National Greenhouse Gas Emission Reduction Potential from Adopting Anaerobic Digestion on Large-Scale Dairy Farms in the United States.

Waste-to-energy systems can provide a functional demonstration of the economic and environmental benefits of circularity, innovation, and reimagining existing systems. This study offers a robust quantification of the greenhouse gas (GHG) emission reduction potential of the adoption of anaerobic digestion (AD) technology on applicable large-scale dairy farms in the contiguous United States. GHG reduction estimates were developed through a robust life cycle modeling framework paired with sensitivity and uncertainty analyses. Twenty dairy configurations were modeled to capture important differences in housing and manure management practices, applicable AD technologies, regional climates, storage cleanout schedules, and methods of land application. Monte Carlo results for the 90% confidence interval illustrate the potential for AD adoption to reduce GHG emissions from the large-scale dairy industry by 2.45-3.52 MMT of CO2-eq per year considering biogas use only in renewable natural gas programs and as much as 4.53-6.46 MMT of CO2-eq per year with combined heat and power as an additional biogas use case. At the farm level, AD technology may reduce GHG emissions from manure management systems by 58.1-79.8% depending on the region. Discussion focuses on regional differences in GHG emissions from manure management strategies and the challenges and opportunities surrounding AD adoption.

Phenotypic and genomic characterization of Methanothermobacter wolfeii strain BSEL, a CO2-capturing archaeon with minimal nutrient requirements.

A new variant of Methanothermobacter wolfeii was isolated from an anaerobic digester using enrichment cultivation in anaerobic conditions. The new isolate was taxonomically identified via 16S rRNA gene sequencing and tagged as M. wolfeii BSEL. The whole genome of the new variant was sequenced and de novo assembled. Genomic variations between the BSEL strain and the type strain were discovered, suggesting evolutionary adaptations of the BSEL strain that conferred advantages while growing under a low concentration of nutrients. M. wolfeii BSEL displayed the highest specific growth rate ever reported for the wolfeii species (0.27 ± 0.03 h-1) using carbon dioxide (CO2) as unique carbon source and hydrogen (H2) as electron donor. M. wolfeii BSEL grew at this rate in an environment with ammonium (NH4+) as sole nitrogen source. The minerals content required to cultivate the BSEL strain was relatively low and resembled the ionic background of tap water without mineral supplements. Optimum growth rate for the new isolate was observed at 64°C and pH 8.3. In this work, it was shown that wastewater from a wastewater treatment facility can be used as a low-cost alternative medium to cultivate M. wolfeii BSEL. Continuous gas fermentation fed with a synthetic biogas mimic along with H2 in a bubble column bioreactor using M. wolfeii BSEL as biocatalyst resulted in a CO2 conversion efficiency of 97% and a final methane (CH4) titer of 98.5%v, demonstrating the ability of the new strain for upgrading biogas to renewable natural gas.IMPORTANCEAs a methanogenic archaeon, Methanothermobacter wolfeii uses CO2 as electron acceptor, producing CH4 as final product. The metabolism of M. wolfeii can be harnessed to capture CO2 from industrial emissions, besides producing a drop-in renewable biofuel to substitute fossil natural gas. If used as biocatalyst in new-generation CO2 sequestration processes, M. wolfeii has the potential to accelerate the decarbonization of the energy generation sector, which is the biggest contributor of CO2 emissions worldwide. Nonetheless, the development of CO2 sequestration archaeal-based biotechnology is still limited by an uncertainty in the requirements to cultivate methanogenic archaea and the unknown longevity of archaeal cultures. In this study, we report the adaptation, isolation, and phenotypic characterization of a novel variant of M. wolfeii, which is capable of maximum growth with minimal nutrients input. Our findings demonstrate the potential of this variant for the production of renewable natural gas, paving the way for the development of more efficient and sustainable CO2 sequestration processes.

Policy Analysis of CO2 Capture and Sequestration with Anaerobic Digestion for Transportation Fuel Production.

Low carbon fuel and waste management policies at the federal and state levels have catalyzed the construction of California's wet anaerobic digestion (AD) facilities. Wet ADs can digest food waste and dairy manure to produce compressed natural gas (CNG) for natural gas vehicles or electricity for electric vehicles (EVs). Carbon capture and sequestration (CCS) of CO2 generated from AD reduces the fuel carbon intensity by carbon removal in addition to avoided methane emissions. Using a combined lifecycle and techno-economic analysis, we determine the most cost-effective design under current and forthcoming federal and state low carbon fuel policies. Under many scenarios, designs that convert biogas to electricity for EVs (Biogas to EV) are favored; however, CCS is only cost-effective in these systems with policy incentives that exceed $200/tonne of CO2 captured. Adding CCS to CNG-producing systems (Biogas to CNG) only requires a single unit operation to prepare the CO2 for sequestration, with a sequestration cost of $34/tonne. When maximizing negative emissions is the goal, incentives are needed to either (1) fund CCS with Biogas to EV designs or (2) favor CNG over electricity production from wet AD facilities.

Microbial community dynamics of a sequentially fed anaerobic digester treating solid organic waste.

A 50-kg scale, high solids anaerobic digester (AD) comprising six sequentially fed leach beds with a leachate recirculation system was operated at 37°C for 88 weeks. The solid feedstock contained a constant fibre fraction (a mix of cardboard, boxboard, newsprint, and fine paper) and varying proportions of food waste. Previously, we reported on the stable operation of this digestion system, where significantly enhanced methane production from the fibre fraction was observed as the proportion of food waste increased. The objective of this study was to identify relationships between process parameters and the microbial community. Increasing food waste led to a large increase in the absolute microbial abundance in the circulating leachate. While 16S rRNA amplicons for Clostridium butyricum were most abundant and correlated with the amount of FW in the system and with the overall methane yield, it was more cryptic Candidatus Roizmanbacteria and Spirochaetaceae that correlated specifically with enhanced methane from the fiber fraction. A faulty batch of bulking agent led to hydraulic channeling, which was reflected in the leachate microbial profiles matching that of the incoming food waste. The system performance and microbial community re-established rapidly after reverting to better bulking agent, illustrating the robustness of the system.

Comparative Life Cycle Evaluation of the Global Warming Potential (GWP) Impacts of Renewable Natural Gas Production Pathways.

Renewable natural gas (RNG) sources are being considered in future energy strategy discussions as potential replacements for fossil natural gas (FNG). While today's supply of RNG resources is insufficient to meet U.S. demands, there is significant interest in its viability to supplement and decarbonize the natural gas supply. However, the studies compare the life cycle global warming potential (GWP) of various RNG production pathways are lacking and focus mostly on a singular pathway. This effort is an attempt to close this gap and provide a comparison between the life cycle GWP of three major RNG pathways and the FNG pathway. The three RNG pathways evaluated are anaerobic digestion (AD), thermal gasification (TG), and power-to-gas (P2G) using various feedstocks. The functional unit is 1 MJ of compressed RNG ready for injection into the natural gas transmission network. The results show that RNG production is not always carbon neutral or negative. Depending on the pathway, the GWP impact of RNG production can range from -229 to 27 g CO2e/MJ compressed RNG, with AD of animal manure and AD of municipal solid waste being the least and the most impactful pathways, respectively, compared to the 10.1 g CO2e/MJ impact for compressed FNG.

Market Potential for CO2 Removal and Sequestration from Renewable Natural Gas Production in California.

Bioenergy with carbon capture and sequestration (BECCS) is critical for stringent climate change mitigation but is commercially and technologically immature and resource intensive. State and federal fuel and climate policies can drive first markets for BECCS in California. We develop a spatially explicit optimization model to assess niche markets for renewable natural gas (RNG) production with carbon capture and sequestration (CCS) from waste biomass in California. Existing biomass residues produce biogas and RNG and enable low-cost CCS through the upgrading process and CO2 truck transport. Under current state and federal policy incentives, RNG-CCS can avoid 12.4 mmtCO2e/year (3% of California's 2018 CO2 emissions), of which 2.9 mmtCO2/year are captured and sequestered. It simultaneously produces 93 PJ RNG/year (4% of California's 2018 natural gas demand) with a profit maximizing objective, resulting in profits of $11/GJ. Distributed RNG production with CCS can potentially catalyze markets and technologies for CO2 capture, transport, and storage in California.

The role of microbial ecology in improving the performance of anaerobic digestion of sewage sludge.

The use of next-generation diagnostic tools to optimise the anaerobic digestion of municipal sewage sludge has the potential to increase renewable natural gas recovery, improve the reuse of biosolid fertilisers and help operators expand circular economies globally. This review aims to provide perspectives on the role of microbial ecology in improving digester performance in wastewater treatment plants, highlighting that a systems biology approach is fundamental for monitoring mesophilic anaerobic sewage sludge in continuously stirred reactor tanks. We further highlight the potential applications arising from investigations into sludge ecology. The principal limitation for improvements in methane recoveries or in process stability of anaerobic digestion, especially after pre-treatment or during co-digestion, are ecological knowledge gaps related to the front-end metabolism (hydrolysis and fermentation). Operational problems such as stable biological foaming are a key problem, for which ecological markers are a suitable approach. However, no biomarkers exist yet to assist in monitoring and management of clade-specific foaming potentials along with other risks, such as pollutants and pathogens. Fundamental ecological principles apply to anaerobic digestion, which presents opportunities to predict and manipulate reactor functions. The path ahead for mapping ecological markers on process endpoints and risk factors of anaerobic digestion will involve numerical ecology, an expanding field that employs metrics derived from alpha, beta, phylogenetic, taxonomic, and functional diversity, as well as from phenotypes or life strategies derived from genetic potentials. In contrast to addressing operational issues (as noted above), which are effectively addressed by whole population or individual biomarkers, broad improvement and optimisation of function will require enhancement of hydrolysis and acidogenic processes. This will require a discovery-based approach, which will involve integrative research involving the proteome and metabolome. This will utilise, but overcome current limitations of DNA-centric approaches, and likely have broad application outside the specific field of anaerobic digestion.

The implications of facility design and enabling policies on the economics of dry anaerobic digestion.

Diverting organic waste from landfills provides significant emissions benefits in addition to preserving landfill capacity and creating value-added energy and compost products. Dry anaerobic digestion (AD) is particularly attractive for managing the organic fraction of municipal solid waste because of its high-solids composition and minimal water requirements. This study utilizes empirical data from operational facilities in California in order to explore the key drivers of dry AD facility profitability, impacts of market forces, and the efficacy of policy incentives. The study finds that dry AD facilities can achieve meaningful economies of scale with organic waste intake amounts larger than 75,000 tonnes per year. Materials handling costs, including the disposal of inorganic residuals from contaminated waste streams and post-digester mass (digestate) management, are both the largest and the most uncertain facility costs. Facilities that utilize the biogas for vehicle fueling and earn associated fuel credits collect revenues that are 4-6x greater than those of facilities generating and selling electricity and 10-12x greater than facilities selling natural gas at market prices. The results suggest important facility design elements and enabling policies to support an increased scale of organic waste handling infrastructure.

Biological Co-treatment of H2S and reduction of CO2 to methane in an anoxic biological trickling filter upgrading biogas.

This study investigated the feasibility of co-treating H2S and CO2 in a biological trickling filter (BTF) inoculated with hydrogenotrophic methanogens (HMs) and nitrate-reducing, sulfur-oxidizing bacteria. This was accomplished by introducing a pure culture of Thiobacillus denitrificans in a BTF that was successfully upgrading a biogas mimic (60:40 CH4:CO2) to >97% methane using an enriched HM consortium. Nitrate was fed as the electron acceptor to oxidize H2S. The results revealed that a severe competition for hydrogen's electrons occurred between carbon dioxide and nitrate. Due to this competition, N:S loading rates of 16:1 were required to achieve >98% H2S removal, a ratio which is four times greater than the theoretical N:S ratio for complete sulfur oxidation. However, such high nitrate loading rates (>50 g N-NO3- m-3 h-1) had a negative impact on the BTF's biogas upgrading performance. An electron balance illustrated the increasing diversion of H2 electrons towards nitrate reduction as nitrate loading increased. Overall, this study showed that simultaneous biogas upgrading and H2S removal in a single bioreactor is possible, but that achieving high yields for both reactions requires further research in process and culture optimization.

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 kWe 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.

Bioconversion of carbon dioxide to methane using hydrogen and hydrogenotrophic methanogens.

Biogas produced from organic wastes contains energetically usable methane and unavoidable amount of carbon dioxide. The exploitation of whole biogas energy is locally limited and utilization of the natural gas transport system requires CO2 removal or its conversion to methane. The biological conversion of CO2 and hydrogen to methane is well known reaction without the demand of high pressure and temperature and is carried out by hydrogenotrophic methanogens. Reducing equivalents to the biotransformation of carbon dioxide from biogas or other resources to biomethane can be supplied by external hydrogen. Discontinuous electricity production from wind and solar energy combined with fluctuating utilization cause serious storage problems that can be solved by power-to-gas strategy representing the production of storable hydrogen via the electrolysis of water. The possibility of subsequent repowering of the energy of hydrogen to the easily utilizable and transportable form is a biological conversion with CO2 to biomethane. Biomethanization of CO2 can take place directly in anaerobic digesters fed with organic substrates or in separate bioreactors. The major bottleneck in the process is gas-liquid mass transfer of H2 and the method of the effective input of hydrogen into the system. There are many studies with different bioreactors arrangements and a way of enrichment of hydrogenotrophic methanogens, but the system still has to be optimized for a higher efficiency. The aim of the paper is to gather and critically assess the state of a research and experience from laboratory, pilot and operational applications of carbon dioxide bioconversion and highlight further perspective fields of research.