A settling tank for receiving an influent stream separated into first and second influent streams and recombined under pressure in the settling tank. The settling tank comprises first and second pipes for conveyance of the first and second influent streams to a turbulent mixing zone wherein the first and second pipes terminate in first and second longitudinally parallel manifolds, each having a plurality of longitudinally arrayed holes defining first and second hole arrays. Each of the first and second arrays is directed in opposition to the other of the first and second arrays such that influent flows from the respective holes thereof can collide to cause turbulent mixing in said turbulent mixing zone.