Water and wastewater treatment — where is dissolved organic matter primarily removed? Identify the treatment unit process that is most effective for removing dissolved organic matter (biodegradable BOD) from water or wastewater.

Introduction / Context:
Dissolved organic matter measured as biochemical oxygen demand (BOD) is best removed using biological treatment processes. Physical filtration units primarily target suspended solids and some colloids; they are not designed to substantially reduce dissolved organics. Understanding which unit does what is crucial in designing effective treatment trains.

Given Data / Assumptions:

• We are considering removal of dissolved biodegradable organic matter (BOD), not just particulates.
• Conventional units in the options include filtration (slow/rapid/dual media) and biological attached-growth systems (trickling filters).
• Influent is municipal wastewater or organics-laden water.

Concept / Approach:
Trickling filters are attached-growth biological reactors. Wastewater is distributed over a media bed where biofilms oxidize dissolved organics. In contrast, slow and rapid sand filters (including dual media) are finishing steps predominantly for turbidity, fine suspended matter, and pathogens (slow sand) in water treatment, not for high dissolved BOD removal. Plain sedimentation only settles heavier suspended solids and does not remove dissolved organics.

Step-by-Step Solution:
Classify each unit: physical filtration vs biological oxidation.Match the removal mechanism to dissolved BOD: biological oxidation is required.Select trickling filters as the appropriate unit for significant dissolved organic removal.

Verification / Alternative check:
Process design texts show typical BOD removals: trickling filters can remove a major fraction of soluble BOD; filters like rapid sand or dual media usually follow coagulation/clarification and remove remaining turbidity, not soluble BOD.

Why Other Options Are Wrong:
Slow sand filters: can reduce pathogens and turbidity in water treatment; limited effect on soluble BOD loads.
Rapid sand/dual media: mainly physical straining and adsorption; not designed for high dissolved organics.
Sedimentation: removes settleable solids only.

Common Pitfalls:

• Confusing dissolved organics removal (biological) with particulate removal (physical filtration).
• Assuming any filter reduces BOD substantially; without biological activity, impact on soluble BOD is small.

Final Answer:
Trickling filters