Journal of Environmental Engineering

This work investigates the use of membrane air-stripping (MAS) contactors containing microporous polypropylene hollow-fiber membranes to remove volatile organic compounds from water into the gas phase. Experiments using countercurrent and cross-flow flow configurations to remove trichloroethylene (TCE) from water into air were performed. The effects of influent TCE concentration, liquid and gas flow rates, and air-to-water ratio were investigated. In each case studied, it was found that mass transfer in MAS contactors is controlled by liquid-phase resistance. Maximum removal of TCE was obtained at air-to-water ratios that were significantly lower than those required for conventional packed-tower stripping. Mathematical models based on empirical correlations for the liquid-phase convective mass transfer coefficients were developed and validated with experimental data, and subsequently used to perform simulations of full-scale MAS contactors. At the operating conditions explored, full-scale cross-flow contactors were shown to be superior to countercurrent-flow contactors: for the same gas and liquid flow rates and number of fibers, cross-flow contactors removed more than 95% of the original TCE in the feed liquid whereas countercurrent-flow contactors removed less than 50%. In comparison with packed-tower stripping, MAS contactors are shown to be an alternative feasible technology.

The mechanism of pore blockage plays an important role in competitive adsorption of trace organic compounds by microporous materials, such as activated carbon. An accurate description of the pore blockage effect of natural organic matter (NOM) on adsorption kinetics is a necessary part in the development of kinetic models for process design and performance prediction. In this study, a bisolute kinetic model based on homogeneous surface diffusion was developed to predict atrazine adsorption in a continuous flow powdered activated carbon (PAC)/membrane filtration system, incorporating the pore blockage effect. In order to elucidate the pore blockage effect, the system used in this study was simplified by representing the pore-blocking fraction of NOM with a single compound, poly(styrene sulfonate) with a nominal molecular weight of 1,800 Dalton. The surface diffusion coefficient of atrazine was modeled as a function of the surface concentration of the pore-blocking compound, which was also calculated by the model. The adsorption process was modeled for PAC application as either a pulse input or a step input; the quality of membrane backwash water was also used as a variable. Part II of this study, which appears in the companion paper, presents the model verification with a bench-scale PAC/microfiltration system, as well as model simulations that show optimal operating conditions for this system.

The bisolute model developed in Part I of this study to describe the effect of pore blockage on trace organic compound adsorption was verified and then used to study the roles of various design and operating variables on process performance efficiency. Continuous flow experiments were conducted with atrazine and a pore-blocking macroelectrolyte, poly(styrene sulfonate) $\left(\mathrm{PSS}-1.8k\right)$ using two powdered activated carbons (PACs) in a bench-scale PAC/microfiltration (MF) system. The model was calibrated with one set of experimental data, and verified with additional data obtained at different operating conditions for both PACs. PAC B, which has a large mesopore surface area and volume, was found to be less affected by pore blockage at a given $\mathrm{PSS}-1.8k$ surface loading compared to PAC A, which has relatively small mesopore surface area and volume. However, it was also shown that when PAC B was fully loaded with $\mathrm{PSS}-1.8k$, the pore blockage effect on the rate of atrazine diffusion was even greater than that for PAC A due to the higher $\mathrm{PSS}-1.8k$ adsorption capacity of PAC B. Model simulations were conducted to investigate the effects of reactor hydraulic retention time, membrane cleaning interval, influent $\mathrm{PSS}-1.8k$ concentration, PAC dosage, PAC dosing scenario, and the quality of membrane cleaning water on system performance. Optimal PAC/MF system operating conditions were also determined based on model simulations. The study results showed the effects that different concentrations and adsorbent loadings with pore blocking compounds, such as the pore blocking fractions of natural organic matter, can have, and the importance of mesopores in mitigating the detrimental effect of pore blockage.

Chlorine residuals of drinking water have long been recognized as an excellent indicator for studying water quality in the distribution network. This research applied factor analysis and cluster analysis to determine the spatial diversity of chlorine residual in the distribution system of Feng-Yuan city. Thirteen sampling sites were established. From the results of factor analysis, the sampling sites of the study area could be classified into three groups: residential zone, mixed zone, and commercial zone. The spatial diversity of chlorine residual was found to correlate with the daily lifestyle of the inhabitants and the commercial activities. From the results of cluster analysis, the sampling sites of the study area could also be classified into three groups: high-chlorine-level zone, medium-chlorine-level zone, and low-chlorine-level zone. By combining the results of factor analysis and cluster analysis, the worst case scenarios for drinking water quality in the distribution network also could be determined.

Drinking water utilities may be required to change disinfectant to improve water quality and meet more stringent disinfection regulations. This research was conducted to assess and compares chlorine dioxide to free chlorine and chloramines on bacterial water quality monitored within model distribution systems (i.e., annular reactors). Following colonization with nondisinfected water, annular reactors containing either polycarbonate or cast iron coupons were treated with free chlorine, chlorine dioxide or chloramines. Two disinfectant doses (low/high) were tested for each disinfectant. Under specific environmental conditions, bacterial inactivation varied as a function of the disinfectant type and dose, sample type (bulk water versus biofilm bacteria) and coupon material. The ranking by efficiency was as follows: chlorine dioxide $>$ chlorine $>$ chloramines. On preformed biofilms of ${10}^6-{10}^7\mathrm{cfu}/{\mathrm{cm}}^2$, the continuous application of a disinfectant led to a log removal of heterotrophic bacteria concentrations for suspended and biofilm bacteria ranging from 1.1 to 4.0, and from 0.2 to 2.5, respectively. Doubling the amount of disinfectant doses led to an additional log inactivation of 1–2.5 of heterotrophic bacteria levels. This study demonstrates that bacterial inactivation in distribution systems is governed by various inter-related parameters. The data indicate that chlorine dioxide represents a viable alternative for secondary disinfection in distribution systems.

The trickling filter solids contact water pollution control facility for the city of Ames, Iowa has successfully nitrified wastewater with trickling filters for the past decade. Both first stage, carbonaceous biochemical oxygen demand removing trickling filters (TFs) and second stage, nitrifying TFs (NTFs) remove significant quantities of ammonia from the wastewater. Based on operating data from January 1999 through December 2001, the average specific ammonia removal rate for the TFs was $1.5\times {10}^{-4}\mathrm{kg}N/\left(d{m}^2\right)$. Most probable number testing confirmed the presence of nitrifiers in the top media layer of both stages of trickling filters. An experiment was performed whereby flows to the TFs and NTFs were varied to test ammonia removal capabilities of the facility. During the experiment, the TFs removed an average of $2.4\times {10}^{-4}\mathrm{kg}N/\left(d{m}^2\right)$ and the NTFs removed an average of $1.5\times {10}^{-5}\mathrm{kg}N/\left(d{m}^2\right)$ due to low loading. Data collected during the study varied with operating conditions. It was compared to and used to calibrate NTF models. An empirical design model poorly fit the data, and a theoretically based model could not be calibrated well with apparent ammonia removal rates. A best-fit equation, dependent on hydraulic loading and influent ammonia concentration (adjusted for recirculation), was regressed directly to the data and is useful for describing nitrification in the Ames WPCF TFs.

The influence of four industrial wastewater types (organic chemical, dye, leather, and winery industries) on the exopolymeric substances (EPSs) constituents [protein, polysaccharide, and deoxyribonucleic acid (DNA)] and physicochemical properties of flocs (contact angle, surface charge, and bound water) was determined. Four laboratory scale activated sludge systems were used; they were fed with real wastewaters and were operated continuously under steady-state conditions. The EPS analysis showed that between 70 and 80% of the extracellular organic material could be attributed to protein. The proteins in the bound form were quantitatively greater than the soluble protein form in winery industry floc EPSs. The soluble protein ratios were higher than bound protein ratios in organic chemical, dye, and leather industries flocs. Lower amounts of protein and high DNA content were found in the EPS flocs grown on wastewaters containing more complex substrates (organic chemical, leather, and dye). Changing the wastewater from chemical, leather, and dye to that of winery industry resulted in an increase in protein and a decrease in DNA level of floc EPSs. The sludge surface of winery industry flocs were less hydrophobic (small contact angle) and more highly negatively charged compared to chemical, dye, and leather industry flocs. A higher sludge volume index (SVI), an indication of poorer settleability, was associated with both small amounts of EPS and protein and a high amount of DNA. Both high hydrophilic and negatively charged surfaces corresponded to lower levels of SVI and good settling properties. The results of this study demonstrated that wastewater type influenced the EPS composition and physicochemical properties of sludge. The surface properties (surface charge, contact angle, and bound water) and composition of EPS, apart from the carbohydrate content, governed the settleability of sludge and were important in controlling the SVI.

Eight rainfall-runoff events were examined from each of two small paved urban transportation land use watersheds ($A=544m^2$ and $300m^2$) in an attempt to distill multiple definitions of the first flush phenomenon into a consistent framework and examine common volumetric capture requirements. Results indicated that two separate criteria must be employed to describe the delivery of suspended sediment concentration (SSC) and total dissolved solids (TDS) as aggregate indices of entrained particulate and dissolved matter. The concentration-based first flush criterion is defined by high initial SSC or TDS concentration in the early portion of a rainfall-runoff event with a subsequent rapid concentration decline. In contrast, the mass-based first flush (MBFF) has several published forms, shown to be equivalent herein. The MBFF is defined generally as a disproportionately high mass delivery in relation to corresponding flow volume. For mass-limited events, mass delivery was skewed towards the initial portion of the event while the mass delivery in flow limited events tended to follow the hydrograph. This study also investigated published estimates of the water quality volume (WQV); assuming that an in-situ Control Strategy or Best Management Practice (BMP) captures and treats only this WQV, while flows in excess of this volume bypass the BMP. For the two watersheds, results indicate that a relatively large runoff volume must be captured to effect meaningful reductions in mass and concentrations (as event mean concentrations) despite a disproportionately high mass delivery early in the event. Results suggest the potential for misinterpretation of overall BMP effectiveness may be significant based on use of a number of these common published estimates based on a WQV.

A mathematical model of pollutant removal by wet ponds was developed based on the mass balance principle and the release–storage equation. The release–storage equation can be linear or nonlinear depending on the wet pond shape and the spillway crest features. If the exponent index of the storage relationship $\left(d\right)$ is equal to the index of the outflow equation $\left(b\right)$, the wet pond hydrological routing model is linear. Otherwise, the model is nonlinear. Substituting the release–storage equation into the continuity equation produces a nonlinear ordinary differential equation (ODE). A Runge–Kutta method was used to solve the resulting nonlinear ODE. When the ratio of indexes $d/b=2/3$, the hydrologic wet pond model reduces to a special case that leads to an implicit analytical solution. The pollutant removal and flow routing models were tested with data obtained from an actual wet pond for treating highway runoff. The predicted flow discharges and pollutant concentrations compared well with the observed data.

A method is presented to compute sensitivities of in-stream dissolved oxygen (DO) with respect to perturbations in the load vector and the reaction coefficients that make up the eutrophication cycle. It is shown that the direct sensitivity method, i.e., the repetitive solution of the direct problem, produces the desired information, however at a large computational cost. The utilization of the adjoint sensitivity method proves to be a much more efficient way to compute these sensitivities as large subsets of the sensitivity information domain can be easily extracted with just a few runs. It is found that for the given problem setup, in-stream DO is most sensitive to ammonia, effluent DO, algae, and carbonaceous biochemical oxygen demand (CBOD) loads. Additionally, the computed sensitivities vary considerably in their general trend over the simulation time. Sensitivities also are computed with respect to the reaction coefficients (26 total) that govern the interdependency of all constituents. Sediment oxygen demand proves to be the coefficient with the highest influence that is three orders of magnitude higher than the next set of coefficients comprised of reaeration, CBOD degradation, nitrification, and denitrification. All other coefficients have a negligible influence on DO concentrations. Computations are carried out using a two-dimensional model formulation applied to a long rectangular channel with varying width and slope and periodic but unsteady flow conditions.

This paper presents the development of an efficient strategy for achieving in-stream dissolved oxygen (DO) water quality standards (WQSs) via optimized point-load control strategies using the adjoint method. To this end, a least-squares-type objective function is formulated that measures the difference between desired (WQSs) and current DO concentrations at strategically selected monitoring points in the domain. The goal is to minimize the difference between actual DO concentration and the WQS, hence allowing time-variant loadings to utilize the assimilative capacity of the receiving water body at an optimal level. Time-variant discharge rates for a number of discharge locations are considered as control parameters, while different zone-specific critical DO levels are imposed as constraints. The selection of the control is kept flexible and a number of different scenarios are tested. First, only carbonaceous biochemical oxygen demand is used, which allows for a reduction of the number of equations that need to be solved. In the other tests, all constituents are switched on and different variables at each load node are selected as a control by first varying the concentrations individually, and then linking them through control of the volumetric flow rate. Optimization is achieved using a conjugate gradient search method, for which the gradients are computed through the solution of both the direct and adjoint problems. It is shown that the large amount of gradient information (parameter space has a dimension of several thousands) can be computed very efficiently using the adjoint, and that optimized results are achieved after only a few iterations irrespective of the initial guess. Computations are carried out using both two-dimensional model formulation applied to a long rectangular channel with varying width and slope and a model for the upper Potomac River estuary.

This study addresses the feasibility of using a continuum modeling approach to simulate pump-and-treat remediation of nonaqueous phase liquid (NAPL) contaminated fractured permeable formations. A simplified discrete fracture model, which incorporates permeable blocks with embedded parallel equidistant constant aperture fractures, was used to simulate the NAPL dissolution in an idealized fractured permeable formation. The applicability of this model is defined by the ranges of a dimensionless mobility number and interphase mass transfer coefficient. A continuum based model able to simulate phenomena predicted by the discrete fracture model has also been used. Three dimensionless parameters referring to organic solute advection and dispersion, and the continuum interphase mass transfer coefficient govern the performance of the continuum model. The nonlinear relationships between the discrete fracture and continuum model have been identified and formulated. However, the simplified conceptual models of this study may be inapplicable to many types of fractured formations. Ranges of possible use of the continuum modeling were determined in terms of dimensionless parameters. The discrete fracture and continuum approaches of this study can be useful for the preliminary evaluation of ideas concerning optimization of the remediation of NAPL contaminated fractured permeable formations.

Batch and electrokinetic experiments were conducted to investigate the removal of three different heavy metals, chromium(VI), nickel(II), and cadmium(II), from a clayey soil by using ethylenediamine tetraacetic acid (EDTA) as a complexing agent. The batch experiments revealed that high removal of these heavy metals $(62-100\%)$ was possible by using either a $0.1M$ or $0.2M$ EDTA concentration over a wide range of $\mathrm{pH}$ conditions $(2-10)$. However, the results of the electrokinetic experiments using EDTA at the cathode showed low heavy metal removal efficiency. Using EDTA at the cathode along with the $\mathrm{pH}$ control at the anode with $\mathrm{NaOH}$ increased the $\mathrm{pH}$ throughout the soil and achieved high (95%) $\mathrm{Cr}\left(\mathrm{VI}\right)$ removal, but the removal of $\mathrm{Ni}\left(\mathrm{II}\right)$ and $\mathrm{Cd}\left(\mathrm{II}\right)$ was limited due to the precipitation of these metals near the cathode. Apparently, the low mobility of EDTA and its migration direction, which opposed electroosmotic flow, prevented EDTA complexation from occurring. Overall, this study found that many complicating factors affect EDTA-enhanced electrokinetic remediation, and further research is necessary to optimize this process to achieve high contaminant removal efficiency.

A one-dimensional contaminant fate and transport model was developed to simulate reductive dechlorination of perchloroethene (PCE) in an anaerobic aquifer supplied with hydrogen via a gas-permeable membrane curtain. The model predicted that providing hydrogen at transfer rates equal to the reducing-equivalent demand associated with the groundwater PCE flux would mineralize 75% of the PCE-bound chlorine to chloride and, furthermore, that 0.55 moles of chloride would be released per mole of hydrogen transferred. Supplying higher hydrogen transfer rates was predicted to result in slightly lower dechlorination efficiencies and significantly lower dechlorination yields due to greater methanogenic growth and concomitant displacement of dehalorespirers away from the hydrogen-supply membranes. The model also predicted that high hydrogen-utilizing biomass concentrations would develop near the membranes, resulting in minimal hydrogen dispersal. Model predictions were qualitatively similar to results attained in experimental soil column studies; however, incorporation of homoacetogenesis and acetate utilization by dehalorespirers, as well as hydrogen production via fermentation of biomass decay products, would have improved agreement between model simulations and experimentally observed dechlorination performance.

The factors affecting sulfide buildup in gravity sewers are complex, consisting of biological and physical processes, both in the aqueous and the gas phases of the sewer. The rate of each of these processes varies (among other parameters) according to flow characteristics, temperature, and $\mathrm{pH}$. Under fast and turbulent flow conditions, the stripping of hydrogen sulfide into the gas phase may become the dominant process. The paper presents a semiempirical approach to the problem of quantifying hydrogen sulfide emission rates in sewers. Kinetics of hydrogen sulfide emission as a function of hydraulic parameters was measured in the laboratory using methods adopted from flocculation theory. A flocculation unit was used to impart a selected velocity gradient $\left(G\right)$ into the water, and sulfide concentration was measured with time. The process was repeated for a number of $G$ values. Regression analysis was then used to fit the rate of hydrogen sulfide emission equation against $G$. An equation was developed linking $G$ to $H_L$ (head loss) in sewers assuming plug flow conditions. The hydraulic model and the kinetic model were linked (via $G$) to give the desired rate equation for hydrogen sulfide emission along a sewer line. The model was used to predict $H_{2}S$ emission from a uniform flow sewer and the effect of parameters such as $\mathrm{pH}$, sewer slope and degree of fullness was studied. As expected, results show that low $\mathrm{pH}$, high slope, and low degree of fullness enhance emission rates. Reasonable agreement was attained when the model output was compared with measured results from a field test sewer in Virginia, South Africa, under conditions where sulfide stripping was the rate-dominant process.

This paper presents a life-cycle inventory (LCI) for solid waste composting. Three LCIs were developed for two typical municipal solid waste (MSW) composting facilities (MSWCFs) and one typical yard waste (YW) composting facility (YWCF). Municipal solid waste was assumed to comprise three organic components, food wastes, yard wastes, and mixed paper, as well as various inorganic components. Total costs, combined precombustion, and combustion energy requirements and 29 selected material flows—also referred to as LCI coefficients—were calculated by accounting for both the processes involved in originally producing, refining and transporting a material used in the facility as well as consumption during normal facility operation. Total costs ranged from $\$15/t$ to $\$50/t$ and energy requirements from $29\mathrm{kw}h/t$ to $167\mathrm{kw}h/t$ for a YWCF and a high quality MSW composting facility, respectively. More than 90% of the overall ${\mathrm{CO}}_2$ emissions in all facilities were due to the biological decomposition of the organic substrate, while the rest was due to fossil fuel combustion.

A bench-scale study was completed to assess the potential for the bioremediation of smear zone soils at a leaking underground storage tank site in Fort Drum, N.Y. The study demonstrated that the vertical distribution of total petroleum hydrocarbons (TPHs) within the smear zone corresponded to differences in microbial biomass and activity. The soil core featured the lowest TPH and highest biomass levels at the top of the smear zone, and the highest TPH and median biomass levels at the bottom of the smear zone. Phospholipid fatty acid biomarkers indicate that gram-positive bacteria and fungi were associated with the in situ TPH biodegradation. The microcosm study shows that microbial respiration at the top of the smear zone was much stronger than that at the bottom of the smear zone. At the top of the smear zone, microbial mineralization was faster under the saturated condition, whereas at the bottom of the smear zone, microbial mineralization was faster under the unsaturated condition. The column study shows that the biodegradation rates of bioventing and biosparging for the specific site did not differ substantially.

A low-temperature $O_2$ trap was designed in order to achieve strict anoxic conditions. The work was motivated by observation of slow oxidation of $\mathrm{Fe}\left(\mathrm{II}\right)$ in an anaerobic chamber, leading to an estimated $3.8\times {10}^{-7}\mathrm{atm}$ $O_2$ (well below the $O_2$ monitor detection limit) despite recirculation of the $N_2:H_2$ atmosphere across a $\mathrm{Pd}$ catalyst. Very low $O_2$ activity inside an “anaerobic” chamber can result in erroneous conclusions regarding oxidation-reduction reactivities in anoxic environments. The $O_2$ trap consisted of two sequential barrier suspensions with $93.2\mathrm{mM}$ $\mathrm{Fe}\left(\mathrm{III}\right)$ as ferric hydroxide, $0.90\mathrm{mM}$ ${\mathrm{FeCl}}_2$, and pH 8.1. The partial pressure of $O_2$ was estimated to be less than $7.5\times {10}^{-9}\mathrm{atm}{O}_2$ when reactors were attached to the traps, based on no observed oxidation of $\mathrm{Fe}\left(\mathrm{II}\right)$.