Fallstudie von Econs SA
DEPONIEGAS-MIGRATION IM SUBSOIL: ERFAHRUNGEN VON KONTROLLE UND SANIERUNG
The migration of biogas outside sanitary landfills for municipal solid wastes is a very common issue. Migrations occur very often even in modern, so-called “controlled sanitary landfills”. The reason for this is that, in spite of our firm belief that a controlled tipping site is efficiently isolated from the surrounding environment, this proves not to be true for the great majority of existing landfills. The walls of the landfills usually are their weakest point: both in design and construction indeed, the emphasis is set on protecting the subsoil and reinforcing the bottom of the site, but the walls are even more delicate points and always more difficult to deal with. As a consequence, countless wall linings rupture every day in landfills throughout the world. They are weaker than the bottom liners and at the same time subject to much stronger strains (just think of the friction brought about by the waste settling and moving down a fixed membrane lying on a slope of natural soil). Moreover, our modern, compacted landfills tend to stratify horizontally, and the permeability to fluids (both liquid and gaseous) of the deposited waste becomes much greater horizontally than vertically. This makes lateral migration even easier, and causes the formation of supended water layers inside the landfill body. Many of the failures in the wall linings thus result in the migration of biogas outside the landfill, if one or both of the following conditions occur:
• the natural soil outside the walls of the landfill offers a possible migration path due to unsaturated permeable layers (resting above the groundwater table);
• the biogas extraction system operating inside the landfill is not sufficient to compensate, in the peripheral portions of the waste body, for the positive pressure caused by the generation of biogas.
This last condition is very often the case also in well managed landfills: biogas extraction systems indeed seldom recover all the biogas that is produced, generally due to underestimation of the landfills production capacity. Even if they can vent all the gas generated, they are usually operated at less than their full capacity to prevent the adverse effects that would result if atmospheric oxygen penetrated inside the landfill (dilution of the biogas, with a subsequent lowering of its heating capacity, creation of explosive mixtures, disturbing of the methanogenic bacteria, composting of waste or oxidation of methane with production of excessive heat, etc.). Biogas is essentially composed of a mixture of methane (ranging between 45 and 65 % in volume) and carbon dioxide (between 35 and 55 %), with other minor components (2-4 % of water vapour and less than 1 % of trace elements). Methane is responsible for the explosivity and flammability of biogas, carbon dioxide for its asphyxiating potential, and the trace components may be toxic (like, for example, carbon monoxide or hydrogen sulphide, or some chlorinated hydrocarbons at very low concentrations).
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