Rapid sand filters – cause of air binding Air binding in a rapid sand filter is primarily caused by which condition?

Introduction / Context:
Air binding is a serious operational issue in rapid sand filters. Entrained or released air accumulates within the media, blocking pores and sharply reducing filtration rate until the bed is backwashed.

Given Data / Assumptions:

• Standard gravity or pressure-type rapid sand filter.
• Normal filtrate temperature and dissolved gas content.

Concept / Approach:
When local pressure within the bed drops below the dissolved gas pressure, air is released from solution. This is most likely under excessive negative head (suction) conditions near the underdrain or due to over-rapid filtration/insufficient submergence, causing air pockets and channeling.

Step-by-Step Solution:
Identify the mechanism → pressure in the pores falls → gases come out of solution.Consequent effect → pores clog with air → sudden headloss and poor effluent quality.Therefore, choose “excessive negative pressure” as the cause.

Verification / Alternative check:
Operating manuals warn against excessive suction at outlets and recommend adequate submergence and rate control to prevent air release.

Why Other Options Are Wrong:

• (b) Positive pressure does not release dissolved air.
• (c) High turbidity contributes to solids loading but is not the primary cause of air binding.
• (d) “High water pressure” at outlet is not the causal mechanism.
• (e) Cannot be true because only (a) states the correct mechanism.

Common Pitfalls:
Confusing mudball formation with air binding; both harm filter performance but have different causes and remedies.

Final Answer:
Excessive negative pressure within the filter leading to air release from water