Paul J. Van Geel

Modeling of biological clogging in unsaturated porous media

A two-dimensional unsaturated flow and transport model, which includes microbial growth and decay, has been developed to simulate biological clogging in unsaturated soils, specifically biofilters. The bacterial growth and rate of solute reduction due to biodegradation is estimated using the Monod equation. The effect of microbial growth is considered in the proposed conceptual model that relates the relative permeability term for unsaturated flow to the microbial growth. Two applications of the model are presented in this study. Using the model, the clogging mechanism in different soils has been simulated. The results of the model indicate that the time to reach a clogged state is influenced by the hydraulic properties of the soil. Clogging is delayed in soils with higher saturated hydraulic conductivities, and higher porosities. For the relative permeability model proposed, higher van Genuchten n values lead to a delay in clogging. The model was also used to simulate the progressive clogging of a septic bed as the biomat initially forms at the up-gradient end of the distribution pipe, displacing wastewater infiltration and biomat formation further down-gradient over time.

Hydraulics of Peat Filters Treating Septic Tank Effluent

Peat is an alternative filter medium for the treatment of various waste streams including septic tank effluent. The water holding capacity and adsorption capacity of peat make it a favorable filter medium over sand or gravel which are commonly used as the filter medium for the drainage field of septic systems. This paper presents the results of a field study to evaluate the hydraulics of a peat filter used to treat the septic tank effluent from a public school west of Ottawa, Canada. An experimental box was placed within the filter during its construction to provide access to the vertical profile of the peat layer. The filter is periodically pulsed with septic tank effluent, which is distributed over the top of the filter and flows vertically through the peat. The filter was instrumented with tensiometers and transducers to monitor the pore water pressures in response to a pulse of septic tank effluent. An in-depth study of the hydraulics of the system was completed. The soil moisture retention curve and the hydraulic conductivity as a function of density were determined in laboratory experiments. A one-dimensional unsaturated flow model was used to predict the pressure response due to a pulse. A comparison of the field and model results illustrates the impact of the density variations, and the corresponding hydraulic conductivity variations, on the model predictions. The compaction of the peat is an important design consideration for the filter since it directly impacts the flow characteristics and the hydraulic retention time within the filter.

Effect of saline water and sludge addition on biodegradation of municipal solid waste in bioreactor landfills

Bioreactor landfills require sufficient moisture to optimize the biodegradation processes and methane generation. In arid regions, this is problematic given the lack of fresh water supplies. Saline water can be used but may inhibit the biodegradation of the municipal solid waste (MSW) in landfills. Sludge may be used to enhance the biodegradation of MSW under saline conditions. For this study, two groups of laboratory-scale bioreactor cells were used to study the impact of saline water and sludge addition on the biodegradation of MSW in bioreactor landfills. The first group (four bioreactors) operated without sludge addition. The second group (four bioreactors) operated with the addition of sludge. The salt concentrations in the two groups were 0, 0.5, 1 and 3% (w/v), respectively. All bioreactors were operated at neutral pH levels with leachate recycling. The methane yield was 70.6, 61.7 and 47.5 L kg—1 dry waste for bioreactors R1, R2 and R4, respectively; and 84.7, 78.7, 72.6 and 59 L kg— 1 dry waste for bioreactors R5, R6, R7 and R8, respectively. The high salt content (3%) inhibited the MSW biodegradation as evidenced by the methane yield, the percentage reduction in leachate concentration and the settlement that occurred during the study. Sludge addition was able to improve the methane yield at all salt concentrations.

Development of a vertical TDR probe to evaluate the vertical moisture profile in peat columns to assess biological clogging

Time domain reflectometry (TDR) is used to monitor the moisture content in soils including peat. The objective of this study was to develop a TDR-based method to measure the vertical moisture profi...

A proposed transient model for cometabolism in biofilm systems

A dynamic model was developed to describe the behaviour of primary and secondary substrates in a biofilm reactor. The model incorporates structured kinetics to describe the generation and consumption of reducing power in the catabolic and respiratory subsystems, respectively. Secondary substrate transformation through oxygenolytic or reductive mechanisms can be modelled under either single or dual limitation of the electron donor and electron acceptor substrates. An example simulation of a theoretical biofilm system was performed.

Potential concerns related to using octadecyltrichlorosilane (OTS) in rendering soils and porous ceramics hydrophobic

The treatment of hydrophilic porous ceramics to render them hydrophobic and wetting to non-aqueous phase liquids (NAPLs) is frequently needed in multiphase flow experiments to control the flow or to measure the pressure of the NAPL. In addition, research dealing with soil wettability implies a need for hydrophobic or NAPL-wet soils. The traditional procedure, which has been widely used in literature, to render hydrophilic porous ceramics and soils hydrophobic is achieved by placing the hydrophilic solid in a 5% (by volume) octadecyltrichlorosilane (OTS) solution in ethanol followed by rinsing in ethanol. This research assesses the use of this procedure as it was found that this treatment procedure resulted in excess OTS on the surface of treated hydrophobic solids which can dissolve in an organic phase and in turn alter the wettability condition of adjacent hydrophilic soils. A modified procedure, which results in hydrophobic solids free of excess OTS, is presented.

Heat budget for a waste lift placed under freezing conditions at a landfill operated in a northern climate

A landfill operated in Ste. Sophie, Québec, Canada was instrumented to better understand the waste stabilization process in northern climates. Instrument bundles were placed within the waste to monitor temperature, settlement, oxygen, moisture content, total load, mounding of leachate and electrical conductivity. A finite element model was developed to simulate the heat budget for the first waste lift placed in the winter months and was calibrated using the first 10.5 months of collected temperature data. The calibrated model was then used to complete a sensitivity analysis for the various parameters that impact the heat budget. The results of the analysis indicated that the heat required for phase change to thaw the liquid fraction within frozen waste had a significant impact on the heat budget causing sections of waste to remain frozen throughout the simulation period. This was supported by the data collected to date at Ste. Sophie and by other researchers indicating that frozen waste placed during the winter months can remain frozen for periods in access of 1.5 years.

Simulating settlement during waste placement at a landfill with waste lifts placed under frozen conditions

Twelve instrument bundles were placed within two waste profiles as waste was placed in an operating landfill in Ste. Sophie, Quebec, Canada. The settlement data were simulated using a three-component model to account for primary or instantaneous compression, secondary compression or mechanical creep and biodegradation induced settlement. The regressed model parameters from the first waste layer were able to predict the settlement of the remaining four waste layers with good agreement. The model parameters were compared to values published in the literature. A MSW landfill scenario referenced in the literature was used to illustrate how the parameter values from the different studies predicted settlement. The parameters determined in this study and other studies with total waste heights between 15 and 60 m provided similar estimates of total settlement in the long term while the settlement rates and relative magnitudes of the three components varied. The parameters determined based on studies with total waste heights less than 15m resulted in larger secondary compression indices and lower biodegradation induced settlements. When these were applied to a MSW landfill scenario with a total waste height of 30 m, the settlement was overestimated and provided unrealistic values. This study concludes that more field studies are needed to measure waste settlement during the filling stage of landfill operations and more field data are needed to assess different settlement models and their respective parameters.

Simulating waste temperatures in an operating landfill in Québec, Canada

A bioreactor landfill operated in Sainte-Sophie, Québec, Canada was instrumented to better understand the waste stabilization process in northern climates. Instrument bundles were placed within the waste to monitor temperature, oxygen, moisture content, settlement, total load, mounding of leachate and electrical conductivity. A finite element model was developed to simulate the heat fluxes to and from the waste, as well as heat generation within the waste from both anaerobic and aerobic processes. The results of the analysis suggest the majority of the aerobic activity occurs in the top portion of the waste lift exposed to ambient air. In addition, the model indicates that frozen waste lifts require a significant amount of heat to thaw the liquid fraction. The model also demonstrates that when a lift of cold waste is placed at the bottom of the landfill, the subsurface acts as a significant source of heat.

SEQUENCE Visualization of Natural Attenuation Trends at Hill Air Force Base, Utah

For monitored natural attenuation to be considered as an acceptable remedial approach, the proponent must clearly document converging lines of evidence that illustrate the effectiveness of this measure. SEQUENCE, a visualization tool based on a modified radial diagram approach, is ideally suited for evaluating spatial and temporal trends that provide supporting evidence for the efficacy of monitored natural attenuation. SEQUENCE was applied to evaluate the natural attenuation of benzene, toluene, ethylbenzene, and total xylene (BTEX) concentrations observed in groundwater at Hill Air Force Base, Utah. SEQUENCE-BTEX maps provided an efficient means of documenting the declining BTEX concentrations downgradient from the source area. SE-QUENCE-Redox maps were used to facilitate a correlation between elevated BTEX concentrations; decreasing electron acceptor concentrations (oxygen, nitrate, and sulfate); and elevated metabolic byproduct concentrations (iron(II) and methane) providing a second line of evidence ...