The Activated Sludge (AS) process was developed as an alternative to biological filters, and is particularly useful for large populations where land is at a premium. More recent research however, has shown that the process can be operated in many different modes, making it a more flexible process than biological filtration. The Activated Sludge process is an accelerated natural biological treatment process. It is a complex mix of microbiology and biochemistry involving many different sorts of bugs. 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, the bugs 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 bugs. The organic matter, or food, sticks to the activated sludge. The oxygen dissolved in the water allows the bugs 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 Final Settlement Tanks (FSTs) 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.