Facultative ponds have found wide application in wastewater treatment as an economical systemwhere land area is available at reasonable cost. Different approaches are available in the literature for the design of facultative ponds. Most research have dealt with only suspended biomass and considered it as the major form of biomass responsible for substrate removal. However, the side walls and bottom of the facultative pond can provide support for the growth of attached (biofilm) biomass which also aids in the degradation of organic matters (substrate). This study demonstrates the significance of biofilm biomass growing on the side walls and bottom of these ponds to substrate utilization. A model for substrate utilization in facultative ponds is proposed which encompasses first-order reactions of both suspended and biofilm biomass. The biofilm activity is described with a diffusion type model, while the dispersed flow model is used for the pond hydraulics to include a wide range of pond dimensions and operating conditions. The proposed model, validated with observed data of two full-scale facultative ponds located in Bangkok, Thailand, and in New Mexico, U.S.A., was able to predict effluent BOD5 concentrations of these two ponds reasonably well.

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