Life cycle assessment of integrated solid state anaerobic digestion and composting for on-farm organic residues treatment

Sustainable valorization of macroalgae residual biomass, optimization of pyrolysis parameters and life cycle assessment

The major challenges for the current climate change issue are an increase in global energy demand, a limited supply of fossil fuels, and increasing carbon footprints from fossil fuels, which have necessitated the exploration of sustainable alternatives to fossil fuels. Biorefineries offer a promising path to sustainable fuel production, converting biomass into biofuels using diverse technologies. Aquatic biomass, such as macroalgae in this context, represents an abundant and renewable biomass resource that can be cultivated from water bodies without competing with traditional agricultural land. Despite this, the potential of macroalgae for biofuel production remains largely untapped, with very limited studies addressing their viability and efficiency. This study investigates the efficient conversion of unexplored macroalgae biomass through a biorefinery process that involves lipid extraction to produce biodiesel, along with the production of biochar and bio-oil from the pyrolysis of residual biomass. To improve the effectiveness and overall performance of the pyrolysis system, Response Surface Methodology (RSM) was utilized through a Box-Behnken design to systematically investigate how alterations in temperature, reaction time, and catalyst concentration influence the production of bio-oil and biochar to maximize their yields. The results showed the highest bio-oil yield achieved to be 36 %, while the highest biochar yield reached 45 %. The integration of Life Cycle Assessment (LCA) in the study helps to assess carbon emission and environmental burdens and identify potential areas for optimization, such as resource efficiency, waste management, and energy utilization. The LCA results contribute to the identification of potential environmental hotspots and guide the development of strategies to optimize the overall sustainability of the biofuel production process. The LCA results indicate that the solvent (chloroform) used in transesterification contributes significantly to greenhouse gas emissions and climate change impacts. Therefore, it is crucial to explore alternative, safe solvents that can mitigate the environmental impacts of transesterification.

Nutrient recovery from digestate: Pilot test experiments

A series of technologies have been employed in pilot-scale to process digestate, i.e. the byproduct remaining after the anaerobic digestion of agricultural and other wastes, with the aim of recovering nutrients and reducing the load of solids and organics from it, hence improving the quality of digestate for potential subsequent reuse. In this case the digestate originated from a mixture of dairy and animal wastes and a small amount of agricultural wastes. It was processed by the application of several treatments, applied in series, i.e. microfiltration, ultrafiltration, reverse osmosis, selective electrodialysis and combined UV/ozonation. The initially applied membrane filtration methods (micro- and ultra-filtration) removed most of the suspended solids and macromolecules with a combined efficiency of more than 80%, while the reverse osmosis (at the end) removed almost all the remaining solutes (85-100%), producing sufficiently clarified water, appropriate for potential reuse. In the selective electrodialysis unit over 95% of ammonium and potassium were recovered from the feed, along with 55% of the phosphates. Of the latter, 75% was retrieved in the form of struvite.

Livestock waste management for energy recovery in Brazil: a life cycle assessment approach

Livestock farming has exerted intense environmental pressure on our planet. The high emissions to the environment and the high demands of resources for the production process have encouraged the search for decarbonization and circularity in the livestock sector. In this context, the objective of this study was to evaluate and compare the environmental performance of two different uses for biogas generated in the anaerobic digestion of animal waste, either for electricity generation or biomethane. For this purpose, a life cycle assessment approach was applied to evaluate the potential of anaerobic digestion as a management technology for three different livestock wastes, related to beef cattle, dairy, and sheep in the Brazilian animal production context. The results suggest that the treatment scenarios focusing on biomethane generation were able to mitigate the highest percentage of damages (77 to 108%) in the global warming category when compared to the scenarios without the use of anaerobic digestion (3.00·102 to 3.71·103 kgCO2 eq) or in the perspective of electricity generation (mitigation of 74 to 96%). In terms of freshwater eutrophication, the generation of electricity (- 2.17·10-2 to 2.31·10-3 kg P eq) is more favorable than the purification of biogas to biomethane (- 1.73·10-2 to 2.44·10-3 kg P eq), due to the loss of methane in the upgrading process. In terms of terrestrial ecotoxicity, all scenarios are very similar, with negative values (- 1.19·101 to - 7.17·102 kg 1,4-DCB) due to the benefit of nutrient recovery, especially nitrogen, associated with the use of digestate as fertilizer, which was one of the critical points in all scenarios. Based on these results, it is evident that proper management of all stages of the treatment life cycle is the key to decarbonization and circularity in livestock waste management. The biogas use does not present different effects on the environmental performance of the scenarios studied, demonstrating that the purpose should be chosen according to the needs of each plant or management system.

A Critical Review of Data Science Applications in Resource Recovery and Carbon Capture from Organic Waste

Municipal and agricultural organic waste can be treated to recover energy, nutrients, and carbon through resource recovery and carbon capture (RRCC) technologies such as anaerobic digestion, struvite precipitation, and pyrolysis. Data science could benefit such technologies by improving their efficiency through data-driven process modeling along with reducing environmental and economic burdens via life cycle assessment (LCA) and techno-economic analysis (TEA), respectively. We critically reviewed 616 peer-reviewed articles on the use of data science in RRCC published during 2002-2022. Although applications of machine learning (ML) methods have drastically increased over time for modeling RRCC technologies, the reviewed studies exhibited significant knowledge gaps at various model development stages. In terms of sustainability, an increasing number of studies included LCA with TEA to quantify both environmental and economic impacts of RRCC. Integration of ML methods with LCA and TEA has the potential to cost-effectively investigate the trade-off between efficiency and sustainability of RRCC, although the literature lacked such integration of techniques. Therefore, we propose an integrated data science framework to inform efficient and sustainable RRCC from organic waste based on the review. Overall, the findings from this review can inform practitioners about the effective utilization of various data science methods for real-world implementation of RRCC technologies.

Environmental impacts and optimization simulation of aerobic anaerobic combination treatment technology for food waste with life cycle assessment

After the implementation of waste sorting policy in Shanghai, the amount of food waste (FW) separation and treatment demand has increased significantly. It is necessary to establish the life cycle assessment (LCA) to assess the environmental impacts of various treatment technologies comprehensively, thus provide support for sorting, recycling, treatment and disposal strategies of FW. In this study, a local FW treatment plant in Shanghai, using typically aerobic anaerobic combination treatment technology was selected to analyze the environmental impacts with LCA. The process mainly included pretreatment, power, aerobic composting, anaerobic digestion, and further process systems. LCA results showed that the environmental impacts mainly came from the power and aerobic composting systems on the fine particulate matter formation and eutrophication, and freshwater ecotoxicity and terrestrial acidification, respectively. Considering the carbon footprint, the aerobic composting system contributed 3.61E + 02 kg CO₂ eq and represented the largest source of carbon emission. The soil conditioner yielded both environmental benefits on eutrophication and terrestrial ecotoxicity, and ecological benefits of 75.33 million CNY per year being the major revenue for the treatment plant. It also suggested that the biogas generation capacity of anaerobic digestion could be increased to achieve electricity self-sufficiency, thus save about 7.12 million CNY per year in electricity costs, and avoid corresponding environmental impacts caused by coal-fired. In summary, the aerobic anaerobic combination treatment could be further optimized and applied in FW treatment to reduce the environmental impacts, and enhance resource recovery and secondary pollution control.

Composting Processes for Agricultural Waste Management: A Comprehensive Review

Composting is the most adaptable and fruitful method for managing biodegradable solid wastes; it is a crucial agricultural practice that contributes to recycling farm and agricultural wastes. Composting is profitable for various plant, animal, and synthetic wastes, from residential bins to large corporations. Composting and agricultural waste management (AWM) practices flourish in developing countries, especially Pakistan. Composting has advantages over other AWM practices, such as landfilling agricultural waste, which increases the potential for pollution of groundwater by leachate, while composting reduces water contamination. Furthermore, waste is burned, open-dumped on land surfaces, and disposed of into bodies of water, leading to environmental and global warming concerns. Among AWM practices, composting is an environment-friendly and cost-effective practice for agricultural waste disposal. This review investigates improved AWM via various conventional and emerging composting processes and stages: composting, underlying mechanisms, and factors that influence composting of discrete crop residue, municipal solid waste (MSW), and biomedical waste (BMW). Additionally, this review describes and compares conventional and emerging composting. In the conclusion, current trends and future composting possibilities are summarized and reviewed. Recent developments in composting for AWM are highlighted in this critical review; various recommendations are developed to aid its technological growth, recognize its advantages, and increase research interest in composting processes.

Optimizing the environmental sustainability of alternative post-harvest scenarios for fresh vegetables: A case study in Spain

The aim of this research is to define different scenarios that optimize the environmental sustainability of the post-harvest stage of vegetable products (cauliflower and brassicas mix). These scenarios considered different packaging materials; energy generation technologies for the processing plant (standard electricity mix vs. renewable options); organic waste management (composting, anaerobic digestion, and animal feeding); and refrigerated transportation (local, national, and international, using diesel, natural gas, and hybrid trucks and railway). The analysis has been carried out based on a foreground inventory provided by a company that operating internationally, in accordance with the International Organization for Standardization (ISO) 14,040 methodological framework and following the latest Product Environmental Footprint (PEF) protocols. The analysis describes four midpoint categories, single score (SS) using EF3.0 life cycle impact assessment (LCIA) methodology and the Cumulative Energy Demand. The carbon footprint (CF) of the post-harvest stage for a base case scenario ranged between 0.24 and 0.29 kg CO₂ eq/kg of vegetable, with a strong contribution associated to the production of packaging materials (57.8-65.2 %) and the transport stage (national range in conventional diesel vehicles) (31.5-38.0 %). Comparatively, lower emissions were associated with the energy consumed at the processing factory (up to 4.1 %) while the composting of organic waste management produced some impact savings (up to -3.5 %). Although certain differences were observed, the dominance of the transport stage and the packaging materials is sustained in all the other environmental impact and energy categories evaluated. The most effective measures to reduce the environmental footprint of the post-harvest stage involve: i) using reusable packaging materials; ii) reducing the transport range and using vehicles running on natural gas or hybrid technologies; iii) the incorporation of renewable energy to supply the factory; and iv) the utilization of the organic residues in higher value applications such as animal feeding. Implementing the measures proposed in this study would reduce the post-harvest CF of fresh vegetables by 90 %.

Swine manure treatment technologies as drivers for circular economy in agribusiness: A techno-economic and life cycle assessment approach

Anaerobic digestion has been employed as a technology capable of adding value to waste coupled with environmental impact mitigation. However, many issues need to be elucidated to ensure the systems viability based on this technology. In this sense, the present study evaluated technically, environmentally, and economically, four configurations of swine waste treatment systems focused on the promotion of decarbonization and circularity of the swine chain. For this, a reference plant, based on a compact treatment process named SISTRATES® (Portuguese acronym for swine effluent treatment system) was adopted to serve as a model for comparison and validation. The results showed the importance of prioritization of the energy recuperation routes through anaerobic digestion, providing increased economic benefits and minimizing environmental damage. Thus, the SISTRATES® configuration was the one that presented the best designs in a circular context, maximizing the recovery of energy and nutrients, along with the reduction of greenhouse gas emissions, ensuring the sustainability of the pig production chain.

Environmental sustainability analysis of dairy bedding regeneration system based on emergy evaluation and life cycle assessment methods

Dairy farm bedding can be produced by composting technology using dairy manure, which offers advantages in terms of cost, availability, and economic value. However, few information is available on the environmental sustainability and impacts for manure recycling systems based on different composting methods. The resource-environmental impact and eco-economic sustainability of two manure bedding regeneration systems: forced-ventilation static-stack aerobic fermentation (FVSSAF) system (Scenario A) and bedding recovery unit (BRU) system (Scenario B) were evaluated in this study. The life cycle assessment yielded a combined environmental impact potential of 0.01032 for scenario B, much lower than the 0.02656 for scenario A. The emergy evaluation showed that scenario B can handle more dairy manure than scenario A due to 57% increase of emergy input. Form the emergy indices of the two systems, scenario B had lighter environmental pressure and higher sustainability. Therefore, the BRU system had economic advantages and ecological sustainability, which was more suitable for large dairy farms. The trade-offs between environmental consequences, resource efficiency, and economic benefits were analyzed from several perspectives in this study, which would help stakeholders to have a new understanding when choosing a bedding recycling system.

Bioreactor-scale production of rhamnolipids from food waste digestate and its recirculation into anaerobic digestion for enhanced process performance: Creating closed-loop integrated biorefinery framework

Reaching industrially relevant productivities in bioprocesses and their efficient integration in the existing industrial infrastructure remain as important challenges in the circular economy to create closed loop sustainability framework. Using anaerobic digestion (AD) biorefinery as a model, the present work addressed these problems via integration of next-generation rhamnolipids production with AD. A high rhamnolipids concentration of 10.25 ± 1.34 g/L was obtained by fed-batch fermentation using food waste digestate as medium. Digestate-derived rhamnolipids contained Rha-C₁₀-C₁₀ and Rha-Rha-C₁₀-C₁₀ as the predominant congeners. These were used back in single-phase AD to demonstrate their effect on sludge solubilization and digestion efficiency. A dosage of 0.02 g rhamnolipids/g total suspended solids was found to be optimal which enhanced the hydrolysis-acidogenesis reactions to up to 27% over control. It however retarded methane production which could be overcome by the prolongation of digestion time. Finally, the value chain appreciation by the proposed process was demonstrated by a feasibility analysis.

A comparative environmental life cycle assessment of rice straw-based bioenergy projects in China

Bioenergy is a promising solution for greenhouse gas (GHG) emissions mitigation. However, the emissions resulting from the different production stages must be quantified and evaluated. The life cycle assessment (LCA) method was used to compare and quantify the environmental burdens of three rice straw (RS) utilization scenarios for producing biogas, briquette fuel, and syngas. To our knowledge, this is the first study that applies the LCA approach to assess these three bioenergy scenarios in a single study where the main goal was to determine the most sustainable option. A total of 10 mid-point impact categories were investigated. The results indicated that the three scenarios achieved net positive energy and net negative GHG balances. The briquette fuel scenarios had the highest net energy balance (11,115 MJ/tonne dry RS), while the syngas scenario had the highest net GHG (-2,315 kg CO₂-eq./tonne dry RS). Moreover, the syngas scenario was the most beneficial to the environment, achieving negative results in 9 out of the 10 impact categories; the largest was marine ecotoxicity (-853,897 kg 1,4-DB-eq./tonne dry RS). The biogas scenario achieved emission savings in 3 out of the 10 categories. Although the briquette fuel scenario had no negative values in the 10 categories, its overall contribution to environmental burdens was relatively low. Overall, the order of the three scenarios in terms of the most sustainable option is syngas > briquette fuel > biogas.

High-rate in-vessel continuous composting of olive mill byproducts

The increasing production of agro-industrial organic residues in modern society is extremely concerning. One of the most polluting procedures in the agricultural industry is the production of olive oil. This process creates a large amount of waste with high organic load and phytotoxic components. In this study, composting of two-phase olive pomace (OP), olive leaves (OL) and dewatered anaerobic sludge (DAS) from an olive mill wastewater anaerobic digestion process was conducted in a pilot-scale in-vessel high-rate continuous composter. Five different feed scenarios were studied with different OP/OL ratio in the feed material, while the effect of the addition of pine tree bark pieces (PB) and DAS was examined. The OP:OL 95:5 % w/w ratio exhibited the best results in terms of product quality, while OL proved capable of acting as a bulking agent for the better aeration of the material. The final product in the optimum feed ratio was free of Salmonella spp., was stable in terms of static respiratory index (lower than 0.5 g O₂ kg^-1 VS h^-1) but contained elevated E. coli levels (3.5 × 10⁴ CFU g^-1 with a limit of 1 × 10³ CFU g^-1), which was the only EU proposed compost quality criteria not met. The addition of a more easily degradable material in the feed mixture is expected to lead to elevated composting temperature and amend the presence of pathogens.

Pathways of Nitrogen and Phosphorus Utilization and Removal from Cyanobacteria Wastewater by Combining Constructed Wetlands with Aerobic Reactors

Due to its low C/N ratio and high concentrations of nitrogen and phosphorus, the effluent of anaerobic cyanobacteria fermentation cannot be directly discharged without further treatment. To effectively reduce nutrient loads and utilize the nutrient resources of biogas slurry generated from the anaerobic digestion of stored algae, two different aerobic treatment units (AUs) were combined with an ecological treatment unit (EU) to create two different treatment systems. The two AUEU systems paired a constructed wetland (CW) with either a cascade biological contact reactor (CBCR) or a carrousel oxidation ditch reactor (CODR). In this paper, the water quality characteristics of biogas slurry were measured, and comprehensive experiments on the two trial-treatment systems were carried out to validate their performance in removing pollutants and utilizing resources. Furthermore, the pollutant removal efficiencies of the combined systems, along with the removal mechanisms and utilization of the nitrogen and phosphorus in the CWs, were also investigated. The results showed that the CWs, with aquatic vegetation, took up the majority of removed nitrogen and phosphorus by absorption, which effectively reduced the concentration of pollutants in the effluent and enabled the nitrogen and phosphorus to be reused in plants. Biomass assimilation by the absorption by vegetation took up 75.8%, 66.1%, 70.3%, and 86% of the removed NH4+-N, NOx−-N, TN, and TP, respectively.

Environmental Life Cycle Assessments of Chicken Manure Compost Using Tobacco Residue, Mushroom Bran, and Biochar as Additives

As an environmental management method, the (life cycle assessment) LCA method can be used to compare the differences between various waste treatment processes in order to provide an environmentally friendly and economically feasible method for waste management. This study focused on the reutilization of typical organic waste to produce organic fertilizer in southwest China and used the life cycle assessment method to evaluate three aerobic chicken manure composting scenarios modified with three additives (biochar, mushroom bran, and tobacco residue) from an environmental and economic perspective. The results show that the total environmental loads of the optimized treatments using mushroom bran and biochar mixed with mushroom bran as additives were reduced by 30.0% and 35.1%, respectively, compared to the control treatment (viz. chicken manure composted with tobacco residue). Compared to the control treatment, the optimized composting treatment modified by mushroom bran with and without biochar improved the profit by 23.9% and 35.4%, respectively. This work reflected that the combined composting mode of chicken manure, tobacco residue, mushroom bran, and biochar is an environmentally friendly and economically feasible composting process, which is more suitable for the resource utilization of the typical organic waste in southwest China.

Research trend analysis of composting based on Web of Science database

Bibliometric analysis was used in this study for the quantitative evaluation of current research trends on composting. The research articles indexed from the Science Citation Index-Expanded in Web of Science database published from 2000 to 2019 were investigated. The USA, China and Spain were the top three countries considering the number of papers. Amongst the research institutes, CSIC of Spain, Chinese Academy of Sciences and Agriculture & Agri-Food Canada ranked the top three in total publication amount. Journals that published a significant number of literature regarding topics of composting included Environmental Sciences & Ecology, Agriculture and Engineering. In terms of research content, keywords such as heavy metal, heavy metal and biodegradation appeared frequently. In addition, the analysis of keywords revealed the following research hotspots in future studies: investigation of heavy metal passivator, optimisation of composting conditioner, development of all kinds of microorganisms, rational management of the composting process and improvement of solid waste life cycle assessment. To some extent, it helps to understand the current global status and trends of the related research.

Poultry litter stabilization by two-stage composting-vermicomposting process: Environmental, energetic and economic performance

Poultry litter (PL) is a heterogeneous mixture that contains bedding materials, antibiotics, dead skin, feed scraps, water, feathers and the resulting microbiota from poultry production cycle. Although its treatment does not receive attention, it is an important organic resource generated in the north-east region of Santa Fe Province since animal primary production is one of the main economic activities there and in the whole country. The objective of this work was to analyse the economic and energy aspects and the environmental impact of two scenarios corresponding to treat two different mixtures of organic wastes: Eucalyptus sp. sawdust (ES), rice hulls (RH) and PL. PL was considered in two different volume proportions: 1RH:3 PL for scenario 1 and 0.5RH:1 ES:2 PL for scenario 2. The two-stage combined system of composting + vermicomposting was applied to both scenarios and compared; on one hand, the current regulated practice for waste final deposition (landfill) and on the other hand, a base line scenario, which describes the current situation, where the raw material is transported and spread over the field as fertilizer without previous stabilization. The scenario construction was based according to on-site data, lab-scale experiments previously published and software databases. The environmental study was carried out with life cycle assessment; and the energy study, using cumulative energy demand methodology and the energy balance. Based on the results, it was possible to affirm the importance of the integrated agricultural waste treatment to address environmental benefits, especially related to the base line scenario. Furthermore, there were no significant differences between the two proposed scenarios. Economic and environmental results were contradictory when no long-term socio-economic consequences are considered. Combined composting and vermicomposting strategy reduce the organic matter and nutrients content not only to avoid discharges into the environment, but also to replace the fossil fuels consumption during unsustainable techniques as sanitary landfill, producing improved substrates for agricultural use.

Life cycle assessment of the integration of anaerobic digestion and pyrolysis for treatment of municipal solid waste

The integration of anaerobic digestion (AD) and pyrolysis (Py) could be a solution to economically utilize the organic fraction of municipal solid waste (OFMSW). However, it is not clear whether the environmental impact of the integrated pathway always outperforms the two single technologies. In this study, two integrated pathways (AD-Py, Py-AD) were compared with single AD and Py from the life cycle environmental impacts point of view. The results indicate that the environmental impacts of the four pathways are heavily dependent on their energy inputs and outputs. AD-Py is more environmentally friendly (-11.53 of total environmental impact /kg OFMSW) than single AD or Py. Py-AD exhibites the heaviest environmental burden (2.75 of total environmental impact /kg OFMSW) in all pathways. Therefore, AD-Py can be the top priority of treating OFMSW among the four pathways from the environmental viewpoint. This work could provide a theoretical support for the utilization of OFMSW.

Anaerobic digestion of agricultural wastes from liquid to solid state: Performance and environ-economic comparison

Performance and environ-economic impacts were compared for anaerobic digestion (AD) of dairy manure and cucumber residues from liquid to solid state. Environ-economic evaluation of the overall AD process at different total solids (TS) was performed with the estimated treatment capacity of 9600 tons/year and service life of 20 years. Results showed that TS increase from 6% to 22% enhanced both cumulative and volumetric methane (CH₄) production. Further TS increase to 25%, however, reduced CH₄ yield. Environ-economic assessment indicated that TS increase enhanced volumetric waste treatment capacity and thus AD environmental footprints. Environmental credits from digestate and biogas utilization could compensate the adverse environmental impacts of other processes in AD plants. Furthermore, biogas and nutrients in digestate determined AD net-present value. As a result, solid state AD was more profitable with higher CH₄ yield and more nutrients in both biosolids and digested effluent of digestate than its liquid and hemi-solid counterparts.

Solid digestate disposal strategies to reduce the environmental impact and energy consumption of food waste-based biogas systems

This study examined the environmental impacts and energy consumption of three solid digestate treatment scenarios to quantify their impacts on the entire food waste (FW)-based biogas system: (1) incineration; (2) composting, and; (3) landfill. The results showed that composting had the largest net energy consumption, but least total environmental impact of 57.3 kWh and 8.75 E-03, respectively, whereas landfill showed the opposite pattern. Moreover, there were significant differences (p < 0.05) and relatively high contributions between the digestate treatment subunits among the three scenarios. The most significant contributions of digestate subunits in methods 1-3 to the 100-year global warming potential (GWP₁₀₀) were 70.5%, 52.5%, and 103.4%, respectively. The results indicated that solid digestate treatment had a significant impact, and reasonable disposal of solid digestate could significantly reduce the environmental impacts and energy consumption of the entire FW-based biogas system.

Linking Perceived Policy Effectiveness and Proenvironmental Behavior: The Influence of Attitude, Implementation Intention, and Knowledge

Residents' behavior is the result of the combined effect of external environment factors and internal psychological factors. Based on the theory of planned behavior (TPB) and the attitude-behavior-condition (ABC) theory, this study aims to explore the impact of policy support on residents' psychological factors and proenvironmental behavior. This study developed an extended TPB and ABC model and replaced the behavioral intention in the TPB model with implementation intentions to enhance the ability of the variables to explain and predict proenvironmental behavior. The longitudinal research method was adopted to collect data through a two-stage questionnaire survey of 1145 Shanghai residents. Results demonstrated that perceived policy effectiveness has a significant and positive impact on attitude, implementation intention, and proenvironmental behavior. This means that proenvironmental behavior tends to appear in people with a high perception of policy effectiveness, positive attitude, and strong implementation intention. Moreover, this study points out for the first time that high waste management knowledge weakens the relationship between perceived policy effectiveness and attitude. For residents with high waste management knowledge, the effect of simple policy publicity is limited. The findings suggest that the government should increase the breadth and depth of policy support and policy publicity to cover the entire waste management process.

Carbon footprint of anaerobic digestion combined with ultrasonic post-treatment of agro-industrial organic residues

Anaerobic digestion (AD) of organic waste, although widely practiced, may require suitable accompanying treatments to enhance the degradability of complex materials. Since these may require significant efforts in terms of energy and chemical demand, careful assessment of their overall environmental sustainability is mandatory to evaluate their full-scale feasibility. The study aims to represent the environmental profile of ultrasonication (US) applied as a post-treatment of anaerobic digestion of agro-industrial organic residues. There is an interest in the US treatment for the processing of complex organic materials prior to AD in order to enhance the hydrolysis of complex organic substrates and increase the biogas yield of the biological process. An attributional, process-based life cycle assessment (LCA) study was applied to quantify and compare the potential environmental impacts of an AD plant, the biogas utilization options as well as the different digestate processing alternatives grouped into a set of 16 scenarios. Based on the results, upgrading of biogas and bio-methane use as vehicle fuel instead of energy generation from CHP or fuel cell was recommended due to the lower impact on GWP. Similarly, composting was a suitable option to reduce environmental impacts compared to belt drying. From the uncertainty analysis, AD without US as post-treatment proves to be more sustainable in terms of GWP compared to when US is used, showing net savings in GHG emissions especially when upgrading of biogas is applied. The analysis provides useful indications to policy makers to define sustainable management alternatives for organic residues as well as identify the environmental advantages associated with biogas utilization and digestate treatment and disposal alternatives.

Greenhouse gas emissions from inorganic and organic fertilizer production and use: A review of emission factors and their variability

Fertilizers have become an essential part of our global food supply chain and are necessary to sustain our growing population. However, fertilizers can also contribute to greenhouse gas (GHG) emissions, along with other potential nutrient losses in the environment, e.g. through leaching. To reduce this environmental impact, tools such as life cycle assessments and decision support systems are being used to aid in selecting sustainable fertilization scenarios. These scenarios often include organic waste-derived amendments, such as manures, composts and digestates. To produce an accurate assessment and comparison of potential fertilization scenarios, these tools require emission factors (EFs) that are used to estimate GHG emissions and that are an integral part of these analyses. However, such EFs seem to be very variable in nature, thereby often resulting in high uncertainty on the outcomes of the analyses. This review aims to identify ranges and sources of variability in EFs to provide a better understanding of the potential uncertainty on the outcomes, as well as to provide recommendations for selecting EFs for future studies. As such, an extensive review of the literature on GHG emissions from production, storage, transportation and application of synthetic fertilizers (N, P, K), composts, digestates and manures was performed. This paper highlights the high variability that is present in emissions data and confirms the great impact of this uncertainty on the quality and validity of GHG predictions related to fertilizers. Variability in EFs stem from the energy source used for production, operating conditions, storage systems, crop and soil type, soil nutrient content, amount and method of fertilizer application, soil bacterial community, irrigation method, among others. Furthermore, a knowledge gap exists related to EFs for potassium fertilizers and waste valorization (anaerobic digestion/composting) processes. Overall, based on this review, it is recommended to determine EFs on a case by case basis when possible and to use uncertainty analyses as a tool to better understand the impact of EF variability.

GHG Emissions and Efficiency of Energy Generation through Anaerobic Fermentation of Wetland Biomass

We conducted the Life Cycle Analysis (LCA) of energy production from biogas for maize and three types of wetland biomass: reed Phragmites australis, sedges Carex elata, and Carex gracilis, and “grassy vegetation” of wet meadows (WM). Biogas energy produced from maize reached over 90 GJ ha−1, which was more than four times higher than that gained from wetland biomass. However, an estimation of energy efficiency (EE) calculated as a ratio of energy input to the energy produced in a biogas plant showed that the wet fermentation (WF) of maize was similar to the values obtained for dry fermentation (DF) of sedge biomass (~0.30 GJ GJ−1). The greenhouse gases (GHG) emissions released during preparation of the feedstock and operation of the biogas plant were 150 g CO2 eq. kWhel.−1 for DF of sedges and 262 g CO2 eq. kWhel.−1 for WF of Phragmites. Compared to the prevailing coal-based power generation in Central Europe, anaerobic digestion (AD) of wetland biomass could contribute to a reduction in GHG emissions by 74% to 85%. However, calculations covering the GHG emissions during the entire process “from field to field” seem to disqualify AD of conservation biomass as valid low-GHG energy supply technology. Estimated emissions ranged between 795 g CO2 eq. kWhel.−1 for DF of Phragmites and 2738 g CO2 eq. kWhel.−1 for the WM and, in most cases, exceeded those related to fossil fuel technologies.

Anaerobic digestion of different agricultural wastes: A techno-economic assessment

In this work, techno-economic evaluation of anaerobic digestion (AD) system (8000 metric tons (MT)/year) with singular (dairy manure), binary (dairy manure and corn stalk), and ternary mixture (dairy manure, corn stalk, and tomato residues) under bio-methane and combined heat and power (CHP) pathways based on a plant service life of 20 years were carried out. Solid state-AD (SS-AD) of ternary mixture improved the efficiency of investment, benefited the digestate price, and was shown to be economic viability. The introduction of a CHP unit highly improved the economics of SS-AD. SS-AD of the binary mixture under CHP pathway was able to compensate the initial required investment, however was not financially attractive under bio-methane pathway. Besides, SS-AD of the ternary mixture under CHP pathway had higher net present value (NPV) ($0.60 million vs $0.40 million) and internal rate of return (IRR) (23% vs 20%) than that under bio-methane pathway.

Anaerobic digestion effluent treatment using microalgae and nitrifiers in an outdoor raceway pond with fluidized carriers

Combining microalgae and nitrifiers in a single photobioreactor has attracted attention as an alternative approach for conventional nitrogen removal from wastewater. However, nitrifiers are known to be sensitive to light exposure. This study demonstrated the effectiveness of using fluidized carriers to mitigate light stress in nitrifiers. An outdoor raceway pond containing microalgae and nitrifiers with fluidized carriers was used to treat two-fold diluted anaerobic digestion effluent (785 mg-N L^-1 as a form of dissolved total Kjeldahl nitrogen: TKN) over 50 days. The average daily sunlight intensity reached the inhibition level of nitrifiers (423 μmol photons m^-2 s^-1); however, stable nitrification with a specific ammonium oxidation rate of 55 mg-N g-total suspended solid^-1 day^-1 was observed. TKN was mostly removed via nitrifier metabolism (ammonium oxidation and uptake: 40.1%) and partially via microalgae uptake (5.7%). Different microalgae-based processes including that of this study were compared in terms of tolerances to a high dissolved TKN concentration and strong light. Our system showed a relatively higher resistance to not only light exposure but also TKN because the nitrification process decreased the free ammonia level to less than 0.25 mg L^-1, which allowed microalgae to grow despite the high ammonium concentration.

Factors affecting gaseous emissions, maturity, and energy efficiency in composting of livestock manure digestate

This study investigated the interplay effects of key operational factors on maturity, gaseous emissions, and energy efficiency during composting of livestock manure digestate. Orthogonal experiments were conducted with three factors: digestion duration (15, 30, 45 days), corn stalk addition (15%, 25%, 35%, wet weight based), and aeration rate (0.12, 0.24, 0.48 L kg^-1 dry matter (DM) min^-1). Results showed that digestion duration was the main factor influencing the compost germination index (GI), greenhouse gases (GHGs) emission, and net energy. Digestion duration of 30 days was favored for compost GI and GHG reduction, while digestion duration of 45 days exhibited 18% higher daily net energy. Increasing corn stalk addition and aeration rates improved compost GI, but increased energy consumption. Corn stalk addition of 25% and aeration rate of 0.24 L kg^-1 DM min^-1 could ensure desirable compost maturity and save energy consumption. Thus, digestion for 30 days, 25% corn stalk addition, and aeration rates of 0.24 L kg^-1 DM min^-1 can be potentially implemented in industry for environmental and cost efficient composting of digestate.

Life cycle assessment of bio-based levoglucosan production from cotton straw through fast pyrolysis

This study aimed to evaluate the environmental impacts (i.e. global warming potential (GWP) and resource depletion (RD)) of the bio-based levoglucosan production process through fast pyrolysis of cotton straw via life cycle assessment (LCA). An LCA model consisting of feedstock transportation, biomass pretreatment, fast pyrolysis, bio-oil transportation, bio-oil recovery and levoglucosan extraction was developed. Results indicated that GWP and RD of bio-based levoglucosan production were approximately 2 and 32.5 times less than that of the petroleum-based counterpart. Sensitivity analysis showed that the GWP and RD of levoglucosan production were highly sensitive to plant size, hydrochloric acid usage, cooling energy, levoglucosan yield and bio-oil yield. The results of this research could provide a framework for robust decision making at an industrial level, which is useful for the commercial-scale production of levoglucosan.

Methane production from the co-digestion of pig manure and corn stover with the addition of cucumber residue: Role of the total solids content and feedstock-to-inoculum ratio

This study investigated performance and stability of increasing total solids (TS) content (10-30%) and feedstock-to-inoculum (F/I) ratios (1, 2) on anaerobic co-digestion of agricultural wastes. The cumulative methane yields generally decreased with the increasing TS content except for the TS content of 30% at F/I ratio of 1 and TS content of 10% at F/I ratio of 2. This was consistent with the maximum methane production rate (Rmax) and rate of the hydrolysis (Kh) stage in reactors. The pH, VFAs and NH4+-N content were positively correlated with increasing TS contents and F/I ratios. Economic analysis results indicated the net present value generally increased with increasing TS contents and TS content of 30% at F/I ratio of 1 had the highest net present value (5.7 million US$) and internal rate of return (41.9%). This indicated solid-state anaerobic digestion was financially attractive under analyzed conditions.

Digestate management for high-solid anaerobic digestion of organic wastes: A review

Digestate management for anaerobic digestion (AD) is becoming a bottleneck of the sustainability of AD plants when the use of digestate for agricultural application is restricted due to nutrient surplus and low market acceptance. Digestate quality and treatment in high solid anaerobic digestion (HSAD) can be better than conventional low-solid system. The rheological behavior of digestate in high solid anaerobic digestion (HSAD) can have a great impact on the energy consumption of digestate management. After post-conditioning guided by rheological parameters, the solid digestate can be further treated based on the integrated solutions to enhance the energy efficiency or nutrients recovery. The environmental impacts for some core parts of those integrated systems were also evaluated in this study. This article presented a critical review of recent investigations of digestate management for HSAD, especially focusing on the rheology of HSAD digestate, integrated solutions and their environmental performances.

Biogas production by means of an anaerobic-digestion plant in France: LCA of greenhouse-gas emissions and other environmental indicators

The present article assesses the environmental profile of a real-scale anaerobic-digestion plant that has been developed in France. The system utilises 13652 t of different types of feedstock related to food industry, agriculture, etc. The study is based on Life Cycle Assessment (LCA) according to Global Warming Potential (GWP), Cumulative Energy Demand (CED), ReCiPe midpoint/endpoint and USEtox. The life-cycle inventory includes real data from various sources of waste as well as the transportation distances. By considering the impact of both anaerobic digestion and transportation for the whole system, the following findings have been found: 6430 t CO_2.eq (GWP 100a); 67194 GJ_prim (CED); 231100 Pts (ReCiPe endpoint single-score: Human health), 146932 Pts (ReCiPe endpoint single-score: Ecosystems), 171568 Pts (ReCiPe endpoint single-score: Resources). Furthermore, USEtox results, for the whole system and by taking into account both anaerobic digestion and transportation, show that based on: 1) Human toxicity/cancer, anaerobic-digestion phase has around 21 times higher value comparing to transportation, 2) Ecotoxicity, anaerobic-digestion phase presents about 77 times higher value than transportation. Regarding the impact of both phases (anaerobic digestion; transportation) per t of waste or per MWh of electricity, the findings show values of 0.5-0.6 t CO_2.eq per t of feedstock (or digestate) or per MWh of electricity produced (not net). A separate subsection with comparisons of the present findings with literature studies about LCA of anaerobic-digestion plants has been included. In general, a good agreement has been observed. Moreover, comparisons of the impact of the electricity produced by means of the present biogas system with the impact of conventional electricity mixes of several countries are presented and discussed, proving the environmental benefits of the proposed anaerobic-digestion plant.

Techno-economic and life cycle analysis of a farm-scale anaerobic digestion plant in Iowa

There is growing interest in the use of anaerobic digestion to increase revenues in rural areas and reduce greenhouse gas emissions. This study evaluates the economic and environmental feasibility of a farm-scale anaerobic digestion (AD) combined heat and power (CHP) plant co-located with a cattle feedlot. The study evaluates two different scenarios with six cases - Biomass Only (BO) scenario and Biomass and Glycerin (BG) scenario, targeting a power capacity of 950 kW_e using combinations of manure, biomass, and crude glycerin. Beef cattle manure with approximately 10.15 wt% of biomass and 10 wt% of glycerin is added into the system. The internal rate of return (IRR) and greenhouse gas emissions (GHG) were calculated for six cases. The IRR ranges between 3.51% and 5.57%, and the GHG emissions range between -82.6 and 498.52 g CO₂e/kWh. Glycerin reduces the operating cost by 32%. These results indicate that AD CHP could be profitable at the farm-scale depending on various parameters. Sensitivity analysis indicates that power efficiency, operating capacity and waste generation per cattle have the strongest impact on the IRR, affecting it by over 40%, while glycerin and manure emission factors are the most important for GHG emissions affecting it by over 15%. Uncertainty analysis describes the role of feedstock choice and process performance on minimizing commercialization risks.

Reactor performance and economic evaluation of anaerobic co-digestion of dairy manure with corn stover and tomato residues under liquid, hemi-solid, and solid state conditions

Anaerobic co-digestion of tomato residues, dairy manure, and corn stover at ratios of 20:48:32, 40:36:24, and 60:24:16 (volatile solid basis) were compared for liquid anaerobic digestion (L-AD), hemi-solid state AD (HSS-AD), and solid state AD (SS-AD) systems. The highest methane yield (353.5 L/kg-VS_added) and volumetric methane productivity (24.5 m³_methane/m³_{reactor volume}) were both obtained with 20% tomato residues addition under L-AD and HSS-AD conditions, respectively. Total solid and feedstock mixing ratio affected the degradation of protein and lipids during AD, but not cellulose and hemicellulose. Economic analysis results indicated that capital and labor costs have the dominant effect on total investment. SS-AD of tomato residues, dairy manure, and corn stover at ratios of 20:48:32 (VS basis) has the highest net present value (2.6 million US$) and shortest payback period (10.1 year), which indicated SS-AD was financially attractive under analysis conditions.