Efficiency for a large diameter air-lift pump is achieved with a scalloped air liberation ring, inwardly projecting portions having orifices to liberate air with good distribution through the liquid flow area. The under sides of the projections slope steeply toward the tips and are gently rounded to avoid catching elongated strands. Liberation of air at near the lowest level possible, for efficiency, is achieved by first liberating air at a higher level until flow causes dropped liquid head pressure, permitting the same air pressure to be used for the deeper liberation.
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