Biogeochemistry of landfill leachate plumes

Physical constraints affecting bacterial habitats and activity in unsaturated porous media – a review

Acid mine drainage (AMD), characterized by low pH and high concentrations of sulfate and heavy metals, is an important and widespread environmental problem related to the mining industry. Sulfate-reducing passive bioreactors have received much attention lately as promising biotechnologies for AMD treatment. They offer advantages such as high metal removal at low pH, stable sludge, very low operation costs, and minimal energy consumption. Sulfide precipitation is the desired mechanism of contaminant removal; however, many mechanisms including adsorption and precipitation of metal carbonates and hydroxides occur in passive bioreactors. The efficiency of sulfate-reducing passive bioreactors is sometimes limited because they rely on the activity of an anaerobic microflora [including sulfate-reducing bacteria (SRB)] which is controlled primarily by the reactive mixture composition. The most important mixture component is the organic carbon source. The performance of field bioreactors can also be limited by AMD load and metal toxicity. Several studies conducted to find the best mixture of natural organic substrates for SRB are reviewed. Moreover, critical parameters for design and long-term operation are discussed. Additional work needs to be done to properly assess the long-term efficiency of reactive mixtures and the metal removal mechanisms. Furthermore, metal speciation and ecotoxicological assessment of treated effluent from on-site passive bioreactors have yet to be performed.

Passive treatment of acid mine drainage in bioreactors using sulfate-reducing bacteria: critical review and research needs.

Landfill leachate contains a variety of pollutants that may potentially contaminate the ground water and affect the quality of surface waters and well waters. The literature has been critically reviewed in order to assess the attenuation processes governing the contaminants in leachate‐affected aquifers. After an introductory section on leachate composition, the physical and chemical frameworks for the attenuation processes are discussed in terms of dilution/dispersion and redox zones in the plume, respectively. A separate section focuses on the microbiology in terms of the occurrence of bacteria in plumes, the fate of pathogens, and microbial mediation of redox processes. In individual sections, the attenuation of dissolved organic matter, anthropogenic‐specific organic compounds, inorganic macrocomponents as anions and cations, and heavy metals are discussed. The focus is on laboratory experiences and field investigations. The review shows that most leachate contamination plumes are relatively ...

Attenuation of landfill leachate pollutants in aquifers

The biological clogging of natural porous media, often in conjunction with physical or chemical clogging, is encountered under a wide range of conditions. Wastewater disposal, artificial groundwater recharge, in situ bioremediation of contaminated aquifers, construction of water reservoirs, or secondary oil recovery are all affected by this process. The present review provides an overview of the techniques that are used to study clogging in the laboratory, or to monitor it in field applications. After a brief survey of the clogging patterns most commonly observed in practice, and of a number of physical and chemical causes of clogging, the various mechanisms by which microorganisms clog soils and other natural porous media are analyzed in detail. A critical assessment is also provided of the few mathematical models that have been developed in the last few years to describe the biological clogging process. The overall conclusion of the review is that although information is available on several aspects of the biological clogging of natural porous media, further research is required to predict its extent quantitatively in a given situation. This is particularly true in cases that involve complicating factors such as predation or competition among organisms.

Environmental impact and mechanisms of the biological clogging of saturated soils and aquifer materials

An experimental investigation was conducted to quantify the permeability reduction caused by enhanced biological growth in a porous medium. Studies were conducted using sand-packed column reactors for which variations in piezometric head, substrate concentration, and biomass measured as organic carbon were monitored in space and time. Methanol was used as a growth substrate. Permeability reductions by factors of order 10−3 were observed. The data show that a limit on permeability reduction exists, having a magnitude of 5 × 10−4 in the present study. The limit on permeability reduction and the existence of high densities of bacteria in substrate depleted zones are explained with an open pore model. Permeability reduction was observed to correlate well with biomass density for values less than about 0.4 mg/cm3, and exhibited independence at higher densities.

Biofilm growth and the related changes in the physical properties of a porous medium: 1. Experimental investigation

Growth of a biofilm in a porous medium reduces the total volume and the average size of the pores. The change in the pore size distributions is easily quantified when certain geometric assumptions are made. Existing models of permeability or of relative permeability can be manipulated to yield estimates of the resulting reduction in permeability as a function of biofilm thickness. The associated reductions in porosity and specific surface can be estimated as well. Based on a sphere model of the medium, the Kozeny-Carman permeability model predicts physically realistic results for this problem. Using a cut-and-random-rejoin-type model of the medium, the permeability model of Childs and Collis-George yields qualitatively reasonable results for this problem, as does a generalization of the relative permeability model of Mualem. Permeability models of Kozeny-Carman and of Millington and Quirk lead to unrealistic results for a cut-and-random-rejoin-type medium. The Childs and Collis-George and the Mualem models predict that the permeability reduction for a given volume of biomass is greatest when the porous medium has uniform pore sizes.

Biofilm growth and the related changes in the physical properties of a porous medium: 2. Permeability

A model describing the transport of substrate and biomass in porous media is formulated which accounts for the transport, growth and decay of biomass suspended in the water phase and attached to the solid matrix as a biofilm. Interphase transport of biomass between the water phase and attached biofilm phase due to fluid mechanical shear and filtration is considered as well. Special attention is given to changes in the porosity, permeability, and dispersivity resulting from the biofilm altering the microscopic geometry of the pore space. A numerical solution to the governing equations is obtained by the finite element method. The model is calibrated and verified against experimental data. An application is presented which assesses the effects of pulsed substrate loading, flow rate, and flow duration on the clogging of porous media by biomass.

Substrate and biomass transport in a porous medium

In the first part of this paper the change in dispersivity resulting from the growth of a biofilm in a porous medium is derived from an existing model of dispersivity and a cut-and-random-rejoin-type model of the pore geometry. The change in dispersivity due to a biofilm is also estimated from experimental data. Tracer experiments were conducted in biofilm column reactors and dispersion coefficients estimated by solving the inverse solute transport problem by nonlinear, least squares regression. Due to the presence of the biofilm in the porous media, solute flux into the biofilm is an important transport process and is given special attention. Both the dispersivity model and experimentally estimated dispersivities show order of magnitude increases in dispersivity as a result of significant biofilm growth. In the second part of the paper the models for the biofilm-affected permeability, porosity, and specific surface derived in a companion paper (Taylor et al., this issue) are verified using data from biofilm column reactors. These models are used to parameterize an equation describing the transport of substrate in the experimental columns. Numerical simulations were performed and compared to observed substrate data. Results show that the models for permeability and porosity can be used to make estimates of these parameters, while the model for specific surface appears to be inadequate.

Biofilm growth and the related changes in the physical properties of a porous medium. 3. Dispersivity and model verification.

A model describing the enhanced in‐situ biodegradation of an organic substrate in ground water is presented. This model simulates the transport and aerobic utilization of substrate and oxygen; the transport and growth of biomass dispersed in the water phase and in the biofilm; changes in porous‐media properties as a result of biofilm growth; and biofilm shearing and filtration. The model is applied to a recharge well to simulate the injection of an electron donor (substrate) and electron acceptor (oxygen) into an aquifer. Results show that a porous medium having a high porosity, wide range of pore sizes, and a small maximum pore radius is most susceptible to biofouling; and alternately pulsing the electron donor and acceptor reduces the biofouling propensity. The model is also applied to a hypothetical aquifer to simulate the process of bioremediation. Results show that increasing the oxygen concentration in the injection water, increasing the well‐pumping rate, and introducing oxygen through multiple inj...

Stewart W. Taylor

Papers

Biofilm growth and the related changes in the physical properties of a porous medium: 1. Experimental investigation

Biofilm growth and the related changes in the physical properties of a porous medium: 2. Permeability

Substrate and biomass transport in a porous medium

Biofilm growth and the related changes in the physical properties of a porous medium. 3. Dispersivity and model verification.

Enhanced in-situ biodegradation and aquifer permeability reduction