Stabilization ponds in the tropics: promotion of the sanitary reuse of water.

This article reports the results of efforts by PAHO's Environmental Health Program to lay the groundwork for designing wastewater stabilization ponds suited to tropical areas. The resulting design concepts are flexible (permitting pond construction to be adapted to a broad range of terrains, community sizes, and working temperatures) and directed at meeting WHO standards established for the microbial quality of effluents used in agriculture and aquaculture. The WHO standards (1) call for treated wastewater being used in agriculture or aquaculture to contain less than 1,000 fecal coliforms per 100 ml. The plans outlined here, which are calculated to meet those standards, seek to minimize the pond area and volume needed by using two high-load primary anaerobic ponds discharging into an elongated secondary pond with a length-to-width ratio of at least 15:1. In rough terrain the elongated pond can be allowed to meander, following the land's natural contours. However, where the terrain is level it is recommended that a system of partitions be used to maintain the 15:1 ratio for operating purposes while changing the actual ratio of the secondary pond's outer dimensions to something like 5:3. Basic equations presented in the text will be helpful in estimating the pond sizes needed to cope with different climatic conditions, community sizes, per capita rates of wastewater generation, and per capita biochemical oxygen demands. While these equations should prove of great assistance during initial planning and development of preliminary criteria, nothing in this article supplants the topographic and geotechnologic studies that will be needed in order to clearly determine what is feasible in any given case.