Journal of Environmental Engineering

Eleven statewide waste characterization studies were compared to assess variation in the quantity and composition of waste after separation of recyclable and compostable materials, i.e., discarded waste. These data were also used to assess the impact of varying composition on sequestered carbon and methane yield. Inconsistencies in the designation of waste component categories and definitions were the primary differences between study methodologies; however, sampling methodologies were consistent with recommended protocols. The average municipal solid waste (MSW) discard rate based on the statewide studies was $1.90\text{kg}\text{MSW}{\text{person}}^{-1}{\text{day}}^{-1}$ , which was within the range of two national estimates: 2.35 and $1.46\text{kg}\text{MSW}{\text{person}}^{-1}{\text{day}}^{-1}$ . Dominant components in MSW discards were similar between studies. Organics (food waste, yard trimmings), paper, and plastic components averaged $23.6\pm 4.9\%$ , $28.5\pm 6.5\%$ , and $10.6\pm 3.0\%$ of discarded MSW, respectively. Construction and demolition (C&D) waste was $20.2\pm 9.7\%$ of total solid waste discards (i.e., MSW plus C&D). Based on average statewide waste composition data, a carbon sequestration factor (CSF) for MSW of 0.13 kg C dry $\text{kg}{\text{MSW}}^{-1}$ was calculated. For C&D waste, a CSF of 0.14 kg C dry kg C and D ${\text{waste}}^{-1}$ was estimated. Ultimate methane yields $\left(L_o\right)$ of 59.1 and $63.9{\text{m}}^3$ ${\text{CH}}_4$ wet Mg ${\text{refuse}}^{-1}$ were computed using EPA and state characterization study data, respectively, and were lower than AP-42 guidelines. Recycling, combustion, and other management practices at the local level could significantly impact CSF and $\left(L_o\right)$ estimates, which are sensitive to the relative fraction of organic components in discarded MSW and C&D waste.

Pressure-treated wood is often disposed of in landfills in the United States, very frequently in construction and demolition (C&D) debris landfills. C&D debris landfills in many states are not equipped with liner systems to protect groundwater. With the voluntary withdrawal of chromated copper arsenate (CCA) treated wood for most residential applications in January 2004, copper-based wood preservatives, including alkaline copper quaternary (ACQ), are more widely used. To evaluate the impact of metal losses from ACQ-treated wood disposed in C&D debris landfills and compare to those of CCA-treated wood under similar conditions, leachates from three simulated C&D debris landfills (lysimeters) were collected and analyzed for over a period of one year. The wood component in one lysimeter (the control lysimeter) contained pallet wood; the second lysimeter contained CCA-treated wood, and the third contained ACQ-treated wood. Each lysimeter was buried in an active landfill for temperature control. Several batch leaching tests [including the standardized toxicity characteristic leaching procedure (TCLP) and the synthetic precipitation leaching procedure (SPLP)] were also conducted for comparison purposes. Although the two lysimeters containing treated wood had elevated copper concentrations within the waste matrix, the concentration in the leachate samples from these lysimeters was below detection for Cu $\left(<4\mu \text{g}/\text{L}\right)$ throughout the duration of the experiment, likely a result of precipitation as copper sulfide mineral in the reducing conditions of the simulated C&D landfills. As expected, the lysimeter containing CCA-treated wood showed elevated concentrations of arsenic and chromium, with maximum concentrations of 1.16 mg/L and 0.2 mg/L respectively. Greater amounts of boron (B) leached from ACQ-treated wood than CCA-treated wood or pallet wood debris. The results suggest that copper leaching will not be a major concern upon the disposal of ACQ-treated wood in C&D debris landfills. Arsenic leaching from CCA-treated wood remains a concern for unlined C&D debris landfills.

Changes in chlorine residual concentrations in water distribution systems could be used as an indicator of microbial contamination. Consideration is given on how to model the behavior of chlorine within the distribution system following a microbial contamination event. Existing multispecies models require knowledge of specific reaction kinetics that are unlikely to be known. A method to parameterize a rate expression describing microbially induced chlorine decay over a wide range of conditions based on a limited number of batch experiments is described. This method is integrated into EPANET-MSX using the programmer’s toolkit. The model was used to simulate a series of microbial contamination events in a small community distribution system. Results of these simulations showed that changes in chlorine induced by microbial contaminants can be observed throughout a network at nodes downstream from and distant to the contaminated node. Some factors that promote or inhibit the transport of these chlorine demand signals are species-specific reaction kinetics, the chlorine concentration at the time and location of contamination, and the system’s unique demand patterns and architecture.

The effect of conventional and booster chlorination on chlorine residuals and trihalomethane (THM) formation in drinking water distribution systems was modeled using the EPANET hydraulic modeling software. The model results suggest that booster chlorination may allow utilities to meet disinfection goals better by carrying chlorine residuals to remote points in the distribution system while lowering the total mass of chlorine applied to the system. The model results suggest that booster chlorination may provide the greatest advantages to points in the distribution system located near storage tanks by providing a more consistent chlorine residual and possibly reducing THM formation. A new version of the EPANET model, the EPANET Multispecies model, was also used to compare chlorine decay due to reactions in the bulk fluid and reactions occurring at the pipe wall. The results suggest that chlorine decay due to wall reactions can be very significant at remote points in the distribution system. Additionally, if THMs are assumed to form primarily through reactions in the bulk fluid, use of the new EPANET Multispecies software allows for calculation of THM formation based solely on chlorine reactions in the bulk fluid rather than on overall chlorine decay.

The shield or shadow effect of large particles on bacteria undergoing irradiation from ultraviolet light in effluent of treated wastewater would greatly reduce the effectiveness of ultraviolet disinfection. The objective of this work is to evaluate the newly proposed method of applying a high pressure water jet (HPWJ) as a pretreatment step in combination with ultraviolet irradiation to improve the disinfection efficiency of wastewater by breaking up the large particles (reducing the shield or shadow effects). Pretreatment experiments with two HPWJ units having different water pressure output (6.9 and 13.1 MPa) and nozzles were conducted in the study on primary clarifier overflow (PCO). The experiment results revealed that the particle size distributions (PSDs) in the test samples were apparently shifted toward the small size range and the total number of particles has been greatly increased after undergoing treatment with the HPWJ. In general it is assumed that bioparticles larger than a certain size $\left(10\mu \text{m}\right)$ are difficult to disinfect by ultraviolet. After the HPWJ treatment the relevant particle size range $>12.5\mu \text{m}$ in the test samples of the PCO was reduced on average by half for tested HPWJ settings. Also, a primary comparison showed that under a similar amount of energy consumption, the HPWJ performed better than the ultrasonic method did in terms of effectively breaking up large particles. As expected, the changes of PSD notably affect the efficiency of ultraviolet disinfection as tested with the collimated beam. The E. coli and total coliform tests showed that pretreated PCO by the HPWJ enhanced the efficiency of ultraviolet disinfection by more than 1 log unit compared to treatment by ultraviolet alone when the ultraviolet dose was in the $20–30\text{mJ}/{\text{cm}}^2$ range. Multiple pretreatment with the HPWJ was also tested, which showed the advantage of breaking up slightly more particles, but with less enhancement of ultraviolet disinfection compared to a single pretreatment. This could possibly be due to the reduced ultraviolet transmission in the test sample caused by the presence of a larger number of suspended particles. The E. coli test results indicated that the well known “tailing region” of typical patterns of the ultraviolet dose-response curve was eliminated or decreased after the PCO was treated by HPWJ. The application of a HPWJ step might also be beneficial in terms of cost-effectiveness. With HPWJ pretreatment it only takes about one-third of the ultraviolet light exposure time to achieve the same disinfection results as ultraviolet treatment alone in the collimated beam test. The experimental results in this study showed some degree of variation for similar experiments. This is not unusual for this kind of study, due to many uncontrolled factors such as characteristics of tested PCO samples, random error in the microbiology tests and so on. However, these experiments clearly showed that the proposal of using the HPWJ to pre-treat wastewater could be a very attractive alternative to enhance ultraviolet disinfection efficiency.

A probabilistic approach to obtain theoretical residence-time distribution (RTD) functions for series of reactors with possible stagnation, bypassing, and recycle is presented. It is shown that most known RTD functions can be obtained from probability arguments alone, thus avoiding abstract Laplace transform mathematical techniques and providing additional physical insight. Several new RTD models for reactors in series are derived, based exclusively on using the binomial probability distribution to describe the passage of a particle through the treatment train with bypassing and stagnation possible at each individual reactor. The proposed RTD models are validated with travel time computer simulation of a large number of particles through the series of reactors. A MSExcel based computer procedure was programmed to obtain the nonideal flow parameters by minimizing the squared sum of the differences between tracer test data and the derived unit’s RTD function. The least squares parameter estimation procedure was used to fit theoretical RTDs to tracer data collected from real water treatment units with different hydraulic behavior at two locations in Mexico.

Settling velocity is an important constitutive parameter of particulate matter (PM) transported by runoff. Settling velocity is either explicitly or implicitly utilized when designing or modeling unit operations, and in situ or watershed controls for urban rainfall-runoff. Utilizing two common settling devices, a settling column and an Imhoff cone, settling velocities of discrete noncolloidal particles in source area urban rainfall-runoff were measured. A comparison of settling models applicable to discrete (Type I) PM settling was developed. Models were compared to measured results across the noncohesive silt- and sand-size PM gradation from 2 to $2,000\mu \text{m}$ , utilizing measured particle-size distributions (PSDs) and specific gravity. Results indicate that Newton’s Law can reproduce measured settling velocity when measured inputs of PM diameter, specific gravity, and temperature are utilized. Alternative models to Newton’s Law (in the Stokesian regime) did not improve agreement with measured settling velocities determined using PSDs from laser diffraction. Settling velocity distributions using Newton’s Law were applied for two limiting classes of storm events loading a screened hydrodynamic separator (HS) at an urban watershed. Results indicate that for a low flow and high flow event, Newton’s Law and a simple ideal overflow model of the HS could reproduce PM separation and the PSD of eluted PM (2 to $\sim 250\mu \text{m}$ ) within 17% of measured results on a gravimetric basis.

Recent studies have demonstrated that the annual rates of consumer battery litter on urban pavements can be as high as 215 batteries per hectare of pavement and 0.4 batteries per meter of curb. As littered batteries deteriorate, they release components of potential environmental significance (Ag, Ba, Cd, Cr, Cu, Hg, Li, Mn, Na, Ni, Pb, Ti, Zn) and add to urban storm water contamination. However, consumer batteries come in many sizes, brands, and power chemistries, all of which may deteriorate at different rates in response to hostile physical and chemical conditions. The work presented here examines the impact of tire load cycles on battery litter. Tests were conducted on 735 individual cells representing 42 brands of AAA, AA, C, and D cell batteries. Results indicate that there are significant differences in the deterioration properties of battery brands and power chemistry types. Alkaline batteries are most resistant to deformation under tire loading. Nearly 50% survive their first tire load and about 30% survive 100 tire load cycles without rupturing. Zinc chloride/zinc carbon (ZnCl/ZnC) batteries are less resistant to deformation. Only approximately 10% survive their first load cycle unruptured. Cell size appears to have little influence on traffic-induced rupture. Based on general battery litter profiles, mass loading calculations for moderate traffic areas should assume that 70% of littered cells have experienced rupture release. This estimate should be increased to 85% for heavy traffic areas. These estimates may be refined if more detail is available on the brand and type distribution of site battery litter.

While constructed treatment wetlands are very efficient at polishing nitrate from secondary effluent, they are much less effective at removing ammonia. A key factor that limits ammonia oxidation via biological nitrification in vegetated wetlands is low levels of dissolved oxygen. This study evaluated the effectiveness of side-stream oxygenation to enhance ammonia removal in replicate surface-flow experimental mesocosms containing wetland sediment and plants (Typha spp.). Mesocosms had a water volume of 29.5 L, a hydraulic retention time of 5 days, and a hydraulic loading rate of 4.3 cm/d, and were loaded with synthetic secondary effluent contain 10 mg-N/L of ammonia. Relative to nonoxygenated controls, oxygenation increased ammonia removal rates by an order of magnitude. Areal removal rates increased from $40\text{mg-N}/{\text{m}}^2/\text{d}$ to $450\text{mg-N}/{\text{m}}^2/\text{d}$ , concentration removal efficiency increased from 10 to 95%, and area-based first-order removal rates increased from $<2\text{m}/\text{year}$ to 50–75 m/year. Ammonia removal rates in oxygenated mesocosms were 2- to 4-fold higher than rates reported for full-scale constructed wetlands treating secondary effluent. Results show that oxygen-activated nitrification wetlands, a hybrid of conventional oxygenation technology and wetland ecotechnology, hold promise in economically enhancing rates of ammonia removal and shrinking the wetland area needed to polish ammonia-dominated secondary effluent. Further study is needed to confirm that oxygenation can promote high rates of ammonia removal at the field scale.

The effect of subsurface barrier on the motion of the saltwater—freshwater interface in coastal aquifers is analyzed for wide ranging freshwater pumping scenarios. A Galerkin finite-element model considering sharp interface approach is used for this purpose. A semi-pervious subsurface barrier extending up to impervious bottom of the aquifer is considered at certain distance inland, parallel to the seacoast. The effect of barrier is analyzed in checking the advancement of the saltwater-freshwater interface under different scenarios of freshwater withdrawals at seaward and landward locations of the barrier and compared with nonbarrier conditions. The results indicated that barrier is able to check the advancement of the intrusion significantly and in certain cases, the progress is completely stalled for withdrawals on the landward side. Also, marked variations in the interface profile are observed as compared to no barrier condition, especially, for the seaward freshwater developments. From the model, nearest possible locations from the seacoast have been worked out for the safe withdrawal of freshwater where their effects are negligible on the saltwater advancement.

The coexistence of heavy metals and polycyclic aromatic hydrocarbons (PAHs) at many of the contaminated sites poses a severe threat to public health and the environment. Very few technologies, such as soil washing/flushing and stabilization/solidification, are available to remediate such sites; however, these technologies are ineffective and expensive to treat contaminants in low permeability clayey soils. Previous studies have shown that electrokinetic remediation has potential to remove heavy metals and organic compounds when they exist individually in clayey soils. In the present study, the feasibility of using surfactants and organic acids sequentially and vice versa during electrokinetic remediation was evaluated for the removal of both heavy metals and PAHs from clayey soils. Kaolin was selected as a model clayey soil and it was spiked with phenanthrene and nickel at concentrations of 500 mg/kg dry each to simulate typical field mixed contamination. Bench-scale electrokinetic experiments were performed with the sequential anode conditioning with: (1) 1 M citric acid followed by 5% Igepal CA-720; (2) 1 M citric acid followed by 5% Tween 80; and (3) 5% Igepal CA-720 followed by 1 M citric acid. A periodic voltage gradient of 2 V/cm (with 5 days on and 2 days off cycles) was applied in all the tests. A removal of about 96% of phenanthrene was observed in the test with 5% Igepal CA-720 followed by 1 M citric acid sequence. Most of the nickel $\left(>90\%\right)$ migrated from anode to cathode in this test; however, it precipitated in the section very close to the cathode due to the high pH conditions. Conversely, the removal efficiency of nickel was about 96 and 88% in the tests with 1 M citric acid followed by 5% Igepal CA-720 sequence and 1 M citric acid followed by 5% Tween 80 sequence, respectively. However, the migration and removal efficiency of phenanthrene in both of these tests were very low. Overall, it can be concluded that the sequential use of 5% Igepal CA-720 followed by 1 M citric acid may be an effective remedial strategy to remove coexisting heavy metals and PAHs from clayey soils.

Cost-efficient design for remediation of light nonaqueous phase liquids (LNAPL) from subsurface has become an essential task for site managers in North America. Although many related studies were conducted, there was still lack of efficient and computationally attractive method in such an area. In this study, a response-surface-based optimization approach was developed for enhancing remediation of LNAPL recovery. A two-dimensional numerical model was provided to simulate LNAPL transport during remediation. A dual response regression model was proposed for establishing a linkage between remediation actions and system responses. A nonlinear VFPR management model was then established for generating desired operating conditions. A petroleum-contaminated site in western Canada was used to demonstrate the applicability of the proposed method. The results demonstrated that the DRS model could be used as an effective proxy for simulation models and the optimal solutions from management model led to better cost-effectiveness compared with those from nonoptimized ones. The proposed optimization method was computationally attractive and required simple mathematical manipulations. It was of great practical values for supporting site remediation actions.

Iron sulfide minerals commonly found in natural anoxic Fe-S systems have been shown to reductively transform chlorinated hydrocarbons including trichloroethylene (TCE). In the present study, we tested the feasibility of applying an Fe(II) solution to a TCE-contaminated aquifer groundwater under simulated sulfide reducing conditions to enhance reductive transformation of TCE to nontoxic compounds. To achieve this goal, iron sulfide particles were precipitated under a range of pH and Fe:S molar ratios in aquifer groundwater samples from the Dugway Proving Grounds, Utah. Batch tests for abiotic reductive dechlorination of TCE were performed using the precipitates to establish the conditions for most favorable solids for dechlorination. Under all experimental conditions, the solids formed consisted mainly of mackinawite, a tetragonal reduced iron monosulfide ${\text{FeS}}_{1\text{-}x}$ . However, the precipitation conditions strongly affected the reactivity of the mackinawite particles formed. The results indicated that addition of Fe(II) to a sulfur-rich groundwater could reduce free sulfide concentration to levels below what is considered inhibitory for biotic dechlorination of TCE, and that higher pH, an Fe:S ratio close to 1, and the addition of citrate produced the most effective mackinawite for abiotic TCE dechlorination.

Among petroleum-hydrocarbon pollutants, fuel-oil is more difficult to treat compared to gasoline and diesel fuel. The objectives of this bench-scale study were to: (1) develop a two-stage remedial system consisting of surfactant washing followed by Fenton-like oxidation process to remediate fuel-oil contaminated soils; (2) evaluate the effects of residual surfactant and soil organic matter (SOM) on the efficiency of Fenton-like oxidation; (3) evaluate the effect of potassium dihydrogen phosphate $\left({\text{KH}}_2{\text{PO}}_4\right)$ addition on the stability of ${\text{H}}_2{\text{O}}_2$ and oxidation efficiency; and (4) evaluate the possible oxidation products after the oxidation process. In the surfactant washing stage, biodegradable surfactant, Simple Green (SG) $\left(50\text{g}{\text{L}}^{-1}\right)$ , was applied to flush fuel-oil contaminated soils with initial total petroleum-hydrocarbons (TPHs) concentration of $50,000\text{mg}{\text{kg}}^{-1}$ . Results show that approximately 90% of TPH could be removed after washing with 45 pore volumes (PVs) of SG followed by 25 PVs of deionized water, while the soil TPH concentration dropped from 50,000 to $4,950\text{mg}{\text{kg}}^{-1}$ . In the Fenton-like oxidation stage with initial soil TPH concentration was approximately $4,950\text{mg}{\text{kg}}^{-1}$ , TPH removal efficiency can be significantly increased with increased ${\text{H}}_2{\text{O}}_2$ concentrations. Results also reveal that residual SG and SOM would compete with TPH for oxidants and cause the decrease in oxidation efficiency. An “oxidation-sorption-desorption-oxidation” scheme for soil TPH was observed in this experiment due to the initial sorption of TPH on SOM. Results show that an addition of 2.2 mM of ${\text{KH}}_2{\text{PO}}_4$ could increase the stability and half-life of ${\text{H}}_2{\text{O}}_2$ , but caused the decrease in TPH removal efficiency. The oxidation potential of Fenton-like process was not capable of completely oxidizing fuel-oil to nontoxic end products. The observed by-products after oxidation process contained carboxyl groups with molecular weights similar to their parent compounds. Results from this study indicate that the two-stage remedial system is a promising technology for fuel-oil contaminated soil treatment.

Identification of bacteria that can utilize a wide range of nitroaromatic compounds will allow development of more effective biological treatment methods in industrial wastewater treatment processes and environmental remediation efforts. A new strain of Rhodococcus opacus capable of mineralizing or transforming nitroaromatic and nitramine compounds of importance was isolated. Specifically, the bacterium were found to utilize 2,4,6-trinitrophenol (TNP) as a sole carbon and nitrogen source and have a yield coefficient of 0.16 g cells-N/g TNP-N. The Edwards model was found to provide the best fit to the data and the estimated inhibited growth parameters ${\mu }_{\text{max}}$ , $K_S$ , and $K_I$ were $0.58{\text{h}}^{-1}$ , 25, and $112\text{mg}/\text{L}$ , respectively. It was found that the TNP-degraders could degrade 2,4-dinitrophenol as the sole carbon and nitrogen source and utilize 1,3,5-trinitrobenzene as the sole nitrogen source. Additionally, the results show that the isolates are able to cometabolize 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 4-nitrophenol (4-NP), and 2,4- and 2,6-dinitrotoluene (2,4 and 2,6-DNT) to some extent when TNP is also present.

This research developed and demonstrated a technology to liberate Hg adsorbed onto powdered activated carbon (PAC) by the TOXECON process using pilot-scale high temperature air slide (HTAS) and bench-scale thermogravimetric analyzer (TGA). The HTAS removed 65, 83, and 92% of Hg captured with PAC when ran at $900°\text{F}$ , $1,000°\text{F}$ , and $1,200°\text{F}$ , respectively, while the TGA removed 46 and 100% of Hg at $800°\text{F}$ and $900°\text{F}$ , respectively. However, addition of ${\text{CuO}–\text{Fe}}_2{\text{O}}_3$ mixture and CuCl catalysts enhanced Hg removal and PAC regeneration at lower temperatures. ${\text{CuO}–\text{Fe}}_2{\text{O}}_3$ mixture performed better than CuCl in PAC regeneration. Scanning electron microscopy images and energy dispersive X-ray analysis show no change in PAC particle aggregation or chemical composition. Thermally treated sorbents had higher surface area and pore volume than the untreated samples indicating regeneration. The optimum temperature for PAC regeneration in the HTAS was $1,000°\text{F}$ . At this temperature, the regenerated sorbent had sufficient adsorption capacity similar to its virgin counterpart at 33.9% loss on ignition. Consequently, the regenerated PAC may be recycled back into the system by blending it with virgin PAC.

This research investigated the sorption and transport of MS-2 in saturated sand under a wide range of salinities using one-dimensional column experiments. The salinity varied from 0 ppt (fresh water) to 30 ppt. The MS-2 in the fresh water showed very weak adsorption due to having the same negative charge as the sand. Increasing the salinity concentrations dramatically enhanced MS-2 adsorption. The MS-2 breakthrough revealed the existence of reversible and irreversible sorption sites in the sand. Salinity increased MS-2 attachment by compressing the double layers of MS-2 and reversible sorption sites. The salinity also changed some reversible sorption sites into irreversible sorption sites by reversing to positive surface charges of silica powder. An advection-dispersion-sorption model with a two-site reversible-irreversible kinetic sorption was developed to describe MS-2 breakthrough under different salinity conditions. The sorption parameters were estimated and their independence was evaluated by minimizing the total squared error of the MS-2 data. The proposed model showed good agreement with the experimental data for a wide range of salinity levels from fresh water to near seawater. The strong sorption shown in the MS-2 breakthrough at high salinity levels above 8 ppt was able to distinguish the proposed model from other sorption models. This study promotes the understanding of the viral sorption with salinity and provides a useful model for coastal management of viral migration in saline coastal groundwater.

The electrooxidation of polynuclear aromatic hydrocarbons (PAHs) in solution was investigated. Most of the PAHs compounds are toxic and hardly biodegradable, so that a chemical or physicochemical treatment is required. In this paper, we reported treatment of synthetic creosote oily effluent (COE) containing several PAHs by using $\text{Ti}/{\text{IrO}}_2$ and $\text{Ti}/{\text{SnO}}_2$ circular or cylindrical mesh anode electrodes. COE was prepared with distilled water and a commercial creosote solution in the presence of an amphoteric surfactant (CAS). In addition to anode material, different operating parameters were investigated such as current density, reaction time, recycling flow rate, and oxygen injection flow rate. The first series of experiments carried out in the recirculating batch reactor showed that circular $\text{Ti}/{\text{SnO}}_2$ electrode was found to be more effective in removing PAHs than circular or cylindrical $\text{Ti}/{\text{IrO}}_2$ electrodes. Current density and retention time played important roles for PAHs degradation efficiency, whereas circulation flow rate and oxygen injection slightly influenced the removal efficiency. Finally, the best and simplest operating conditions (82–84% of PAHs removal) determined for PAHs degradation in COE were obtained at a current density of $15\text{mA}/{\text{cm}}^2$ through 90 min of treatment with a recycling rate of 3.6 L/min but without ${\text{O}}_2$ injection in the close loop. Likewise, in the recirculating batch tests, PAHs decomposition exhibited behaviors of the fist-order reaction with a rate coefficient $\left(k\right)$ of $0.015{\text{min}}^{-1}$ . The energy consumption was $7.5\text{kWh}/{\text{m}}^3$ . The second series of experiment using successively batch and continuous treatment of COE shows that the percentage of PAHs degradation could be maintained above 80% up to 18 h of treatment, thereafter, removal efficiency decreased owing to the formation of organic substances on the electrodes surface.

Selective precipitation of phosphate from fluoride-containing wastewater was studied using magnesium salts. Wastewater sampled from a semiconductor manufacture was found to contain 936 mg/L of fluoride $\left({\text{F}}^{-}\right)$ , 640 mg/L of sulfate $\left({\text{SO}}_4^{2-}\right)$ , 118 mg/L of phosphate $\left({\text{PO}}_4^{3-}\right)$ , and 26.72 mg/L ammonium $\left({\text{NH}}_4^{+}\right)$ . Magnesium chloride $\left({\text{MgCl}}_2\right)$ was more effective than magnesium oxide (MgO) in inducing precipitation reactions between magnesium and phosphate. Effects of both molar ratio $\left(\left[{\text{Mg}}^{2+}\right]:\left[{\text{PO}}_4^{3-}\right]\right)$ and pH were examined and experimental results were compared with those from equilibrium modeling by PHREEQC. A total of 41.72% of phosphate was removed and recovered when at molar ratio of 3:2 and pH of 10. Precipitation of phosphate increased with increasing molar ratio and 66.19% of phosphate was recovered when molar ratio was 3:1. Precipitation of phosphate increased as pH changed from 8 to 10. However, it decreased as pH increased from 10 to 12, probably because of competition between phosphate and hydroxyl $\left({\text{OH}}^{-}\right)$ ions. Solid precipitates were characterized by field emission scanning electron microscope with energy dispersive spectrometer and X-ray diffraction. In accordance with theoretical modeling, it was found that the precipitate in pH range of 8–10 was predominantly bobierrite $\left[{\text{Mg}}_3{\left({\text{PO}}_4\right)}_2.8{\text{H}}_2\text{O}\right]$ and some amorphous precipitates. However, brucite $\left[\text{Mg}{\left(\text{OH}\right)}_2\right]$ precipitate would start to form when pH became higher than 10. Results showed that ${\text{MgCl}}_2$ can be a selective precipitation reagent for phosphate removal from semiconductor wastewater since it does not form precipitate with fluoride, sulfate, and ammonium.

In this paper, the feasibility and efficacy of chromium (Cr(VI)) removal using three different kinds of tree leaves viz. Emblica officinalis, Azadirachta indica, Eucalyptus agglomerata, and the activated carbon is examined through batch and continuous flow experiments. Pretreatments were given to the selected tree leaf powders to remove the natural pigments and lignin present. Batch and continuous flow experiments have been conducted to study the kinetics of adsorption, effects of pH, adsorbent dose, contact time, bed depth, flow rate, and initial Cr(VI) concentration on Cr(VI) adsorption onto the selected adsorbents. The adsorption capacity is observed higher for Emblica officinalis followed by Eucalyptus agglomerata and Azadirachta indica. The adsorption equilibrium is reached in less than 30 min and the maximum Cr(VI) uptake occurred at pH 3.0 under the test conditions. The results are also compared with the commercially available activated carbon. A mathematical model incorporating diffusion, advection, and mass transfer mechanisms available in the literature has been simplified and is then tested to simulate the laboratory and literature data. A simple method for the determination of saturation Cr(VI) concentration along the length of column has been presented. The study reveals that the model incorporating the molecular diffusion and the mass transfer mechanisms simulates better the Cr(VI) adsorption onto tree leaf powders than the literature model and the advection term plays only a negligible role due to low flow rates applied during the experiments. The model parameters, i.e., axial dispersion coefficient, “ $D_L$ ” and the external mass transfer coefficient, “ $k_f$ ” are found in the order of ${10}^{-5}–{10}^{-6}{\text{m}}^2/\text{s}$ and ${10}^{-9}–{10}^{-11}\text{m}/\text{s}$ , respectively.