Slow sand filtration – identify the incorrect operational statement Which one of the following statements about slow sand filters is incorrect for standard municipal operation and maintenance?

Introduction / Context:
Slow sand filters are biologically active filters that rely on a developed surface layer (schmutzdecke). Their operation and cleaning procedures differ fundamentally from rapid gravity filters, and misunderstanding this leads to poor performance.

Given Data / Assumptions:

• Conventional slow sand filtration (SSF) with fine sand media and low filtration rates.
• Pre-treatment achieves low turbidity to prevent premature clogging.
• Head loss is monitored to schedule cleaning.

Concept / Approach:
SSF develops a biological layer that captures particles and inactivates pathogens. As the layer matures, effluent quality typically improves until excessive head loss necessitates cleaning. Cleaning is performed by scraping off the top few centimetres of sand, not by backwashing. Backwash would disrupt the biological layer and is not feasible at the low rates and fine media used in SSF.

Step-by-Step Solution:
Identify the statement about cleaning method → SSF uses scraping, not backwashing.Confirm the role of head loss and schmutzdecke maturity in effluent quality.Recognize typical water depth over the bed is maintained for hydraulic stability (site-specific values vary but the concept is sound).Therefore, the incorrect statement is that SSF are cleaned by backwashing.

Verification / Alternative check:
Operational manuals describe draining the unit, scraping 1–2 cm of sand, and re-leveling; after several scrapings, sand is replenished.

Why Other Options Are Wrong:

• (a) SSF prefer low-turbidity influent; heavy coagulation is not standard for SSF.
• (b) Maintaining adequate supernatant depth is common practice.
• (c) Mature schmutzdecke often improves quality until head loss triggers cleaning.
• (e) Not all statements are correct because (d) is wrong.

Common Pitfalls:
Treating SSF like rapid filters; they have different hydraulics and O&M.

Final Answer:
Slow sand filters are cleaned by backwashing the bed like rapid gravity filters