value of the predicted and calculated methane yield were 0.962 and 0.920, respectively. The high R2 in the present study confirms a good correlation between the model and experimental value. Hence, the model is of significance because it is applicable in predicting the performance of methane production of systems of the same design used in different locations, thereby arriving at the same constant values. From the study, the ambient weather factors (ambient temperature, relative humidity, and global horizontal irradiance) affected the methane production. Additionally, the indoor parameters (pH, gas temperature, slurry bottom and slurry top temperature) impacted on the yield of the methane production because the scaling factors associated with these quantities are non-zero real numbers. Hence, the scope of the study did not consider the volume of the biogas digester as an input parameter to the response.

Modified Anaerobic Digestion Model No. 1 for modeling methane production from food waste in batch and semi-continuous anaerobic digestions

A modified Anaerobic Digestion Model No. 1 (ADM1) with optimized kinetic parameters was presented to model methane production in the anaerobic digestion of food waste. Experimental data from batch and semi-continuous fermentations were used to calibrate and verify the model. Modified ADM1 simulation was carried out using AQUASIM 2.0 software. Sensitivity analysis was used to identify and evaluate the most sensitive kinetic parameters during biogas production. The decay constant of microorganisms, the disintegration constant, the hydrolysis constant of carbohydrates, the Monod maximum specific substrate uptake rate, and the half-saturation constants affected biogas production significantly. The optimized values of these parameters were 0.001, 0.16, 3, 1 and 0.23, respectively. Optimization results were validated using batch and semi-continuous experiments. The modified ADM1 well-predicted methane production, with R² values for the validation experiments all above 90%. These results can be used as basic data to simulate methane production in full-scale reactors.

Techno-economic and environmental assessment of biogas production from banana peel (Musa paradisiaca) in a biorefinery concept

Two scenarios for the biogas production using Banana Peel as raw material were evaluated. The first scenario involves the stand-alone production of biogas and the second scenario includes the biogas production together with other products under biorefinery concept. In both scenarios, the influence of the production scale on the process economy was assessed and feasibility limits were defined. For this purpose, the mass and energy balances were established using the software Aspen Plus along with kinetic models reported in the literature. The economic and environmental analysis of the process was performed considering Colombian economic conditions. As a result, it was found that different process scales showed great potential for biogas production. Thus, plants with greater capacity have a greater economic benefit than those with lower capacity. However, this benefit leads to high-energy consumption and greater environmental impact.