Which of the following statements is NOT correct with respect to the slow sand filter in the drinking water treatment plant?
1
It removes bacteria effectively up to 90%.
2
Backwashing is used for cleaning the filter bed.
3
The effective size of sand used is in the range between 0.15 and 0.3 mm.
4
Slow sand filtration is suitable when raw water turbidity does not exceed 30 NTU, EXCEPT occasionally for a few days.