Modeling TOC Breakthrough in Granular Activated Carbon Adsorbers

Linear regression techniques were used to develop practical models to predict total organic carbon (TOC) breakthrough in bituminous granular activated carbon (GAC) adsorbers. Models were developed for two field-scale GAC sizes (8×30 and 12×40?mesh) and two empty bed contact times (EBCTs) (10 and 20 min). Model input parameters include two water quality variables, influent TOC concentration (TOC0) and pH, that impact performance. The dependent variables for the models were normalized breakthrough time, throughput in bed volumes, to six fractional (TOC/TOC0 = 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7) and three mass (TOC = 1.0, 1.5, and 2.0 mg/L) effluent concentrations. Model development was performed using small-scale breakthrough data from 35 different source waters; external model validation was performed with small-scale breakthrough data from 14 source waters; a sensitivity analysis was performed to ensure that the models effectively capture expected breakthrough trend; and a scalability test was performed to verify the models’ ability to predict breakthrough for field-scale GAC adsorbers.     

Activated Carbon Addition to a Submerged Anaerobic Membrane Bioreactor: Effect on Performance, Transmembrane Pressure, and Flux

This study examined the effect of the addition of activated carbon to three, 3 L submerged anaerobic membrane bioreactors (SAMBRs) in terms of chemical oxygen demand (COD) removal, flux, and transmembrane pressure (TMP). The feed was a synthetic substrate with a COD of 460?mg?L?1, with one reactor run as a control, one with 1.7?g?L?1 of powdered activated carbon (PAC), and the third with 1.7?g?L?1 of granular activated carbon (GAC). While COD removal was high in all reactors (>90%), in comparison to the control (SAMBR1), the average COD removal in SAMBR2 (PAC) increased by 22.4%, while SAMBR3 with GAC was not significantly better. Because PAC has a significantly greater surface area per mass than GAC, it is probable that this difference was primarily due to the greater absorbance of fine colloidal particles and high molecular weight organics onto the carbon surface. These effects manifested themselves by SAMBR2 having lower TMPs and higher fluxes than both SAMBR3 and SAMBR1. Volatile fatty acids in the effluent from all three SAMBRs were extremely low (<18?mg?L?1), even during step changes in hydraulic retention tune, and most of the soluble COD in the effluent was soluble microbial products. Biochemical methane potential assays showed that biomass in the SAMBRs was less active than the seed sludge, and it appears that the addition of activated carbon to Reactors SAMBR2 and SAMBR3 provided a solid support for growth, and hence reduced floc breakage.     

Investigating Role of Growing Adsorbent Bed in a Dead-End PAC/UF Process

A two-stage mathematical model was developed to describe adsorbate removal in a dead-end powdered activated carbon/ultrafiltration (PAC/UF) membrane process. Para-nitrophenol (PNP) was used as the model organic compound. The first stage accounted for adsorbate removal during transport from the initial PAC contact with the PNP solution to the membrane system, and the second stage accounted for additional PNP removal due to the retention of the PAC in a growing bed on the membrane surface. The PAC adsorptive capacity was described using the Langmuir isotherm, whose parameters were estimated from isotherm experiments. Transport of the PNP through the PAC particle was described using the homogeneous surface diffusion model and the surface diffusivity was estimated from batch experiments. The two stage model predicted the effluent concentrations from the PAC/UF process during the early stages of the experiments, but model improvements are required to more accurately predict the latter stages. A batch model can be used to describe the effluent PNP concentration from the PAC/UF process if dispersion is neglected.     

Adsorption of RDX and HMX in Rapid Small-Scale Column Tests: Implications for Full-Scale Adsorbers

The high explosive (HE) compounds royal demolition explosive or hexahydro-1,3,5-trinitro-1,3,5-triazocine (RDX) and high melting explosive or octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) have been detected as groundwater contaminants at many military facilities. This research evaluated adsorption of RDX and HMX with granular activated carbon (GAC) to provide guidance for the design and operation of GAC adsorbers for treatment of HE-contaminated groundwater. Five GACs were screened using rapid small-scale column tests (RSSCTs), after which additional tests were performed with the two GACs that most effectively treated mixtures of RDX and HMX (Calgon F400 and Northwestern LB-830). GAC service life as a function of empty-bed contact time (EBCT) was determined using RSSCTs for a range of simulated full scale EBCTs with influent concentrations of 2,200 μg RDX/L and 350 μg HMX/L. Increasing the influent concentration of either contaminant significantly reduced the predicted service life, as did preloading GAC with groundwater natural organic matter. In batch isotherm tests, RDX was less adsorbable than HMX under all conditions studied. Concurrent loading of natural organic matter reduced the Freundlich K for RDX, whereas adsorption of HMX was not affected. Of the GACs tested, Calgon F400 most effectively removed RDX and HMX.     

Effect of Hexavalent Chromium on Performance of Membrane Bioreactor in Wastewater Treatment

The presence of toxic hexavalent chromium poses a great challenge in biological wastewater treatment. In this study, the performance of a membrane bioreactor (MBR) for the treatment of synthetic domestic wastewater in the presence of chromium was investigated. The carbonaceous pollutant removal is not affected by Cr(VI) with concentration ranging from 0.4 to 10 mg/L; it becomes slightly lower when the Cr(VI) is 50 mg/L. The nitrification efficiency of above 99% can be achieved when the waste stream is free of the metal or contains 0.4 mg/L chromium. When its concentration is 10 mg/L, nitrification efficiency above 50% is found; however, it becomes deteriorated in the presence of 50 mg/L chromium. The positive biomass growth, though lower than conventional activated sludge process, can be achieved at Cr(VI) concentration less than 10 mg/L; a decline in the cell growth occurs when the metal concentration is increased to 50 mg/L. Significant accumulation for the metal is observed when its concentration is 0.4 mg/L; however, almost no metal removal is observed when the concentration is above 10 mg/L. During eight-month continuous operation, the presence of Cr(VI) has an insignificant effect on the flux. The nitrifiers in the MBR are more sensitive to the presence of Cr(VI) than heterotrophs.     

Electrochemical Reactivation of Granular Activated Carbon: Effect of Electrolyte Mixing

Bench-scale experiments investigated the effect of electrolyte mixing on the effectiveness of an electrochemical reactor for the reactivation of granular activated carbon (GAC). Two different GACs (F-400 and WV-B) were loaded with phenol via batch adsorption tests, then electrochemically reactivated and finally reloaded with phenol. Reactivation was conducted in a recirculating flow reactor with a 0.1?M NaCl solution as the electrolyte. Cathodic reactivation was more efficient than the anodic reactivation and increasing the degree of electrolyte mixing decreased the cathodic reactivation efficiencies, while there was no significant change in the anodic reactivation efficiencies. Higher degrees of electrolyte mixing decreased the local pH at the cathode and consequently reduced the desorption driving force and therefore reduced the reactivation efficiency. The electrolyte mixing lowered the cell voltage. However, this advantage was overshadowed by the increased energy consumption required for the electrolyte pumping, the reduction of the oxidation rate of phenol, and a 20% reduction in the reactivation efficiencies. Thus, electrolyte mixing of the electrolyte is not recommended in the electrochemical reactivation of GAC.     

Predicting Minimum Carbon Usage for PAC Adsorption of Trace Organic Contaminants from Natural Water

Powdered activated carbon (PAC) is an excellent adsorbent for drinking water treatment of many trace organic contaminants. To evaluate and design a PAC adsorption process for a particular application, it is necessary to know the minimum (lowest economical) carbon (adsorbent) usage (MCU) defined thermodynamically. In this work, an explicit relationship is developed for predicting the MCU required for a desirable level of treatment of a target trace organic compound (TC). The adsorption processes considered are PAC slurry contactors idealized either as batch reactors, plug flow reactors, or continuous-flow stirred tank reactors. Comparing with the ones previously available in the literature, this newly developed relationship, as a predictive tool for practical uses as well, is more accurate because it does not need to assume that the MCU required for target TC removal can always reduce the competing background natural organic matter (NOM) to a level much less than the NOM initial/influent concentration. Applications of the relationship developed herein to PAC adsorption of typical trace organic contaminants in natural water are demonstrated with isotherm data from multiple literature sources.     

Adsorption of Carbon Disulfide (CS2) in Water by Different Types of Activated Carbon—Equilibrium, Dynamics, and Mathematical Modeling

Adsorption equilibrium and kinetics of carbon disulfide in water by granular activated carbon (GAC), powdered activated carbon (PAC), and activated carbon fiber (ACF) were investigated and compared in an effort to elucidate the fundamentals for optimizing the control process design. It has been shown that the BET expression can satisfactorily describe the adsorption equilibrium of carbon disulfide (CS2) on GAC, PAC, and ACF and the corresponding kinetic experimental data properly correlated with the second-order kinetic model, which indicates that the CS2 adsorption is the rate-limiting step. A two-phase mathematical model was developed to simulate CS2 transfer in fixed-bed operation filled with the GAC, PAC, and ACF, and the equilibrium and kinetics information is subsequently used in the model to characterize the dynamics of adsorption. The model includes mechanisms such as axial dispersion, advection, liquid-to-solid mass transport, and intraparticle mass transport by pore and surface diffusion. It is manifested that the model was able to predict the dynamic breakthrough curve of CS2 in a fixed-bed adsorption column filled with GAC, PAC, and ACF at varied conditions (standard deviations for 1.5?cm/min is 12.13% and for 2.2?cm/min is 16.12%), based on BET-3 equilibrium and second-order kinetics, which indicates that the methodology proposed by this work could be employed for adsorbents selection, adsorption design, and process optimization for CS2 waste-water emission control.     

Characteristics of Ground Granular Activated Carbon for Rapid Small-Scale Column Tests

The rapid small-scale column test (RSSCT) has become a popular method for sizing granulated activated carbon (GAC) systems and columns for water treatment facilities. In this procedure, the GAC is ground and a specific size fraction is used for the RSSCT. Since GAC is produced and activated using different processes from different starting materials (e.g., bituminous, lignite, wood, etc.), the possibility exists that the extent of activation and, hence, the adsorptive capacity and surface reactivity may vary throughout the GAC particles. This would be the case if there were less activated inner cores in the GAC particles. If there is a variation in the sorption properties throughout the GAC particles, then grinding the GAC may result in smaller particles that have different properties than the bulk GAC. This study was carried out to test this commonly assumed hypothesis that the limited-sized ground particles represent the same adsorptive properties as the bulk GAC. Four activated carbons (manufactured from different source carbons) were studied. Gas adsorption tests determined the physical morphology, Boehm’s titrations checked the chemical nature of the surface oxides, and the Mohs hardness test was performed on all bulk GACs and ground fractions. No apparent differences were found in the total surface area, cumulative pore volume, or pore size volume of fractions generated by grinding activated carbons. In addition, the Boehm technique did not identify any significant differences in the chemical nature of the surfaces of the various size fractions of GAC. The Mohs hardness test did not indicate any variations in the hardness of the bulk GAC, the ground fractions, and the unground core. Based on the methods and materials used, the underlying assumption in the RSSCT analysis—that there are no variations in the different size fractions of the ground GAC—appears to be correct.     

Performance Assessment of Hydraulic Efficiency Indexes

The analysis of residence time distribution functions originating from tracer studies is one of the main tools for the assessment of hydraulic performance in water and wastewater treatment units. In order to simplify the analysis, hydraulic indexes extracted from these functions are normally used. In general, these indexes are divided into two categories: short circuit and mixing indicators. However, some indexes may be related to more than one physical phenomenon (i.e., short circuit, mixing, recirculation, dead zones), leading to erroneous interpretation. In this work their capability to evaluate short-circuit and mixing levels in water and wastewater treatment units is assessed. Among the indexes analyzed, t10, which is the time necessary for 10% of the tracer to leave the unit, is recommended as a short-circuit indicator and the dispersion index (σ2) and the Morril index as mixing indicators, depending on the mixing level.     

Evaluation of Water Reuse Technologies for the Textile Industry

Treatment technologies were evaluated for application in water reuse for the textile industry. Technologies tested included electrochemical oxidation, hypochlorite oxidation, ozonation, granular activated carbon (GAC) adsorption, bisulfite catalyzed sodium borohydride reduction, Fenton’s reagent, coagulation, and anaerobic biodegradation. Bench-scale side-by-side tests were conducted using a spent dyebath wastewater from a jigg dyeing operation. The dyebath contained three reactive dyes and auxiliary chemicals (e.g., common salt, soda ash, acetic acid, and caustic). Each technology was evaluated for its effectiveness at removing color and chemical oxygen demand (COD) and anticipated operating costs. Ozone, GAC, and electrochemical oxidation produced high-quality effluent, suitable for reuse. Although hypochlorite oxidation and sodium borohydride reduction resulted in significant color removal, it was not sufficient to meet reuse criteria. Results were either insignificant or inconclusive for coagulation, Fenton’s reagent, and anaerobic biodegradation. Auxiliary chemicals had great impact on the performance of many of the alternatives evaluated.     

Effect of Organic Surface Load on Process Performance of Pilot-Scale Algae and Duckweed-Based Waste Stabilization Ponds

Removal efficiencies in pilot scale algae-based ponds (ABPs) and duckweed-based ponds (DBPs) were assessed during two periods of 4 months each. During Periods 1 and 2, the effect of low and high organic loading was studied. A linear correlation between ponds organic surface loading rates and the corresponding biochemical oxygen demand (BOD) removal rates was observed in both systems. For both periods, higher BOD and total suspended solids (TSS) removal efficiencies were found in DBPs compared to ABPs. Nitrogen removal rates (λr) in ABPs were linearly correlated with BOD surface loading rates (λs,BOD) and nitrogen loading rates (λs,N), while in DBPs, N removal rates were almost constant irrespective of λs,BOD or λs,N. Overall N removal rate in the algae system was significantly higher than that in duckweed system. Organic loading had no effect on total phosphorus removal efficiency in both systems. Higher P removal efficiency was achieved in the duckweed system than in the algae system. In ABPs as well as DBPs, fecal coliforms were better removed during low organic loading in comparison with high organic loading. During the two operational periods, higher fecal coliform removal efficiency in the algae system than in the duckweed system was observed.     

Replacing Outliers and Missing Values from Activated Sludge Data Using Kohonen Self-Organizing Map

Modeling the activated sludge wastewater treatment plant plays an important role in improving its performance. However, there are many limitations of the available data for model identification, calibration, and verification, such as the presence of missing values and outliers. Because available data are generally short, these gaps and outliers in data cannot be discarded but must be replaced by more reasonable estimates. The aim of this study is to use the Kohonen self-organizing map (KSOM), unsupervised neural networks, to predict the missing values and replace outliers in time series data for an activated sludge wastewater treatment plant in Edinburgh, U.K. The method is simple, computationally efficient and highly accurate. The results demonstrated that the KSOM is an excellent tool for replacing outliers and missing values from a high-dimensional data set. A comparison of the KSOM with multiple regression analysis and back-propagation artificial neural networks showed that the KSOM is superior in performance to either of the two latter approaches.     

Use of Dubinin–Astakhov Equation to Describe Preloading Effect of Natural Organic Matter

This note presents a simple model to quantify the preloading effect of naturally occurring organic matter (NOM) in water on the adsorption capacity of activated carbon for a trace synthetic organic chemical (SOC). The model was developed from the Dubinin–Astakhov (DA) equation based on the assumption that the NOM preloading irreversibly reduced the limiting adsorption pore volume for the target SOC. Given that the DA-n value equal to one, the model reduces to a form similar to the one obtained by modifying the Freundlich equation directly. By assuming that the reduction of the limiting adsorption pore volume was proportional to the volume of NOM adsorbed, the NOM preloading effect was correlated directly to the amount of total organic carbon preloaded on the carbon. The resulting model was then compared with the experimental data in the literature. This simple model may be useful for certain practical applications that require only the estimation of the NOM preloading effect on the adsorption capacity of a target SOC from natural water.     

Disinfection By-Product Precursor Adsorption as Function of GAC Properties: Case Study

Four different granular activated carbons (GACs) were tested at the bench scale for the adsorption of disinfection by-product (DBP) precursors and were found to be spent at different rates for the Lincoln (Nebraska) water system. This study examined the value of several physical and chemical tests for ranking the potential of different GACs for DBP precursor removal for one water utility. The surface area in the micro- and mesopore range and tannin adsorption were found to be useful indicators of DBP precursor adsorption potential. GACs with the largest surface in the 5?to?50?? pore-width range were able to treat the largest amount of water before being spent. A high value obtained in the tannin adsorption test was observed for the GACs that treated large water volumes.     

Application of Natural Clayey Soil as Adsorbent for the Removal of Copper from Wastewater

Use of clayey soil has been explored in the laboratory scale experiment as a low cost adsorbent for the removal of copper from wastewater. The influence of metal ion concentration, weight of adsorbent, stirring rates, influence of temperature, pH are also evaluated and the results are fitted using adsorption isotherm models. From the experimental results it is observed that almost 90–99% copper can be removed from the solution using clay at optimized pH 5.5. Langmuir adsorption isotherm, Freundlich isotherm and Tempkin isotherm model have been used to describe the distribution of copper between the liquid and solid phases in batch studies and it has been observed that Langmuir isotherm better represents the phenomenon. From the experimental results rate constant, activation energy, Gibbs free energy, enthalpy, and entropy of the reaction are calculated to determine the mechanism of the sorption process. Thomas, Adams-Bohart, and Yoon-Nelson models are applied to the experimental data to determine the characteristic parameters of the column for process design.     

Activated Sludge Process Modification for Sludge Yield Reduction Using Pulp and Paper Wastewater

Implications of conventional activated sludge (CAS) process modification to a low sludge production (LSP) process have been studied for treating pulp and paper wastewaters. The activated sludge process is modified to a two-stage design to establish a microbial food chain that would result in reduced sludge production. The return activated sludge in the LSP process bypasses the first (dispersed growth) stage to be received only by the second (predatory) stage. The resulting once-through operation of the dispersed growth (DG) stage makes it potentially susceptible to bacterial washout under hydraulic shock conditions. A sensitivity analysis of the DG stage operation was performed by varying its hydraulic residence time. The experimental data revealed that the optimal DG stage hydraulic residence is between 3 and 5?h, with bacterial washout likely to be initiated within 2?h. Based on laboratory results, it appears that a well-designed LSP system is likely to be able to handle day-to-day variations in hydraulic and organic loading rates. The LSP process produced 36% less sludge than the CAS process while consuming approximately 25% more oxygen. The treatment performance of the two systems was comparable except that the LSP sludge had much better settling and dewatering properties.     

Wastewater Treatment with Biomass Attached to Porous Geotextile Baffles

A bench-scale study used nonwoven geotextiles as a compact biomass host media to treat wastewater from a combined sewer system. The geotextile coupons were used as baffles and suspended in an aerated reactor. Each baffle was offset in succession to form a sinuous channel with permeable boundaries. Filtering the total suspended solids (TSS) and micro-organisms formed a biomass floc in the interior of the baffles, which grew to emerge on the surface. Suspended and nonsettleable colloidal solids in the influent wastewater were captured by both filtration and adsorption from the channel flow. This bench-scale setup, named the geotextile baffle contact system, consistently provided secondary treatment to influent concentrations up to 318 mg/l of TSS and 114 mg/l of biological oxygen demand. Ammonia (NH3–N) concentrations were reduced over 90%, and mineralization of the nitrate (NO3–N) was also observed when the biofilm aged and thickened. Some of the influent TSS and sloughed biomass from the baffles settled to the bottom of the tank.     

SSPRSD Using a Filamentous Fungal Strain Penicillium expansum BS30 Isolated from Wastewater Sludge

A filamentous fungal strain (Penicillium expansum BS30) isolated from a municipal wastewater treatment plant was used in this study to simultaneously reduce sludge solids, pathogens, and improve the sludge settling and dewaterability [simultaneous solids and pathogens reduction, settling and dewatering (SSPRSD)] in shake flask and 10-L bioreactor experiments. The fungal strain role in the SSPRSD process was evaluated at different temperatures and inoculum (spores) concentrations. The best performance of the process was achieved at incubation temperature of 25°C and inoculum concentration of 106?spores/mL. At these optimal conditions, suspended solids (SS) and volatile SS were degraded >50 and >53%, respectively. The capillary suction time value recorded (<13?s) was lower than that required for sludge dewaterability (<20?s). The populations of total coliforms and Salmonella (pathogen indicators) were reduced by two and four log cycles, respectively. A study on molecular screening of penicillin biosynthesis gene cluster and toxic organic compounds degrading machinery of the fungal strain was also conducted. It was found that the fungal strain possessed the penicillin-producing gene and toxic organic compounds degrading genes, and therefore may be helpful in degrading these compounds.     

Mechanisms of Ballasted Floc Formation

Ballasted flocculation processes were studied at laboratory scale to evaluate the mode of transport of ballasting agent from bulk liquid into the chemical floc and establish a new model for assessing the mechanisms of ballasted floc formation. For this purpose, a test program consisting of bench scale observations, microscopic observations, density tests, and centrifugal settling tests was developed. Ballasted flocs were found to be formed by a mechanism in which the ballasting agent is incorporated into the floc matrix by inertial forces, and the ballasting agent taking the place of the bound water content (ΦW). Bulk density of ballasted flocs (ρb) was found to be linearly related to the ballasting agent dose (M). This study provided a better understanding of the mechanisms controlling the interaction between ballasting agent and coagulant/polymer chemical floc.