Journal of Environmental Engineering

Experiments were conducted on the oxygen transfer coefficient, k_La₂₀, through surface aeration in geometrically similar square tanks, with a rotor of diameter D fitted with six flat blades. An optimal geometric similarity of various linear dimensions, which produced maximum k_La₂₀ for any rotational speed of rotor N by an earlier study, was maintained. A simulation equation uniquely correlating k = k_La₂₀(ν/g²)^1/3 (ν and g are kinematic viscosity of water and gravitational constant, respectively), and a parameter governing the theoretical power per unit volume, X = N³D²/(g^4/3ν^1/3), is developed. Such a simulation equation can be used to predict maximum k for any N in any size of such geometrically similar square tanks. An example illustrating the application of results is presented. Also, it has been established that neither the Reynolds criterion nor the Froude criterion is singularly valid to simulate either k or K = k_La₂₀/N, simultaneously in all the sizes of tanks, even through they are geometrically similar. Occurrence of “scale effects” due to the Reynolds and the Froude laws of similitude on both k and K are also evaluated.

Eleven types of single circular biplane grids with different diameter (d) and mesh (M) were vertically and constantly oscillated inside a 2 L square jar. The velocity components were measured using a 2D laser doppler anemometer. The average root-mean-square turbulent velocity q′ values were found to be relatively constant at both vertical and horizontal points of measurement—a condition that could not be achieved in the case of impeller mixing. Since the mixing intensity was uniform within the jar, the average volume velocity gradient $\overline{G}$ could be applied as the surrogate mixing intensity parameter. It was also found that q′ was linearly related to the vertical grid speed and grid physical characteristics, indicating that the mixing was easily controlled. The macro length scale (L) was calculated and was found to be constant and proportional to d or M, as it should be in the case of turbulent mixing. This study shows the potential of grids as the mixing devices that can be expected to produce an optimum mixing environment for the flocculation process.

Nitrite accumulated in denitrifying activated sludge mixed liquor when the carbon and electron source, acetate, was limited. If acetate was added to obtain a carbon-to-nitrogen (C:N) ratio in the range of 2:1 to 3:1, nitrate was completely consumed at the same rate with no nitrite accumulation, indicating that nitrate concentration controlled the respiration rate as long as sufficient substrate was present. However, when acetate was reduced to a C:N ratio of 1:1, while nitrate continued to be consumed, >50% of the initial nitrate-nitrogen accumulated as nitrite and 29% persisted as nitrite throughout an endogenous denitrification period of 8–9 h. While nitrite accumulated during acetate-limited denitrification, the specific nitrate reduction rate increased significantly compared with the rate when excess acetate was provided as follows: 0.034 mg-NO₃-N/mg-mixed liquid volatile suspended solids/h versus 0.023 mg-NO₃-N/mg-mixed liquid volatile suspended solids/h, respectively. This may be explained by nitrate respiration out-competing nitrite respiration for limited acetate electrons. Complete restoration of balanced denitrification and elimination of nitrite accumulation during denitrification required several weeks after the C:N ratio was increased back to 2:1.

Soil washing processes and in-situ immobilization techniques have been often-suggested technologies for remediation of soils contaminated with heavy metals such as chromium. Laboratory batch extraction studies were conducted with synthetically contaminated soils [with Cr(VI)] to determine the ability of extractants such as 0.12 M Na₄P₂O₇, 0.1 M NaF, and 1.0 M HCl to remove chromium from three types of soils with varying soil pH (5.8–8.1), organic matter (1.0–5.0%), and clay content (10–35%). Up to 73% of the total chromium present in the synthetically contaminated soil was removed by 0.12 M Na₄P₂O₇. The synthetically contaminated soils were also treated with FeSO₄ and NaOH solutions to immobilize chromium and investigated for leachability of Cr by Cr-free water. Up to 80% of the chromium could be retained in the soil following the immobilization process. The presence of soil organic matter and clay content enhanced the immobilization of chromium in both chromium flushing and immobilization experiments.

A simple methodology has been developed for evaluating the technical capabilities of potential sites for disposal of mixed low-level radioactive waste. The results of the evaluation are expressed as “permissible radionuclide concentrations in disposed waste.” The methodology includes an analysis of three separate pathways: (1) releases of radionuclides to groundwater; (2) releases of potentially volatile radionuclides to the atmosphere; and (3) the consequences of inadvertent intrusion into a disposal facility. For each radionuclide, its limiting permissible concentration in disposed waste is the lowest of the permissible concentrations determined from each of the three pathways. These permissible concentrations in waste at an evaluated site can be used to assess the capability of the site to dispose of waste streams containing multiple radionuclides.

Ten sites were evaluated as potential locations for disposal of mixed low-level radioactive waste using a simple methodology. Three environmental pathways (water, atmospheric, and inadvertent intruder) were analyzed using generic transport models that were modified for site-specific conditions. The results were summarized by grouping the 58 evaluated radionuclides according to their half-lives and environmental mobility and by their limiting pathway (i.e., the pathway providing the lowest permissible radionuclide concentration in disposed waste of the three evaluated pathways). The results indicate that all evaluated sites have the technical capability for disposal of some radionuclides in the waste. For most radionuclides, the intruder scenarios were more important in determining permissible radionuclide concentrations than the other pathways, particularly for arid sites. For humid sites, if the water pathway is not the most limiting and the permissible radionuclide concentration is high, a more sophisticated and rigorous analysis of the water pathway may not be warranted. However, if the permissible concentration is relatively low, more refined analyses may produce higher permissible concentrations based on additional site characterization data.

The problem of optimal water distribution to a range of retention reservoirs in an urban sewer network during rainfall events is considered in this paper. The goal of the control actions is the minimization of overflows and eventually the reduction of their polluting impact on receiving waters. A multilayer control structure consisting of an adaptation, an optimization, and a direct control layer, is proposed for the solution of this complex control problem. Several approaches have been proposed with regard to the optimization layer. This paper proposes a linear multivariable feedback regulator that is developed via a systematic design procedure, including a simplified model, a quadratic minimization criterion, and subsequent application of the linear-quadratic optimization method. Inflow predictions are accommodated through feedforward terms in the control law. The control design guidelines facilitate a quick and efficient regulator design for a broad class of sewer network control problems. Simulation tests for a particular large network and various inflow scenarios indicate that significant overflow reductions are achieved by the application of the linear-quadratic regulator.

The kinetics of thermal and oxidative decomposition of printed circuit boards is investigated under various heating rates (5, 10, 15, 20, and 25 K/min) and oxygen concentrations (5, 10, and 15%) in the nitrogen-oxygen atmosphere by means of thermogravimetric measurements. Results show that there is only one reaction stage in the inert atmosphere, with an initial reaction temperature of 564–584 K and an activation energy at 181.59 ± 21.80 kJ/mol. But there are two reaction stages when oxygen is present in the carrier gas. The activation energy is in the range of 170.79–184.98 kJ/mol for the first-stage reaction and 113.69–138.85 kJ/mol for the second-stage reaction. The fractional residue is reduced by 14.1–18.8% in the oxidative atmosphere. The rate equations for various oxygen concentrations can be modeled by Arrhenius-type equations, from which kinetic parameters such as activation energy, preexponential factor, and reaction orders for unreacted material and oxygen concentration are determined using an extended Friedman's method. The cutoff conversion factor between the first and second reactions is in the range of 0.46–0.54, depending on the oxygen concentration. The complete rate equations when two-stage reactions are involved can be obtained by summing the individual weighted rate equations; the weighting factor are determined from the cut-off conversion factor.

The feasibility of bench scale sequencing batch reactors (SBRs) in treating a bottling plant effluent is demonstrated at various influent chemical oxygen demands (CODs) ranging from 300 to over 4,000 mg/L. The SBRs removed over 75% of the biodegradable fraction of the influent COD at a fixed react time of 5.5 h. An easy to use process model is presented to predict the temporal variations of substrate, biomass, and dissolved oxygen concentrations during the fill and react periods in SBRs. The model satisfactorily predicted effluent COD and final biomass concentrations, as well as temporal variations of COD, biomass, and dissolved oxygen throughout the fill and react cycles.

An alternative approach for the interpretation of the residence time distribution (RTD) curves is proposed. The method has semiempirical basis, supported upon a gross fit of the tracer curve area by two triangles in order to obtain a single parameter (φ index) that indicates whether the system tends to a completely mixed or to a plug flow hydraulic behavior. In order to verify its applicability, 28 experiments were carried out in a laboratory tank (138 L). In these experiments, the influence of aeration, packing, and number of baffles on the tank hydraulic behavior was assessed using the index as well as conventional models. The general results obtained in the laboratory tank experiments showed that mixed tanks tend to have a larger dead volume fraction (DVF) than plug-flow tanks and that packed tanks tend to have a smaller DVF than nonpacked tanks. This index provides a reliable, quick, and simple method for the interpretation of the RTD curves when hydraulic behavior trends or tank modifications for hydraulic improvements are evaluated.