Abstract

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. The majority of Imhoff tanks within the U.S. have since been abandoned or modified to adapt to changing treatment objectives and regulations. However, within the Central American country of Honduras they continue to represent a significant portion of the wastewater treatment infrastructure and are routinely designed and constructed where populations are small and terrain is limited.

In Honduras, stormwater inflow and infiltration into wastewater treat-ment systems pose significant challenges. The drainage flows in urban areas mix with raw sewage creating dilute, but large volumes of, wastewater that requires treatment. Imhoff tanks in Honduras use design parameters more suitable for dryer regions of the world that do not experience such consequences of stormwater. These tanks are therefore currently not successful in providing treatment to regulated levels.

Since 2006, the authors have researched improved maintenance and operation of Imhoff tanks in Honduras. The 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 (CEPT) within the tanks. In addition to this detailed work, this chapter also presents the authors’ survey of several Honduran communities with Imhoff tanks in order to assess common design features and system failures.

This chapter contains a brief background of the status of wastewater treatment and Imhoff tanks within Honduras. It also succinctly discusses the governing technical considerations involved with treating wastewater in an Imhoff tank. It identifies some of the challenges that Honduran mu-nicipalities face while maintaining these systems, and highlights some of the low cost solutions implemented in other locations. Through awareness of these issues, stormwater management plans and easy-to-use design models could be developed. In-depth technical details on project aspects such as site specific flows and loads and CEPT may be found in Herrera (2007), Mikelonis (2008) and Hodge (2008).

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