(December 2009). Metabolic wastes or excretes are substances left over from excretory processes, which cannot be used by the organism (they are surplus or have lethal effect), and must therefore be excreted.This includes nitrogen compounds, water, CO2, phosphates, sulfates, etc.December 2009). Metabolic wastes or excretes are substances left over from excretory processes, which cannot be used by the organism (they are surplus or have lethal effect), and must therefore be excreted.