Journal of Environmental Engineering

In this paper a new mathematical steady-state model specifically aimed at the design of biological phases in single sludge predenitrification systems is proposed. The model is based on the mass balance equations of four substrates (dissolved and suspended biodegradable organic matter, ammonia nitrogen, and nitrate nitrogen) and two biomasses (heterotrophic and autotrophic microorganisms). For each specific design problem, if both the influent flow composition and the kinetic parameter values are known and the effluent nitrogen compound concentrations are fixed, the model makes possible an a priori assessment of system applicability and gives the explicit expressions of the biological phase volumes. A sensitivity analysis is also presented in order to evaluate the effect of variations in the influent and effluent characteristics on the biological phase volumes.

A dynamic mathematical model for the high purity oxygen activated sludge process, which incorporates structured biomass, gas–liquid interactions and control systems, was developed. The model was calibrated using pilot plant data associated with the development of the West Point Treatment Plant near Seattle, Wash. The calibrated model was used to simulate oxygen transfer rates for various operating conditions. Simulations showed that an optimal control system can reduce aerator power by 33% as compared to a conventional design, and reduce average oxygen feed gas by as much as 18%. Vent gas purity control dramatically reduced the peak aerator horsepower required to maintain set point dissolved oxygen concentration during high loadings. Step feed operation reduced the stag-to-stage variation in aerator horsepower and also reduced the required peak power. Predicted power savings for a $605\text{,}000m^3/\mathrm{day}$ plant were $500,000 per year at current power costs.

The potential of biological processes during aquifer storage to reduce disinfection by-products (DBP), and DBP precursors were examined under controlled conditions. Finished water treated by conventional water treatment practice was pumped into a sand media column for up to 34 days of residence time. Two experiments were conducted where the finished water was chlorinated or ozonated prior to injection. Chlorination of water withdrawn from simulated aquifer storage conditions resulted in reduced formation of trihalomethane (THM) concentrations for all three treated water types. Ozonation of finished water resulted in a 70% decrease in TTHM formation. Aquifer storage of finished water resulted in a 26–28% reduction in TTHM formation and the removal of preformed THM species was as high as 40%. Overall, aquifer storage of chlorinated finished water resulted in a 44% reduction in TTHM formation when additionally chlorinated after withdrawal. Bromate formed during ozonation was reduced by approximately 54%. This study indicates that the sequencing of chlorination or ozonation with respect to aquifer storage and recovery operations can impact DBP formation.

The control of struvite precipitation was investigated using a range of chemical inhibitors and chelating agents. Initial experiments investigated the mass of struvite that precipitated on jar tester impellers used to mix anaerobically digested sludge liquors. Mixing the liquors resulted in degassing of ${\mathrm{CO}}_2$ increasing the pH from below 7 to above 8. It was found that the greater the liquors’ supersaturation ratio (Ω) the greater the mass of struvite formed on the impellers. Liquors with a $\Omega =1.7$ and 5.3 produced mean final masses of 73.8 and $136g/m^2\text{,}$ respectively, after 40 h of mixing. Experiments using the same impellers were performed with a range of scale inhibitors and chelating agents. The scale inhibitors did not prevent struvite from forming, with only one product, Polystabilis, showing a reduction in the mass of scale formed on the impellers. Of the chelating agents tested, ethylenediaminetetraacetic acid (EDTA) was found to be the most effective. Further experiments showed that an EDTA: struvite ratio of 2:1 dissolved $>90\%$ of struvite back into solution even at pH values greater than 7.5.

The exploratory research of applying an acoustic standing wave to a sediment flow stream to fractionate and segregate particles was investigated. Using fundamental physics of particles in an acoustic field, a mathematical model was developed to calculate trajectories of deflected particles due to the application of acoustic standing waves. Then at the bench scale, the above technology was implemented by building a flow chamber with two transducers at opposite ends to generate an acoustic standing wave. The technology was evaluated using uniform size silicon dioxide and silicon carbide particle suspensions in de-ionized water. Due to the acoustic force field, ${\mathrm{SiO}}_2$ particles migrated toward the pressure nodes at half wavelength intervals at an optimum frequency of 333 kHz and 40 W power. Dark lines representing particle columns were formed after the application of the acoustic field, which was recorded in videotape. However, due to the small particle size of ${\mathrm{SiO}}_2\text{,}$ particle trajectories could not be recorded, hence the slightly larger sized SiC was used to track particle trajectories. The displacements of SiC particles due to an acoustic force were compared with the mathematical model predictions. For input power level between 3.0 and 5.0 W, the experimental data were comparable to mathematical model predictions. Also, from the experimental data it was possible to develop a relationship between input power and acoustic energy in the resonance chamber. Hence based on preliminary results it can be concluded that the acoustic field can be used either to segregate or fractionate fine particles.

Fly ash from municipal solid waste incineration (MSWI) is considered as hazardous waste that calls for a robust, reliable, and reasonable treatment technique. This investigation aims to assess the impact of ${\mathrm{CO}}_2$ partial pressure, water addition, time, and temperature on the stabilization of MSWI fly ash with particular emphasis on Pb, Zn, Cd, and Cr. Carbonation and element mobility were studied by applying thermal analysis and leaching assays on fly ash samples treated according to a $2^4$ factorial design. The relationship between the factors and the response variables was evaluated using partial least squares modeling. Chemical equilibrium calculations were performed so as to complement the experimental findings. Decalcification of carbonated fly ash in a typical Swedish landfill was estimated at $0.13\mathrm{mm}\cdot {\mathrm{yr}}^{-1}$ Treatment through carbonation reduced the availability of Pb and Zn about 100 times and also the carbonate alkalinity of $7.4\mathrm{eq}\cdot (\mathrm{kg}\cdot \mathrm{FS}{)}^{-1}$ (FS represents the fixed solids) was remedied successfully. However, shortcomings that need to be resolved are the remobilization of Cr with time and the mobilization of Cd.

Mercury was removed from a field-contaminated soil by a combination of redox and complexation processes with iodide/iodine and electrokinetic mobilization. Iodide added to the cathode compartment was transported into the soil and oxidized to iodine near the anode. Mercury was mobilized and transported to the anode as a mercury–iodide complex. After 5 days, some 50% of the total mercury content had migrated to the anode compartment, and another 25% was recovered from the soil water in the vicinity of the anode. No volatile mercury was formed. Electromigration is the dominant transport process for the (charged) mercury–iodide complex, since electro-osmosis would have moved the mercury toward the cathode. The combination of iodide as complexing agent and an electric field for physical mobilization could be developed to a new method for in situ remediation of mercury contaminated soil.

Time-variable (periodic) flow over a lake bed, and the associated boundary layer development, have the potential to control or at least influence rates of mass transfer across the sediment/water interface. An analysis for instantaneous and time averaged flux of a material across the sediment/water interface for infinite supply in the water and infinite sink in the sediment is presented. The water flow above the interface is characterized by the shear velocity $\left(U_{*}\right)$ which is a periodic function of time with a maximum amplitude of $\left(U_{*0}\right)$ as may be typical of an internal seiche (internal standing wave) motion in a density stratified lake. The relationship between the shear velocity on the lake bed and the wind shear on the lake surface is illustrated for an extremely simplified two-layered lake of constant depth. For a less restrictive analysis, shear velocities on a lake bed have to be obtained either from field measurements or from a three-dimensional lake circulation model driven by atmospheric forcing including wind. Smaller and wind-sheltered lakes will have lower $\left(U_{*0}\right)$ and periodicities (T). The response of the diffusive boundary layer was related to the period of the periodic motion (T), Schmidt number $\left(\mathrm{Sc}\right)\text{,}$ and shear velocity $\left(U_{*}\right)\text{.}$ The vertical diffusive flux at the sediment/water interface was expressed by a Sherwood number $\left(\mathrm{Sh}\right)\text{,}$ either instantaneous or time averaged. The mean Sherwood number $\left({\mathrm{Sh}}_{\mathrm{ave}}\right)$ varies with shear velocity of the wave motion over the sediment bed, Schmidt number $\left(\mathrm{Sc}\right)$ and the period (T) due to the response of the diffusive boundary layer to the time variable water velocity. Effective diffusive boundary layers develop only at low shear velocities. Where they do, maximum and minimum boundary layer thickness depends on all three independent variables (T, $\mathrm{Sc}\text{,}$ and $U_{*0})\text{.}$ The diffusive boundary layer strongly affects sediment/water mass transfer, i.e., Sherwood numbers. Mass transfer averaged over a period can be substantially less than that produced by steady-state flow at the same $U_{*0}$ and $\mathrm{Sc}\text{.}$ At $\mathrm{Sc}=500\text{,}$ typical for dissolved oxygen, the mass transfer ratio can be reduced to 60% of steady state, depending on the internal wave period (T).

The transient storage model (TSM) is the most commonly used model for stream–subsurface exchange of solutes. The TSM provides a convenient, simplified representation of hyporheic exchange, but its lack of a true physical basis causes its parameters to be difficult to predict. However, the simple formulation makes the model a useful practical tool for many applications. This work compares the TSM with a physically based pumping model. This comparison is advantageous for two reasons: Advective pumping is known to be an important hyporheic exchange process in many streams, and the pumping model can be used to derive dimensionless transient storage parameters that are properly scaled with important physical stream parameters. Transient storage model parameters are shown to be dependent on both the timescale of observation and the shape of the breakthrough curve, i.e., on the temporal evolution of the solute concentration in the surface water. This indicates that the transient storage model can, in practice, lead to incorrect predictions when model parameters are obtained without consideration of the stream flow dynamics, the properties of the stream bed, or the process timescale. This work emphasizes the limitations of simplified models for hyporheic transport, and indicates that such models need to be carefully applied.

Infiltration of storm water through detention and retention basins may increase the risk of groundwater contamination, especially in areas where the soil is sandy and the water table shallow, and contaminants may not have a chance to degrade or sorb onto soil particles before reaching the saturated zone. Groundwater from 16 monitoring wells installed in basins in southern New Jersey was compared to the quality of shallow groundwater from 30 wells in areas of new-urban land use. Basin groundwater contained much lower levels of dissolved oxygen, which affected concentrations of major ions. Patterns of volatile organic compound and pesticide occurrence in basin groundwater reflected the land use in the drainage areas served by the basins, and differed from patterns in background samples, exhibiting a greater occurrence of petroleum hydrocarbons and certain pesticides. Dilution effects and volatilization likely decrease the concentration and detection frequency of certain compounds commonly found in background groundwater. High recharge rates in storm water basins may cause loading factors to be substantial even when constituent concentrations in infiltrating storm water are relatively low.

Recovery of hydrofluoric acid (HF) from waste semiconductor acid solution by ion exchange was investigated. Strong base anionic and strong acid cationic ion exchange resins were involved in the recovery process. The former was used to capture the fluoride in the waste acid solution while the latter was employed for exchanging hydrogen ion for sodium in the NaF solution resulting from regeneration of the anion exchanger with NaOH. The ion exchange process consisted of two ion exchange steps and two regeneration steps. Batch experiments were conducted to determine the equilibrium ion exchange capacities and to develop ion exchange isotherms. Column tests were made to examine the performances of continuous ion exchange operations under conditions of different feed flow rates and inlet acid concentrations. A simplified model based on the general logistic function was adopted and tested for describing the solute breakthrough behaviors of the exit aqueous solution. Regeneration of exhausted ion exchange resins was also examined to test their effectiveness for repeated use. The hydrofluoric acid recovered by the ion exchange process would be available for reuse. It could also be converted by calcium chloride to high-purity calcium fluoride.

Heat and volatile liquid transport in soils are physically coupled processes. Soil water movement driven by thermal gradients has received considerable attention both experimentally and via models. This is appropriate since the thermal regime of underground electric power cables, pipelines, agricultural soils, and nuclear-waste repositories influences their ability to function effectively. However, available experimental liquid concentration data are of relatively low resolution, generally about one measurement per cm. By using synchrotron x rays we were able to nondestructively obtain high (50 μm) spatial resolution, time-lapsed data for the thermally driven movement of dibromomethane (DBM) in soil. Under repeated temperature cycling during a 22 h period, DBM movement within a sealed, 25-mm-long quartz sand $(D_{50}=0.25 \mathrm{mm})$ soil column was highly repeatable. About four complete scans of the column were completed each hour. Average DBM vol % within the column as a whole was 4.0 and the porosity was 0.31. Measurements of DBM concentration at the same position and equivalent times during different cycles usually differed by less than 0.2 vol %. Experimental data of this precision will permit better comparison with detailed model predictions.