What is Activated Sludge?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 10 November 2019
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Activated sludge is a term used both to refer to a widely utilized wastewater treatment process, and to the solid compounds which result from that process. The activated sludge technique is one of the most commonly used methods for handling human waste in municipal settings around the world, and it can also be employed in the treatment of industrial wastewater. The goal is to remove as much solid organic material from the wastewater as possible, to facilitate further stages in the water treatment.

This process was developed in the early 20th century, as researchers worked to cope with wastewater on a large scale in a hygienic and efficient way. Prior to wastewater treatment, effluent was simply released into waterways, carrying a heavy load of bacteria and other unwanted organisms along with it, and this caused numerous human health problems along with issues in the natural environment. Wastewater treatment plants were devised to cope with this problem.


In the activated sludge method, wastewater is moved into tanks which are aerated with pure oxygen or regular atmospheric air. The aeration of the wastewater facilitates the growth of numerous microorganisms which will work to break down the material in the wastewater. These organisms also contribute to the consolidation of solids into a sludge, through a process known as flocculation, which will settle to the bottom of a tank so that it can easily be siphoned off for further treatment and eventual disposal. Meanwhile, the water at the top of the tank can be separated for treatment and reuse.

Once the activated sludge has separated out from the wastewater, it can be subjected to additional treatments which will kill microorganisms to make the sludge safe to handle. The solids can be used for things like fertilizing, as they are rich in nutrients, and they can also be utilized in other ways. A small amount of the activated sludge is usually pumped back into the system to seed it with beneficial microorganisms to start the process all over again.

This technique has a number of advantages, which is why it has become so popular. However, as visitors to wastewater treatment plants have noted, it does come with some distinct drawbacks. The processing method is often quite smelly, because the microorganisms generate gases as part of their metabolisms, and the wastewater can acquire curious and sometimes radical hues as populations of various organisms wax and wane in the water.


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