Solute transport under steady and transient conditions in biodegraded municipal solid waste

Abstract

The transport of a conservative tracer (lithium) in a large (3.5 m3) undisturbed municipal solid waste sample has been investigated under steady and fully transient conditions using a simple model. The model comprises a kinematic wave approximation for water movement, presented in a previous paper, and a strict convective solute flux law. The waste medium is conceptualized as a three‐domain system consisting of a mobile domain (channels), an immobile fast domain, and an immobile slow domain. The mobile domain constitutes only a minor fraction of the medium, and the access to the major part of medium is constrained by diffusive transport. Thus the system is in a state of physical nonequilibrium. The fast immobile domain is the part of the matrix which surrounds the channels and forms the boundary between the channels and the matrix. Owing to its exposure to mobile water, which enhances the biodegradation process, this domain is assumed to be more porous and loose in its structure and therefore to respond faster to a change in solute concentration in the mobile domain compared to the regions deep inside the matrix. The diffusive mass exchange between the domains is modeled with two first‐order mass transfer expressions coupled in series. Under transient conditions the system will also be in a state of hydraulic nonequilibrium. Hydraulic gradients build up between the channel domain and the matrix in response to the water input events. The gradients will govern a reversible flow and convective transport between the domains, here represented as a source/sink term in the governing equation. The model has been used to interpret and compare the results from a steady state experiment and an unsteady state experiment. By solely adjusting the size of the fraction of the immobile fast domain that is active in transferring solute, the model is capable of accurately reproducing the measured outflow breakthrough curves for both the steady and unsteady state experiments. During transient conditions the fraction of the immobile fast domain that is active in transferring solute is found to be about 65% larger than that under steady state conditions. It is therefore concluded that the water input pattern governs the size of the fraction of the immobile fast domain which, in turn, governs the solute residence time in the solid waste. It can be concluded that the contaminant transport process in landfills is likely to be in a state of both physical, hydraulic, and chemical nonequilibrium. The transport process for a conservative solute is here shown to be dominated by convective transport in the channels and a fast diffusive mass exchange with the surrounding matrix. This may imply that the observed leachate quality from landfills mainly reflects the biochemical conditions in these regions. The water input pattern is of great importance for the transport process since it governs the size of the fraction of the immobile fast domain which is active in transferring solute. This may be the reason for leachate quality to be seasonally or water flux dependent, which has been observed in several investigations. The result also has a significant practical implication for efforts to enhance the biodegradation process in landfills by recycling of the leachate.