High-pressure anaerobic digestion up to 100 bar: influence of initial pressure on production kinetics and specific methane yields

Pressure-centric regulation for efficient anaerobic digestion: State-of-the-art, challenges and prospects

Anaerobic digestion (AD) is an environmentally friendly technology that simultaneously stabilizes biowaste and produces biogas. Conventional AD faces challenges such as inadequate substrate degradation and low methane purity. Pressure-centric regulation serves as an AD optimization strategy that can enhance the digestion efficiency and generate higher-energy-value biogas. However, limited reviews have been undertaken to focus on this technology. This review is designed to discuss innovations in ex-situ high-pressure pretreatment and in-situ high-pressure anaerobic digestion (HPAD) processes. Moreover, comprehensive understandings on the intrinsic mechanisms of HPAD are critically examined, including physicochemical reaction principles and microbial responses. The constraints currently curtailing these technologies and potential mitigation strategies are also scrutinized. Additionally, current knowledge gaps and future research directions on mechanisms, model fitting, and engineering practices are presented. Overall, this work highlights the feasibility of pressure-centric regulated AD and provides novel insights to overcome existing technical barriers in its application.

Upflow anaerobic sludge blanket reactor operation under high pressure for energy-rich biogas production

Autogenerative high-pressure digestion has an advantage of producing CH₄-rich biogas directly from the reactor. However, its continuous operation has rarely been reported, and has never been attempted in an upflow anaerobic sludge blanket reactor (UASB). Here, UASB was continuously operated at 10 g COD/L/d with increasing pressure from 1 to 8 bar. As the pressure increased, the CH₄ content in the biogas increased gradually, reaching 96.7 ± 0.8% at 8 bar (309 MJ/m³ biogas). The pH was dropped from 8.2 to 7.2 with pressure increase, but COD removal efficiency was maintained > 90%. The high pressure up to 8 bar did not adversely impact the physicochemical properties of granules, which was due to the increased production of extracellular polymeric substances (EPS), particularly, tightly bound EPS (34% increase). With pressure increase, there was no changes in the microbial community and ATPase gene expression, but 41% increase in carbonic anhydrase gene expression was observed.

Two-stage anaerobic digestion: State of technology and perspective roles in future energy systems

Two-stage anaerobic digestion (TSAD) systems have been studied on a laboratory scale for about 50 years. However, they have not yet reached industrial scale despite their potential for future energy systems. This review provides an analysis of the TSAD technology, including the influence of process parameters on biomass conversion rates. The most common substrate (35.2% of the 38 selected studies) used in the analysed data was in the category of rapidly hydrolysable industrial waste with an average dry matter content of 7.24%. The highest methane content of 85% was reached when digesting food waste in a combination of two mesophilic continuously stirred tank reactors with an acidic (pH 5.5) first stage and alkaline (pH 7) second stage. Therefore, the review shows the limitations of the TSAD technology, future research directions, and the effect of integration of TSAD systems into the current strategy to reduce greenhouse gas emissions.

Feasibility Study of Anaerobic Codigestion of Municipal Organic Waste in Moderately Pressurized Digesters: A Case for the Russian Federation

Anaerobic digestion (AD) is a promising option to obtain renewable energy in the form of biogas and reduce the anthropogenic impact on the environment. In recent years there has been increasing interest in using pressurized digesters to improve the quality of biogas. However, maintaining high overpressure increases the requirements for the explosion safety of digesters. Consequently, there are natural limitations in the available technologies and facilities suitable for full-scale operation. In this work, we aimed to evaluate the possibility of using overpressure in the digester to improve the efficiency of codigestion of common municipal organic waste–sewage sludge and the organic fraction of municipal solid waste. Three levels of moderate excess pressure (100, 150 and 200 kPa) were used to meet requirements of existing block-modular anaerobic bioreactors based on railway tanks, which are widely utilized for AD in the Russian Federation. There was no significant change in methane content in biogas (65% ± 3%) at different values of overpressure, hydraulic retention time (HRT) and organic loading rate (OLR). The maximum methane and energy production rates (2.365 L/(L·day) and 94.27 kJ/(L·day), respectively) were obtained at an overpressure of 200 kPa, HRT of 5 days and OLR of 14 kg VS/(m3·day). However, the maximum methane yield (202.44 mL/g VS), energy yield (8.07 kJ/g VS) and volatile solids (VS) removal (63.21%) were recorded at an overpressure of 150 kPa, HRT of 7 days and OLR of 10.4 kg VS/(m3·day). The pressured conditions showed better performance in terms of AD stability at high OLRs.

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.

Performance Evaluation of Pressurized Anaerobic Digestion (PDA) of Raw Compost Leachate

Anaerobic digestion (AD) represents an advantageous solution for the treatment and valorization of organic waste and wastewater. To be suitable for energy purposes, biogas generated in AD must be subjected to proper upgrading treatments aimed at the removal of carbon dioxide and other undesirable gases. Pressurized anaerobic digestion (PDA) has gained increasing interest in recent years, as it allows the generation of a high-quality biogas with a low CO2 content. However, high pressures can cause some negative impacts on the AD process, which could be accentuated by feedstock characteristics. Until now, few studies have focused on the application of PAD to the treatment of real waste. The present work investigated, for the first time, the performance of the pressurized anaerobic digestion of raw compost leachate. The study was conducted in a lab-scale pressurized CSTR reactor, working in semi-continuous mode. Operating pressures from the atmospheric value to 4 bar were tested at organic loading rate (OLR) values of 20 and 30 kgCOD/m3d. In response to the rise in operating pressure, for both OLR values tested, a decrease of CO2 content in biogas was observed, whereas the CH4 fraction increased to values around 75% at 4 bar. Despite this positive effect, the pressure growth caused a decline in COD removal from 88 to 62% in tests with OLR = 20 kgCOD/m3d. At OLR = 30 kgCOD/m3d, an overload condition was observed, which induced abatements of about 56%, regardless of the applied pressure. With both OLR values, biogas productions and specific methane yields decreased largely when the pressure was brought from atmospheric value to just 1 bar. The values went from 0.33 to 0.27 LCH4/gCODremoved at 20 kgCOD/m3d, and from 0.27 to 0.18 LCH4/gCODremoved at 30 kgCOD/m3d. Therefore, as the pressure increased, although there was an enhanced biogas quality, the overall amount of methane was lowered. The pressured conditions did not cause substantial modification in the characteristics of digestates.

Storage and Upgrading of Biogas by Physicochemical Purification in a Sudano-Sahelian Context

The global energy trends are currently dominated by a massive use of fossil non-renewable energy sources which are progressively depleting. In this way, the production of second-generation biogas production from organic wastes by the dark fermentation process offers, therefore, an attractive solution to diversify the present energy mix. The development of biogas production units has led to an increase in the quantity of biomethane, but it contains impurities. A biomethane purification and storage system was developed in this work to improve the quality of this biofuel. Solutions were first developed to capture carbon dioxide, hydrogen sulfide, water, and volatile organic compounds found in the initial biogas. These solutions were based on a system of purification made up of water absorption reactions and iron oxide, activated charcoal, and steel wool adsorption. Thus, the biomethane obtained after purification has been stored in an inflatable balloon before being compressed into a refrigerant bottle of R134a. The treatment system was used to release a biogas with 95 % biomethane and a law heating value (LHV) of 54 MJ/kg after purification. It also emerges that purification of 2 m3 of biogas requires 0.15 m3 of water at 20oC to produce 1.4 m3 of biomethane. This biomethane can meet an energy demand of 1624 Wh or 0.2 m3 of daily biomethane requirements. The system as a whole can allow customers with a biodigester to produce their own energy (cooking or electric) while reducing the production of green-house gases in the atmosphere.

Comparison of the microbial communities in anaerobic digesters treating high alkalinity synthetic wastewater at atmospheric and high-pressure (11 bar)

High-pressure anaerobic digestion is an appealing concept since it can upgrade biogas directly within the reactor. However, the decline of pH caused by the dissolution of CO₂ is the main barrier that prevents a good operating high-pressure anaerobic digestion process. Therefore, in this study, a high-pressure anaerobic digestion was studied to treat high alkalinity synthetic wastewater, which could not be treated in a normal-pressure anaerobic digester. In the high-pressure reactor, the pH value was 7.5 ~ 7.8, and the CH₄ content reached 88% at 11 bar. Unlike its normal-pressure counterpart (2285 mg/L acetic acid), the high-pressure reactor ran steadily (without volatile fatty acids inhibition). Furthermore, the microbial community changed in the high-pressure reactor. Specifically, key microbial guilds (Syntrophus (11.2%), Methanosaeta concilii (50.9%), and Methanobrevibacter (26.8%)) were dominant in the high-pressure reactor at 11 bar, indicating their fundamental roles under high-pressure treating high alkalinity synthetic wastewater.

Significance of Pretreatment in Enhancing the Performance of Dry Anaerobic Digestion of Food Waste: An Insight on Full Scale Implementation Strategy with Theoretical Analogy

The aim of this study was to treat food waste containing 25% total solids (TS) through dry anaerobic digestion (dry AD) process at various pressures (0.5 to 2.5 kg/cm2) and different time duration (20 to 100 min) to understand the impact of pretreatment in enhancing the methane generation potential along with insights on scale up. The findings revealed that vs. reduction and methane yield of 60% and 0.25 L CH4/(g VSadded) can be achieved with pretreated food waste at two kilograms per square centimeter, while pretreatment of food waste at 2 kg/cm2 for 100 min enhanced the vs. reduction from 60% to 85% and methane yield from 0.25 to 0.368 L CH4/(g VSadded). However, the net energy indicated that 40 min of pre -treatment at two kilograms per square centimeter can be a suitable option as methane yield and vs. reduction of 0.272 L CH4/(g VSadded) and 70%, respectively was achieved. The vs. reduction and the methane yield of 45% and 0.14 L CH4/(g VSadded), respectively was obtained from untreated food waste which illustrated that pretreatment had significantly impacted on the enhancement of methane generation and organic matter removal which can make the dry AD process more attractive and feasible at commercial scale.

A mini-review on the metabolic pathways of food waste two-phase anaerobic digestion system

Food waste (FW) disposal has become a global social, environmental, and economic problem. The current practice of landfilling is undesirable due to its potential emission of greenhouse gas, nutrient recycling, and pollution of water resources. Anaerobic digestion (AD), particularly two-phase AD is a promising option to manage FW and recover energy in the form of methane and obtain value-added by-products. However, most current review literature focuses on operating conditions while often placing little emphasis on improving conversion efficiency through regulating intermediate products. The AD process involves complex metabolic reactions carried out by several microbial groups. Therefore, understanding of these metabolic pathways existing in AD is the key to design effective strategies for enrichment of specific microbial groups which can produce desired intermediates for methane production, which can possibly be achieved by an understanding of the influence of critical process parameters on these metabolic pathways. Thus, it is the aim of this review to describe the effect of process conditions on underlying metabolic pathways in order to allow an efficient manipulation of these pathways for enhancing methane production.