Waste discharge requirements depend on which of the following metrics?

• A. microbial count
• B. energy content
• C. pollutant mass per liter of water
• D. color and odor

Answer: (C)

The regulatory standard for waste discharge is driven by how much pollutant is present in the water per unit volume, because the ecological and health impact depends on the concentration that the receiving body experiences. Expressing limits as pollutant mass per liter of water directly ties the amount of pollutant to the amount of water it’s mixed with, which is why regulators specify concentration limits (for example, mg/L for common pollutants). While sometimes permits reference total mass loading (kg/day) to account for flow, the metric shown here—mass per liter—best captures the concentration-based approach used to set discharge limits. Microbial count, energy content, and color/odor relate to other aspects of water quality, but they are not the primary quantitative basis for most discharge requirements.