ABSTRACT
The design of pollution control systems requires knowledge of the mass and mass flow rate of all materials that move through the system. Most pollutants are measured as concentrations, and there are a variety of ways that these quantities can be expressed, depending on whether the material is a gas, liquid, or solid. The basic SI units that are used to measure mass, length, volume, and time are the kilogram (kg), meter (m), cubic meter (m3) or liter (L), and second (s). Bulk density refers to the material as it is delivered or as it flows. Concentrations may be expressed using several choices of units, and the choice may change if the concentration refers to water or wastewater, sludge, soil, or gas. The volume fractions for gases in a mixture do not change when the gas expands or contracts. This is not true for mass concentrations.
