Rapid gravity filters – typical maximum bacterial removal without disinfection Considering only the filtration step (no downstream chlorination), up to what percentage of bacterial impurities can rapid gravity (rapid sand) filters usually remove?

Introduction / Context:
Filtration is a key barrier in drinking-water treatment. Rapid gravity filters excel at turbidity removal with proper coagulation but have limited standalone bacterial removal compared with slow sand filtration. Understanding realistic removal helps set appropriate disinfection targets.

Given Data / Assumptions:

• Coagulation–flocculation–sedimentation upstream is effective.
• No credit is taken for chlorination in this question.
• Typical filter media and rates (rapid gravity filtration).

Concept / Approach:
Rapid filters remove particles primarily by straining, interception, and adhesion within the media. While they can reduce bacterial counts substantially, their maximum removal without subsequent disinfection is commonly quoted around 80% (occasionally approaching 90% under ideal conditions). In contrast, slow sand filters achieve much higher pathogen reductions due to biological schmutzdecke.

Step-by-Step Solution:
Acknowledge variability but use conservative typical values from practice.Select the upper bound commonly credited in design texts for rapid filters alone → about 80%.

Verification / Alternative check:
Design references differentiate between turbidity removal (very high) and bacterial removal (moderate) for rapid filters, emphasizing the necessity of post-filtration disinfection.

Why Other Options Are Wrong:

• 50–70%: Too conservative for well-operated rapid filters.
• 90%: Achievable in some cases but not a reliable maximum to claim without disinfection.

Common Pitfalls:
Assuming filtration alone guarantees microbiological safety; disinfection is essential to inactivate pathogens that pass the filters.

Final Answer:
80%