The phenomenon in a rapid sand filter where dissolved gases come out of solution due to negative pressure, forming bubbles that stick to the sand grains and stop the filter's operation is known as
- AAir bindingCorrect
- BMud ball formation
- CCracking of the filter bed
- DFloc breakthrough
Solution
Concept
Air binding phenomenon in sand filters
Formula
Negative Head = Frictional Head Loss - Available Water Depth
Step-by-step solution
1. Impurities lodged in a rapid sand filter increase head loss as the run proceeds. 2. When frictional head loss exceeds water depth above the media, negative head occurs in the bed. 3. Under negative head, dissolved gases release as air bubbles that lodge in sand voids. 4. This phenomenon chokes the filter bed and is termed air binding. Answer: A
Explanation
Air binding occurs in rapid sand filters when accumulation of suspended matter causes high head loss that exceeds static water depth, creating negative pressure (negative head) in the bed. Dissolved air and gases come out of solution as bubbles, becoming entrapped in media voids and choking water passage.
Why the other options are wrong
B) Mud balls form when sticky floc aggregates with sand due to poor backwash agitation. C) Cracking occurs when fine surface silt shrinks or settles unevenly. D) Floc breakthrough occurs when excessive filtration velocity shears flocs through media pores.
Shortcut
Negative head releasing gas bubbles in filter bed = Air Binding.
Common mistake
Confusing air binding with mud ball formation caused by inadequate backwashing.
Correct answer: A) Air binding