Water disinfection

Abstract

Water disinfection is a necessary process for the control of the pathogenic micro-organisms. Chlorination, chloramination, ozonation, and ultraviolet system are the most common methods used for drinking water and wastewater treatment However, the use of chemical disinfectants leads to the formation of disinfection by-products (DBPs).,trihalomethanes (THMs), halogenic acetic acids, haloacetonitrils (HAN), halo-aldehydes and haloketons consist mainly of DBPs. DBPs provide an unintended health hazard. The overall purpose of this paper was to introduce several disinfection methods ，some disinfection by-products (DBPs) and different techniques for the removal of DBPs.

Keywords: Disinfection methods; Disinfection by-products; removing DBPs

1. Disinfection methods

Disinfection is usually the final stage in the water treatment process in order to limit the effects of organic material, suspended solids (SS), and other contaminants like pathogenic micro-organisms. The primary methods used for the disinfection of water in very small (25,500 people) and small (501–3,300 people) treatment systems are ozonation , ultraviolet irradiation (UV), and chlorination. The disinfection process has been routinely carried out since the dawn of the 20th century to eradicate and inactivate the pathogens from water used for drinking purpose. Disinfectants in addition to removing pathogens from drinking water, serve as oxidants in water treatment. They are also used for :(1) removing taste and color ;(2) oxidizing iron and manganese;

(3) improving coagulation and filtration efficiency;(4) preventing algal growth in sedimentation basins and filters; Chlorine and its compounds are the most commonly used disinfectants for water treatment. Chlorine’s popularity is not only due to lower cost, but also to its higher oxidizing potential, which provides a minimum level of chlorine residual throughout the distribution system and protects against microbial recontamination. The disinfection process is affected by different physicochemical and biological factors. The disinfection efficiency (Ct) is a product of residual disinfectant and the contact time of chlorine in the water. This product is used as a design parameter for the disinfection facility. Disinfectants have varying capacities to inactivate or kill pathogens. The types and nature of organisms as well as the process conditions, including temperature and pH, also affect disinfection. Generally, inactivation of organisms’ increases with increasing Ct.. For a specific contact time, required chlorine doses for disinfection are consequently higher in winter than in summer conditions. However, in most drinking water utilities the application of a disinfectant (such as chlorine) in addition maintains adequate residuals to avoid the reappearance of microorganisms in the water distribution system. The disinfectant residuals deplete rapidly when the water temperature is high, which explains the difficulty of maintaining minimum residual level in the large distribution systems during summer. Also, microbial activity within distribution systems is higher in warm than in cold waters. To maintain an adequate level of residual disinfectant in the distribution system, higher disinfectant doses are applied during the summer. Generally, the conditions affecting the disinfection efficiency and the requirements to maintain disinfectant residuals in the distribution systems simultaneously affect

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[1]

the formation of DBPs.

There are numerous alternative disinfectants that have been less widely used in small and very small water treatment systems, including chlorine dioxide, potassium permanganate, chloramines and peroxone (ozone/hydro-gen peroxide [HP]).An effective disinfectant should be able to:

(1) destroy all types of pathogens in whatever number present in the water(2) destroy the pathogens within the time available for disinfection, function properly regardless of any fluctuations in the composition or condition of the water(3) function within the temperature range of the water(4) not cause the water to become toxic or unpalatable(5) be safe and easy to handle

(6)determine its concentration in the water and provide residual protection against recontamination.

1.1 Chlorination

Chlorine has been successfully used for the control of waterborne infectious diseases for nearly a century,and chlorination is one of the most effective public health measures ever undertaken . Chlorine is used to disinfect water in either gaseous form (Cl2) , or as hypochlorite salts. All forms of chlorine react with water to produce hypochlorous acid (HOCl), which rapidly dissociates to form the hypochlorite ion according to the following .In addition to HOCl and the hypochlorite ion (OCl+),chlorine may also be found in the form of monochloramine (NH2Cl) and dichloramine (NHCl2). The dominant form of chlorine depends upon the combination of parameters such as temperature, pH and ammonia concentrations. As the pH increases the concentration of the hypochlorite ion relative to hypochlorous (HOCl) acid increases, while the presence of ammonia tends to increase the concentration of monochloramine. Knowledge of the dominant form of chlorine in a particular disinfection process is important. With the differing forms come varying oxidizing strengths and thus biocidal efficiencies. The chlorine disinfection process occurs primarily through oxidation of cell walls leading to cell lysis (bacterial) or inactivation of functional sites on the cell surface. Hypochlorous acid is the most potent of the four main oxidizing forms. In addition to differences in oxidizing strengths between forms of chlorine, the disinfection effectiveness varies across the range of micro-organisms. Protozoans, helminths, and viruses are the most resistant, followed by bacterial pathogens, with each species varying in resistance. Chlorine is very effective against enteric bacteria, but less effective against other bacterial species . 【2】

1.2 Chloramination

Chloramines are formed during a reaction between chlorine (Cl2) and ammonia (NH3). Chloramines are amines, which contain at least one chlorine atom that is directly bonded to nitrogen atoms (N). Inorganic chloramines are formed when dissolved chlorine and ammonia react. During this reaction, three different inorganic chloramines are formed: monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3). There are many similarities between chlorine and chloramine. The most important fact is that both of them provide effective residual disinfection with minimal risk to public health. The difference is that monochloramine is 200 times less effective as a disinfectant than chlorine. On the other hand, chlorine forms many by-products, including trihalomethanes (THM) and haloacetic acids (HAA), whereas chloramine forms a significantly lower amount of THMs and HAAs, but also forms N-nitrosodimethylamine (NDMA)

【3】. 2 / 11

1.3 Chlorine dioxide

Chlorine dioxide (ClO2) is used both as a disinfectant and an oxidant in water treatments. It has several distinct chemical advantages, which complement the traditional use of chlorine in water treatments [8].Chlorine dioxide is highly effective in controlling waterborne pathogens while minimizing halogenated DBPs. Also, a broad-spectrum micro biocide is as effective as chlorine against viruses, bacteria, and fungi, and more effective than chlorine for the inactivation of the encysted parasites Giardia and Cryptosporidium. Furthermore it is an effective control strategy for taste, odor, color, and iron and manganese removal. Chlorine dioxide presents several other advantages than other disinfectants, which can be summarized as follows:

(1) the bactericidal efficiency is relatively unaffected by pH value between 4 and 10

(2) Chlorine dioxide is clearly superior to chlorine in the destruction of spores, bacteria, viruses, and other pathogenic micro-organisms on an equal residual base

(3) the required contact time for ClO2 is lower

(4) chlorine dioxide has better solubility (5) no corrosion associated with high chlorine concentrations

(6) chlorine dioxide does not react with ammonia

(7) it destroys THM precursors and increases coagulation

(8) ClO2destroys phenols and has no distinct smell

(9)it is better at removing iron and magnesia compounds than chlorine, especially complex bounds. 【4】

1.4 Ozonation

Ozone has been used for water disinfection for about 80years in France, Germany, and other European countries. It is now undergoing a critical evaluation as a possible alternative to chlorine when used alone or in conjunction with other disinfection systems. Ozone is produced when oxygen (O2 ) molecules are dissociated by an energy source into oxygen atoms and subsequently collide with an oxygen molecule to form an unstable gas, ozone (O3). Disinfection by ozonation is achieved using the formation of free radicals as oxidizing agents. The method is more effective against viruses and bacteria than chlorination. The low solubility of ozone in water is the main factor that greatly reduces its disinfection capacity, and any ozone residual produced rapidly dissipates as a consequence of its reactive nature. The absence of a lasting residual may also be seen as a disadvantage as this may allow possible microbial re-growth and make it difficult to measure the efficiency of the disinfection process. Ozone is the most efficient chemical disinfectant currently applied in drinking water treatment. Even for microorganisms such as protozoa which are difficult to inactivate with other disinfectants, ozone provides adequate inactivation with reasonable doses and contact times.

The mechanisms of disinfection using ozone include, direct oxidation of the cell wall with leakage of cellular constituents outside of the cell, reactions with radical by-products of ozone decomposition. The effectiveness of disinfection depends on the susceptibility of the target micro-organisms, the contact time, and the concentration of ozone . The advantages of the method could be summarized as follows:(1) It is more effective than chlorine in destroying viruses and bacteria;(2) it requires a short contact time (approximately 10–30min);(3) there are no harmful residuals that need to be removed after ozonation because ozone decomposes rapidly ;(4) ozone is

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[5]

generated onsite, and thus, there are fewer safety problems associated with shipping and handling The disadvantages of the method could be: ;(1) low dosage may not effectively inactivate some viruses, spores, and cysts;(2) it is not economical for wastewater with high levels of SS, biochemical oxygen demand (BOD), chemical oxygen demand, or total organic carbon;(3) it is extremely irritating and possibly toxic, so off-gases from the contactor must be destroyed to prevent workers’ exposure;(4) the cost of treatment can be relatively high in capital and in power intensiveness

1.5 UV disinfection

Disinfection by ultraviolet light (UV) is considered as a cost-effective and easily implementable system for drinking water disinfection. Interest in UV disinfection process has been increased sharply in drinking water industry, since researchers demonstrated that even very low dosage of UV light could inactivate Cryptosporidium effectively in the late 1990s. UV spectrum is divided into four regions; vacuum UV(100w200 nm, hereafter VUV), UV-C (200w280 nm), UV-B(280w315 nm), and UV-A (315w400 nm). UV disinfection primarily occurs due to the germicidal action of UV-B and UV-C light on microorganisms. Although VUV can disinfect microorganisms, it is not efficient to use VUV for water disinfection because it rapidly dissipates through water in very short distances (EPA, 2006). VUV is also known to breakdown bonds of organic carbons Two UV systems are generally applied for drinking water disinfection process. Monochromatic low pressure UV (here-after LPUV) emits single wavelength at 254 nm which is close to the maximum microbial action spectrum. Polychromatic medium pressure UV (hereafter MPUV) emits a wide range of wavelength including UV-A, -B, -C and visible light. Special LPUV emitting two wavelengths at 185 and 254 nm (hereafter LPUV for TOC) is applied to remove TOC for producing ultra-pure water. Although these UV systems are inactivate most of microorganisms effectively except for some viruses, they cannot guarantee biological safety of tap water because the effect of UV irradiation cannot be maintained throughout distribution system. On the contrary, UV disinfection is concerned to have negative effects on water quality by UV photolysis. Many researchers have reported that UV irradiation can modify DOM structure and increase biodegrad-ability[8]. Especially, VUV irradiation is known to be more effective than UV-C irradiation in formation of biodegradable compounds and mineralization. UV-A and UV-B can also splits large NOM molecules into organic acids with lower molecular weight. This change of DOM structure can increase biodegradability, which stimulates microbial re-growth and biofilm formation in distribution system[9]. Increase of biofilm can also cause taste and odor problems and reduction of hydraulic capacity. Therefore, sequential disinfection process with additional chemical disinfectant such as chlorine or monochloramine was applied to prevent microbial re-growth in the distribution system. With chlorination as secondary disinfection process, UV treatment is often expected to reduce chlorine demand and DBPs formation. Liu et al. (2006), however, reported that the DBPs formation of four organic waters was increased by chlorination after UV irradiation. The effect of UV irradiation on water quality depends on many factors, such as characteristics of source water

quality, UV wavelength and applied dosage. Previous studies have often been carried out under bench-scale conditions, and organic water with relatively high DOC level and high UV dosage of used, which were not the conditions used for drinking water disinfection process . [7][6]

2. Disinfection by-products

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Since the 1970s, research in the drinking water field has significantly focused on documenting and understanding the occurrence of disinfection by-products (DBPs) in drinking water. The application of disinfection agents to drinking water reduces the microbial risk but poses chemical risk in the form of their by-products. The DBPs are formed when the disinfectant reacts with natural organic matter (NOM) andyor inorganic substances present in water. More than 250 different types of DBPs have already been identified. The DBP concentrations may vary in orders of magnitude during different disinfection processes. Some DBPs reported have not been identified in field-scaled studies, however, they were observed in laboratory-scaled studies [11]. The formation of chlorinated DBPs in drinking water like trihalomethanes (THMs) has emphasized the need for exploring alternate disinfectants and new treatment technologies. Because organicyinorganic substances act as precursors for DBPs, their removal prior to disinfection has proven to be an effective method for reducing chlorinated DBP formation potential. The NOM can be partially removed using a conventional treatment (coagulation, flocculation, sedimentation and filtration) or by combining /replacing its components with more efficient processes such as granular activated carbon (GAC) filtration, enhanced coagulation and membrane filtration[12]. Another effective method to control chlorinated DBPs in drinking water is the use of alternative disinfectants—ozone, chloramines, chlorine dioxide and more recently ultraviolet (UV) light—alone or in combination with chlorine. The use of various disinfectant alternatives to chlorination must be considered, however, they may form non-chlorinated DBPs. Finally, a better control of operational factors (e.g. control of pH or disinfection contact time) may contribute to a reduction in the formation of DBPs[13]. For chlorination, generally chlorine gas (Cl2) is 2 bubbled into pure water and rapid hydrolysis to hydrochloric (HCl) and hypochlorous acid (HOCl) takes place. The HOCl undergoes subsequent reactions resulting in the formation of THMs. HOCl oxidizes the bromide (Br- )present in the water, which reacts readily with NOM to form brominated THMs. Similar parameters that affect the disinfection efficiency (Ct) and residual depletion in the distribution system affect the rate and the degree of THM formation. THM occurrence is influenced by chlorine dose, concentration and nature of NOM (mainly humic substances), chlorine contact time (water residence time in distribution system), pH, temperature of water, and bromide ion. In general, higher THM concentrations are expected at higher levels of the above-mentioned parameters. In temperate environments, THM levels in drinking water are significantly affected by seasonal conditions. In the winter months and in some cases where the ice cover protects surface raw waters, the THM concentrations are lower due to lower water temperature and NOM. In these conditions, the chlorine demand is lower, therefore, the chlorine dose required to maintain adequate residual in the distribution system is also less important[14]. Moreover, higher DBPs concentrations have been observed particularly in the extremities of water distribution systems, especially in the summer months .The type of raw water also affects the THM levels. Generally, ground waters are naturally protected from runoff NOM, while the difference in occurrence of DBP precursors in river and lakes depends on geological, physical and environmental factors. For the DBPs associated to alternative disinfectant to chlorine (chloramines, chlorine dioxide, ozone), the similar operational factors influence the formation of the associated by-products (dose, pH, temperature, reaction time). Chloramines produce similar DBPs than chlorine but with much lower concentrations [15]. For disinfection with chlorine dioxide (ClO2 ), there is no evidence of reactions with 2 humic acids to form trihalomethanes. However, the inorganic DBPs such as chlorite and chlorate are formed and they also have human health risk

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[11][10]

implications. Finally, in ozonation, the most important by-product formed is bromate, which depends on the presence of bromide and ammonia ion concentrations.

2.1 Trihalomethanes (THMs) and haloacetic acids (HAAs)

Trihalomethanes (THMs) and haloacetic acids (HAAs) are the most important groups of CDBPs (others are haloacetonitriles, haloketones and chloropicrin). THMs include chloroform, bromodichloromethane (BDCM), dibromochloromethane (DBCM) and bromoform. Total THMs (TTHMs) refers to the sum of these four substances. HAAs include nine substances, the most common being dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA), other compounds, found generally at lower levels, are bromochloroacetic acid (BCAA), dibromoacetic acid (DBAA),monochloroacetic acid (MCAA) and monobromoacetic acid (MBAA). In the US, the sum of DCAA, TCAA,DBAA, MCAA and MBAA is commonly denoted asHAA5[16]. Due to their potential link to human health effects, maximum acceptable concentration (MAC) for THMs in drinking water have recently been established in several countries and regions of the world. In most cases the MAC refers to TTHMs, although the World Health Organization has published guidelines for each of the four THM species[17]. Some countries and regions have also established MAC for HAAs. Canadian drinking water quality guidelines (Health Canada, 1996) set the MAC for TTHMs at 100mg/L, while in the province of Quebec—since 2002—the drinking water quality regulations (DWQR) mandate that utilities comply with a maximum acceptable level of 80mg/L for TTHMs, based on the annual average of four samples collected quarterly (one per trimester) at the extremity of the distribution system[18]. This MAC was largely inspired by Stage 1 maximum contaminant level (MCL) for TTHMs of the US Environmental Protection Agency (USEPA) Disinfectant/Disinfection by-product (D/DBP) rule (USEPA, 1998). However, no MAC for HAAs has been established in Quebec’s drinking water regulations, based on the argument that these substances have a common origin with THMs. As for TTHMs, the USEPA did include (in Stage 1 of the D/DBP rule) MCLs of 60mg/L for HAA5 based on a running annual average of quarterly samples collected at four distribution system sites. Stage 2 will favor a locational approach for both TTHMs and HAA5 monitoring (USEPA, 2003). The locational running annual average will require compliance at each distribution system site. That is, based on quarterly monitoring at four locations, four separate running annual averages will be computed (one for each site). Compliance with the MCL will need to be ensured at each location. The occurrence of CDBPs in treated and distributed drinking water varies according to the quality of the water source and the operations carried out in thtreatment plant. Generally speaking, the main influential factors are the nature and amount of natural organic matter (NOM)—in particular humic substances, concentrations of bromide (which mainly impact the speciation of CDBP species), pH of water, water temperature and residence time of water in the distribution system . In temperate northern environments where important seasonal changes in water temperature and water quality occur, changes to the operational parameters of the treatment process are necessary, therefore important variations of CDBP levels can also occur. Whereas recent studies have documented the formation of CDBPs (mainly THMs) using bench-scale controlled chlorination , there is currently relatively little information about the impact of seasonal water quality changes and operational water treatment strategies on the simultaneous occurrence of THMs and HAAs, and on the preponderance of one or the other in full-scale distribution systems[19].

2.2 N-DBPs in drinking water

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N-Nitrosodimethylamine (NDMA), a disinfection by-product and potent carcinogen, has recently been observed in drinking water supplies . The USEPA integrated risk information system (IRIS) database lists an estimated 10 ?6 lifetime cancer risk level for NDMA in drinking water of 0.7ng/L [20].It has been reported that the use of chloramines or chlorine as a primary disinfectant may increase NDMA concentrations in drinking water treatments . NDMA, one of nitrosamines, can be formed through a chemical reaction between monochloramine and an organic nitrogen compound. Monochloramine is used directly or formed in chlorination of drinking water in the presence of ammonia. Gerecke and Sedlak demonstrated that the yield of NDMA from chloramination of dimethylamine(DMA) was about 0.6% in natural waters. DMA is one of themost frequently detected organic nitrogen compounds in surface water [21]. Also, they suggested that chloramination of surface waters with high dissolved organic carbon(DOC)concentration could result in elevated NDMA formations. In addition, Mitchand Sedlak , and Choi and Valentine showed that NDMA formation during chlorination could occur through unsymmetrical dimethylhydrazine (UDMH) as an intermediate. The overall rate of NDMA formation by the UDMH oxidation is very slow due to slow reactions in the preliminary step. Mitch et al. studied that the NDMA formation rate varied with pH with a maximum formation rate between pH 7 and

8.They pointed out that NDMA formation via the UDMH pathway has significant implications for disinfection in water and wastewater, because its formation is maximized at pH between 6 and 9, which are typical values during water and wastewater treatment. The rate of UDMH formation increases with pH [22]. Longer contact time during drinking water disinfection and water distribution systems may result in higher NDMA formation . Previous investigations indicated that there would likely be relationships between NDMA formation and several parameters such as NDMA precursors and inorganic concentrations.Mitch and Sedlak[23] reported that NDMA formation rate was positively correlated with monochloramine concentration and increased linearly over time due to the very slow reaction rate. Until now, it was possible to assume NDMA formation from assumption of NDMA yield. However, there is a limitation of use in high molar ratio (monochloramine/dimethylamine) and presence of other NDMA precursors[24].

2.3 Other Emerging DBPs

Brominated DBPs are now being recognized as toxicologically important. Many brominated DBPs have been shown to be more carcinogenic than their chlorinated analogs. Preliminary studies are also indicating that iodinated compounds may be more toxic than their brominated analogs [25]. Brominated and iodinated DBPs form because of the reaction of the disinfectant (such as chlorine) with natural bromide or iodide present in source waters. Coastal cities, the ground and surface waters of which can be impacted by saltwater intrusion, and some inland locations, the surface waters of which can be impacted by natural salt deposits from ancient seas, are examples of locations that can have high bromide and iodide levels. In addition to comparisons that can be made for brominated versus chlorinated DBPs from animal cancer studies[26], Plewa et al. have conducted systematic genotoxicity and cytotoxicity studies of brominated and chlorinated DBPs using mammalian cell assays . Speci?cally, in a study of brominated versus chlorinated haloacetic acids (including mixed bromo-chloro species), the brominated HAAs were found to be more cytotoxic and genotoxic than their chlorinated analogs.

Disinfection by-products include (non-)brominated organic compounds and the inorganic compounds bromate, iodate (and chlorate). Most of the low-molecular weight non-brominated

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organic by-products are mineralized during biologically activated carbon filtration, which is typically performed after an ozonation process. Therefore, they are of minor concern. Information is lacking on higher molecular weight organic by-products. Bromate is currently the by-product that causes most concern due to its potential carcinogenicity. Because of the low drinking water standards , an optimization of the ozonation process is required in certain cases to minimize its formation[27]. Once formed, its removal is non-economical. The best bromate minimization strategies appear to be lowering of the pH or ammonia addition. Iodate, quantitatively formed by oxidation of naturally occurring iodide by ozone, is of no toxicological concern. It is rapidly metabolised after ingestion. Chlorate is only formed during ozonation if a preoxidation of the water with chlorine and/or chlorine dioxide is applied. The toxicological impact of chlorate is unclear and more studies are required to permit regulation.

3. Different techniques for the removal of DBPs

Various options are applicable for the removal of water pollutants included reverse osmosis, ion exchange, coagulation, co-precipitation, catalytic reduction, herbal filtration, electrodialysis and

【】adsorption 28. Chlorine based treatment techniques have the property to remain active in the water as long as they are not consumed by either inactivation or competitive reaction. Water treatment designs and operators have only alternative seither limit the formation of disinfection by products by innovative chlorination strategiesorto develop process for the removal of organic and

【】other chlorine sensitive compound33. Synthetic and natural organic contaminants are mostly

found in drinking water. These compounds include taste and odor causing synthetic organic chemicals, pesticides, herbicides color and trihalomethane precursors. NOM is produced in the biological degradation of organic substances viz. amino acids, fatty acids, phenols, steroids, sugars, hydrocarbons, urea, porphyrines and polymers. The polymers include polypeptides, lipids,

【】polysaccharides and humic substances 34. Drinking water sources contain 2–10mg/l of NOM,

【】although much higher levels are also reported 35. Therefore the introduction of adsorption

process is found to be the most suitable and significant for its removal. Granular activated carbon (GAC) and powdered activated carbon (PAC) have been used to remove these organic compounds from the water. It is suggested that the formation of the compounds with NOM and chlorine are referred as THM precursors and the complete reaction is known as trihalomethane formation potential (THMFP) and can be determined directly or by measuring total organic carbons Therefore the removal of THM precursor and THMs from drinking water by PAC is variable. The fraction of NOM that produces THMs varies in the different water sample. However the data indicate that there is no correlation between the level of THM precursor and TOC in the different type of water Granular activated carbon has been suggested due to its greater efficiency in the

【】removal of NOM, THMs, odor, color containing compound and other toxic chemicals 36. Iron

oxide coated sand was found to be useful as sorbent for accumulating NOM from water source and for removing DBP precursor from water supply. Humic substances are known to be DBP precursors , a substantial fraction of its formation potential can be removed by sorption of NOM on to oxide surfaces which form during coagulation with iron and aluminum salt. The use of Fe(II) noticed to be a most promising of excess ClO2 removal technique and has been successfully used

【】in pilot and full scale studies 38. Other alternatives have been studied including adsorption on

activated carbon, coagulation with polymer, alum, lime or iron sulfate, ion exchange and

【】membrane process for the removal of DBPs 39.Bacterial growth potential and trihalomethane

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formation potential(THMFP) were investigated in relation to DOM before and after alum treatment, and results indicated that DBP of terrestrially derived DOM can be high in comparison with that of overall DOM in natural water and alum coagulation was not found to be sufficient to

【】produce microbiologically stable drinking water40.It is established that the activated carbon

obtained from apricot stones by pyrolysis can be used for removal of trihalomethanes from water

【】treated with chlorine 41.

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