Q.What is the key difference between primary and secondary sewage treatment?

Step 1. Primary treatment is purely physical: floating debris is removed by sequential filtration, and grit (soil and small pebbles) is removed by sedimentation, leaving a primary sludge and a clarified effluent — no microbial action is involved at this stage.

Step 2. Secondary (biological) treatment, by contrast, is a microbiological process: the primary effluent is passed into aeration tanks where it is agitated and pumped with air, allowing aerobic microbes to grow vigorously into floc and consume the remaining dissolved and suspended organic matter. …

Contrast the physical mechanism of primary treatment with the microbiological mechanism of secondary t …

• Describing both stages as 'removing pollutants' without distinguishing the physical (primary) versus biological/microbial (secondary) mechanism. …