Journal of Environmental Engineering

The influence of seepage velocity and groundwater temperature on the dechlorination rates of trichloroethylene (TCE) and tetrachloroethylene (PCE) by zero-valent iron $\left({\mathrm{Fe}}^0\right)$ were investigated by running laboratory column tests at seepage velocities ranging from 31 to $\mathrm{1,884}\mathrm{m}∕\mathrm{year}$ at temperatures of 10 and $23°\mathrm{C}$ . By increasing the seepage velocity from 31 to $\mathrm{1,884}\mathrm{m}∕\mathrm{year}$ at $10°\mathrm{C}$ , there were approximately seven- and nine-fold increases in the normalized dechlorination rate constants $\left({\overline{k}}_{\mathrm{SA}}\right)$ of TCE and PCE, respectively. Similarly, a four-fold increase in the ${\overline{k}}_{\mathrm{SA}}$ of TCE and PCE was also observed at $23°\mathrm{C}$ when increasing the seepage velocity from 103 to $\mathrm{1,183}\mathrm{m}∕\mathrm{year}$ . Raising the groundwater temperature from 10 to $23°\mathrm{C}$ at a given seepage velocity resulted in 2.7 and 1.1 times increases in the TCE ${\overline{k}}_{\mathrm{SA}}$ and PCE ${\overline{k}}_{\mathrm{SA}}$ , respectively. With the application of the Arrhenius equation, activation energies of $70.3\mathrm{kJ}∕\mathrm{mol}$ for TCE and $38.6\mathrm{kJ}∕\mathrm{mol}$ for PCE dechlorination were determined, indicating domination of the electron transfer process over the mass transfer as a major rate-limiting step of the dechlorination reactions by ${\mathrm{Fe}}^0$ .

The hydraulic design of certain types of subsurface drains has recently been put on a more rational footing, and deficiencies in earlier design methods have been demonstrated. However, significant limitations remain in hydraulic design methods for geosynthetic and aggregate subsurface drains. It is important to decouple the groundwater hydrology from the internal hydraulics of the drain, and properly design subsurface drains for open-channel rather than pressurized conditions. Present design methods can inadvertently result in pressurized flow. The assumption of uniform flow (Manning’s equation alone) is also improperly made in some present design methods. Consequences can include unintended pressurized flow and attendant nonuniformity of inflow on the one hand and uneconomical design on the other. Current standard guidelines provide relatively little guidance for the design of geosynthetic and aggregate drains. A current ASTM standard, commonly referenced by geosynthetic manufacturers, has significant limitations. Deficiencies and qualifications are identified for present design methods. Guidance is given for the improved design of geosynthetic and aggregate subsurface drains based on sound hydraulic principles.

Using transform techniques, analytical expressions for potential losses by volatilization and degradation are developed for several organic compounds in dual porosity porous media. A sensitivity analysis is conducted to study the importance of different physical/chemical processes on volatilization, degradation, and leaching losses. To obtain estimates for overall solute behavior, expressions for effective Péclet numbers and degradation rates for organic contaminants are presented using method of moments. Results indicate that large fractions of many organic compounds are likely to volatilize into the atmosphere for sandy and clayey soils under typical flow conditions. It is found that nondimensional degradation influences both advective and dispersive effects. Thus, the Péclet number from effective-parameter equation tends to be enhanced when the nondimensional degradation is rather high. The simple expressions for moments and effective parameters can be used as screening tools to assess the behavior of volatile compounds in vadoze zone of soils.

A novel treatment process for arsenic removal from contaminated groundwater has been developed for use as a reactive barrier or a small drinking water treatment unit. In this study, modified porous media was made by the deposition of colloidal iron oxide onto sand grains at intermediate pH and ionic strength. $K_d$ values from column experiments were $0.016–0.37\mathrm{L}∕\mathrm{kg}$ for $\mathrm{As}\left(\mathrm{III}\right)$ and $0.023–0.85\mathrm{L}∕\mathrm{kg}$ for $\mathrm{As}\left(\mathrm{V}\right)$ , being lower than those of batch experiments (0.50 and $1.30\mathrm{L}∕\mathrm{kg}$ for $\mathrm{As}\left(\mathrm{III}\right)$ and $\mathrm{As}\left(\mathrm{V}\right)$ , respectively) due to lower availability of surface adsorption sites in the packed column. Media-independent $K_d$ values reflect the enhancement of arsenic adsorption with an increase of colloidal iron oxide coated sand fraction, apparently due to adsorption equilibration during arsenic transport under the same flow column conditions. The heterogeneous composition of two groundwater samples also reduced arsenic adsorption. Therefore, arsenic elution near the initial breakthrough was regulated by available adsorption surface in a porous coated sand media as well as the effects of competing oxyanions. The exhaustion of adsorption capacity near the critical contamination level is sensitive to geochemical and remedial properties of the contaminants.

A long-term comparative study on the kinetics of enhanced biological phosphorus removal (EBPR) was carried out in pilot scale membrane-assisted and conventional biological phosphorus removal processes, by monitoring system performance, phosphorus mass balances, and maximum specific rates in off-line batch tests. The two systems exhibited similar performance in the removal of soluble phosphorus (P) from the influent wastewater, in the specific P release observed in the anaerobic zone, and in the maximum specific P release and volatile fatty acid (VFA) uptake rates. However, when the VFA in the influent was limiting, the conventional EBPR (CEBPR) process performed significantly better than the membrane (MEBPR) counterpart, and this behavior was also reflected in the kinetics of P release. Denitrifying dephosphatation was observed to be significant in both processes during periods of satisfactory P removal. When the aerobic recycle ratio was reduced to a minimum level, the anoxic P uptake activity in the CEBPR sludge was lower than that of the MEBPR sludge. Finally, the biomass decay rates of the two sludge types were estimated to be comparable, with significant reduction of the decay under unaerated conditions.

Modeling the activated sludge wastewater treatment plant plays an important role in improving its performance. However, there are many limitations of the available data for model identification, calibration, and verification, such as the presence of missing values and outliers. Because available data are generally short, these gaps and outliers in data cannot be discarded but must be replaced by more reasonable estimates. The aim of this study is to use the Kohonen self-organizing map (KSOM), unsupervised neural networks, to predict the missing values and replace outliers in time series data for an activated sludge wastewater treatment plant in Edinburgh, U.K. The method is simple, computationally efficient and highly accurate. The results demonstrated that the KSOM is an excellent tool for replacing outliers and missing values from a high-dimensional data set. A comparison of the KSOM with multiple regression analysis and back-propagation artificial neural networks showed that the KSOM is superior in performance to either of the two latter approaches.

A robust eutrophication and sediment diagenesis model has been developed for the Patuxent Estuary to study the impact of different nutrient loadings on phytoplankton biomass and dissolved oxygen (DO) levels. The modeling approach was to begin with an existing water quality model (CE-QUAL-W2) for the Patuxent Estuary (hereafter referred to as the Estuary). First, formulations for the water column kinetics were completely replaced with routines based on the WASP/EUTRO5 water quality model. Then, a sediment diagenesis component was added to simulate the accumulation and mineralization of organic matter in the sediment, the generation of sediment oxygen demand, and the flux of phosphate and ammonia from the sediment. Loadings from the tributaries for nutrients and flow were based on a combination of watershed modeling and sampling by scientists at the Smithsonian Environmental Research Center. The new model was able to reproduce the ambient water quality data from 1997 to 1999 by adequately simulating the high concentrations of phytoplankton and low DO levels in the Estuary. The model was then used to evaluate the response to various hypothetical nutrient loading scenarios. Model results show that phytoplankton growth in the upper Estuary is much more sensitive to nutrient loading from tributaries than in the lower estuary. Further, model results indicate that DO concentrations in the lower Estuary are largely influenced by levels of nutrients and organic carbon at the mouth of the Estuary.

Inorganic (minerogenic) particles received from streams during runoff events cause undesirable increases in turbidity ( $T_n$ ; i.e., the light scattering coefficient, $b$ ) in many lakes and reservoirs. Quantification of particle inputs in the context of turbidity impacts and representative light scattering calculations (submodel), the necessary components in the development of a mechanistic multiple particle class model to simulate these impacts, are described for Schoharie Creek, N.Y. Light scattering attributes of minerogenic particles, including number concentration $\left(N\right)$ , size distribution (PSD), composition, and projected area per unit volume $\left({\mathrm{PAV}}_m\right)$ , are quantified for three runoff events through analyses by scanning electron microscopy interfaced with automated X-ray microanalysis and image analysis (SAX). The combined credibility of the light scattering submodel and SAX to represent the light scattering features of minerogenic particles is depicted by the consistency of calculated scattering efficiencies with theory and the strength of the reported $T_n-{\mathrm{PAV}}_m$ relationship. Particles in the size range of $2–10\mu \mathrm{m}$ were responsible for $b$ and $T_n$ in the stream, with greater contributions by the larger particles of this range at elevated stream flows. Empirical relationships are developed to predict $N$ and PSD from $T_n$ or stream flow.