Waste-to-energy systems
can play an important role in diverting
organic waste from landfills. However, real-world waste management
can differ from idealized practices, and emissions driven by microbial
communities and complex chemical processes are poorly understood.
This study presents a comprehensive life-cycle assessment, using reported
and measured data, of competing management alternatives for organic
municipal solid waste including landfilling, composting, dry anaerobic
digestion (AD) for the production of renewable natural gas (RNG),
and dry AD with electricity generation. Landfilling is the most greenhouse
gas (GHG)-intensive option, emitting nearly 400 kg CO2e per tonne of organic waste. Composting raw organics resulted in
the lowest GHG emissions, at −41 kg CO2e per tonne
of waste, while upgrading biogas to RNG after dry AD resulted in −36
to −2 kg CO2e per tonne. Monetizing the results
based on social costs of carbon and other air pollutant emissions
highlights the importance of ground-level NH3 emissions
from composting nitrogen-rich organic waste or post-AD solids. However,
better characterization of material-specific NH3 emissions
from landfills and land-application of digestate is essential to fully
understand the trade-offs between alternatives.
Food waste and biopolymers, plastics derived from plants, are unexploited sources of energy when discarded in landfills without energy recovery. In addition, polylactic acid (PLA) and food waste have complimentary characteristics for anaerobic digestion; both are organic and degrade under anaerobic conditions. Lab-scale reactors were set up to quantify the solubilization of pretreated amorphous and crystalline PLA. Biochemical methane potential (BMP) assays were performed to quantify CH4 production from both treated and untreated PLA in the presence of food waste and anaerobic digested sludge. Amorphous and crystalline PLA reached near-complete solubilization at 97% and 99%, respectively, when alkaline pretreatment was applied. The PLA that received alkaline treatment produced the most of CH4 throughout the run time of 70 days. The PLA without treatment resulted in 54% weight reduction after anaerobic digestion. Results from this study show that alkaline pretreatment has the greatest solid reduction of PLA and maximum production of CH4 when combined with food waste and anaerobic digested sludge.
Water management plays a major role in any city, but applying alternative strategies might be more or less feasible depending on the urban form and water demand. This paper aims to compare the environmental performance of implementing rainwater harvesting (RWH) systems in American and European cities. To do so, two neighborhoods with a water-stressed Mediterranean climate were selected in contrasting cities, i.e., Calafell (Catalonia, Spain) and Ukiah (California, US). Calafell is a high-density, tourist city, whereas Ukiah is a typical sprawled area. We studied the life cycle impacts of RWH in urban contexts by using runoff modeling before (i.e. business as usual) and after the implementation of this system. In general, cisterns were able to supply >75% of the rainwater demand for laundry and toilet flushing. The exception were multi-story buildings with roofs smaller than 200m, where the catchment area was insufficient to meet demand. The implementation of RWH was environmentally beneficial with respect to the business-as-usual scenario, especially because of reduced runoff treatment needs. Along with soil features, roof area and water demand were major parameters that affected this reduction. RWH systems are more attractive in Calafell, which had 60% lower impacts than in Ukiah. Therefore, high-density areas can potentially benefit more from RWH than sprawled cities.
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