Foam Flow Model of Municipal Solid Waste and Its Application in Landfill Gas Pressure Prediction

Foam, as a group of disconnected gas bubbles wrapped with liquid film, also exists in municipal solid waste (MSW) landfills. In this study, a foam flow model in MSW is established, in which the population balance equation is adopted to calculate the dynamic process of foam generation, bursting, and transport. The status of foam is characterized using the number density, which is defined as the number of bubbles per unit volume of pore. The relative permeability and viscosity of foam are corrected according to its number density. Then, the proposed model is verified through the foam displacement experiment conducted in an MSW column. The foam flow model can accurately simulate the transient change of pressure drop and liquid saturation during foam displacement. The values of coefficients of foam generation and bursting rates are fitted to be suitable for MSW. Finally, the foam flow model is applied to predict the gas pressure development in an MSW landfill. Foam is able to maintain the landfill gas pressure at a high value even for aged MSW with low gas generation rate. The S-shaped curve of gas pressure versus depth suggests that the foam flow model is capable of simulating the phenomenon of high gas pressure accumulated in the deep layer of landfills because it takes into account the nonuniform distribution of foam number density. The advantage of the foam flow model over the gas flow model is discussed with respect to landfill gas pressure prediction.