In rapid sand filter design, high-quality filter sand (quartzite) should have minimal acid solubility. The percentage loss in weight after 24 hours in hydrochloric acid should not exceed:

Introduction / Context:
Filter media quality directly influences the performance and longevity of rapid gravity filters in water treatment. Chemical durability is assessed via acid solubility; excessive dissolution implies weaker grains that can break down, create fines, and compromise filtration and backwashing.

Given Data / Assumptions:

• Media: quartzite filter sand for rapid sand filters.
• Test: immersion in hydrochloric acid (HCl) for 24 hours.
• Criterion: maximum allowable percentage loss in weight.

Concept / Approach:
A chemically durable filter medium resists dissolution. Standard water-works specifications typically limit acid solubility to a small percentage, ensuring long-term integrity, minimal fines production, and low headloss growth.

Step-by-Step Solution:

Verification / Alternative check:
Specifications from water utilities and standard texts align with the 5% criterion for first-grade media, with less stringent grades allowing slightly higher loss but not preferred for critical installations.

Why Other Options Are Wrong:

• 8%, 10%, 12%, 15%: These are too lenient and risk inferior media that deteriorate during service, leading to higher turbidity breakthrough and operational issues.

Common Pitfalls:
Confusing acid solubility limits with uniformity coefficient or effective size specifications; assuming all “silica sands” behave identically without quality testing.

Final Answer:
5%