Journal of Environmental Engineering

Degradation of a mortar specimen exposed to an acid sulfate solution was studied. The pH of the solution was maintained in the range of 4–5 for 8 days and 2–3 for 73 days. Data from the experiment were used to investigate the efficacy of a diffusion-reaction based model with a moving boundary for the corrosion process. Acid neutralization rates in the solution were used to calculate the corrosion rate constant for the mortar sample and to determine the effective diffusion rate of sulfuric acid in the corrosion layer. Visual observation of the cross section of the sample at the end of the experiment indicated that 0.82 mm was corroded. The data established the validity of the moving boundary paradigm for sulfide corrosion of concrete when the effective diffusion rate was a decreasing function of the age of the corrosion product being formed.

Degradation of a mortar specimen exposed to an acidic sulfate solution was studied using iron pins set within the sample with their ends close to the surface. Electrochemical impedance spectroscopy and the open circuit potential of the pins were used to monitor corrosion behavior. An experiment in which the pH of the test solution was maintained in the range of 4–5 for 8 days and 2–3 for 73 days was performed. The experimental data were used to investigate the efficacy of a diffusion-reaction based model with a moving boundary for the corrosion process. The open circuit potential of the pins indicated initiation of active corrosion of the pins closest to the surface of the mortar after 36 days in the second experiment. Visual observation of the cross section of the mortar sample at the end of the experiment indicated that 0.82 mm of the mortar was corroded. The data established the validity of the moving boundary paradigm for sulfide corrosion of concrete. Electrochemical impedance spectroscopy and open circuit potential were found to be valuable tools for monitoring corrosion of the iron pins in the specimen.

The pyrolysis of sewage sludge, with and without ZnCl₂ activation, was monitored using TGA-FTIR, with the results utilized in a proposed decomposition mechanism. For the pyrolysis of sludge without ZnCl₂ activation, three global stages of mass loss occurred and are proposed as dehydration, initial depolymerization, and deep degradation. For the pyrolysis of sludge with ZnCl₂ activation, apart from these three stages, an additional mass loss stage could be also observed at high temperature. It is suggested this results from the evaporation of ZnCl₂. To make a comparison between these two cases, it was found that the pyrolysis of sludge with ZnCl₂ activation had a longer dehydration phase and a shorter degradation phase. Based on the speculated mechanism, a kinetic model of four consecutive competitive reactions was adopted to simulate the pyrolysis of sewage sludge with and without ZnCl₂ activation. Globally, the proposed model was able to fit the experimental data very well for both cases, with the correlation coefficients above 0.99.

Chlorine dioxide inactivation of Cryptosporidium parvum oocysts was studied at bench-scale in oxidant demand-free 0.05 M phosphate buffer at pH 6, 8, and 11, and temperature from 1°C to 37°C. Animal infectivity using neonatal CD-1 mice was used for evaluation of oocyst infectiousness before and after treatment. Survival curves of the oocysts following treatment declined linearly with the chlorine dioxide C_avgt product. Temperature was critical for C. parvum inactivation, while pH was found not to be a significant factor at pH 6–11. Inactivation kinetics at different temperatures was expressed as a Chick-Watson model with different reaction rate constants adjusted by van't Hoff-Arrhenius relationship. Between 1°C and 37°C, for every 10°C decrease in temperature, the reaction rate constant decreased by a factor of 2.3, corresponding to an activation energy of 54.9 kJ/mol. Design criteria targeting 0.5 to 2.0 log-units of inactivation of C. parvum were developed for different water temperatures and their 90% confidence intervals were provided.

Oxygen transfer into water at low-drop weirs was studied by using a prototype-scale, recirculating hydraulic flume with a weir width of 0.61 m, and a downstream channel width of 1.22 m. An enhanced-oxygen environment was used to maintain sufficient difference between saturation and upstream dissolved oxygen concentrations to reduce the experimental uncertainty. The variation in the deficit ratio r as a function of tailwater depth H was evaluated by using unit discharges q of 167, 334, and 502 m³/hr⋅m and weir heights W of 0.54, 1.04, and 1.36 m. A semitheoretical equation for predicting r as a function of jet Froude number F_J, drop height h, and H was fitted to observed values by nonlinear regression analysis. An assumed value of 0.667 for maximum H/h (found in the literature) proved acceptable. We concluded that tailwater depths are important to predict oxygen transfer at low-drop weirs, but that designing for optimum tailwater depth to maximize oxygen transfer may not be as important as was previously thought, because large optimum ranges for H/h were observed in this study.

Wastewater generated in the table olive processing industries has been treated by means of an integrated wet air oxidation–aerobic biodegradation process. Chemical oxygen demand (COD) conversion in the range of 30–60% (6 h of treatment) has been achieved by using relatively mild conditions (443–483 K and 3.0–7.0 MPa of total pressure using air). Use of the homogeneous catalysts [copper (II)] or radical promoters (hydrogen peroxide) resulted in a higher efficiency of the process (roughly between 16 and 33% COD removal improvement depending on operating conditions). Biodegradability tests, conducted after the oxidation pretreatment, showed a negative effect with the addition of copper to the reaction media. Promoted experiments resulted in the highest values of biodegradability (measured as the ratio of biological oxygen demand to COD). The rate of the COD biodegradation was compared to the kinetics of the aerobic process without a previous chemical preoxidation. The calculated kinetic parameters showed the positive effect of the pretreatment (maximum growth rate of 0.03 ± 0.006 h⁻¹ and 0.014 ± 0.00014 h⁻¹). Acclimation of microorganisms to oxygenated species formed in a chemical preoxidation step enhanced the efficacy of the biodegradation (maximum growth rate of 0.04 ± 0.007 h⁻¹).

Photosonolysis of 1,1,1-trichloroethane (TCA), trichloroethylene (TCE), and tetrachloroethylene (PCE) in water was investigated using a cup-horn, flow-through reactor system. Water containing titanium dioxide was deliberately contaminated with a mixture of volatile organic compound (VOC)—TCA, TCE, and PCE—and individual VOC. These solutions were irradiated with ultraviolet light (UV) and ultrasonic waves (US), independently and concurrently (UVUS). The values of the first-order degradation rate constant and the removal efficiency for the VOC were evaluated for the UV, US, and UVUS treatments. The results showed that the concurrent use of UV and US increased the VOC degradation rate and removal efficiency beyond the additive effect of UV and US, suggesting that the UVUS effect on the decomposition of the VOC is synergistic under certain conditions. TCE and PCE were more readily degraded than TCA, suggesting that double-bond cleavage is one of the initial degradation pathways. With the reactor specifications used, the photosonolysis process can produce water meeting the drinking water standard (MCLs of 5 μg/L) for TCE and PCE with a removal efficiency of approximately 90%.

A coupled flow and transport model (VENT3D) was used to simulate the performance of soil vapor extraction at a jet-fuel (JP-4) contaminated site. The contaminant mixture was approximated with 10 surrogate components categorized by carbon number class. The contaminant was distributed heterogeneously throughout the soil. The initial total contaminant mass was the only parameter that was adjusted, and it was fit to the observed removal during the pilot tests. The predicted total mass removal using the most general approach matched the observed hydrocarbon mass removal within ±4%. The effects of the following selected simplifying assumptions were examined with the model: (1) uniform contaminant distribution; (2) 2D horizontal flow; and (3) single equivalent contaminant. The uniform soil contamination distribution and the horizontal flow assumptions yielded similar results—both estimated removal rates that were initially higher by 30% and eventually overpredicted the total removal by 10%. The equivalent single-component approximation underpredicted the removal by as much as 50% during the first half of the operation and then overpredicted the removal, ultimately overpredicting the total removal by <10%.

Soil and ground water in a shallow aquifer were contaminated by petroleum hydrocarbons, mostly comprised of toluene. The degrees of soil and ground-water contamination were closely correlated. The vertical profile of soil contamination showed an exponential or semilinear decrease both upward and downward from the mean water table, and this was closely related to water table fluctuation. Vertical and seasonal variations in the concentrations of petroleum hydrocarbon contaminants and their degradation products were analyzed in detail. Total natural attenuation rate and the contribution of biodegradation to the rate were estimated. Biodegradation was found to be an effective attenuation process at this site.

The energy-effectiveness of packed-bed and nonpacked-bed plasma reactors for the destruction of toluene vapors is evaluated on the basis of the power consumption of the reactor system. The operation and design parameters being evaluated include the applied voltage, inlet gas flow rate, and toluene concentration. The optimal operational conditions for the target destruction of toluene vapors, with minimum power consumption, are determined. The results showed that a packed-bed plasma reactor has a higher toluene destruction efficiency and a lower power consumption than a nonpacked-bed plasma reactor. The toluene destruction efficiency is increased as both the inlet gas flow rate and the toluene gas concentration are decreased. For a plasma reactor 2 cm in diameter and packed with 5-mm glass pellets, >90% of toluene destruction can be reached at an applied voltage of 8.7 kV, inlet toluene concentrations <1,000 ppm, and gas flow rates of <680 cm³/min. The corresponding energy-effectiveness is about 15 g toluene/kW⋅h.

The affect of nitrate concentration and reactor backwashing on biofilter performance is evaluated using a dynamic mathematical model of the biodegradation process of volatile organic compounds in a trickle bed biofilter packed with uniform synthetic solids. Experimental observations from a bench-scale biofilter system treating ether were used to develop and validate the model. Experience acquired in biofiltration of volatile organic compounds has demonstrated that although these two factors—nitrate and backwashing—are secondary when organic packing material is used, they are essential when the packing media is synthetic. The operation of a synthetic media packed reactor requires the addition of nutrients necessary for biodegradation. Since nitrate was utilized as the nitrogen source in this system, it was included in the model as a limiting substrate (nutrient). A negative affect of excessive accumulation of biomass in the reactor on biofilter performance has also been observed in highly loaded synthetic media biofilters. This problem was solved by removing excessive biomass via full media fluidization and backwashing of the reactor. The affect of periodic backwashing was included in the model as a reduction in the biofilm thickness and a new approach to calculate the reactor specific surface area after backwashing. The unknown model parameters that correspond to nitrate limitations were estimated. The mathematical model was then used for simulation and analyses of the affect of these two factors on the biodegradation process.

The leachate from a Hong Kong landfill, containing 15,700 mg/L of chemical oxygen demand (COD) and 2,260 mg/L of ammonia nitrogen (NH₃–N), was first treated in a UASB (upflow anaerobic sludge blanket) reactor at 37°C. The process on average removed 90.4% of COD with 6.6 days of hydraulic retention at an organic loading rate of 2.37 g of COD/L⋅day. The UASB effluent was further treated by the Fenton coagulation process using H₂O₂ and Fe²⁺. Under the optimal condition of 200 mg of H₂O₂/L and 300 mg of Fe²⁺/L and an initial pH of 6.0, 70% of residual COD in the UASB effluent was removed, of which 56% was removed by coagulation/precipitation and only 14% by free radical oxidation. It is obvious that H₂O₂ and Fe²⁺ had a strong synergistic effect on coagulation. The average COD in the final effluent was 447 mg/L. Removing each gram of COD required 0.28 g of Fe²⁺ and 0.18 g of H₂O₂.