French term (edited): ensemble de V « multiples » de récupération ...

The book I referred to earlier shows one system for sludge removal: the bottom of the tank is not flat but has a series of parallel triangular-section "troughs" or "hoppers" formed by triangular-section concrete profiles called "sludge concentrators" which effectively concentrate sludge at the bottom of the vee from where it can, in principle, be removed through a pipe at the the end of the vee bottom. "In principle" because apparently the sludge tends to cake and a "bypass" forms directly to the outlet, allowing water to be withdrawn while the sludge stays put. hence only low confidence since hopefully techniques have moved on ...

Why multiples is in quotes I cannot fathom. That also suggests that it might be something quite different entirely.