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Activated Sludge Treatment Process

What is it?

Activated sludge refers to a mass of microorganisms cultivated in the treatment process to break down organic matter into carbon dioxide, water, and other inorganic compounds. The activated sludge process has three basic components: 1) a reactor in which the microorganisms are kept in suspension, aerated, and in contact with the waste they are treating; 2) liquid-solid separation; and 3) a sludge recycling system for returning activated sludge back to the beginning of the process. There are many variants of activated sludge processes, including variations in the aeration method and the way the sludge is returned to the process.

When to use it?

While many activated sludge treatment works have been built in developing countries, very few work as well as intended. Activated sludge can be appropriate where high removal of organic pollution is required, funds and skilled personnel are available for operation and maintenance, and land is scarce or expensive. Since activated sludge requires the continuous operation of oxygen blowers and sludge pumps, a steady energy supply is a key requirement. The system usually needs some form of pretreatment, such as screening and primary sedimentation.


Efficient removal of BOD, COD and nutrients when designed and professionally operated according to local requirements. The process itself has flexibility and numerous modifications can be tailored to meet specific requirements (e.g. for nitrogen removal). Activated sludge is the best documented and most widely used form of secondary wastewater treatment.


Expensive in terms of both capital and O&M costs, requires a constant energy supply, needs trained operators who can monitor the system and react to changes immediately, and the availability of spare parts and chemicals may be an obstacle. The track record of activated sludge plants in the developing world is very poor, and few operate as designed or intended.

Technical features and requirements:

There is a vast literature on the design of various forms of the activated sludge treatment process. General considerations include: wastewater characteristics, local environmental conditions (including temperature), possible presence of toxic or other inhibitory substances (will the process receive industrial effluents or septage, for instance), oxygen transfer requirements and reaction kinetics (detention time in the system, related to quality and quantity of wastewater received, effluent requirements, sludge treatment requirements and other factors listed above).

Return Activated Sludge (4)

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