Process selection for a large city: For treating municipal sewage at metropolitan scale, which process train is generally recommended?

Introduction / Context:

Large cities require robust, scalable, and controllable wastewater treatment capable of meeting stringent effluent standards. The process must handle variable flows and loads while allowing energy-efficient operation and nutrient removal options.

Given Data / Assumptions:

• Metropolitan-scale flows with significant industrial and commercial contributions.
• Regulatory compliance for BOD, TSS, and potentially nutrients.
• Existing best practices in urban wastewater treatment.

Concept / Approach:

Primary sedimentation removes settleable solids and reduces organic loading to the biological stage. The activated sludge process (ASP) offers high-rate biological oxidation with process control (MLSS, SRT, DO), modularity (plug-flow, step-feed), and the ability to integrate nitrification-denitrification and biological phosphorus removal. Trickling filters or Imhoff tanks are typically suited to smaller plants or specific contexts and offer less process control at very large scales.

Step-by-Step Solution:

Verification / Alternative check:

Most large municipal WWTPs worldwide use some variant of ASP (conventional, extended aeration, step-feed, oxidation ditches, SBRs) after primary clarification, validating the selection.

Why Other Options Are Wrong:

• Imhoff + low-rate trickling filters: Viable for small/medium towns; scaling and control are limited for megacities.
• High-rate trickling filters only: Can be used, but control and footprint for nutrient removal are less favorable than ASP at very large scales.
• None of these / Septic tanks: Not appropriate for metropolitan systems.

Common Pitfalls:

• Underestimating the need for process control, redundancy, and flexibility in large plants.
• Ignoring sludge handling (thickening, digestion, dewatering) integral to the overall train.

Final Answer:

Primary sedimentation followed by activated sludge (secondary) treatment