Q.Study the given diagram of Sewage Treatment Plant (S.T.P.) and answer the questions that follow :

Concept: Effectiveness of a Sewage Treatment Plant (S.T.P.) depends on how many stages of biological treatment it includes — a plant with secondary (biological) treatment removes far more organic pollution (BOD) than one with only primary (physical) treatment.

(a) Without the specific diagram, the general principle is: whichever of the two S.T.P.s includes a secondary treatment stage — an aeration tank where aerobic microbes oxidise and reduce the BOD of the sewage — is the more effective one for treating human excreta in municipal waste. A plant offering only primary (physical settling) treatment removes solids but leaves most of the dissolved/suspended organic load untreated, so it is far less effective.

(b) Once BOD is reduced significantly in step Z (the secondary/aeration stage), the following steps are carried out:

• The mixture is passed into a settling tank, where the microbial biomass (activated sludge) settles out from the now-clear treated effluent.
• A portion of this activated sludge is recycled back into the aeration tank to maintain active microbial digestion. …

Sewage treatment is fundamentally about making wastewater safe enough to return to the environment or reuse. The key measure of pollution in sewage is Biochemical Oxygen Demand (BOD) — the amount of oxygen that microorganisms will consume while decomposing organic matter in the water. High BOD means heavy organic pollution; as microbes break down waste, they deplete oxygen, suffocating aquatic life. Effective sewage treatment must bring BOD down to safe levels.

A typical Sewage Treatment Plant works in stages. Primary treatment is purely physical: large debris is filtered out, grit settles, and the sewage is allowed to stand so that solids settle to the bottom as primary sludge while oils float to the top. This removes about 30–40% of suspended solids but does very little to reduce BOD because the dissolved and finely suspended organic matter remains untouched.

Secondary treatment is where the real transformation happens. This is a biological process. The partially clarified effluent from primary treatment is pumped into large aeration tanks where it is constantly agitated and supplied with oxygen. Aerobic bacteria and other microbes thrive in these conditions, feeding voraciously on the organic matter. They break down proteins, carbohydrates, and fats into carbon dioxide, water, and microbial biomass. This microbial action slashes the BOD — often by 90% or more — because the organic pollutants are being oxidized and converted into harmless end products. Without this step, sewage remains dangerously polluted.

1. Which S.T.P. is more effective? The S.T.P. that carries the sewage through secondary treatment — the aeration tank where aerobic microbes actively digest the organic matter — is the more effective one for treating human excreta and municipal waste. A plant offering only primary (physical settling) treatment removes solids but leaves most of the dissolved and finely suspended organic load intact, so its BOD remains high and it cannot be considered adequate on its own. Human excreta is rich in organic content, so only the plant with biological (secondary) oxidation can bring its BOD down to a safe level.
   Note

   This answer states the general principle that determines which of the two labelled plants (A or B) is more effective; matching it to the specific labels in the printed diagram requires the diagram itself.

2. Steps in treatment after BOD reduction in step Z Once the aerobic microbes in the aeration tank (step Z) have done their work and BOD is significantly reduced, the effluent moves to a secondary settling tank or clarifier. Here, the process unfolds in a sequence:

• Settling and separation: The mixture from the aeration tank contains treated water and a large mass of microbial biomass (the "flocs" of bacteria that have multiplied while consuming the waste). In the settling tank, gravity does the work — the dense microbial flocs settle to the bottom as activated sludge. …