What issue could gravel displacement in a filter bed from rapid backwash cause?

• A. Channeling
• B. A sand boil
• C. Clogging
• D. Back pressure

Answer: (B)

Gravel displacement in a filter bed from rapid backwash can lead to the formation of a sand boil. This occurs when the upward flow of water during backwashing is so forceful that it displaces the gravel and finer media, allowing water to escape through the surface in a localized manner. This creates instability within the filter bed and can lead to pockets of sand being washed away, contributing to the development of a boil effect where water and media create a turbulent area at the surface.

In contrast, while channeling refers to the flow of water creating a path through the filter media without adequately treating the water, and clogging typically relates to the accumulation of debris in a manner that restricts flow, these conditions arise from different causes and are not directly a result of gravel displacement due to rapid backwashing. Back pressure involves resistance to flow within the system, which can be influenced by several factors but is not specifically a direct consequence of gravel being displaced in the filter bed. Thus, the key issue with gravel displacement is its potential to create a sand boil, disrupting the filtering process and affecting water quality.