## Core Concept
Slow sand filtration is a method used for water purification. The process involves passing water through a layer of sand at a relatively slow rate, allowing for physical and biological removal of contaminants. This method is effective in removing bacteria, viruses, and protozoa due to the combination of physical straining and biological activity.

## Why the Correct Answer is Right
The correct answer, , indicates that a significant percentage of water can be rendered bacteria-free through slow sand filtration. This process works because the sand layer provides a physical barrier that traps particulate matter and microorganisms. Additionally, a biological layer or "schmutzdecke" forms on the surface of the sand, which consists of living organisms that consume bacteria and other microorganisms, further purifying the water. Studies and practical applications have shown that slow sand filtration can achieve high levels of bacterial removal, often in the range of 90% to 99.9% or more, depending on the quality of the influent water and the operation and maintenance of the filter.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a lower effectiveness compared to the correct answer. Without specific numbers, it's hard to directly refute, but given that slow sand filtration is known for its high efficiency in removing bacteria, an option suggesting significantly lower effectiveness (e.g., less than 90%) would be incorrect.
- **Option B:** Similar to Option A, if this suggests a lower percentage than the correct answer, it would be incorrect based on the known efficacy of slow sand filtration.
- **Option D:** If this option suggests a higher or lower percentage than the correct answer without a basis in the effectiveness of slow sand filtration, it would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that slow sand filtration not only removes particulate matter but also significantly reduces the bacterial load in water through both physical and biological mechanisms. This method is particularly useful in areas where access to more advanced water treatment technologies is limited. The schmutzdecke layer, which forms on the surface of the sand, is crucial for the biological removal of contaminants.

## Correct Answer: C.