Clogging of unsaturated gravel permeated with landfill leachate

The results of an experimental investigation into the clogging of unsaturated, uniformly graded 50 mm gravel permeated with municipal solid waste landfill leachate are reported. The flow of leachate within the unsaturated gravel was heterogeneous and occurred in free-draining flow pathways. The leachate experienced reductions in the concentrations of both the organic and inorganic constituents after passing through the unsaturated gravel although there was very little clogging within the unsaturated gravel. The average drainable porosity was reduced by 8% after 8 years permeation. The biofilm was limited to areas on the gravel where leachate could be retained; predominantly on top of lateral gravel surfaces and near particle-to-particle contacts. As a result, only a small fraction of the total surface area of the unsaturated gravel was covered with biofilm. The short leachate retention time and the sporadic distribution of biofilm limited the degree of contact between the bacteria and the leachate and hence limited biologically induced clogging within the unsaturated gravel. The data suggest that leachate collection systems should be designed and operated such that the drainage material of the leachate collection system remains unsaturated for as long as possible.