Correct Answer - Option 2 : 60 mg/l
Explanation:
Slow sand filter:
(i) The first filter to be used on a widespread basis for water purification were slow sand filters. These filters were constructed of fine sand with an effective size of about 0.2 mm.
Efficiency of Slow Sand Filters:
1. Bacterial Load:
(i) The slow sand filters are highly efficient in the removal of bacterial load from water. It is expected that they may remove about 98 to 99% of bacterial load from raw water. However, for complete removal of bacteria, disinfection is essential.
2. Colour:
(i) The slow sand filters are less efficient in the removal of colour of raw water. It is estimated that they remove about 20 to 25% colour of raw water.
3. Turbidity:
(i) The slow sand filters can remove turbidity to the extent of about 50 ppm. For water having greater turbidity than 60 ppm, it is necessary to give preliminary treatment.