Activated sludge

Activated sludge is a wastewater-treatment process in which aerated microbial flocs consume organic pollutants and are separated from treated water by settling.

Aeration tank: A treatment basin where wastewater is mixed with air and microbial biomass. It supplies oxygen and mixing for the flocs that break down organic pollutants.
Heterotrophic bacteria: Bacteria that obtain carbon and energy by consuming organic compounds. They make up much of the biomass that removes organic pollutants.
Extended aeration: An activated-sludge configuration using long aeration times and low organic loading. It treats wastewater with a simple process that produces relatively stabilized sludge.
Trickling filter: A biological treatment system in which wastewater flows over a surface supporting attached microbial growth. Unlike activated sludge, it keeps most treatment biomass attached to media.
Activated sludge floc: A cluster of microorganisms and particles that forms suspended biomass in wastewater treatment. These aggregates consume pollutants and settle out of the treated water.
Nitrification: The microbial oxidation of ammonia to nitrite and then nitrate. A sufficiently long sludge age can support these slower-growing organisms.
Sequencing batch reactor: A wastewater-treatment system that performs fill, reaction, settling, and decanting in timed batches. It carries out activated-sludge treatment in one tank through staged operation.
Rotating biological contactor: A biological treatment device with rotating surfaces that alternately contact wastewater and air. Its attached growth contrasts with the suspended flocs of activated sludge.
Secondary clarifier: A settling tank that separates biological solids from treated wastewater. It returns settled biomass to the process while allowing clarified water to leave.
Dissolved oxygen: Oxygen dissolved in water and available to aquatic organisms and chemical reactions. Aeration maintains the oxygen needed for aerobic breakdown in the tank.

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