- The primary treatment consisting of sedimentation or sieving operation to remove than the size of the colloidal particles. At the same time remove 30-60% of the BOD.
Primary treatment consists of the collection of the waste water using draining under gravitation. The large objects are removed, This is realized with grills and sieves. Deposits are stored in pits or incinerated. After that waste water are decanted in decanters, in which are put the smaller solid parts. The bottoms of these decanters are tilted and their flow speed is small, these coagulants are added to remove finer suspensions.