system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
A slow sand filter has the ability to filter out bacteria. Which of the following components of the filter is responsible for providing bacteria free water?
assistant
The correct answer is  The vital layer is layer of slimy gelatinous material which covers the surface of the sand bed after a few days. It consists of thread like algae, plankton, diatoms and bacteria. When fully formed it extends for 2 - 3 cm into the top poion of the sand bed. It removes organic material, holds back bacteria and oxidises ammoniacal nitrogen into nitrates and helps in yielding a bacteria free water. Ref: Park, Edition 21, Page - 654