Slow sand filter – ripening period after cleaning After scraping and cleaning a slow sand filter, for how long is the filtered water typically not put into supply until an adequate biological ripening is achieved?

Introduction / Context:
Slow sand filters rely on a biologically active layer (schmutzdecke) formed on top of the sand to achieve high removals of bacteria and turbidity. After scraping for maintenance, this layer must re-form before the filtrate is considered safe and compliant for distribution.

Given Data / Assumptions:

• Typical slow sand filter operation without advanced pre-treatment.
• Normal raw-water quality conditions.
• Standard practice to discard or recycle initial filtrate after cleaning until ripened.

Concept / Approach:
The biological ripening time depends on temperature, nutrient load, and flow rate, but a common operational rule is to wait roughly one to two days after cleaning, during which filtrate is bypassed. This period allows re-establishment of the schmutzdecke and restoration of target turbidity and bacteriological performance.

Step-by-Step Solution:
Recognize that immediately after scraping, bacterial removal efficiency is poor.Operationally the ripening period is commonly 24–36 hours before water is directed to supply.Among the options, '24 to 36 hours' best matches standard practice.

Verification / Alternative check:
Commissioning records and operating manuals for slow sand plants typically quote 24–48 hours; the 24–36 hours choice sits within this accepted window and is a frequently cited exam value.

Why Other Options Are Wrong:

• 6–12, 12–18, and 18–24 hours are usually too short for full biological activity to return.
• Exactly 48 hours may be conservative in many cases; operators often resume within 24–36 hours given acceptable quality tests.

Common Pitfalls:
Sending post-cleaning filtrate directly to supply; failing to monitor turbidity and coliforms during ripening; running excessive rates that delay schmutz layer formation.

Final Answer:
24 to 36 hours