Slow sand filter operations – cleaning cycle:\r What is the usual period between successive cleanings (scrapings) of a slow sand filter under normal raw water quality and loading?

Introduction / Context:
Slow sand filtration relies on a biologically active schmutzdecke layer for turbidity and pathogen removal. Over time, accumulated deposits increase headloss, requiring periodic surface scraping to restore throughput. The interval between cleanings is a key operational parameter.

Given Data / Assumptions:

• Conventional slow sand filter with typical rates of 0.1 to 0.3 m/h.
• Normal raw water turbidity and temperature conditions.
• Headloss limit triggers cleaning.

Concept / Approach:
Under usual conditions, slow sand filters run several weeks before headloss reaches the allowable maximum. Operator practice commonly reports scraping intervals from about one to three months, depending on loading rate, pretreatment quality, and season. Extremely short cycles signal pretreatment issues or overloading.

Step-by-Step Reasoning:
Consider biological maturation and deposition rates.Observe that headloss builds gradually at slow hydraulic loading.Select the widely cited range of one to three months as the most representative interval.

Verification / Alternative check:
Operational manuals document scraping intervals in the month scale under steady conditions; shorter periods are associated with unusually high raw turbidity or cold temperatures that alter schmutzdecke behavior.

Why Other Options Are Wrong:

• 5 to 10 days and two to three weeks are too short for normal conditions.
• Three to six months or one year are too long and risk excessive headloss or breakthrough.

Common Pitfalls:

• Confusing slow sand filters with rapid gravity filters, which have much shorter backwash cycles measured in days or hours.
• Not resetting the filter properly after scraping, leading to suboptimal initial filtrate quality.

Final Answer:
One month to three months