Water treatment: For trace concentrations of organic contaminants (e.g., < 200 mg/L COD), which method is generally the most effective for removal?

Introduction / Context:
Wastewater often contains small amounts of dissolved organic compounds that are not easily removed by sedimentation or simple biological treatment. Selecting the right polishing step is crucial to meet discharge or reuse standards.

Given Data / Assumptions:

• Organic contaminants present at low (trace) levels, below roughly 200 mg/L.
• Goal: high-efficiency removal for polishing/tertiary treatment.
• Methods considered: adsorption, coagulation, biological ponds, lagooning.

Concept / Approach:
Activated carbon exhibits high surface area and affinity for a wide range of dissolved organics (e.g., phenols, dyes, taste/odor compounds). At low concentrations, adsorption performs better than bulk biological processes or coagulation, which are less selective for dissolved, non-particulate organics.

Step-by-Step Solution:
1) Identify target phase: dissolved organics (not suspended solids).2) Match process: adsorption removes dissolved species effectively at low concentrations.3) Consider alternatives: coagulation targets colloids/particulates; ponds/lagooning require long retention and may not reach low residuals.4) Choose activated carbon adsorption as best option.

Verification / Alternative check:
Design manuals list granular activated carbon (GAC) filters or powdered activated carbon (PAC) dosing as standard polishing for low-level organics, often following biological treatment and clarification.

Why Other Options Are Wrong:
Chemical coagulation: Limited effect on dissolved, non-ionizable organics.Biological oxidation pond/Lagooning: Space- and time-intensive; variable performance at trace levels.

Common Pitfalls:
Expecting primary or secondary treatment alone to meet stringent organics limits.Ignoring carbon exhaustion and the need for regeneration/replacement.

Final Answer:
Activated carbon adsorption