Concern about energy use at wastewater treatment facilities has developed well after many of the plans were made for management of water pollution in the U. S. With changing standards and technology, infor mation on energy requirements for 190 to 1.9 X 104 m3/ d (0.05 to 5 mgd) wastewater treatment systems is needed to avoid future errors and to provide information to assist in designing and planning. Several estimates have been made for large systems, usually in the range of 1.9 to 38 X 104 m3/d (5 to 100 mgd), but because hundreds of small systems are being used, it is imper ative that information be gathered on the energy re quirements for wastewater treatment in small systems. This paper summarizes the energy requirements for all viable alternatives presently available for the treat ment of flow rates of 190 to 1.9 X 104 m3/d (0.05 to 5 mgd) of wastewater. Various treatment combinations are compared, and the energy requirements for the most viably alternatives are presented in tabular form. The data can be combined to produce estimates of the en ergy requirements for all currently available unit op erations and processes.