BOD (biochemical oxygen demand): It is actually a measure of the amount of oxygen microorganisms will consume to convert organic material, so it is a measure of the "strength" of wastewater. The standard test measures BOD for 5 days (BOD5).

Clarifier/Settling basin: a basing in which the force of gravity is used to settle/separate particles from the water.

Eutrophication: the "aging" of a surface water due to the excess deposition of nutrients such as phosphorus and nitrogen.

Floc: an aggregate of particles. In the case of wastewater treatment, it is an aggregate of microorganisms. They are cultivated this way to assist in settleability (i.e. help separate the clean/treated water from the microorganisms).

Pretreatment: the first portion of primary treatment that includes bar screens, grit chambers, and equalization basins, handles raw sewage.

Primary Treatment: treatment methods that remove settleable or floating pollutants.

Secondary Treatment: the second phase of treatment that seeks to eliminate the soluble/dissolved pollutants from the wastewater. Biological treatment is most commonly used at this stage (i.e. activated sludge)..

SRT (Solids retention time): is the main criteria used in designing wastewater treatment plants (WWTP). It refers to the average amount of time that any solid floc stays within the system.

Tertiary/Advanced Treatment: treatment methods applied after secondary treatment to get the water at or close to drinking level standards. Usually requires chemical treatment methods.