Journal of Environmental Engineering

Subsurface spills of high-molecular weight, multicomponent, dense nonaqueous-phase liquids (DNAPLs) are intractable for remediation by conventional techniques. This paper introduces the concept of biostabilization of the DNAPL source region as a means of achieving risk reduction at DNAPL-contaminated sites. Successful biostabilization depends upon the interplay among dissolution, degradability, and toxicity of various DNAPL constituents, difficult to predict a priori for the mixture. Bench-scale screening tests are proposed for identifying those DNAPLs that are amenable to biostabilization. The screening protocols compare four criteria: (1) microbial activity; (2) composition of the DNAPL residue; (3) aqueous phase contaminant concentrations; and (4) aggregate aqueous phase toxicity—across unbiotreated controls and in mixed versus unmixed biometers. The unmixed system represents slow dissolution from DNAPL pools in the quiescent subsurface. The protocols are developed and evaluated with DNAPL coal tar in the first paper of this set (Part I). Unmixed coal tar biometers, characterized by slow mass transfer and low-level microbial activity, exhibited reduced, aqueous-phase contaminant concentrations and aggregate toxicity, as well as stable DNAPL composition, consistently indicating favorable potential for in situ biostabilization.

Aroclors are dense nonaqueous-phase liquids (DNAPLs) composed of polychlorinated biphenyls, which are common subsurface contaminants. Because complete remediation of Aroclor is very difficult, biostabilization may offer an alternative where risk reduction can be achieved without destruction of the DNAPL mass. The potential for aerobic in situ biostabilization of Aroclor 1242 was evaluated using laboratory protocols similar to those described in the companion paper. Total microbial concentrations increased and stabilized in both mixed and unmixed systems, while the respiring cells did not stabilize in either system. After 100 days, the DNAPL in mixed biometers was depleted in dichlorobiphenyls; the DNAPL composition in unmixed biometers did not change significantly. The total aqueous polychlorinated biphenyl concentration was lower in the unmixed than mixed biometers; both were below the predicted equilibrium concentration. After 100 days, the chronic toxicity of the aqueous phase to Cerodaphnia was greater in the biotreated systems than in the unbiotreated systems. The results indicate that aerobic microbiological activity may be insufficient to fully stabilize Aroclor in the subsurface, in contrast to the clear biostabilization potential of coal tar.

Federal and state policies governing contaminated site investigations and remediation strategies have evolved over the past several years. Advances in treatment technologies, especially in situ methods of remediation, are largely responsible for this evolution. However, public opinion has also influenced environmental regulators and helped to shape policy. In dealing with issues of contamination, especially in conjunction with on-going construction projects, making appropriate choices on how to handle petroleum-contaminated soil requires more than comparing costs of treatment methods and weighing the advantages and disadvantages of various cleanup options. Selection of a remediation technique to clean up a site must take into account the community in which the site is located. Local governmental agencies and public opinion can play a strong role in determining what remediation actions are allowable, especially regarding highly visible ex situ treatments. The ability to choose among methods of soil treatment may be quite limited for some responsible parties, depending on whether construction activities are taking place through the zone of contamination and what the current regulatory climate is. It is crucial, especially for state agencies like transportation departments who serve the public, to involve communities early on in cleanup efforts. How a remediation project is implemented and received by the public can have great influence on its acceptance in the future. The objective of this paper is to describe the Minnesota Department of Transportation's experience in dealing with petroleum-contaminated soil and how changing philosophies from environmental regulators and local politics have influenced the process. Descriptions of treatment methods used by the Minnesota Department of Transportation are provided. The paper concludes with a discussion of an emerging issue that may have a significant impact on remediation projects in the future throughout the nation. Paramount to making sound cleanup decisions is the availability of meaningful site contamination data and a scientifically defensible framework for interpreting the data. State and national work groups have already instituted, or are currently evaluating, a unique approach to quantify and interpret total petroleum hydrocarbon analyses that can be used to determine if a site should be remediated and the appropriate cleanup thresholds. This technique can assist in making scientifically defensible remediation decisions that are based on risk determinations to human health and ecological endpoints

The mass transfer of ozone from the gaseous phase into the aqueous phase plays an important role in determining the efficiency of ozonation. In this study, a mass-transfer model has been developed to simultaneously predict concentrations of the dissolved and outlet gaseous ozone in a semibatch reactor. A model in which the liquid phase is described as well mixed and the gas phase is described as a plug-flow system was used to estimate the mass transfer of ozone in both laboratory and pilot reactors. The self-decomposition of ozone was also incorporated into the model. The mass-transfer coefficient was found to increase with increasing gas flow rate, temperature, and ionic strength in the solution. Using a sensitivity analysis, the partition coefficient α was found to be the most sensitive factor that affects the concentration profile of dissolved ozone. However, the outlet gaseous concentration of ozone is quite insensitive to all physical parameters investigated in this study.

A system dynamics model is developed and tested to determine the significant processes and appropriate level of detail required to capture dynamic behavior important in managing biodegradation in landfills. Uniform, spherical, solid waste particles are assumed to hydrolyze from the outer surface to produce simple sugars; fermentation, acetogenesis, and methanogenesis produce methane and carbon dioxide as end products. Representative reactions, sufficient to carry all hydrolyzed small chain carbon molecules to stabilization, are used for stoichiometric relationships. Microbial populations consume their respective substrates according to classical Monod kinetics. Results of simulation experiments suggest that hydrogen inhibition of fermentation and acetogenesis is critical to the empirically observed time course of landfill gas generation. In addition, an uninhibited anaerobic pathway producing CO₂ without the production of H₂ is required to produce the large CO₂ peak often seen early in the stabilization process. Further simulation experiments suggest appropriate model refinement that allows efficient exploration of management strategies to enhance landfill stabilization. For example, forced ventilation during any phase of degradation can be shown to decrease efficiency or inhibit overall degradation.

Experiments were conducted to determine the effect of solid levels on natural sedimentation of swine manure. Total solids (TS) levels of 0.5, 1.0, 2.0, 4.0, and 6.0% were evaluated. Natural sedimentation was impeded at higher than 2.0% and also at lower than 1.0% TS concentrations. Two flocculants (ferric chloride and aluminum sulfate) commonly used in the municipal wastewater treatment industry were then evaluated for their enhancement of natural sedimentation and concomitant removal of phosphorus from swine manure. Each flocculant was evaluated at five levels—0 (control), 500, 1,000, 1,500, and 2,000 mg/L—on swine manure with an adjusted TS level of 1.0%. At dosage levels of 1,500 mg/L (5.4 mM Fe⁺³), ferric chloride removed 76% suspended solids (SS) and 86% phosphorus, while aluminum sulfate at the same dosage level removed 96% SS and 78% phosphorus. Unaided natural sedimentation at this TS concentration removed 66% and 42% of the SS and phosphorus, respectively. Chemical flocculation can, therefore, be an effective method of removing solids and phosphorus in swine manure.

Alcohol (cosolvent) flooding is a relatively new, in situ technique for enhancing the removal of organic solvents from groundwater aquifers. Recovery of the alcohol for reinjection is important for ensuring the cost-effectiveness of this technology. The research goal was to investigate the ability of activated carbon to remove tetrachloroethylene (PCE) from alcohol solutions, allowing reuse of the alcohol in the cosolvent-flooding process. Laboratory studies were conducted using PCE (concentrations ranged from 10 to 100,000 mg/L) and isopropyl alcohol or ethanol. Alcohol concentrations ranged from 0 to 80% (v/v). A field application conducted at the Dover National Test Site in Delaware consisted of treating 60 to 70% (v/v) ethanol solutions containing PCE concentrations ranging from 500 to 2,000 mg/L with carbon-filled drums. PCE adsorbed onto activated carbon even at high alcohol concentrations; however, the Freundlich coefficient (K_f) was reduced by almost two orders of magnitude with increasing alcohol concentrations. In the field, activated carbon successfully treated ethanol solutions, allowing alcohol reinjection in the cosolvent flood. The results demonstrated technical and economic feasibility of the reuse of alcohol from cosolvent-flooding activities.

Adsorption of Cr(VI) onto hydrous concrete particles in synthetic groundwater was investigated with batch equilibrium adsorption experiments. Results show marked adsorption under acidic conditions, and the amount of Cr(VI) adsorbed decreases with increasing solution pH and surface loading. A surface complex formation model was employed to describe the adsorption of Cr(VI) onto concrete particles. The adsorption-free energy of ${\mathrm{HCrO}}^{{\mathit{-}}_{\mathit{4}}}$ and ${\mathrm{CrO}}^{\mathit{2}{\mathit{-}}_{\mathit{4}}}$ species are −6.16 and −8.53 kcal/mol, respectively. Results also indicate that specific chemical interaction is the major mechanism responsible for the adsorption process.

This study employed pilot-scale filters to examine the fate of Mn in MnO_x(s)-coated filter media as a function of filter-applied pH and backwash conditions. A key operational issue for continuous efficient Mn(II) removal was maintenance of appropriate levels of free chlorine to ensure coated media regeneration. Neutral or slightly acidic pH promoted Mn(II) sorption and subsequent oxidation on MnO_x(s)-coated media. Alkaline influent pH (pH > 7) allowed some soluble Mn(II) oxidation by free chlorine prior to filtration, resulting in significant Mn removal by MnO_x(s) particle filtration. Increased backwash rates removed greater amounts of MnO_x(s) from the filter media. The combination of MnO_x(s) accumulation on filter media during filtration and its partial removal during backwash maintained a net amount of MnO_x(s) coating sufficient for catalyzing further soluble Mn(II) removal, yet it did not significantly alter the size of the media.

Increasing evidence on the detrimental health effects of suspended fine particulates has prompted the introduction of new ambient air quality standards for particles with diameters smaller than 2.5 and/or 10 μm and has created the need for size-specific inventories. To address this need, “generic” cumulative weight fraction data have been fitted with lognormal particle size distributions, and the resulting mean diameter d_m and standard deviation σ_g values compiled for many sources of practical interest. In addition, nomographs are developed for assessing the mass fraction of particles with diameters <2.5, 6, 10, 15, and 30 μm as a function of the applicable d_m and σ_g. The above data and tools allow easy extension of total particulate matter emission inventories for uncontrolled sources into size-specific ones. Supplemented with published typical efficiencies of control systems in the ranges 0–2.5, 2.5–6, and 6–10 μm, they also allow easy compilation of approximate size-specific inventories for controlled sources. In addition, used in conjunction with graphical models for selected control systems that predict the total efficiency and lognormal size distribution of penetrating particles, they offer a unique rigorous approach for size-specific inventories from controlled sources.