Modelling species fate and porous media effects for landfill leachate flow

Abstract

A numerical, multiple-species, reactive transport model, coupled to models of kinetic biodegradation, precipitation, and particle attachment and detachment for predicting landfill leachate-induced clogging in porous media for one-dimensional flow systems, is described. The finite-element method is used for transport modelling, with reactions incorporated into point-source or sink terms. The species modelled include three volatile fatty acids, active and inert suspended biomass, dissolved calcium, and inorganic particles. The clog matter consists of active biofilm, inert biofilm, and inorganic solids. A biofilm model is used to simulate the growth and decay of active biomass and removal of substrate. Precipitate accumulation and calcium removal are simulated by a model of calcium carbonate precipitation. Interphase movement between clog matter and fluid includes the processes of attachment and detachment. A geometric representation of the porous media allows porosity and specific surface to be estimated from the thickness of the accumulated clog matter. The porosity of the media can thus change spatially and temporally. The behaviour of the model is demonstrated with a hypothetical example.Key words: clogging, landfills, leachate collection systems, modelling, biofilms, mineral precipitation.

Modelling landfill leachate-induced clogging of field-scale test cells (mesocosms)

“…Detailed description of the modelling of species fate and accumulation of clog matter by BioClog may be found in Cooke et al (2005a) and for 2D flow, including a moving mesh, in Cooke and Rowe (2008). A very brief summary is provided in the following subsections.…”

“…The clog matter consists of five separate films: a biofilm for each of the active degraders (propionate degraders, acetate degraders, and butyrate degraders), an inert biofilm, and an inorganic solids film. The reactions and film thickness equations are given in Cooke et al (2005a).…”

“…It was assumed that the inorganic film was protected to some extent from shear stress detachment while the active and inert films were not, according to the relationship given in Cooke et al (2005a) where protection parameter P is 0.00104 cm.…”

“…The active and inert biofilm density was assumed to increase as porosity decreased according to the experimentally determined formula in Cooke et al (2005a) for clog material accumulated under similar conditions. The variable density parameters were A X = 247 and B X = 72 mg VS/cm 3 .…”

“…When permeated with landfill leachate, the porous media has been found to clog due to biomass growth, mineral precipitation, and sedimentation of suspended solids (Bass 1986;Brune et al 1991;McBean et al 1993;Koerner et al 1994;Rowe et al 1995Rowe 1998;Fleming et al 1999;Maliva et al 2000;Bouchez et al 2003). A numerical model, BioClog, was developed to predict the rate at which clogging occurs in the drainage layers beneath engineered landfills due to exposure to leachate (Cooke et al 2005a;Cooke and Rowe 2008). For field cases, BioClog represents the mound of leachate within the drainage layer in profile from top of slope to bottom using two-dimensional (2D) models of fluid flow and solute transport.…”