Definitions for "Primary treatment"
the first, and usually the most important, treatment.
The first stage of treating sewage, usually the removal of big solids such as pieces of wood, as well as rags and plastics.
Primary treatment involves a physical and/or chemically-enhanced settlement of suspended solids that is not removed by preliminary treatment. The Directive sets percentage reduction figures for biochemical oxygen demand of the influent by at least 20% and a reduction in total suspended solids in the influent by at least 50% before discharge to receiving waters.