The question of creating cyclical production/waste cycles is critical to a theory of urban metabolic justice.  “Waste” itself is does not represent a “biophysical sphere” in the same sense that soil, water, and air do with defined edges pertaining largely to their physical material state.  Rather, waste can be thought more of being a component of metabolic processes, either linear or cyclical.  In a linear metabolism, waste is equal to output, discharge, emissions – unwanted and toxic materials (trash, wastewater, particulates, CO2, heat) externalized to the atmosphere, sewers, incinerators, and landfills.  While some resource recovery is possible from linear metabolic waste, it is done only with considerable energetic expense, and usually results in a recovered material with high toxic concentrations (biosolids from sewage treatment, landfill gas) – albeit serving as a profitable revenue source for an elite few.  Furthermore, in keeping with an environmental justice analysis, it is commonly lower-income and marginalized populations who bear the greatest risk of exposure to these toxic and noxious outputs on account of their proximity.  Increasingly, the flow-paths for metabolic waste outputs are growing longer, with toxic burdens being externalized to communities living far from their points of production.  This has long been the case for airborne pollutants, which can be transported long distances through atmospheric “grasshopper” effects, but more so now for solid waste which is being shipped greater distances as more and more peri-urban landfills reach capacity.