Modeling of trihalomethane cometabolism in nitrifying biofilters

The computer program AQUASIM was used to model biofilter experiments seeded with Lake Austin, Texas mixed-culture nitrifiers. These biofilters degraded four trihalomethanes (THMs) (trichloromethane (TCM) or chloroform, bromodichloromethane (BDCM), dibromochloromethane (DBCM), tribromomethane (TBM) or bromoform) commonly found in treated drinking water. Apparent steady-state data from the biofilter experiments and supporting batch experiments were used to estimate kinetic parameters for TCM, DBCM and ammonia degradation. Subsequently, the model was verified against other experimental biofilter data. To allow for full-scale simulations, BDCM and TBM rate constants were estimated using data from batch kinetic studies. Finally, the model was used to simulate full-scale filter performance under different filter surface loading rates and THM speciation seen in practice. Overall, total THM removals ranged from 16% to 54% in these simulations with influent total THM concentrations of 75-82microg/L, which illustrates the potential of THM cometabolism to have a significant impact on treated water quality.     

Cometabolism of trihalomethanes by mixed culture nitrifiers

Three mixed-culture nitrifier sources degraded low concentrations (25-450 microg/L) of four trihalomethanes (THMs) (trichloromethane (TCM) or chloroform, bromodichloromethane (BDCM), dibromochloromethane (DBCM), tribromomethane (TBM) or bromoform) commonly found in treated drinking water. Individual THM rate constants (k1THM) increased with increasing THM bromine-substitution with TBM>DBCM>BDCM>TCM and were comparable to previous studies with the pure culture nitrifier, Nitrosomonas europaea. A decrease in temperature resulted in a decrease in both ammonia and THM degradation rates with ammonia rates affected to a greater extent than THM degradation rates. The significant effect of temperature indicates that seasonal variations in water temperature should be a consideration for technology implementation. Product toxicity, measured by transformation capacity (T(c)), was similar to that observed with N. europaea. Because both rate constants and product toxicities increase with increasing THM bromine-substitution, a water's THM speciation is an important consideration for process implementation during drinking water treatment. Even though a given water is kinetically favored, the resulting THM product toxicity may not allow stable treatment process performance.     

Performance and biofilm activity of nitrifying biofilters removing trihalomethanes

Nitrifying biofilters seeded with three different mixed-culture sources removed trichloromethane (TCM) and dibromochloromethane (DBCM) with removals reaching 18% for TCM and 75% for DBCM. In addition, resuspended biofilm removed TCM, bromodichloromethane (BDCM), DBCM, and tribromomethane (TBM) in backwash batch kinetic tests, demonstrating that the biofilters contained organisms capable of biotransforming the four regulated trihalomethanes (THMs) commonly found in treated drinking water. Upon the initial and subsequent increased TCM addition, total ammonia nitrogen (TOTNH₃) removal decreased and then reestablished, indicating an adjustment by the biofilm bacteria. In addition, changes in DBCM removal indicated a change in activity related to DBCM. The backwash batch kinetic tests provided a useful tool to evaluate the biofilm’s bacteria. Based on these experiments, the biofilters contained bacteria with similar THM removal kinetics to those seen in previous batch kinetic experiments. Overall, performance or selection does not seem based specifically on nutrients, source water, or source cultures and most likely results from THM product toxicity, and the use of GAC media appeared to offer benefits over anthracite for biofilter stability and long-term performance, although the reasons for this advantage are not apparent based on research to date.     

Human health risk assessment of chlorinated disinfection by-products in drinking water using a probabilistic approach

The presence of chlorinated disinfection by-products (DBPs) in drinking water is a public health issue, due to their possible adverse health effects on humans. To gauge the risk of chlorinated DBPs on human health, a risk assessment of chloroform (trichloromethane (TCM)), bromodichloromethane (BDCM), dibromochloromethane (DBCM), bromoform (tribromomethane (TBM)), dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) in drinking water was carried out using probabilistic techniques. Literature data on exposure concentrations from more than 15 different countries and adverse health effects on test animals as well as human epidemiological studies were used. The risk assessment showed no overlap between the highest human exposure dose (EXP(D)) and the lowest human equivalent dose (HED) from animal test data, for TCM, BDCM, DBCM, TBM, DCAA and TCAA. All the HED values were approximately 10(4)-10(5) times higher than the 95th percentiles of EXP(D). However, from the human epidemiology data, there was a positive overlap between the highest EXP(D) and the lifetime average daily doses (LADD(H)) for TCM, BDCM, DCAA and TCAA. This suggests that there are possible adverse health risks such as a small increased incidence of cancers in males and developmental effects on infants. However, the epidemiological data comprised several risk factors and exposure classification levels which may affect the overall results.     

Decomposition of trihaloacetic acids and formation of the corresponding trihalomethanes in drinking water

The decomposition of trihaloacetic acids [bromodichloroacetic acid (BDCAA), dibromochloroacetic acid (DBCAA), tribromoacetic acid (TBAA)], and the formation of the corresponding trihalomethanes [bromodichloromethane (BDCM), dibromochloromethane (DBCM), tribromomethane (TBM)] were studied. Like TBAA, the two mixed chlorobromo-species, BDCAA and DBCAA, were found to decompose to form BDCM and DBCM, respectively, via a decarboxylation pathway. The decomposition of BDCAA, DBCAA and TBAA in water at neutral pH follows a first-order reaction, with rate constants of 0.0011, 0.0062 and 0.040 day(-1) at 23 degrees C, respectively; and 0.000028, 0.00014 and 0.0016 day(-1) at 4 degrees C, respectively. The activation energies for the decomposition reaction of BDCAA, DBCAA and TBAA in water at neutral pH were found to be 35.0, 34.5 and 29.2 kcal/mol, respectively. The effect of pH in the range of 6-9 and the effect of a drinking water matrix on the decomposition of BDCAA, DBCAA, and TBAA in water were found to be insignificant. Measurement and health implications due to decomposition of trihaloacetic acids and formation of the corresponding trihalomethanes were discussed. By applying the technique of quantitative structure-activity relationships (QSAR), the decomposition rate constants of six iodinated trihaloacetic acids were estimated.     

Analysis of trihalomethanes in drinking water using headspace-SPME technique with gas chromatography

In many drinking water treatment plants, the chlorination process is one of the main techniques used for the disinfection of water. This disinfecting treatment leads to the formation of trihalomethanes (THMs) such as chloroform, dichlorobromomethane, chlorodibromomethane and bromoform. In this study, headspace-solid-phase microextraction (HS-SPME, 85 microm carboxen/polydimethylsiloxane fiber) technique was applied for the analysis of THMs in drinking water. The effects of experimental parameters such as kinds of SPME fiber, the volume ratio of sample to headspace, the addition of salts, magnetic stirring, extraction temperature, extraction time and desorption time on the analysis were investigated. Analytical parameters such as linearity, repeatability and limit of detection were also evaluated. The results of THMs from the survey of Seongnam (Korea) drinking water samples showed that the highest total trihalomethane and chloroform were 24.03 and 13.34 microg/l, which were well within the Korean drinking water quality standard of 100 and 80 microg/l, respectively.     

Use of regression models to link raw water characteristics to trihalomethane concentrations in drinking water

A study was conducted to examine the effect of raw water bromide on the formation and distribution of trihalomethanes (THMs) in finished drinking water. Twenty major water supplies in East Tennessee were selected for their significant levels of bromine-containing THMs, as demonstrated by previous studies. The cities were sampled quarterly for raw water pH, temperature, NVTOC, and bromide content, as well as finished water pH, NVTOC, and applied chlorine dose, coupled with conjunctive measurement for 7-day THMs.Few data for bromide levels in natural waters are currently available since most conventional colorimetric applications lack desired sensitivity, as does direct ion chromatography (IC). Although Fishman's kinetic permanganate method is adequate in the 10–100 μp 1⁻¹ range, it is time-consuming, and prone to certain interferences. An IC method using a sample preconcentration column has been evaluated. The method produced a 1 μg 1⁻¹ minimum detection level using deionized water based standards, with 3% relative precision completed at a standard concentration of 1000μg 1⁻¹. Bromide levels in the raw waters sampled were found to range from 10 to 225 μg 1⁻¹.Concurrent with the field sampling, laboratory chlorination experiments were conducted using Tennessee River water, under controlled laboratory conditions of bromide level, chlorine dose, pH, ionic strength, temperature, and organic precursor concentration. Resultant THM formations were monitored over a 96 h reaction period. The results concurred with observations made in previous work by such researchers as Bird and Rook.Nonlinear regression models for THM formation were generated using the laboratory chlorination data with respect to pH, temperature, chlorine dose, bromide, and NVTOC level. Actual values for these variables were substituted into the regression models, using the seasonal field data. Resultant predictive THM values were then compared with actual THM values for those data sampled. In general, these models were found to give acceptable fits. Overall 74.1% of the predicted values were within ± 15% of the measured values.     

Carbohydrates as trihalomethanes precursors. Influence of pH and the presence of Cl(-) and Br(-) on trihalomethane formation potential

Upon chlorination carbohydrates can give trihalomethanes (THMs). In the present work, we have studied the influence of pH, chloride or bromide concentration on the formation of THMs from carbohydrates. We have observed that THMs are not formed at acidic pH, while basic pH values only increase slightly the THM content, although the consumption of chlorine increases up to 100% with respect to pH 8. The presence of chloride in ppm increases the THM formation from carbohydrates without influence of the chlorine consumption. In the same manner, the presence of bromide ions in ppb also increases remarkably the THMs formed upon chlorination of saccharides. Even more, we have observed that at bromide concentrations below 100ppb, complete incorporation of bromide in THMs occurs. Overall, the results obtained show that saccharides widely present in natural waters can give rise to significant THM concentrations in the disinfection process by chlorine.     

Electrochemical removal of bromide and reduction of THM formation potential in drinking water

Trihalomethanes (THMs), a by-product of the chlorination of natural waters containing dissolved organic carbon and bromide, are the focus of considerable public health concern and regulation due to their potential as a carcinogen by ingestion. This paper presents a promising new water treatment process that lowers the concentration of bromide in drinking water and thus, lowers the THM formation potential. Bromide is oxidized by electrolysis to bromine and then the bromine apparently volatilized. The electrolyzed water, when chlorinated, produces measurably lower amounts of THMs and proportionately fewer brominated THMs, which are of greater public health concern than the chlorinated THMs. Removing bromide should also reduce the formation of other disinfection by-products such as bromate and haloacetic acids.     

Effect of bromide on the formation of disinfection by-products during wastewater chlorination

The effect of bromide ion on the formation and speciation of trihalomethanes (THMs) and haloacetic acids (HAAs) during the chlorination of biologically treated wastewaters was investigated. The experimental results showed that the formation of total THMs and total HAAs during chlorine disinfection increased with increasing bromide levels in wastewater. The formation of CHBr₃ increased nearly linearly with increasing bromide ion levels, while CHCl₂Br and CHClBr₂ increased with increasing bromide concentration from 0 to 3.2 mg L⁻¹ and thereafter remained constant or slightly decreased. Increasing initial bromide levels up to 12.8 mg L⁻¹ resulted in sharp decrease of the concentration of CHCl₃ and chloro- HAAs. The mixed bromochloro- HAAs and bromo-only species replaced chloro- HAAs as the dominated species of HAA with increasing bromide levels. The distribution of monohalogenated, dihalogenated and trihalogenated species of HAAs in chlorinated wastewater at high concentration of bromide (>2 mg L⁻¹) is different from that of drinking/natural water. The values of the bromine incorporation factors, n (Br) and n′ (Br), increased with increasing bromide concentration and remained constant or slightly decreased with increasing contact time under the studied range of bromide ion concentrations during chlorination. Moreover, the bromine incorporation into THMs was higher than that of HAAs with bromide levels ranging from 1.0 to 12.8 mg L⁻¹, indicating the dissimilar formation mechanisms of THMs and HAAs involving bromide.     

Effect of bromide ions on genotoxicity of halogenated by-products from chlorination of humic acid in water

Genotoxicity of halogenated by-products obtained by chlorination of humic acid in water was evaluated in the presence of bromide ions (Br⁻). After the halogenated humic acid solution was made to flow through CSP800 cartridge, absorbed substances were eluted with dimethyl sulfoxide or acetone, and subjected to mutagenicity assays and to analysis of trihalomethanes (THMs). Mutagenic activity was measured by Ames tests using S. typhimurium TA100 strain without metabolic activation, and by the frequencies of micronuclei formation using cultured Chinese hamster lung cells (CHL/IU) in vitro. A powerful effect of bromide ions in chlorinated humic acid solutions was observed on the reverse mutation and micronuclei formations. The formations of total THMs and more brominated THMs were also enhanced in the presence of bromide ions. The ratio of [Br⁻/Cl⁻] regulated the composition and concentrations of THMs intensely, and the rate of substitution of Br⁻ was greater than that of chloride ions (Cl⁻). The increments of the mutagenicity and total THMs formed in chlorinated solutions were observed in parallel with the concentration of Br⁻ or Cl⁻. From the observations, it was concluded that the increasing mutagenicity might be caused by the increasing chlorinated and/or brominated by-products.     

Trihalomethanes formation in water treated with chlorine dioxide

Formation of trihalomethanes (THMs) was investigated in water treated with chlorine dioxide (ClO₂) and/or chlorine (Cl₂) where humic acid (HA) was used as THMs precursors. When ClO₂ was used as the only disinfectant, no THMs were detected in bromide-free water; while only CHBr₃ was formed in water containing bromide ion because ClO₂ could oxide bromide to form hydrobromous acid which subsequently reacted with HA, and the yield CHBr₃ increased with bromide concentration and ClO₂ dosing. When water was treated with ClO₂ combined with Cl₂, only CHCl₃ was formed in the absence of bromide, however, all four species of THMs were formed in the presence of bromide; the THMs formation potential decreased gradually with an increase in the ratio of ClO₂ to Cl₂ because ClO₂ reacted with HA to render them unreactive or unavailable for THMs production. When water (with or without bromide ion) was irradiated by light, the yield of THMs was increased as a function of irradiation time to a maximum, and thereafter decreased markedly; the possible mechanism is that irradiation could activate the THMs precursors in HA, and at the same time destroy the reactivity of ClO₂ or Cl₂. The same results could be collected from natural water treated by ClO₂ with or without irradiation.     

Cancer risk assessment from trihalomethanes in drinking water

This study intends to calculate the lifetime cancer risks resulting from intakes of trihalomethanes (THMs) in drinking water based on the presence of each THM species. The slope factors for each THM species are used, combined with exposure model and Monte Carlo simulations, to calculate the cancer risks with consideration of different exposure routes (oral ingestion, inhalation and dermal absorption). The results revealed that the highest risk comes from the inhalation exposure to chloroform during showers, which also dominates the total risk associated with chloroform exposure. For dichlorobromomethane and chlorodibromomethane, inhalation exposure also plays an important role for total risks; however, contribution from the oral consumption cannot be ignored for these two compounds. Bromoform contributes the least cancer risk among the four THM species, with a risk factor two orders of magnitude smaller than the other three THM species. For all of the four THM species, exposure from dermal absorption is not significant when compared with oral ingestion and inhalation exposures. This study also uses the THMs data collected from Taiwan to calculate the cancer risks associated with THM exposures in different areas of Taiwan. Due to the variations of the THMs compositions, it is observed that higher concentrations of total THMs do not necessarily lead to higher cancer risks. Areas with higher bromide concentration in raw water and often with higher total THM concentration may actually give lower cancer risk if the THMs formed shift to bromoform. However, this also leads to the violation of THM standards since bromoform has much higher molecular weight than chloroform. Based on the results of the cancer risks calculated from each THM species, the regulatory issue of the THMs was also discussed.     

Seasonal variation in drinking water concentrations of disinfection by-products in IZMIR and associated human health risks

Seasonal variation in concentrations of two different disinfection by-product groups, trihalomethanes (THMs) and haloacetonitriles (HANs), was investigated in tap water samples collected from five sampling points (one groundwater and four surface water sources) in Izmir, Turkey. Estimates of previously published carcinogenic and non-carcinogenic risks through oral exposure to THMs were re-evaluated using a probabilistic approach that took the seasonal concentration variation into account. Chloroform, bromoform, dibromochloromethane and dichloroacetonitrile were the most frequently detected compounds. Among these, chloroform was detected with the highest concentrations ranging from 0.03 to 98.4 microg/L. In tap water, at the groundwater supplied sampling point, brominated species, bromoform and dibromoacetonitrile, were detected at the highest levels most probably due to bromide ion intrusion from seawater. The highest total THM and total HAN concentrations were detected in spring while the lowest in summer and fall. The annual average total THM concentration measured at one of the surface water supplied sampling points exceeded the USEPA's limit of 80 microg/L. While all non-carcinogenic risks due to exposure to THMs in Izmir drinking water were negligible, carcinogenic risk levels associated with bromodichloromethane and dibromochloromethane were higher than one in million.     

Trihalomethanes in the drinking water of Concepción and Talcahuano,Chile

Formation of trihalomethanes (THMs) during water disinfection has been related to several health problems, although the magnitude of these effects is under discussion. This paper quantifies the THMs in drinking water from the Bío‐Bío Region of central Chile, the first since the modification of the national reference value (Nch 409/05) to include maximal values for THMs. THMs were quantified using a solid phase micro‐extraction (SPME) method and GC‐MS. The concentration ranges were 9.7–111.6, 0.1–1.0 and 0.9–25.5 μg/L for chloroform (CHCl₃), and dibromochloromethane (CHClBr₂) and bromodichloromethane (CHCl₂Br), respectively. Bromoform was not detected in any sample. There were good correlations (R²=0.91–0.98, P<0.001) between the THMs and the residence time of the water, the distance from the treatment plant and an inverse correlation to free chlorine in the water. The Additive Toxicity Index Value (0.07–1.00) showed that all samples were within the Chilean reference value for THMs in drinking water. However, several values were close to exceeding the maximum permitted concentration (200, 100, 100 and 60 μg/L for CHCl₃, CHBr₃, CHClBr₂ and CHCl₂Br, respectively), which may occur when the water demand is low and thus residence times are longer.     

Disinfection by-product formation following chlorination of drinking water: Artificial neural network models and changes in speciation with treatment

Artificial neural network (ANN) models were developed to predict disinfection by-product (DBP) formation during municipal drinking water treatment using the Information Collection Rule Treatment Studies database complied by the United States Environmental Protection Agency. The formation of trihalomethanes (THMs), haloacetic acids (HAAs), and total organic halide (TOX) upon chlorination of untreated water, and after conventional treatment, granular activated carbon treatment, and nanofiltration were quantified using ANNs. Highly accurate predictions of DBP concentrations were possible using physically meaningful water quality parameters as ANN inputs including dissolved organic carbon (DOC) concentration, ultraviolet absorbance at 254 nm and one cm path length (UV₂₅₄), bromide ion concentration (Br⁻), chlorine dose, chlorination pH, contact time, and reaction temperature. This highlights the ability of ANNs to closely capture the highly complex and non-linear relationships underlying DBP formation. Accurate simulations suggest the potential use of ANNs for process control and optimization, comparison of treatment alternatives for DBP control prior to piloting, and even to reduce the number of experiments to evaluate water quality variations when operating conditions are changed. Changes in THM and HAA speciation and bromine substitution patterns following treatment are also discussed.     

不同净水工艺对含溴水体消毒副产物生成势的影响

以溴离子和有机物浓度不同的5个水厂原水和各工艺段出水为研究对象,考察了不同净水工艺对三卤甲烷(THMs)和卤乙酸(HAAs)这两类典型消毒副产物生成势和种类分布的影响。结果表明,仅采用常规处理工艺对THMs和HAAs生成势的控制效果不明显,而增设生物预处理和臭氧氧化预处理工艺能显著提高常规工艺对THMs和HAAs前体物的去除效果,臭氧/生物活性炭(O3/BAC)深度处理工艺能进一步去除THMs和HAAs的前体物。增设预处理和O3/BAC深度处理工艺,并采取砂滤池后置的净水工艺流程对THMs和HAAs生成势的控制效果最好。对于含溴水体,溴离子浓度越高,有机物中亲水性组分所占比例越高,经氯消毒后生成的溴代THMs和HAAs所占比例就越高。随着处理工艺流程的进行,THMs和HAAs的生成势逐渐降低,但是它们的溴结合因子逐渐增大,即毒性更大的溴代组分所占比例逐渐增大。     

Inhalation exposure to THMs from drinking water in south Taiwan

Trihalomethanes (THMs) are important disinfection byproducts (DBPs) in drinking water. To understand the magnitude of exposure to THMs for the people in southern Taiwan, models are used to estimate the inhalation exposure associated with drinking water based on raw water quality. Two parts of models are used in this study, one for estimating THM concentration from raw water quality, and one for estimating inhalation exposure to people. Important raw water quality and operational parameters, including TOC, UV254, pH, temperature, chlorine dosage, and water residence time of a major water treatment plant in south Taiwan were collected. An empirical THM formation model was then employed to predict the THM concentration at consumers' dwellings based on the parameters collected. Differences between the predicted results and experimental data were found to be small, indicating that the model is appropriate. The predicted THM concentration distribution was served as input parameters for the exposure models. Three major scenarios associated with probable inhalation exposure of THMs, including shower, pre- and post-cooking activities, and cooking processes, were considered in the exposure models. The model results show that the mean inhalation exposure of THMs for shower, pre- and post-cooking activities, and cooking processes are 26.4, 1.56, 3.29 micrograms/day, respectively. The total inhalation exposure (summation of the three scenarios) was found to be comparable with that for direct ingestion, indicating that inhalation is an important pathway for THM exposure from drinking water.     

Factors affecting formation of haloacetonitriles, haloketones, chloropicrin and cyanogen halides during chloramination

Effects of contact time, monochloramine doses, monochloramine application modes, pH, temperature and bromide ion concentrations on formation of disinfection by-products (DBPs), including haloacetonitriles, haloketones, chloropicrin, cyanogen halides and trihalomethanes, during chloramination were investigated using model solutions containing 5 mg/L (as DOC) Suwannee River natural organic matter (NOM). Chloramine speciation and some DBPs were measured using membrane introduction mass Spectrometer (MIMS). Longer reaction times led to continued formation over time for dichloroacetonitrile (DCAN), 1,1-dichloro-2-propanone (1,1-DCP) and chloroform. Cyanogen chloride (CNCl) formation occurred over time, but after reaching a peak concentration CNCl concentrations decreased over longer time periods. Linear relationships were observed between the formation of DCAN, 1,1-DCP, CNCl or chloroform and the dosage of monochloramine. Chloramination modes (addition of preformed monochloramine or variable sequential additions of free chlorine and ammonium salts) exhibited the largest impact on chloroform formation but displayed little effect on the formation of DCAN, 1,1-DCP and CNCl. Over the range in pH from 4 to 9 profound differences in DBP formation were observed; pH values between 5 and 6 resulted in the highest DBP concentrations. An increase in temperature enhanced the formation of chloroform but did not affect DCAN, 1,1-DCP and CNCl formation. Chloropicrin concentrations were always low (around detection limits) under all conditions. Increasing the concentrations of bromide ions enhanced the formation of bromine-substituted DBPs.     

DPB formation in breakpoint chlorination of wastewater

The formation of trihalomethanes (THMs) and haloacetic acids (HAAs), two major disinfection by-products (DBPs), from the breakpoint chlorination of three diluted yet buffered (pH 7.0) wastewater effluents was studied. The concentrations and distributions of THMs and HAAs species varied among different effluents at different zones of the breakpoint curves. Nevertheless, some common trends were observed. The formation of chloro-only THMs and HAAs, after normalization with the carbon contents of the effluents, increased with increasing the specific UV absorbance (SUVA) of the effluents but the dependency is not valid for bromo- or bromochloro-DBPs. The formation of THMs and HAAs showed no significant inclination with increasing chlorine dosages up to the breakpoint, but increased sharply beyond the breakpoint dosing level. Bromine incorporations into THMs and HAAs increased with an increasing bromide to DOC molar ratio. In addition, the bromine incorporation was also found to be highly dependent on the chlorine dosage and the bromide to ammonia ratio. A longer reaction time increased the yields of THMs and HAAs and was found to favor the formation of dihalogenated HAAs. A two-stage correlation between the total THMs and the total HAAs was found for each wastewater effluent.