Journal of Environmental Engineering

This study reports the sustainability of a large semicontinuous anaerobic reactor (3,000 L) for biogas production from food wastes for a period of 324 days. The methane potential ranged from 418.3 to $13.9{\mathrm{mL}\mathrm{CH}}_4/{\mathrm{g}\mathrm{VS}}_{\mathrm{added}}$ during the experiment. An average 54.8% reduction was observed in biogas production during the winter season compared with summer. Results showed that food wastes could be utilized in a low-temperature range (0°C–15°C) as well as a high-temperature range (15°C–27°C). The total microbial community analysis revealed mixed consortia representing acetotrophic, hydrogenotrophic, and methylotrophic archaea (e.g., Methanobacterium, Methanosarcina, and Methanospirillum), indicating a syntrophic pathway for methane production. The bacterial community was well represented by two major phyla: Proteobacteria (31.3%) and Bacteroidetes (14.92%). The highest maximum specific biogas production ($R_m$) and maximum biogas production potential ($A$) were $134.5{\mathrm{mL}\mathrm{CH}}_4/\mathrm{g}\mathrm{VS}/\mathrm{day}$ and $34.1{\mathrm{L}\mathrm{CH}}_4/\mathrm{g}\mathrm{VS}$ for modified Gompertz and first-order equations, respectively. The methane production data were modeled, and showed substantial agreement with experimental results; however, the first-order equation had the best agreement with the experimental data ($R^2=0.99$).

Unit operations (UO) for urban water clarification are commonly implemented for management of particulate matter (PM), PM-associated chemicals, and microbial species, primarily through sedimentation. In this study, an Eulerian-equilibrium Eulerian computational fluid dynamics (CFD) model is developed to simulate the transport and fate of dilute PM suspensions in UO clarification systems. PM sedimentation and resuspension are considered with PM bed boundary conditions. For these simulations, two Robin boundary conditions are proposed for the capacity-limited resuspension (no-flux condition) of the PM bed layer and nonequilibrium sedimentation conditions. The applicability of the numerical model to PM transport is tested through four case-study databases including two laboratory flumes and two full-scale commercial UO systems designed for PM sedimentation. The numerical implementation of the proposed model is based on an open-source C++ framework. The detailed numerical structure of the model is given. For PM sedimentation with no resuspension, the numerical and physical models of PM separation are in excellent agreement for these systems. For PM resuspension, numerical and physical models of eluted PM concentration as a function of hydraulic stress (flow rate) and PM gradation are in good agreement. The numerical and mechanistic robustness of the proposed model represents an improvement compared with a representative Lagrangian particle method of previous studies. The proposed model can be implemented for design iterations and regulatory certification of UO clarification systems.

The preparation of the water-accommodated fraction (WAF) is important for evaluating the toxicity of oil. The Chemical Response to Oil Spills: Ecological Research Forum method, which uses a magnetic stirrer in aspirator bottles, is commonly used. Thus, it is investigated herein focusing on the hydrodynamics. The particle image velocimetry technique was applied to measure the fluid velocity and mixing characteristics of three bottles 2, 4, and 20 L in volume, respectively. The hydrodynamics and mixing performance of cases without an air tube and cases in which the air tube occupied approximately 22.5% of the water height were considered. The energy dissipation rates were highest near the center of the bottles, and the average values ranged from $5\times {10}^{-5}\mathrm{W}/\mathrm{kg}$ in the no-air-tube bottles to approximately $1.1\mathrm{W}/\mathrm{kg}$ in the air-tube bottles. The latter value is comparable to non-breaking waves or small breakers (0.4 m in height). Knowing the rotation speed, one can use the results and the computed power number, $K_T$, to obtain the energy dissipation rates in comparable bottles. A new concept, vessel efficiency ($VE$), was introduced and represents the product of the energy dissipation rate in the impeller region and the residence time of water in this region. The $VE$ of cases without and with air tube vessels showed that $VE$ significantly increased with an air tube in each bottle. The $VE$ for cases with an air tube also increased with bottle size due to the high residence time in the impeller region. The hydrodynamics investigation herein provides a foundation for interpreting the droplet size distribution in actual WAF and chemically enhanced water-accommodated fraction tests.

Pseudomonas aeruginosa is an opportunistic pathogen that can grow and proliferate in biofilms in premise plumbing systems and is capable of causing infections through the ocular, inhalation, and dermal routes of exposure. When the pathogen colonizes showerheads and faucets, there is risk of exposure to contaminated water through the ocular route during hand and face-washing events and the inhalation route during showering events. P. aeruginosa can cause an infection of the lungs, a more severe risk for immunocompromised individuals, and is one of the main causes of bacterial keratitis. A reverse quantitative microbial risk assessment (QMRA) was completed to determine the threshold concentrations of P. aeruginosa associated with the US Environmental Protection Agency’s maximum allowable risk for waterborne pathogens of 1 infection in 10,000 persons per year. The ocular exposure route poses a more significant risk of infection to the average user at the tap, as indicated by the median threshold concentrations of $6.04\times {10}^{11}$, 0.92, and 37 colony forming units (CFU) per liter for the showering, face-washing, and handwashing events, respectively. The reverse QMRA methodology followed in this analysis provides distributions of concentrations of concern that will aid decision makers in prioritizing mitigation strategies and monitoring plans for premise plumbing systems.

This work studied the effect of total solids content on methane production potential from pulp and paper mill sludge and its kinetic modeling. The modified Gompertz, logistic function, and transference function models were used for the model prediction. The results showed that food:microorganisms ($F/M$) ratios of 0.5–2.0 produced significant increments in methane production and biodegradability. Conversely, a $F/M$ ratio of 2.5 was observed with reduced methane production. This decrement was because of the presence of higher solids content that makes the hydrolysis stage difficult and mass transfer problems for the degraded organic matter. A $F/M$ ratio of 2.0 produced higher biodegradability and cellulose removal of about 66% and 47% respectively. Three models fitted the experimental data with $R^2>0.98$. A significant difference was observed for the maximum methane production rate among the three models. The modified Gompertz and logistic models had the best fit and mimicked the experimental data, whereas the transfer function model was in slight accord with the experimental data.

This study involves the removal of the olive-mill wastewater (OMW), which is a significant pollutant, in a batch reactor under aerobic conditions by mixed cultures. It also includes comparison of different substrate inhibition models, calculation of kinetic parameters, and testing the merit of the chosen mathematical model with respect to OMW concentration and changes with time. Average initial microorganism concentration ($X_0$) in the batch reactor was $100\pm 15\mathrm{mg}{\mathrm{L}}^{-1}$. During the study the treatment of OMW with initial chemical oxygen demand ($S_0$) concentration between $10\mathrm{and}\mathrm{1,000}\mathrm{mg}\mathrm{COD}{\mathrm{L}}^{-1}$ was investigated. Maximum specific growth rate (${\mu }_{\max }$) was reached with $130\mathrm{mg}\mathrm{COD}{\mathrm{L}}^{-1}$ initial concentration and 3 h experimental duration produced 57.17% chemical oxygen demand (COD) removal efficiency. Other experiments with increasing initial concentration increased COD removal duration, with $927\mathrm{mg}\mathrm{COD}{\mathrm{L}}^{-1}$ initial concentration regressing to 8.88% removal efficiency after 3 h. The biokinetic equations of Aiba, Haldane, Tseng, and Yano and Koga were chosen to relate $S_0$ concentration to specific growth rate ($\mu$) and the biokinetic parameters in these equations were calculated. The most appropriate biokinetic equation was the Haldane model in terms of $R^2$ value and the Haldane equation parameters $0.43{\mathrm{h}}^{-1}$, $45.34\mathrm{mg}{\mathrm{L}}^{-1}$, and $207.97\mathrm{mg}{\mathrm{L}}^{-1}$ were calculated for ${\mu }_{\max }$, half-saturation constant ($K_s$) and inhibition constant ($K_i$), respectively. Also, a mathematical biokinetic model including the Haldane equation was used to test the OMW removal performance and it was seen that the chosen model was well able to reflect system behavior. After the calibration of ${\mu }_{\max }$, the most-sensitive parameter of the Haldane model, to $0.22{\mathrm{h}}^{-1}$, the model gave a better fit for all tested conditions.

A pilot-scale study conducted in a municipal wastewater treatment plant (WWTP) for short-cut biological nitrogen removal (SBNR) from anaerobic reject water (ARW) was evaluated based on the microbial population and activity to assess optimum operational modes and effective parameters. Two main periods, one- and two-stage sequential batch reactor (SBR) systems, were operated at multiple-feeding mode + high sludge age + ambient temperature (during hot and cold seasons) and two separate process/reactor configurations with back feeding + controlled temperature (20°C, 25°C, 30°C, and 35°C) conditions, respectively. One-stage SBR operation at high temperature, moderate hydraulic retention time (HRT), and multiple-feeding mode promoted high sludge age and bacterial count and activity at the highest level for ammonium-oxidizing bacteria (AOB) and denitrifying bacteria (DB) where other species common in municipal WWTPs, Actinobacteria (filamentous bacteria) and Accumulibacter (phosphate-removing bacteria), dropped substantially in population number. Nitrospira was the only nitrite-oxidizing bacteria (NOB) species detected, ranging at the lowest level during the one-stage SBR operation period, showed a tendency to wash out, and rose in population in the full aerobic SBR of the two-stage operation. An overall bacterial activity and viability loss obtained in the two-stage SBR operation resulted in reduced sludge age values with a severe deterioration in the denitrifying anoxic SBR (AnSBR), which was correlated with the limited level of suitable substrate, thus, starving condition. The contribution of the anaerobic bacteria activity and slowly degrading organic matter oxidation in the one-stage SBR operation was pronounced in the study where the sludge age obtained was found as the most significant parameter in comparison with temperature and HRT.

In this paper, the validation of models for the organic and nitrogenated substrates utilization rate (SUR) in a sequential batch reactor (SBR) are proposed. The treatments tested are (1) Treatment 1: oxic, cycle duration of 24 h; (2) Treatment 2: oxic, cycle duration of 6 h; (3) Treatment 3: anaerobic-oxic, cycle duration of 24 h; (4) Treatment 4: anaerobic-oxic, cycle duration of 6 h; and (5) Treatment 5: anoxic-oxic-anoxic, cycle duration of 12 h. The novelty consists of the mathematical adjustment for each stage into the proposed treatments for SBR. The SURs of chemical demand oxygen (COD), ${\mathrm{NH}}_4^{+}$, and TKN (total Kjeldahl nitrogen) for the Treatments ${\mathrm{T}}_1$ to ${\mathrm{T}}_5$ have given successful $R_{adjusted}^2$ in the calibration and validation stages and vary between 75% and 100%, allowing the estimation of these variables into the predenitrification, aeration, conventional, and simultaneous denitrification stages of SBR operation.

To select and improve techniques capable of eco-restoration and reconstruction of soils, it is vital to determine the impacts of different engineered eco-restoration techniques on communities of soil microbes. For this purpose, slopes of four engineered eco-restoration techniques [thick layer base material spraying technique (TBS), vegetation concrete base spraying technique (CBS), external soil spray seeding technique (ESSS), and frame beam + external soil (FBES)] were studied and compared with slopes of natural forest (NF) at Xiangjiaba Hydropower Station in China. The results indicated that the engineered eco-restoration techniques had different effects on the physical and chemical properties of the soil and the structure and functional diversity of the microbial community. Phosphorus content and potential of hydrogen (pH) were higher in soil of all artificially restored slopes than those in soil of the natural forest. Whether it was soil physicochemical properties and microbial utilization of carbon source or average well color development (AWCD) reflecting soil microbial metabolic functional diversity, the soil of TBS was significantly better or closer to soil of the natural forest. However, the soil’s AWCD values, microbial utilization carbon source capacity, and diversity index of the other three technologies were lower than those of natural forests. Among them, the soil of CBS had the lowest AWCD value and McIntosh diversity index ($U$). The redundancy analysis and correlation analysis indicated that the content of available phosphorus (AP) and total phosphorus (TP) showed a negative correlation with most carbon sources’ utilization and McIntosh diversity index and were significantly positively correlated with the evenness index. The content of total nitrogen (TN) was positively associated with the utilization of the most carbon sources and the diversity index. In addition, there was a positive correlation between the content of soil organic carbon (SOC) and the utilization of most carbon sources. These results suggest that the high phosphorus content and low nitrogen content in the substrates of engineered eco-restoration techniques would inhibit the microbial activity.

The hydraulic habitat preference of the nuisance benthic alga Cladophora glomerata was investigated in a shallow river using an unmanned aerial vehicle (UAV) and acoustic Doppler current profiler (ADCP). Three separate ADCP surveys were completed over a 1-km channel segment during peak algal growth to correlate in situ velocity data with remotely sensed algal spatial cover. By pairing data from more than 60 transects and 3,500 ADCP ensembles, it was established that two distinct velocity distributions exist for Cladophora ($p<0.001$). In algal covered areas, the velocity profile averaged $0.67\mathrm{m}{\mathrm{s}}^{-1}$, whereas it was found to be $0.41\mathrm{m}{\mathrm{s}}^{-1}$ in uncovered locations. Groupings are considered to be optimal (former) and resource-limiting (latter) values. Corresponding shear velocities averaged 0.071 and $0.050\mathrm{m}{\mathrm{s}}^{-1}$. An upper threshold perhaps also exists at which Cladophora will not colonize, but we were unable to quantify it in this study. Based on our findings, the integration of ADCP and remote sensing appears to be a worthwhile pairing for benthological study.

A novel adsorbent of chemically modified moso bamboo (CMMB) for removing Pb(II) from an aqueous solution was prepared from twice-mercerized moso bamboo (MMB) using pyromellitic dianhydride (PMDA) as the modifying agent. The characteristics of prepared adsorbents were studied by scanning electron microscopy (SEM), X-ray diffractometry (XRD), and Fourier transform infrared spectrophotometry (FTIR). The effects of contact time, pH, and adsorbent dosage on the adsorption capacity were investigated by batch adsorption experiments. The equilibrium adsorption isotherms and kinetics of CMMB adsorbent and activated carbon were compared. Adsorption kinetics and all equilibrium data followed the pseudo-second-order model and Langmuir isotherm model, respectively. The maximum adsorption capacities of CMMB for Pb(II) were higher than that of activated carbon for Pb(II) at different temperatures. The thermodynamic analysis illustrated an endothermic and spontaneous process of Pb(II) removal using the CMMB adsorbent, and the regeneration studies suggest that the CMMB was a promising regeneration potential adsorbent.

Submerged gravel wetlands (SGWs) are subsurface-flow wetlands that can act as effective stormwater control measures (SCMs). To investigate SGW nitrogen (N) and phosphorus (P) performance and impacts from green waste compost (derived from leaves and grass) addition, indoor mesocosm studies were conducted using bioretention soil media (BSM) alone and mixed with 15% and 30% compost, by volume. The unamended mesocosm demonstrated effective and consistent N and P treatment. Compost-amended mesocosms were found to leach N and P above that of the system without compost. Maximum total N concentrations of 16 and $6.4\mathrm{mg}\text{-}\mathrm{N}/\mathrm{L}$ were reached after 1.7 and 3.0 cm of equivalent rainfall for 30% and 15% compost, respectively. Maximum total P concentrations of 2.9 and $0.52\mathrm{mg}\text{-}\mathrm{P}/\mathrm{L}$ were both reached after 2.5 cm of equivalent rainfall for 30% and 15%, respectively. Particulate P was more prevalent than dissolved P in effluent samples from compost systems, while N species were mixed among ammonium, nitrate, and organic N. Initial leaching of N and P from compost was counterbalanced by treatment of N and P in synthetic stormwater influent, with the 15% compost mesocosm reaching cumulative net-zero export of P after the equivalent of 6.1 cm of rainfall and of N after 12.7 cm. The 30% compost mesocosm did not achieve net-zero cumulative export of N or P after 22 cm of applied water. N treatment was attributed to denitrification and plant and microbial uptake. P treatment was attributed primarily to adsorption.

North America’s water distribution systems are aging and incurring increased pipe breaks. These breaks pose a serious threat to urban drinking water security, leading to service interruptions, loss of revenue, and increasing risk of water contamination. Prediction models have been developed to help identify when individual underground water pipes are expected to break, helping utilities develop pipe renewal projects and avoid costly pipe breaks that impact water supply reliability. This paper provides an in-depth comparison of the two leading statistical pipe-break modeling methods: machine-learning and survival-analysis algorithms. A gradient-boosting decision tree machine-learning model and a Weibull proportional hazard survival-analysis model are used to predict time to next break for cast-iron pipes in a major Canadian water distribution system. Results indicate that removal of censored events from the machine-learning model biases the model to predict earlier pipe breaks than occur. Overall, water utilities concerned with short-term security arising from impacts of pipe breaks on water security may favor the machine-learning approach, but the survival-analysis models’ ability to incorporate right-censored data makes it more appropriate for long-term asset management planning.

This study investigates the inactivation of adenovirus by silver and copper in aqueous solution, compares silver and copper disinfection of adenovirus to the MS2 phage, and quantifies MS2 transport through porous ceramic filters fabricated with silver nitrate and copper nitrate added prior to firing the clay-sawdust-water mixture. The MS2 phage is commonly used as a model virus for testing the efficacy of point-of-use water treatment technologies. However, results show that MS2 phage is not a representative virus, specifically as a surrogate for silver ion inactivation of viruses. At safe drinking water concentrations, after 8 h of exposure, silver and copper resulted in a 0.91- and $1.61\text{-}{\log }_{10}$ reduction of viable adenovirus, respectively. Under the same conditions, silver and copper resulted in a 0.42- and $1.46\text{-}{\log }_{10}$ reduction of viable MS-2 phage, respectively. Transmission electron microscopy (TEM) images suggest the mechanism of adenovirus inactivation may be related to damage/removal of the viral fibers. Removal of MS2 phage by ceramic water filters is modest, 71.95% and 75.98% by silver- and copper-embedded filters, respectively. These results indicate that ionic silver is a better viral disinfectant than previously known based on data for the MS2 phage alone and that copper, both alone and in combination with silver, may further improve viral disinfection.

The purpose of this study is to develop a modeling framework to assess the impact of drinking water distribution system water-age dynamics on population-weighted disinfection by-product (DBP) risks. This approach was demonstrated by utilizing methodology from a water treatment plant model and incorporating the relative health indicator (RHI) as metric for quantifying cumulative trihalomethane and haloacetic acid risks. Distribution system network hydraulic modeling revealed that water-age dynamics can impact population-weighted DBP exposures. Using treatment modeling, it was demonstrated that DBP precursor removal at the treatment plant provides greater public health protection, as demonstrated by lower population-weighted RHI values, than granular activated carbon or aeration applied to lower DBP concentrations within the distribution system. When these population-weighted RHI reductions were normalized to the treatment process flowrates, the risk reduction of distribution system technologies tended to be equivalent to those applied at the treatment plant. It was also demonstrated that moderate population-weighted RHI reductions can be achieved for scenarios in which booster chlorination is implemented for secondary disinfection rather than as a DBP compliance strategy.

Membrane bioreactors (MBRs) have been gaining acceptance as the best available technology for treating domestic and industrial wastewater. Relatively high operational costs, however, limit a broader adoption of MBRs. Air scouring, which is commonly used as a strategy to alleviate membrane fouling, suffers from inherent limitations such as low shear at the membrane surface. Vibration-based approaches offer promise as alternative or complementary methods of fouling mitigation. In this study, we evaluated magnetically induced membrane vibration (MMV) as a means of controlling membrane fouling in a pilot-scale MBR equipped with reinforced hollow-fiber membranes. Two vibration frequencies (30 and 150 Hz) were tested at a subcritical permeate flux of $26\mathrm{L}/({\mathrm{m}}^2·\mathrm{h})$. Membrane vibration retarded the transmembrane pressure buildup and saved $\sim 8\%$ of the costs associated with chemical and energy consumption.

Straw retention is an important practice increasing crop production worldwide. It also can reduce nitrous oxide (${\mathrm{N}}_2\mathrm{O}$) gas emissions from agricultural soils. But it is very difficult to predict ${\mathrm{N}}_2\mathrm{O}$ gas discharge amounts when straw is incorporated at different soil depths. This research combined wheat growing season data from the Agricultural Meteorological Experiment Station of Nanjing University of Information Science and Technology, China, with the denitrification-decomposition model (DNDC model) to determine whether the DNDC could simulate the ${\mathrm{N}}_2\mathrm{O}$ emission rate, annual discharge, and effect of incorporation depth on ${\mathrm{N}}_2\mathrm{O}$ emissions. Straw was retained on the soil surface, and incorporated at 10, 20, and 30 cm depths. The sensitivity of ${\mathrm{N}}_2\mathrm{O}$ emissions to several site-specific and environmental factors was also examined. The results were as follows: (1) the DNDC model was able to simulate the ${\mathrm{N}}_2\mathrm{O}$ emission rate and annual emission rate from different straw incorporation depths; (2) ${\mathrm{N}}_2\mathrm{O}$ gas emissions are sensitive to annual mean temperature, soil pH, and amounts of soil organic carbon, fertilization, and retained straw, which have different impacts under different straw incorporation depths; and (3) the direct influence of soil internal factors was more important than environmental factors in the surface and 10-cm treatments; however, the influence of straw retention decreased and the influence of environmental factors increased in the 20- and 30-cm treatments.

Water cyber-physical systems (CPSs) have gained increasing interest to improve operational efficiency and reliability. However, due to growing exposure to cyber-physical attacks, cybersecurity and resilience against the attacks have become significant concerns. Efforts have been made to improve cybersecurity in water CPSs, but only a few attempts to investigate resilience against cyber-physical attacks. This study contributes to characterizing the resilience of a water CPS and investigating potential resilience strategies. An advanced resilience measure that integrates the withstanding, absorptive, adaptive, and restorative capabilities of a system is proposed and applied to the C-town water distribution network (WDN) for 15 failure set scenarios during a pressure-driven hydraulic simulation. The results identify the failure sets and unfavorable operational conditions that make the system more vulnerable to cyber-physical attacks and, in turn, produce low resilience and capabilities. Additionally, after the recovery of a disrupted component, adjustment of overall operational interactions across system components is found to be needed for the complete restoration of disrupted functionality. The findings provide insights into infrastructure investments with resilience strategies in cyber and physical water system domains.

Sedimentation tanks are one of the most important and costly parts of the water treatment process in which particles remaining from previous stages settle down due to the force of gravity. In this research, particle-laden flows in secondary settling tanks are numerically investigated by considering the effect of the free surface using the volume of fluid (VOF) method. Comparison of the results of the current study with available experimental results shows the importance of size distribution and particle diameter deviation from the mean particle size $D_{50}$. Moreover, examination of the effects of inlet position as well as inlet Froude number reveals that (1) a decrease in inlet Froude number exerts a positive effect by reducing the volume of the recirculation zone and a negative effect by increasing the maximum velocity. Therefore, an optimum Froude number can be obtained for each geometry that maximizes the tank efficiency. (2) The maximum efficiency occurs when the inlet position is close to the free surface; however, for this case, the recirculation zone is larger than that for cases in which the inlet is in the middle of the tank or close to the bed.

Octahedral and spherical ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles (NPs) were synthesized and used as heterogeneous Fenton-like catalysts to degrade methylene blue (MB). Their catalytic activities were investigated by evaluating the effects of pH, catalyst dosage, ${\mathrm{H}}_2{\mathrm{O}}_2$, and MB concentration on the removal efficiency of MB and total organic carbon (TOC). Nearly complete color removal was achieved in 20 min at pH 3.0 in both Fenton-like systems with a low ${\mathrm{H}}_2{\mathrm{O}}_2$ concentration of $1.0\mathrm{mmol}{\mathrm{L}}^{-1}$. Removal of $>90\%$ MB was achieved in the fifth and third runs by octahedral and spherical ${\mathrm{Fe}}_3{\mathrm{O}}_4$ NPs, respectively, and the two catalysts eliminated 77.1% and 62.9% of TOC in solution in 30 min of the first cycle. The removal rates of MB as a function of surface area were slower in the presence of spherical than octahedral ${\mathrm{Fe}}_3{\mathrm{O}}_4$ NPs, indicating shape-dependence of the NPs’ Fenton-like catalytic activity. The catalytic activity and reusability of octahedral ${\mathrm{Fe}}_3{\mathrm{O}}_4$ NPs were superior to that of spherical NPs. HO· was found to be the main active species in the Fenton-like system.

In the present study, a highly active reaction system of nanoscale zero-valent (nZVI) was successfully established by enhancing nZVI with a biodegradable complexing agent (CA) and applied to hexavalent chromium [Cr(VI)] removal. Both iminodisuccinic acid (IDS) and N,N-bis(carboxymethyl)glutamic (GLDA) are good biodegradable CAs with strong ability to Cr(III)/Fe(II)/Fe(III). Cr(VI) ($10.0\mathrm{mg}{\mathrm{L}}^{-1}$) removal efficiency by nZVI increased from 60.1% to 100.0% with the addition of $4.0\mathrm{mmol}{\mathrm{L}}^{-1}$ IDS or GLDA for 60 min. A characterization analysis suggested that nZVI and CA enhanced Cr(VI) removal by decreasing the surface passivation layer, which was expected to be $({\mathrm{Cr}}_{\mathrm{x}}{\mathrm{Fe}}_{1\text{-}\mathrm{x}}){(\mathrm{OH})}_3$. In addition, the Cr(VI) removal efficiency decreased with increasing initial pH, and appropriate CA dosage ($4.0\mathrm{mmol}{\mathrm{L}}^{-1}$) and temperature (25.0°C) were beneficial for the efficient reduction of Cr(VI). An amount of 94.8% of Cr(VI) removal was achieved within the initial 2 min when the experiment conditions were as follows: $0.1\mathrm{g}{\mathrm{L}}^{-1}$ nZVI, $4.0\mathrm{mmol}{\mathrm{L}}^{-1}$ GLDA, the temperature at 25.0°C, an initial pH of 7.0, and stirring at $200\mathrm{r}{\min }^{-1}$. And the Cr(VI) removal by GLDA improved nZVI maintained above 84.3% after three consecutive cycles. The results indicate CA-enhanced-nZVI could be a promising approach for the Cr(VI) removal in a solution.

This research presents two hybrid models based on the Streter-Phelps equation associated with nonstationary conditions $dS/dt\ne 0$ and advective $dS/dZ\ne 0$, statistically adjusted and applicable for the design of anaerobic upflow filters separated in two and three phases, operated on a laboratory scale using landfill leachate as substrate under a $3^3$ experimental factorial design. The factors included (1) volumetric organic load $(\mathrm{VOL})=2.25$, 3.45, and $4.64\mathrm{kg}\mathrm{COD}{\mathrm{m}}^{-3}{\mathrm{d}\mathrm{a}\mathrm{y}}^{-1}$ [chemical oxygen demand (COD)]; (2) temperature $(T)=20°\mathrm{C}$, 27°C, and 34°C; and (3) height ratios ($D_1$ = height phase 1, $D_2$ = height phase 2, and $D_3$ = height phase); $\mathrm{DI}\text{-}\mathrm{FAFS}$ (Spanish acronym that means Upflow Anaerobic Filter separated in Two Phases): $D_1/D_2=20\%/80\%$, $50\%/50\%$, and $80\%/20\%$, and TRI-FAFS (upflow anaerobic filter separated in three phases): $D_1/D_2/D_3=4\%/16\%/80\%$, $10\%/10\%/80\%$, and $16\%/4\%/80\%$. Operating conditions included (1) hydraulic retention time (HRT) between 16 and 18 h; (2) flow rates of $3.5–4.0\mathrm{mL}{\min }^{-1}$; (3) surface hydraulic load of $1.82{\mathrm{m}}^3{\mathrm{m}}^{-2}{\mathrm{d}\mathrm{a}\mathrm{y}}^{-1}$; and (4) filter total depth of 1.2 m, filled with plastic material with a specific surface area of $476.35{\mathrm{m}}^2{\mathrm{m}}^{-3}$. Fifty-four tests were performed, obtaining efficiency between 27% and 73% in the DI-FAFS and between 84% and 94% in the TRI-FAFS (Spanish acronym that means Upflow Anaerobic Filter separated in Three Phases). Maximum efficiencies were achieved with $D_1/D_2$ ratios $20\%/80\%$ and $D_1/D_2/D_3=10\%/10\%/80\%$, respectively, with temperatures $\ge 27°\mathrm{C}$ and $\mathrm{VOL}\ge 3.45{\mathrm{kgCODm}}^{-3}{\mathrm{d}\mathrm{a}\mathrm{y}}^{-1}$.

The present study quantifies the pollution potential of the leachate and its effects on surrounding groundwater in the vicinity of four nonengineered dump sites in the study regions of Solan, Mandi, Sundernagar, and Baddi in Himachal Pradesh in India. The analysis primarily focused on the determination of leachate characteristics for determination of the leachate pollution index (LPI), groundwater characterization at different downstream distances using the water quality index (WQI), and heavy metal pollution index (HPI) to study the effects of leachate pollution on groundwater. Characterization of leachate samples revealed that most of the physicochemical parameters and heavy metals were in excess of the permissible limits for the study regions of Himachal Pradesh. The water quality improved with an increase in downstream distance from the dumpsite per the indexing method. Principal component analysis (PCA) was carried out to determine the components arising from natural and anthropogenic components while hierarchical cluster analysis (HCA) was used to determine and identify the regions of low, moderate, and high pollution zones in the groundwater.