Journal of Environmental Engineering

The transport timescales residence time, exposure time, and age of water are evaluated for the St. Louis Bay estuary, Mississippi, to investigate the impacts of freshwater inflows and tidal dynamics from Mississippi Sound on the estuary’s transport characteristics. The timescales are explicitly defined and computed using a hydrodynamic model and tracer experiments for a set of 11 hydrologic scenarios designed to represent permanent low, average, and high flow conditions. Results indicate that (1) the estuary’s residence time can vary between 2.0 and 134.5 days during high and low flow conditions, respectively; (2) under low flow conditions dispersive processes caused by the tidal dynamics at the estuary’s open boundary may be dominant, and the returning flows can increase the exposure times up to 30% in relation to the residence times; (3) during high flow conditions advective transport caused by the freshwater flows may be dominant resulting in exposure times similar to the residence times; and, (4) there are important spatial variations in the estuary's flushing characteristics as evaluated by the age of water.

Rainwater tanks are a fundamental component of integrated water cycle management, but the quality of harvested water is little understood and is potentially affected by many factors such as roofing material, site temperature, frequency of tank use, and surrounding land use. There has been limited study of the first flush, overflow water and harvested water quality from rainwater tanks in Australia. This paper presents the water quality results of a yearlong data collection and analysis study involving five rainwater tanks located in Western Sydney, Australia. The results show that tank water quality was impacted by the construction materials used to fabricate both the tanks and roofs. Furthermore, the overflow and first flush water quality were found to be of a lower standard than the tank samples, with implications for the cleaning of the first flush device and overflow diversion. Finally, the issues identified with the maintenance of the tanks are highlighted and discussed.

A major water-quality concern in a potable water service reservoir is the potential loss of chlorine residual, which is closely related to the flow pattern. In this article, the effects of operation and baffling on the flow pattern and chlorine-concentration distribution in a potable water service reservoir are reported. Using a computational fluid dynamics (CFD) method coupled with dynamic meshes, actual service conditions are mimicked, which have seldom been reported in the literature. First, this article provides an insight into why manipulating the valve located at the inlet can be beneficial in enhancing water quality in the long run. An explanation based on the simulation results suggests that manipulating the valve located at the inlet can lead to the evolution and migration of the vortices in the service reservoir, which would then allow water with prolonged age to flow out of the reservoir. This is significant for the operation of service reservoirs. Second, it explores the possibility of retrofitting a service reservoir with baffle walls to minimize the probability of seriously diminished water quality resulting from poor mixing and excessive aging. Although adding baffle walls in the flow-recirculation regions is found to break up the vortices and thus shorten the flow path, the fluid-velocity magnitude is reduced after flowing past the baffle walls. The outcome of these conflicting effects may eventually lead to a diminished effluent chlorine concentration after adding the baffle walls.

The aims of the current study were to evaluate the ability of three types of Thai bentonite (TNBC) for the removal of lead ions [Pb(II)] from aqueous solutions. Batch-adsorption kinetic experiments revealed that the sorption of Pb(II) onto TNBCs was very fast and reached equilibrium after 30 min. The adsorption mechanisms in the Pb(II)/TNBC system follow pseudo–second order reactions. The adsorption isotherm of Pb(II) follows both Langmuir and Freundlich models. The maximum adsorption capacities for the three tested clays were 45.46, 40.00, and $47.62\mathrm{mg}/\mathrm{g}$, respectively. The results also revealed that the sorption energy ($E$-value) were 15.18, 11.85, and $17.25\mathrm{kJ}/\mathrm{mol}$, respectively, for the three tested clays. Desorption of Pb(II) ions and regeneration of the natural clays were attained simultaneously by acid elution. Even after four cycles of adsorption–elution, the adsorption capacity was maintained, and the decline in efficiency was less than 10%. It is concluded that Thai bentonite has an affinity to lead ions from aqueous solutions and might have a potential to be used effectively to remove Pb(II) wastewater.

The lack of information on constructed-wetland performance in the tropics is among the factors that have hindered the adoption of low-cost wastewater-treatment technologies as alternatives to conventional wastewater treatment. A pilot scale study was undertaken in Juja (Kenya) to assess the performance of horizontal subsurface flow–constructed wetlands (HSSF-CWs) under tropical conditions. Primary domestic wastewater effluent was continuously fed into three replicate wetland cells, each with an area of $22.5{\mathrm{m}}^2$ ($7.5\times 3\mathrm{m}$) and with gravel as substrate. The study revealed successful performance of the wetlands in terms of compliance with local discharge standards with respect to chemical-oxygen demand (COD), ${\mathrm{BOD}}_5$, total suspended solids (TSS), and ${\mathrm{SO}}_4^{2-}\text{-}\mathrm{S}$ at an average mass removal efficiency between 58.9 and 74.9%. Moderate removal of ${\mathrm{NH}}_4^{+}\text{-}\mathrm{N}$ and total phosphorus (TP) was recorded. The estimated first-order aerial-rate constant and the ${\mathrm{BOD}}_5$ background concentration showed the HSSF-CW to be area-requirement competitive. The good performance in organic matter and suspended solids removal reveals that HSSF-CW can help to alleviate the current environmental pollution problems experienced in developing countries caused by the discharge of partially treated or untreated domestic wastewater.

Equations were developed to predict arsenate sorption on iron oxide coated sand, on the basis of the constant capacitance surface complexation model, assuming formation of bidentate surface complexes between arsenate and iron oxyhydroxide surface sites. The developed equations can predict arsenate adsorption when initial arsenate concentration, adsorbent concentration, and solution pH are known. The average discrepancy between experimental data and modeling results was less than 5%. The maximum arsenate sorption capacity is the only parameter required that needs to be estimated from experimental data. The developed equations were validated by modeling arsenate adsorption on iron oxide sand over the pH range 5–8, under various initial arsenate concentrations and iron oxide coated sand solid concentrations. The developed predictive equation can also be used for calculating arsenate adsorption performance on iron impregnated activated carbon. Finally, the developed equation can be used to calculate sorption media dose requirements by specifying initial arsenate concentration, arsenate removal goal, and solution pH.

Experiments were conducted to determine ammonium removal kinetics in an operating biologically active sand filter at a waterworks treating anaerobic groundwater. The ammonium load varied between $0.7–3\mathrm{g}\mathrm{N}/\mathrm{h}/{\mathrm{m}}^2$ (concentration ranged from $0.23–0.78\mathrm{mg}\mathrm{N}/\mathrm{L}$), and the inlet water flux varied between $1.7–5.1\mathrm{m}/\mathrm{h}$. Ammonium profiles and salt breakthrough curves were obtained in four different locations of the filter six times during two filter runs. The experiments show that the nitrification in the rapid sand filter was heterogeneous. The ammonium profiles exhibited variation in time and in space, vertically and laterally within the filter. The nitrification rate constants were calculated by using the ammonium profiles and the local pore velocity data obtained from the breakthrough curves. The local pore velocities were estimated to vary from $0.02–20.5\mathrm{m}/\mathrm{h}$. The nitrification rate constants varied randomly in time, and it was not possible to determine a clear order of the nitrification reaction. The average zero-order nitrification rate constant was $7\mathrm{mg}\mathrm{N}/\mathrm{L}/\mathrm{h}$, and the average first-order nitrification rate constant was $28{\mathrm{h}}^{-1}$ with an average $1.5\mathrm{g}/\mathrm{h}/{\mathrm{m}}^2$ ammonium load. The first-order nitrification rate constant was closely related to the water pore velocity, which implies that the rate is strongly determined by the resistance to mass transport in the diffusion boundary layer around the sand grains.

Elevated levels of total dissolved gas (TDG) may occur downstream of dams, leading to increased incidence of gas bubble disease in fish. Accelerating the dissipation of supersaturated TDG in the downstream river can mitigate this problem; however, data useful for modeling the dissipation of supersaturated TDG through the free surface in natural rivers are limited. Lacking data to the contrary, prior modeling studies have assumed (1) dissolved oxygen (DO) is a reasonable proxy for TDG; and (2) unsaturated reaeration is sufficiently similar to supersaturated dissipation such that the same rate coefficients may be applied to either process. To test the validity of these assumptions and motivate future research, laboratory experiments were conducted to estimate the first-order dissipation rate coefficients for supersaturated DO and TDG and the reaeration rate coefficients for DO under identical turbulence conditions. The results indicate the dissipation process is quantitatively different from the reaeration process, and TDG behavior is quantitatively different from DO. Comparison of laboratory results with prior field research leads to speculation that increasing river turbulence and/or decreasing in water depth may be possible methods for promoting the TDG dissipation rate and reducing the length of a river affected by supersaturation.

The scarcity of water resources in Egypt has triggered the need for using different types of low-quality water. Agricultural drainage water (ADW) is considered as a strategic reserve for coping with increasing fresh water demands. In this paper, an organized and general methodology for determining the optimum reuse discharges is presented. The physical properties of a drainage network are modeled by using a developed simulation water quality model. The genetic algorithm technique is used as an optimization tool for finding the optimum ADW reuse quantities at specific points of withdrawal. The simulation-optimization process was applied twice during the summer and winter seasons. The results show that the reuse quantities represent 75.50 and 87.12% of the available ADW in the study area for summer and winter, respectively, representing 40.57 and 43.52% of seasonal canal delivery. The use of the simulation-optimization model is suitable for decision support leading to near-optimum reuse selection of ADW for irrigation.

The competitive adsorption of phenol and $p$-hydroxybenzoic acid (4HBA) has been investigated on activated carbon (AC) for a wide range of concentrations under unbuffered conditions. The results show a preferential adsorption of 4HBA, which can be explained by the lower solubility of 4HBA and the electrostatic interactions between the AC and the ionic form of the molecule in this range of pH. The Langmuir isotherm was found suitable to describe the single-component adsorptions, indicating a monolayer adsorption in accordance with the microporous nature of the AC. Then the empirical extended Langmuir model and the predictive ideal adsorption solution theory model have been compared for competitive adsorption. When using parameter values optimized for single pollutants, both models show rather poor agreement with mixture data. However, after fitting the extended Langmuir parameters with the whole data set, better results can be obtained, showing that there is some peculiar behavior of the mixture under oxic conditions, probably tied to the effect of 4HBA on the irreversible adsorption of phenol.

In this study, the mechanism involved in the removal of U(VI) from aqueous solution by the green unicellular microalgae, Chlorella vulgaris, was examined through a series of batch experiments. The effect of various operation parameters such as solution pH, biomass dose, temperature, and different concentrations of U(VI) on the uptake capacity and removal efficiency of C. vulgaris was evaluated. The adsorption process could be defined well by both the Langmuir and Freundlich isotherms with $R^2$ of 0.99 and 0.97, respectively. The kinetic data were fitted to a pseudo second-order model with the $R^2$ value of 0.99. The thermodynamic equilibrium parameters obtained at different temperatures were used to evaluate the thermodynamic constants, $\mathrm{\Delta }{G°}^{}$, $\mathrm{\Delta }{H°}^{}$, and $\mathrm{\Delta }{S°}^{}$. The negative value of Gibbs free energy, $\mathrm{\Delta }{G°}^{}$, demonstrated the endothermic nature of the biosorption process. The adsorption–desorption cycle results demonstrated that this species of algae can be regenerated using 1 M of HCl. The recovery yield of U(VI) ions was found to be 90%, and the biosorption had good reusability after four consecutive adsorption–desorption cycles. The biosorption of U(VI) was found to be adversely affected also by the presence of Al(III) cations, whereas other ions seemed to have negligible effect on the process. The results suggested that C. vulgaris biomass is suitable as a sorbent materials for recovery and biosorption of U(VI) ions from aqueous solution.

One-stage autothermal thermophilic aerobic digestion (ATAD) is a promising process for sewage sludge stabilization. This study investigated semicontinuous operation in simulated one-stage ATAD digesters and analyzed the effects of hydraulic retention time (HRT) on sludge stabilization. During semicontinuous operation, the average volatile solids (VS) removals at HRTs of 13 and 16 days were 40.1% and 41.8%, respectively. Because HRTs were 6 and 10 days, the maximum VS removal was no more than 33.2%, less than 38% required by Class A digested sludge, so a suitable HRT level is 13–16 days for a one-stage ATAD system. The digesters with 13- and 16-day HRTs maintained relatively low soluble chemical oxidation demand (SCOD), total nitrogen, ${\mathrm{NH}}_4^{+}\text{-}\mathrm{N}$, and total phosphate, whereas the digester at HRT 6 days had high SCOD concentration ($<6,300\mathrm{mg}{\mathrm{L}}^{-1}$) and low oxidation reduction potential (ORP), ranging between $-335$ and $-245\mathrm{mV}$. Volatile fatty acids (VFAs) accumulated rapidly at short HRTs, and acetic, propionic, isobutyric, and isovaleric acids were the major components of VFAs in the digester with a 6-day HRT. Digestion systems at HRTs of 13 and 16 days had lower specific capillary suction time than the other two digesters at HRTs of 6 and 10 days, indicating that extending retention time favors the dewaterability of the digested sludge.

The industrial solvent trichloroethylene (TCE) is among the most ubiquitous chlorinated compounds found in groundwater contamination. The objective of this pilot-scale study was to evaluate the effectiveness of an in situ reductive dechlorination process to remediate TCE-contaminated groundwater at a TCE-spill site using specific gene analysis. An injection well was installed inside the TCE plume for substrate and inocula injection. Brown sugar and anaerobic activated sludge (collected from an industrial wastewater-treatment plant with influent containing TCE) as electron donor (primary substrate) and inocula, respectively, were injected to enhance the TCE biodegradation rate through anaerobic reductive dechlorination. Three monitor wells were installed downgradient of the injection well along the groundwater flow path to monitor the TCE degradation trend. Polymerase chain reaction (PCR) analytical results reveal that the major TCE degrader (Dehalococcoides) and TCE-degrading genes (vcrA and tceA) were not observed in site groundwater before sludge injection. Thus, anaerobic sludge was injected for inoculation. After sludge injection, Dehalococcoides and the vcrA and tceA genes were detected in the injection and downgradient monitoring wells. Results show that brown sugar could be fermented to volatile fatty acids (VFAs), which could be used as substrates to enhance TCE reductive dechlorination. Up to 97% of TCE removal was observed after 204 days of operation. This reveals that appropriate substrates and inocula are required to effectively enhance the anaerobic reductive dechlorinating rate of TCE. Results indicate that the enhanced in situ bioremediation is a promising technology to remediate TCE-contaminated groundwater, and gene analysis is a necessity to evaluate the feasibility and occurrence of enhanced bioremediation.

Large-scale marine reclamation is expected to have some effects on the marine environment, but these effects vary depending on the reclamation layout schemes. For the offshore airport programming of Dalian, China, an insular artificial island scheme and a peninsula artificial island scheme are considered. To contrast the effects of these two schemes on the marine environment, a two-dimensional (2D) hydrodynamic model was applied on the basis of the characteristics of the tides, currents, waves, and sediments in the Jinzhou Bay. Furthermore, a water quality model of suspended sediment convection-diffusion was developed on the basis of the hydrodynamic model, and suspended sediments were the source for identifying the intensity of sewage outfalls during the sea-filling process. Data for tides, currents, waves, and sediments from field observations carried out in 2009 at six stations were used for calibration and validation of the models, which showed a positive agreement with the simulated results. Moreover, on the basis of the verified models, the comparative analysis of the effects of the two schemes on hydrodynamics, morphology evolution, water pollution, and biological resource loss was comprehensively carried out in the paper. The results indicate that the effects on the marine environment of the insular artificial island scheme are less than that of the peninsula scheme. The conclusions provide an important foundation for the programming of the Dalian offshore airport and can be used as a reference for similar coastal engineering and marine environment protection.

This paper reports on results from steady-state Reynolds-averaged Navier-Stokes simulation (RANSS) of the flow and passive nonreactive tracer transport inside a multichambered, laboratory-scale ozone contactor using a structured, collocated, finite-volume discretization. Simulations are posed following a physical laboratory experiment. Simulations performed on different grids lead to varying degrees of short-circuiting and dispersion, which ultimately leads to differences in the residence time distribution (RTD) of the tracer (released at the inflow as a pulse) and in particular $t_{10}$ index. The predicted $t_{10}$ index depends on grid density and RANS turbulence model. For fine grids that resolve the viscous sublayer, a model with proper behavior within this sublayer has to be applied for accurate prediction of $t_{10}$. Simulations on all considered grids with proper turbulence model yield cumulative RTD and associated $t_{10}$ index in good agreement with the experimental data despite the underresolution of the flow by the RANSS methodology relative to a large-eddy simulation (LES) of the same flow.

Because of the difficulty of specifying optimal design parameters of rectangular sedimentation tanks using a forward numerical model, an inverse calculation model based on an optimization algorithm and a forward model were developed in this paper. Using this model, a numerical simulation of the sedimentation tank involving determination of the velocity field and the concentration distribution of the particles by the forward model was conducted. The eddy viscosity was obtained with the aid of $k-\epsilon$ turbulence model equations. The inverse model was applied to the optimal design of the horizontal baffle location in a given sedimentation tank. As a one-dimensional nonlinear optimization method, Brent’s method was used. Moreover, the effect of the horizontal baffle location on flow and solids removal ratio was also discussed. Compared with the traditional forward model for specifying optimal design parameters for rectangular sedimentation tanks, the inverse calculation model can search directly for the optimum design parameters and can obtain good results.