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The GSE Drainage Design Manual Chapter 4 — Design Methods And Concepts The authors believe that the risk for chemical clogging in the landfill gas collection layer should be similar to that in the leak detection systems. Water vapor within the landfill gas that is collected by the landfill gas collection layer will often condense, resulting in liquid which is referred to as condensate. This condensate often has similar characteristics to landfill leachate; therefore, the authors recommend using a range similar to the LDS (Leakage Detection System) range of 1.1 to 1.5, since the layer will have a level of exposure to leachate similar to that of the LDS. In the absence of available information, the designer is urged to choose a conservative value (i.e., 1.5). Table 4.4 Chemical clogging and biological clogging reduction factors (GRI GC81. The designer should evaluate the soils he/she anticipates using in the protective layer of the liner system and the materials anticipated in the overlying waste, in order to judge the risk of chemical clogging of the primary leachate collection layer. GRI-GC8 recommends using values in the range of 1.5 to 2.0 for chemical clogging in the leachate collection system (see Table 4.4). Values at the lower end of the range should be used for municipal solid waste. GRI-GC8 recommends using values in the range of 1.1 to 1.5 for chemical clogging in the leakage detection layer (see Table 4.4). The designer should evaluate the soils he/she anticipates using in the protective layer of the liner system and the typical materials anticipated in the waste stream, in order to evaluate the risk of chemical clogging. 4.3.3 Biological Clogging Reduction Factor, RFBc The biological clogging reduction factor accounts for the reduction of flow in the geonet due to the growth of biological organisms such as fungi or algae, or root penetration through the overlying soil. GRI-GC8 recommends using values in the range of 1.2 to 3.5 for biological clogging in the final cover system (see Table 4.4). The authors consider the upper end of this range to constitute a condition whereby improper design of the cover allows root penetration to occur. Coordination is required with a plant biologist and the cover system designer to understand how prolific and how deep roots may develop. The reduction factor for biological clogging should then represent anticipated design conditions. GRI-GC8 does not provide explicit recommendations for selecting a biological clogging reduction factor for a landfill gas collection layer. A comparison to the other functions described in GRI-GC8 can nonetheless be made. The authors believe that the risk for biological clogging in the landfill gas collection layer should be similar to the leakage detection layer. The authors recommend using a range similar to that in the leakage detection layer range of 1.1 to 1.3, since the layer's level of exposure to leachate will be similar to that of the LDS. In the absence of available information, the designer is urged to choose a conservative value (i.e., 1.3). Page 4-15