A static assembly admixes a gas e.g. air to a fluid e.g. water. The assembly consists of an upright pipe (2) around fixed upright helical (3) strip of corrugated or fan-folded material whose width is identical to that of the pipe inner diameter. The upright pipe (5) has an incoming gas pipe (5) at the base with air holes. When used for the treatment of effluent water, the helical strip (3) also serves as a growth substrate for a microbiological colony.
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