Rapid gravity filter (RGF) operating range:\r A rapid gravity (rapid sand) filter is typically used after coagulation–flocculation–sedimentation. What influent turbidity range (expressed in NTU) can an RGF reliably handle under normal design practice?

Introduction / Context:
Rapid gravity filters provide final clarification and polishing after upstream treatment. Correctly specifying the turbidity band for influent water ensures acceptable filter run lengths, headloss progression, and effluent quality.

Given Data / Assumptions:

• Filter is used post-coagulation and sedimentation (i.e., clarified water enters the RGF).
• NTU (nephelometric turbidity units) is the standard turbidity measure.
• We seek a practical influent range routinely cited for design/operation.

Concept / Approach:
Design texts indicate RGFs are suited to clarified waters typically below about 30–35 NTU. Higher influent turbidity reduces run length and risks turbidity breakthrough, unless enhanced coagulation or dual-media/advanced configurations are used.

Step-by-Step Reasoning:
Confirm process train: coagulation → flocculation → sedimentation → RGF.Adopt a conservative, commonly accepted band for influent to RGF.Select 30 to 35 NTU as a representative normal operating range.

Verification / Alternative check:
Many utility practices target much lower average influent turbidities to maximize filter performance; however, 30–35 NTU serves as an upper practical limit in standard references for post-clarifier water under non-extreme conditions.

Why Other Options Are Wrong:

• 15–25 or 25–30 NTU are possible but on the lower side; the question asks what an RGF can reliably handle, for which 30–35 NTU is a well-cited upper range.
• 35–40 NTU and “50 NTU or more without pre-treatment” overstate capability for conventional single-stage RGFs.

Common Pitfalls:

• Confusing raw water turbidity limits with clarified water limits.
• Assuming filters can compensate for inadequate coagulation/settling.

Final Answer:
30 to 35 NTU