James C. Thomas

The effect of various environmental and design parameters on methane oxidation in a model biofilter.

Methane from landfills built with RCRA (Resource Conservation and Recovery Act) covers is frequently vented directly to the atmosphere. Alternatively, landfill gasses could be vented through a layer of soil that could serve as a biofilter to oxidize CH4 to carbon dioxide and water. Properly designed soil biofilters may reduce atmospheric CH4 emissions from landfills and help reduce the accumulation of greenhouse gasses in the atmosphere. This study was conducted to investigate the performance of a lab-scale model biofilter system using soil as the filter-bed medium in packed columns to measure the effect of a variety of environmental and design factors on the CH4 oxidation capacity of a soil biofilter. Biofilter performance was tested under a variety of environmental and design conditions. The optimum soil moisture content for CH4 oxidation in a loamy sand was 13% by weight. Addition of NO3-N did not affect the CH4 oxidation rate. Soil depths of 30 cm and 60 cm were equally efficient in CH4 oxidation. When the CH4 loading rate was decreased, the percentage of CH4 oxidized increased. The maximum CH4 oxidation rate was 27.2 mol m-2 d-1 under optimum conditions.

Effect of desiccation cracking on the hydraulic conductivity of a compacted clay liner

Despite our best efforts to reduce the waste stream, there will always remain some residues which cannot be further treated and must be disposed in landfills. One critical aspect of landfill construction is the integrity of the landfill liner. Current landfill liner technology includes a composite liner which consists of a FML component and a compacted soil component. The primary characteristic for selecting a soil for use in composite liner construction is that the soil have a saturated hydraulic conductivity of 1 × 10−7 cm s−1 or less. In the present study the effects of desiccation cracks on the hydraulic conductivity of the compacted soil were measured. Two soils of diverse mineralogy and typical of soils used for clay liner construction were selected for use. Each was tested in its native state plus after the addition of 30% sand. Laboratory measurements were made of the volumetric shrinkage of each soil. In addition, the hydraulic conductivity was determined using 10 cm diameter fixed wall permeameters. Additional conductivity measurements were made using 60 cm diameter fixed wall double ring permeameters which had been exposed to 0, 1, and 2 periods of desiccation prior to hydraulic conductivity determinations. The data show that laboratory measurements using 10- cm diameter fixed wall permeameters underestimate the hydraulic conductivity of the same soils when packed in large diameter permeameters. It was also found that exposure to two cycles of desiccation resulted in large increases in hydraulic conductivity. The time required to reach a steady outflow volume decreased as the amount of desiccation increased. The hydraulic conductivities of soils which had been allowed to dry were greater than those which were not allowed to dry prior to measurement. The relationship between volumetric shrinkage and the increase in hydraulic conductivity after desiccation indicates that soils which exhibit less than 11% shrinkage in the laboratory, exhibit increases in K of less than a factor of 2 upon desiccation. Clay soils with greater than 11% shrinkage can potentially be amended with sand to decrease the volumetric shrinkage and their response to desiccation.

Conductivity of compacted clay soils to water and organic liquids

Four clay soils, selected to represent a range of mineralogies, were permeated with water and seven selected organic liquids in fixed wall permeameters. The conductivity of each soil/water followed by the chosen organic liquid was measured. The tested organic liquids included an acid and a base, and both polar and non-polar solvents. All four compacted soils had conductivities to water of less than 1 × 10−7 cm s−1. Acetic acid initially decreased the conductivity; however, after sufficient acid had permeated the soils, the conductivity generally increased to approximately the values measured with water. Soils treated with aniline exhibited conductivities of one to two orders of magnitude greater than values measured with water. Organic solvents, i.e. methanol, acetone, ethylene glycol, heptane and xylene, had conductivities of two to three orders of magnitude greater than their respective conductivities to water. While the miscible organics may undergo dilution by water in a field situation, the water immiscible solvents heptane and xylene would not be diluted by leachate and may reach a clay liner at or very near full strength. Conductivities of the soils to the organic solvents were much greater than could be explained by the densities and viscosities of the liquids. Visual observation of the soils permeated with organic solvents indicated the presence of cracks and voids in the centre of the soil mass which were not observed after the soils were permeated with water.

Diffusion Coefficients of Organics in High Density Polyethylene (HDPE)

Abstract Only limited data are available on the diffusion of volatile organic solvents through flexible membrane liners (FMLs) used for lining impoundments and landfills. To expand this database, a rapid, inexpensive method is needed to measure the diffusion coefficients of volatile organic solvents through FML materials. An absorption method has been developed to determine the diffusion coefficients of volatile organic solvents through FML materials. The method is based on the depletion of an organic compound from an aqueous solution due to absorption by a submerged sample of FML. A numerical solution of Ficks second law of diffusion was used to develop a graph which can be used to determine the diffusion coefficient from the time dependent concentration data. The diffusion coefficients obtained from the absorption tests were validated by comparing them with coefficients determined using a two chamber diffusion cell. The diffusion coefficients determined for toluene and xylene in high density polyethylene (HDPE) were 5.1 × 10 −9 cm 2 s −1 and 1.0 × 10 −9 cm 2 s −1 by the two methods, respectively. The data indicate that the coefficient of distribution (K d ) between the FML and the organic solution, a value which is needed to calculate the diffusion coefficient from the data, can be estimated from the log of the octanol-water partition coefficient (K ow ), a commonly measured and reported value for many chemicals.

Mutagenicity of three agricultural soils

A chemical and biological testing protocol was employed to evaluate the mutagenic potential of the organic compounds extracted from three agricultural soils. The analytical procedures used included bioassays with Salmonella typhimurium and Aspergillus nidulans for the detection of point mutations and a gas chromatography/mass spectrometry/computer system to identify major organic constituents. The extracts of all three soils exhibited mutagenic response in the bioassays. At a dose level of 1000 micrograms per plate, the organic extract of the Bastrop clay induced 434 net revertants; while at the same dose level the Norwood sandy clay and the Sassafrass sandy loam induced 35 and 178 net revertants, respectively, in the Salmonella assay with metabolic activation. In the Aspergillus assay, the extract of the Norwood and Bastrop soils induced a positive response without metabolic activation; this effect was reduced or eliminated in the presence of metabolic activation. Chemical analysis identified a variety of initiators, promotors, inhibitors, and cocarcinogens; however, there were no mutagenic compounds identified in any of the soil extracts. The results of this combined testing protocol indicate that the agricultural soils tested had an inherent level of mutagenic activity, which was not detected by GC/MS analysis alone, and this activity may be related to the past history of agricultural practices, including biocide applications, fertilization, and cultivation.

The Use of Biofilters to Reduce Atmospheric Methane Emissions from Landfills: Part I. Biofilter Design

Previous laboratory research has shown that biofilters have the potential to reduce CH4 emissions from landfills by as much as 83%. However, to achieve this level of CH4 reduction biofilters must be properly designed. The present study was conducted to develop a method for properly designing biofilters based on landfill size and location. A quadratic equation was developed to describe the dependence of CH4 oxidation rate in a sandy loam textured soil as a function of soil temperature, soil moisture and ammonium nitrogen concentration. Using this equation and the average monthly soil temperature and moisture contents for the largest cities of each of the 48 contiguous states, the monthly CH4 oxidation rate at each location was calculated. Then, assuming a standard landfill depth of 27.6 m, and a standard area of 121,500 m2, the required biofilter size was calculated. Finally, the ratio of biofilter size to landfill size was calculated. Design calculations for biofilters located in the states of Alabama, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina and Texas where the CH4 oxidation rates are relatively high throughout the year indicate that the necessary biofilter sizes are small. In addition, biofilters in these states may be expected to be effective throughout the year. In contrast, the calculations indicated biofilter systems in the states of Idaho, Minnesota and North Dakota will have much lower efficiencies during much of the year due to unfavorable soil moisture and temperature ranges. Given proper design, installation and management, a biofilter should be capable of achieving a significant reduction in atmospheric CH4 emission as compared to emissions from the same landfill without a biofilter.

The Effectiveness of Capillary Barriers to Hydraulically Isolate Salt Contaminated Soils

The use of capillary barriers overlayed with topsoil to hydraulically isolate highly saline soils was investigated in a greenhouse study. Capillary barriers of 8 cm or greater thickness effectively prevented salt from migrating upward into the topsoil, thus allowing vegetation to be established. The water which drained from the capillary barrier had electrical conductivities on the order of one third that of the underlying saline soil, and should be acceptable for stream discharge. The capillary barriers reduced the flux of water and salt to the groundwater to about one third of that of the control. The results of the study show that capillary barriers can be used effectively to contain salt contaminated soils.

Mutagenic activity of soils amended with two refinery wastes

The mutagenic potential of the acid, base, and neutral fractions of petroleum sludge amended soil was determined using the Salmonella/microsome assay and Aspergillus methionine assay. Organic compounds were extracted from two different soils amended with either storm-water runoff impoundment or combined API-separator/slop-oil emulsion solids waste. Application of either waste to soil reduced the mutagenic activity of organic compounds extracted from equal weights of soil. However, biodegradation increased both the total and the direct-acting mutagenicity of all fractions residual in the waste-amended soil. The maximum level of mutagenic activity per milligram residual C was detected in the sample collected 360 days after waste application for the acid and base fractions from the storm-water runoff impoundment amended soils and the acid, base, and neutral fractions of the combined API separator/slop-oil emulsion waste amended soils.A comparison of the results based on equivalent weights of soil indicates that the mutagenic potential of both wastes was reduced by soil incorporation. The results from the Salmonella assay indicate that while the bulk of the solvent extractable organics in both wastes was rendered non-mutagenic, the mutagenic potential of the organic compounds in the acid fraction from the storm-water runoff impoundment sludge amended soil was increased. The results from the Aspergillus assay of both wastes indicate that the mutagenic potential of all three fractions was eventually reduced to a level that would be considered non-mutagenic. Thus, while degradation may have increased the mutagenic potential of specific organic compounds that were residual in the soil, the overall effect of degradation was to reduce the weighted activity of the waste amended soil.

The influence of amendments on the volumetric shrinkage and integrity of compacted clay soils used in landfill liners

Clay liners remain an important component of composite liners used in landfill construction. Because their hydraulic integrity is frequently lost due to desiccation cracking, either during construction or shortly thereafter. The present study was initiated to evaluate the effects of common soil additives including lime, cement, and sand on the shrinkage and hydraulic conductivity of compacted clay soils commonly used in clay liner construction. Three soils having predominant clay minerals of smectite, illite and kaolinite were amended with varying amounts of lime, cement or sand; compacted using the Harvard miniature compactor; and the volumetric shrinkage was measured on the compacted samples. Additional samples of each treated soil were compacted according to ASTM 698 and used for measurement of the hydraulic conductivity. The results show that the majority of shrinkage occurs when the samples were dried to 25 °C with little additional shrinkage at temperatures up to 105 °C. The amendments of either 4% lime or 40 to 50% sand resulted in reduced shrinkage and increased hydraulic conductivity. The addition of 3% cement reduced shrinkage by up to 50% and simultaneously reduced hydraulic conductivity by 2 orders of magnitude. Thus, amendment of clay soils having a high shrink-swell potential with Type I Portland cement has the greatest poetential for field application as an amendment to help maintain the integrity and improve the long term performance of compacted clay liners.

The influence of application rate on the bacterial mutagenicity of soil amended with municipal sewage sludge.

The mutagenic potential of two soils amended with a municipal sewage sludge at two application rates was monitored over a 2-year period using Salmonella/microsome mutagenicity assay. Samples were collected from undisturbed monolith lysimeters of Weswood sandy clay (Fluventic Ustochrept) and Padina sandy loam (Grossarenic Paleustalf) amended with dried sewage sludge at 50 and 100 Mg/ha. Soil samples were collected and sequentially extracted with methylene chloride and methanol. The residues from these extracts were tested for mutagenicity at five doses with and without metabolic activation in Salmonella strain TA98. In general, the mutagenic potential of the amended soils of both application rates for the first 8 weeks following sludge application increased and then slowly decreased. The maximum mutagenic response observed in the soil extracts was 222 revertants at a dose of 10 mg of residue. This response was induced by the methanol extract from the Weswood soil collected 56 days after the application of 50 Mg/ha sewage sludge as compared to the 100 Mg/ha application which induced 202 revertants/mg. The mutagenicity of all fractions extracted from the sludge-amended soil at both application rates collected 717 days after application were not appreciably different from extracts from the unamended soils. The data indicate that chemicals that were mutagenic in bacteria persist in the soil and that at the higher application rates, as much as 2 years may be required for the mutagenic potential of the soil to return to background levels.