In rapid gravity (rapid sand) filters, what is the usual maximum permitted terminal headloss (loss of head) across the filter bed before initiating backwashing?

Introduction / Context:
As filtration proceeds, particulates accumulate and headloss rises. The terminal headloss is the threshold beyond which performance or safety is compromised, necessitating backwash. Knowing typical values helps operators schedule efficient filter runs and maintain effluent quality.

Given Data / Assumptions:

• Rapid gravity filters with typical media depth and effective size.
• Conventional treatment pre-steps (coagulation, flocculation, sedimentation).
• Normal design practice values from standard references.

Concept / Approach:
Initial headloss is low; as solids load builds, the headloss reaches a terminal value (commonly around 2.5 to 3.0 m) at which the filter is backwashed to restore permeability. Operating beyond this increases risk of media disturbance or breakthrough during rate fluctuations.

Step-by-Step Solution:

Verification / Alternative check:
Codes and manuals often cite terminal headloss of about 2.5–3.0 m; the nearest option provided is 3 m.

Why Other Options Are Wrong:

• 1–2 m: too conservative for typical designs; would shorten filter runs excessively.
• 4–5 m: risk undue stress and poor effluent quality.

Common Pitfalls:
Confusing initial with terminal headloss; ignoring temperature effects on viscosity and headloss progression; delaying backwash beyond alarms.

Final Answer:
3 m