Question

Choose the correct statement about Sewage Treatment Plants (STP):

• A. STP treats wastewater to reduce the volume of the wastewater and reduce it to a semi-solid byproduct
• B. STP treats wastewater to make it into environmentally safe water, which is safe to discharge
• C. In primary and secondary treatment of sewage in STP, both aerobic and anaerobic processes are used
• D. STP processes mainly involve thickening, stabilising and dewatering

Hint:

Remember the primary target of an STP: \[ Raw Sewage (High BOD/TSS) \xrightarrow{STP Main Line} Environmentally Safe Effluent (Safe for Discharge)

Answer 1

The Correct Option is B

Concept: The fundamental objective of a Sewage Treatment Plant (STP) is to treat municipal or industrial wastewater to remove contaminants, organic solids, and pathogens. The goal is to produce treated effluent that satisfies stringent environmental standards so that it can be safely released back into natural water bodies or repurposed for secondary applications like landscape irrigation without causing environmental degradation or public health hazards.

Step 1: Analyzing the Primary Purpose of an STP.
Let us examine the basic operational flowchart of sewage treatment to determine its true core function:
• Raw sewage enters the treatment facility carrying suspended solids, heavy organic matter, pathogenic microorganisms, and chemical pollutants.
• Through progressive mechanical, biological, and chemical unit operations, the concentration of these harmful pollutants is lowered until the biochemical oxygen demand (BOD) and total suspended solids (TSS) drop within permissible ecological ranges.
• Consequently, the principal overarching definition of an STP's success is converting raw, hazardous wastewater into environmentally stable, purified water that can be safely discharged into the ecosystem. This aligns directly with the statement in Option B.

Step 2: Evaluating why the remaining choices are incorrect or non-integral.
Let us systematically evaluate the technical inaccuracies present in the other options:
• Option A: The primary target of an STP is not merely the volumetric reduction of the fluid mass into a semi-solid material. It is the chemical and biological purification of the liquid carrier stream itself.
• Option C: While primary treatment is strictly a physical separation process (sedimentation and screening) that does not use biological activity, secondary treatment is predominantly aerobic (e.g., activated sludge process or trickling filters). Anaerobic digestion occurs separately in sludge handling units rather than during mainstream primary or secondary treatment phases.
• Option D: Processes like thickening, stabilizing, conditioning, and dewatering belong explicitly to the secondary stage of sludge processing, not to the main primary wastewater treatment line.