Journal of Environmental Engineering

The potential for changes in the wettability of a subsurface system following exposure of sands to a variety of non–aqueous-phase liquids (NAPLs) was assessed. Many of the NAPLs were collected from field sites with on-going NAPL recovery systems. Three methods were used to evaluate the wetting conditions: contact angle measurements, a qualitative bottle test, and capillary pressure-saturation curves. The results clearly show that a wide range of wetting conditions can be expected following spills of complex NAPL mixtures to the subsurface. NAPLs comprised of higher molecular weight constituents—such as creosote—or with added surfactants—such as gasoline—had a greater impact on the system wettability than the lower molecular weight NAPLs. Neat solvents did not have a significant impact on the wettability of quartz surfaces. Many of the petroleum products tested resulted in the formation of weakly water-wet surfaces. Under these conditions, the potential recovery of NAPL from the subsurface would be maximized. Much lower recoveries would be expected for the NAPL-wetting creosote and coal tar contaminants, however.

In preparation for renegotiation of contracts with its haulers, the Broward County, Fla., waste management authority commissioned a sample survey designed to produce a “statistically defensible” estimate of the total weight of solid waste generated annually by the typical residential unit in a target area consisting of about 40,000 such residences. At the time of the study, initiated on Oct. 1, 1990, the annual unit weight figure used for contractual and payment purposes was 1,837 kg/yr/residence (4,050 lb/yr/residence), with an unknown margin of error. The survey, using a selection scheme and estimation methods based on full probability sampling, was completed on Sept. 30, 1991. It produced estimated annual figures of 1,378.92 kg/yr/residence (3,040 lb/yr/residence) of regular household trash and garbage, 89.85 kg/yr/residence (198.1 lb/yr/residence) of bulk trash, and 37.92 kg/yr/residence (83.6 lb/yr/residence) of recyclable material—a total of 1,506.69 kg (3,321.7 lb) per year, per residential unit, and a reduction of 18% from the earlier contractual figure. Furthermore the rigorous statistical procedures that were employed permitted the estimation of a standard error of 30.89 kg (68.1 lb) for the overall weight.

Siting a landfill typically requires processing a significant amount of spatial data with respect to various siting rules, regulations, factors, and constraints. Manually performing such a spatial analysis with drawing tools is generally tedious. A modern geographical information system (GIS), although capable of manipulating spatial data to facilitate the analysis, lacks the ability to locate an optimal site when compactness and other factors are simultaneously considered. An appropriate siting model was therefore explored for use with a raster-based GIS. A mixed-integer programming model was developed to obtain a site with optimal compactness. A comparison was made between the model and two other previously proposed models in terms of their applicability and simplicity for raster-based data. The compactness model was further extended to include multiple siting factors with weights determined using map layer analysis functions provided by a GIS. This multifactor model was applied to analyze the effects of varied weights and factors on making a siting decision.

Two parallel diffusion mechanisms, pore and surface, can control the rate of contaminant adsorption. The two mechanisms are different functions of temperature and adsorbate concentration. To develop a mechanistic design model for adsorption processes, the two mechanisms must be evaluated separately. In this paper, we show that the mechanisms can be separated accurately using a stepwise linearization technique. The technique can easily be incorporated in adsorption diffusion modeling. Two phenolic compounds were used in this study: p-chlorophenol (PCP) and p-nitrophenol (PNP). The application of the linearization technique is illustrated using two types of reactors: a completely mixed batch reactor and a differential reactor. The study results show that pore and surface diffusivity can be determined accurately using the linearization technique. Furthermore, the tortuosity for the absorbent can be estimated from the pore diffusivity. For PCP that is strongly adsorbed by the adsorbent, surface diffusion is the dominant mechanism controlling the intraparticle transport. For weakly adsorbed PNP, neither surface nor pore diffusion is dominant.

A numerical method is developed to minimize the environmental impact of one or multiple contaminant point sources in a vertically mixed river or estuary. The algorithm is based on a finite-element solution of the adjoint to the fate and transport equation that permits efficient selection of optimal locations and mass loadings of outfalls in the presence of tidal action, secondary currents, river branches, and other two-dimensional factors. The evaluation is based on the cumulative effects of contaminants in one or more arbitrary target points in the system. The model is validated by comparison to repeated trial solutions and is shown to yield information comparable to that resulting from a large number of hypothetical simulations. Practical applications are presented for the Fox River and the upper Potomac Estuary verifying the method's applicability to rivers of arbitrary geometry and bathymetry.

An existing two-dimensional estuary model was modified to incorporate processes important to the transport and fate of copper in San Francisco Bay. These processes include advection, dispersion, partitioning with suspended particles, settling, and resuspension of adsorbed copper. A systematic calibration of these processes was made. The simulated advection was first shown to match tidal stages, time lag of slack waters, and currents. The model's dispersion was then calibrated by matching observed total dissolved solids. Finally, the model was calibrated to match total suspended solids, total copper, dissolved copper, and sediment copper. The model simulated the recently observed copper concentration in the bay under current point and nonpoint source loadings. The model predicted that a reduction of copper load in winter storm-water runoff would lower copper concentration in the summer.

A simple and inexpensive method of stabilization/solidification (S/S) of hazardous wastes using C-grade fly ash as the only binding agent is presented. The process involves mixing the waste and fly ash and compacting the mixture for less than 3 s at 1.4–6.9 MPa to form a monolith. Toxicity characteristic leaching procedure (TCLP) results for a simulated mixed waste showed that the chromium concentration in the leachate is well below current Environmental Protection Agency (EPA) limits. For this particular waste, TCLP results were not significantly affected by the ratio of waste:fly ash in the block due to the limited range of waste concentration that can be processed. Furthermore, it is shown that the optimum operating conditions are at a waste pH of 9.2 and an applied pressure of 4.65 MPa.

A laboratory-scale investigation at ambient temperature was performed to examine the applicability of a four-stage across-the-flow rotating biological contactor (RBC) reactor for the treatment of synthetic wastewater containing phenol and thiocyanate. The composition was so maintained that the phenol to thiocyanate ratio approximated that usually found in coal carbonization effluent. In the combined treatment of the toxicants, it was noted that the toxicants were removed in sequence. Phenol was mostly removed in the earlier (first and second) stages, while thiocyanate-fed microbes acted more efficiently in the latter stages (third and fourth). The overall reaction rate coefficient for phenol biodegradation ( K _PH) varied in the range of 10.20–3.50 g/m ²⋅d in presence of 0–240 g/m ³ of influent thiocyanate concentration. The same coefficient for thiocyanate biodegradation ( K _TH) was found to be in the range of 8.20–0.30 g/m ²⋅d in presence of 0–240 g/m ³ of influent phenol concentration. The effects of various operating and process parameters such as disk rotational speed, temperature of the reactor liquid, and substrate load on the removal of phenol and thiocyanate were also examined. The microbes, predominantly found in the earlier stages, were identified as Pseudomonas putida, Pseudomonas stutzeri and Candida tropicalis, whereas in the latter stages, Thiobacillus thioparus and Nocardia sp. populations predominated.

The retrofit of an existing activated sludge system with a biological nitrogen removal system requires the installation of a baffle wall to separate the biological nitrogen removal anoxic and oxic zones. The baffle wall consists of a weir and a submerged orifice. An experimental investigation of the hydraulics of the anoxic/oxic baffle wall is presented. It is shown that backflow from the oxic zone to the anoxic zone can occur if the baffle wall is not designed properly. This backflow will reduce the efficiency of the biological nitrogen removal process. Based on the experimental data, a design procedure that eliminates backflow over the anoxic/oxic baffle wall is developed. The design procedure introduces a backflow parameter that is a dimensionless number dependent on the total discharge, orifice area, tank depth, and oxic zone air concentration. The resulting design offers an economical alternative to designs implementing a valve option.