Pretreatment is performed in order to prepare raw sewage for the biological treatment stage.
Municipal sewage may contain large objects known as debris that must be removed prior to biological treatment.
The debris is removed with 4 bar screen type strainers comprised of parallel steel bars spaced 15 mm apart. The debris that has been collected is disposed of at a municipal landfill site.
The sewage also contains relatively heavy inert materials such as sand, stones and gravel. These materials are known as grit and are removed by discharging the wastewater into four clarifiers.
The settled grit is collected at the bottom of the basins with an air compressor and is also disposed of at a municipal landfill site.
After disposal of the grit, the wastewater is discharged into two aerated pretreatment basins for purposes of improving the properties of the sludge precipitate, thereby improving performance of the primary clarifiers.
In the photograph: Mechanical bar screens used to extract debris