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The results of a laboratory column test conducted to gain insight regarding the clogging mechanisms within porous media are presented. Tests were conducted under saturated, anaerobic conditions using synthetic leachate that models Keele Valley Landfill (KVL) leachate in chemical compositions but has negligible suspended solids and bacterial concentrations compared with KVL leachate. Comparisons were made between tests conducted with synthetic leachate and similar column tests conducted with KVL leachate to assess the presence of suspended solids and bacterial loading on the rate of clogging. It is shown that the synthetic leachate columns behaved similarly to the KVL leachate columns in terms of nutrients removed from the leachate as it passed through the porous media and clog properties (bulk densities and chemical composition). Excessive microbially induced clogging near the influent end of the column (but likely near the collection pipe in a field situation) resulted in a decrease in drainable porosity to less than 10% of the initial value and a decrease in hydraulic conductivity by seven orders of magnitude. Key words: leachate collection, clogging, porous media, anaerobic, laboratory study.

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A numerical, multiple-species, reactive chemical transport model (BioClog) developed to predict clogging in landfill leachate collection systems is used to interpret results from experiments conducted with gravel-packed columns permeated with landfill leachate. The model predicts changes to the microbial community and leachate chemistry, including the concentrations of volatile fatty acids, suspended biomass, dissolved calcium, and suspended inorganic solids. The calculated quantity and composition of the clog matter (biomass and mineral), along with the associated decrease in porosity, are compared to the measured values. The modelled clogging is in reasonable agreement with that observed in the gravel column experiments. By identifying and quantitatively linking many microbiological, chemical, and transport mechanisms, the model helps elucidate the phenomena controlling the rate and extent of clogging.Key words: clogging, landfills, leachate collection systems, biofilms, mineral precipitation.

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