The function of pretreatment is 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 waste 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 settlement basins. The settled grit is collected at the bottom of the basins with an air compressor and is also disposed of at a municipal waste site.
Following removal of the grit, the sewage is discharged into two aerated pretreatment basins for the purpose of providing uniformity to the wastewater and adding oxygen to it in order to prevent anoxic (lacking oxygen) wastewater from reaching the biological treatment system. This oxygen is added with the use of ceramic aerators at the bottom of the aeration basins.
The picture below shows the sand removal system in the pretreatment process.
Sand washer with bucket conveyer