Journal of Environmental Engineering

A review of recent research conducted on the size distributions of indoor nonviable airborne allergens is presented here. An overview is provided on immunological techniques, such as the enzyme-linked immunosorbent assay (ELISA) and radioimmunoassay (RIA), which have been successfully used for the quantification of specific nonviable aeroallergens in indoor environments. Studies of mite and cat aeroallergens containing relevant size distribution information are analyzed and interpreted from an aerosol science perspective. Recommendations for future research are also presented.

The photocatalytic oxidation of 2,4,6-trichlorophenol (TCP) on the surface of titanium dioxide (TiO₂) at varying water contents was investigated to provide fundamental data for incorporating photocatalysts into the surface of pavements to promote the destruction of spilled organic chemicals. 2,4,6-Trichlorophenol, spiked onto a thin layer of TiO₂, was degraded to 20% of its original concentration over 24 h; the release of chloride confirmed the degradation of the parent compound on the surface of the dry TiO₂. Addition of water (≥25% by weight) to the TiO₂ increased the rate of photocatalysis resulting in degradation of TCP to below detectable levels after 20 h. Based on competition studies using the hydroxyl radical scavengers, bicarbonate and 1-octanol, the proposed mechanisms for the dry phase photocatalytic degradation of TCP was oxidation by the valence band hole on the surface of the TiO₂ particle or dehalogenation by superoxide radical anions. Competition studies also confirmed that the more rapid TCP oxidation on wet TiO₂ was primarily the result of generation of hydroxyl radicals through oxidation of water by the valence band hole. The results show that dry phase TiO₂-mediated photocatalysis may be a potential system for the in situ surface destruction of chemicals that can be oxidized by nonhydroxyl radical mechanisms, such as valence band electron holes and dehalogenation processes.

Chlorinated organic compounds such as trichloroethylene (TCE) are the most prevalent contaminants found in soil and ground water, and pose serious health risks even at trace concentrations. This research reports a new chemical treatment technique for rapid degradation of TCE in strong alkaline solutions. Batch kinetic reactions between TCE and NaOH indicate that TCE can be rapidly and completely dechlorinated in NaOH at elevated temperatures. The reaction can be described by a pseudo-first-order rate kinetics with an estimated activation energy of ∼85 kJ/mol. The half-lives for TCE degradation in 2M NaOH at 40, 60, 80, and 100°C were approximately 347, 48.8, 4.0, and 2.4 min, respectively. The reaction end-products are primarily Cl⁻ anions and Na-glycollate, both of which are nonhazardous. This treatment technique is applicable for degrading other halogenated organic compounds wherein a nucleophilic substitution or elimination is the major reaction mechanism or pathway. Potential applications of this technology includes the removal and destruction of vapor-phase chlorinated volatile organic compounds (VOCs) in off-gases when soil vapor extraction or air-stripping techniques are used for remediating VOC-contaminated soils and ground water. A bench-scale alkaline destruction module was tested, and results indicated that ∼90% of TCE was destroyed when TCE vapor (10 mg/L) was passed through a destruction column with a retention time of ∼1 min at 95°C.

The presence of 2,4,6-trinitrotoluene (TNT) and its transformation products in surface soil, the vadose zone, and ground water can present serious environmental problems. This situation is exacerbated because the processes that control the mobility and transformation of TNT are not well understood. The objective of this study was to determine the effects of redox potential (Eh) and pH on the fate and transformation of TNT in soil. An initial investigation of soil components responsible for the observed TNT transformation was also conducted. Laboratory investigations consisted of testing at four separate redox potentials and four pH levels. An 18:1 (water:soil) suspension spiked with 100 μg/g TNT was used. Results indicated that TNT was unstable under all redox and pH conditions, and was least stable under highly reducing conditions at all four pH values. Greater amounts of TNT were incorporated into soil organic matter under anaerobic than under aerobic conditions. Results of the soil component study indicated that the presence of Fe⁺² sorbed to clay surfaces may account for the rapid disappearance of TNT at reduced redox potentials. TNT in ground water moving into areas of intense reduction would not persist for long, but would undergo transformation and binding by soil organic matter.

Electrophoresis of clay particles from dilute suspensions is an innovative technology to seal leaks in operating surface impoundments that does not require removal of impoundment contents, exposure of workers to contaminants, or prior knowledge of the leak locations. A suspension of clay particles is added to the impoundment liquid. A cathode (negative electrode) is placed inside and an anode (positive electrode) is placed outside the leaking impoundment. A direct current (DC) electric field is imposed externally across the geomembrane liner through the leaks. The clay particles migrate to the leaks under the influence of the imposed electric field to form a clay cake seal. The results of laboratory experiments to evaluate the use of a DC electric field to direct migration of clay particles into a leak and the hydraulic integrity of the resulting seal are presented in this paper. The effects of clay type, clay particle concentration in suspension, size of leak, and electric field strength on the migration of clay particles and process of cake formation are evaluated. The sealing effectiveness and internal structure of the resulting clay cakes are examined by hydraulic conductivity measurements and nuclear magnetic resonance imaging. Electrophoretic mobilities of bentonite particles in different chemical environments were also measured to evaluate the feasibility of the technology in practical situations.

Microbial granulation was examined in four lab-scale upflow anaerobic sludge blanket (UASB) reactors under varied microbial load indices (MLIs). The dimensionless MLI is defined as the ratio of the sludge loading rate to the specific methanogenic activity, and represents the relative substrate sufficiency subjected to by the sludge. A synthetic feed consisting of glucose, peptone, and meat extract was used as substrate and anaerobic digested sludge as seed. The four reactors were designed with the same diameters yet different heights, aiming at achieving a different biological selection pressure (solely MLI) yet a similar physical selection pressure in the reactors. Granulation under the high MLIs of 0.8 and 0.6 developed well within three and four months of operation, respectively, whereas that under the low MLIs of 0.3 and 0.2 did not even after six months. The granulation process was characterized by three phases of acclimation, granulation, and maturation. The MLI provides a quantitative approach toward stepwise sludge loading rate increases during start-up, with the appropriate values of 0.6–0.8 for fast and well-proceeded granulation.

A jet flocculator/powdered activated carbon (PAC) adsorption pilot plant was designed, built, and operated in order to evaluate a water treatment process capable of simultaneous adsorption of benzene and flocculation of PAC and silica clay particles. The jet flocculation/PAC system successfully combined, in the same reactor, flocculation of suspended solids by using jet mixing, solids retention using pall rings, and benzene removal by PAC adsorption. The advantages of this process included operational simplicity, reliability, and low energy consumption. The jet flocculator/PAC adsorption system was able to achieve suspended solids and benzene removals of approximately 80 and 95%, respectively, from an influent containing 70 mg/L of suspended solids (silica clay and PAC) and 100 μg/L of benzene.

This paper reviews the way in which scaling protocols for solid-liquid separators have developed in the last 25 years and shows how modelers of hydrodynamic separators can be faced with a dilemma when choosing the appropriate dimensionless groups used for scaling. Laboratory results from a series of geometrically similar hydrodynamic separators (operating with no baseflow) are presented and correlations between efficiency and several nondimensional groups (conventionally used to compare the performance of solids liquid separators) are discussed. It is concluded that the use of single dimensionless groups such as Froude number has limited applicability in predicting full-scale separator performance. However it is shown that a hybrid protocol can successfully predict the efficiency of solids removal from full-scale hydrodynamic separators over a wide range of flow conditions.

Discharge of iron coagulation sludge to sewers was demonstrated to provide multiple benefits at a wastewater treatment plant at low cost. Adding iron sludge to the collection system virtually eliminated hydrogen sulfide within anaerobic digester gas and prevented the formation of struvite throughout the solids handling facilities. Sulfides were removed via a direct reaction with the sludge, whereas struvite formation was presumably controlled by removal of soluble orthophosphate. Adding alum sludge to sanitary sewers had relatively little impact on sulfide production or struvite formation. Disadvantages of the process included elevated trace metal concentrations in the wastewater and increased solids loading.

The Surface Water Treatment Rule under the Safe Drinking Water Act and its amendments require that the water utilities maintain a detectable disinfectant residual throughout the distribution system at all times. A new computer model is presented to directly calculate the chlorine concentrations needed at the source(s) to have specified residuals at given locations in a pipe network in unsteady flow conditions by using an inverse method. Unlike the forward solution method, which calculates the source chlorine concentration by a trial-and-error procedure, the inverse solution technique presented in this paper directly calculates the source concentration required to meet a specified value at a particular point in the network. The dynamic flows and pressures are first calculated by solving the governing equations by an implicit finite-difference scheme subject to appropriate boundary conditions. For chlorine concentrations at various pipe sections and at each node, the one-dimensional transport equation is modified for the advection dominated flow. This modified transport equation is then discretized by using the four-point implicit finite-difference scheme and solved simultaneously together with the junction mass balance equations. A first-order reaction rate for chlorine decay is assumed and a flow-weighted averaging procedure is used to calculate the complete mixing of chlorine at the junctions. For the verification of the inverse method, the computed results are compared with those determined by direct simulations. An excellent agreement is observed between the inverse method and direct simulation technique.

The kinetics of thermal decomposition of epoxy resin are investigated under various heating rates (2, 5, 10, and 20 K/min) and oxygen concentrations (5, 10, and 20%) in the nitrogen-oxygen atmosphere by means of thermogravimetric measurements. Results show that, unlike only one stage of reaction in the inert atmosphere, two reaction stages are involved when oxygen is present in the carrier gas. The initial reaction temperature, in the range of 470–572 K, decreases with increasing oxygen concentration and decreasing heating rate. 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 Friedman's method. The activation energy is 129.6–151.9 kJ/mol for the first reaction and 103–117.8 kJ/mol for the second reaction, and decreases with increasing oxygen concentration. The cut-off conversion factor between the first and the second reactions is in the range of 0.71–0.74. The complete rate equation, when two-stage reactions are involved, can be obtained by summing the individual weighted rate equations; the weighting factors are determined from the cut-off conversion factor.

At the present time, the sewage treatment plants in the United Kingdom produce about 25,000,000 t of sewage sludge each year at a concentration of 4 solids. New regulations in the United Kingdom forbid sea dumping, and in the near future new incinerators will be required to dispose of about 5,000,000 t per year. The research program at the Sheffield University consisted of design and construction of a small-scale cylindrical rotating fluidized bed incinerator with an inside diameter of 200 mm and an axial length of up to 200 mm. The fluidizing medium was silica sand (0.6–0.7 mm) and the bed depth 30–50 mm. The rotational speed of the bed varied between 100 and 1,000 rpm (which produced 1–100 g loading). Coal, oil, sewage sludge, and chicken manure (biomass) were burnt in the bed at temperatures between 700° and 900°C. Measurements of heat transfer rate, bed temperature, and exhaust gas composition were carried out over a range of operating conditions. The influence of rotational forces on basic fluidization characteristics (pressure drop, minimum fluidization, and particle mixing) were also investigated on a perspex model. The results obtained from the research work demonstrated that the rotating fluidized bed incinerator can produce considerably higher combustion intensities (i.e., process intensification), has a wide turn-down range (i.e., versatile), has low toxic emissions (i.e., clean technology), and has a more easy and rapid start-up than conventional stationary fluidized bed incinerators. Other advantages of the system include: good mixing, simpler fuel injection, low thermal inertia, and good heat transfer rates. Other potential applications for this system include boilers, open cycle gas turbines, and combined gas/steam cycles.

Effective treatment of digested biosolids (sewage sludges), by heat or other means, is likely to reduce virus concentrations to below economically realistic detection limits. This paper describes how such data can be interpreted using a sample size varying version of the negative binomial distribution, derived as a Poisson stopped-sum of the logarithmic distribution. A dispersion parameter is determined from samples taken prior to treatment and then used to formulate one-sided upper confidence limits for the mean density of viruses after treatment. To illustrate the approach, a continuously mixed heat-treatment system with a mean retention time of 30 min is considered. Viruses were not detected after treatment at either 60° or 70°C. Confidence limits for mean virus densities after treatment ranged from 25.2 plaque forming units per g dry weight for treatment at 60°C with 95 confidence (41.3 g assayed) to 2.8 plaque forming units per g dry weight for treatment at 70°C with only 50 confidence (96.1 g assayed). Since viruses were not recovered after heat treatment at either temperature, this range of confidence limits was due to the desired statistical confidence level and the biosolids mass collected and measured rather than to treatment temperature.

The number of samples necessary for characterization of highway storm-water runoff concentrations is examined. Using extensive field monitoring results available from Minnesota, the statistical modeling results demonstrate that approximately 15 to 20 samples are required to provide reasonable estimates of the mean concentrations of runoff events for total suspended solids, total dissolved solids, total organic carbon, and zinc.