The activated sludge process is used to speed up the rate of decomposition of waste material in water. It involves the following steps:

- To remove organic pollutants from primary treated water air or oxygen and micro-organisms are added.
- The resulting sludge is captured in a settling tank.
- The remaining waste water is further cleaned in downstream processes.
- Part of the sludge is returned to the aeration tank to reuse the micro-organisms. This is called the return activated sludge.
- The rest of the sludge is typically mixed with sludge from primary water treatment and further treated to be used for other applications such as fuel production and composting.
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