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The overall degradation efficiencies (%) for protein, carbohydrates, and fat in the full-scale biogas plants; FW = food waste, TD = thermophilic dry digestion, AW = plant-based agricultural waste, AWM = agricultural waste + manure, WWTP = wastewater treatment plant
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Background
This study examines the destiny of macromolecules in different full-scale biogas processes. From previous studies it is clear that the residual organic matter in outgoing digestates can have significant biogas potential, but the factors dictating the size and composition of this residual fraction and how they correlate with the residual...
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Nowadays, generating energy is discussed as a paramount option in waste management, because the share of energy from waste is increasing annually. e ever-increasing need for energy and limitations of using fossil resources, and also the increase in environmental pollution from consuming these resources have made the use of renewable energies of esp...
Citations
... Moreover, this also points out that there is room for integration between biogas yield criteria and digestate quality parameters (Logan and Visvanathan, 2019). As proteins are the main macromolecule left in the digestate, a better degradation could simultaneously generate additional biogas and increase the ammonium N content in the digestate (Ekstrand et al., 2022;Mazzini et al., 2020;Tambone et al., 2017). ...
... 2 As conventional organic wastes, such as sewage sludge and food wastes are increasingly utilized in AD, new, as of yet unused, substrates need to be introduced, pushing the industry into uncharted territories for design, optimization, and operation of AD systems. 3 An unbalanced AD process initially manifests itself as accumulation of intermediate microbial degradation products (i.e., volatile fatty acids (VFA; acetic, propionic, iso-butyric, butyric, iso-valeric, n-valeric, iso-caproic, and caproic acid)), 4 which further amplify the disturbance, resulting in decreased organic matter degradation, biogas production, and process efficiency. Additionally, unanticipated shifts in reactor fluid behavior (i.e., rheology) or foaming may occur, both of which are mostly difficult and expensive to rectify. 5 Simultaneously achieving a high efficiency and stability of the AD process is therefore far from being straightforward, yet it is vital for high production capacities to support future societal needs for integrated waste management, renewable energy, and nutrient recovery. ...
Anaerobic digestion (AD) is an important technology for achieving sustainability, but it faces challenges in meeting rising production demands while remaining economically profitable. One difficulty is the lack of a comprehensive understanding of the many interactions within anaerobic digesters, which makes it challenging to fully optimize them. This is particularly notable when considering the interlinked dynamics between micronutrient availability and fluid behavior. This study addresses this gap by focusing on key operational parameters affecting the efficiency of the process in continuous stirred-tank biogas reactors, which are the most used AD technology today. It does so by proposing and evaluating a novel conceptual model of the mechanisms behind how different parts of AD processes interact upon disturbance, highlighting strategies for preventing process failure. This article aims to improve our understanding of the complexity of AD biotechnology and to provide a starting point for developing advanced strategies for operational optimization.
... The amount of lipids in sludge decreased by 58% during anaerobic digestion. It has been shown experimentally that anaerobic digestion is able to biodegrade nearly 90% of fats, resulting in close to zero percent crude fat in the digestate [56]. The suboptimal reduction indicated that lipid digestion did not occur efficiently at WWTP M. It has been shown that the degradation of lipids starts to occur after 8-10 days of anaerobic digestion treatment [43]. ...
... The mean cellulose content in ESS was 2.3 ± 1.1% TS, whereas DSS contained 2.6 ± 0.7% TS cellulose. Typical values for cellulose content are between 0.3% TSS and 9-10% VS in ESS [56,58], and between 1.0% TSS and 13.8% TS for DSS [23,58]. ...
... In this study, the amount of cellulose in sludge was reduced by 89% during anaerobic digestion. The extent of cellulose biodegradation following anaerobic treatment is around 60% [56]. ...
Sewage sludge is a type of waste that has high health and environmental risks associated with its reuse. Moreover, sludge has been neglected in global circular economy targets because it is generated in considerably lower quantities than municipal solid waste. At the same time, European Union's transition towards circular economy has set the need to reduce the amount of waste and to promote the production of secondary raw materials. Many countries have developed national strategies for sludge management to reach their sustainability goals. In Latvia, the current sludge management approaches include land application, composting and anaerobic digestion which all utilize sludge as an organic fertilizer. As an alternative to current management practices, resource recovery is put forward as a solution that is in agreement with EU policy. Carbohydrates (including cellulose), proteins and lipids were selected as candidates for energy and materials recovery from sludge. For the first time, this study demonstrates a comprehensive assessment of Latvian municipal sewage sludge composition and offers the theoretical yields of secondary resources on a yearly basis. Primary, secondary, and anaerobically digested sludge from 13 wastewater treatment plants (WWTPs) in Latvia was characterized in this study. The most abundant sludge type – secondary sludge – contained 18.5% proteins, 9.8% lipids and 2.6% cellulose per TS. On a yearly basis, secondary sludge from all Latvian WWTPs could provide 2530 t proteins, corresponding to 750 t protein-based fertilizer. Primary sludge contained 23.9% proteins, 9.1% lipids and 7.1% cellulose per TS. Primary sludge could provide 763 t/a carbohydrates, including 545 t/a cellulose. The currently available secondary and digested sludge would yield 727 t bioethanol, corresponding to 4.0% of the national biofuel consumption. This work applies the concept of resource recovery to the Latvian wastewater sector and shows the potential of simultaneously addressing waste and wastewater management issues.
... However, in circular economy paradigm, sewage sludge can be used for agricultural needs after post-treatment activities. For example, mixing sewage sludge with wood waste by composting allows you to get organic fertilizer [31][32][33][34]. ...
... Optimal conditions provide high efficiency in the growth of microorganisms [32]. According to Ekstrand et al. [33] composition of residual digestate fractions significantly The anaerobic digestion process depends on the parameters and technology implemented in the biogas plants. The most critical parameters of the process are temperature, pH, carbon-to-nitrogen ratio, organic loading rate, hydraulic retention time and feedstock composition. ...
... Optimal conditions provide high efficiency in the growth of microorganisms [32]. According to Ekstrand et al. [33] composition of residual digestate fractions significantly correlates with various operational and chemical parameters affecting the residual methane potential. ...
In agricultural biogas plants, besides biogas, the by-product digestate is also produced. Due to its high moisture content and organic origin, it can successfully be applied in the hydrothermal carbonization process to avoid the fate of landfilling. This paper reviews the properties of agricultural digestate and its hydrothermal conversion (HTC) into hydrochar and process water. The type of feedstock and the parameters of the HTC process, such as temperature, pressure and residence time, affects the physical and chemical characteristics of hydrochar. Therefore, its possible application might be as a biofuel, fertilizer, soil improver, adsorber, or catalyst. In this paper, the properties of hydrochar derived from agricultural digestate are widely discussed.
... The measurement of this residual methane potential under lab-conditions compared to the initial methane potential from the feedstocks can help in evaluating full-scale process effectiveness giving an index of the biological efficiency achieved [5]. A well-digested feedstock will decrease GHG emissions associated with subsequent digestate storage reaching environmental goals [6]. ...
... The average daily amount of residual methane at the full-scale biogas plant (V CH4 , R ) is calculated using equation (6) as reported in literature [11][12][13]: ...
... In this study, significant differences among feedstock categories were not observed, even though the lowest average SMRY of BGP14-16 seems be associated with maize silage utilization at quite low OLR (2.70 ± 0.78 kgVS m − 3 day − 1 ) and high HRT (95 ± 27 days) (Supplementary Data S1). Similar SMRY (75 ± 29 L N CH 4 kgVS − 1 ) was measured by Ekstrand and co-workers in full-scale biogas plant fed with crop silage (46%), corn silage (26%), sugar beets (16%), cereals (12%) at OLR of 3 kgVS m − 3 day − 1 and 85 days of HRT [6]. ...
Agricultural biogas plants should operate at the highest biological efficiency, with a fast anaerobic conversion of organic feedstocks leading to a low Residual Methane Potential (RMP). Optimal biochemical conditions boost the metabolic activity of microorganisms, providing better process performances. In this study, RMP of 16 full scale agricultural biogas plants operating with different agri-based feedstock categories was used as indicator of biological efficiency and linked to their operational parameters such as Organic Loading Rate (OLR), Hydraulic Retention Time (HRT) and to chemical elements concentration in the digestate. RMP ranged between 1% and 26% of the total biomethane produced revealing a clear relation with HRT. The Principal Component Analysis (PCA) showed how specific trace elements supplementation, in the order Se > Mo > Co, and their concentration ratios (Ni/Co and Ni/Se) lead to RMP reduction in full-scale agricultural biogas plants with low HRT (<48 days) and high OLR (>3.61 kgVS m−3 day−1) mainly fed with manure and agro-industrial by-products.
... However, further efforts are needed to elucidate the antagonism/synergies that influence the properties of the final digestate. Accordingly, identifying the exogenous (i.e., substrate) and the endogenous (i.e., microbial-derived) compounds that persist in the digestate after AD can help in better understanding these pathways and enhancing their valorization [4,5]. ...
... The particulate OM (POM) of the microbial biomass is mainly recalcitrant, where its partial solubilization and conversion could influence the diversity of the SMP and the quality of the digested OM [4]. Fluorescence spectroscopy is a relevant tool not only for identifying SMP but also for assessing the complexity level of a sample. ...
Efforts to characterize the organic matter (OM) properties in anaerobic digestates, comprising recalcitrant substrate (exogenous) and microbial-derived (endogenous) compounds, require further investigation. This study evaluated the identification of endogenous compounds during the anaerobic digestion (AD) of well-defined substrates with a subsequent starvation period. Substrates with different biodegradability levels and inocula from diverse origins were tested. Then, the digested OM was analyzed via ISBAMO fractionation and mid-infrared spectroscopy. The analysis of extracted fractions from the particulate OM of the inoculum revealed potential transfers of compounds to the dissolved OM after AD. The fluorescence complexity index of the dissolved OM fraction globally increased after AD and starvation periods when simple substrates were employed. Blank tests indicated that this increase was likely due to prolonged endogenous phases. Contrarily, partially biodegradable substrates accumulated slowly biodegradable protein-like and impeded the identification of soluble microbial products (SMP) via 3D fluorescence regardless of the inoculum type. Despite the systems fed with fully biodegradable substrates enabled the characterization of SMP, their profiles were influenced by multiple factors such as the substrate and inoculum type, as well as the digestion period. However, the impact of the selected substrates on the particulate OM was not detectable using Fourier transform infrared spectroscopy ratios or ISBAMO fractionation. Overall, the identification of endogenous compounds during anaerobic biodegradation was explored, contributing to a better understanding of the composition of digestates that could be valorized.
Graphical Abstract
... Over time, high organic loading leads to an increase in the population of acidogenesis bacteria that produces acids. However, the methanogenic bacteria would not be able to consume the acids as quickly, resulting in the accumulation of acids causing a large proportion of organic matter to remain in the digestate after the treatment [14]. ...
Foaming has impacted the digestion process causing severe process failures. The
basis of foaming is often unclear due to multiple foaming compounds in a single system. This study aims to elucidate the effect of protein concentration on foaming in anaerobic sludge and cow manure (CM) digesters inoculated with synthetic wastewater as a substrate. Gelatine was used to alter the protein concentration in the digesters. The foaming tendency, biogas production, protein, and chemical oxygen demand (COD) removal were measured to comprehend the findings. The results show that the foaming tendency increased with protein concentration in anaerobic sludge digesters. It also had the highest biogas production. There was no evident process inhibition observed in the anaerobic sludge digester, even at higher protein and COD concentrations. For CM digester, the foaming tendency and biogas production increased to 9 g/L; however, the biogas production decreased with increased substrate protein concentration, indicating process inhibition. The results indicate that the biogas production, foaming tendency, COD, and protein removal increased steadily for both digesters. However, the trend decreased for CM digester at increased protein concentrations. This study also concluded that protein creates foaming when inoculated with anaerobic sludge and CM even at low substrate protein concentrations.
... Over time, high organic loading leads to an increase in the population of acidogenesis bacteria that produces acids. However, the methanogenic bacteria would not be able to consume the acids as quickly, resulting in the accumulation of acids causing a large proportion of organic matter to remain in the digestate after the treatment [14]. ...
Foaming has impacted the digestion process causing severe process failures. The basis of foaming is often unclear due to multiple foaming compounds in a single system. This study aims to elucidate the effect of protein concentration on foaming in anaerobic sludge and cow manure (CM) digesters inoculated with synthetic wastewater as a substrate. Gelatine was used to alter the protein concentration in the digesters. The foaming tendency, biogas production, protein, and chemical oxygen demand (COD) removal were measured to comprehend the findings. The results show that the foaming tendency increased with protein concentration in anaerobic sludge digesters. It also had the highest biogas production. There was no evident process inhibition observed in the anaerobic sludge digester, even at higher protein and COD concentrations. For CM digester, the foaming tendency and biogas production increased to 9 g/L; however, the biogas production decreased with increased substrate protein concentration, indicating process inhibition. The results indicate that the biogas production, foaming tendency, COD and protein removal increased steadily for both digesters. However, the trend decreased for CM digester at increased protein concentrations. This study also concluded that protein creates foaming when inoculated with anaerobic sludge and CM even at low substrate protein concentration.
... For example, digested effluent can be combined with other waste materials such as wood chip, straw or green wastes prior to composting to provide a pasteurized productthis is the best way to improve ecological safety and make valuable fertilizer. In addition, a significant amount of produced methane remains dissolved in the effluents -it can be recovered by special post-treatment techniques [22]- [25]. ...
