Journal of Environmental Engineering

The “funnel/gate system” is a developing technology for passive ground-water plume management and treatment. This technology uses sheet pilings as a funnel to force polluted ground water through a highly permeable zone of reactive porous media (the gate) where contaminants are degraded by biotic or abiotic heterogeneous reactions. This paper presents a new analytical nonequilibrium model for solute transport in saturated, nonhomogeneous or mixed porous media that could assist efforts to design funnel/gate systems and predict their performance. The model incorporates convective/dispersion transport, dissolved constituent decay, surface-mediated degradation, and time-dependent mass transfer between phases. Simulation studies of equilibrium and nonequilibrium transport conditions reveal manifestations of rate-limited degradation when mass-transfer times are longer than system hydraulic residence times, or when surface-mediated reaction rates are faster than solute mass-transfer processes (i.e., sorption, film diffusion, or intraparticle diffusion). For example, steady-state contaminant concentrations will be higher under a nonequilibrium transport scenario than would otherwise be expected when assuming equilibrium conditions. Thus, a funnel/gate system may fail to achieve desired ground-water treatment if the possibility of mass-transfer-limited degradation is not considered.

The limited success and high cost of traditional “active” ground-water-contaminant plume management efforts (i.e., pump-and-treat systems) has stimulated a search for less expensive “passive” plume interception and in-situ treatment technologies. The “funnel/gate system,” which uses heterogeneous (surface-mediated) reactions on porous media to degrade dissolved contaminants, is one passive technology under consideration. Research on a heterogeneous reaction is presented in this paper, which can be extended to facilitate the design of engineered porous media systems (i.e., funnel/gates). Results are examined from batch and flow-through column experiments involving nitrobenzene degradation in a surface-mediated reaction with granular metallic iron. A nonequilibrium transport model that incorporates solute mass-transfer resistance near reactive iron surfaces is shown to simulate breakthrough curves (BTCs) from column systems, using model parameters estimated from batch systems. The investigation shows pseudo first-order degradation-rate coefficients increasing with higher solid:liquid ratios and with greater iron concentrations. In addition, nitrobenzene degradation is found to be faster in batch systems than in comparable column systems, indicating the presence of mass-transfer limitations in the flow-through systems. Finally, the present study provides insights on conditions pertinent to the design of engineered in-situ treatment zones, such as how mass-transfer, hydraulic, and reaction kinetic conditions affect ground-water-contaminant fate and transport through reactive porous media.

A technique for remediating and protecting ground-water and drinking-water wells from contamination is presented. A ground-water remediation well design is proposed in which ground water is drawn into the well bottom, treated in the well casing, and returned clean to the aquifer at the well top. The hydraulics of ground-water circulation around imperfectly penetrating wells were examined experimentally in pilot-scale tanks and mathematically with numerical models in a two-dimensional domain. Ambient ground-water velocities of from 1 to 3 m/day, typical for coarse sand and gravel aquifers, were simulated in combination with in-well vertical velocities of from 2 to 16 m/day. Under some of these flow conditions, contaminants were intercepted and drawn into the wells for treatment. Hydraulic problems identified with the experimental apparatus and simulated by the computer models included blow through of contaminant at the well intake by high ambient ground-water velocities and submergence by the well hydraulics of surface contaminant plumes without interception. Both types of problems were corrected by adjusting internal well velocities. Important design parameters include ambient ground-water velocity and well pumping rate.

Numerical simulations were used to evaluate the effects of vapor extraction operations on contaminant loadings to the atmosphere, the ground water, and to the vapor extraction well under application of vapor extraction. A two-dimensional numerical model for simulating migration of a volatile contaminant partitioned between the nonaqueous phase liquid, air, water, and solid phases in the unsaturated zone was developed and used in this study. The model was based on an explicit finite difference scheme. The sensitivity of vapor extraction efficiency and contaminant migration to system parameters was investigated. Results from model simulations show that well vacuum, nonaqueous phase liquid-air mass transfer rate, and inhomogeneities in soil air conductivity have a significant influence on contaminant flux and the time required for contaminant removal. Contaminant loadings to the atmosphere and ground water were strongly dependent on well vacuum and soil air conductivity, but were almost independent on nonaqueous phase liquid-air mass transfer rate.

The overall objective of this study is to investigate the technical feasibility of removal and destruction of organic contaminants in water using adsorption and photocatalytic oxidation. The process consists of two consecutive operational steps: (1) removal of organic contaminants using fixed-bed adsorption; and (2) regeneration of the spent adsorbent using photocatalytic regeneration. To facilitate the regeneration, some of the adsorbents were impregnated with photocatalysts, which would allow them to act as both an adsorbent for capturing the organics and a photocatalyst for destroying the organics during regeneration. Laboratory-scale adsorption processes were conducted using rapid small-scale column tests (RSSCTs) for the loading of F-400 GAC (60X80) and Ambersorb 563 (60X80) with trichloroethene (TCE) and para-dichlorobenzene (p-DCB), respectively. The spent adsorbents were regenerated in a plate photoreactor and the organic-free water was used. For the impregnated F-400 carbon, catalyst loading was observed to have little impact on the adsorption capacity for TCE. The Pt-TiO ₂ impregnated F-400 carbon loaded with TCE (1,000 μg/L) could be regenerated using artificial light with a regeneration time identical to the adsorption time. About 30 loss in capacity was observed during the four adsorption/regeneration cycles. For Pt-TiO ₂ impregnated Ambersorb 563, the capacity loss from the first regeneration was about 40 and no further loss was observed for the remaining adsorption/regeneration cycles. Based on these studies, the regeneration process was found to be limited by the reaction rate at the beginning of the regeneration cycle and then desorption of adsorbates from the interior adsorbent surface limited the rate. This was especially true for p-DCB, which is very strongly adsorbed. Upon further testing, it was observed that increasing the temperature significantly improved the regeneration rate. The laboratory studies confirmed that both F-400 and Ambersorb 563 could be regenerated using photocatalytic oxidation; however, a low photoefficiency was observed as compared to photocatalysis alone because the desorption of the organics from the interior of the adsorbent was very slow even at elevated temperatures.

The long-term leaching of metals from concrete products made with spent abrasive media was investigated using a sequential extraction procedure employing both an acidic extraction fluid and seawater. By using seawater, leaching behavior under conditions encountered in the environment (especially coastal areas) can be determined. Chromium, cadmium, and lead concentrations were substantially less for the seawater sequential extractions than for the acidic sequential extractions. The environment created during the acidic sequential extractions resulted in the leaching of substantial amounts of alkalinity from the concrete, and leachate pH levels dropped below 4 where metals are highly soluble. Also, as only one-tenth of the alkalinity that leached in the acidic extractions leached in the seawater extractions, the integrity of the calcium matrix within the concrete seemed to play a role in the successful stabilization of cadmium and lead. Acidic extraction tests provide more severe conditions than concrete products are likely to encounter in real-world applications.

Organic colloids resulting from sediment diagenesis in porewaters are transported across the sediment-water interface via Brownian diffusion. The sediment selected for this study was from the University Lake, Baton Rouge, La. The effects of electrolytes on the sediment-to-water flux of colloids varied depending on the type and concentration of the electrolyte. An ionic strength of 0.5 M reduced the flux of colloids; the effect being greater for KCl than for NaCl at the same ionic strength. The effects of different electrolytes (viz., NaCl, KCl, and CaCl ₂ ) on the porewater colloid concentrations were studied in batch experiments. Below the ionic strength of 0.5 M, the electrolytes were effective in increasing the binding of the colloids to sediment surfaces, and hence decreased their pore-water concentrations. Above the ionic strength of 0.5 M, the amount of colloids released to the porewater was greatest in the presence of CaCl ₂ . The diffusive flux of colloids from sediment to water was modeled using an effective diffusivity as the single adjustable parameter. The effective diffusivity ( D _eff ) was obtained using two complementary techniques. The first method utilized the sediment-to-water flux of colloids, which was fit to a model to extract D_eff . For the second method, postmortem cores of the sediment were obtained after 82 d, the porewater colloids were extracted and analyzed. The porewater colloid profile as a function of sediment depth was used to obtain D _eff . The effects of sediment diagenesis on the porewater colloids and the sediment total organic carbon (TOC) in a natural environment are illustrated using data from New Bedford Harbor (NBH), Mass. For this specific case only 0.43% of the organic matter delivered to the sediment was found to reappear in the water column as “dissolved” species.

Characterization of the types and concentrations of heavy metals in their various phases is needed to assess potential adverse effects on the aquatic environment and its ecosystems, and thereby to assure cost-effective mitigation and waste-allocation strategies. A two-dimensional vertically averaged finite-element model was extended to predict the spatial and temporal distribution of cohesive sediments and associated toxic heavy metals as a result of a pollutant input into an estuarine system. Constitutive relationships for cohesive sediment deposition and erosion, heavy metal adsorption and desorption, and the governing hydromechanical transport mechanisms were incorporated in an existing sediment transport model. Formulation and validation of a new relationship for deposition was based on data derived from experiments carried out by previous investigators. The fate and transport of cohesive sediments and associated nickel was simulated for south San Francisco Bay. The comparison of model performance against field observations indicated that the model is capable of representing important phenomena governing the fate of cohesive sediments and associated nickel.

While different forms of swirl and vortex technologies have been developed during the last thirty years, their major function has been the dual purpose of flow regulation and settleable-solids concentration for combined sewer overflows. A variety of opinions have developed regarding the application of these technologies, which vary from overwhelming support to reservations of their effectiveness. Performance of swirl/vortex devices depends on the settling characteristics of the suspended solids and the fraction of dissolved solids in the storm flow. When correctly installed with other controls of the combined-sewerage or separately sewered storm-water system, swirl/vortex devices can play an important role in combined sewer overflow and storm-water discharge pollution control. Reliable determination of performance depends principally upon accurate sampling techniques, suspended solids and other pollutant analyses, and settling-velocity distribution of the influent and effluent. Simultaneous flow-rate measurement synchronized to sampling times is also necessary. This paper discusses design, evaluation, and application practice enhancements for the use of swirl/vortex technologies as part of a combined sewer overflow and storm-water pollution control system.

The objective of this study was to determine the anaerobic degradability of acrylic acid in both acidogenic and methanogenic systems. Enriched acetate, propionate, and glucose cultures were used in batch serum bottles and chemostats. The results showed that the acrylic acid was degraded to propionate and acetate, and without acclimation it completely inhibited propionate degradation and partially inhibited acetate degradation. The maximum specific rate constant k and the half-saturation constant K _S, for the acrylic acid degradation were 0.22 d⁻¹ and 7.2 mg/L, 0.40 d⁻¹ and 16.8 mg/L, and 1.2 d⁻¹ and 22.9 mg/L for the acetate, propionate, and glucose cultures, respectively. The inhibition coefficient K_I for acetate degradation in the presence of acrylic acid was 4.2 mg/L. In both the methanogenic and acidogenic chemostats, 98% of the acrylic acid was removed. However, the acidogenic chemostat had a significantly higher loading rate (416 mg/L⋅d) than the methanogenic chemostat (16.7–66.7 mg/L⋅d). Even at low acrylic acid loading, the propionate utilizers required a long time to acclimate to acrylic acid. Without such acclimation, acrylic acid fed anaerobic systems can lead to process failures.

The paper-manufacturing industry generates more than 90,000,000 t of sludge annually in the United States, the disposal of which is a complex environmental issue. Test burns of pelletized and dried paper-mill sludge were performed in a small institutional boiler for the purpose of determining environmental feasibility of the operation. The sludge was from a facility processing 100 secondary fiber (40 postconsumer), and was co-fired with wood chips. Emission rates of various criteria and hazardous air pollutants were measured during the burns and compared to regulatory limits. Results indicated that criteria pollutant emissions from this 2.9 MW (10,000,000 Btu/h) operation were well within regulatory limits with the exception of particulate material. Emission control such as a baghouse or electrostatic precipitator would be needed for a facility performing energy recovery from sludge. Organic hazardous air-pollutant emissions were also well below mass rates that would qualify the boiler as a major source under the Clean Air Act Amendments of 1990, with the largest emissions being formaldehyde, followed by acetaldehyde, acrolein, and naphthalene.

Evaporation is often the dominant mechanism for mass loss during the first few days following an oil spill. The initial boiling point of the oil and the rate at which the boiling point changes as the oil evaporates are needed to initialize some computer models used in spill response. The lack of available boiling point data often limits the usefulness of these models in actual emergency situations. A new computational method was developed to estimate the temperature at which a crude oil boils as a function of the fraction evaporated using only standard distillation data, which are commonly available. This method employs established thermodynamic rules and approximations, and was designed to be used with automated spill-response models. Comparisons with measurements show a strong correlation between results obtained with this method and measured values.

A technique is presented for determining the electrode configuration required for developing a subsurface electric field with specific direction and intensity. The complex potential offers a convenient means to describe the electric field associated with a particular electrode configuration. The electrode configurations required to construct various electric fields are described. Generation of specific electric fields in saturated fine-grain soils and electrochemical environments allows one to evaluate if the associated electrokinetic forces might drive pollutant species in particular directions to confine the pollutant or establish a barrier. The subsurface electric field can also be used with specific bacteria for electrobioremediation.

In a laboratory study, iron oxide-coated sand showed promise as a medium for use in small systems or home-treatment units in developing areas of the world, for removing arsenic(III) and arsenic(V) from ground water. A low-cost, simple, home arsenic removal unit (material and fabrication cost: Rs. 200, cost of medium: Rs. 80, and regeneration cost: Rs. 5; 1 U.S. dollar = Rs. 35), containing 6 kg (4 L) of iron oxide-coated sand, produced 625 and 610, and 780 and 760 L of water in two cycles of runs when the influent arsenic(III) or arsenic(V) concentration was 1.0 mg/L. A detailed study addressing the effects of some important factors (selectivity of arsenic(III) and arsenic(V) over one another for removal, water pH, concentration and type of competing anions, and cations) on the process is needed. The home arsenic removal unit should be subjected to field trial to assess long-term effects on performance.