Tjalfe G. Poulsen

Gas Permeability in Undisturbed Soils: Measurements and Predictive Models

Accurate prediction of changes in the gas permeability during variable soil-moisture conditions is a prerequisite for improved simulation and design of soil-venting systems for removal of volatile organic chemicals in polluted soils. Air permeability, k, as a function of soil air-filled porosity,

Colloid Mobilization and Transport in Undisturbed Soil Columns. I. Pore Structure Characterization and Tritium Transport

While it is recognized that preferential flow may increase the transport of colloids, less is known about the actual influence of preferential flow on colloid mobilization in situ. Changes in pore structure upon soil exposure to drying and rewetting may additionally affect colloid mobilization. Information about the pore structure and the active flow volume, as well as the changes in these properties, are therefore important when investigating colloid mobilization. We investigate the pore structure characteristics and the transport of tritium ( 3 H 2 O) during steady unsaturated flow conditions. A total of 54 soil columns sampled along a natural clay gradient representing six clay contents (12, 18, 24, 28, 37, and 43% clay) were equilibrated to three different initial matric potentials (IMP), ψ = −2.5, −100, and −15500 hPa. Pore structure characteristics were deduced from water retention characteristics and measurements of air-filled porosity and air permeability. Tracer experiments were conducted at 1 mm h −1 and with a suction of 5 hPa. A mobile–immobile region model (MIM) and a three-region model (2MIM) with two mobile and one immobile region were used for describing the breakthrough curves (BTCs). The 2MIM model was able to fit the data well and predicted the existence of two mobile flow regions, most pronounced at higher clay content. The 12% clay soil exhibited matrix-dominated flow behavior, which is probably attributable to a large fraction of drained pores disconnecting the rapidly conducting flow system. Soils with ≥18% clay exhibited asymmetrical BTCs with early breakthrough and tailing and an increasing amount of immobile water, indicating preferential flow. Drying and rewetting, because of associated changes in the pore structure, significantly reduced the degree of preferential flow.

Optimizing feed composition for improved methane yield during anaerobic digestion of cow manure based waste mixtures

This study investigated methane yield via anaerobic digestion of multi-component substrates based on mixtures of biodegradable single-component substrates with cow dung as main component. Bench and full-scale digestion experiments were carried out for both single and multi-component substrates to identify the relationship between methane yield and substrate composition. Results from both bench- and full-scale experiments corresponded well and showed that using multi-component substrates increases the methane yield much more than what would be expected from digestion of single substrates. Process stability as indicated by gas production, pH and NH(4)(+) concentration variations were also improved by using multi-component substrates compared to digestion of single-component substrates. The results, thus, suggest that assessment of methane yield for multi-component substrates cannot reliably be based on methane yields for corresponding single-component substrates but should instead be measured directly.

Organic micropollutant degradation in sewage sludge during composting under thermophilic conditions.

Degradation of 12 common organic micropollutants in sewage sludge representing bactericides, flame retardants, fragrances, vulcanizers, and plasticizers (part of many common products) during thermophilic composting was investigated. Micropollutant concentrations, compost temperature, water content, and organic matter content were measured over 24 days in a full-scale compost windrow made from digested sewage sludge, yard waste, and horse manure. Composting took place indoors, and the windrow was turned several times during the experimental period. Concentrations of all 12 micropollutants decreased during composting, and degradation was statistically significant for 7 of the 12 micropollutants. Metabolites (galaxolidone and methyl-triclosan) were produced from two micropollutants (galaxolide and triclosan) during composting, indicating microbial degradation. Pollutant concentrations early in the experiment were more variable than those experienced for the chemical method development. This was likely due to compost heterogeneity. After the second compost turning, concentrations became more stable as compost became more homogeneous.

Evaluating effects of wind-induced pressure fluctuations on soil-atmosphere gas exchange at a landfill using stochastic modelling

The impact of wind turbulence-induced pressure fluctuations at the soil surface on landfill gas transport and emissions to the atmosphere at an old Danish landfill site was investigated using stochastic modelling combined with soil property and gas transport data measured at the site. The impacts of soil physical properties (including air permeability and volumetric water content) and wind-induced pressure fluctuation properties (amplitude and temporal correlation) on landfill gas emissions to the atmosphere were evaluated. Soil-air permeability and pressure fluctuation amplitude were found to be the most important parameters. Wind-induced gas emissions were further compared with gas emissions caused by diffusion and by long-term pressure variations (due to passing weather systems). Here diffusion and wind-induced gas transport were found to be equally important with wind-induced gas transport becoming the most important at lower soil-air contents.

SPATIAL AND TEMPORAL DYNAMICS OF AIR PERMEABILITY IN A CONSTRUCTED FIELD

Soil air permeability in undisturbed soil is closely related to soil structure and heterogeneity and hydraulic properties. Knowledge of air permeability behavior for different types of soils is, therefore, valuable. In this study, we investigated the variability and spatial correlation structure of soil air permeability and soil textural properties in an undisturbed constructed field at Hiroshima University, Hiroshima, Japan. Air permeability was measured on undisturbed soil samples along two 70-m-long transects in the top 10 cm of a field constructed approximately 20 years ago from sandy loam soil collected from a nearby mountain. Air permeability (ka) showed spatial correlation along both transects, with a range of approximately 20 m. Measurements of ka on large (3140 cm3) samples were similar, showing a soil with little small-scale heterogeneity. Local-scale measurements of ka within a 1.6- by 1.6-m grid taken 4 months after the transect sampling suggested that soil structure and pore size distribution had changed significantly over time, as a result of tilling and precipitation, causing an increase in ka. No spatial drift in soil physical parameters within the 1.6- by 1.6-m grid was found. On-site ka measurements after 1- to 2-day rainy periods compared well with laboratory measurements at a soil-water potential of −100 cm H2O, suggesting that natural field capacity occurs at this potential. Measurements of ka as a function of air-filled porosity (&epsis;) at the mid-point of the two transects were compared with predictions by two recently presented ka (&epsis;) models, and good agreement between simulated values and measurements was found.

Energy production, nutrient recovery and greenhouse gas emission potentials from integrated pig manure management systems

Improper management of pig manure has resulted in environmental problems such as surface water eutrophication, ground water pollution, and greenhouse gas emissions. This study develops and compares 14 alternative manure management scenarios aiming at energy and nutrient extraction. The scenarios based on combinations of thermal pretreatment, anaerobic digestion, anaerobic co-digestion, liquid/solid separation, drying, incineration, and thermal gasification were compared with respect to their energy, nutrient and greenhouse gas balances. Both sole pig manure and pig manure mixed with other types of waste materials were considered. Data for the analyses were obtained from existing waste treatment facilities, experimental plants, laboratory measurements and literature. The assessment reveals that incineration combined with liquid/solid separation and drying of the solids is a promising management option yielding a high potential energy utilization rate and greenhouse gas savings. If maximum electricity production is desired, anaerobic digestion is advantageous as the biogas can be converted to electricity at high efficiency in a gas engine while allowing production of heat for operation of the digestion process. In conclusion, this study shows that the choice of technology has a strong influence on energy, nutrient and greenhouse gas balances. Thus, to get the most reliable results, it is important to consider the most representative (and up-to-date) technology combined with data representing the area or region in question.

Strategic environmental assessment of alternative sewage sludge management scenarios

Strategic environmental assessment (SEA) of sewage sludge management in a Danish municipality (Aalborg), with 160,000 inhabitants using alternative methods for aggregation of environmental impacts was performed. The purpose is to demonstrate the use of SEA in relation to sludge management and to improve SEA methodology. Six different scenarios for management of sewage sludge within the Aalborg municipality involving thermal treatment, composting and landfilling of sludge were evaluated. Environmental impact categories considered were global warming, non-renewable resources (nutrients and fossil fuels) and land use. Impact categories human health, ecotoxicity and soil quality were excluded as methodology for their assessment is not yet fully developed. Thermal sludge treatment with energy utilisation was shown to be a promising option for sewage sludge management in Aalborg. Sensitivity of the relative environmental impacts with respect to calculation methodology and input parameter values were evaluated to identify important parameters and calculation methods. The analysis showed that aggregation procedures, sludge biogas potential and sludge production were very important whereas sludge transport was not.

Air permeability of compost as related to bulk density and volumetric air content.

Compost air permeability controls air flow through compost during composting or when using compost as biofilter material. Air permeability is therefore an important characteristic of compost. The relationships between air permeability (ka) in compost and compost dry bulk density (ρb), gravimetric water content (ω), and volumetric air content (ε) was investigated for two types of composts. The composts used were produced from a digested sewage sludge—straw mixture and from garden waste and measurements were conducted on sieved and repacked 100 cm3 compost samples. Results showed a linear relation between log(ka) and ρb at constant values of ω for both composts, indicating an exponential relationship between ka and ρb. The slopes of these relationships generally became more negative with increasing ρb. The results further showed a linear relationship between log(ka) and log(ε) for both composts as also often observed for soils. It was observed that the log(ka) and log(ε) relationships for the garden waste compost all intercepted at the same location despite having very different slopes. This means that it is possible to predict the entire ka—ε relationship using only one measurement of corresponding (ka, ε) for garden waste. It was not possible to determine whether this was also the case for the sewage sludge compost due to difficulties in sample preparation at low and high water content.

Relating landfill gas emissions to atmospheric pressure using numerical modelling and state-space analysis

Landfill gas (CO2 and CH4) concentrations and fluxes in soil adjacent to an old, unlined Danish municipal landfill measured over a 48-hour period during the passage of a low-pressure weather system were used to identify processes governing gas fluxes and concentrations. Two different approaches were applied: (I) State-space analysis was used to identify relations between gas flux and short-term (hourly) variations in atmospheric pressure. (II) A numerical gas transport model was fitted to the data and used to quantify short-term impacts of variations in atmospheric pressure, volumetric soil-water content, soil gas permeability, soil gas diffusion coefficients, and biological CH4 degradation rate upon landfill gas concentration and fluxes in the soil. Fluxes and concentrations were found to be most sensitive to variations in volumetric soil water content, atmospheric pressure variations and gas permeability whereas variations in CH4 oxidation rate and molecular coefficients had less influence. Fluxes appeared to be most sensitive to atmospheric pressure at intermediate distances from the landfill edge. Also overall CH4 fluxes out of the soil over longer periods (years) were largest during periods with rapidly decreasing atmospheric pressures resulting in emission of large amounts of CH4 during short periods of time. This effect, however, was less significant for the CO2 fluxes.