Slow sand filter operation — terminal head loss for cleaning A slow sand filter should be taken out for cleaning (scraping) when the filter head (loss of head) rises beyond which range?

Introduction / Context:
Slow sand filters remove turbidity and pathogens primarily through the biologically active schmutzdecke and fine sand matrix. Over time, clogging increases head loss. A maximum permissible head loss (terminal head) is specified to decide when to stop and clean the unit.

Given Data / Assumptions:

• Conventional slow sand filter (not rapid sand filter).
• Standard bed depth and effective size sand in municipal practice.
• Cleaning by scraping 1–2 cm of top sand.

Concept / Approach:
The filter runs until head loss approaches the terminal value. Typical terminal head for slow sand filters lies around 0.7 m to about 1.0–1.2 m of water column, beyond which operation is inefficient and risk of breakthrough increases during rate upsets.

Step-by-Step Solution:
Monitor inlet and outlet heads to compute loss of head across the bed.When head loss approaches 70–120 cm, schedule scraping and resting.After cleaning, restore bed depth with make-up sand at intervals.

Verification / Alternative check:
Operating manuals and exam standards cite ≈0.75–1.0 m as the trigger, aligning with the 70–120 cm range.

Why Other Options Are Wrong:

• 10–40 cm and 40–70 cm are too low; frequent cleaning would be uneconomic.
• 120–150 cm is above typical recommended terminal head for slow filters.

Common Pitfalls:
Confusing slow sand filters (biological) with rapid sand filters (higher rates, different head-loss criteria). Running past terminal head increases risk of media disturbance and short-circuiting.

Final Answer:
70 cm to 120 cm