The ripening of a filter after it backwashes depends on ________.

• A. the attraction of sticky particles to the media and then those attached particles attracting even more sticky particles until the filter is ripened
• B. the bed slowly settling and compacting, which prevents turbidity breakthrough
• C. the sand collecting particles first by straining action; then once anthracite starts collecting particles, the filter is ripened
• D. fluid drag jostling the irregular-shaped anthracite particles into positions that eventually prevent most turbidity breakthrough

Answer: (A)

The main idea being tested is how a rapid filtration bed becomes effective after backwashing as a filter cake forms on the media. Right after a backwash, the bed is clean and has limited ability to remove small particles. As filtration resumes, fine particles in the water start sticking to the media and to each other. This initial, sticky layer serves as a nucleation site, so attached particles attract more particles and form a cohesive cake. That cake becomes the actual filtration barrier, increasing removal of particles and reducing turbidity until the filter is fully ripened.

The other ideas don’t fit this mechanism. Simply settling and compacting the bed isn’t the primary driver of ripening, though it can affect headloss. The notion of ripening arising from a staged shift between media types or from rearranging media via fluid drag doesn’t capture the essential process of cake formation and particle adhesion that enhances filtration efficiency.