The findings of a series of studies investigating the clogging of leachate collection and drainage systems for municipal landfill sites are described. Batch tests, column studies, and field-scale "mesocosm" test cells were used to study the development of a thick microbial biofilm covering the surfaces and filling the pore space of the granular drainage media. Within this biofilm, anaerobic microbial consortia effect the stabilization of the organic load (expressed as chemical oxygen demand), with the concurrent precipitation within the granular drainage system of various minerals dominated by calcite.Key words: municipal waste, leachate, clogging, drainage, biofilm, calcite.

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The findings from a rare opportunity to exhume, examine, and sample a granular underdrain leachate collection system at a large municipal landfill site are reported. The "clear stone" drainage blanket was constructed from uniform, coarse gravel (with a nominal 50 mm particle size) obtained by crushing dolomitic limestone. After exposure to municipal landfill leachate for 1-4 years, the drainage stone was found to contain a considerable mass and volume of clog and slime materials. These were composed of mineral precipitates, fine granular particulate, and biofilm, growing under the ambient anaerobic conditions prevalent below the landfilled waste. The spatial distribution, physical and hydraulic properties, and chemical and microbiological composition of this material were examined and compared with similar material recovered from a laboratory mesocosm. The findings suggest a theoretical framework for a model of clogging behaviour of leachate collection drains at municipal solid waste landfill sites.Key words: municipal waste, leachate, clogging, drainage, biofilm, cementation.

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