In a typical air-scour sequence, when is backwash water introduced?

• A. After
• B. During
• C. Before
• D. Simultaneously

Answer: (C)

In a typical air-scour sequence, backwash water is introduced before the air-scour process begins. This is essential because the introduction of the backwash water helps to loosen dirt and debris trapped in the filter media. By saturating the media with water prior to the air scour, the process becomes more effective in removing particles that could be stuck in the pores of the medium. This initial water application helps to maximize the efficiency of the subsequent air scour by ensuring a clean and prepared environment for the air to displace particles.

Introducing backwash water simultaneously, during, or after the air scour would not provide the same level of cleaning effectiveness because the air bubbles would not have an optimal medium to work against. Instead, pre-saturating the filter bed allows for better agitation and dislodging of impurities during the air-scour sequence, leading to a more thorough cleaning process.