Terminology — The activated sludge process (ASP) is sometimes referred to as which type of biological oxidation system?

Introduction:
Biological wastewater processes can be broadly categorized as suspended-growth (cells freely suspended in mixed liquor) or attached-growth (biofilms on media). The activated sludge process is the archetypal suspended-growth system, sustained in a well-mixed, aerated basin.

Given Data / Assumptions:

• Aeration provides mixing and oxygen to keep flocs suspended.
• Secondary clarifier returns biomass to maintain MLSS.
• Terminology emphasizes hydrodynamic state of the biomass.

Concept / Approach:
Because the microbial solids are suspended and “fluidized” by aeration-induced mixing, ASP is sometimes described (informally) as a fluid-bed biological oxidation system. In contrast, fixed-bed systems (e.g., trickling filters, rotating biological contactors) rely on biofilms attached to media and differ in hydrodynamics and design equations.

Step-by-Step Solution:
Classify ASP as suspended growth (cells in the fluid phase).Relate suspended state to a “fluid-bed” notion sustained by aeration.Select the option that reflects suspended/fluidized behavior rather than fixed media.

Verification / Alternative check:
Design texts contrast ASP (completely mixed, suspended growth) with fixed-film processes; the descriptive phrase “fluid bed” reflects the maintained suspension of flocs by air-induced mixing.

Why Other Options Are Wrong:

• (b) Fixed bed refers to attached-growth biofilms, not ASP.
• (c) “Turning bed” is not a standard term for ASP.
• (d) Incorrect because a descriptive choice exists.
• (e) Trickling filters are attached-growth, not ASP.

Common Pitfalls:
Assuming “fluidized bed” strictly means granular media fluidization; here the phrase is descriptive of suspended growth, not a literal sand-fluidized reactor.

Final Answer:
Fluid bed biological oxidation system (suspended growth kept in suspension by aeration)