Honduran Imhoff Tanks: Potentials and Pitfalls

An Imhoff tank is a structure originally designed to provide primary wastewater treatment. It is a sedimentation tank with a steeply sloped floor resting above a sludge digester. During the 1930s, Imhoff tanks represented 50% of all wastewater treatment facilities in the United States. While the majority of Imhoff tanks within the U.S. have since been abandoned or modified to adapt to changing treatment objectives and regulations, within Honduras they continue to represent 40% of wastewater treatment infrastructure.

Hondurans face many challenges politically, economically, and geographically. Much of the mountainous terrain lacks electricity and communities have a lack of skilled engineers. In these situations, an Imhoff tank potentially provides an appropriate gravity-driven solution to primary treatment and sludge disposal. However, given the current state of use and maintenance of these systems there is significant research and educational outreach that needs to be performed in order to improve and maintain them as a viable mechanism for providing wastewater treatment.

The Honduran municipality of Las Vegas uses two Imhoff tanks in parallel to treat wastewater produced by the city’s urban center that has a population of approximately 3,600 residents. The existing structures were built in 1992 with capital funds from the Honduran Social Investment Fund (FHIS in Spanish) and are currently ineffective at treating the wastewater produced by residents. Since 2006, Masters of Engineering students at the Massachusetts Institute of Technology and the University of Texas-Austin have researched improved maintenance and operation of the Imhoff tanks in the Honduran municipality of Las Vegas. Their work includes, but is not limited to, monitoring wastewater flow rates, wastewater quality testing, improving gate and baffle designs, and bench-scale and pilot testing the use of solid aluminum sulfate as a coagulant for use in chemically enhanced primary treatment within the tanks. In addition to detailed work in Las Vegas, the authors have also surveyed additional communities with Imhoff tanks in order to assess common design features and system failures.

The presentation and paper will contain a brief background on the status of wastewater treatment and Imhoff tanks within Honduras. It will also succinctly discuss the governing technical considerations involved with treating wastewater in an Imhoff tank. The municipality of Las Vegas will be used as a case study of common design and maintenance issues and low-cost solutions that have worked to improve the system. Areas for future work will also be identified.