Sewage is treated in sewage treatment plants to remove pollutants and retrieving clean water for its reuse again. This treatment is carried in two stages.
1. Primary treatment. It is a physical process of removal of small and large particles through filtration and sedimentation.
2. Secondary treatment. It is a biological process that employs bacteria. The activated sludge after primary treatment is pumped into a large tank called anaerobic sludge digester where anaerobic bacteria act on organic matter in anaerobic conditions to produce mixture of gases (biogas). The latter can be used as source of energy as it is inflammable.
