Journal of Environmental Engineering

A numerical model of gravity sedimentation and thickening was developed from the governing two-phase flow equations for the liquid and solid phases. The inertial and gravity terms in the solid and liquid momentum equations were retained in the gravity sedimentation and thickening model. An implicit, space-staggered finite-difference algorithm was developed for the resulting coupled partial differential equations. Constitutive relationships describing the physical properties of the slurry were required to solve the numerical model. These constitutive properties describing the relationship between effective stress and porosity and between permeability and porosity were determined experimentally and by model calibration. The model was calibrated and verified using the data of dynamic porosity profiles of gravity sedimentation and thickening of kaolin suspensions in distilled water.

A gravity drainage model was developed that included terms for cake resistance, filter resistance, and initial ponding. Laboratory scale batch drainage tests were performed at sludge volume loadings between 0.013 and 0.006 m³/m². Results were compared with limits established from the model for ponded and nonponded sludge drainage. At low volumetric loadings, the series of drainage curves represented a smooth family of curves with increased drainage rates at the volume was increased. However, at higher volumetric loadings, a discontinuity in the family of curves was observed in which the rate of filtrate flow decreased with an increase in sludge volume. The change in the shape of the filtrate curves corresponded with the observance of standing water on top of the forming sludge cake, or ponding. The effects of plowing during drainage were investigated. One effect of plows was to diminish the effects of ponding, noted as an increased drainage rate, for sludge loading conditions that had a tendency to pond in the absence of plows. A second effect was to cause the release of more water from sludges that were not prone to ponding than that seen in the absence of plows.

In this study, the applied liquid pressure was used to characterize the rate and extent of activated sludge dewatering in the expression phase. By characterizing the liquid pressure, three important features of expression phase dewatering were determined. First, as the depth of sludge cake increased, the ability to express water from the sludge cake was greatly reduced. Second, iron and lime were found to be superior to polymer, especially as the applied pressure increased. Third, for either type of conditioner, increased pressure resulted in a decreased rate of expression. This was probably due to the highly compressible nature of activated sludge, which responds to pressure by becoming less porous.

A bench-scale pilot plant was operated to evaluate the potential of using microfiltration hollow fiber membrane modules as an air diffuser and solid/liquid separator in an alternate cycle within a bioreactor treating domestic wastewater. Two modules capable of air backwashing were immersed in a bioreactor. Compressed air backwashing and filtration by suction were effected alternatively. The experimental results reveal that application of the air backwashing technique to submerged membrane modules is capable of not only declogging the membranes but also of aerating the mixed liquor. Thus, better filtration flux rate and aeration without a separate aeration device were attained simultaneously. It was also noted that the introduction of an anoxic zone enhanced the removal of nitrogen. In addition, operation at low hydraulic retention time with high sludge concentration and the absence of a sedimentation tank indicate considerable savings on the plant area.

This paper describes the hydrologic evaluation of leachate production and quality model, a screening-level tool to simulate contaminant leaching from a confined disposal facility (CDF) for dredged material. The model combines hydraulics, hydrology, and equilibrium partitioning, using site-specific design specifications, weather data, and equilibrium partitioning coefficients from the literature or from sequential batch or column leach tests of dredged material. The hydraulics and hydrology are modeled using Version 3 of the hydrologic evaluation of landfill performance model. The equilibrium partitioning model includes provisions for estuarine sediments that have variable distribution coefficients resulting from saltwater washout. Model output includes contaminant concentrations in the CDF profile, contaminant concentration and mass releases through the bottom of the CDF, and contaminant concentrations and masses captured by leachate collection systems. The purpose of the model is to provide sound information for evaluating the potential leachate impacts on ground water at dredged material CDFs and the effectiveness of leachate control measures.

A landfill siting analysis typically requires evaluating various rules, factors, constraints, and numerous spatial data. A modern geographical information system, although capable of rapidly processing a massive amount of spatial data, lacks the ability to locate an optimal site when compactness and other factors are simultaneously evaluated. A previously developed grid-based model could not be applied to resolve this inability for irregularly shaped spatial data. Therefore, an enhanced spatial siting model is proposed herein for general spatial data. A compactness index is applied to ensure the integrity of selected sites. Two case studies are presented to demonstrate the applicability of the proposed model. The proposed model and two models developed previously are compared in the first case study based on the single factor of land cost. A single factor model is perhaps quite unacceptable with respect to other factors. The second case study is thus presented to demonstrate the flexibility of the model for considering additional factors, land slope, and road network accessibility. Moreover, results obtained from various models and siting factors are compared and discussed.

Vaporization and soil adsorption are the two mass transfer mechanisms that control contaminant recovery rates for soil vapor extraction (SVE) systems. At most soil remediation sites, contaminants are distributed among three phases, namely, soil particles, pore water, and soil vapor. Contaminant mass transfer from adsorption sites into a convective vapor stream involves desorption, diffusion through pore water, and vaporization into soil vapor. An SVE design model is proposed to describe this three-phase mass transfer process and assist the design and evaluation of SVE systems. The model contains analytical solutions developed to estimate contaminant concentrations in the vapor phase and predict contaminant removal rates. Monitoring data from two full-scale SVE systems were used for model development and calibration. The results suggest that contaminant diffusion through the pore water is the rate-limiting step and leads to remediation inefficiency of an SVE system. Mass transfer retardation from molecular diffusion in water is likely the major contributing component to the venting efficiency coefficient of Staudinger et al.

Microbial reductive dechlorination is a naturally occurring transformation process tetrachloroethene (PCE) undergoes in aquifers. It contributes significantly to natural attenuation of chlorinated solvents. Stimulation of this process has been considered as a method of enhanced bioremediation. Experiments on the stimulation of reductive dechlorination were carried out in a large-scale quasi-2D sandbox model. The transformation of PCE to cis-1,2-dichloroethene (DCE) was attempted by inoculation with Dehalospirillum multivorans and that of DCE to ethene with a mixed culture. Ethanol used as the electron donor was introduced into the inlet of the domain, whereas water loaded with PCE was injected into a well. Limitations due to insufficient mixing could not be observed as high-permeability lenses enhanced the transverse exchange of the compounds. Both reductive dechlorination and competitive microbial reactions led to the acidification of the domain. The artificial aquifer was buffered by the concurrent injection of sodium sulfide with the electron donor. Under these conditions bioaugmentation of Dehalospirillum multivorans was successful, whereas stable dechlorination of DCE could not be achieved.

Steady-state models of nitrifying biofilms are developed taking into account the mass transfer of neutral and ionic species, electroneutrality, pH-dependent Monod kinetics, chemical equilibrium, and the presence of a boundary layer. Due to the assumption of bulk conditions, the rigorous biofilm model often predicts the washout of one of the biofilm species involved in a series of sequential biofilm transformations such as nitrification. Coupling the rigorous biofilm model with reactor mass balances yields the rigorous reactor model, which calculates the bulk conditions where both of the biofilm species remain and is thus more widely applicable. Substantial changes in the pH across the boundary layer and the biofilm are predicted; therefore, rates of nitrification may be improved by increasing bulk pH to high (alkaline) values, which results in optimum (neutral) pH within the biofilm. Varying influent conditions such as pH and buffer capacity can result in significant changes in reactor performance with respect to complete nitrification. The validity of the nitrification models are tested by comparison of profile predictions with that of experimental data, with encouraging results.

Steady-state models of nitrifying biofilms have been developed that take into account the mass transfer of neutral and ionic species, electroneutrality, pH-dependent Monod kinetics, chemical equilibrium, and the presence of a boundary layer. Under the conditions investigated, the models predict significant changes in the biomass speciation and percent conversions in nitrifying biofilms as functions of pH, buffer capacity [in terms of total inorganic carbons], and, for the reactor models, volumetric flow rate per unit surface area of biofilm and total ammonium loading. To a lesser extent, the models also predict sensitivity to biofilm thickness, total ammonium, total nitrite, dissolved oxygen concentration, and total biomass density. Nitrogen conversions are only a very weak function of boundary layer thickness, total nitrate concentration, and the ratio of the chemical species' diffusivity in the biofilm to its diffusivity in water.