Q.Explain the biological treatment of primary effluent when passed into the large aeration tanks in a sewage treatment plant (STP).

When the primary effluent enters the large aeration tanks, it is mixed with a population of aerobic microorganisms — mainly bacteria and fungi — that are already present in the tank. These microbes are the workhorses of biological treatment. The tank is continuously agitated and air is pumped in, ensuring plenty of dissolved oxygen for the microbes to respire.

Under these aerobic conditions, the microorganisms consume the dissolved and suspended organic matter (the pollutants) as their food. They break down this organic material into harmless byproducts — primarily carbon dioxide and water — and use the energy to grow and multiply. As they feed, the organic load of the effluent drops sharply. …

Once the primary effluent — the liquid that has already passed through sedimentation to remove settleable solids — enters the large aeration tank, the real biological work begins. This stage is the heart of secondary treatment, and it relies entirely on the activity of aerobic microorganisms. The tank is designed to be a controlled environment where these microbes can thrive and consume the organic pollutants that remain dissolved or suspended in the water.

The key to this process is aeration. Air is continuously bubbled through the tank, either by mechanical agitators or by diffusers at the bottom. This does two critical things: it supplies the oxygen that aerobic bacteria need to respire, and it keeps the tank contents well-mixed so that the microbes stay in constant contact with the organic matter. Without this steady oxygen supply, the beneficial bacteria would die off and the treatment would fail.

As the microbes feed on the organic matter — mainly carbohydrates, proteins, and fats — they break it down through cellular respiration. The organic carbon is converted into carbon dioxide and water, while the energy released is used by the bacteria to grow and reproduce. This means the population of microbes increases dramatically inside the tank. The dissolved organic load, measured as Biochemical Oxygen Demand (BOD), drops sharply because the bacteria are literally eating the pollution.

The process is not instantaneous. The effluent stays in the aeration tank for several hours — typically 4 to 8 hours in a well-designed plant — to give the microbes enough time to do their work. The tank is large enough to hold the effluent for this duration while fresh primary effluent continuously enters and treated water overflows to the next stage. …