Sewage treatment clarification: in primary sedimentation, which commonly added conditioning chemical is used to speed up the settling of suspended solids and colloids without altering the basic treatment flow?

Introduction / Context:
Primary sedimentation in sewage treatment removes settleable suspended solids before biological steps. Operators often dose a conditioning chemical (coagulant/alkalinity agent) to accelerate particle aggregation and settling, thereby improving effluent clarity and protecting downstream units.

Given Data / Assumptions:

• Unit process: primary sedimentation (clarification).
• Goal: speed up settling of suspended solids and colloids.
• Typical aids: lime, alum, ferric salts, and polymer coagulant aids.

Concept / Approach:
Coagulation and flocculation convert fine, slow-settling particles into larger, faster-settling flocs. Lime supplies alkalinity, improves pH for optimal hydrolysis of metal coagulants, and can directly aid sweep floc formation with natural alkalinity and hardness, thus enhancing sedimentation rates.

Step-by-Step Solution:

Verification / Alternative check:
Plants that dose lime or alum ahead of primary clarifiers routinely report higher suspended solids removal compared to no chemical aid. Jar tests confirm improved floc size and settling velocity.

Why Other Options Are Wrong:

• Hydrochloric acid: acidifying would destabilize alkalinity control and is not standard for speeding sedimentation.
• Copper sulphate: algicide; not a primary sedimentation coagulant.
• Sodium sulphate: inert salt at typical doses; not used to accelerate settling.
• Alum: a valid coagulant but the question emphasizes commonly added conditioning chemical; lime is routinely cited for sedimentation pH/alkalinity conditioning.

Common Pitfalls:
Confusing algicides or generic salts with coagulant conditioning agents; ignoring the role of alkalinity and pH in floc formation; assuming any chemical addition will improve settling.

Final Answer:
Lime