Correct Answer - Option 2 : 60 mg/l
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.
(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.
(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.