True about slow sand filter is –
**Core Concept**
Slow sand filtration is a type of water treatment process that uses a combination of physical and biological mechanisms to remove contaminants from water. This method involves passing water through a bed of sand, which is typically 0.1-0.5 meters deep, at a slow rate of 0.1-0.3 meters per hour.
**Why the Correct Answer is Right**
The slow sand filter's effectiveness is due to the formation of a biological layer called the "schmutzdecke" or "biological crust" on the surface of the sand. This layer is composed of microorganisms, algae, and other biological agents that help to break down and remove organic matter, bacteria, and other pathogens from the water. The slow rate of filtration allows for sufficient time for the biological activity to occur, making the process highly efficient in removing contaminants.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the mechanism of slow sand filtration. While sand is indeed a key component, the process is not simply a matter of physical filtration.
* **Option B:** This option is incorrect because it misrepresents the role of the biological layer in slow sand filtration. The schmutzdecke is not merely a passive barrier, but an active participant in the removal of contaminants.
* **Option C:** This option is incorrect because it implies that slow sand filtration is a rapid process, which is not the case. The slow rate of filtration is a critical aspect of the process.
**Clinical Pearl / High-Yield Fact**
A key advantage of slow sand filtration is its ability to remove a wide range of contaminants, including bacteria, viruses, and parasites, making it a highly effective method for providing safe drinking water in developing communities.
**Correct Answer:**