Video narration
If sewage is filtered, isn't it already clean? Not quite. Think of Primary Treatment as the physical first step: it removes large and small solid particles by filtering and settling. No aeration is usually needed, and microbes play no major role. The solids that settle become primary sludge. But after this stage, the liquid still contains dissolved organic matter. This is the turn: both treatments clean sewage, but they solve different problems. In Secondary Treatment, the primary effluent enters an aeration tank. Air is pumped in continuously, supporting aerobic microbes. They break down the remaining organic matter. Bacterial and fungal threads form flocs; when these settle, they produce Activated Sludge. As this biological breakdown increases, BOD-the oxygen demand created by organic matter-drops significantly. So remember: Primary means filtering and settling, making primary sludge. Secondary means air, microbes, and flocs-reducing BOD.