Activated sludge process — identify the critical design/operating factors (Consider flow peaking and organic loading while sizing aeration and clarifier units.)

Introduction / Context:
The activated sludge process is sensitive to both hydraulic surges and organic loading. Aeration basin sizing, oxygen transfer, and secondary clarifier capacity must accommodate peak flows and the highest expected substrate (BOD/COD) loads. This question asks which factors are truly critical for reliable design and operation.

Given Data / Assumptions:

• Conventional activated sludge system with primary treatment upstream.
• Performance depends on mixed liquor (MLVSS), F/M ratio, sludge age (SRT), and clarifier solids flux.
• Design must withstand short-term peaks without washout or loss of effluent quality.

Concept / Approach:
Critical design conditions are tied to the worst credible scenarios. Hydraulic peaking (maximum hourly flow) governs reactor detention time reduction and clarifier surface overflow rate. Maximum daily organic load governs oxygen demand, F/M ratio, and risk of filamentous growth or bulking if under-aerated. Minimum flows are generally less critical, as control strategies (airflow, recycle) can be turned down safely without overloading units.

Step-by-Step Solution:
1) Identify hydraulic driver → peak hour flow dictates minimum effective HRT and clarifier SOR.2) Identify organic driver → max daily BOD/COD sets oxygen transfer rate and diffuser/motor sizing.3) Confirm sludge handling → return activated sludge (RAS) and waste (WAS) rates must remain feasible at peaks.4) Therefore pair the two maxima: peak hour flow + peak daily organic load.

Verification / Alternative check:
Check solids flux at the secondary clarifier under Q_peak with elevated MLSS; confirm that the clarifier limiting flux is not exceeded and that aeration power covers the highest OUR (oxygen uptake rate) day.

Why Other Options Are Wrong:
Only maximum flow (a) ignores oxygen demand; (b) adds minimum flow which is less critical; (d) focuses on minima which rarely govern design.

Common Pitfalls:
Using average-day values; ignoring I/I-driven peaks; not coordinating aeration power turndown and RAS/WAS strategies.

Final Answer:
maximum hourly flow rate and maximum daily organic load