EXPERIMENTAL EVALUATION OF THE BIOLOGICAL CLOGGING OF DRAINAGE GEOCOMPOSITE AT THE BOTOM OF LANDFILL
12Pages

EXPERIMENTAL EVALUATION OF THE BIOLOGICAL CLOGGING OF DRAINAGE GEOCOMPOSITE AT THE BOTOM OF LANDFILL Yves Durkheim, Afitex, France Stéphane FOURMONT, Afitex, France Carole BLOQUET, Sita, France ABSTRACT This paper presents a study of the biological clogging potential of drainage geocomposites installed in the bottom of landfills, as a replacement to 0.20 cm of granular material. Ìn a first section of the document, the drainage capacity actually needed is analyzed with respect to the biological clogging potential. The general principles governing experimental evaluation of biological clogging are presented in a second step. The apparatus used to conduct the evaluation is then presented, along with the results obtained after several months of monitoring. The results presented involved circulation of a fresh leachate, pumped in a class 2, non-hazardous landfill into 9 cells, and monitoring of the flow through the system, either geocomposites or granular drainage layer. Drainage geocomposites are more and more used by Waste Storage Centers operators who use them in replacement of granular material and more often for capping. View sites replace a part of the granular layer at the bottom with a drainage geocomposite due to risks of variability of long term hydraulic performance. Partial replacement of the granular layer at the bottom by a geocomposite permits to save granular material which is more and more difficult to find and involve significants costs. Moreover, this solution increases the storage capacity of the cell and reduces the truck traffic. Design of the geocomposite must be specific to this type of application since it will be subjected to high stress and an aggressive environment due to leachate. For this type of application, a safety factor of 10 on the drainage capacity of the geocomposite is generaly considered to take into account the biological clogging of the filter. However, there is curently no specific study for determining the long term properties of the drainage layer and this value of 10 was set following a principle of circumspection. This study aims at determining a bacterial clogging factor for a geocomposite with mini-pipes specifically developped for use at the bottom of landfill. The long term integrity of the geocomposite was validated by video inspection in real situation (Fourmont et al., 2008). French legislation for Waste Storage Centers recommends a 0.50 m thick gravel layer with a -4 conductivity superior to 10 m/s to drain leachate at the bottom of landfill. Height of leachate in the bottom must not exced 0.30 m. The last 0.20 m of gravel above the maximum permissible height of leachate is a security layer witch are more considered as a mechanical protective layer against puncture of very large objects Insofar as this aspect is related to operating methods of the site and not to the design of the sealing device, it will not be considered here.

The replacement of these 0.20 m of gravel with a drainage geocomposite specifically designed for this application and having superior properties will increase the storage capacity of the cell, reduce the truck traffic on site without override the leachate collection principles defined in the legislation. To proof that the real hydraulic capacity of the drainage layer (0.30 m of gravel + geocomposite) will always be superior to the needs of the application, we have to validate that the drainage layer will be able to evacuate the flow of leachate with a hydraulic head less than 0.30 m. In...

4. CONSTRAINTS WHICH CAN REDUCE THE LIFE OF DRAINAGE SYSTEMS The constraints that can reduce the life of drainage systems are: - Creep in compression, - Clogging: biological, mineral, etc. All polymeric materials under compression are likely to creep. To design drainage geocomposites which the drainage layer is a geonet, it is often considered: - Factor of safety of 2 or more on the mechanical properties of the geonet, and more precisely, the compressive strength - In-plane flow capacity required with standard ISO 12958 but under a stress a least 2 times greater than the operating stress....

Therefore, to be relevant to the site conditions, the essential points are: - place the experimental device on a waste storage center to have fresh leachate going into the experimental cells, - have a leachate temperature as close as possible form the temperature at the bottom of the cell. Indeed, the types of bacteria growing at 30°C or more are different from those at 1520°C. Then a study made with a temperature of leacahte of 20°C could not be relevant if the temperature at the bottom of the cell is superior, - operating conditions are relevant to reality, especially the oxygen supply...

Figure 2. Bungalow near the well Test cells The test device has been developped to force the leachate passing through the filter of the geocomposite then into the mini-pipe as shown on the figure 3. This system permits to assess the behaviour of the global system: - passing through the pores geotextile, - enter into the mini-pipe through the perforations, - circulation into the mini-pipe. The test device is composed of 9 square cells 0.25 m width. The total length of the test device is 3 m and its width is 1 m. Waterproof foam Distribution grid Granular material Geocomposite Figure 3. Test...

Moreover, a vertical stress of 100 kPa has been applied on the system to reproduce mechanical conditions at the bottom of landfill. This stress is not enougth to damage the geocomposite. In our study, it will be assumed that only the biological clogging will be responsible of a decrease of the drainage capacity of the system. The stress effect has been studied in previous testing related in litterature (Fourmont et al. 2008). The stress is applied thanks to calibrated springs as shown on the figure 4. Figure 4. Application of the normal stress Leachate admission The quantity of leachate...

Inlet flow Intermediate reservoir Valves Dosing reservoir Valves To test cells Figure 5. Leachate admission System performances During the experimentation, the flow through the cells is measured by a speed of discharge of the cell. This measurement permits to define a ratio between the speed of discharge and the head loss through the system. The measure is based on a falling head transmissivimeter test. This measure shows the global performance of the system, since a bacteriological glogging of any part of the system will induce a decrease of the speed of discharge. At the end of the...