Diatomaceous earth filtration is used for raw water that has a maximum turbidity of ____.

• A. 5 ntu
• B. 20 ntu
• C. 40 ntu
• D. 10 ntu

Answer: (D)

Diatomaceous earth filtration uses a fine DE coating to create a deep, highly efficient filtration bed that traps very small particles after pretreatment. This polishing step works best when the incoming water isn’t highly turbid, because the DE bed can effectively remove fine colloids without clogging too quickly or driving up backwash frequency.

A practical upper limit for raw water turbidity entering a DE filter is about 10 NTU. Beyond this level, the filter cake tends to clog more rapidly, head loss increases, and backwashing becomes frequent, which reduces performance and efficiency. So the maximum turbidity suited for DE filtration is around 10 NTU.