For 30 water treatment plants using coagulation, sedimentation and rapid sand filtration detailed cost engineering studies were made by collecting design, operating and economic data in actual field interviews. The sample comprised plants in two capability bands, the smaller in the region of 0.5 mgd and the larger surrounding 8.0 mgd. With tolerance for certain compromises involving an adequate size of sample the smaller size plant represents the median size treatment plant of this type among all such plants in the nation, and the larger size plant represents the median size in amount of water produced among community water supplies in the nation.