Clogging of Gravel Drainage Layers Permeated with Landfill Leachate

Ten flow cells, called mesocosms, are used to investigate the effect of different gravel sizes (38 and 19 mm) and operating conditions on clogging of leachate collection systems. These mesocosms simulated in real time and real scale the two-dimensional leachate flow conditions representative of a section of a continuous 300-mm-thick gravel drainage blanket adjacent to a leachate collection pipe in a primary leachate collection system. The tests were terminated after 6–12 years of operation. In some mesocosms the full 300 mm of gravel was saturated. In others, the leachate level was initially set at 100 mm and the upper 200 mm were unsaturated. Although the flow through all mesocosms was similar, the clogging in the fully saturated gravel was substantially more than in the partially saturated gravel. After 6 years of operation, typically, less than 10% of the initial pore space was filled with clog material in the unsaturated gravel. For the saturated zone, 45% of the initial pore space was filled with clog material in the fully saturated design as compared to only 31% in the partially saturated design. The 38 mm gravel performed much better than the 19 mm gravel. For example, it maintained a hydraulic conductivity that was higher than the 19 mm gravel even after operating for twice as long. Up to four mesocosms were placed in series, with the effluent from one mesocosm being the influent for another. The reduction in mass loading within the first mesocosm reduced the amount of clogging within the mesocosm later in series. There was a clear progression of decreasing amounts of initial pore space filled with clog material in the last mesocosm in series, and most of the clogging was due to the vertically percolating leachate.