Filtration in Water Purification for Bacteria Removal

The question asks about a crucial stage in water purification where a significant percentage of bacteria are removed. Understanding the various stages of water treatment helps in identifying this specific process.

Water Purification Stages

Water purification involves several steps to make water safe for consumption. Each stage targets different types of impurities.

• Coagulation and Flocculation: Chemicals are added to make small particles stick together, forming larger, heavier particles called flocs.
• Sedimentation: The heavier flocs settle at the bottom of a large tank, separating them from the clear water above.
• Filtration: Water passes through a filter, typically made of layers of sand, gravel, and charcoal. This physical barrier traps remaining suspended particles, including a very high percentage of bacteria, protozoa, and other microorganisms.
• Disinfection: After filtration, disinfectants like chlorine or UV light are used to kill any remaining bacteria, viruses, and other pathogens.
• Storage: The treated water is then stored in reservoirs or tanks before being distributed.

Understanding Filtration for Bacterial Removal

Filtration is a highly effective physical process in water treatment. During this stage, water is passed through porous media, such as layers of sand and gravel. The small pores within the filter media physically trap suspended particles, including bacteria, protozoa, and other larger microorganisms. This mechanical straining action is incredibly efficient.

Slow sand filters, for instance, are known for their ability to remove over 99% of bacteria and viruses, along with other particulate matter, due to both physical straining and biological processes occurring within the filter bed. Rapid sand filters also achieve high levels of bacterial removal, especially when preceded by proper coagulation and sedimentation.

The question states that 98-99 percent of the bacteria are removed. This high level of removal specifically points to the filtration stage, which acts as a primary barrier against microbial contaminants. While disinfection kills remaining pathogens, filtration physically removes a large bulk of them.

Analyzing Other Options

• Disinfection: This stage primarily aims to kill or inactivate pathogenic microorganisms that have passed through previous stages. While crucial for safety, it's not the stage that *removes* 98-99% of bacteria by physical means; filtration handles the bulk physical removal.
• Storage: This refers to holding the purified water before distribution. It is not a purification stage that removes bacteria.
• Filter Bottom: This is a component of a filter bed, usually consisting of gravel layers or underdrain systems, which supports the filter media and collects filtered water. It is not a stage of purification itself.

Conclusion

Based on the effectiveness in removing a very high percentage (98-99%) of bacteria through a physical separation process, filtration is the correct stage in water purification.