The fractal dimension of the aggregates can be modified by the addition of polyelectrolytes. A low fractal dimension, indicating a more open aggregate structure, is necessary to permit sufficient drainage to allow cake development in sludge filtration and centrifugation. Small angle light scattering may be particularly useful as an on-line control tool. In the 1950s and 1960s, solid waste was commonly disposed of in open dumps. By the 1970s sanitary landfilling developed in which 6 inches of daily soil cover were placed over the trash.
Clay cover rather than sand reduced infiltration and leachate generation.
It was assumed in NA sites that leachate was neutralized by a clay interface, with its cation exchange capacity, removing exchangeable contaminants. In containment landfills leachate is intercepted by a collection system for removal and treatment. Two leachate collection systems developed chronologically. The first was a perimeter drain system extending around the trash boundary and the second, a piping system collecting leachate along the base of the trash deposit. NA landfills commonly lacked a constructed liner and a leachate collection system.