In slow sand filters, the turbidity of raw water can be removed only up to

Explanation:

Slow Sand Filters

Slow sand filters are effective in treating raw water by removing turbidity, but their efficiency is limited to a certain threshold. This threshold is determined by the filter's ability to handle suspended solids without clogging and maintaining a proper filtration rate. The maximum turbidity level that slow sand filters can handle is discussed below:

• Key Design Considerations: Slow sand filters are designed for low turbidity levels, making them suitable for water with limited suspended solids.

• Maximum Turbidity: The raw water turbidity that can be effectively treated by slow sand filters is up to 60 mg/litre.

• Clogging Issues: Higher turbidity levels (above 60 mg/litre) can lead to clogging of the filter bed, reducing efficiency.

Analyzing the Given Options

1. Option 1: 60 mg/litre (Correct Answer)

   • Slow sand filters are designed to handle a turbidity of up to 60 mg/litre efficiently without clogging.

   • This is the recommended limit for ensuring optimal performance.

2. Option 2: 75 mg/litre (Incorrect)

   • While slow sand filters might function at 75 mg/litre, their efficiency drastically decreases.

   • This turbidity level can cause frequent clogging, requiring more maintenance.

3. Option 3: 100 mg/litre (Incorrect)

   • 100 mg/litre is beyond the capacity of slow sand filters to operate effectively.

   • This level of turbidity would require pre-treatment before filtration.

4. Option 4: 150 mg/litre (Incorrect)

   • 150 mg/litre is far beyond the design capacity of slow sand filters.

   • Such high turbidity levels would lead to immediate clogging and failure of the filter bed.