Anaerobic Digestion Research Articles

Ammonia-stressed anaerobic digestion: Sensitivity dynamics of key syntrophic interactions and methanogenic pathways-A review.

The problematic anaerobic digestion (AD) of protein-rich substrates owing to their high ammonia content continues to hinder optimum methanation despite their high potential for offsetting greenhouse gas (GHG) emissions. This review focuses on the analyses of the sensitivity dynamics of key AD processes as well as the microbial interactions and exchanges that occur with them. Aside from the apparent increased risk associated with thermophilic ammonia-rich substrate AD, the marginally higher energy generation compared to mesophilic systems is not commensurate to the energy requirement. Moreover, while comparable FAN thresholds have been confirmed, TAN thresholds are susceptible to physical chemistry and so vary greatly. Profiling of the metabolic capability of front-end AD microbiome revealed Bacteroidetes, Firmicutes, and Synergistetes as some of the ammonia-resilient bacteria groups while Proteobacteria and Actinobacteria were the most fragile taxa. Besides the predominance of incomplete propionate oxidizing bacteria under ammonia stress conditions, syntrophic propionate oxidation (SPO) is usually shifted from the methylmalonyl CoA to the dismutation pathway. Furthermore, besides their different recoverability potentials, distinct methanogenic groups are differentially impacted by different ammonia species. Prevailing literature evidence suggests that conductive material assisted bioaugmentation with SAO-HM consortia, and in-situ H2 supplementation are the most effective for expediting electron transfer and relieving ammonia stress. These valuable insights should inform the design of targeted ammonia inhibition mitigation strategies.

Microbial diversity and biosafety judgment of digestates derived from different biogas plants for agricultural applications.

The composition of microbial communities is the key to effective anaerobic digestion (AD). The microbiome driving the AD process has been extensively researched, whereas the influence of specific substrates on the microbiome of digestate remains insufficiently investigated. Digestate has considerable potential for use in soil fertilization and bioremediation, therefore its biological safety should be monitored. Moreover, the knowledge about the composition of microbial communities and their interconnections in digestate should be extended, due to the impact on soil microbiota and its functionality. The aim of this study was a comprehensive assessment of the (1) sanitary quality, (2) core microbiome, and (3) microbial interactions in digestates collected from three full-scale agricultural biogas plants, with particular emphasis on their applicability from the perspective of the resident microbiota. Analyzed samples of digestate were derived from various substrates used for AD, including plant- and animal-based materials, and industrial waste. The study demonstrated that the phyla Bacillota, Bacteroidota, and Cloacimonadota were the most dominant in digestates regardless of the composition of the processed substrates, however, member composition at the genus level differed significantly between samples. In addition, we observed that microbial genera belonging to the less prevalent phyla play an integral role in the forming of microbial community interactions. Dominant microbial taxa with broad metabolic capabilities, potentially improving soil quality and functionality, have been identified. Moreover, we confirmed, that digestate samples were free of analyzed pathogenic bacteria and parasites. The study results indicate that digestate may have an immense fertilizing and bioremediation potential that has not been fully availed of to date.

Assessing the Biogas Potential of Slaughterhouse Waste in Tenerife: A Sustainable Approach to Waste Management in an Outermost European Region

The Canary Islands, as an outermost region of Europe with fragmented territory, benefit from an exemption under Commission Regulation (EU) No 142/2011 allowing for animal by-products disposal in landfills. However, this practice has resulted in environmental and economic challenges, including reduced landfill capacity, increased greenhouse gas emissions, and missed opportunities for renewable energy production. Specifically, waste management in Tenerife faces significant challenges due to the burgeoning meat production and subsequent waste generation in its primary slaughterhouses. With an average annual production of 5,000 tons of waste, landfill capacity is nearing its limit. This study therefore aims to assess the biogas production potential from slaughterhouse waste in Tenerife and evaluate anaerobic digestion as an alternative approach. Various combinations of viscera, blood, and slaughterhouse sludge from cattle, pigs, goats, sheep, rabbits, and chickens were analyzed to determine their biogas potential. Results indicate that comprehensive valorization of these wastes could generate 12.86 GWh of energy, equivalent to approximately 20% of the energy consumed in the municipality's industrial sector. Moreover, this approach could reduce emissions by 5.88 Gg of CO2 equivalent and alleviate landfill pressures, thereby mitigating current waste management impacts. Given the region's reliance on imported fuels and limited territorial space, anaerobic digestion presents an environmentally and economically sustainable solution for managing slaughterhouse waste within a closed-loop system.

Superstructure optimization of hydrothermal liquefaction for microalgae biorefinery considering environmental impacts and economics

Microalgae biorefinery superstructure connections from algae to muti-products are addressed. The superstructure connections affect eCO2 emissions, product variety, and profits. Through the superstructure optimization algorithm for the illustrated four scenarios within the same lipid content of 25% and 200 kg weight of microalgae, (i) Scenario-1 for maximizing the net profit, the best net profit of 32.54 USD is achieved due to producing the green electricity and biodiesel, (ii) Scenario-2 for minimizing total eCO2 emissions, the lowest total eCO2 emissions with 78.28 kg eCO2 is guaranteed if the power generation from biogas is assumed to fully cover load demands of the process units but the corresponding net profit is down to 10.84 USD, (iii) Scenario-3 for balancing net profit and eCO2 emissions, the net profit is returned to 17.65 USD since the acetone/butanol/ethanol by the ABE fermentation and biooil/green diesel are added, (iv) Scenario-4 for maximizing biooil production, the total revenue is higher than Scenarios 1-3 by 16∼164% but the corresponding eCO2 emissions is also higher than Scenarios 1-3 by 65∼160%. Although the superstructure optimization problem induces the trade-off results, it is validated that the combination of ABE fermentation and HTL is superior to the combination of transesterification and anaerobic digestion.

Comprehensive assessment of sludge wet air oxidation and its combination with anaerobic digestion

Wet air oxidation (WAO) is a promising sludge treatment technology but has yet to be used widely. This study evaluates a new integrated pathway of AD plus WAO, juxtaposing against the standalone AD and WAO, from the aspects of carbon emissions, energy efficiency, and cost-benefits. The assessment unit is 1 t dewatered sludge with a water content of 80%, and the greenhouse gases are converted to equivalent CO2. WAO can recover the thermal heat released from organic oxidation, resulting in the lowest carbon emissions, 10.92 to −36.86 kg CO2-eq/t. AD plus WAO also performs well with 32.21 to −48.16 kg CO2-eq/t. All three ways can produce renewable energy through biogas utilization or thermal recovery. WAO has the highest energy efficiency of 35%–42% due to efficient thermal energy recovery, and AD plus WAO has a second-ranked efficiency of 15%–30%. However, WAO needs expensive facilities and external electricity, resulting in a higher expenditure than AD. The integrated systems also require a high investment, but can simplify digested sludge treatment. According to the normalized scores in the three aspects, WAO is recommended chiefly, especially for treating low-organic-content sludge. AD plus WAO ranked second and can be used for high-organic-content sludge.

Surface engineering-based S, N co-doped biochar for improved anaerobic digestion: Enhancing microbial-pollutant and inter-microbial electron transfer synergistic EPS protection

Enhancing extracellular electron transfer (EET) efficiency is crucial for improving the anaerobic digestion (AD) system's capability to treat recalcitrant wastewater. In this study, a novel S, N co-doped biochar (S-N-BC) was prepared through surface engineering to optimize EET within AD systems. The addition of S-N-BC significantly enhanced the performance of a mesophilic AD system treating Congo red wastewater, increasing the decolorization rate by 78%, COD degradation rate by 82%, and methane yield by 87% compared to the control. Additionally, the shock resistance of anaerobic granular sludge was improved, as evidenced by the formation of the protective extracellular polymeric substances (EPS) barrier and the enhanced activities of the electron transport system. Mechanistic analysis revealed that adding S-N-BC did not alter the Congo red decolorization pathway but significantly enriched various electrochemically active bacteria and established EET pathways between microbial-pollutant and inter-microbial. This significantly accelerated EET efficiency within the AD system, ensuring stable and efficient operation under challenging conditions. This study proposed a novel approach using S-N-BC to simultaneously enhance "dual-pathway EET" between microbial-pollutant and inter-microbial while constructing an EPS protective barrier, addressing the issues of low efficiency and fragile stability of AD systems for treating recalcitrant wastewater.

Optimization of Parameters Responsible for the Rate of Gas Generation Through Mixed Anaerobic Digestion

Optimization of Parameters Responsible for the Rate of Gas Generation Through Mixed Anaerobic Digestion

Enhancing hydrogen production from anaerobic digestion of pretreated fruit and vegetable peels using Clostridium butyricum NE133

This study aimed to investigate the feasibility of hydrogen production (HP) by Clostridium butyricum NE133 from different fruit and vegetable peels (FVPs) as substrates. In addition, the kinetic parameters of hydrogen production and optimization of the anaerobic dark fermentation conditions were analyzed. Clostridium butyricum NE133 was isolated from domestic wastewater and selected as the front runner hydrogen-producer from glucose with maximum hydrogen production (Hmax) of 1778.00 ± 15.03 mL/L, maximum production rate (Rmax) of 961.95 mL/L/h and lag phase (λ) of 28.12 h. NE133 was genetically identified (accession number PP581793) and shown to harbor the Fe-Fe hydrogenase gene. This isolate showed a high potential to produce hydrogen from anaerobic fermentation of watermelon peels with Hmax of 1062.67 ± 11.92 mL/L, Rmax of 268.01mL/L/h and λ of 33.92 h. The watermelon peels were subjected to different pretreatment methods to enhance the dark fermentation by C. butyricum NE133. It was revealed that the combined physicochemical treatment (0.05 M H₂SO₄/121°C) significantly increased hydrogen yield, with 2300.33 ± 0.88 mL/L, Rmax of 1065.56 mL/L/h and λ of 22.39 h with a high accuracy of R2 (0.9999). The study emphasizes the effectiveness of using C. butyricum NE133 for sustainable biohydrogen production. The findings also indicate the feasibility of converting agricultural waste into valuable energy sources, contributing to waste management and renewable energy solutions.

The collaboration and competition between indigenous microorganisms and exogenous anaerobic digester sludge in anaerobic treatment of pickled mustard wastewater at different salinities

The highly concentrated pickled mustard wastewater presents significant potential for energy recovery, but the stress effect of high osmotic pressure on cell integrity and activity seriously impedes the methane production by anaerobic microorganisms. The survival ability of indigenous microorganisms (IM) in pickled mustard wastewater supports the establishment of anaerobic treatment. Moreover, inoculation of anaerobic digester sludge is a common start-up strategy. However, the effects of exogenous anaerobic sludge on IM are unclear, especially in hypersaline environment. This research aimed to investigate the influence of exogenous anaerobic sludge on the construction, performance, and microbiota at 3% and 5% salinity. And the research focused on the collaboration and competition between exogenous anaerobic sludge and IM. The neutral community model (which explains the formation and evolution of biological communities) indicated that the interaction between exogenous digester sludge microorganisms and IM dominated community assembly. At 3%, the digester sludge collaborated with IM to increase daily COD reduction and biogas production compared with IM group. However, at 5%, the competitive relationship reduced daily COD reduction and biogas production compared with IM group. This study provides a new perspective for the selection of inoculation strategies for exogenous anaerobic digester sludge under different salinity, in order to realize energy conversion from salinity organic wastewater.

Machine Learning for Enhancing Prediction of Biogas Production and Building a VFA/ALK Soft Sensor in Full-Scale Dry Anaerobic Digestion of Kitchen Food Waste

Based on operational data collected over 1.5 years from four industrial-scale dry anaerobic digesters used for kitchen food waste treatment, this study adopted eight typical machine learning algorithms to distinguish the best methane prediction model and to develop a soft sensor based on the VFA/ALK ratio. Among all the eight tested models, the CatBoost (CB) algorithm demonstrated superior performance in terms of prediction accuracy and model fitting. Specifically, the CB model achieved a biogas production prediction accuracy (R2) ranging from 0.604 to 0.915, and a VFA/ALK R2 between 0.618 and 0.768 on the test dataset. Furthermore, the feature importance analysis revealed that biomass amount into the dry anaerobic digester was the primary factor influencing biogas production. Chemical oxygen demand (COD) and total ammonia nitrogen (TAN) were identified as the most significant factors impacting the VFA/ALK indicator during dry anaerobic digestion, collectively contributing to nearly 50% of the influence. Overall, this study verifies the feasibility of using machine learning to predict biogas production in industrial-scale dry anaerobic digestion and provides a crucial foundation for monitoring the stability of dry anaerobic digesters.

Study of a theoretical approach to bioprocess modeling for the performance of an anaerobic digester

Anaerobic digestion represents one of the major challenges of sustainable development and circular economy in the concept "from waste to energy". Given the great diversity of organic waste, its development requires the optimization of co-digestion. Hence the need to develop simple tools to characterize substrates and to predict the performance of digesters in order to optimize their operation. In this work, an approach based on a simple mathematical model to be able to calculate the biogas yield was studied. The models studied (AM2 and ADM1), serve as a basis for the development of monitoring and regulation strategies, offering more or less detailed levels of description of the digestion process. We have identified a difference in complexity between these two models explained by the different objectives for which they were developed. Thus, our study aims to propose a theoretical approach to the modeling of bioprocesses by choosing between the two aforementioned models. The objective being to provide a certain ease to the biogas producer and designer of digesters in the rural world, the simple model chosen is justified by intermediate stages of lesser importance than the final stage of methanogenesis. Thus, the information collected and estimated can subsequently be used for control strategies to optimize the operation of the digester, or they can be used by monitoring and diagnostic systems. This model will give a synthetic vision of anaerobic digestion, which limits its use to the modeling of the treatment of simple effluents. On the other hand, its formulation will allow a thorough mathematical analysis, and its simple structure would be well suited to the development of control strategies or monitoring procedures.

Bioplastic’s Valorisation by Anaerobic Co-Digestion with WWTP Mixed Sludge

Bioplastics are designed to degrade at the end of their lifecycle, but effective management of their end-of-life phase and integration into existing organic waste management systems remain significant challenges. Some bioplastics decompose under anaerobic conditions, with the anaerobic digestion (AD) process being a potential solution for their disposal. AD is a promising technology for valorising organic wastes, enabling biomethane production, reducing carbon footprints, and promoting product circularity. This study focuses on evaluating the continuous co-digestion of bioplastics with mixed sludge from an urban wastewater treatment plant (WWTP). Polyhydroxybutyrate (PHB) was the selected bioplastic, as various studies have reported its high and rapid degradation under anaerobic mesophilic conditions. PHB’s biodegradability under typical WWTP anaerobic digestion conditions (35 °C, 20-day retention time) was assessed in batch tests and the results indicate that PHB degradation ranged from 68 to 75%, depending on particle size. To further explore the potential of AD for PHB valorisation, the feasibility of anaerobic co-digestion of PHB with WWTP sludge was tested on a continuous laboratory scale using two digesters: a conventional digester (CSTR) and an anaerobic membrane bioreactor (AnMBR). The results indicated complete degradation of PHB, which led to higher biomethanisation percentages in both digesters, rising from 58% to 70% in the AnMBR and from 44% to 72% in the CSTR. The notable increase observed in the CSTR was attributed to changes in microbial populations that improved sludge biodegradability.

Microbial community evolution in a lab-scale reactor operated to obtain biomass for biochemical methane potential assays.

Biochemical methane potential (BMP) test is an important tool to evaluate the methane production biodegradability and toxicity of different wastes or wastewaters. This is a key parameter for assessing design and feasibility issues in the full-scale implementation of anaerobic digestion processes. A standardized and storable inoculum is the key to obtain reproducible results. In Uruguay, a local enterprise dedicated to design and install anaerobic digesters operated a lab-scale bioreactor as a source of biomass for BMP tests, using a protocol previously described. This reactor was controlled and fed with a mixture of varied organic compounds (lipids, cellulolytic wastes, proteins). Biomass was reintroduced into the reactor after BMP assays to maintain a constant volume and biomass concentration. The aim of this work was to evaluate how the microbial community evolved during this operation and the effect of storing biomass in the refrigerator. The composition of the microbial communities was analyzed by 16S rRNA amplicon sequencing using primers for Bacteria and Archaea. The methanogenic activity was determined, and the methanogens were quantified by mcrA qPCR. One sample was stored for a 5-month period in the refrigerator (4°C); the activity and the microbial community composition were analyzed before and after storage. Results showed that applying the reported methodology, a reliable methanogenic sludge with an acceptable SMA was obtained even though the reactor suffered biomass alterations along the evaluated period. Refrigerating the acclimatized biomass for 5months did not affect its activity nor its microbial composition according to the 16S rRNA gene sequence analysis, even though changes in the mcrA abundance were observed. KEY POINTS: • The applied methodology was successful to obtain biomass suitable to perform BMP assays. • The microbial community was resilient to external biomass addition. • Biomass storage at 4°C for 5months did not alter the methanogenic activity.

Climate impact of alternative organic fertilizers using life cycle assessment

Abstract Anaerobic digestion is a common method for managing liquid manure and other biomasses, generating biogas as a renewable energy source. The resulting digestate can be processed into organic fertilizers to enhance nutrient recycling, but its environmental impact warrants investigation. In this study, a life cycle assessment was conducted to examine the impact of fertilizers derived from cattle slurry and grass–clover co-digestion on global warming (measured in CO2 equivalents) compared to untreated cattle slurry (CA). The different treatments analyzed include CA, digestate, liquid fractions (LFs) from digestate separation, and an enriched liquid nitrogen–sulfur product derived from post-processing of biogas and drying of the solid fraction. The functional units of this study were 100 kg of total nitrogen in the final organic fertilizer (FU1) with the cradle-to-processing gate boundary, and the harvesting of 1 ton of spring barley dry matter (FU2) with the cradle-to-field application boundary. The carbon footprint ranged from 24% to 49% of the baseline scenario for FU1, and from −6% to 177% of the baseline scenario for FU2. The main contributors to the carbon footprint of fertilizers included greenhouse gas emissions from storage and field application. However, biogas production from anaerobic digestion, together with the concurrent mitigation of CH4 emissions during storage, contributed most to a reduction in the overall global warming potential associated with anaerobic digestate and its LF. This study showed large climate prospects in replacing untreated slurry as organic fertilizer with alternatives resulting from its anaerobic digestion and post-treatment.

Lipase pretreatment enhances biochar‐assisted anaerobic digestion of food waste

AbstractAnaerobic digestion is a promising technology for the treatment of food waste (FW) but it requires optimization to improve its performance. This study investigated whether combining the addition of biochar with lipase pretreatment could improve the anaerobic digestion of FW. The results showed that biochar stimulated the release of soluble substances, resulting in an increase in the methane yield of groups with added biochar by 16.24% to 19.36% in comparison with the group without biochar. Lipase pretreatment substantially shortened the fermentation time required to complete the anaerobic digestion, from 15 to 9 days. Lipase pretreatment further improved microbial abundance and promoted the enrichment of functional microbes in the anaerobic digestion of FW mediated by biochar. It also changed the methane production pathway from acetotrophic to hydrogenotrophic, thereby ensuring the stability of the anaerobic digestion system in the presence of a high ammonia nitrogen concentration.

Classical Batch Distillation of Anaerobic Digestate to Isolate Ammonium Bicarbonate: Membrane Not Necessary!

The excessive mineralization of organic molecules during anaerobic fermentation increases the availability of nitrogen and carbon. For this reason, the development of downstream processing technologies is required to better manage ammonia and carbon dioxide emissions during the storage and land application of the resulting soil organic amendment. The present work investigated classical distillation as a technology for valorizing ammoniacal nitrogen (NH4+-N) in anaerobic digestate. The results implied that the direct isolation of ammonium bicarbonate (NH4HCO3) was possible when applying the reactive distillation to the food waste digestate (FWD) with a high content of NH4+-N, while the addition of antifoam to the agrowaste digestate (AWD) was necessary to be able to produce an aqueous solution of NH4HCO3 as the distillate. The reason was that the extraction of NH4HCO3 from the AWD required a higher temperature (>95 °C) and duration (i.e., steady state in batch operation) than the recovery of the inorganic fertilizer from the FWD. The titration method, when applied to the depleted digestate, offered the quickest way of monitoring the reactive distillation because the buffer capacity of the distillate was much higher. The isolation of NH4HCO3 from the FWD was attained in a transient mode at a temperature below 90 °C (i.e., while heating up to reach the desired distillation temperature or cooling down once the batch distillation was finished). For the operating conditions to be regarded as techno-economically feasible, they should be attained in the anaerobic digestion plant by integrating the heat harvested from the engines, which convert the biogas into electricity.

Insights into the Acute Stress of Glutaraldehyde Disinfectant on Short-Term Wet Anaerobic Digestion System of Pig Manure: Dose Response, Performance Variation, and Microbial Community Structure

The outbreak of epidemics such as African swine fever has intensified the use of disinfectants in pig farms, resulting in an increasing residual concentration of disinfectants in environmental media; however, the high-frequency excessive use of disinfectants that damage pig farm manure anaerobic fermentation systems and their mechanisms has not attracted enough attention. Especially, the complex effects of residual disinfectants on anaerobic fermentation systems for pig manure remain poorly understood, thus impeding the application of disinfectants in practical anaerobic fermentation systems. Herein, we explored the effects of glutaraldehyde disinfectant on methane production, effluent physicochemical indices, and microbial communities in a fully automated methanogenic potential test system (AMPTSII). The results show that adding glutaraldehyde led to remarkable alterations in methane production, chemical oxygen demand (COD), volatile solids (VS), and polysaccharide and phosphorus concentrations. During the anaerobic process, the production of methane displayed a notable decrease of 5.0–98% in all glutaraldehyde treatments, and the trend was especially apparent for treatments containing high levels of glutaraldehyde. Comparisons of the effluent quality showed that in the presence of 0.002–0.04% glutaraldehyde, the COD and total phosphorus (TP) increased by 12–310% and 15–27%, respectively. Moreover, the addition of 0.01–0.08% glutaraldehyde decreased the ammonium (NH4+-N) concentration and VS degradation rate by 7.7–15% and 4.9–26.2%. Furthermore, microbiological analysis showed that the glutaraldehyde treatments had adverse effects on the microbial community. Notably, certain functional bacteria were restrained, as highlighted by the decreases in relative abundance and microbial diversity by 1.3–17% and 0.06–21%, respectively. This study provides a theoretical basis for the rational use of disinfectants in anaerobic fermentation systems.

Assessing the Amending and Fertilizing Properties of Digestates From Anaerobic Digestion of Faecal Sludge in Burkina Faso, West Africa

The aim of this work is to contribute to the agronomic utilization of digestate from the anaerobic treatment of faecal sludge at the Office National de l’Eau et de l’Assainissement (ONEA) Kossodo plant in Burkina Faso. Digestates were sampled and dried in the laboratory (25±2 °C). Physico-chemical and microbiological parameters were determined using standard methods. The degree of maturity of the digestate was determined using respirometry and humification tests. The effects of digestate on plants and on environment were studied using phytotoxicity, emergence, growth tests and microbiological analysis. The results revealed a pH of 8.54, electrical conductivity of 7300 µS/cm, 25.42% dry matter, 83.31% organic matter, and 489.8 g/kg total organic carbon. The C/N ratio was 9.7 and the NO3/NH4 ratio was 23.34, indicating mature compost despite other indicators suggesting that the digestate was only partially stabilized. Nutrient contents were 50.48 g/kg total nitrogen, 0.02 g/kg total phosphorus, and 4.72 g/kg total potassium. Water-soluble minerals included 1.97 g/kg ammonium, 45.99 g/kg nitrate, 0.02 g/kg nitrite, 0.35 g/kg phosphate, and 5.90 g/kg sulfate. Phytotoxicity tests showed that cabbage seeds had a germination rate of 89% with digestate filtrate compared to 100% in the control, while tomato seeds had a 100% germination rate with digestate filtrate. The germination indices were 137.58 for tomato and 82.10 for cabbage. Tests on tomato plant growth showed significant effects on biomass, height, leaf number, and branch number, with the 50% digestate proportion demonstrating the highest values. Microbiological analysis detected total aerobic flora at 4.75E+07 CFU/g, yeasts and molds at 7.77E+04 CFU/g, and coliforms at various levels, with potential pathogens such as S. aureus and Salmonella sp. reaching 6.14E+04 and 2.65E+06 CFU/g respectively. The presence of these pathogens suggests a need for further treatment of digestate to ensure safety before field application.

The Path to Net-Zero in Dairy Production: Are Pronounced Decreases in Enteric Methane Achievable?

Achieving net-zero greenhouse gas (GHG) emissions in dairy production will require >50% reduction in enteric methane (CH4) emissions together with elimination of emissions from feed production, additional carbon sequestration, reduction in manure emissions, anaerobic digestion of manure, and decreased reliance on fossil fuel energy. Over past decades, improved production efficiency has reduced GHG intensity of milk production (i.e., emissions per unit of milk) in the United States, but this trend will continue only if cows are bred for increased efficiency. Genetic selection of low-CH4-producing animals, diet reformulation, use of feed additives, and vaccination show tremendous potential for enteric CH4 mitigation; however, few mitigation strategies are currently available, and added cost without increased revenue is a major barrier to implementation. Complete elimination of CH4 emissions from dairying is likely not possible without negatively affecting milk production; thus, offsets and removals of other GHGs will be needed to achieve net-zero milk production.

Inventory analysis and environmental life cycle impact assessment of hotel food waste management for bio-circular economy development in Zimbabwe.

This study is an inventory analysis and environmental life cycle assessment of hotel food waste management that seeks to inform efforts towards the development of bio circular economy in Zimbabwe. An audit of food waste generation and an inventory analysis of the prevailing food waste management practices at three selected hotels was undertaken. The greenhouse gas emissions from the prevailing disposal of food waste at dumpsites and the proposed biological treatment of food waste were evaluated using the Tier 1 FOD method and Tier 1 method of biological treatment using the 2019 refined 2006 Intergovernmental Panel on Climate Change guidelines. Environmental life cycle assessment was also conducted for the open dumping, composting, and anaerobic digestion. The average food waste generation within the Zimbabwean hospitality industry was estimated at 1.63kg/guest/day, with a minimum and maximum of 1.01 and 2.25kg/guest/day, respectively. Source-separated food waste is currently being collected indiscriminately by municipal waste collection trucks for final disposal at the official landfills or dumpsites. This calls for the need for an offtake system of the source-separated food waste in the form of composting or anaerobic digestion. Study results showed that the disposal of food waste at solid waste disposal sites contributes to the highest greenhouse gas emissions, followed by composting with a 75% reduction in greenhouse emissions. Anaerobic digestion brings about a maximum reduction in greenhouse emissions of 97%. Environmental life cycle assessment results also show that anaerobic digestion is the best method leading to net negative environmental impacts.