Following the aeration pretreatment process, the wastewater is discharged into six primary clarifiers in which the wastewater is kept for several hours (see picture). These clarifiers have a total surface area of about 3,000 m². While in the basins, some of the suspended solids settle and the oily scum is removed. The settled solids are collected and form part of the sludge collected during the wastewater treatment process. These are handled separately from the liquid fraction, as will be explained later in the section dealing with sludge treatment. During primary sedimentation, the quantity of contaminated materials in the wastewater is reduced. The primary sedimentation process reduces on average about 35% of the organic material, which is decomposed in a biological process (BOD), and about 55% of the suspended solids in the raw sewage.
Primary sedimentation tanks