The anaerobic digestion of microalgae is a prospective environmentally feasible option for creating a renewable source of energy for industrial and domestic needs. Microalgae anaerobic digestion is a ...key unit process that integrates efficiency and beneficially into the production of microalgae derived biofuels. Anaerobic digestion culminating in methane fermentation improves the economic viability of microalgae liquid biofuel production and presents an opportunity for power generation from wastewater derived microalgae. However the anaerobic digestion of microalgae biomass is not straight forward due to several technical restraints including low concentration of digestible biodegradable substrate, recalcitrant substrate constituents, cell wall degradability, low carbon to nitrogen ratio, ammonia toxicity and effects from salinity and associated metal ions.
Current production methods for liquid biofuel production from microalgae produce approximately 60–70% residual biomass that is currently a byproduct. Anaerobic digestion provides biogas, but it can also provide essential nutrient recovery from lipid extracted microalgae biomass. The biogas produced from the anaerobic digestion process can be used to generate onsite electrical power or thermal heat to offset biomass processing and extraction processes. When both of these processes are integrated and operated simultaneously, the benefits to microalgae biofuel production and wastewater treatment derived energy production are increased significantly. To consider the integration of anaerobic digestion into a commercial-scale integrated microalgae production and biofuel refinery facility or wastewater treatment plant we present a review of the literature, the current state of the art and future directions for research.
Extracellular polymer proteins have been reported to play an important role in enhancing sludge dewaterability during anaerobic digestion in our previous study. However, how the proteins in sludge ...determine sludge dewaterability remains to be determined. In this work, proteins from digested sludge were identified using label free proteomics analysis, and its hydrophilicity/hydrophobicity properties and functional groups were analysed. We determined that the microbial community variation between the three stages during the anaerobic digestion process was responsible for enhancing sludge dewaterability; The transformation from hydrophilicity to hydrophobicity of digested sludge surface is the result of functional groups distribution variation which caused by the proteins and microbial communities. This study provides a new insight into the development of anaerobic digestion based on sludge dewaterability.
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•Conversion among microbial communities might be responsible for protein variation.•Molecular functional proteins help mechanically bound moisture releasing.•Sludge hydrophobicity was caused by protein and microbial community distribution.•Functional groups distribution varies with protein and microbial community.•Anaerobic digestion process can be optimised according to sludge dewaterability.
A bioaugmentation approach was used to enhance the performance of anaerobic digestion (AD) using cow manure (CM) as the substrate in a continuous system. To obtain the desirable microbial culture for ...bioaugmentation, a biochemical methane potential test (BMP) was used to evaluate three commonly used inocula namely (1) municipal solid waste (MSW), (2) wastewater treatment plant (WWTP), and (3) cow manure digester (CMMD) for their hydrolytic capacity. The highest lignocellulose removal (56% for cellulose and 50% for hemicellulose) and the most profusion of cellulolytic bacteria were obtained when CM was inoculated with CMMD. CMMD was thus used as the seed inoculum in a continuously operated reactor (Ra) with the fiber fraction of CM as the substrate to further enrich cellulolytic microbes. After 100 days (HRT: 30 days), the Bacteria fraction mainly contained Ruminofilibacter, norank_o_SBR1031, Treponema, Acetivibrio. Surprisingly, the Archaea fraction contained 97% ‘cellulolytic archaea’ norank_c_Bathyarchaeia (Phylum Bathyarchaeota). This enriched consortium was used in the bioaugmentation experiment. A positive effect of bioaugmentation was verified, with a substantial daily methane yield (DMY) enhancement (24.3%) obtained in the bioaugmented reactor (Rb) (179 mL CH4/gVS/d) than that of the control reactor (Rc) (144 mL CH4/gVS/d) (P < 0.05). Meanwhile, the effluent of Rb enjoyed an improved cellulose reduction (14.7%) than that of Rc, whereas the amount of hemicellulose remained similar in both reactors' effluent. When bioaugmentation stopped, its influence on the hydrolysis and methanogenesis sustained, reflected by an improved DMY (160 mL CH4/gVS/d) and lower cellulose content (53 mg/g TS) in Rb than those in Rc (DMY 144 mL/CH4/gVS/d and cellulose content 63 mg/g TS, respectively). The increased DMY of the continuous reactor seeded with a specifically enriched consortium able to degrade the fiber fraction in CM shows the feasibility of applying bioaugmentation in AD of CM.
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•Bioaugmentation was used to boost the anaerobic digestion performance of cow manure.•The bioaugmentation culture was enriched from the inoculum treating dairy waste.•The culture contained cellulolytic bacteria and Bathyarchaeota after long-term cultivation.•Bioaugmentation enhanced both hydrolysis and methanogenesis in the continuous test.•When bioaugmentation stopped, improved hydrolysis and methanogenesis were still observed.
This paper is a critical evaluation of the effect of mixing on biogas production rates in an anaerobic digester. Mixing plays a prominent role in determining the efficiency of the anaerobic digestion ...process. This review analyzes the miscellaneous effects of mixing (on the microbial community, methane content and volatile fatty acids) at various mixing intensities and during different stages of the digestion process. Intermittent mixing (mixing at intervals) seems preferable in terms of the quality and quantity of biogas produced, and results in lower power consumption and maintenance costs associated with large-scale biogas production. Preferable mixing time (the length at intervals) and the intensity depends on the geometry of the digester and impeller.
The conclusion is drawn that the study of the slurry rheology is very crucial in the designing of the mixing equipment, the shape and size of the digester, and the pipe transport system which can assist in minimizing the initial investment and operational costs. Accordingly, this paper focuses on the parameters which determine the potency of mixing, such as viscosity, total solid content and digester design. Empirical data demonstrated by various researchers regarding rheological characteristics is compared and reviewed. Consequently, close attention should be paid toward the optimization of mixing in terms of its speed, mixing time and impeller geometry, especially during different stages of the digestion process (hydrolysis, acidogenesis, acetogenesis and methanogenesis). Finally, readers will be guided to the extensive publications regarding optimization, directions of future research, and troubleshooting of the mixing operation in an anaerobic digester. This investigation will help to improve mixing efficiency with biogas plants.
Highlights
Effect of shear rate on different stages of an anaerobic digestion process.
Methane content varies with the variation in mixing speeds.
Mixing effect is significant when the total solid content is higher.
Intermittent mixing is favorable when compared to continuous mixing.
The geometry of the digester and mixer is essential to evaluate digester mixing.
Wastewater treatment processes with granular sludge are compact and are becoming increasingly popular. Interest has been accompanied by the development of mathematical models. This contribution ...simultaneously reviews available models in the scientific literature for anaerobic, aerobic and partial nitritation-anammox granular sludge reactors because they comprise common phenomena (e.g. liquid, gas and granule transport) and thus pose similar challenges. Many of the publications were found to have no clearly defined goal. The importance of a goal is stressed because it determines the appropriate model complexity and helps other potential users to find a suitable model in the vast amount of literature. Secondly, a wide variety was found in the model features. This review explains the chosen modelling assumptions based on the different reactor types and goals wherever possible, but some assumptions appeared to be habitual within fields of research, without clear reason. We therefore suggest further research to more clearly define the range of operational conditions and goals for which certain simplifying assumptions can be made, e.g. when intragranule solute transport can be lumped in apparent kinetics and when biofilm models are needed, which explicitly calculate substrate concentration gradients inside granules. Furthermore, research is needed to better mechanistically understand detachment, removal of influent particulate matter and changes in the mixing behaviour inside anaerobic systems, before these phenomena can be adequately incorporated in models. Finally, it is suggested to perform full-scale model validation studies for aerobic and anammox reactors. A spreadsheet in the supplementary information provides an overview of the features in the 167 reviewed models.
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•Granular sludge reactor models for various processes present common features.•Differences are explained by different reactor types and modelling goals.•Some assumptions seem habitual for certain reactor types.•Knowledge gaps were identified.•A spreadsheet with assumptions of the 167 models reviewed is provided.
Biogas systems are often multi-functional and involve several actors in different sectors, requiring these actors to collaborate closely in order to implement such systems. In this paper, a study is ...presented where the theory of institutional capacity building is used to guide interventions with public and private actors to facilitate the development of local biogas systems in Norrköping, Sweden. The interventions were performed in the form of a workshop series, where local actors with potential to influence biogas developments actively took part. The workshop series generated knowledge on Norrköping’s significant potential for both producing and using biogas, which was traced, in part, to its high concentration of bio-based industries and its good position as a hub for transports. The interventions also created a shared understanding that cooperation and coordination to distribute resources and knowledge about biogas, both geographically and across sectors, was critical for realizing this potential. The municipal organization was identified as an important actor for coordinating these efforts. Observations during the workshops and survey responses indicate that the interventions contributed to building institutional capacity and initiation of efforts to develop local biogas solutions. Ideas put forth in this study enable interventions to target the intangible internal capacities of emerging industrial symbiosis networks. In addition, institutional capacity building serves as a useful analytical framework capable of capturing progress within emerging networks in the short-term even when material, water or energy synergies are yet to be realized.
This study developed an intermittent oxidation-reduction potential (ORP)-controlled micro-aeration system for high solids anaerobic digestion (AD) of lignocellulosic biomass without volatile fatty ...acids (VFA) accumulation at high organic loading rate (OLR). Traditional AD of Napier grass, a model lignocellulosic biomass, at an OLR of 5 g volatile solids (VS)/L/day resulted in an accumulation of total VFA concentration up to 9.2 g/L as acetic acid (HAc) equivalent, causing rapid drops in pH and methane yield, and driving the digester to the verge of failure. Once intermittent (every 24 h) ORP-controlled micro-aeration (at ORP of −470 mV) was initiated, the total VFA concentration rapidly decreased to 3.0 g HAc/L and the methane yield improved, resulting in stable digester performance without the need for alkalinity supplementation or OLR reduction. By combining reactor performance results, mass balance analyses, microbial community characterization data, and a bioenergetic evaluation, this study suggested that rapid VFA conversion and CH4 production were carried out by facultative anaerobes and hydrogenotrophic methanogens under micro-aerobic conditions. This novel operating approach can be applied as an effective control strategy for high OLR AD processes especially in the event of VFA accumulation.
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•Increasing OLR resulted in accumulation of VFA and potential reactor failure.•Micro-aeration reduced VFA concentration and recovered reactor performance.•Micro-aeration created niches of facultative heterotrophs and anaerobic methanogens.
Small-scale anaerobic digestion (SSAD) is a promising technology for the treatment of livestock manure and the organic fraction of municipal wastes, especially in low population communities or in ...stand-alone waste treatment facilities. SSAD systems can transform organic matter into biogas (a mixture, mainly composed of carbon dioxide and methane), making the technology suitable for a variety of applications in energy, agriculture and, potentially, the emerging bio-products and bio-processes sector. Small-scale farming processes can further exploit the portable and flexible options made available by implementing SSAD systems to effect on-demand conversion of organic waste streams to useful heat (and, potentially, electricity), with significant economic benefits accruable (especially when such energy carriers are exported). SSAD is particularly applicable to the European agricultural sector, where the average individual farm sizes and land productivities are currently insufficient to meet the feedstock requirements of medium and large-scale plants. Despite the apparent benefits of SSAD, the technology is still not well utilised. Much of the research previously conducted has focused on large-scale systems.
This study explores the current status of SSAD technology in Europe by identifying process design and operational characteristics, influential EU policies, the recent progress related to SSAD, and the issues encountered. The study sheds light on an area with limited research by providing an overview of the technology's present status in Europe by identifying areas of future study.
•Small-scale anaerobic digestion; a promising renewable energy technology for rural agricultural industries.•Beneficial for its capability to produce bio-energy, bio-fertiliser, and mitigate emissions.•Items related to plant design, energy utilisation, policy implications, and limiting barriers were examined.
Anaerobic digestion is increasingly used worldwide to generate energy from biogas, bringing significant economic and environmental benefits. In particular, in the European Union (EU), biogas can ...contribute significantly in several countries to reach the renewable energy targets. This study provides an assessment of the spatial distribution of the biogas potential of farm manure from livestock and poultry in Europe, which is a key issue for the location and economic performances of a bioenergy plant. Biogas estimates provided in this study are computed through a spatial analysis algorithm that uses data of livestock and poultry, manure production and collection, leading to the evaluation of the spatial distribution of biogas potential at 1 km spatial resolution. Following this analysis, the theoretical biogas potential of manure was estimated at 26 billion m3 biomethane in Europe (23 billion m3 biomethane in the EU) and the realistic biogas potential, counting on collectible manure, was assessed at 18 billion m3 biomethane in Europe (16 billion m3 biomethane in the EU). Several maps provide the suitable locations and capacity of manure-based biogas plants in two different scenarios. Between 13,866 and 19,482 biogas plants could be built in Europe, with a total installed capacity between 6144 MWe and 7145 MWe, and an average capacity between 315 kWe and 515 kWe.
•This is an assessment of the spatial distribution of the biogas potential of manure at 1 km spatial resolution in Europe.•Biogas estimates are computed through a spatial analysis algorithm that uses data of manure production and collection.•The theoretical biogas potential of manure was estimated at 26 billion m3 biomethane in Europe (23 billion m3 in the EU).•The realistic biogas potential was assessed at 18 billion m3 biomethane in Europe (16 billion m3 in the EU).•Between 13866 and 19482 biogas plants could be built in Europe, with an installed capacity between 6144 MWe and 7145 MWe.
The increasing world population necessitates the production of larger amounts of food in a safe and environmentally sustainable manner, while concomitantly managing an increasing amount of food waste ...similarly. These needs can theoretically be met by the recycling of the nutrients in food waste via anaerobic digestion, which also produces renewable energy. This hypothesis is proven by the growing of a commonly consumed leafy vegetable, xiao bai cai (Brassica rapa), by the addition of food waste anaerobic digestate in place of commercial fertilizer. Different concentrations of the digestate were tested, as well as different heat treatments to simulate hygienization, and the results for most part (aerial fresh weight, dry weight, chlorophyll content) are not significantly different from growth utilizing commercial inorganic 15:15:15 NPK fertilizer. Microbial analysis of the growth media was also carried out to explicate digestate effects and to show that some common foodborne disease pathogens were not detected.
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•The increasing global population requires more food, food waste management and energy.•Food waste was anaerobically digested to produce energy and nutrient-rich digestate.•Digestate was used, including hygienization, to grow a commonly consumed green leafy vegetable, xiao bai cai, successfully.•The digestate was able to replace commercial fertilizer to achieve comparable aerial fresh and dry weight and increased chlorophyll index.•Anaerobic digestion can be used to manage food waste, produce biogas and to grow food.
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