Journal of Environmental Engineering

A previously developed model that describes leaching of heavy metals from contaminated soils is applied to four hazardous-waste-site soils contaminated with Pb. Processes included in the model are intraparticle diffusion, rate expressions for irreversibly and reversibly sorbed fractions, and metal complexation by ions in solution. The model is calibrated using laboratory experimental data in the pH 1–3 range, liquid-to-solid mass ratios from 5 to 20, and leaching times of 24 h. Parameters for the model are estimated through a combination of independent experiments, literature correlations, and mathematical optimization. Equilibrium data were used to estimate site density and an adsorption equilibrium constant. Two kinetic rate coefficients, a particle tortuosity factor, and a distribution coefficient (α_a) that defined the amount of Pb in two contaminant fractions were adjusted to match kinetic leaching data. Using one set of parameter estimates for each soil, the model successfully simulated experimental data collected under different leaching conditions. The fraction of Pb associated with easily leachable, irreversibly sorbed fraction (1 −α_a) provides some insight to the geochemical distribution of Pb in the soils tested. The model is used to explore effects of process variables such as liquid-to-solid ratio and sequential washes. The model should be useful for simulating ex-situ soil washing processes and may, with further development, have applications for in-situ flushing processes.

An Fe-rich soil from the site of a former leather tannery, heavily polluted with Cr, was studied using a combination of wet chemical analysis, high gradient magnetic separation (HGMS), and electron probe microanalysis (EPMA). It is demonstrated that such a combination is a powerful tool for the characterization of polluted soils, especially in cases where the pollution is present as discrete particles. Both EPMA and magnetic separation data indicated that the Cr pollution was present as a hydrous Cr-oxide phase. The Cr does not correlate with the Fe minerals, most likely as a result of the initial high Cr concentrations in the soil, which lead to precipitation of separate hydrous Cr-oxide minerals and Fe minerals. The Cr-containing material is present as (layered) aggregates, which are formed around larger quartz grains or around very small other particles that served as precipitation nuclei. Magnetic separation tests show that the Cr pollution can largely be removed by HGMS.

Investigation on Cr(VI) reduction was conducted using bacteria isolated from soil samples receiving electroplating wastewater. Chromium reduction capacity of these isolates was compared with that of Pseudomonas aeruginosa, a pure culture procured from the Institute of Microbial Technology, as well as Bacillus circulans, a laboratory isolate from garden soil. Bacillus coagulans, isolated and identified from chromium polluted soil, gave maximum reduction potential among all organisms studied. Malate was found to yield maximum biotransformation out of four electron donors employed. B. coagulans was able to reduce Cr(VI) even at 10 mM initial Cr(VI) concentration. With an increase in initial cell density, Cr(VI) reduction capacity was also increased; however, maximum specific biotransformation occurred at low cell densities. The optimum pH for Cr(VI) reduction was 7. Sulphates and nitrates did not compete with Cr(VI) for accepting the electrons. The presence of respiratory inhibitors like DNP and NaN₃ marginally reduced the Cr(VI) reduction capacity. Cell free extracts were able to reduce Cr(VI) even in the absence of external electron donors; however, marginal increase was found in the presence of malate, nicotinamide adenine dinucleotide, and reduced nicotinamide adenine dinucleotide. The contribution of cell membrane in Cr(VI) reduction was negligible. Hence, the soluble enzymes appear to be responsible for Cr(VI) reduction by B. coagulans.

Nonaqueous-phase liquids less dense than water (LNAPLs) form pools at the ground-water table, when leaked in sufficient quantities into the subsurface. Subsequent fluctuations of the ground-water table create a smear zone in the vicinity of the water table with the LNAPLs trapped in the pore spaces in the form of blobs. In addition to the pool, these blobs participate in dissolution and biodegradation processes in the saturated zone. A quantitative method is developed to evaluate mass loss of LNAPLs in the saturated zone under fluctuating water table conditions. A mathematical scheme is used to transform the moving domain to a stationary domain and the transformed advective-dispersive-reaction equation is solved numerically. The mass loss is computed in terms of two components: (1) Biodegradation within the domain; and (2) advective flux leaving the domain. The results indicate that the fluctuating water table conditions enhance the mass loss inasmuch as the entrapped blobs contribute to dissolution and biodegradation processes. Analysis of mass loss as a function of the Damköhler number (Da) indicates that biodegradation increases the mass removal from the system only after Da exceeds 0.1. Furthermore, for Da> 0.1, the biodegradation of solute dissolved from the entrapped blobs enhances the mass loss from the pool to a much greater extent than when no entrapped blobs exist under static water table conditions.

Nutrient simulations of a municipal wastewater land treatment system were performed using the ground-water loading effects of agricultural management systems model. A total of 33 best management practices (BMPs) were simulated and compared to a baseline simulation of the current reed canary grass management practice. BMPs consisted of various combinations of crop types and cropping practices. Effectiveness of BMPs was compared based on the amount of nitrogen leached through the root zone and the harvested market value of each crop. Based on the model predictions and local market conditions, recommendations for the facility were given as the following: reed canary grass cut four times per year, orchard grass cut three or four times per year, or silage corn with a winter cover crop under low wastewater loading. Overloading the reed canary grass and orchard grass, especially for short periods of time, was simulated to have no adverse effects on the ground water. The ground-water loading effects of agricultural management systems model was demonstrated as a useful tool for the optimization of a municipal wastewater land treatment facility.

To overcome two of the main problems intrinsic to upflow anaerobic filters, clogging and poor efficiency in the upper zone, a new feeding strategy consisting of distributing the influent flow through several entrances along the system was studied. Two identical upflow anaerobic filters, a single-fed reactor (SFR) and a multi-fed reactor (MFR), were used with synthetic substrate of ∼10-kg chemical oxygen demand (COD)/m³ at an organic loading rate of up to 32-kg COD/m³$\dot{s}$day. After more than 1 year of operation, the MFR strategy proved to be more efficient than the SFR, in terms of COD removal as well as with regard to stability against hydraulic and organic overloads. The MFR developed a more homogeneous bacteria consortium throughout the filter, therefore the specific methanogenic and nonmethanogenic activities are very similar in the bottom and the upper part of the filter. The MFR does not need a recycling flow to increase mixing. Moreover, the working volume of the MFR was higher than 85%, while only 65% of SFR remained active after 410 days.

Eleven samples of typical solid waste components, ranging from high to low predicted degradability, were analyzed prior to burial within three landfills representing different climates. Sets of these samples were retrieved periodically over 6 years and analyzed for changes in moisture content, dry weight, cellulose (holocellulose), and lignin. Results were that carbohydrates such as cellulose (holocellulose) and starch were the quickest degrading portions of the waste, and the samples with high lignin contents degraded more slowly than those samples with lower lignin contents. Finally, degradation was found to increase with higher moisture contents and higher temperatures. The uniqueness of this research is that it was performed in full-scale landfills, yet having baseline values for the samples.

Chlorination is the most widely practiced form of disinfection in the United States. It is highly effective against most microbiological contaminants. However, there is concern that the disinfection by-products (DBPs) formed by the use of chlorine might be carcinogenic. One class of DBPs that are formed and the only class of DBPs that currently are regulated are total trihalomethanes (TTHMs). Therefore, much effort is being expended in developing models that can be used to predict both TTHMs and chlorine residual levels in treated drinking water. This paper presents a model that predicts both TTHMs and chlorine residuals based on the consumption of chlorine and can be used to assist in evaluating the complex balance between microbial and DBP risks associated with disinfecting drinking water with chlorine. The parameters of the model have been found to be functions of total organic carbon, pH, temperature, and initial chlorine residual level. Bromide and the subsequent formation of brominated by-products were not considered in this paper.

Results from a potable water microfiltration (MF) pilot study employing untreated surface water are reported. The effects of filtrate flux and recovery on direct flow, outside-inside, hollow fiber MF fouling rates, and backwash effectiveness are presented. Constant flux experiments suggested the existence of a critical recovery below which MF fouling rates were low and effectiveness of backwashes was high and relatively independent of the recovery. However, in the range of experimental conditions investigated, fouling rates increased dramatically and backwash effectiveness decreased steeply when this critical recovery was exceeded regardless of the flux. In general, for a fixed recovery, specific flux profiles analyzed on the basis of volume filtered per unit membrane area were insensitive to filtrate flux. Fouling was accelerated by operating membranes at constant flux rather than at constant pressure, in part, because of membrane compaction and cake compression. Changing the mode of filtration between constant flux and constant pressure is shown to have no effect on MF filtrate water quality. For any given capacity, membrane area requirements are decreased, and power requirements are increased when membranes are operated at constant flux rather than at constant pressure.

A simple model for the design of selective catalytic reduction (SCR) process is presented. The two-dimensional mass transfer equation shown in Bai and Chwu (1997) is simplified and the analytic solution is obtained. The analytic solution is verified by the solution of the numerical two-dimensional model as well as published experimental data. Then it is applied for the design of the SCR reactor. Effects of operating temperature, inlet molar ratio of NH₃/NO, channel pitch, and gas flow rate on the SCR reactor volume are evaluated based on a designed NO removal efficiency. The NH₃ slip problem of a given inlet ratio of NH₃/NO also is discussed. It is shown that the injection ratio of NH₃/NO must be slightly higher than the desired NO removal efficiency so that the reactor volume can be reduced and the problem of NH₃ slip is not a major concern.

This paper is intended to provide the necessary information to construct and properly operate a simple streaming potential apparatus capable of measuring the zeta potential of granular media. Through an understanding of the principles of the technique, such a device can readily be used in the study of a variety of granular materials including unstable and dynamic surfaces, as shown for surface coatings and even biofilm growth. The simplified mathematical theory of the streaming potential is herein explained, as well as operational factors and the general reasons for nonreproducible measurements such as electrode maintenance, electrode polarization, nonzero rest potentials, and sample equilibration. This specific apparatus design is intended for use with granular materials and has intentionally been developed in a simplistic fashion to encourage wider usage of the technique. The overall precision of this instrument was ±10% based on a 95% confidence interval. In addition, the accuracy of the device was found to be very good based on comparisons with literature values.