A reactor chamber for placement in a wastewater treatment system, where the reactor chamber has a chamber formed from a sidewall which forms an interior of the chamber. The sidewall has a top portion and a bottom portion and fixed channel growth media positioned in the interior of the chamber. The fixed channel growth media is positioned below the top of the sidewall, so that when positioned in a treatment system, most of the growth media is located below the water level in the treatment system. The top of the sidewall near the water level (when positioned in a treatment system) is substantially impermeable to wastewater. The reactor includes an air distribution manifold system having a series of air release sites positioned below the fixed channel growth media and adapted to release air which disperses upwardly through the fixed channel growth media. Influent is discharged into the treatment tank near the air discharge sites.