The organic impurities from sewage are removed by   0 pointsPreliminary treatmentPrimary treatmentSecondary treatmentTertiary treatment

The organic impurities from sewage are primarily removed during the Secondary treatment.

Here's a step-by-step breakdown of the sewage treatment process:

1. Preliminary Treatment: This is the first stage of sewage treatment where large solids and grit are removed from the sewage through screening and sedimentation.

2. Primary Treatment: In this stage, the sewage is held in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment.

3. Secondary Treatment: This is where the removal of organic impurities primarily occurs. The sewage is aerated in a tank, which promotes the growth of bacteria and other microorganisms that consume the organic matter. This process can remove up to 90% of the organic matter in sewage.

4. Tertiary Treatment: This is the final cleaning process that improves wastewater quality before it is reused, recycled or discharged to the environment. The treatment removes remaining inorganic compounds and substances, such as nitrogen and phosphorus.

So, the answer to your question is Secondary treatment.