What would be the immediate effect if virgin greensand was not skimmed of fines after initial backwashing?

When virgin greensand is not skimmed of fines after initial backwashing, the immediate effect is shorter run times due to void clogging. This occurs because the presence of fines in the filtration media can lead to the filling of the void spaces that normally allow water to flow through the filter. As these finer particles accumulate, they create areas of reduced permeability, leading to increased pressure within the system and a decline in the efficiency of the filtering process.

As these voids become more clogged with fines, the overall flow of water through the filter decreases, which can lead to the need for more frequent backwashing or even replacement of the filter medium sooner than anticipated. This ultimately results in shorter operational cycles and increased maintenance requirements, affecting the system's performance.

The other options do not accurately reflect the consequences of failing to skim fines. Increased filtration speed would be highly unlikely, and water clarity would suffer due to finer particulates remaining in the system rather than being removed. Furthermore, the lifespan of the filter would not be extended; rather, it could decrease due to the excessive strain from the clogging and additional maintenance needed.