Coagulants in sewage treatment: Which of the following is generally not used as a coagulant for treating sewage (wastewater)?

Introduction / Context:
Chemical coagulation is sometimes applied to wastewater for enhanced primary treatment or tertiary polishing. Knowing which coagulants are typical for sewage, as opposed to potable water, helps in selecting cost-effective chemicals and avoiding side effects.

Given Data / Assumptions:

• Sewage characteristics: higher organics, variable pH/alkalinity compared with raw surface water.
• Goal: identify coagulants conventionally adopted for sewage.

Concept / Approach:
In wastewater, ferric salts (ferric chloride/sulfate) and alum are commonly used for chemical phosphorus removal, suspended solids reduction, and enhanced clarification. “Chlorinated copperas” (chlorinated ferrous sulfate) has niche or historical use, more associated with water treatment/odor control contexts, and is generally not a standard coagulant for sewage treatment trains.

Step-by-Step Solution:
List typical sewage coagulants: alum, ferric chloride, ferric sulfate, PAC.Assess chlorinated copperas: not commonly deployed for municipal sewage coagulation.Select the exception.

Verification / Alternative check:
Practice guidelines and plant case studies widely report ferric and alum chemistry for sewage; chlorinated copperas seldom appears as a standard choice for coagulation in municipal wastewater.

Why Other Options Are Wrong:
Alum, ferric chloride/sulfate, and PAC are widely used in wastewater for solids and phosphorus removal.

Common Pitfalls:

• Assuming chemicals popular in water treatment are equally prevalent in wastewater.
• Ignoring impacts on alkalinity and sludge production when choosing coagulants.

Final Answer:
Chlorinated copperas