What is primary sludge explain how it is treated

Primary sedimentation tank called a clarifier and allowed to stand for 10 to 12 hours most of the suspended solids(like faeces) settle down at the sloping bottom of the primary sedimentation tank in the form of primary sludge. The primary sludge is removed from the bottom of the primary sedimentation tank with scrapper and shifted into a large closed tank called digester tank.