Journal of Environmental Engineering

The genetic algorithm (GA) is used as an optimization tool to estimate water quality model parameters in a calibration scenario. The GA is found to be a useful calibration tool, capable of providing least-squares parameter estimates while incorporating field observations as constraints and accumulating useful information about the response surface. Because the GA provides a directed, randomized search using a population of points, a database of information about the response surface, parameter correlation, and objective function sensitivity to model parameters is obtained. Synthetic data with and without error are used initially to investigate the potential of the GA for model calibration applications. A case study is then carried out to confirm GA performance with field data. Constraints are included successfully in the GA search using either a penalty function or a special decoding operation. However, results show that the GA with the penalty function outperforms the GA with the decoder. Furthermore, parameter estimation is found to be improved by the inclusion of multiple-response data. For ill-posed problems, the GA provides several parameter estimates, all performing equally well mathematically.

This paper presents an unsteady depth-integrated simulation for solving a hydrodynamic engineering problem in the estuary of the Yangtze River. The computation is based on the advanced turbulence depth-averaged two-equation $\tilde{\mathit{k}}$−$\tilde{\mathit{w}}(\tilde{\mathit{k}}\text{,}$ depth-averaged turbulent kinetic energy parameter; $\tilde{w}$, depth-averaged vorticity fluctuation parameter of turbulence) model. Variations of bottom topography and water surface elevation were taken into account. The model has the ability to simulate and predict flow fields and pollutant transport driven by tidal flows within a coarse grid resolution. The distributions and variations of velocity, temperature, and concentration fields caused by discharged pollutants from seven sources, two of which are submerged, were simulated over a full tidal cycle. The velocity and temperature fields computed by turbulence $\tilde{\mathit{k}}-\tilde{\mathit{w}}$ modeling have been compared with experimental results and field data. It was found that the simulation by using the turbulence $\tilde{\mathit{k}}-\tilde{\mathit{w}}$ two-equation closure can provide more details of flow fields and concentration distributions than ones by using phenomenological algebraic formulas of eddy viscosity and diffusivity.

Sedimentary oxygen demand (SOD) is the uptake of dissolved oxygen by sediments. The oxygen is removed from the water column by chemical oxidation and by the respiration of microbes in the sediments. In 1994, Nakamura and Stefan published a theory relating SOD to flow velocity using boundary layer concepts. This paper is an experimental validation and extension of those results. In this study, SOD is investigated in laboratory experiments in which sediments are exposed to water flowing at different velocities. The experiments were performed in a recirculating channel with well defined flow characteristics. The results verify that SOD increases linearly with the velocity of the water above the sediments when the velocities are low. As the velocity is increased, the SOD reaches an upper bound. Both the rate of increase with velocity as well as the upper bound of SOD were found to depend strongly on the sediment material, the benthic biology, and the temperature. Characterization of the highly variable benthic biology proved to be particularly difficult. SOD is approximated by linear and Michaelis-Menten type equations with velocity being the independent variable.

Anaerobic biodegradation of a group of 12 chlorophenols (2-CP; 3-CP; 4-CP; 2,3-DCP; 2,6-DCP; 3,4-DCP; 3,5-DCP; 2,3,6-TCP; 2,4,5-DCP; 2,3,4,6-TeCP; 2,3,5,6-TeCP; and PCP) was examined in an unacclimated digester sludge culture and in a phenol-enriched, 2-CP and 3-CP acclimated methanogenic culture. The phenol-enriched culture degraded all chlorophenols except 4-CP, whereas the digester sludge culture did not degrade PCP or 2,3,4,6-TeCP. Dechlorination products were observed in the phenol-enriched culture but not in the fresh sludge. Based on the observed dechlorination products, degradation pathways for chlorophenols in the phenol-enriched culture were proposed. The phenol-enriched culture degraded chlorophenols at much higher rates than the digester sludge by dechlorinating at the ortho and meta positions. In both cultures, the rate of degradation depended on the number as well as the ring position of chlorine substituents. Higher rates were generally observed with ortho dechlorination and for compounds with lower numbers of chlorine substituents. The average rate of degradation did not vary significantly between mono- and dichlorophenols or between tri- and tetrachlorophenols. Analysis with the Haldane expression revealed that the phenol-enriched culture was less susceptible to inhibition caused by chlorophenols and that it possessed a higher affinity for substrate than the digester sludge culture.

Denitrification was integrated in a nitrifying biofilm airlift suspension reactor by intermittent aeration. A total nitrogen-removal efficiency of 75% was reached at an aerobic ammonia load of 5 kg_N ⋅ m⁻³ ⋅ d⁻¹. Acetate was added to the reactor during the anoxic periods. The system was operated at sufficiently long anoxic hydraulic retention times to allow suspended growth of denitrifying heterotrophs. However, the heterotrophic biomass did not grow in suspension but grew as protuberances and layers on the nitrifying biofilms. Nitrification and denitrification occurred via nitrite as an intermediate, possibly because denitrifiers outcompeted the nitrite oxidizers. Because of the establishment of the “nitrite route,” a low average COD/N ratio of 2.6 kg_COD ⋅ ${\mathrm{kg}}_N^{-1}$ was obtained. When denitrifying operation with acetate addition was started, initially almost all nitrogen removed was emitted as nitrous oxide. Subsequently, the nitrous oxide emission strongly decreased to almost zero in a period of 30 days, while the nitrogen-removal efficiency was maintained.

Freezing and thawing transform alum sludge flocs into granular particles that rapidly settle. One objective of this paper is to help modelers and designers of the freeze/thaw conditioning process visualize the process at a microscopic scale. A second objective is to report a phenomenon occurring at the ice/water interface, namely, the fragmentation of flocs by the advancing freezing front. To meet these objectives, directional freezing of alum sludge was observed using a microscope at 40× power. A Bridgman growth apparatus was designed to allow various rates of directional freezing to be observed. The interface interacted with the flocs in various ways, depending on the freezing rate. At high freezing rates, dendrites either pierced the flocs or bypassed them, effectively confining the flocs and fragmenting them. At low rates, the interface was smooth and planar. Rather than being incorporated in the ice, flocs were rejected by the planar interface. At intermediate freezing rates, the interface was rough, and flocs were fragmented as they were rejected. The fragmentation of the flocs helps explain why some researchers have noted a decrease in floc size following freeze/thaw conditioning.

The objective of the paper is to investigate the effect of temperature and caustic soda concentration in the bauxite digestion process on the settling behavior and the physical, chemical, and mineralogical characteristics of the derived red mud. The study was carried out on Jamaican bauxite. The results demonstrate the influence of the goethite-hematite conversion on the settling rate of red mud. This iron mineral transformation affects the specific gravity, which is one of the factors that affects the settling rate of the red muds. The iron conversion in turn is mainly affected by the temperature in the digestion step despite the variation of caustic soda concentration. The results also demonstrate that the specific surface area and the settling rate of red muds are inversely proportional and that a positive correlation exists between the amorphous and cryptocrystalline iron (Fe_d) and the settling rate of the Jamaican red muds.

A novel method is proposed in this study for determining the dissolved oxygen (DO) diffusion coefficient in industrial wastewaters. The method is based on the principle of measuring the average DO concentration during the diffusion process. A specially designed experimental apparatus with an appropriate operating procedure and a mathematical equation are necessary in the determination of the DO diffusion coefficient. The accuracy of the estimated DO diffusion coefficient using the present method was compared with that reported in the literature and was found to be quite good. Extended test runs were conducted to examine the effects of various types of surfactants on the DO diffusion coefficient. Determination of the DO diffusion coefficient in the raw textile wastewater was demonstrated as a practical example.

Membrane filtration coupled with ozonation of the retentate was used for the treatment of colored textile wastewater. The textile wastewater examined was simulated from a commercial batch formula and contained organic dyes, sodium chloride, and copper among its components. A selected membrane filtration process generated a permeate with over 99 of the color and copper removed, while 85 of salt by mass and 85 of the original water were reusable. The effect of pressure, cross-flow velocity, feed concentration, and membrane fouling on permeate flux also was investigated. Subsequent ozonation of several different concentrated retentates each followed first-order reaction kinetics and removed color effectively. However, the decolorization rate constant decreased with increasing initial dye color. An empirical correlation was established for the apparent first-order rate constant of ozonation and the initial dye concentration. The relationship between the apparent rate constant and the dosage of ozone input also was investigated.

A mathematical model is developed for metal leaching from contaminated soils subjected to acid extraction in a batch reactor. The model considers transport by pore diffusion and film transfer, leaching of metal bound to reversible and irreversible phases, and metal complexation by ions in solution. As currently developed, the model is semiempirical, with the primary objective of exploring the relative significance of chemical kinetics and diffusional transport under acidic leaching conditions. Simulation results and sensitivity analyses show that leaching kinetics vary according to the metal binding mechanism and location within a soil particle. The effects of pH, pore diffusion, film transfer, chemical reaction rate, particle size, and initial metal distribution are examined using parameter estimates derived from experimental data for a hazardous-waste-site soil contaminated with lead. Depending on leaching conditions, diffusion, reaction, or both may control metal leaching for timescales of interest in soil washing. Both the distribution of contaminant metal between the reversibly and irreversibly sorbed fractions and the initial spatial distribution within the soil particle are important in determining leaching behavior.

This paper presents the results of a laboratory investigation performed to study the role of various air sparging system parameters on the removal of benzene from saturated soils and ground water. A series of one-dimensional column experiments was conducted with predetermined contaminant concentrations and predetermined injected air flow rates and pressures to investigate the effects of: (1) the soil type; (2) the use of pulsed air injection; and (3) the synergistic effects of co-contaminants on air sparging removal efficiency. This study demonstrated that the grain size of the soils affects the air sparging removal efficiency. A threshold value exists for effective particle size (D₁₀), which is equal to 0.2 mm; above this threshold value, the rate of removal is linearly proportional to the D₁₀ value; while below this value, there is a drastic increase in the time required for contaminant removal. Additionally, it was demonstrated that pulsed air injection did not offer any appreciable advantages over continuous injection for the coarse sand; however, pulsed air injection led to substantial reductions in system operating time for the fine sand. Finally, it was found that benzene and toluene existing alone had similar removal rates; however, they were removed slightly more quickly when they coexisted in the test soil.