Effect of mixing on biogas production during mesophilic anaerobic digestion of screened dairy manure in a pilot plant

High-solid digestion - A comparison of completely stirred and plug-flow reactor systems

High-solid digestion (HSD) for biogas production is a resource-efficient and sustainable method to treat organic wastes with high total solids content and obtain renewable energy and an organic fertiliser, using a lower dilution rate than in the more common wet digestion process. This study examined the effect of reactor type on the performance of an HSD process, comparing plug-flow (PFR) type reactors developed for continuous HSD processes, and completely stirred-tank reactors (CSTRs) commonly used for wet digestion. The HSD process was operated in thermophilic conditions (52 °C), with a mixture of household waste, garden waste and agricultural residues (total solids content 27-28 %). The PFRs showed slightly better performance, with higher specific methane production and nitrogen mineralisation than the CSTRs, while the reduction of volatile solids was the same in both reactor types. Results from 16S rRNA gene sequencing showed a significant difference in the microbial population, potentially related to large differences in stirring speed between the reactor types (1 rpm in PFRs and 70-150 rpm in CSTRs, respectively). The bacterial community was dominated by the genus Defluviitoga in the PFRs and order MBA03 in the CSTRs. For the archaeal community, there was a predominance of the genus Methanoculleus in the PFRs, and of the genera Methanosarcina and Methanothermobacter in the CSTRs. Despite these shifts in microbiology, the results showed that stable digestion of substrates with high total solids content can be achieved in both reactor types, indicating flexibility in the choice of technique for HSD processes.

A comprehensive review on anaerobic digestion with focus on potential feedstocks, limitations associated and recent advances for biogas production

With the escalating energy demand to accommodate the growing population and its needs along with the responsibility to mitigate climate change and its consequences, anaerobic digestion (AD) has become the potential approach to sustainably fulfil our demands and tackle environmental issues. Notably, a lot of attention has been drawn in recent years towards the production of biogas around the world in waste-to-energy perspective. Nevertheless, the progress of AD is hindered by several factors such as operating parameters, designing and the performance of AD reactors. Furthermore, the full potential of this approach is not fully realised yet due the dependence on people's acceptance and government policies. This article focuses on the different types of feedstocks and their biogas production potential. The feedstock selection is the basic and most important step for accessing the biogas yield. Furthermore, different stages of the AD process, design and the configuration of the biogas digester/reactors have been discussed to get better insight into process. The important aspect to talk about this process is its limitations associated which have been focused upon in detail. Biogas is considered to attain the sustainable development goals (SDG) proposed by United Nations. Therefore, the huge focus should be drawn towards its improvements to counter the limitation and makes it available to all the rural communities in developing countries and set-up the pilot scale AD plants in both developing and developed countries. In this regard, this article talks about the improvements and futures perspective related to the AD process and biogas enhancement.

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.

The pump-mixed anaerobic digestion of pig slurry: new technology and mathematical modeling

Biogas production is a relatively novel and developing branch of the renewable fuel sector, which allows agricultural waste, and more, to be used as a feedstock. New technologies have been integrated into the process to improve its efficiency. In this study, a pump-mixed anaerobic digestion concept is considered for both experimental and modeling approaches. The experiment included a total of nine configurations with the same geometry (140 dm³ of total reactor volume) but different hydraulic retention times and mixing intervals. The measurements were used to create and optimize a mathematical model. The complete-stirring assumption, which underlies most anaerobic digestion (AD) simulations, is no longer valid in this case. Thus, the novel concept is developed by assuming that the liquid phase is split into three separate sections, which approximates the concentration gradient in a real reactor. This method allows partial differential equations to be avoided, which could potentially affect the calculation efficiency. The final mean accuracy of the model in the tested range was estimated to be 86.60% while, in selected parts of the scope, was close to 90%. The pump-mixed anaerobic digestion technique in the experiment achieved high production performance (above 8 dm³ of product per 1 dm³ of feedstock) while maintaining a high methane content (approximately 65%). The comparison between the reactor stirred by an impeller, and the pump-mixed, indicated that the proposed configuration ensures better production stability. Additionally, it was possible to achieve a higher biogas production rate with the same feedstock concentration.

Effect of mixing duration on biogas production and methanogen distribution in an anaerobic digester

Mixing has been shown to have effect on biogas production in anaerobic digestion systems. To further examine this impact, a study was designed to evaluate nearly continuous mixing (mixing for 15 min followed by no mixing for 15 min, CON), intermediate mixing (mixing for 15 min followed by no mixing for 45 min, INT) and no mixing (unmixed, NO) on biogas production in three 208 L pilot-scale tank reactors. The experiments were conducted in triplicates at a controlled temperature of 37 ± 1°C, with a total solids percentage of 5%, a hydraulic retention time (HRT) of 21 days, and an organic loading rate (OLR) of 2 kg VS m^-3 d^{- 1}. Digesters with NO mixing had greater solids build up in the bottom quarter of the digester after four weeks' retention time. The methane percentage in biogas produced from digesters with INT and CON mixing were 63% and 62%, respectively, which were 4% and 5% higher than that from digesters with NO mixing (58%). The specific methane yield for digesters with NO, INT and CON mixing was 1.15, 1.15, and 1.49 m³-methane per kg-VS destroyed, however, those differences were not statistically significant (p > 0.05). Digesters had the least amount of Methanosarcinales of the methanogens measured under all treatments. However, the Methanosarcinales, Methanosarcinaceae, Methanomicrobiales, and the total amount of methanogens were less in digesters with INT mixing compared to NO and CON mixing treatments.

Effects of mixing time on methane production from anaerobic co-digestion of food waste and chicken manure: Experimental studies and CFD analysis

The relationship between mixing time and methane production was investigated by anaerobically co-digesting food waste (FW) and chicken manure (CM) at four different organic loading rates. The mixing pattern and turbulence intensity obtained from CFD were adopted to evaluate the mixing performance in digesters with different viscosities. The simulated mixing time from CFD was selected as a reference for the first time to analyze the methane yield. The results showed that if the digester was well mixed under intermittent mixing mode with relatively short mixing time, then extending mixing time or changing intermittent mixing to continuous mixing would have no substantial effects on methane production. By contrast, continuously mixed digesters performed better when the intermittent mixing modes cannot make the digester get to a high degree of uniformity. Hence, the simulated mixing time from CFD can be used as a reference to determine the experimental mixing time in different cases.

Effect of Mixing Regimes on Cow Manure Digestion in Impeller Mixed, Unmixed and Chinese Dome Digesters

This study examines the effect of mixing on the performance of anaerobic digestion of cow manure in Chinese dome digesters (CDDs) at ambient temperatures (27–32 °C) in comparison with impeller mixed digesters (STRs) and unmixed digesters (UMDs) at the laboratory scale. The CDD is a type of household digester used in rural and pre-urban areas of developing countries for cooking. They are mixed by hydraulic variation during gas production and gas use. Six digesters (two of each type) were operated at two different influent total solids (TS) concentration, at a hydraulic retention time (HRT) of 30 days for 319 days. The STRs were mixed at 55 rpm, 10 min/hour; the unmixed digesters were not mixed, and the Chinese dome digesters were mixed once a day releasing the stored biogas under pressure. The reactors exhibited different specific biogas production and treatment efficiencies at steady state conditions. The STR 1 exhibited the highest methane (CH4) production and treatment efficiency (volatile solid (VS) reduction), followed by STR 2. The CDDs performed better (10% more methane) than the UMDs, but less (approx. 8%) compared to STRs. The mixing regime via hydraulic variation in the CDD was limited despite a higher volumetric biogas rate and therefore requires optimization.

Innovative operational strategies for biogas plant including temperature and stirring management

Finding the optimum operational conditions (mainly temperature and stirring) inside the fermenters is crucial in the field of anaerobic digestion. This study was conducted to contribute to the research area of anaerobic digestion process optimization and is an example for other biogas plants to improve efficiency. The research aimed at finding the optimum operational conditions in a large-scale biogas plant located in Lower Saxony, Germany, which started operation in 2011. The optimization activities were performed by operating the fermenters under different operational conditions: the temperature inside the fermenters ranged between 40°C and 43°C, while applying several stirring scenarios. These changes led to an increase in specific electricity yield of 11.7% and a decrease in internal energy consumption of 10.4%. The total internal energy consumption of the biogas plant was in the range of 6.3-7.2% (the average monthly internal energy consumption was 6.7% in 2013 and 6.0% in 2014) from their own production, and 28% of this energy consumption was used by the stirrers before optimization. Therefore, finding the optimum operational conditions leads to high energy harvesting and lower internal energy consumption.

Influence of mixing on anaerobic digestion efficiency in stirred tank digesters: A review

A comprehensive review was carried out on the influence of mixing on anaerobic digestion (AD) efficiency in stirred tank anaerobic digesters. Though traditionally, stirred tank digesters operated as continuous stirred tank reactors (CSTRs), this review revealed that there is no motivation to continue to operate stirred tank anaerobic digesters as CSTRs if AD energy efficiency is to be improved. AD energy production efficiency can be achieved with optimized intermittent mixing. AD efficiency should include an assessment of the net energy production efficiency and should be the criteria in determining the mixing mode, mixing intensity, mixing time and mixing interval for every anaerobic digestion operating plan.

Quantitative characterization and analysis of granule transformations: Role of intermittent gas sparging in a super high-rate anaerobic system

Knowledge of leveraging biomass characteristics is essential for achieving a microbial community with a desired structure to optimize anaerobic bioreactor performance. This study investigates the successive granule transformations in a high-rate anaerobic system with intermittent gas sparging and sequential increases in organic loading rates (OLRs), by establishing the correlations between the granule microstructures and reactor operating parameters. Over the course of a 196-day lab-scale trial, the granules were visualized in various stages using scanning electron microscopy, and digital image processing was applied for further quantifying their surface properties. Correlation analyses revealed that irregularities of the granule microstructures (surface properties, specific surface area and pore volume) emerged at stage 4 when the OLR was 13.31 kg COD/m³·day and in stage 5 in the absence of gas sparging. The loading ratio (substrate surface loading to upward velocity) was identified to be the main parameter controlling the granule transformations, and the surface structures were classified into three categories for further interpretation. Confocal laser scanning microscopy analyses showed that the granule core started to hollow out from stage 4. It is also found that a rough granule surface helped accelerate the growth of the granular diameter under gas sparging. Overall, this study not only establish quantitative correlations between the granules microstructures and reactor operating parameters, but also shed light on the use of intermittent gas sparging to control the surface properties of anaerobic granules in high-rate anaerobic bioreactors.

Bioenergy recovery from cattle wastewater in an UASB-AF hybrid reactor

New data on biogas production and treatment of cattle wastewater were registered using an upflow anaerobic sludge blanket-anaerobic filter (UASB-AF) hybrid reactor under mesophilic temperature conditions (37 °C). The reactor was operated in semi-continuous mode with hydraulic retention times of 6, 5, 3 and 2 days and organic loading rates of 3.8, 4.6, 7.0 and 10.8 kg COD_t m^-3 d^-1. Biogas volumes of 0.6-0.8 m³ m^-3 d^-1 (3.8-4.6 kg COD_t m^-3 d^-1) and 1.2-1.4 m³ m^-3 d^-1 (7.0-10.8 kg COD_t m^-3 d^-1), with methane concentrations between 69 and 75%, were attained. The removal of organic matter with values of 60-81% (COD_t) and 51-75% (COD_s) allowed methane yields of 0.155-0.183 m³ CH₄ kg^-1 COD_t and 0.401-0.513 m³ CH₄ kg^-1 COD_s to be obtained. Volatile solids were removed in 34 to 69%, with corresponding methane yields of 0.27 to 0.42 m³ CH₄ kg^-1 VS_removed. The good performance of the novel hybrid reactor was demonstrated by biogas outputs higher than reported previously in the literature, along with the quality of the gas obtained in the various experimental phases. The hybrid reactor investigated in this study presents comparative advantages, particularly in relation to conventional complete mixture units, considering economic factors such as energy consumption, reactor volume and installation area.

Pseudo catalytic transformation of volatile fatty acids into fatty acid methyl esters

Instead of anaerobic digestion of biodegradable wastes for producing methane, this work introduced the transformation of acidogenesis products (VFAs) into fatty acid methyl esters (FAMEs) to validate the feasible production of short-chained fatty alcohols via hydrogenation of FAMEs. In particular, among VFAs, this work mainly described the mechanistic explanations for transforming butyric acid into butyric acid methyl ester as a case study. Unlike the conventional esterification process (conversion efficiency of ∼94%), the newly introduced esterification under the presence of porous materials via the thermo-chemical process reached up to ∼99.5%. Furthermore, the newly introduced esterification via the thermo-chemical pathway in this work showed extremely high tolerance of impurities: the conversion efficiency under the presence of impurities reached up to ∼99±0.3%; thus, the inhibition behaviors attributed from the impurities used for the experimental work were negligible.

The effects of different mixing intensities during anaerobic digestion of the organic fraction of municipal solid waste

Mixing inside an anaerobic digester is often continuous and is not actively controlled. The selected mixing regime can however affect both gas production and the energy efficiency of the biogas plant. This study aims to evaluate these effects and compare three different mixing regimes, 150 RPM and 25 RPM continuous mixing and minimally intermittent mixing for both digestion of fresh substrate and post-digestion of the organic fraction of municipal solid waste. The results show that a lower mixing intensity leads to a higher biogas production rate and higher total biogas production in both cases. 25 RPM continuous mixing and minimally intermittent mixing resulted in similar biogas production after process stabilization, while 150 RPM continuous mixing resulted in lower production throughout the experiment. The lower gas production at 150 RPM could not be explained by the inhibition of volatile fatty acids. Cumulative biogas production until day 31 was 295 ± 2.9, 317 ± 1.9 and 304 ± 2.8N ml/g VS added during digestion of fresh feed and 113 ± 1.3, 134 ± 1.1 and 130 ± 2.3N ml/g VS added during post digestion for the 150 RPM, 25 RPM and minimally mixed intensities respectively. As well as increasing gas production, optimal mixing can improve the energy efficiency of the anaerobic digestion process.

Microbial anaerobic digestion (bio-digesters) as an approach to the decontamination of animal wastes in pollution control and the generation of renewable energy

With an ever increasing population rate; a vast array of biomass wastes rich in organic and inorganic nutrients as well as pathogenic microorganisms will result from the diversified human, industrial and agricultural activities. Anaerobic digestion is applauded as one of the best ways to properly handle and manage these wastes. Animal wastes have been recognized as suitable substrates for anaerobic digestion process, a natural biological process in which complex organic materials are broken down into simpler molecules in the absence of oxygen by the concerted activities of four sets of metabolically linked microorganisms. This process occurs in an airtight chamber (biodigester) via four stages represented by hydrolytic, acidogenic, acetogenic and methanogenic microorganisms. The microbial population and structure can be identified by the combined use of culture-based, microscopic and molecular techniques. Overall, the process is affected by bio-digester design, operational factors and manure characteristics. The purpose of anaerobic digestion is the production of a renewable energy source (biogas) and an odor free nutrient-rich fertilizer. Conversely, if animal wastes are accidentally found in the environment, it can cause a drastic chain of environmental and public health complications.

Biogas production performance of mesophilic and thermophilic anaerobic co-digestion with fat, oil, and grease in semi-continuous flow digesters: effects of temperature, hydraulic retention time, and organic loading rate

Anaerobic co-digestions with fat, oil, and grease (FOG) were investigated in semi-continuous flow digesters under various operating conditions. The effects of hydraulic retention times (HRTs) of 12 and 24 days, organic loading rates (OLRs) between 1.19 and 8.97 gTVS/Ld, and digestion temperatures of 37 degrees C and 55 degrees C on biogas production were evaluated. It was proposed that, compared to anaerobic digestion with wastewater treatment plant sludge (primary raw sludge), semi-continuous flow anaerobic co-digestion with FOG could effectively enhance biogas and methane production. Thermophilic (55 degrees C) co-digestions exhibited higher biogas production and degradation of organics than mesophilic co-digestions. The best biogas production rate of 17.4 +/- 0.86 L/d and methane content 67.9 +/- 1.46% was obtained with a thermophilic co-digestion at HRT = 24 days and OLR = 2.43 +/- 0.15 g TVS/Ld. These were 32.8% and 7.10% higher than the respective values from the mesophilic co-digestion under similar operating conditions.

Methane capture from livestock manure

It has been estimated that livestock manure contributes about 240 million metric tons of carbon dioxide equivalent of methane to the atmosphere and represents one of the biggest anthropogenic sources of methane. Considering that methane is the second biggest contributor to global warming after carbon dioxide, it is imperative that ways and means are developed to capture as much of the anthropogenic methane as possible. There is a major associated advantage of methane capture: its use as a source of energy which is comparable in 'cleanness' to natural gas. The present review dwells upon the traditional ways of methane capture used in India, China, and other developing countries for providing energy to the rural poor. It then reviews the present status of methane capture from livestock manure in developed countries and touches upon the prevalent trends.