Environmental life cycle assessment of treatment and management strategies for food waste and sewage sludge.

A consequential life cycle assessment (LCA) was utilized to compare the environmental impacts of food waste and sewage sludge management strategies. The strategies included a novel two-phase anaerobic digestion (AD) system and alternatives including landfill, waste-to-energy, composting, anaerobic membrane bioreactor, and conventional AD (wet continuous stirred-tank reactor [CSTR]). The co-management of food waste with sewage sludge was also considered for the two-phase AD system and for a conventional AD reactor. A multidimensional LCA approach was taken, considering the five-midpoint impact categories of global warming, smog, human health particulate, acidification, and eutrophication estimated using the U.S. EPA Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts. Co-management of food waste and sewage sludge using the novel two-phase AD system was shown to maximize energy recovery and had a net global warming benefit while reducing other environmental impacts compared with the alternative management strategies. It had similar relative environmental advantages across all categories as conventional AD, with the advantage of a smaller physical footprint. However, both approaches featured net environmental burdens when the background electric grid intensity fell below 0.25 kg CO2-eq kWh-1, as could be expected in a decarbonized electric future. Upgrading the biogas produced from AD to renewable natural gas can displace the use of fossil natural gas for other non-electricity energy requirements that are difficult to decarbonize and may extend the time period of significant environmental benefits of utilizing AD for organic waste management. Treatment of the nutrient-rich supernatant generated by the novel two-phase AD system could be an obstacle for utilities with stringent nutrient discharge limits. Future research and full-scale implementation are needed to demonstrate the benefits of the two-phase AD system predicted through this analysis.

Reducing CO2 Emissions from U.S. Steel Consumption by 70% by 2050.

The steel sector emits 25% of global industrial greenhouse gases, and the U.S. is the world's second-largest steel consumer. In this article, we determine how CO2 emissions attributable to U.S. steel consumption can be cut by 70% by 2050. We vary four key steel cycle parameters (U.S. steel stocks per capita, recycling rate, product lifespan, and manufacturing yield) in a dynamic material flow analysis to determine a range of values for annual steel demand and the scrap available for recycling. We combine these data with steelmaking technology and trade scenarios to calculate potential U.S. steel sector emissions in each year to 2050. Only 20% of the pathways we modeled for the U.S. steel sector achieved the emissions target. Emissions in 2050 are most sensitive to the CO2 released per kilogram of steel produced and the steel stocks per capita. Deployment of emerging low carbon steelmaking technology alone is insufficient to achieve the emissions cut; conversely, reducing stocks per capita from the current ∼11 tons/capita toward levels in the U.K. and France, ∼8 tons/capita, would enable the emissions cut to be achieved under a range of foreseeable steelmaking technology scenarios and steel cycle parameters. If action to reduce per capita steel stocks is delayed by more than five years, then it is likely infeasible for the U.S. steel sector to stay within its 2050 CO2 budget because of the increased demand for emissions-intensive steel made from iron ore.

Sourcing of Steam and Electricity for Carbon Capture Retrofits.

This paper compares different steam and electricity sources for carbon capture and sequestration (CCS) retrofits of pulverized coal (PC) and natural gas combined cycle (NGCC) power plants. Analytical expressions for the thermal efficiency of these power plants are derived under 16 different CCS retrofit scenarios for the purpose of illustrating their environmental and economic characteristics. The scenarios emerge from combinations of steam and electricity sources, fuel used in each source, steam generation equipment and process details, and the extent of CO2 capture. Comparing these scenarios reveals distinct trade-offs between thermal efficiency, net power output, levelized cost, profit, and net CO2 reduction. Despite causing the highest loss in useful power output, bleeding steam and extracting electric power from the main power plant to meet the CCS plant's electricity and steam demand maximizes plant efficiency and profit while minimizing emissions and levelized cost when wholesale electricity prices are below 4.5 and 5.2 US¢/kWh for PC-CCS and NGCC-CCS plants, respectively. At prices higher than these higher profits for operating CCS retrofits can be obtained by meeting 100% of the CCS plant's electric power demand using an auxiliary natural gas turbine-based combined heat and power plant.

Analysis of Costs and Time Frame for Reducing CO2 Emissions by 70% in the U.S. Auto and Energy Sectors by 2050.

Using a least-cost optimization framework, it is shown that unless emissions reductions beyond those already in place begin at the latest by 2025 (±2 years) for the U.S. automotive sector, and by 2026 (-3 years) for the U.S. electric sector, 2050 targets to achieve necessary within-sector preventative CO2 emissions reductions of 70% or more relative to 2010 will be infeasible. The analysis finds no evidence to justify delaying climate action in the name of reducing technological costs. Even without considering social and environmental damage costs, delaying aggressive climate action does not reduce CO2 abatement costs even under the most optimistic trajectories for improvements in fuel efficiencies, demand, and technology costs in the U.S. auto and electric sectors. In fact, the abatement cost for both sectors is found to increase sharply with every year of delay beyond 2020. When further considering reasonable limits to technology turnover, retirements, and new capacity additions, these costs would be higher, and the feasible time frame for initiating successful climate action on the 70% by 2050 target would be shorter, perhaps having passed already. The analysis also reveals that optimistic business-as-usual scenarios in the U.S. will, conservatively, release 79-108 billion metric tons of CO2. This could represent up to 13% of humanity's remaining carbon budget through 2050.

Compliance by Design: Influence of Acceleration Trade-offs on CO2 Emissions and Costs of Fuel Economy and Greenhouse Gas Regulations.

The ability of automakers to improve the fuel economy of vehicles using engineering design modifications that compromise other performance attributes, such as acceleration, is not currently considered when setting fuel economy and greenhouse-gas emission standards for passenger cars and light trucks. We examine the role of these design trade-offs by simulating automaker responses to recently reformed vehicle standards with and without the ability to adjust acceleration performance. Results indicate that acceleration trade-offs can be important in two respects: (1) they can reduce the compliance costs of the standards, and (2) they can significantly reduce emissions associated with a particular level of the standards by mitigating incentives to shift sales toward larger vehicles and light trucks relative to passenger cars. We contrast simulation-based results with observed changes in vehicle attributes under the reformed standards. We find evidence that is consistent with firms using acceleration trade-offs to achieve compliance. Taken together, our analysis suggests that acceleration trade-offs play a role in automaker compliance strategies with potentially large implications for both compliance costs and emissions.

A stability assessment tool for anaerobic codigestion.

Anaerobic codigestion allows for greater resource recovery from organic substrates and provides opportunities for more stable operation than mono-digestion. Despite these benefits, the adoption of codigestion is limited because it can introduce operational complexity and suffers from some of the same challenges as mono-digestion, such as ammonia inhibition and nutrient imbalances. There is a need for rapid and cost-effective assessments that can provide insight to design engineers as they explore the valorization of local organic waste streams and seek to maintain or improve digester stability. To address this need, we developed and tested a tool that can yield useful stability indicators, performance predictions, and substrate selection protocols for codigestion. This tool uses quantitative, empirical data on stability indicators within an assessment framework to evaluate a digester's process stability. The tool's accuracy was tested using real and simulated digester data, and the importance of the nitrogen and lipid composition of a substrate was identified. The resulting stability assessment tool improves our fundamental understanding of codigestion, provides a mechanism to reduce the number of experiments, and guides selection of appropriate substrate combinations that can maximize energy recovery during codigestion without compromising process stability.

Reassessing the Efficiency Penalty from Carbon Capture in Coal-Fired Power Plants.

This paper examines thermal efficiency penalties and greenhouse gas as well as other pollutant emissions associated with pulverized coal (PC) power plants equipped with postcombustion CO2 capture for carbon sequestration. We find that, depending on the source of heat used to meet the steam requirements in the capture unit, retrofitting a PC power plant that maintains its gross power output (compared to a PC power plant without a capture unit) can cause a drop in plant thermal efficiency of 11.3-22.9%-points. This estimate for efficiency penalty is significantly higher than literature values and corresponds to an increase of about 5.3-7.7 US¢/kWh in the levelized cost of electricity (COE) over the 8.4 US¢/kWh COE value for PC plants without CO2 capture. The results follow from the inclusion of mass and energy feedbacks in PC power plants with CO2 capture into previous analyses, as well as including potential quality considerations for safe and reliable transportation and sequestration of CO2. We conclude that PC power plants with CO2 capture are likely to remain less competitive than natural gas combined cycle (without CO2 capture) and on-shore wind power plants, both from a levelized and marginal COE point of view.

Membrane biofilm development improves COD removal in anaerobic membrane bioreactor wastewater treatment.

Membrane biofilm development was evaluated to improve psychrophilic (15°C) anaerobic membrane bioreactor (AnMBR) treatment of domestic wastewater. An AnMBR containing three replicate submerged membrane housings with separate permeate collection was operated at three levels of membrane fouling by independently controlling biogas sparging for each membrane unit. High membrane fouling significantly improved permeate quality, but resulted in dissolved methane in the permeate at a concentration two to three times the equilibrium concentration predicted by Henry's law. Illumina sequencing of 16S rRNA targeting Bacteria and Archaea and reverse transcription-quantitative polymerase chain reaction targeting the methyl coenzyme-M reductase (mcrA) gene in methanogens indicated that the membrane biofilm was enriched in highly active methanogens and syntrophic bacteria. Restoring fouled membranes to a transmembrane pressure (TMP) near zero by increasing biogas sparging did not disrupt the biofilm's treatment performance, suggesting that microbes in the foulant layer were tightly adhered and did not significantly contribute to TMP. Dissolved methane oversaturation persisted without high TMP, implying that methanogenesis in the biofilm, rather than high TMP, was the primary driving force in methane oversaturation. The results describe an attractive operational strategy to improve treatment performance in low-temperature AnMBR by supporting syntrophy and methanogenesis in the membrane biofilm through controlled membrane fouling.

Market-driven emissions from recovery of carbon dioxide gas.

This article uses a market-based allocation method in a consequential life cycle assessment (LCA) framework to estimate the environmental emissions created by recovering carbon dioxide (CO2). We find that 1 ton of CO2 recovered as a coproduct of chemicals manufacturing leads to additional greenhouse gas emissions of 147-210 kg CO2 eq , while consuming 160-248 kWh of electricity, 254-480 MJ of heat, and 1836-4027 kg of water. The ranges depend on the initial and final purity of the CO2, particularly because higher purity grades require additional processing steps such as distillation, as well as higher temperature and flow rate of regeneration as needed for activated carbon treatment and desiccant beds. Higher purity also reduces process efficiency due to increased yield losses from regeneration gas and distillation reflux. Mass- and revenue-based allocation methods used in attributional LCA estimate that recovering CO2 leads to 19 and 11 times the global warming impact estimated from a market-based allocation used in consequential LCA.

Navigating wastewater energy recovery strategies: a life cycle comparison of anaerobic membrane bioreactor and conventional treatment systems with anaerobic digestion.

The objective of this study was to evaluate emerging anaerobic membrane bioreactor (AnMBR) technology in comparison with conventional wastewater energy recovery technologies. Wastewater treatment process modeling and systems analyses were combined to evaluate the conditions under which AnMBR may produce more net energy and have lower life cycle environmental emissions than high rate activated sludge with anaerobic digestion (HRAS+AD), conventional activated sludge with anaerobic digestion (CAS+AD), and an aerobic membrane bioreactor with anaerobic digestion (AeMBR+AD). For medium strength domestic wastewater treatment under baseline assumptions at 15 °C, AnMBR recovered 49% more energy as biogas than HRAS+AD, the most energy positive conventional technology considered, but had significantly higher energy demands and environmental emissions. Global warming impacts associated with AnMBR were largely due to emissions of effluent dissolved methane. For high strength domestic wastewater treatment, AnMBR recovered 15% more net energy than HRAS+AD, and the environmental emissions gap between the two systems was reduced. Future developments of AnMBR technology in low energy fouling control, increased flux, and management of effluent methane emissions would make AnMBR competitive with HRAS+AD. Rapid advancements in AnMBR technology must continue to achieve its full economic and environmental potential as an energy recovery strategy for domestic wastewater.

Lumped pathway metabolic model of organic carbon accumulation and mobilization by the alga Chlamydomonas reinhardtii.

Phototrophic microorganisms have significant potential as bioenergy feedstocks, but the sustainability of large-scale cultivation will require the use of wastewater as a renewable resource. A key barrier to this advancement is a lack of bioprocess understanding that would enable the design and implementation of efficient and resilient mixed community, naturally lit cultivation systems. In this study, a lumped pathway metabolic model (denoted the phototrophic process model or PPM) was developed for mixed phototrophic communities subjected to day/night cycling. State variables included functional biomass (XCPO), stored carbohydrates (XCH), stored lipids (XLI), nitrate (SNO), phosphate (SP), and others. PPM metabolic reactions and stoichiometry were based on Chlamydomonas reinhardtii , but experiments for model calibration and validation were performed in flat panel photobioreactors (PBRs) originally inoculated with biomass from a phototrophic system at a wastewater treatment plant. PBRs were operated continuously as cyclostats to poise cells for intrinsic kinetic parameter estimation in batch studies, which included nutrient-available conditions in light and dark as well as nitrogen-starved and phosphorus-starved conditions in light. The model was calibrated and validated and was shown to be a reasonable predictor of growth, lipid and carbohydrate storage, and lipid and carbohydrate mobilization by a mixed microbial community.

Psychrophilic anaerobic membrane bioreactor treatment of domestic wastewater.

A bench-scale anaerobic membrane bioreactor (AnMBR) equipped with submerged flat-sheet microfiltration membranes was operated at psychrophilic temperature (15 °C) treating simulated and actual domestic wastewater (DWW). Chemical oxygen demand (COD) removal during simulated DWW operation averaged 92 ± 5% corresponding to an average permeate COD of 36 ± 21 mg/L. Dissolved methane in the permeate stream represented a substantial fraction (40-50%) of the total methane generated by the system due to methane solubility at psychrophilic temperatures and oversaturation relative to Henry's law. During actual DWW operation, COD removal averaged 69 ± 10%. The permeate COD and 5-day biochemical oxygen demand (BOD(5)) averaged 76 ± 10 mg/L and 24 ± 3 mg/L, respectively, indicating compliance with the U.S. EPA's standard for secondary effluent (30 mg/L BOD(5)). Membrane fouling was managed using biogas sparging and permeate backflushing and a flux greater than 7 LMH was maintained for 30 days. Comparative fouling experiments suggested that the combination of the two fouling control measures was more effective than either fouling prevention method alone. A UniFrac based comparison of bacterial and archaeal microbial communities in the AnMBR and three different inocula using pyrosequencing targeting 16S rRNA genes suggested that mesophilic inocula are suitable for seeding psychrophilic AnMBRs treating low strength wastewater. Overall, the research described relatively stable COD removal, acceptable flux, and the ability to seed a psychrophilic AnMBR with mesophilic inocula, indicating future potential for the technology in practice, particularly in cold and temperate climates where DWW temperatures are low during part of the year.

Perspectives on anaerobic membrane bioreactor treatment of domestic wastewater: a critical review.

Interest in increasing the sustainability of water management is leading to a reevaluation of domestic wastewater (DWW) treatment practices. A central goal is to reduce energy demands and environmental impacts while recovering resources. Anaerobic membrane bioreactors (AnMBRs) have the ability to produce a similar quality effluent to aerobic treatment, while generating useful energy and producing substantially less residuals. This review focuses on operational considerations that require further research to allow implementation of AnMBR DWW treatment. Specific topics include membrane fouling, the lower limits of hydraulic retention time and temperature allowing for adequate treatment, complications with methane recovery, and nutrient removal options. Based on the current literature, future research efforts should focus on increasing the likelihood of net energy recovery through advancements in fouling control and development of efficient methods for dissolved methane recovery. Furthermore, assessing the sustainability of AnMBR treatment requires establishment of a quantitative environmental and economic evaluation framework.

Life cycle comparison of environmental emissions from three disposal options for unused pharmaceuticals.

We use life cycle assessment methodology to compare three disposal options for unused pharmaceuticals: (i) incineration after take-back to a pharmacy, (ii) wastewater treatment after toilet disposal, and (iii) landfilling or incineration after trash disposal. For each option, emissions of active pharmaceutical ingredients to the environment (API emissions) are estimated along with nine other types of emissions to air and water (non-API emissions). Under a scenario with 50% take-back to a pharmacy and 50% trash disposal, current API emissions are expected to be reduced by 93%. This is within 6% of a 100% trash disposal scenario, which achieves an 88% reduction. The 50% take-back scenario achieves a modest reduction in API emissions over a 100% trash scenario while increasing most non-API emissions by over 300%. If the 50% of unused pharmaceuticals not taken-back are toileted instead of trashed, all emissions increase relative to 100% trash disposal. Evidence suggests that 50% participation in take-back programs could be an upper bound. As a result, we recommend trash disposal for unused pharmaceuticals. A 100% trash disposal program would have similar API emissions to a take-back program with 50% participation, while also having significantly lower non-API emissions, lower financial costs, higher convenience, and higher compliance rates.

Comparison of life cycle emissions and energy consumption for environmentally adapted metalworking fluid systems.

A number of environmentally adapted lubricants have been proposed in response to the environmental and health impacts of metalworking fluids (MWFs). The alternatives typically substitute petroleum with vegetable-based components and/or deliver minimum quantities of lubricant in gas rather than water, with the former strategy being more prevalent than the latter. A comparative life cycle assessment of water- and gas-based systems has shown that delivery of lubricants in air rather than water can reduce solid waste by 60%, water use by 90%, and aquatic toxicity by 80%, while virtually eliminating occupational health concerns. However, air-delivery of lubricants cannot be used for severe machining operations due to limitations of cooling and lubricant delivery. For such operations, lubricants delivered in supercritical carbon dioxide (scCO2) are effective while maintaining the health and environmental advantages of air-based systems. Although delivery conditions were found to significantly influence the environmental burdens of all fluids, energy consumption was relatively constant under expected operating conditions. Global warming potential (GWP) increased when delivering lubricants in gas rather than water though all classes of MWFs have low GWP compared with other factory operations. It is therefore concluded that the possibility of increased GWP when switching to gas-based MWFs is a reasonable tradeoff for definite and large reductions in aquatic toxicity, water use, solid waste, and occupational health risks.

Optimization of metalworking fluid microemulsion surfactant concentrations for microfiltration recycling.

Microfiltration can be used as a recycling technology to increase metalworking fluid (MWF) life span, decrease procurement and disposal costs, and reduce occupational health risks and environmental impacts. The cost-effectiveness of the process can be increased by minimizing fouling interactions between MWFs and membranes. This paper reports on the development of a microfiltration model that establishes governing relationships between MWF surfactant system characteristics and microfiltration recycling performance. The model, which is based on surfactant adsorption/desorption kinetics, queueing theory, and coalescence kinetics of emulsion droplets, is verified experimentally. An analysis of the model and supporting experimental evidence indicates that the selection of surfactant systems minimally adsorb to membranes and lead to a high activation energy of coalescence results in a higher MWF flux through microfiltration membranes. The model also yields mathematical equations that express the optimal concentrations of anionic and nonionic surfactants with which microfiltration flux is maximized for a given combination of oil type, oil concentration, and surfactant types. Optimal MWF formulations are demonstrated for a petroleum oil MWF using a disulfonate/ ethoxylated alcohol surfactant package and for several vegetable oil MWFs using a disulfonate/ethoxylated glyceryl ester surfactant package. The optimization leads to flux increases ranging from 300 to 800% without impact on manufacturing performance. It is further shown that MWF reformulation efforts directed toward increasing microfiltration flux can have the beneficial effect of increasing MWF robustness to deterioration and flux decline in the presence of elevated concentrations of hardwater ions.

Structural aspects of surfactant selection for the design of vegetable oil semi-synthetic metalworking fluids.

This paper presents a set of surfactant-selection guidelines that can be used to design bio-based semi-synthetic metalworking fluid (MWF) microemulsions as a renewable alternative to conventional petroleum formulations. Ten surfactant classes (six anionic and four nonionic) with different head and tail structures and three vegetable base oils (canola oil, soybean oil, and a fatty acid trimethylolpropane ester) were investigated as representatives of oil and surfactant options currently under consideration in the MWF industry. All combinations of these surfactants and oils were formulated at the full range of oil to surfactant ratios and surfactant concentrations. The stability of each formulation was evaluated based on visual transparency, light transmittance, and droplet diameter. The experimental results yield the following guidelines that produce stable bio-based MWF microemulsions with minimum necessary concentrations of surfactants: (1) a combination of two surfactants, one nonionic and one water soluble co-surfactant (either nonionic or anionic) is preferred over a single surfactant; (2) the nonionic surfactant should have a carbon tail length greater than or equal to the nominal carbon chain length of the fatty acids in the oil as well as a head group that is not excessively small or large (e.g., 10-20 ethylene oxide groups for a polysorbitan ester, ethoxylated alcohol, or ethoxylated glyceryl ester); (3) the difference in tail lengths between the surfactant and the co-surfactant should be less than 6 to maximize the feasible range of oil to surfactant ratios yielding stable emulsions. These guidelines are consistent with general results of micelle solubilization theory and evidence is provided to suggest that common semi-synthetic MWF systems can be thought of as swollen micelle systems.

Modeling of porous filter penneability via image-based stochastic reconstruction of spatial porosity correlations.

A methodology for producing a pore-scale, 3D computational model of porous filter permeability is developed that is based on the analysis of 2D images of the filter matrix and first principles. The computationally reconstructed porous filter model retains statistical details of porosity and the spatial correlations of porosity within the filter and can be used to calculate permeability for either isotropic or 1D anisotropic porous filters. In the isotropic case, validation of the methodology was conducted using 0.2 and 0.8 microm ceramic membrane filters,forwhich it is shown that the image-based computational models provide a viable statistical reproduction of actual porosity characteristics. It is also shown that these models can predict water flux directly from first principles with deviations from experimental measurements in the range of experimental error. In the anisotropic case, validation of the methodology was conducted using a natural river sand filter. For this case, it is shown that the methodology yields predictions of filtration velocity that are similar or better than predictions offered by existing filtration models. It was found for the sand filter that the deviation between observation and prediction was mostly due to swelling during the preparation of the sand filter for imaging and can be reduced significantly using alternative methods reported in the literature. On the basis of these results, it is concluded that the computational reconstruction methodology is valid for porous filter modeling, and given that it captures pore-scale details, it has potential application to the investigation of permeability decline underthe influence of pore-scale fouling mechanisms.