Anaerobic co-digestion of agricultural wastes toward circular bioeconomy

Recycling of domestic sludge cake as the inoculum of anaerobic digestion for kitchen waste and its benefits to carbon negativity

Organic waste recovery has been a concerning issue in line with resource conservation. In the present study, the kitchen waste of vegetables, fish, and beef was digested anaerobically using domestic sludge as the inoculum, the methane and carbon dioxide were monitored, and the environmental benefits of the anaerobic digestion (AD) process were evaluated. AD using sludge cake as the inoculum was shown to treat kitchen waste effectively. Raw beef was found to produce more gas than raw fish or vegetables. Investigations also indicated that celluloses within vegetables were not as readily biodegradable as the proteins in beef and fish. Moreover, cooking altered the protein structures in beef and fish, thus increasing methane production. Meanwhile, oil inhibited methane generation as carbon dioxide generation remained, implying that the hydrolysis and acedogenesis still proceeded in the digestion process containing oil. Anaerolineaceae and Synergistaceae are the two most abundant microbial species observed in an anaerobic digestion system. However, the carbon conversions to liquid (i.e., leachate), solid (i.e., digestate), and gaseous (i.e., methane and carbon dioxide) occurred in the AD process, showing a diverse transforming process from waste to reusable valuables. Moreover, the kitchen waste treatment by domestic sludge cake was shown to have positive effects on reducing carbon dioxide emissions compared to the conventional treatment of kitchen waste and domestic sludge. More environmental benefits could be expected if the resulting products (i.e., methane gas, leachate, digestate) were applied as an energy source, liquid fertilizers, and soil conditioners.

Towards a circular bioeconomy to produce methane by co-digestion of coffee and brewery waste using a mixture of anaerobic granular sludge and cattle manure as inoculum

Coffee processing wastes, such as solid (pulp and husk) and wastewater, co-digested with industrial brewery wastewater, serve as excellent substrates for generating methane in the anaerobic digestion process. This study compared methane production using different compositions of cattle manure (CM) and granular sludge from an Upflow Anaerobic Sludge Blanket (UASB) reactor used in poultry wastewater treatment (GS). Four anaerobic batch reactors (500 mL) were assembled, A (50% CM and 50% GS), B (60% CM and 40% GS), C (70% CM and 30% of GS) and D (60% CM and 40% GS). Equal concentrations of substrates were added to all reactors: pulp and husk pretreated by hydrothermolysis (1 g L-1), coffee (10 g COD L-1) and brewery (1.5 g COD L-1) wastewaters. Assays A, B and C were supplemented with 2 g L-1 of yeast extract, except for assay D. The reactors were operated at 37 °C and pH 7.0. In assay B, the highest CH4 production of 759.15 ± 19.20 mL CH4 g-1 TS was observed, possibly favored by the synergistic interactions between cellulolytic bacteria Christensenellaceae_R-7_group and Methanosaeta archaea, as inferred by genes encoding enzymes related to acetoclastic methanogenesis (acetyl-CoA synthetase). Consequently, the electricity production potential of assay B (45614.08 kWh-1 year-1) could meet the energy demand of a farm producing coffee and beer, contributing to a positive energy balance concerning methane generation.

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.

Biofilm cultivation of chlorella species. MUR 269 to treat anaerobic digestate food effluent (ADFE): Total ammonia nitrogen (TAN) concentrations effect

Microalgal-based treatment of anaerobic digestate food effluent (ADFE) has been found to be efficient and effective. However, turbidity and high total ammonia nitrogen (TAN)) content of ADFE is a major setback, requiring significant dilution. Although the possibility of growing microalgae in a high-strength ADFE with minimal dilution has been demonstrated in suspension cultures, such effluents remain highly turbid and affect the light path in suspension cultures. Here, the feasibility of growing Chlorella sp.MUR 269 in biofilm to treat ADFE with high TAN concentrations was investigated. Six different TAN concentrations in ADFE were evaluated for their effects on biofilm growth and nutrient removal by Chlorella sp. MUR 269 using the perfused biofilm technique. Biomass yields and productivities of this alga at various TAN concentrations (mg N NH3 L-1) were 55a (108 g m-2 and 9.80 g m-2 d-1)>100b > 200c = 300c = 500c > 1000d. Growth was inhibited, resulting in a 28% reduction in yield of Chlorella biofilm when this alga was grown at 1000 mg N NH3 L-1. A survey of the photosynthetic parameters reveals evidence of stress occurring in the following sequence: 55 < 100<200 < 300<1000. A significant nutrient removal was observed across various TAN concentrations. The removal pattern also followed the concentration gradients except COD, where the highest removal occurred at 500 mg N NH3 L-1. Higher removal rates were seen at higher nutrient concentrations and declined gradually over time. In general, our results indicated that the perfused biofilm strategy is efficient, minimizes water consumption, offers easy biomass harvesting, and better exposure to light. Therefore, it can be suitable for treating turbid and concentrated effluent with minimal treatment to reduce the TAN concentration.

Valorization of crop residues and animal wastes: Anaerobic co-digestion technology

To switch the over-reliance on fossil-based resources, curb environmental quality deterioration, and promote the use of renewable fuels, much attention has recently been directed toward the implementation of sustainable and environmentally benign 'waste-to-energy' technology exploiting a clean, inexhaustible, carbon-neutral, and renewable energy source, namely agricultural biomass. From this perspective, anaerobic co-digestion (AcoD) technology emerges as a potent and plausible approach to attain sustainable energy development, foster environmental sustainability, and, most importantly, circumvent the key challenges associated with mono-digestion. This review article provides a comprehensive overview of AcoD as a biochemical valorization pathway of crop residues and livestock manure for biogas production. Furthermore, this manuscript aims to assess the different biotic and abiotic parameters affecting co-digestion efficiency and present recent advancements in pretreatment technologies designed to enhance feedstock biodegradability and conversion rate. It can be concluded that the substantial quantities of crop residues and animal waste generated annually from agricultural practices represent valuable bioenergy resources that can contribute to meeting global targets for affordable renewable energy. Nevertheless, extensive and multidisciplinary research is needed to evolve the industrial-scale implementation of AcoD technology of livestock waste and crop residues, particularly when a pretreatment phase is included, and bridge the gap between small-scale studies and real-world applications.

Meso- and thermophilic posttreatment of press water coming from a thermophilic municipal solid waste digester

An efficient biogas production out of organic (waste) materials is important to contribute to a carbon-neutral future. In this study, thermophilic press water (PW) coming from an organic fraction of the municipal solid waste digester was further digested in a thermo- and mesophilic posttreatment approach using two semicontinuous 14 L digesters. The results showed that the PW can still have considerable high biogas potential-at least during the touristic high season in central Europe. The change in temperature led to an increase in volatile fatty acid concentrations and a decrease in biogas production in the mesophilic approach in the first days. However, the losses in biogas production at the beginning could be compensated thus there were no considerable differences in biogas production between thermo- and mesophilic posttreatment at the end of incubation. This can most probably be contributed to a change in the microbial community, and potentially problematic intermediates like valerate could be better degraded in the mesophilic reactor. Especially the abundance of representatives of the phylum Bacteroidota, like Fermentimonas spp., increased during mesophilic anaerobic digestion.

A comprehensive review on food waste anaerobic co-digestion: Research progress and tendencies

Food waste (FW) anaerobic digestion systems are prone to imbalance during long-term operation, and the imbalance mechanism is complex. Anaerobic co-digestion (AcoD) of FW and other substrates can overcome the performance limitations of single digestion, allowing for the mutual use of multiple wastes and resource recovery. Research on the AcoD of FW has been widely conducted and successfully applied to a practical engineering scale. Therefore, this review describes the research progress of AcoD of FW with other substrates. By analyzing the problems and challenges faced by AcoD of FW, the synergistic effects and influencing factors of different biomass wastes are discussed, and improvement strategies to improve the performance of AcoD of FW are summarized from different reaction stages of anaerobic digestion. By combing the research progress of AcoD of FW, it provides a reference for the optimization and improvement of the performance of the co-digestion system.

Organic waste derived bioethanol supply chain network: Multiobjective snapshot model with a real-Korea case study

Bioethanol, a promising biofuel gasoline additive, was recently produced by a new technology using acetic acid derived from organic waste. This study develops a multiobjective mathematical model with two competing minimization objectives: economy and environmental impact. The formulation is based on a mixed integer linear programming approach. The configuration of the organic-waste (OW)-based bioethanol supply chain network is optimized in terms of the number and locations of bioethanol refineries. The flows of acetic acid and bioethanol between the geographical nodes must meet the bioethanol regional demand. The model is validated in three real-scenario case studies with different OW utilization rates (30%, 50%, and 70%) in South Korea in the near future (2030). The multiobjective problem is solved using the ε-constraint method and the selected Pareto solutions balance the trade-off between the economic and environmental objectives. At the "best-choice" solution points, increasing the OW utilization rate from 30% to 70% decreased the total annual cost from 904.2 to 707.3 million $/yr and the total greenhouse emissions from 1087.2 to -15.7 CO₂ equiv./yr.

Persistence and remote sensing of agri-food wastes in the environment: Current state and perspectives

Food demand is expected to increase globally by 60-110% from 2005 to 2050 due to diet shifts and population growth. This growth in food demand leads to the generation of enormous agri-food wastes (AFWs), which could be classified into pre-consumption and post-consumption. The AFW represents economic losses for all stakeholders along food supply chains, including consumers. It is reported that the direct financial, social, and environmental costs of food waste are 1, 0.9, and 0.7 trillion USD/year, respectively. Diverse conventional AFW management approaches are employed at the different life cycle levels (entre supply chain). The review indicates that inadequate transportation, erroneous packaging, improper storage, losses during processing, contamination, issues with handling, and expiry dates are the main reason for the generation of AFWs in the supply chain. Further, various variables such as cultural, societal, personal, and behavioral factors contribute to the AFW generation. The selection of a specific valorization technology is based on multiple physicochemical and biological parameters. Furthermore, other factors like heterogeneity of the AFWs, preferable energy carriers, by-products management, cost, end-usage applications, and environmental legislative and disposal processes also play a crucial role in adopting suitable technology. Valorization of AFW could significantly impact both economy and the environment. AFWs have been widely investigated for the development of engineered added-value biomaterials and renewable energy production. Considering this, this study has been carried out to highlight the significance of AFW cost, aggregation, quantification, and membrane-based strategies for its management. The study also explored the satellite remote sensing data for Spatio-temporal monitoring, mapping, optimization, and management of AFW management. Along with this, the study also explained the most recent strategies for AFW valorization and outlined the detailed policy recommendation along with opportunities and challenges. The review suggested that AFW should be managed using a triple-bottom-line strategy (economic, social, and environmental sustainability).

Energy and Nutrients from Apple Waste Using Anaerobic Digestion and Membrane Technology

The worldwide increment of food waste requires innovative management solutions, aligned with sustainability, energy, and food security. Anaerobic digestion (AD), followed by nutrient recovery, may be considered an interesting approach. This study proposed a co-digestion of apple pomace (AP) with swine manure (SM) to study the effect of different proportions of AP (0, 7.5, 15, and 30%, on a volatile solids (VS) basis) on the methane production and the stability of the process. Subsequently, the gas-permeable membrane (GPM) technology was applied to recover nitrogen (N) as ammonium sulfate (bio-based fertilizer) from the digestates produced after the AD of 7.5% of AP and SM, and SM alone. The results showed that the co-digestion of 7.5% and 15% of AP with SM presented a methane production similar to the AD of SM alone (with 412.3 ± 62.6, 381.8 ± 134.1, and 421.7 ± 153.6 mL g VS^-1 day^-1, respectively). The later application of the GPM technology on the resulting digestates, with SM alone and with 7.5% of AP with SM, showed total ammoniacal N recovery rates of 33 and 25.8 g N m^-2 d^-1, respectively. Therefore, the AP valorization through the AD process, followed by N recovery from the digestate, could be a good management strategy.

Investigating microbial dynamics and potential advantages of anaerobic co-digestion of cheese whey and poultry slaughterhouse wastewaters

Resource recovery and prevention of environmental pollution are key goals for sustainable development. It is widely reported that agro-industrial activities are responsible for the discharge of billions of liters of wastewater to the environment. Anaerobic digestion of these energy rich agro-industrial wastewaters can simultaneously mitigate environmental pollution and recover embedded energy as methane gas. In this study, an assessment of mono- and co-digestion of cheese whey wastewater (CWW) and poultry slaughterhouse wastewater (PSW) was conducted in 2.25-L lab-scale anaerobic digesters. Treatment combinations evaluated included CWW (R1), PSW (R2), 75:25 CWW:PSW (R3), 25:75 CWW:PSW (R4), and 50:50 CWW:PSW (R5). The digestion efficiencies of the mixed wastewaters were compared to the weighted efficiencies of the corresponding combined mono-digested samples. R4, with a mixture of 25% CWW and 75% PSW, achieved the greatest treatment efficiency. This corresponded with an average biodegradability of 84%, which was greater than for R1 and R2 at 68.5 and 71.9%, respectively. Similarly, R4 produced the highest average cumulative methane value compared to R1 and R2 at 1.22× and 1.39× for similar COD loading, respectively. The modified Gompertz model provided the best fit for the obtained methane production data, with lag time decreasing over progressive treatment cycles. PCoA and heatmap analysis of relative microbial abundances indicated a divergence of microbial communities based on feed type over the treatment cycles. Microbial community analysis showed that genus Petrimonas attained the highest relative abundance (RA) at up to 38.9% in the first two cycles, then subsequently decreased to near 0% for all reactors. Syntrophomonas was highly abundant in PSW reactors, reaching up to 36% RA. Acinetobacter was present mostly in CWW reactors with a RA reaching 56.5%. The methanogenic community was dominated by Methanothrix (84.3–99.9% of archaea). The presence of phosphate and Acinetobacter in CWW feed appeared to reduce the treatment efficiency of associated reactors. Despite Acinetobacter being strictly aerobic, previous and current results indicate its survival under anaerobic conditions, with the storage of phosphate likely playing a key role in its ability to scavenge acetate during the digestion process.

A waste-to-wealth initiative exploiting the potential of Anabaena variabilis for designing an integrated biorefinery

The current research work was an innovative approach providing dual advantages of waste bioremediation and an effective biorefinery. The study attempted to exploit wastewater like aqua discharge and solid wastes like poultry litter/cow dung for cyanobacterial cultivation. Aqua discharge appended with 7.5 g L⁻¹ poultry litter turned out as the best combination generating 46% higher carbohydrate yield than BG-11 control. A. variabilis cultivation in this waste-utilized medium also revealed its excellent bioremediation ability. While 100% removal was observed for nitrite, nitrate, and orthophosphate, a respective 74% and 81% reduction was noted for ammonium and total organic carbon. Chemical and biological oxygen demands were also reduced by 90%. This work was also novel in developing a sequential design for the production of bioethanol and co-products like exopolysaccharides, sodium copper chlorophyllin, C-phycocyanin, and poly-β-hydroxybutyrate from the same cyanobacterial biomass. The developed biorefinery implementing the waste-utilized medium was one of its kind, enabling biomass valorization of 61%. Therefore, the present study would provide a leading-edge for tackling the high production costs that limit the practical viability of biorefinery projects. The recyclability of the bioremediated wastewater would not only curtail freshwater usage, the waste disposal concerns would also be mitigated to a great extent.

The challenges and perspectives for anaerobic digestion of animal waste and fertilizer application of the digestate

The increase in livestock production creates a serious problem of managing animal waste and by-products. Among the wide range of waste valorization methods available, anaerobic digestion is very promising. It is a form of material recycling that also produces renewable energy in the form of biogas, which is reminiscent of energy recycling. The effluent and digestate from the anaerobic digestion process need to be processed further. These materials are widely used in agriculture due to their composition. Both the liquid and solid fractions of digestate are high in nitrogen, making them a valuable source for plants. Before soil or foliar application, conditioning (e.g., with inorganic acids) and neutralization (e.g., with potassium hydroxide) is required to eliminate odorous compounds and microorganisms. Various methods of conducting the process by anaerobic digestion (use of additives increasing activity of microorganisms, co-digestion, multiple techniques of substrate preparation) and the possibility of controlling process parameters such as optimal C/N ratio (15-30), optimal temperature (psychrophilic (<20 °C), mesophilic (35-37 °C) and thermophilic (55 °C) for microorganism activity ensure high efficiency of the process. Literature data describing tests of various digestates on different plants prove high efficiency, determined by yield increase (even by 28%), nitrogen uptake (by 20%) or phosphorus recovery rate (by 43%) or increase of biometric parameters (e.g., leaf area).

Valorization of agro-industrial wastes for biorefinery process and circular bioeconomy: A critical review

Energy recovery from waste resources is a promising approach towards environmental consequences. In the prospect of environmental sustainability, utilization of agro-industrial waste residues as feedstock for biorefinery processes have gained widespread attention. In the agro-industry, various biomasses are exposed to different unit processes for offering value to various agro-industrial waste materials. Agro-industrial wastes can generate a substantial amount of valuable products such as fuels, chemicals, energy, electricity, and by-products. This paper reviews the methodologies for valorization of agro-industrial wastes and their exploitation for generation of renewable energy products. In addition, management of agro-industrial wastes and products from agro-industrial wastes have been elaborated. The waste biorefinery process using agro-industrial wastes does not only offer energy, it also offers environmentally sustainable modes, which address effective management of waste streams. This review aims to highlight the cascading use of biomass from agro-industrial wastes into the systemic approach for economic development.

Indicative Marker Microbiome Structures Deduced from the Taxonomic Inventory of 67 Full-Scale Anaerobic Digesters of 49 Agricultural Biogas Plants

There are almost 9500 biogas plants in Germany, which are predominantly operated with energy crops and residues from livestock husbandry over the last two decades. In the future, biogas plants must be enabled to use a much broader range of input materials in a flexible and demand-oriented manner. Hence, the microbial communities will be exposed to frequently varying process conditions, while an overall stable process must be ensured. To accompany this transition, there is the need to better understand how biogas microbiomes respond to management measures and how these responses affect the process efficiency. Therefore, 67 microbiomes originating from 49 agricultural, full-scale biogas plants were taxonomically investigated by 16S rRNA gene amplicon sequencing. These microbiomes were separated into three distinct clusters and one group of outliers, which are characterized by a specific distribution of 253 indicative taxa and their relative abundances. These indicative taxa seem to be adapted to specific process conditions which result from a different biogas plant operation. Based on these results, it seems to be possible to deduce/assess the general process condition of a biogas digester based solely on the microbiome structure, in particular on the distribution of specific indicative taxa, and without knowing the corresponding operational and chemical process parameters. Perspectively, this could allow the development of detection systems and advanced process models considering the microbial diversity.