Which of the following is the performance efficiency of slow sand filter?
1. It removes about 98 to 99% bacteria.
2. It removes turbidity to the extent of 50 PPM.
3. It does not remove colour of raw water.
4. It removes about 95% colloidal matter.
Statement 1 is CORRECT: Slow sand filters have an exceptionally high bacterial removal efficiency, typically removing
98% to 99% of the bacterial load due to the action of the biological layer (Schmutzdecke).
Statement 2 is CORRECT: They are designed to effectively remove raw water turbidity up to 50 ppm (or mg/L). If raw
water turbidity exceeds 60 ppm, a preliminary treatment (like plain sedimentation) is required to prevent immediate
clogging.
Statement 3 is INCORRECT: While they are less efficient at it compared to rapid sand filters, they do remove a small
fraction of colour (typically around 20% to 25%) rather than failing to remove it entirely.
Statement 4 is INCORRECT: Fine colloidal matter passes through the sand media relatively easily unless chemical
coagulants are used (which are not applied in slow sand filtration). They do not remove anywhere near 95% of colloidal
matter
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