Anaerobic digestion for energy generation and nutrient recycling in the City of Buenos Aires

Abstract

Organic waste is the largest domestic waste category in the City of Buenos Aires (CABA) and is currently landfilled together with other waste streams. Landfilling organic waste not only has a large impact on the environment, but also leads to the loss of the value embedded in organic waste, such as nutrients and energy. In this regard, anaerobic digestion emerges as a potential waste treatment alternative that supports energy generation and nutrient recycling, while avoiding landfilling emissions.
Although AD is a relative mature and widely applied technology for the treatment of a variety of feedstocks (e.g., sewage sludge and animal manure), urban AD systems using biowaste are still in a preliminary stage. Therefore, the present research aims to evaluate the economic performance and the carbon footprint of this technology in CABA in the context of two case studies, where biogas is used to produce bioelectricity (C-1) and bio-CNG (C-2).
The case studies were assessed using a plant design which consisted of a biogas facility treating 23 thousand t/y of substrate, a mixture of OFMSW and recycled liquid fraction. Once biogas is produced as a result of the digestion process, a CHP and a membrane unit are used to produce bioelectricity and bio-CNG, respectively. Moreover, digestate, the material remaining after anaerobic digestion, is pasteurized for its utilization as biofertilizer on land.
The results of the economic analysis suggest that, under defined conditions, a positive NPV, IRR, and payback period can be obtained for both case studies. Nevertheless, there are high chances that the economic performance becomes negative, especially when changes are simulated that directly impact the amount of revenue the project makes.
The results of the carbon footprint indicate that both case studies could lead to substantial carbon savings, given that the avoided GHG emissions are substantially higher than the emitted ones. Large savings are obtained from avoiding the landfilling of organic waste, and replacing conventional energy, fuel, and fertilizers. The results of the carbon footprint are less sensitive to simulations performed, given the margin of avoided emissions over the emitted ones.
The analysis concluded that while both case studies are very likely to present environmental benefits, the economic constraints might impose a drawback for its implementation. Therefore, the support of the government is crucial to promote the adoption of AD, considering all the benefits that are associated with this technology.