Which of the following processes is least likely to occur during primary treatment?

During primary treatment in wastewater management, the main aim is to remove large solids and settleable organic matter from the influent. This is achieved through physical processes rather than biological processes.

Screening of large objects is commonly part of primary treatment as it removes debris such as sticks, leaves, and other large materials that could damage equipment. Settling of solid particles refers to sedimentation, where heavier particles settle to the bottom of a settling tank, which is a critical process during primary treatment. The removal of oil and grease is also typical in this phase, often carried out with skimming devices that separate these substances from the wastewater.

Biological degradation of sludge, however, is not part of primary treatment. Instead, this process occurs during secondary treatment, where microorganisms break down organic matter. Therefore, this option is least likely to occur during primary treatment, as primary processes focus on physical removal of solids rather than biological degradation.