Wastewater coagulation: Which coagulant is most widely used for sewage (wastewater) treatment to aid flocculation and sedimentation?

Introduction / Context:

Coagulants are added in primary and advanced wastewater treatment to destabilize colloids, enhance floc formation, and improve sedimentation. While alum is common in potable water treatment, iron salts are often preferred in sewage treatment due to their performance over wider pH ranges and added benefits in phosphorus removal and odour control.

Given Data / Assumptions:

• Municipal sewage with variable alkalinity and organic load.
• Objective: robust coagulation over a broad pH (≈ 5–9) and effective floc formation.

Concept / Approach:

Ferric chloride hydrolyzes to form ferric hydroxide precipitates that sweep-floc fine particles and adsorb phosphorus. Compared to alum, ferric chloride is less sensitive to low temperatures and low alkalinity; it also aids in odour control by precipitating sulphides. Ferric sulphate behaves similarly but FeCl3 is more ubiquitous in practice.

Step-by-Step Solution:

Verification / Alternative check:

Operational records in many plants show widespread use of ferric chloride for chemical phosphorus removal and enhanced primary treatment; it is stocked commonly and has predictable performance.

Why Other Options Are Wrong:

• Alum works well in water treatment but can be less effective in high-alkalinity variability; it also forms more voluminous sludge.
• Ferric sulphate is viable but is less common than FeCl3 in many sewage applications.
• Chlorinated copperas is specialized and less widely used today.
• PAC is common in potable water; in sewage, iron salts dominate for P-removal and odour control.

Common Pitfalls:

• Assuming the same coagulant is optimal for both drinking water and sewage; wastewater has different chemistry.

Final Answer:

Ferric chloride (FeCl3)