Foam-induced high gas pressures in wet municipal solid waste landfills

Abstract

The assumption that the gas is a connected phase leads to underestimation of the gas pressure in municipal solid waste (MSW) landfills. In this study, the high gas pressure is shown, by way of field investigations and laboratory experiments, to be caused by foam in the landfills. The field investigations indicated that environmental disasters that have occurred at several landfills in China were caused by high gas or pore pressures. During drilling in these landfills, it was found that a mixture of foam and leachate was ejected from the borehole. The laboratory experiments indicated that foam generation in the landfills was attributable to the two-stage anaerobic biodegradation behaviours of MSWs. Volatile fatty acids (VFAs) decreased the surface tension of the leachate, enabling it to generate foam. The foam's composite index was adopted to evaluate foaming ability and foam stability of the leachate. Meanwhile, the liquid film in the foam divided the gas into a disconnected phase and remarkably increased its flow resistance. An experimental apparatus was designed to compare quantitatively the flow resistances of foam and gas in MSWs. When the foam and gas were injected into a saturated waste column at the same rate, the pressure drop of the foam (412 kPa) was 2·54 times that of the connected-phase gas (162 kPa), thereby resulting in lower residual liquid saturation for foam displacement. The unsaturated permeability coefficient of the foam was about an order of magnitude lower than that of the gas. Owing to the low permeability of waste to foam and continuous gas generation, high gas pressures are easily developed in wet landfills.