Trihalomethane Species Forecast Using Optimization Methods: Genetic Algorithms and Simulated Annealing

Chlorination is an effective method for disinfection of drinking water. Yet chlorine is a strong oxidizing agent and easily reacts with both organic and inorganic materials. Trihalomethanes (THMs), formed as a by-product of chlorination, are carcinogenic to humans. Models can be derived from linear and nonlinear multiregression analyses to predict the THM species concentration of empirical reaction kinetic equations. The main objective of this study is to predict the concentrations of THM species by minimizing the nonlinear function, representing the errors between the measured and calculated THM concentrations, using the genetic algorithm (GA) and simulated annealing (SA). Additionally, two modifications of SA are employed. The solutions obtained from GA and SA are compared with the measured values and those obtained from a generalized reduced gradient method (GRG2). The results indicate that the proposed heuristic methods are capable of optimizing the nonlinear problem. The predicted concentrations may provide useful information for controlling the chlorination dosage necessary to assure the safety of water drinking.     

Numerical Method to Elucidate Likely Target Positions of Chlorine Removal in Anaerobic Sediments Undergoing Polychlorinated Biphenyl Dechlorination

Polychlorinated biphenyl (PCB) contamination in the United States is predominately from commercially manufactured Aroclor mixtures. These mixtures consist of approximately 150 congeners and are characterized by chlorination level and congener distribution profile, with some congeners maintaining a constant relative abundance across the chlorination levels. Once introduced into the environment, changes in congener profiles occur, in some cases altering the relative abundance of congeners correlated in the commercial Aroclors. The shifts in the relationships of the correlated congener pairs (trackers) are used to quantify the likelihood of natural remediation processes occurring in the anaerobic sediment and to identify positions where chlorine removal is likely. A numerical model for elucidating the most likely chlorine positions was developed, implemented, and tested on Hudson River sediment data. The model results show that flanked chlorines were most likely to have been removed, followed by meta chlorines. These results are consistent with those reported by laboratory investigation of Hudson River sediments. The findings suggest that the model can successfully determine the most likely positions of chlorine removal, even in the absence of a priori knowledge of the sediment contamination (source Aroclors) or the dechlorinating organisms. Thus the model can be applied, even where limited knowledge exists regarding the contamination source and the nature of the biogeochemical reactions affecting the fate of PCBs in a particular sediment system.     

Trihalomethane and Haloacetic Acid Disinfection By-Products in Full-Scale Drinking Water Systems

Trihalomethane (THM), haloacetic acid (HAA5), and total organic carbon (TOC) data provided by the Missouri Dept. of Natural Resources for drinking water treatment systems in the State of Missouri was analyzed for the years 1997–2001. These data indicated that a significant portion of systems exceeded the current regulatory limits of 80 and 60?μg/L for THM and HAA5 in these years. The vast majority of the treatment plants exceeding the regulatory limits were small plants with service populations less than 10,000 people. No significant temporal trend in either THM or HAA5 was noted for the years 1997–2001. This work suggests that the proposed use of a locational running annual average may have a significant effect on compliance. The use of chloramines (combined chlorine) versus free chlorine (HOCl/OCl?) as a residual disinfectant was shown to significantly reduce both THM and HAA5 in systems that treat their own water (primary systems), but did not have a significant effect in systems which purchase their water from primary systems (secondary systems). Comparison of finished water at the treatment plant versus in the distribution system suggested that a majority of THM and HAA5 may be produced within the plant as opposed to the distribution system. Hence, reducing these chlorinated disinfection byproducts within the treatment plant itself should be a key focus for achieving compliance, and supports Environmental Protection Agency disinfection byproducts compliance guidelines using enhanced coagulation.     

Temporal and Spatial Variations in Bulk Chlorine Decay within a Water Supply System

Modern water treatment must maintain an acceptable balance between the microbial safety of potable water supply, the costs of treatment, and the formation of potentially harmful disinfection by-products (DBPs). In order to achieve the optimum balance, it is essential to understand and predict both the formation of DBP and the decay of chlorine, in relation to source water, treatment processes, storage, and supply. Reported herein are new data which demonstrate the lack of durability, precision, and accuracy associated with earlier empirical chlorine decay rate equations. This work develops an improved methodology for the prediction of variation in chlorine decay rates in distribution systems enabling practical, cost-effective prediction of the effects of both seasonal variations and management interventions on chlorine levels at treatment works and in distribution systems.     

Effects of Bromide Ion and Natural Organic Matter Fractions on the Formation and Speciation of Chlorination By-Products

The impacts of bromide concentration and natural organic matter (NOM) characteristics on the formation and speciation of disinfection by-products (DBPs) in chlorinated NOM fractions were investigated. A total of 20 bulk water NOM fractions with a wide range of specific ultraviolet (UV) absorbance (SUVA254) values were obtained from a source water employing XAD-8 or XAD-4 resin adsorption in completely mixed batch reactors. SUVA was not a good predictor of DBP [trihalomethanes (THMs), haloacetic acids (HAAs), and adsorbable organic halogens (AOX)] formation and speciation. The destruction in the UV254 absorbance from chlorination did not correlate with DBP formation at any bromide level. NOM moieties which do not absorb UV light at 254?nm significantly contributed to DBP formation. Mass balance calculations on halogens using THMs, HAAs, and AOX data indicated that significant amounts of DBPs (>54% of AOX) other than THMs and HAAs were formed in NOM fractions with 60–110?μg/L bromide concentration. The relative occurrence of such other halogenated by-products decreased with increasing bromide concentrations up to 500?μg/L level. NOM in the studied water was more susceptible to the formation of brominated THM species as opposed to brominated HAAs. At constant dissolved organic carbon concentration, chlorine dose and pH, increasing bromide concentrations in NOM fractions increased the total concentrations of DBPs and resulted in a shift toward the formation of brominated species. Further, increasing bromide concentrations increased the spectrum of detected species (i.e., occurrence of all nine HAAs) and provided a competitive advantage to THM and HAA precursors in NOM over precursors of other DBPs.     

Modeling Residual Chlorine Response to a Microbial Contamination Event in Drinking Water Distribution Systems

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.     

Modeling the Impact of Microbial Intrusion on Secondary Disinfection in a Drinking Water Distribution System

The purpose of this study was to quantify the potential level of protection that secondary disinfection may provide in response to an intrusion event. Although several uncertainties exist regarding intrusion events, this study presents an analysis of the inactivation provided by disinfectant residuals by using a distribution system model, inactivation and disinfectant decay models, and conservative assumptions based on available data. A variety of conditions were modeled, including a range of water quality parameters (pH, temperature); inactivation of two microorganisms, Giardia and E. coli O157:H7; and intrusion water dilution ratios. Despite the assumptions inherent in the model, several generalizations were derived from the study. A free chlorine residual of 0.5?mg/L may be insufficient to provide adequate control of disinfectant-resistant Giardia even at low pH (6.5) and high temperature (25°C) conditions that enhance chlorine effectiveness. For E. coli, an organism of “average” disinfectant resistance relative to others, a residual of 0.5?mg/L may provide ample protection against intrusion even assuming that the chlorine residual is reduced within several minutes, such as would be predicted to occur with sewage intrusion at levels below 1% of the total flow. Importantly, chloramines may have a negligible benefit in terms of protecting against intrusion for even relatively susceptible organisms such as E. coli. Consequently, systems should consider protection against intrusion when choosing their secondary disinfectant.     

Spatial Diversity of Chlorine Residual in a Drinking Water Distribution System

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

Secondary Benefits of Aquifer Storage and Recovery: Disinfection By-Product Control

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

Kinetics of Iron Chlorination of Roasted Illmenite Ore,Fe2TiO5 in a Fluidized-Bed Reactor

The preferential chlorination reaction of iron constituents in roasted ilmenite ore was studied in an experimental fluidized bed reactor. The influence of the roasting temperature on the selectivity of chlorination reactions was also examined. It was found that preferential iron chlorination is dependent on roasted product morphology. Effects of chemical and process variables in the fluidized bed reactor, such as chlorination temperature, chlorine gas partial pressure, gas-solid contact time and the quantity of coke added, were investigated in order to study the kinetics of the preferential chlorination reaction. A modified bubble assemblage model was applied to analyze behavior of gases and solids in the reactor. The chlorination reaction rate constant was found to be a function of temperature and coke quantity present.     

Unsteady-State Inverse Chlorine Modeling in Pipe Networks

A model that directly determines the required source concentration(s) to meet specified chlorine residuals at selected nodes is developed for unsteady flows. The model considers even, over-, and underdetermined cases in which the number of available equations is equal to, greater than, or less than the number of unknowns, respectively. The model computes the unsteady-state flows and chlorine concentrations in the system first. Then it utilizes the inverse method to determine the required source concentration(s) so that specified residuals at selected nodes match the calculated ones. To verify the model, chlorine concentrations computed by the forward method are input to the inverse model to determine the source concentration(s) in two hypothetical networks. Results show that the model computed accurately the source concentrations for the even and the overdetermined cases, but the results were less than satisfactory for the underdetermined case.     

Adjoint Sensitivity Analysis of Contaminant Concentrations in Water Distribution Systems

Sensitivity analysis is used to determine how a system state or a model output changes due to a change in the value of a system parameter or a model input. We present the adjoint approach for determining the sensitivity of the concentration of a contaminant in a water distribution system to a change in a system parameter such as the location of the source of contamination, the reaction rate of the contaminant, and others. With the adjoint method, the sensitivity of the model output to any number of parameters can be obtained with one simulation of the adjoint model. If the number of parameters of interest exceeds the number of model outputs for which the sensitivity is desired, the adjoint method is more efficient than traditional direct methods of calculating sensitivities. We develop the adjoint equations for water quality in a water distribution system, verify the adjoint-based sensitivity equation using an analytical example, and demonstrate the numerical calculation of adjoint sensitivities using EPANET.     

Predicting the Formation of Chlorinated and Brominated By-Products

Although disinfection was one of the major public health advances in the last century and continues to be so in the twenty-first century, the disinfectants themselves may react with naturally occurring materials in treated water to form unintended by-products, which may themselves pose risks. This is of particular concern with regard to the use of chlorine. Generation of disinfection by-products (DBPs) has been shown to be a function of various factors including total organic carbon concentration, type of organic precursor, chlorination level, pH, temperature, reaction time, and UV-254 absorbance. Another factor affecting DBP formation is the presence and concentration of the bromide ion in the raw or finished water. Bromine substitutes for chlorine to produce bromine-containing homologues of the more familiar chlorine species. The current list of by-products targeted for regulation contains brominated and mixed bromine-chlorine species of total trihalomethanes and haloacetic acids. These are known to form in bromide-containing waters when chlorinated. To control chlorination DBPs therefore requires an understanding of the factors that influence their formation. This paper presents a model that can be used to predict the formation of chlorinated, brominated, and mixed species compounds based on initial chlorine concentration, chlorine consumption, bromide ion concentration, and pH. The model clearly shows that higher levels of bromide in the water favor the formation of brominated compounds. Brominated compounds also form faster than chlorinated compounds.     

熔盐氯化反应机理的研究

通过熔盐氯化实践的总结、数据分析和试验对反应机理进行研究，提出了熔盐氯化反应和收尘机理新观点——熔盐氯化的反应过程中高钛渣组分与氯气发生氯化反应的同时与熔盐中的组分直接生成低熔点复合氯化物。分析了低熔点复合氯化物生成机理，研究了低熔点复合氯化物的特点和形成低熔点复合氯化物的工艺条件。     

Pore Pressure Response Following Undrained uCPT Sounding in a Dilating Soil

The generation and dissipation of pore fluid pressures following standard piezocone sounding (uCPT) sounding in silty sands are observed to exhibit many of the characteristics of undrained penetration in dilatant materials; steady excess pore pressures may be subhydrostatic, or may become subhydrostatic during dissipation, and are slow to decay. Enigmatic pore pressure dissipation histories which transit from sub- to supra- and again to subhydrostatic before equilibrating at hydrostatic are consistent with a response where undrained pressures are maximally negative remote from the penetrometer tip. This surprising distribution of induced pore fluid pressures is accommodated in cavity expansion models for a dilating soil. A Mohr-Coulomb constitutive model is established for undrained loading of a soil with pore pressure response defined by Skempton pore pressure parameters. Defined in terms of effective stresses, this allows undrained stresses and pore pressures to be determined following cavity expansion in a c–? soil. Pore pressures are conditioned by the shear modulus, Skempton A parameter, and the “undrained shear strength.” The undrained shear strength is additionally modulated by the magnitudes of c, ?, A, and of the initial in situ effective stress, σ0′. Cavity expansion stresses, and pore pressures may be backcalculated. Undrained pore pressures are shown to decay loglinearly with radius from the cavity wall; they may be either supra- or subhydrostatic at the cavity wall, and where suprahydrostatic may become subhydrostatic close to the transition to the elastic region. This initial pressure distribution contributes to the observed switching between supra- and subhydrostatic pore pressures recorded during dissipation. “Type curves” that reflect the dissipation response enable the consolidation coefficient, undrained strength, and shear modulus to be computed from observed pore pressure data, and confirmed against independent measurements. In addition to representing the dilatory response of cohesionless silts, the method applies equally to recovering the pressure generation and dissipation response of overconsolidated clays.     

Chlorination kinetics of ZnO with Ar-Cl2-O2 gas and the effect of oxychloride formation

The chlorination rate of ZnO with Ar-Cl₂-O₂ gas was measured from 1023 K to 1273 K and the effects of temperature and partial pressures of chlorine and oxygen were investigated. The rate-determining step of chlorination was considered to be the dissociation of intermediate between ZnO and Cl₂ from linear relationship between reciprocal values of reaction rate and partial pressure of chlorine. The activation energy of chlorination was 58.2±2.5 kJ/mol. This comparatively low activation energy as a chemically controlled reaction was consistent with estimated rate-determining step of the dissociation of unstable compound. Increasing the partial pressure of oxygen slightly increased the chlorination rate, and this effect is considered to be caused by the increase in the formation rate of zinc oxychloride. To clarify the formation of zinc oxychloride, the equilibrium between Ar-Cl₂-O₂ gas and ZnO was investigated by the transpiration method at 1073 K. Calculated partial pressures of ZnOCl from experimental results were in the same order with or one order of magnitude larger than those estimated from reported Gibbs energy of formation of ZnOCl. Zinc oxychloride formation in ZnO chlorination must be taken into consideration as well as ZnCl₂ and Zn₂Cl₄ formation.     

Incorporating Reservoir Transfer into Treatment Optimization Decisions

The Massachusetts Water Resources Authority (MWRA) supplies unfiltered water from two large surface water reservoirs to the metropolitan Boston area, as well as to three smaller communities in central Massachusetts [the Chicopee Valley Aqueduct (CVA) communities]. Quabbin Reservoir is larger than Wachusett Reservoir, and has traditionally been used to supplement the Wachusett during the summer period. Quabbin water is also of better quality, with lower reactive natural organic matter (NOM). The MWRA began to add chlorine at Wachusett in 1997, and a new facility for adding chlorine at Quabbin for the CVA was also started up in 2000 to meet primary disinfection regulations to meet pathogen inactivation. The reaction of chlorine with NOM produces undesirable disinfection by-products (DBPs). The absorption of ultraviolet light at a wavelength of 254 nm was identified in chlorine decay studies to be the most important raw water quality parameter for predicting chlorine decay and DBP formation. This technical note summarizes the chlorine decay model for Wachusett and Quabbin water. The model is extended to ozonation of Wachusett water for the future Walnut Hill treatment plant. The models allowed the development of a trigger using UV-254 to time the Quabbin transfer to optimize treatment results. It is believed that the model for disinfectant decay and the use of UV-254 as a trigger for water treatment decisions are generalized and applicable to other water utilities.     

The selective carbochlorination of iron from titanlferous magnetite ore in a fluidized bed

The selective chlorination of iron from titaniferous magnetite ore using solid carbon as a reducing agent was studied in a fluidized bed. The effects of chlorination temperature, chlorine gas partial pressure, ratio of ore to carbon particle sizes, and the amount of added carbon were determined. Experimental results indicate that temperatures between 900 and 1000 K were favorable for the selective chlorination of iron. The rate was found to be first order with respect to chlorine concentration, and the observed effects of particle size, temperature, and the amount of carbon added were expressed quantitatively by using a mixed-control model. Formerly Graduate Student with the Department of Metallurgical Engineering, University of Utah     

Process Optimization and Reaction Mechanism of Removing Copper From an Fe-Rich Pyrite Cinder Using Chlorination Roasting

The aim is to remove copper from a pyrite cinder by optimizing the chlorination roasting process using response surface methodology (RSM) and the reaction mechanism of chlorination roasting based on thermodynamic calculation was discussed. A quadratic model was suggested by RSM to correlate the key parameters, namely, dosage of chlorinating agent, roasting temperature and roasting time to the copper volatilization ratio. The results indicate that the model is well consistent with the experimental data at a correlation coefficient (R²) of 0.95, and the dosage of chlorinating agent and roasting temperature both have significant effects on the copper volatilization ratio. However, a roasting temperature exceeding 1170°C decreases the volatilization ratio. The optimum conditions for removing copper from the cinder were identified as chlorinating agent dosage at 5%, roasting temperature at 1155.10 °C and roasting time of 10 min; under such a condition, a copper volatilization ratio of 95. 16% was achieved from the cinder. Thermodynamic calculation shows that SiO₂ in the pellet plays a key role in the chlorine release from calcium chloride, and the chlorine release reactions cannot occur without it.