Slow sand filtration performance: What is the typical bacteriological removal efficiency (percentage of bacteria removed) achieved by a properly operating slow sand filter in municipal treatment?

Introduction / Context:
Slow sand filtration is a biological filtration process renowned for high pathogen removal, especially for bacteria and protozoa. It relies on the schmutzdecke (biological layer) developing on the sand surface to achieve excellent microbial reductions.

Given Data / Assumptions:

• Mature schmutzdecke and stable operation.
• Appropriate filtration rates (very low compared to rapid sand filters).
• Pre-treatment adequate to avoid premature clogging.

Concept / Approach:

Through biological activity, adsorption, and fine mechanical straining, slow sand filters commonly attain extremely high bacterial removal efficiencies approaching 98–99% under steady conditions. This makes them suitable for small to medium systems with sufficient land and careful operation.

Step-by-Step Solution:

Verification / Alternative check:

Historical performance data from municipal plants and WHO design guidance document similar removal percentages for bacteria, with variability based on raw-water quality and temperature.

Why Other Options Are Wrong:

• 80–97% are lower than the commonly reported figures for a healthy, mature filter.
• Values below 75% are characteristic of malfunction or unsuitable loading rates.

Common Pitfalls:

• Confusing slow sand with rapid sand filters, which depend more on coagulation.
• Ignoring the ripening period required for top-layer development.

Final Answer:

98% to 99%.