Landfill GHG Reduction through Different Microbial Methane Oxidation Biocovers

Emissions from daily and final covers of municipal solid waste (MSW) landfills can produce significant impacts on local and global environments. Simplifying, landfills can cause local impacts with odor emissions and global impacts with GHGs. This work focuses on hydrogen sulfide (H2S) and methane (CH4) emissions, with the aim of studying how it is possible to reduce their impacts by means of biofiltration systems. Both field and laboratory investigations have been carried out in Casa Rota Landfill (Tuscany, Italy). In the field trials, four pilot-scale biocovers made of compost from a source-selected organic fraction (SS compost), compost from a mechanical biological treatment plant—the residual fractions of the MSW, a mixed compost (SS-MSW compost) and sand were monitored in the daily cover area of the landfill, where high emissions were detected. Results showed that high CH4 and H2S emissions reductions occurred in the mixed SS-MSW compost plot, given a maximum methane oxidation efficiency of greater than 98% and an average oxidation efficiency of about 75%. To assess the specific oxidation rate, laboratory tests using SS-MSW compost sampled from the biocovers were done.

Predicting the diversion length of capillary barriers using steady state and transient state numerical modeling: case study of the Saint-Tite-des-Caps landfill final cover

Covers with capillary barrier effect (CCBE) have already been proposed to meet regulatory requirements for landfill final covers. Modeling of CCBE can be a relatively complex and time-consuming task. Simpler, albeit conservative, design tools — such as steady state numerical analyses — can, in certain cases, be justified and have a positive impact in practice. In this study, numerical simulations were performed of the experimental CCBE constructed on the Saint-Tite-des-Caps landfill (Quebec). The CCBE consists of a capillary barrier, composed of sand and gravel, on top of which a layer of deinking by-products (DBP) was installed as a protective layer (also to control seepage). The addition of a protective layer over the infiltration control layer (such as a capillary barrier) is required in most jurisdictions. In many European countries, such as Germany and the Netherlands, a thick “recultivation” layer is required. The results of numerical simulations were compared with the in situ behaviour of the Saint-Tite CCBE as well as with analytical solutions. The effectiveness of the capillary barrier was assessed by quantifying the diversion length (DL), which reflects the lateral drainage capacity of the CCBE, i.e., the capacity to drain water laterally. Collection of the water that has drained laterally prevents seepage into the waste mass. This study shows that when the seepage rate reaching the top layer of the capillary barrier is controlled, it is possible to predict the worst-case scenario in terms of seepage (and therefore predict the shortest DL) using steady state numerical simulations. These simpler-to-perform numerical simulations could be adopted in practice, at least in a pre-feasibility study for cases with a similar profile as the one at the Saint-Tite-des-Caps experimental CCBE.

Preferential flow in geosynthetic clay liners exhumed from final covers with composite barriers

Geosynthetic clay liners (GCLs) were exhumed from final covers with composite barriers (geomembrane over GCL) at two municipal solid waste landfills in the USA. Preferential flow and high hydraulic conductivity (>2 × 10−9 m/s) was observed in eight of the 18 GCL samples collected from both sites. At one site, manganese oxide precipitate was concomitant with bundles of needle-punched fibers that conducted preferential flow. Nearly complete replacement of Na by Ca on the bentonite surface occurred in all GCL samples. GCLs with and without preferential flow could not be differentiated by physical and chemical properties commonly used to differentiate GCLs with high and low hydraulic conductivities (exhumed water content, swell index, mole fraction monovalent cations, soluble cation concentrations). The relative abundance of soluble cations in the pore water of GCLs exhibiting preferential flow was comparable to the relative abundance in the subgrade pore water, whereas the pore water in GCLs with distributed flow was more sodic than the pore water in the subgrade. Hydration experiments indicated that bentonite in GCLs initially hydrates in a zone surrounding bundles of needle-punching fibers. Cation exchange during this hydration process may create zones of higher hydraulic conductivity surrounding the fiber bundle, permitting preferential flow.