The hydraulic design of a wastewater treatment plant involves several objectives, the most important of which are: uniform distribution of flow to treatment units; maintenance of relatively constant water level in sedimentation tanks for efficient skimming; uniform withdrawal of effluent from sedimentation tanks in a manner which does not induce short circuiting; and, where practicable, elimination of excessive turbulence and free fall of raw wastewater or primary effluent to minimize the release of odors. Use of submerged effluent collectors in sedimentation tanks permits the attainment of these objectives more completely than is possible with the conventional effluent weir. Although examples presented herein illustrate the application of submerged collectors to rectangular sedimentation units, the general concept applies equally well to circular tanks.