Removal of dissolved gases in water treatment:\r Which process is most effective for removing dissolved carbon dioxide (CO2) from the supply main?

Introduction / Context:
Dissolved gases such as CO2, H2S, and volatile organics can affect taste, odor, corrosivity, and downstream treatment processes. Understanding which unit process targets these gases helps optimize plant performance and corrosion control.

Given Data / Assumptions:

• Water contains dissolved carbon dioxide at levels affecting pH and corrosivity.
• Treatment options include physical (aeration), chemical (chlorination, coagulation), and physical-chemical (sedimentation) processes.

Concept / Approach:
Aeration promotes mass transfer of dissolved gases from water to air by increasing interfacial area and turbulence. Packed towers, cascade aerators, and diffused aeration are standard configurations. CO2 stripping raises pH and reduces corrosivity, aiding subsequent lime softening or stabilization steps.

Step-by-Step Solution:
Identify target: dissolved CO2 (a gas), not particulates or pathogens.Match process: gas removal is achieved by aeration via gas–liquid mass transfer.Select Aeration as the correct process.

Verification / Alternative check:
Design equations for packed-tower aeration use transfer coefficients to predict CO2 removal; field plants routinely deploy aeration before corrosion control, validating effectiveness.

Why Other Options Are Wrong:

• Sedimentation removes settleable solids, not dissolved gases.
• Chlorination disinfects and oxidizes but does not strip CO2.
• Coagulation destabilizes colloids; it does not remove dissolved gases.

Common Pitfalls:

• Confusing pH adjustment by chemical addition with gas stripping; both can raise pH but by different mechanisms.
• Under-designing aeration contact time or surface area, leading to incomplete CO2 removal.

Final Answer:
Aeration