Dynamic Modeling of In-Use Cement Stocks in the United States

Emissions derived from human digestion of food and subsequent excretion are very relevant from a life cycle perspective, and yet they are often omitted from food life cycle assessment (LCA) studies. This article offers a simple model to allocate and include these emissions in LCAs of specific foodstuffs. The model requires basic food composition values and calculates the mass and energy balance for carbon, water, nutrients (mainly nitrogen [N] and phosphorus [P]), and other inorganic substances through different excretion paths: breathing, feces, and urine. In addition to direct excretion, the model also allocates some auxiliary materials and energy related to toilet use, such as flushing and washing and drying hands. Wastewater composition is also an output of the model, enabling water treatment to be modeled in LCA studies. The sensitivity of the model to food composition is illustrated with different food products, and the relative importance of excretion in a product's life cycle is shown with an example of broccoli. The results show that this model is sensitive to food composition and thus useful for assessing the environmental consequences of shifts in diet. From a life cycle perspective, the results show that postconsumption nutrient emissions may dominate the impacts on eutrophication potential, and they illustrate how the carbon cycle is closed with the human emissions after food preparation and consumption.