Low fraction of methane in landfill gas emissions in an industrial waste landfill containing incineration ash and gypsum board waste under anaerobic conditions.

The behaviour of carbon dioxide (CO2) and methane (CH4) emissions at the surface and below the soil cover in an industrial waste landfill under anaerobic operating conditions was evaluated for six years. This landfill contained gypsum board waste and incineration ash - a practice currently allowed because of a change in Japanese regulations. The CO2 and CH4 fluxes decreased throughout the six years of the survey. Almost all of the survey points exhibited fractions of CH4 in landfill gas emissions of <0.5 (mean values: 0.0-0.1 [surface], 0.0-0.3 [subsurface]) under anaerobic conditions. In addition, a relatively high first-order reaction rate constant for the landfill gas emissions (0.3 year-1) was observed. The landfill leachate showed a relatively high sulphate ion (SO4 2-) concentration, although other environmental conditions, such as the pH, oxidation-reduction potential and ammonium concentration, were not at levels that could have inhibited CH4 production. These findings suggest that the low fractions could have been related to the lower amounts of CH4 generation caused by competition between methanogens and sulphate-reducing bacteria (SRB). Therefore, SRB could play a major role in the degradation of organic carbon in the landfill.

The effectiveness of passive gas ventilation on methane emission reduction in a semi-aerobic test cell operated in the tropics.

Two landfill test cells, with and without gas vents, were used to investigate the effectiveness of passive aeration, through basal leachate pipes, in mitigating methane emissions from municipal solid waste disposal in the tropical climate of Thailand. Surface methane emission rate, as well as methane content in the landfill gas, were determined for a period of three years. The results indicate that the average methane emission rate from the test cell with passive gas vents (42.13 g/t dry wt./d) was about half of that from the test cell without gas vents (90.33 g/t dry wt./d). Methane emission rates from both test cells fluctuated and were influenced by precipitation. The emission rate during the wet period in the test cell with gas vents (61.67 g/t dry wt./d) was 3 times as much as that observed during the dry period (20.95 g/t dry wt./d). The emission rate during the wet period in the test cell without gas vents (120.33 g/t dry wt./d), was twice the value of that observed during the dry period (60.32 g/t dry wt./d). The measurements also revealed the formation of methane hotspots in the test cell with passive vents after rainfall events, leading to higher localized surface emissions. Introduction of gas vents helped reduce methane emissions from solid waste landfills in a tropical region. However, rainfall should be limited to avoid turning semi-aerobic conditions into anaerobic conditions.

Influence of oxygen flow rate and compost addition on reduction of organic matter in aerated waste layer containing mainly incineration residue.

Landfilling municipal solid waste incineration (MSWI) residue alkalizes the waste layer, causing a subsequent decrease in microbial activity and a delay in the decomposition of organic matter. In this study, efficiencies of neutralization of the leachate and organic matter decomposition in the waste layer in a column filled with MSWI residue using aeration and compost addition were evaluated. Total organic carbon (TOC) reduction in the waste layer is large at high oxygen flow rate (OFR). To effectively accelerate TOC reduction in the waste layer to which compost was added, a high OFR exceeding that by natural ventilation was required. At day 65, the pH of the leachate when OFR was above 10(2) mol-O2/(day x m3) was lower than that when OFR was below 10(1) mol-O2/(day x m3). At the same OFR, the pH of waste sample was lower than that of waste sample with compost. Although leachate neutralization could be affected by compost addition, TOC reduction in the waste layer became rather small. It is possible that humic substances in compost prevent the decomposition of TOC in MSWI residue.

Influence of oxygen flow rate on reaction rate of organic matter in leachate from aerated waste layer containing mainly incineration ash.

It is known that aeration reduces rapidly the concentration of organic matter in leachate. However, the oxygen flow rate required to attain a certain reaction rate of organic matter should be carefully estimated. In this study, using the oxygen ratio (the ratio of oxygen flow rate by aeration to oxygen consumption rate of waste layer) as a parameter, the reaction rate of organic matter in leachate from landfilled incineration ash and incombustible waste upon aeration was evaluated. Total organic carbon (TOC) in the leachate was reduced rapidly when the oxygen ratio was high. The decomposition rate exceeded the elution rate of TOC in the leachate from the waste layer for several days when the oxygen ratio was above 10(2). The results indicate that the oxygen ratio can be used as a parameter for the aeration operation in actual landfill sites, to rapidly stabilize organic matter in leachate.

Characterization of microorganisms at different landfill depths using carbon-utilization patterns and 16S rRNA gene based T-RFLP.

A borehole core from 20 m depth of a Japanese landfill was characterized chemically and microbially. The borehole core sample was typically divided into 5 waste layers; 2.4-4.0 m, 5.7-8.5 m, 9.25-9.6 m, 9.77-14.9 m, and 15.9-17.86 m depths. The waste layers' ages spanned about 14 years between the bottom and top. Archaeal 16S rRNA gene and eubacterial 16S rRNA gene in the waste samples at their respective levels were 9.8 x 10(5)-7.2 x 10(7) and 1.2 x 10(7)-7.2 x 10(9) copy/g-wet. Similar to populations of viable and culturable bacteria, those populations were high at 7.0 m and 17.5 m depth, but low at 3.0 m depth. The microorganisms' phenotypes and genotypes were evaluated, respectively, using carbon-utilization tests and by eubacterial 16S rRNA gene based T-RFLP. Low dominance of the VFA-utilizing bacteria in samples and low concentrations of VFAs in all waste layers suggest that the organic decomposition in this landfill site remained. Gamma-proteobacteria dominated the microbial community at 17.5 m depth. Clostridia were detected at 7.0, 11.5, and 17.5 m depths, suggesting strict anaerobic conditions in these deep layers. The Shannon-Weaver diversity index showed lower values at 3.0 m and 11.5 m depth with a T-RF pattern. The diversity index calculated from the carbon-utilization pattern increased slightly with depth at the landfill site. The landfill-site waste layers are expected to be mutually isolated and to form unique microbial communities depending on the buried wastes' composition, temperature, moisture content, and pressure inside the landfill.

Improvement of permeability of waste sludge by mixing with slag or construction and demolition waste.

To determine the allowable ratio of waste sludge required to ensure an aerobic zone in the landfill, we investigated sludge permeability, which involved mixing sludge, the major landfill waste in Japan, at different mixing ratios with other wastes (slag and construction and demolition waste (C&D)). We measured parameters of sample permeability and analyzed parameters that exert a large influence on oxygen penetration depth with a simulation model accounting for both diffusion and convection driven by temperature gradients. We also determined the critical volumetric contents in which gas and/or water permeability change significantly when sludge is mixed with sand or gravel. From the results of the simulations, gas permeability of the layer, the difference between inside and outside temperatures and the oxygen consumption rate exert a large influence on the resulting oxygen penetration depth. The allowable ratio of sludge required to ensure an aerobic zone in the landfill was determined by considering the balance of the above three parameters. By keeping volumetric sludge content to below 25%, air convection and oxygen penetration depth of several meters were achieved in the modeling.

Performance evaluation of intermediate cover soil barrier for removal of heavy metals in landfill leachate.

This pilot-scale study evaluated the use of intermediate cover soil barriers for removing heavy metals in leachate generated from test cells for co-disposed fly ash from municipal solid waste incinerators, ash melting plants, and shredder residue. Cover soil barriers were mixtures of Andisol (volcanic ash soil), waste iron powder, (grinder dust waste from iron foundries), and slag fragments. The cover soil barriers were installed in the test cells' bottom layer. Sorption/desorption is an important process in cover soil bottom barrier for removal of heavy metals in landfill leachate. Salt concentrations such as those of Na, K, and Ca in leachate were extremely high (often greater than 30 gL(-1)) because of high salt content in fly ash from ash melting plants. Concentrations of all heavy metals (nickel, manganese, copper, zinc, lead, and cadmium) in test cell leachates with a cover soil barrier were lower than those of the test cell without a cover soil barrier and were mostly below the discharge limit, probably because of dilution caused by the amount of leachate and heavy metal removal by the cover soil barrier. The cover soil barriers' heavy metal removal efficiency was calculated. About 50% of copper, nickel, and manganese were removed. About 20% of the zinc and boron were removed, but lead and cadmium were removed only slightly. Based on results of calculation of the Langelier saturation index and analyses of core samples, the reactivity of the cover soil barrier apparently decreases because of calcium carbonate precipitation on the cover soil barriers' surfaces.

Two-dimensional DNAPL migration affected by groundwater flow in unconfined aquifer.

The dense non-aqueous phase liquid (DNAPL) migration process was experimentally investigated in a laboratory-scale tank (150 cm width, 82.5 cm height, and 15 cm depth) to assess a site characterization on DNAPL contamination below a groundwater table. The heterogeneous ground of the tank model consisted of Toyoura sand (hydraulic conductivity, k = 1.5 x 10(-2) cm/s for void ratio, e = 0.62) and silica #7 sand (k = 2.3 x 10(-3) cm/s for e = 0.72). A series of experiments was carried out with or without lateral groundwater flow. Hydrofluoroether was used as a representative DNAPL. The main results obtained in this study are as follows: (1) the DNAPL plume does not invade into the less permeable soil layer with higher displacement pressure head; (2) the DNAPL plume migrates faster with lateral groundwater flow than without it; (3) lateral groundwater flow does not affect lateral DNAPL migration; rather, it promotes downward migration; and (4) pore DNAPL pressure without groundwater flow is higher than that with it. The above experimental results were compared with numerical analysis. The fundamental behaviors of DNAPL source migration observed experimentally are expected to be useful for assessing the characteristics of two-dimensional DNAPL migration in an aquifer.