Activated sludge process is a process for treating sewage and waste water commonly referred as effluent using bacteria (to degrade the biodegradable organics) and air (Oxygen for respiration).

Activated sludge refers to a mixture of microorganisms and suspended solids. The bacterial culture is cultivated in the treatment process to break down organic matter into carbon dioxide, water, and other inorganic compounds. The typical activated sludge process has following basic components:

1) Primary Clarifier to separate the solids carried along with Sewage/Effluent

2) A reactor in which the microorganisms are kept in suspension, aerated, and in contact with the waste they are treating

3) liquid-solid separation; and

4) 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.

Activated sludge process offers efficient removal of BOD, COD and nutrients when designed professionally and operated properly. The process itself has flexibility and numerous modifications can be tailored to meet specific requirements (e.g. for nitrogen removal).

It is a complex mix of microbiology and biochemistry involving many different sorts of microbes. In the Activated Sludge Plant (ASP) bacteria secrete sticky substances that coat the minute particles carried in sewage. The particles stick together to form flocs of gel-like material, creating a support on, and in which, microbes exist. This is the chocolate-brown coloured activated sludge. The activated sludge is aerated to dissolve oxygen which allows the organic matter (BOD) to be utilised by the bacteria. The organic matter, or food, sticks to the activated sludge. The oxygen dissolved in the water allows the bacteria to use the food (BOD) and also to change the ammonia to nitrate. The tank should be big enough to allow sufficient contact time (retention time) between the sewage and the activated sludge for all the chemical changes to take place.

Return Activated Sludge (RAS)

When the Activated Sludge reaches the end of the process it is still a highly active biomass but is now mixed with purified effluent. It is transferred to Settlement Tanks (Secondary Clarifiers) to allow separation from the purified effluent which may be discharged to the river or to some form of tertiary treatment. The settled biomass, called Return Activated Sludge (RAS), is then returned to the beginning of the aeration process where it will absorb fresh sewage to start the process again. This enables the process to operate as a continuous cycle.

Surplus Activated Sludge (SAS)

As the RAS mixing with the fresh sewage will produce a gradual growth in the activated sludge present it is necessary to waste a certain quantity each day. This Surplus Activated Sludge (SAS) is wasted by continuously withdrawing some of the RAS for sludge disposal.

A typical Flow Sheet is given below depicting all components of Activated Sludge Process

Aeration Methods: 

Diffused Aeration: Sewage liquor is run into deep tanks with diffuser grid aeration systems that are attached to the floor. Air is pumped through the blocks and the curtain of bubbles formed both oxygenates the liquor and also provides the necessary mixing action. Where capacity is limited or the sewage is unusually strong or difficult to treat, oxygen may be used instead of air. Typically, the air is generated by some type of blower or compressor.

Surface aerators: Vertically mounted tubes of up to 1 metre diameter extending from just above the base of a deep concrete tank to just below the surface of the sewage liquor. A typical shaft might be 10 metres high. At the surface end the tube is formed into a cone with helical vanes attached to the inner surface. When the tube is rotated, the vanes spin liquor up and out of the cones drawing new sewage liquor from the base of the tank. In many works each cone is located in a separate cell that can be isolated from the remaining cells if required for maintenance. Some works may have two cones to a cell and some large works may have 4 cones per cell.

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).