Assainissement de la décharge de Colliare à Penthaz – Etude de variantes

The landfill of Colliare is situated on the left bank of the river « la Venoge », at the place of a former gravel pit in the municipality of Penthaz. It was exploited until 1995 and many different wastes were deposited in it, such as incineration slag of household waste, grinding of cars, residues of biostabilizer, waste of construction site and materials of excavation. A water pollution due to the presence of this landfill was noticed at the beginning of the nineties, mainly by ammonium and nitrite. In one measure chromium was also found in rather important concentration. The presence of these compounds in water reflects the existence of biodegradation of the organic matter present in the mass of waste. The concentration of ammonium being higher than the limit imposed by Swiss law, a remediation is necessary. Various projects of remediation were made during the last twenty years, the main being to clean up this former landfill in order to have the possibility of creating a new one for bioactive wastes. This project was rejected by the « Grand Conseil » and the remediation of the landfill was set aside during several years. Today, the question is to study various variants of remediation to define which one would be the best for Colliare. The choice of the variants was limited either to a containment of the landfill’s surface or to bioventing. This work studies the possible implantation of these two variants, by analysing their advantages and disadvantages so as to facilitate the choice of the final solution. For that purpose, laboratory analyses were made to define the list of contaminants present in the mass of waste that risk to contaminate the water near the landfill. It was demonstrated that beside the already turn out organic pollution, the landfill contains a huge quantity of PCB and PAH as well as several heavy metals such as chromium, lead or copper. Furthermore analyses to obtain information on the quantity of biodegradable organic matter were made. It was shown that organic matter constitues only a small percentage of the mass of waste. Landfill containment is not considered as a real remediation because the contaminants stay in the site. The solution is studied here because of the various types of contaminants present in the waste. Furthermore the time it takes to implement is relatively short and it is possible to put vegetation on the site just after the installation of the mineral cover. There are thus few impacts on the fauna and the flora and the contaminants are normally not a risk anymore for the water during all the life of the containment. Bioventing has as main advantage to be a real method of remediation. The problem of its implementation in the case of Colliare is the presence of PCB and PAH in the mass of waste. Indeed biodegradation would have no effect on these compounds and the risk of water contamination would not decrease thanks to bioventing. Furthermore the small percentage of organic matter does not justify the implementation of such an installation. Finally the costs of both variants were compared. Containment costs less than bioventing. Thus the cost of bioventing is too high to implement it in addition to containment except if the decrease of greenhouse gases emissions is enough important for being exchanged on the market of carbon. The solution chosen here is landfill containment, although it is not ideal for a real remediation of the site.