Facultative ponds have found application in wastewater treatment as an economical system where geographical locations are available at reasonable cost. Several design methods have been reported to describe the organic matter removal of a facultative pond and to determine the reaction coefficient (K) in general terms. Therefore, it is important to evaluate if these coefficient values can be used satisfactorily in regional cases. For this purpose the data of two full-scale facultative ponds located in Brazlandia and Samambaia, in the mid-west region of Brazil, were used. A correlation between applied COD load and reaction coefficient (K) was obtained based on a mathematical adjustment using the dispersed flow hydraulic model. The results provide a suggested regional design parameter for facultative ponds in this region in terms of domestic wastewater. (C) 2001 Elsevier Science Ltd. All rights reserved.