Pretreatment is the first stage of sewage treatment: the wastewater is placed in a storage tank, the solids settle to the bottom, where they are collected, and lighter substances such as fats and oils are scraped off the top.
Secondary treatment refers to the place where waste is decomposed by aerobic bacteria that are incorporated into the wastewater treatment system.
Tertiary treatment refers to filtering out nutrients and waste particles that may damage sensitive ecosystems; wastewater passes through additional filtration to remove additional nutrients.