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Monitoring CO 2 concentrations in urban areas is crucial for determining the efficacy of climate change mitigation policies. However, highly heterogeneous land use, local geography, and local convection patterns, which vary throughout the urban landscape, complicate this task. To establish continuous monitoring programs, it is important to first determine the heterogeneity of urban landscapes on the ground. To understand the role these factors play in the distribution of CO 2 over an urban area, we conducted a CO 2 measurement campaign over the Metropolitan Area of Barcelona (AMB) over four urban land uses: impervious, green, forest, and agricultural. There is a clear tendency for CO 2 mixing ratios to decrease as the degree of urban vegetation increases, even in the midst of a developed boundary layer. For example, CO 2 concentrations were 429 and 427 ppm at forest and agricultural sites, respectively, while 485 ppm was reported at urban sites. A decrease in atmospheric CO 2 was observed from 458 to 428 ppm in the gradient from urban to suburban areas, in which the biosphere component increased. The biosphere component of the CO 2 signal was significant and was observed in the gradient from urban to suburban areas, which averaged a reduction from 458 to 428 ppm. Our findings show that the large spatial variability in CO 2 concentrations (ranging from 410 to 495 ppm) is best explained by anthropogenic activity. We propose increasing the spatiotemporal resolution of CO 2 monitoring in the AMB to determine these trends more precisely over longer periods of time. • First data showing CO 2 concentration distribution over the Metropolitan Area of Barcelona. • CO 2 concentrations decrease from urban to suburban areas as biosphere increases. • Local mixing ratios of CO 2 strongly linked to temporal changes in air masses. [ABSTRACT FROM AUTHOR]

• Vulnerabilities are assessed in terms of ecosystem services and urban metabolism. • A stepwise, multi-criteria approach allows evaluating conflicting NBS impacts. • Agricultural growth increased biodiversity vulnerability, except in built-up areas. • Vulnerability to heat was not reduced by agricultural expansion in any scenario. • Recreational vulnerability was decreased despite the reduction in green areas. Nature-based solutions (NBS) are increasingly employed to address urban challenges. Typically, NBS planning emphasizes environmental impacts and ecosystem services, often overlooking their role in addressing vulnerabilities. Our objective is to develop a framework assessing the extent to which NBS alter urban vulnerabilities. For this, we relate ecosystem service and urban metabolism analyses to spatially explicit vulnerabilities. The framework relies on multi-criteria decision analysis to integrate diverse impacts. It follows a stepwise approach including the development of land-use scenarios, selection of vulnerabilities and indicators, normalization and aggregation of indicators, and stakeholder weighting. We apply the framework to the Metropolitan Area of Barcelona to assess the impacts of increasing (peri‑)urban agriculture in terms of critical vulnerabilities: heat, lack of recreational space, biodiversity loss, and lack of local food. Results show that agricultural expansion decreased the vulnerability of lack of local food, increased the vulnerability of biodiversity loss, and increased the heat vulnerability in terms of night temperatures for sensitive areas. Results reveal diverse spatial outcomes and trade-offs in urban vulnerabilities due to shifts in (peri‑)urban agriculture. The framework innovatively evaluates NBS impacts by linking multiple evaluation methods through spatially explicit vulnerabilities, fostering the strategic planning of NBS at the urban metropolitan scale. [ABSTRACT FROM AUTHOR]

• A framework for an integrated vision of dwellers' food-energy-water needs is posed. • The paper considers environmental, social, economic, and spatial dimensions. • Urban forms are more prone to implement different resource production on rooftops. • The scenarios display shares of resource self-sufficiency ranging from 16% to 433%. • Substantial reduction of the carbon footprint by producing their own resources. The external resource dependency of urban areas results in the externalization of environmental and socioeconomic impacts. Implementing food, energy and water production systems on urban rooftops (roof mosaics) can potentially help cities become more self-sufficient but depends on the city's urban morphology. We studied the supply potential and impacts of four roof mosaic scenarios for different urban forms in Cerdanyola, a 58 thousand-inhabitant town in the metropolitan area of Barcelona. We combined spatial analysis of potential rooftops, metabolism analysis, and social and environmental impacts. The municipality has an average rooftop/household potential of 31 m2 on which to implement any of the mosaic scenarios, with the highest potential in the single-family housing typology. The highest level of vegetable self-sufficiency was found in housing estates (32%), and the lowest in originary fabrics (28%). Regarding electricity and water self-sufficiency, the highest self-sufficiency level was found in the single-family housing typology (51% and 14%, respectively) and the lowest in housing estates (26% and 8%, respectively). Regarding impacts, the implementation of the electricity and rainwater harvesting systems depicts the most positive indicators in single-family housing areas. However, for housing estates and originary fabrics typologies, the best performance is shown to be in the implementation of rooftop farming systems. [ABSTRACT FROM AUTHOR]

Sewers are known as longitudinal reactors where gases such as methane, nitrous oxide and hydrogen sulphide can be produced. However, gaseous emissions have been mainly assessed in wastewater treatment plants (WWTP). This article presents a critical review of studies that quantify the generation of these gases in sewers and aims to identify the existing research gaps. Differences in sampling methods and site selection, as well as a limited number of studies, result in incoherent comparisons. To address some of these gaps, sampling campaigns were conducted in two Spanish cities. Results showed that wet wells were the most important sources of gases with concentrations up to 321 μg CH 4 L air −1 and 6.8 μg N 2 O L air −1 . Regarding emission factors, in the case of Calafell, the estimated annual emissions were 18.6 kg CH 4 year −1 and 0.3 kg H 2 S year −1 in summer and 3.8 kg CH 4 year −1 and 0.5 kg H 2 S year −1 in winter. About Betanzos, these values were 24.6 kg CH 4 year −1 and 0.5 kg N 2 O year −1 in summer and 10 kg CH 4 year −1 in winter. The summer campaign resulted in greater gas concentration than in the winter season for both cities, suggesting that temperature is a key parameter. We conclude that gas emissions from sewers are significant compared to those of WWTPs resulting in an important contribution to the carbon footprint. Further work needs to be done to assess the gas production along the entire sewer networks, which can result in very different emission factors depending on the sewer components. [ABSTRACT FROM AUTHOR]

Biogas is rich in methane and can be further purified through biogas upgrading technologies, presenting a viable alternative to natural gas. Landfills and anaerobic digestors treating municipal solid waste are a large source of such biogas. They therefore offer an attractive opportunity to tap into this potential source of natural gas while at the same time minimizing the global warming impact resulting from methane emissions in waste management schemes (WMS) and fossil fuel consumption reduction. This study looks at the current municipal solid waste flows of Spain, Italy, and Austria over one year (2009), in order to determine how much biogas is generated. Then it examines how much natural gas could be substituted by using four different biogas upgrading technologies. Based on current waste generation rates, exploratory but realistic WMS were created for each country in order to maximize biogas production and potential for natural gas substitution. It was found that the potential substitution of natural gas by biogas resulting from the current WMS seems rather insignificant: 0.2% for Austria, 0.6% for Italy and 0.3% for Spain. However, if the WMS is redesigned to maximize biogas production, these figures can increase to 0.7% for Austria, 1% for Italy and 2% for Spain. Furthermore, the potential CO 2 reduction as a consequence of capturing the biogas and replacing fossil fuel can result in up to a 93% reduction of the annual national waste greenhouse gas emissions of Spain and Italy. [ABSTRACT FROM AUTHOR]

Abstract: In 2010, the generation of municipal solid waste (MSW) by the European Unión (EU-27) was 252 million tons, with an estimated organic content of 30–40% by weight. Composting this organic matter would significantly improve waste reduction and mineral fertilizer substitution. We present a Life Cycle Analysis (LCA) and agronomical assessment of the following three fertilization treatments: industrial compost (IC), home compost (HC) and mineral fertilizer (MF), applied to horticultural cauliflower crops. For the IC and HC treatments, we evaluated the entire cycle of the organic matter, starting from the moment it becomes MSW and including collection, production of compost, transportation and application in open field cauliflower crops. For the MF treatment, the analysis includes the raw material extraction, production, transportation and the application of the compost material to crops via irrigation. A higher crop yield was achieved with MF treatment, which was 26% and 91% higher than HC and IC treatment, respectively. However, the application of HC treatment resulted in larger, heavier cauliflowers. No significant differences were found in the nutritional analysis, which included the quantification of the total phenols, glucosilonates and flavonoids. The HC treatment had the best environmental performance with the lowest impact in all categories assessed except for its abiotic depletion potential and eutrophication potential (which was the lowest for IC). The IC treatment had the highest environmental impact in five of the seven categories assessed, whereas the MF treatment had the highest eutrophication and global warming potentials. [Copyright &y& Elsevier]

• Cities benefit from nutrient circularity integrating waste and local agriculture. • Significant benefits from biogas generation and avoiding mixed waste treatment. • Nutrient concentrations in urban agricultural soils limit application of compost. • Local regulations and soil quality hinder nutrient circularity in urban areas. Cities face the challenges of supplying food and managing organic municipal solid waste (OMSW) sustainably amid increasing urbanization rates. Urban agriculture (UA) can help with this effort by producing local crops that are fertilized with nutrients recovered from compost generated from OMSW. This research aims to determine the potential of OMSW compost to supply the nitrogen-phosphorus-potassium (NPK) demand of UA and the environmental benefits of replacing mineral fertilizer from a life cycle perspective. The Metropolitan Area of Barcelona (AMB) serves as the case study given its commitment to reuse biowaste according to the Revised Waste Framework Directive and to promote UA as a signing member of the Milan Urban Food Policy Pact. Based on crop requirements and farmer surveys, we find that the annual NPK demands of the agricultural fields of the AMB that cover 5,500 ha and produce 70,000 tons of crops are approximately 769, 113, and 592 tons of NPK, respectively. Spatial material flow analysis and life cycle assessment were applied to found that the current waste management system can potentially substitute 8 % of the total NPK demanded by UA with compost, reduce the impacts by up to 39 % and yield savings in global warming of 130 %. The more ambitious future scenario of 2025 can potentially substitute 21 % of the total NPK demand and reduce environmental impacts up to 1,049 %, depending on the category considered. Avoiding processing of mixed OMSW, mineral fertilizer replacement and cogeneration of electricity from biogas are the major contributors to these environmental savings. [ABSTRACT FROM AUTHOR]

Despite efforts to increase the selective collection of municipal solid waste (MSW) in developed countries, the amount of unsorted waste remains high, with the consequent difficulty of material recovery and recycling. In 2010, 61% of the MSW generated in the European Union (EU) ended up in landfill and incineration facilities. Autoclaving is a novel technology that can be used to treat unsorted MSW, producing organic fibers that can be composted. The life cycle analysis (LCA) was used to assess the effectiveness of autoclaving unsorted MSW and various alternative methods for treating organic fibers produced through this process. The alternative methods that were considered included composting in tunnels, composting in confined windrow and composting in turning windrow as well as anaerobic digestion. The environmental assessment results were compared to those associated with incineration and landfill. The results of this study showed that autoclaving with sorting, digesting anaerobically and composting had the lowest impact values for eutrophication and the global warming potential. It was also found that autoclaving is justified only if the products of the process, that is, polyethylene terephthalate, ferrous and non-ferrous metals, are recycled to avoid virgin material production and if the remaining mixed plastic wastes are incinerated for energy recovery. [ABSTRACT FROM AUTHOR]

Abstract: An alternative source of methane that can also reduce the greenhouse gas effect is one that comes from the upgrading of biogas. This paper studies eight technologies through life cycle assessment (LCA). Six of the technologies are ones that are already on the market and the two others are novel technologies that use carbon mineralization to store CO2 upon their removal. The two novel technologies include alkaline with regeneration (AwR) and bottom ash upgrading (BABIU). These technologies use waste rich in calcium, from municipal solid waste incinerators (MSWI), to store the CO2 from biogas. Among all conventional technologies, high pressure water scrubbing and chemical scrubbing with amine had the lowest CO2 impacts. Of the novel technologies BABIU saves 10% more CO2 than AwR. An uncertainty analysis and a material flow analysis demonstrated that proximity to a MSWI is an important factor to consider. As well, it was seen that while the technology is promising it cannot be applied to an entire country if the proper infrastructure is not in place. [Copyright &y& Elsevier]

Abstract: This article evaluates the life cycle assessment (LCA) of three biogas upgrading technologies. An in-depth study and evaluation was conducted on high pressure water scrubbing (HPWS), as well as alkaline with regeneration (AwR) and bottom ash upgrading (BABIU), which additionally offer carbon storage. AwR and BABIU are two novel technologies that utilize waste from municipal solid waste incinerators – namely bottom ash (BA) and air pollution control residues (APC) – and are able to store CO2 from biogas through accelerated carbonation processes. These are compared to high pressure water scrubbing (HPWS) which is a widely used technology in Europe. The AwR uses an alkaline solution to remove the CO2 and then the solution – rich in carbonate and bicarbonate ions – is regenerated through carbonation of APC. The BABIU process directly exposes the gas to the BA to remove and immediately store the CO2, again by carbonation. It was determined that the AwR process had an 84% higher impact in all LCA categories largely due to the energy intensive production of the alkaline reactants. The BABIU process had the lowest impact in most categories even when compared to five other CO2 capture technologies on the market. AwR and BABIU have a particularly low impact in the global warming potential category as a result of the immediate storage of the CO2. For AwR, it was determined that using NaOH instead of KOH improves its environmental performance by 34%. For the BABIU process the use of renewable energies would improve its impact since accounts for 55% of the impact. [Copyright &y& Elsevier]

Urban agriculture systems, such as rooftop greenhouses, are attractive alternatives for mitigating the impacts of the extensive food supply chains that currently feed cities. In this study, we study the opportunity that nutrient recirculation offers to improve the environmental performance of agricultural systems. In particular, we analyze the environmental burdens of a hydroponic closed-loop production system that recovers nutrients and reduces water demand by recirculating the irrigation water leaching from the substrate bags along with nutrients that have not been assimilated by the plant. The closed-loop system is compared to a linear system in which there is no nutrient or water recovery. Based on two green bean crop cycles in a Mediterranean rooftop greenhouse, we analyze the yield, climatic variables and water and nutrient balances, and apply life cycle assessment (LCA) to study the environmental impacts. The results of this study indicate that closed-loop systems save daily 40% of irrigation water and between 35 and 54% of nutrients. Moreover, leachate reuse leads to reduced eutrophication impacts, but it can entail nutrient deficiencies. However, implementing a closed-loop system requires additional infrastructure causing larger impacts than linear systems in terms of global warming and fossil resource scarcity. The results of the LCA were highly sensitive to the yield, the crop production period and the meteorological conditions. Based on these results, we design improved scenarios, providing recommendations for reducing the impacts of closed-loop systems for more sustainable cities. Image 1 • Closed systems presented 40% in daily water savings. • Large savings were obtained in closed systems in eutrophication impacts. • High sensitivity was found related to yield, climatic conditions and crop length. • High impacts in closed systems came from the materials required, mainly benches. • Room for improvement exists for closed systems. [ABSTRACT FROM AUTHOR]

Due to population growth and the subsequent increase in the demand for food, low carbon food chain production systems are a necessity to reduce the effects on climate change as much as possible. Urban agriculture is of great interest because of its potential in reducing the indirect CO 2 emissions of a city's food supply by reducing transportation distances, the packaging required and the food losses that occur during transportation. However, intensive urban agriculture production, which often relies on the use of soilless substrates, requires synthetic fertilizers rich in nitrogen, resulting in N 2 O emissions. Presently, there is a lack of studies that determine the generation of N 2 O from soilless crops to properly account for their global warming potential. In this study, an open chamber system was used to quantify N 2 O emissions from lettuce crops with perlite bags as their substrate in a Mediterranean rooftop greenhouse located in the metropolitan area of Barcelona (Spain). N 2 O generation, through nitrifying and denitrifying reactions, was limited by assuring an aerobic environment, negligible water retention, the absence of NH 3 , and controlled dosage of NO 3 − in the most favorable pH conditions for plant assimilation. The emission factor (EF) measured for the soilless lettuce crop (0.0072–0.0085 kg N 2 O −1 per kg N −1 ) was half the EF of the IPCC method (0.0125 kg N 2 O −1 per kg N −1 ) for soil crops, which is commonly used in life cycle assessment (LCA) studies to approximate direct N 2 O emissions, for lack of a better method. Using a more appropriate EF for an LCA study of a tomato crop grown under similar conditions to those used to generate the EF resulted in a 7.5% reduction (0.06 kg CO 2 eq. per kilogram of tomato production) in total global warming potential. This study shows that soilless crops reduce N 2 O emissions when compared to conventional crops, making urban agriculture an attractive practice for reducing GHG emissions. The results highlight the need to determine a standard method for determining an EF applicable to soilless protected crops, which, based on the parameters described here, such as the type of substrate, fertilizers and irrigation system, would allow for a more accurate environmental evaluation of soilless conventional and urban crops. [ABSTRACT FROM AUTHOR]

This paper studies the potential application of a novel biogas upgrading technology called alkaline with regeneration (AwR). This technology uses an alkaline solution, along with carbon mineralization, to remove and store CO 2 from biogas in order to create biomethane, a substitute of natural gas. Three different applications of biogas were explored for their potential economic benefits along three different biogas generation capabilities of landfills in Spain (250 Nm 3 /h, 1000 Nm 3 /h and 5000 Nm 3 /h). The scenarios include upgrading biogas using AwR and injecting the biomethane into the natural gas grid, or selling the gas as a vehicle fuel. The third reference scenario assessed directly burning the biogas for the production of electricity. The latter showed an annual profit of 0.2–5 million € 2012 while upgrading the biogas to obtain biomethane showed an annual loss of 3–50 million € 2012 . This was due to the operational costs involved in AwR, namely the cost of NaOH (principal reagent) and the treatment of wastewater. Increasing revenue can help obtain an annual profit. In order to break-even it would be necessary to raise CO 2 credits to 99 € 2012 /t or, through feed in tariffs, increase the price of the sale of biomethane to 0.25 € 2012 /kWh. [ABSTRACT FROM AUTHOR]

Abstract: Carbon mineralization is a promising process for carbon dioxide (CO2) capture and storage, and it can also be applied for biogas upgrading. This paper uses a life cycle assessment to identify possible methods of improving and reducing the environmental impact of novel biogas upgrading technologies. The two novel pilot-scale technologies that are assessed are alkaline absorption with regeneration (AwR) process and bottom ash for biogas upgrading (BABIU) process. These technologies are still at the pilot–plant scale, which offers the opportunity to identify how their environmental impact can be reduced before industrial-scale production occurs. The AwR technology uses an alkaline solution of either potassium hydroxide or sodium hydroxide to capture the CO2 and air pollution control residues to regenerate the solution. The BABIU process uses bottom ash (BA) from municipal solid waste incinerators to capture CO2 in a direct gas/solid contact process. For the AwR process, focus was placed on the reuse of wastewater (air pollution control residues require pre- and post-washing steps) and reagent type and concentration. Further improvements have focused on reagent and electricity use in AwR and the transport of BA for BABIU. The lowest environmental impact for AwR resulted when NaOH and in-process wastewater was reused. Even in such a process setting, AwR still has a higher environmental impact in 11 of 12 categories compared to other conventional biogas upgrading processes. On average, the BABIU process has a 73% lower impact than AwR and is comparable to conventional technologies. [Copyright &y& Elsevier]

Abstract: Until now, few studies had focused on the environmental impact of the construction phase of a drinking water transport and distribution network (DWTDN). Using the life cycle assessment (LCA) methodology, this article compares the environmental impact of pipes made of different materials as constructive solutions for the DWTDN. Two pipe diameters (90 and 200 mm) commonly used in small to medium-sized cities are analysed. The results show that polyvinyl chloride (PVC), high density polyethylene (HDPE) and low density PE have similar environmental impacts in the case of 90 mm pipe diameter. In the case of 200 mm pipe diameter, ductile iron (DI) and glass fibre reinforced polyester show higher environmental impacts than HDPE and PVC, which in the case of DI are between 3 and 11 times higher than those of HDPE for all the midpoint impact categories. Regarding the different construction phases, installation has a higher percentage of environmental impact for 90 mm pipe diameter (40–68% for HDPE in all the impact categories) than for 200 mm pipe diameter (24–57% for an HDPE) due to the difference in the amount of material required for the manufacture of the pipe. The assessment methodology was applied to calculate the environmental burdens derived from a case study. The impact of the different elements of the case study network has been added to obtain the global impact. The potential reduction of the environmental impacts of the case study has been calculated substituting the whole actual network by less impacting constructive solutions. A potential reduction of between 6 and 16% of the impact has been found for the case study, although the savings might be greater in networks with greater abundance of more impacting pipe materials such as DI. This methodology allows the improvement of the network and the design of more eco-efficient DWTDN. [Copyright &y& Elsevier]