The New Generation of Disinfection Byproducts (DBPs) Term Paper in Biogeochemistry and Pollutant Dynamics Master Studies in Environmental Sciences, ETH Zürich by Stefan Rieder Academic Advisor: T. Hofstetter and U. von Gunten Zürich, November / Dezember 2007
Abstract Disinfection of drinking water is an important procedure to maintain the water quality. There are four common disinfectants; chlorine, chloramine, chlorine dioxide and ozone. Chlorine is the most used. Water treatment leads with all disinfectants to different byproducts. Chlorination, chloramination and chlor dioxination of water can lead to the same DBPs but in different amounts. But there also exist specific DBPs for each disinfectant. Among of them shows toxic effects of human health. Especially brominated DBPs can lead to serious health problems, often they induce cancer. Bromate is one of these toxic DBPs, formed by ozonation of bromide in water. The reduction of such DBPs is one of the main targets of the DBP research. Therefore it is important to understand, how the particular DBPs are formed and which conditions promote their formations. Generally is the formation dependent on the amount and characteristic of organic matter in the water, the pH, the disinfect time and the amount of the disinfectant.
1. Introduction Water resources and it’s maintain of quality are important issues in human life. Many human diseases are related to water-borne pathogens, mainly if the drinking water is affected by wastewater or human or animal excretions. To process water, people have disinfected drinking water since the end of the 19th century [8]. The disinfection of fresh water has the main target to destroy microbial and viral pathogens in order to eliminate waterborne diseases like cholera and typhoid. In many papers is the disinfection of drinking water described as a major public health success of the last century [9]. Before it was widespread used, huge numbers of humans died of cholera, typhoid and other waterborne diseases. The outbreak of these diseases is extremely decreased. Recent outbreaks, for example of Escherichia coli-induced gastroenteritis (Walkerton, Ontario, 2000) or cholera (Peru beginning in 1991), shows the importance of water disinfections [9]. A minor drinking water problem can be the taste and odor from the water. Chlorine, ozone, chloramine and chlorine dioxine are the most used disinfectants to minimize the number of the pathogens in water. This compounds are strong oxidants and convert natural organic matter to disinfection byproducts (DBPs). The first DPB was described in 1974. Many studies showed negative influence of DBPs to human’s health. Animal studies adverted to a correlation between some DBPs and the appearance of cancer. Other concerns showed negative reproductive and developmental effects after the exposure by DBPs [10]. Typical, there exist two sources of raw water for the drinking water production namely groundwater and surface water. Ground water is usually of better quality as compared to surface water (microbial). Additionally surface water (water from rivers, lakes and reservoir) contains higher levels of organic matter and particles. Therefore, lower disinfectants are necessary by groundwater disinfection than by surface water disinfection. The higher amounts of disinfectants combined with the higher amount of organic matter and other compounds in the surface water usually lead to more DBPs than disinfection of groundwater. In the USA, water treatment often involves the disinfection of waste water that often leads to higher concentrations of DBPs, because higher amounts of organic matter are presented and more disinfectant are necessary. In Switzerland, the waste water first will be treated in sewage treatment plants (STP). In this treatment, DBPs can be formed by oxidation of organic compounds by disinfectants. To get drinking water, one disinfects raw water. Raw water treatment processes including usually coagulation, flocculation, sedimentation, filtration and disinfection. In this step, DBPs can be formed too. The literature describes approximately 600-700 DBPs today which are formed by the common disinfectants [2]. In this paper I will try to explore how and in which conditions these products were formed. I will concentrate on the three most used disinfectant, chlorine, chloramines, and ozone. In a second part, I will elect a special DBP, n-nitrosodimethylamine, and I will describe this formation in details. Influence factors can be the disinfectant way, the concentration of the disinfectant, the contact time, the organic compounds in water (DBP precursors), the treatment and the pH [6].
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2. Formation of DBPs by different disinfectants This paper focuses on three common disinfectants, chlorine, chloramines and ozone. I exclude chlorine dioxide, a forth often used disinfectant, of this paper because it forms most the same DBPs like Chlorine. In the last approximately 100 years, chlorine was the usual disinfectant. Chlorine forms many DBPs; some of these are carcinogenic (see point 2.1). In this part, I’ll describe the most used disinfectants and their major DBPs. A recently realized US Nationwide DBPs Occurrence Study ascertained the most priority organic DBPs. They arranged the DBPs in 11 groups. Table 1 shows a part of the results of this study [2]. The seven shown groups are part of the discussed or mentioned DBPs in this paper. The most groups in table 1 include halogenated DBPs. The halogen atom can come from the treated water (for example bromide) or can bring in by the disinfectant (for example Cl by chlorination). Table 1. Priority DBPs Selected for Nationwide Occurrence Study a Not a DBP but included because it is an important source water contaminant.b DBP not originally prioritized (identified in drinking water after initial prioritization) but included due to similarity to other priority compounds.c Not analyzed; not stable in water.d DBP not given a high priority but included for completeness sake to provide more representation to ozone DBPs for occurrence. From Krasner et al. 2006: [2]
chloromethane bromomethane (methyl bromide)a bromochloromethane dibromomethane
halomethanes dichloroiodomethane bromochloroiodomethane b dibromoiodomethane b chlorodiiodomethane
chloroacetonitrile bromoacetonitrile
haloacetonitriles bromodichloroacetonitrile dibromochloroacetonitrile
chloropropanone
haloketones 1-bromo-1,1-dichloropropanone
1,1,3,3-tetrabromopropanoneb
1,1,3,3-tetrachloropropanone 1,1,1,3-tetrachloropropanone
1,1,1,3,3-pentachloropropanone c hexachloropropanone
haloaldehydes b bromochloroacetaldehyde
tribromoacetaldehyde
1,3-dichloropropanone 1,1-dibromopropanone 1,1,3-trichloropropanone
chloroacetaldehyde dichloroacetaldehyde
chloronitromethaneb bromonitromethane b dichloronitromethane
b
monochloroacetamide monobromoacetamideb
b
bromodiiodomethane triiodomethane (iodoform)b carbon tetrachloride tribromochloromethane
tribromoacetonitrile
c
b
halonitromethanes bromochloronitromethaneb dibromonitromethane b bromodichloronitromethane
dibromochloronitromethaneb b tribromonitromethane (bromopicrin)
haloamides dichloroacetamide dibromoacetamideb
trichloroacetamide
b
carbonyls
3
2-hexenal
5-keto-1-hexanal
cyanoformaldehyde d
d
methylethyl ketone
6-hydroxy-2-hexanoned
dimethylglyoxal
2.1. Chlorination Chlorination is the process of adding chlorine (Cl2) into water: Cl2 + H2O → HOCl + HCl Salts of hypochlorous acid (mostly NaOCl) are often used in place of Cl2 due to the lower hazard during handling. The results of the reaction above are hypochlorous acid (HOCl) and hydrochloric acid (HCl). Hypochlorous acids react with many organic components and destroy efficiently many forms of bacteria and viruses. As a result of the chlorine reaction with organic matter, DBPs will be formed. Chlorine was the most used disinfectant for drinking water in the last 100 years. In USA, it is still the main disinfectant. The first observed DBP was formed after raw water chlorination treatment. This was the start of DBP research. The most efforts were spent to find out more about the chlorination treatment [6]. More than 300 reported DBPs are formed from chlorination. The two major classes of formed DBPs are trihalomethanes (THMs) and haloacetic acids (HAAs). These both are the most observed DBPs after the chlorination of wastewater. A third important class is the Haloacetonitriles (HANs). At least four of these compounds are carcinogenic (chloroform, bromodichloromethane, bromoform, MX) [11]. In the follow parts, I'll try to describe the different chlorinated DBPs briefly, especially their formation and effects.
2.1.1. Halomethanes The structure of halomethanes is based of methane. Halogenes take up the binding places of hydrogen atoms. For example the structural formula of Br bromodichloromethane is showed on the left. Cl C H Halomethanes with three haloatomes are called trihalomethanes (THMs), see in table 1 (column in the middle and on the right). Cl Trichloromethane (chloroform; CHCl3), tribromomethane (bromoform; CHBr3) and triiodomethane (iodoform; CHI3) are the most described forms of THMs. Especially chlofororm, bromoform and bromodichloromethane are human hazard. In animal experiments a correlation were found between these three THMs and the appearance of tumors in target organs (e.g. liver, kidney and bladder) [12]. Brominated DBPs are often more carcinogenetic than their chlorinated analogs. Bromide-ions are strong nucleophils and can interact with cellular components, especially with the DAN.
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The origin of chloroform can be illustrated by the interaction between chlorine and propanone. Propanone (an example for organic matter) will be oxidized into trichloropropanone if water became chlorinated: CH3COCH3 + HOCl CH3COCCl3 In the following reaction, trichloropropanone becomes hydrolyzed, especially at high pH: CH3COCCl3 + H2O CH3COOH + CHCl3 The result of both reactions is chloroform. An important influence of THM formation is the organic matter in the water. Hydrophobic organic matter (contains more aromatic compounds, less carboxlic acids and is probably of higher molecular weight compared to hydrophilic organic matter) leads to higher precursors concentrations of THMs than hydrophilic components (contains more carboxylic acids and less aromatic compounds, it is of lower molecular weight compared to hydrophobic organic matter). Due to this, more THMs are produced if in the organic matter is more hydrophobic than hydrophilic matter present [6]. Another factor is the contact time between the waste water and the chlorine. Figure 1 shows the THM4/TOC ratio (TOC = total organic carbon) depending on the contact time. This graphic descend from a observation by Liang and Singer [6]. Under THM4 they collect the sum of the four observed THMs (trichloromethane, bromodichloromethane, chlorodibromomethane, tribromomethane). All experiments were conducted from five water utilities. The water sources were in Tolt, Manatee, Groton, E.St.Louis and Indianapolis.
Fig.1: Formation of THM4 (sum of four diffrent THMs) in raw water depends of chlorine contact time by pH = 8 for five difftent water samples. From Ligan and Singer 2002: [6]
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In figure 1 we can see, that in the first 10 hours the THMs formation is much faster than in the following hours. Chlorinated raw water from Tolt shows the highest THM4/TOC ratio, chlorinated raw water from E. St. Louis and Indianapolis the lowest. How mentions above, THM formation increases with higher amounts of hydrophobic components. Organic matter in Told water shows the highest percentage of hydrophobic fraction (see chapter 3.1.2) and the highest ratio of THM4/TOC. The pH value has an effect of the formation of THMs too. The formation of THMs increases due to a increasing pH from 6 to 8 [6]. Richardson et al. observed that in warmer water, more THMs were produced than in colder [13]. Bromoform and bromodichloromethane can only appear, if bromide has been presented in the disinfected water. The highest concentrations of bromide in water can we find in seawater. Normally, the amount of salt in water reflects the value of bromide. In water by arid region (higher evaporations rates than rainfall) we find the highest concentrations of bromide excluded seawater. A second important point for building brominated trihalomethanes is the concentration of dissolved organic carbon in water. Normally, a high concentration of brominated trihalomethanes is observed at low dissolved organic carbon concentrations.
2.1.2. Haloacetic acids The based forms of haloacetic acids are carboxylic acids in which halogen atoms can take the place of hydrogen atoms in acetic acid. The Cl formula of acetic acid is CH3COOH. For example dichloroacetic acid is OH H shown on the left. There exist 9 common HAAs which we can arrange in 3 different groups of haloacetic acids. The amount of halogen atoms which replaced the hydrogen atoms at the alpha position, define the 3 groups monohaloacetic-, dihaloacetic-, trihaloacetic acids (see table 1). Some HAAs shows toxic effects. Exposure of some HAAs showed mutagenic effects or leaded to tumors. Therefore, the US Environmental Protection Agency (USEPA) and the WHO published guidelines for some HAAs in drinking water [12]. Chlorinated and brominated acetic acids are produced in chlorinated water as often or a bit fewer as those of trihalomethanes [12]. Same by the formation of THMs, hydrophobic organic matter in water leads to higher concentration of HAAs than by hydrophilic components. This is shown in Figure 2 for pH 6 [6]. The HAA9 formations from the hydrophobic fractions were in all cases higher than the corresponding HAA9 formation from the hydrophilic fractions. SUVA (Specific ultraviolet absorbance) is an indicator of natural organic material and a good predictor of aromatic carbon content in water. SUVA is calculated by the water sample’s ultraviolet absorption at a wavelength of 254 nm, times 100, divided by the sample concentration of dissolved organic carbon. As mentioned above, it exist a strong correlation between the SUVA value and the aromatic carbon content of natural organic matter in raw water. The SUVA value of the raw water increases as the percentage of hydrophobic carbon fraction (in raw water) increases [6]. O
Cl C
C
6
Fig.2: Building of HAAs in five different clorinated water during 72 hours by pH = 6. From Ligan and Singer 2002: [6]
In figure 2, the corresponding SUVA values are the numbers above the bars. In all cases, higher SUVA values correlate with higher HAA9/TOC ratios than the samples with the lower SUVA values. The one SUVA value from treated water from Told is more than two times higher than the other. The value of HAA9/TOC is in this water sample almost two times higher than in the waters with the lower SUVA values. The different HAA9/TOC values by chlorination of water from the five source regions, correlate with the values of THM4/TOC (see chapter 2.1.1). Water from Tolt has the highest HAA9/TOC value. The THM4/TOC ratio is in this water higher then in the four other waters. Treated raw water from Groton has the second highest value of THM4/TOC and HAA9/TOC (see figures 1, 2). Another effect to the formation of HAAs has the pH value. Does the pH in water increased from 6 to 8, a very little effect to the formation of monohaloacetic acids and dihaloacetic acids was observed. It seems that the pH in this range has only a significant effect for the formation of trihaloacetic acids. Decrease the pH from 8 to 6, the amount of trihaloacetic acids increase significantly [6]. On other observation was that more HAAs were formed in warmer months than in colder [13]. Brominated haloacetic acids have been observed by chlorination of water with elevated bromide concentrations. By chlorination of seawater the most common produced HAAs are dichloroacetic acid, bromodichloroacetic acid and dibromoacetic acid.
2.1.3. Haloacetonitriles Haloacetonitriles (HANs) are toxic, small nitrogenous water disinfection byproducts and does most exist with two halogen N substitutions [1]. For example dichloroacetonitrile is shown on the C Cl C left. After chlorination, HANs are usually formed at about an order of H magnitude lower concentration than THMs and HAAs [12]. The formation of HANs has been observed; when in chlorinated water nitrogen containing organic material was present. Cl
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I could not find somewhere information about factors which influencing the formation of HANs. I assume that the formation of HANs is not well researched. Muellner analyzed seven HANs due to their acute genotoxycity and chronic cytotoxicity in animal experiments with Chinese hamsters [1]. The seven HANs were iodoacetonitrile (IAN), bromoacetonitrile (BAN), dibromoacetonitrile (DBAN), bromochloroacetonitrile (BCAN), chloroacetonitrile (CAN), dichloroacetonitrile (DCAN) and trichloroacetonitrile (TCAN). The result was that DCAN, BCAN, CAN and TCAN are mutagenic in Salmonella typhimurium. HANs directly induced sister chromatid exchanges in the ovary cells of the inspected animals. But the toxicity of this effect was not the same for each of HANs. The rank order was DBNA > BCNA > TCAN > DCAN > CAN [1]. The results of the experiments are shown in figure 3 + 4. The figure 3 shows the chronic cytotoxic effect of the seven observed HANs by Chinese hamsters ovary cells (CHO) due to their concentration. Measured was the reduction in observed cell density after 72 hours (~ 3 cell divisions) exposure. All seven HANs were cytotoxic. Iodoacetonitrile, dibromoacetonitrile and bromoacetonitrile show by already low concentration strong effects to the cell density. A 50% reduction of the cell density was observed after exposure with these HAAs by a concentration of ~ 5 µM. By the same exposition concentration with chloroacetonitrile, dichloracetonitrile and tricloracetonitrile, no effect was observed. The level of genomic DNA damage was measured with Single cell gel electrophoreses (SCGE). This result is shown in figure 4. Iodoacetonitrile and dibromoacetonitrile shows by already low concentrations strong effects to the cell genomic DNA damage. By a concentration of ~ 50 µM of these two HAAs, more than 60% of the CHO cells show damages. By the same exposition concentration with bromochloroacetonitrile, chloroacetonitrile, dichloracetonitrile and tricloracetonitrile, no effect was observed.
Fig. 3: CHO chronic cytotoxity concentration for seven HANs From Mueller et al. 2007: [1]
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Fig. 4: SCGE genotoxicity concentration, response curves for seven HANs. This figure shows the level of DNA damage in electrophoresed nuclei. From Mueller et al. 2007: [1]
If there bromide or iodite was inside the chlorinated water, there could produced iodoand bromohaloacetonitriles.
2.1.4. Halophenols OH Br
Br
Halophenols are produced during chloronisation of phenol in water. Similar to THMs, there exists chlorine-, bromine- and iodinesubstitueted phenols. In natural water with bromide inside, brominated phenols is the common species. For example it is shown on the left 2, 4-bromophenol. Iodophenols are only to find in very low concentrations in chlorinated drinking water.
2.2. Chloramines If ammonia is added to chlorine, chloramines (for example monochloramine: NH2Cl) were formed [12]: NH3 + HOCl NH2Cl + H2O In this reaction, the nucleophile NH3 attacks the hyperchlorus acid (HOCl; formed by adding chlorine into water, see chapter 3.1). DBPs which are formed during chloramination often contain nitrogen. Another important fact is that in chloraminated water the concentration of formed THMs and HAAs is much lower than in chlorinated water. Chloramines are a weaker and more stable water disinfectant than chlorine. Chloramination of water forms the same DBPs which are producing during chlorination of water. But there are some more like cyanogens halides. The molecule 9
of hydrogen cyanide (HCN) has one hydrogen atom and is mostly produced in a reaction with ammonia, oxygen and methane. If a halogen atom replaces this hydrogen atom, cyanogen halides were formed. On the left is shown Cl C N cyanogen chloride for example. Cyanogen halides are highly toxic compounds. For the formation of cyanogen halides it is important, that ammonia is present in the water. That’s the reason, why cyanogen halides are commonly detected in chloraminated drinking water.
2.2.1. N-nitrosodimethylamine Nitrosamines are a class of compounds which are carcinogenic, mutagenic and teratogenic [3]. N-nitrosodimethylamine (NDMA) is an organic compound and a member of nitrosamines. Since NDMA was observed after chloramination in several water agencies, recently NDMA research had became a very important area. NDMA is one of the most potent detected carcinogen DBPs today but it is not regulated in the USA for drinking water yet [3]. The production of NDMA showed a strong pH dependence [10]. Because NDMAs are one of the strongest carcinogens and it does not exists a limitation of their concentration in drinking water, I will have a detailed view on this DBP in chapter 3.
2.3. Ozone Ozonation is a powerful oxidizing treatment of water. It kills many forms of bacteria’s and parasites which are resistant to conventional disinfectants (chlorination, chloramination). Ozonation minimizes the formation of haloorganic DBPs like THMs and HAAs. Ozone reacts very specific. One assume that ozone reacts most with double bonds, activated aromatic systems and non-protonated amines [14]. Are high concentrations of bromide (Br-) in water (problematic range about 50-100µg/L) the most problematic formation of a disinfection byproduct is bromate (BrO3-). The structure formula of bromate is drawn on the left. Bromate is fixed as a O Class 2B carcinogen by the International Agency for Research on Cancer Br O(IARC); it means that bromat is possibly carcinogens for humans. In animal O experiments realized with rats, exposure with potassium bromate leaded to renal cancer, mesotheliomas and thyroid follicular cell tumors. The United States and the EU determinate the legal limit of bromate in drinking water at 10 g/L [12]. Brominated nitromethane are extremely cytotoxic and genotoxic in mammalian cells [12]. Von Gunten propose two different possible ways to form bromate [4]. The first one includes an oxidation of bromide by ozone (figure 5), the second one is a reaction with ozone and OH radicals (figure 6). Both processes includes up to six oxidation states of bromide. In water, both reactions run simultaneously. To the reaction with ozone (figure 5): This reaction starts with an oxidation of bromide (Br-) by ozone. An oxygen atom binds to bromide and produce hypobromite (OBr-). Ozone oxidize OBr- to BrO2- and following BrO2- to BrO3- (bromate). The free Br- ion at the start of the reaction can form by ozonation too. Ozone oxidizes NH2Br, as a product Br- is formed. To the reaction with ozone and OH radicals (figure 6): OH radicals (OH*) transfer Brto Br radicals (Br*). Br* decay to HOBr/OBr- or can be oxidized by ozone to BrO 10
radicals (BrO*). BrO* decay to BrO2-. The next step is an oxidation with ozone to bromate. HOBr/OBr- undergoes two oxidation with bromate as product or will be transferred with OH* to BrO*.
Fig.5: Bromate formation by ozone From U. von Gunten 2003: [4]
Fig.6: Bromate formation by ozone and OH radicals From U. von Gunten 2003: [4]
The dose of ozone has a significant effect of the formation of bromate. At a dose above 3.1 mg/L, Wert observed a linear relationship between bromat formation and ozone doses [7]. He showed that in Figure 7. The bromate concentration increases approximately about 8 µg/l if the ozone dose increases about 1 mg/L.
Fig. 7: Bromate formation depends of the ozone dose during bench scale (BS) and bench-top pilot plant (BTPP) experiments. From Wert, E.C., et al. 2007: [7]
As shown in figure 7 is the EU and US legal limit for bromate in drinking water (10g/L) to high above an ozone dose of approximately 4.3 mg/L. The formation of bromate is strong pH depended. By same ozone exposure, bromate formation is much higher by pH 8 than by pH 6. Von Gunten propose that the best 11
strategy to minimize bromate concentration in water is to lowering the pH [4]. Another influence to bromate formation is the presence of ammonia in water. Von Gunten showed that the formation of bromate is much lower by addition of 200 µg/L ammonia (approximately a factor two after eight minutes) than by addition of