An overview of the technology used to remove trihalomethane (THM), trihalomethane precursors, and trihalomethane formation potential (THMFP) from water and wastewater

Accepted Manuscript Title: An overview of the technology used to remove trihalomethane (THM), trihalomethane precursors, and trihalomethane formation potential (THMFP) from water and wastewater Authors: Fathiyyah Mohd Zainudin, Hassimi Abu Hasan, Siti Rozaimah Sheikh Abdullah PII: DOI: Reference:

S1226-086X(17)30441-0 http://dx.doi.org/10.1016/j.jiec.2017.08.022 JIEC 3571

To appear in: Received date: Revised date: Accepted date:

27-12-2016 22-7-2017 13-8-2017

Please cite this article as: Fathiyyah Mohd Zainudin, Hassimi Abu Hasan, Siti Rozaimah Sheikh Abdullah, An overview of the technology used to remove trihalomethane (THM), trihalomethane precursors, and trihalomethane formation potential (THMFP) from water and wastewater, Journal of Industrial and Engineering Chemistryhttp://dx.doi.org/10.1016/j.jiec.2017.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

An Overview of the Technology Used to Remove Trihalomethane (THM), Trihalomethane Precursors, and Trihalomethane Formation Potential (THMFP) from Water and Wastewater

1Fathiyyah

1

Mohd Zainudin, 1,2Hassimi Abu Hasan*, and 1Siti Rozaimah Sheikh Abdullah

Department of Chemical and Process Engineering, Faculty of Engineering and Built

Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia 2

Research Centre for Sustainable Process Technology (CESPRO), Faculty of Engineering and

Built

Environment,

Universiti

Kebangsaaan

Malaysia,

43600

UKM

Bangi,

Selangor Darul Ehsan, Malaysia

*Corresponding author: Phone: +603-89216402; Fax: +603-89118345 Email addresses: [email protected] / [email protected] (Hassimi Abu Hasan), [email protected] (Fathiyyah Mohd Zainudin)

Graphical abstract

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Abstract (For online submission) Chlorination is a popular and inexpensive technique for disinfecting water before distribution to consumers. However, the chlorination process results in the formation of low concentrations of toxic THMs. In this paper, an overview of the various technologies for the removal of THM precursors and THMs is presented. Emerging technology using microbes to effectively biodegrade THMs is also discussed. Biodegradation technology has the potential to eliminate THM precursors and THMs from water and wastewater and has the advantages of being a green technology and being inexpensive to operate and maintain compared with other techniques.

Abstract In developing countries such as Malaysia, chlorination is a popular and inexpensive technique for disinfecting water before distribution to consumers. However, the chlorination process results in the formation of low concentrations of toxic trihalomethanes (THMs), which may, over long periods of exposure, lead to adverse effects in consumers. Thus, these compounds should be prevented from forming and removed from drinking water and wastewater through either physicochemical or biological techniques. In this paper, an overview of the various technologies for the removal of THM precursors and THMs is presented. In addition, emerging technology using microbes to effectively biodegrade THMs is also discussed. Biodegradation technology has the potential to eliminate THM precursors and THMs from water and wastewater and has the advantages of being a green technology and being inexpensive to operate and maintain compared with other physicochemical techniques.

Abbreviation AC/NZVI

Activated Carbon/Nanoscale Zero-Valent Iron Page 2 of 55

AOB

Ammonia-Oxidizing Bacteria

AOP

Advanced Oxidation Process

BAC

Biological Activated Carbon

BAF

Biological Aerated Filter

BDOC

Biodegradable Dissolved Organic Carbon

BGAC

Biological Granular Activated Carbon

BOD

Biological Oxygen Demand

CNTs

Carbon Nanotubes

COD

Chemical Oxygen Demand

DBPFP

Disinfection By-Products Formation Potential

DBPs

Disinfection By-Products

DOM

Dissolved Organic Matter

EBCT

Empty Bed Contact Times

EDCs

Endocrine-Disrupting Compounds

EU

European Union

GAC

Granular Activated Carbon

GAC-FA

Granular Activated Carbon-Filter Adsorption

GAC-SAT

Granular Activated Carbon /Soil Aquifer Treatment

HAAs

Haloacetic Acids

HANs

Haloacetonitriles

HRT

Hydraulic Retention Time

HSFW

Horizontal Subsurface Flow Wetlands

IER

Ion-Exchange Resin

MBR

Membrane Bioreactor

MCL

Maximum Contaminant Level

MF

Microfiltration

MIEX®

Magnetic Ion-Exchange Resin

NF

Nanofiltration

NOM

Natural Organic Matter

PAC

Powdered Activated Carbon

PPCPs

Pharmaceutical and Personal Care Products

SAT

Soil Aquifer Treatment

SUVA

Specific Ultraviolet Absorbance

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THMFP

Trihalomethanes Formation Potential

THMs

Trihalomethanes

TOC

Total Organic Carbon

TTHMs

Total Trihalomethanes

UF

Ultrafiltration

US

Ultrasonic

USEPA

United States Environmental Protection Agency

UV

Ultraviolet

VSFW

Vertical Surface Flow Wetlands

WHO

World Health Organization

Keywords: THM Precursors; Trihalomethanes; Drinking Water; Membrane Filtration; Biofiltration; Advanced Oxidation Process

1.0 INTRODUCTION Water is the main resource for life on Earth. In addition to the need to eat and drink, people need water for other uses, such as washing, transportation, industry and agriculture. Other organisms, such as animals and plants, also need water to survive. Water suitable for human consumption is called drinking water or clean water, but unclean water can also be used and/or drunk after undergoing treatment involving filtration, distillation or various other methods. Pollution consists of foreign substances that can have an adverse impact on an ecosystem. There are three broad types of pollution: air, water and land pollution. This review focuses on water pollution, which involves the contamination of water sources, such as rivers, lakes, groundwater and seas. Examples of waste pollutants include heavy metals and organic and inorganic compounds. Organic contaminants include proteins, carbohydrates, fats, and Page 4 of 55

nucleic combinations, which can be measured by the chemical oxygen demand (COD) and biological oxygen demand (BOD), whereas inorganic contaminants include ammonia, phosphate, nitrate and sulphate. In addition, micro-contaminants include pharmaceutical and personal care products (PPCPs), disinfection by-products (DBPs) and endocrine-disrupting compounds (EDCs). Almost all cases of water pollution adversely affect not only individuals and the population but also natural wildlife. In developing countries, almost half the population suffers from health issues associated with the consumption of water contaminated with microorganisms [1]. The chlorination step in the disinfection process is the most vital stage in water processing, but chlorinated compounds should not be consumed [2]. Procedures for water disinfection using chlorine are used worldwide to reduce the health risk associated with the growth of pathogenic microorganisms in drinking water. Despite the importance of this strategy, some classes of undesired disinfection by-products (DBPs) are commonly identified in potable water treated by chlorination [3,4]. This review focuses on trihalomethane (THM) precursors, trihalomethanes (THMs) and the trihalomethane formation potential (THMFP). To date, there has been no overview of the technology used to remove THM precursors and THMs; hence, this technology is extensively discussed in this paper. In addition, a brief review of emerging technology using microbes to effectively biodegrade THMs is also provided.

2.0 DISINFECTION BY-PRODUCTS (DBPs) IN WATER Contaminated water and wastewater are produced by a wide range of activities, such as bathing, cleaning, cooking, manufacturing and industrial production. The contaminants discovered in contaminated water are varied and numerous and include metals, organic materials, salts, pathogens, ammonia, pesticides, endocrine disruptors and pharmaceuticals. As depicted in Figure 1, micropollutants are categorized as DBPs, PPCPs, EDCs and heavy metals.

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The presence of natural organic matter (NOM) and dissolved organic matter (DOM) in the water source is the primary precursor for the formation of DBPs in drinking water. The formation of DBPs occurs through the reaction of NOM and/or DOM with chlorine during the water chlorination process. NOM is a complex mixture of organic compounds derived from the decay of vegetation and animal material [5]. DOM includes numerous compounds, such as carbohydrates, humic substances, carboxylic acids, hydrophilic acids and amino acids [6]. These compounds have been identified as genotoxic mutagens, which can be toxic to humans and aquatic life [7]. Based on epidemiological studies, DBPs are associated with an increase in bladder and colon cancer risk and premature birth and stillbirth [8,9]. In addition, long-term exposure to DBPs can increase potential adverse effects on the reproduction system [10]. Disinfection is a vital process in the elimination of pathogenic microorganisms in drinking water and wastewater. By ensuring drinking water safety, disinfection is a cornerstone in the water treatment process [11,12]. However, since the 1970s, it has been recognized that disinfection can produce harmful by-products and cause health concerns [5]. There are 600 to 700 types of DBPs produced when naturally occurring organic matter interacts with halogens during treatment [13-15]. The reaction of chlorine with NOM in raw water results in the formation of THMs, haloacetonitriles (HANs), haloacetic acids (HAAs) and other chemical compounds [4]. The most common classes of DBPs in chlorinated drinking water are THMs [16],

which

include

chloroform

(CHCl3),

bromodichloromethane

(CHCl2Br),

dibromochloromethane (CHClBr2) and bromoform (CHBr3). Exposure to THMs has been shown to result in adverse reproductive outcomes and digestive cancers and to have negative impacts on the genitourinary systems [10,15,17].

3.0 THM PRECURSORS, THMFP AND THMs 3.1 THM precursors and THMFP

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Most THM precursors are created when both bromide and dissolved organic matter are present, which is responsible for the THM formation potential (THMFP). Among the organic precursors in natural water, 8–11% of the proteins present is estimated to constitute the THMFP [18]. Fortunately, not all THM precursors contribute to the THMFP. Drinking water sources consist of 2–10 mg/L NOM, although much higher levels have been reported [19]. NOM is produced during the biological degradation of organic substances. NOM can be classified as hydrophobic or hydrophilic in nature and can be divided into two fractions: humic substances (fulvic and humic acids) and non-humic substances (carbohydrates, lipids and amino acids) [20]. Humic substances have been identified as the primary precursors of DBPs [21], whereas non-humic substances are generally more biodegradable [22]. Most NOM has the potential to react with chlorine, which is applied for oxidation or disinfection, forming haloforms and other halogenated organic compounds. A high concentration of NOM present in water will result in high concentrations of THMs [23,24]. The quantity of NOM determines the quantity of THMs that can form in the chlorination process. THMFP is defined as the difference between the total THM concentration measured after the chlorination process without treatment (TTHMi) and the total THM concentration measured at regular intervals during water treatment (TTHMf), as shown in Equation (1) [25]:

THMFP  THMFPf  THMFPi

(1)

The THM-forming reaction is presented in Equation (2):

Cl2  THMFP  THM 3.2

(2)

Trihalomethane

THMs are among the most abundant and thoroughly studied DBPs [26]. THM compounds have been identified as genotoxic mutagens, which can be toxic to humans and aquatic life [7]. According to Grossman Rebekah [27], two groups of THMs, i.e., CHCl3 and CHClBr2, are carcinogenic, whereas other groups, such as CHBrCl2, have been identified as mutagens that Page 7 of 55

alter DNA. A mutagen is considered to have a genetic influence on the next generation in addition to being carcinogenic. Table 1 shows the physico-chemical properties of THMs. Chloroform is a volatile liquid that is slightly soluble in water (8.09 g/L at 20 °C) and is commonly found in groundwater and surface water sources, while the quantity of other compounds, such as CHBrCl2, CHClBr2, and bromoform, are sometimes negligible. The overall worldwide production of chloroform through the chlorination of drinking water in the late 1990s was 520,000 tons/year [28]. This chloroform could be discharged into the environment after direct addition to the water, e.g., water chlorination, or as a result of its formation through reactions with other substances [29]. Significant anthropogenic chloroform sources include chemical manufacturing plants, water treatment plants, pulp and paper mills and waste incinerators. In the “CERCLA Priority List of Hazardous Substances,” chloroform ranks 11th and represents a large number of abundant halocarbons in the atmosphere [30]. Chloroform has been used in cough syrups, in toothpastes and as a surgical anaesthetic. It was essentially prohibited from consumer products in 1976 by the Food and Drug Administration following the discovery that chloroform was carcinogenic to laboratory animals [31]. Nevertheless, its use as an industrial solvent nearly doubled from 1980 to 1990 [29]. The amount of THMs in drinking water is higher at the consumption site than at the treatment site [32,33] because reactions between THM precursors and chlorine in the chlorination process may occur for days until either the precursor or the chlorine is exhausted [25]. In the chlorination process, electrophilic attack on the organic matter depends on the conditions of the reaction and the reactants involved, e.g., chlorine (Cl2), chlorine monoxide (Cl2O), H2OCl+ ions or HOCl- ions [34], and the most common reactions are substitution (forming C-chlorinated and N-chlorinated derivatives), oxidation and addition [35]. In most reported cases, THMs are present at the greatest concentrations in disinfected drinking waters, and they are currently regulated in a number of countries, bodies or

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organizations, such as the United States, Canada, Australia, the European Union (EU), Japan, the Netherlands, New Zealand, South Africa, Sweden, the United Kingdom, Malaysia and the World Health Organization (WHO). The United States Environmental Protection Agency (USEPA) [36] has set a regulation on the maximum contaminant level (MCL) for THMs (80 µg/L) because its presence in drinking water may represent a serious health risk to humans. The regulation of THMs in drinking water has been established by different countries, as shown in Table 2, and the Florida Department of Environmental Protection [37] has set a criterion for surface water quality, which is shown in Table 3. Other than the Florida Department of Environmental Protection, no other regulatory bodies have established regulations for THM levels in surface water. However, various countries have set maximum THM concentrations in drinking water, ranging from 50 to 250 µg/L, which are higher than the value set for surface water (≤10.65 µg/L). Human beings are exposed to THMs not only through the direct ingestion of drinking water but also through the volatilization of DBPs during cooking, bathing, showering and various behavioural or lifestyle factors [10,38]. Due to the severe toxicity of THMs, these compounds should be identified, prevented and removed from drinking water and wastewater. Various methods based on chemical or biological technologies can be used to remove THMs. The levels of THMs can be reduced in two ways: (1) using disinfectant agents other than chlorine (preventative action) and (2) removing precursor materials prior to the addition of chlorine (THMFP) and removing THMs after formation (treatment action). This review focuses on technology for the removal of THMs, THM precursors and THMFP. Table 4 lists current removal technologies that have been used for THM precursors, THMFP and THMs. The technologies can be categorized as chemical or biological treatments that act as individual or combined technologies. The removal of contaminants varies based on changes in temperature, pH, retention time, chemical dosage, inoculum size, aeration, type of adsorbent, etc.

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4.0 REMOVAL TECHNOLOGY FOR THM PRECURSORS, THMFP AND THMS Chlorination has been effectively applied as a disinfection agent for the control of water-borne diseases for more than a century. The addition of oxidants to water can generate a variety of DBPs, which have been discovered to be related to adverse health effects. Considerable effort has been invested in strategies to reduce DBP formation through the customization of disinfection techniques and elimination of DBP precursors. These contaminants must be removed or prevented from forming before water can be used by the consumer. This section extensively discusses the current technology for removing THM precursors, THMFP and THMs (Figures 2 and 3).

4.1 Adsorption Adsorption has recently been acknowledged as a productive and economical method for THMFP and THM removal from wastewater. However, these studies are still in the theoretical and experimental phases. Adsorption is generally applied because of its low cost, operational convenience and model simplicity [41-44]. In several circumstances, this process provides highquality treated effluent [41]. Adsorption is a mass transfer process in which the compound is transported and gathered at the interface between two phases (gas–solid or liquid–solid) [45]. The compounds bind to the surface of the solid phase through physicochemical interactions and transform into an adsorbate (the solid on which adsorption occurs is known as the adsorbent). Thus, adsorption is occasionally reversible, and adsorbents can be regenerated by simply using an appropriate desorption process. However, the separation of biosorbents might become challenging after adsorption [41]. Several types of adsorbent, such as granular activated carbon (GAC), powdered activated carbon (PAC), carbon nanotubes (CNTs) and ion-exchange resins (IERs), have been investigated for THM precursor and THM adsorption. Page 10 of 55

Activated carbon is effective in minimizing scent-producing substances and acts as an efficient adsorbent to eliminate THM precursors and THMs [46]. According to Upadhyayula et al. [47], the removal of NOM from raw water can be achieved mainly by carbon-based absorbents, such as GAC, PAC and BAC. The function of carbon surfaces in the adsorption of NOM is related to the physical properties of the carbon material and the chemical composition of NOM [48]. Cationic functional groups present on the surface of activated carbon enhance the adsorption of NOM because the net surface charge of NOM molecules is always negative [48,49]. Larger molecules of organic compounds are more effectively removed during pretreatment, whereas smaller molecules are more effectively eliminated using activated carbon [50]. Therefore, the carbon capacity is easily exhausted in as little as three weeks [51]. Activated carbon has become the most commonly used adsorbent for THM precursor and THM removal. A survey from ScienceDirect [52] and Taylor and Francis [53,54] focusing on THM precursor and THM removal showed that almost 12% of research used activated carbon and adsorbent technologies. 4.1.1 Granular Activated Carbon (GAC) The GAC system is often used for THM precursor removal due to its convenient maintenance, ease of operation [55], regeneration ability, filterability and low cost [56]. According to Kim and Kang [57], hydrophobic GAC materials may preferentially adsorb DOC. In the use of GAC to remove THM precursors, temperature has been reported to play a role in effective removal: a temperature of 35 °C was the most effective condition during the adsorption of dissolved organic matter (DOM) [58]. As reported by Wei et al. [55], the removal of DOC and THMFP using GAC reached 45% and 55.9%, respectively, whereas Yang et al. [59], using a pilot plant, found that the removal of THFMP was only 31.8%. However, GAC has the potential to remove THM precursors, but its efficiency decreases over time due to saturation of the adsorption sites in the GAC. This phenomenon was reported by Gibert et al. [60], who found that the removal

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of total organic carbon (TOC) declined from an initial value of 65% to 40% at the end of the study.

4.1.2 Powdered Activated Carbon (PAC) PAC preferentially adsorbs CHBr3 because it is the largest molecule in the THM group, followed by CHBr2Cl, CHBrCl2 and CHCl3. As shown in Figure 3, THM removal by PAC was in the range of 62.38% to 75.8% [61]. The capacity of activated carbon to adsorb THM compounds increases with increasing boiling point and molecular weight of the compound. Based on an evaluation of THM adsorption on CNTs and PAC, CNTs require less contact time to achieve equilibrium compared with PAC [62]. This difference is mainly because CNTs do not have a porous structure, such as that of PAC, in which the adsorption of THMs shifts from the exterior surface area towards the interior surface area of the PAC pores to reach equilibrium, prolonging the lifetime of the PAC as a result.

4.1.3

Carbon Nanotubes (CNTs)

CNTs are comparatively innovative adsorbents for the adsorption of pollutants in water and have excellent potential for environmental protection applications. The characteristics of CNTs (structure, nature of the surface and purity) can be enhanced through acid treatment, which causes them to become more hydrophilic and improves the adsorption of relatively polar THM molecules. The smallest molecules, such as CHCl3, are preferably adsorbed onto CNTs, followed by CHBrCl2, CHBr2Cl and CHBr3 [61]. The CNT diameter and applied pressure play a role in adsorbing THMs: CNTs with diameters of 5,5 nm have been shown to be able to adsorb all THMs in water [63].

4.1.4 Ion-Exchange Resins

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Alkaline macroporous resins are usually applied in the adsorption process for the removal of organic matter. To the best of our knowledge, this technology is favourable for THM precursor removal but not for THM removal. A distinct reduction in the THM concentration is shown after the removal of NOM, resulting in TTHM reduction percentages of 60.6% to 63.0% [25]. Some types of ion-exchange resins applied for THM precursor removal include gel polystyrene and macroporous acrylic resin [64] and magnetic ion-exchange resin (MIEX®) [65,66].

4.2

Coagulation/Flocculation

Because the presence of NOM is very important in the formation of THMs, studies on the nature of NOM and its removal are common in research on water treatment. The coagulation/flocculation process has been investigated for THM precursor and THMFP removal [7,67-71]. The coagulation process destabilizes the particle charges, and coagulants with positive charges are added to the water to neutralize the negative charges of the dispersed suspended solids, such as organic substances and clay [72]. The suspended particles are capable of sticking together to form microflocs when the charge is neutralized [73]. Through the process of slow mixing, the microflocs contact each other, and this particle collision results in the production of larger flocs. Flocculation involves the use of polymers to form connections among the flocs and combine the particles into much larger agglomerates or clumps [41]. Through additional collisions due to the addition of an organic polymer or the inorganic polymer formed by the coagulant, the particle sizes of the macroflocs continue to increase [73]. Currently, several types of coagulants/flocculants, such as ferric chloride (FeCl3), aluminium sulphate [Al2(SO4)3], and polyaluminium chloride, are extensively applied in THM precursor and THMFP removal. By coagulation/flocculation, as much as 35.1% of the THMFP in municipal wastewater can be effectively removed [74]. Approximately 47% of the TOC in raw water from Lake Roine,

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Tempere, Finland, was removed using alum coagulation [75]. In a laboratory-scale experiment, the removal of compounds related to THMFP, DOC, TOC and specific ultraviolet absorbance (SUVA) using FeCl3 as a coagulant reached 27.2, 45, 40 and 24%, respectively, whereas the removal of THMFP, DOC, TOC and SUVA compounds using regular-grade alum [Al2(SO4)3·14H2O] reached 25.2, 30, 30 and 20%, respectively [76]. DOC can be easily eliminated by applying FeCl3 at doses below 5 mg/L [68]. However, the use of 40 mg/L of polyaluminium chloride, a coagulant, in a set up with a ceramic membrane resulted in the removal of 47.6% DOC [69]. Another study on the coagulant dosage was reported by Zheng et al. [70]: as the Al2(SO4)3 increased, the removal of DOC also increased, and a dosage of 28.7 mg/L Al2(SO4)3 was required for 40% DOC reduction. In a study by Gough et al. [67] on United Kingdom upland water treatment, the removal of THMFP was high and stable due to a high degree of DOC removal (76%). However, in that study, the formation of brominated THM after coagulation/flocculation using Al2(SO4)3 increased two-fold.

4.3

Advanced Oxidation Process (AOP)

AOPs are an alternative group of technologies that have the potential to minimize the formation of DBPs [77]. The application of AOPs has gained interest in the drinking water industry as an additional process for minimizing the formation of DBPs and NOM in drinking water [77,78]. Therefore, AOPs have been documented to reduce the THMFP and TOC contents in raw source water [79] and eliminate THMs. Typically, this technique uses ozone (O3), hydrogen peroxide (H2O2), ultraviolet radiation or combinations thereof. Chlorination can be replaced by UV irradiation as the primary disinfection process, but chlorine can still be applied as a secondary disinfection process [80]. Previous studies on processes such as UV irradiation [78,81], O3 addition [82] and AOP combinations using the oxidant H2O2 and UV (H2O2/UV) [83,84], O3

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and UV (O3/UV) [78,79] or H2O2 and O3 (H2O2/O3) [85] have focused on evaluating the reduction potential of THMs, NOM and DBP formation in finished water.

4.3.1 O3 Ozone is formed naturally in the atmosphere as a colourless gas with a very foul odour. At atmospheric temperature and pressure, ozone is an unstable gaseous state, which breaks down into oxygen molecules. Ozone is known to be a powerful pre-oxidant that can efficiently reduce THM formation. The ozonation process has received further attention because of its potential to minimize the formation of chlorine-related organic DBPs during the disinfection process. Ozonation generally increases the biodegradability of NOM in water through the transformation of larger organic molecules into smaller organic molecules that are more easily biodegraded. Ozone addition not only increases the biodegradability of the dissolved organics but also provides large amounts of oxygen to the water. The application of ozonation in water treatment processes will minimize the formation of THMs in the subsequent chlorination process [20,86,87]. At the beginning of the water treatment process, ozone will not lead to the formation of halogenated compounds, such as THMs, but if THMs are formed, they will be oxidized by ozone [86]. Moreover, it is predicted that the typical conditions of ozonation for the duration of drinking water treatment are not enough for the full mineralization of NOM [89-91] Karnik et al. [20] and De Vera et al. [92] reported that the percent reduction of TTHMs was 13.7 to 36.6% and 25%, respectively. Moreover, ozonation can reduce high organic concentrations by 25 to 30%, and up to 30% of the initial DOC concentration was transformed to biodegradable DOC (BDOC) [93].

4.3.2 UV

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UV systems are very low maintenance and only require bulb replacement yearly. UV light can only kill microorganisms such as bacteria, viruses, fungi, algae, yeast, Cryptosporidium and Giardia in water [94]. UV light does not produce any reaction with chlorine gas, is free and does not produce dangerous fumes [95]. However, UV treatment on its own has very little impact on the organic constituents in raw water [78]. Therefore, for the removal of THM precursors and THMFP, UV treatment is suitable for use in combination with other water treatment techniques. Lamsal et al. [96] reported that the reduction in THMFP upon UV exposure is only approximately 15%, while 70 min of exposure to UV irradiation reduced an initial THM concentration of 200 µg/L to 4 µg/L (95% removal) for CHBr3, CHBr2Cl and CHBrCl2 and to 132 µg/L (34% removal) for CHCl3 [97].

4.3.3 H2O2/UV The combination of H2O2 and the UV process also has a potential effect on THM precursor and THM removal. The combination of H2O2 and UV processes promotes the formation of •OH [83,84]. The formation of •OH radicals is shown in Reaction (3), in which the pollutant solution was treated with H2O2 and UV light irradiation, which causes the homolytic cleavage of H2O2 [98]. •OH is capable of reducing the TOC concentration and DBPFP of raw water [79]. Under strong advanced oxidation conditions, such as long irradiation times or high H2O2 concentrations, NOM is mineralized, as represented by a decrease in TOC and DBPFP [83-85]. Nevertheless, commercial applications apply low or moderate advanced oxidation conditions, as strong treatment conditions may not be economically feasible. Under low or moderate conditions, NOM is partially oxidized and higher molecular weight compounds are transformed into smaller and more biodegradable compounds [99,100]. Changes in the chemical characteristics of NOM also result a reduction in the TOC concentration and alter the characteristics of the DBP precursor to potentially reduce its reactivity with chlorine [96].

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hv H2O2

2•OH

(3)

Rudra et al. [97] reported that 40% w/w H2O2 could remove THMs, but the reduction decreased with increasing dilution of H2O2 due to the presence of unreacted H2O2. A dose of 1200 mJ/cm UV and 6 mg/L H2O2 could remove 99% CHBr3 (< 0.1 µg/L), 80% CHBr2Cl (0.6 µg/L), 26% CHCl3 (47 µg/L) and 53% CHBrCl2 (44 µg/L) [101]. As observed by Rudra et al. [97], Wahman et al. [102] and Chang et al. [101], more brominated THMs (CHBr3 and CHBr2Cl) were eliminated compared with chlorinated THMs (CHCl3) and other types of brominated THMs (CHBrCl2). This pattern is due to the C-Br bond, which serves as the active chromophore [101]. In addition, the UV treatment process showed a 15% reduction in THMFP, while the H2O2/UV treatment process increased the reduction of THMFP by 55% [80] and 77% [82].

4.3.4 H2O2/O3 Comparing •OH radicals with ozone, •OH radicals have a higher oxidation potential and react more readily with most organic compounds [103]. Stoichiometric Equation (4) shows the important reaction for scavenging •OH radicals, and Figure 4 shows the ozonation mechanism of organic compounds. As shown in the figure, Criegee [104] investigated the reaction of ozone through the electrophilic addition to double bonds of organic material and the formation of carboxylic acids, aldehydes or alcohols after several intermediate by-products.

OH  H 2O2  HO2  H 2O

(4) Page 17 of 55

The combination of H2O2 and O3 processes reduced the concentration of TOC by 6– 10%, and the combination of H2O2 and O3 increased the NOM oxidation compared with the O3 process alone [96]. In other studies, the reduction in the NOM concentration using H2O2 alone was found to be negligible [78]. Furthermore, the combination of H2O2 and O3 reduced THMFP by up to 75% [85] compared with the O3 treatment process alone, which reduced THMFP by 69% [96].

4.3.5 UV/O3 UV/O3 is an advanced method for water treatment that relies on the effective oxidation and destruction of refractory organic and toxic compounds in water. This process is more complex than other processes because •OH radicals are generated through several reaction pathways [105], as shown in Equations (5), (6) and (7). O3

k O3 1 → O (D) + O2

(5)

hv

O1 (D) + H2 O → H2 O2

hv

H2 O2 → HO • + • OH

(6)

(7)

Based on the evaluated systems, after UV/O3 treatment, approximately 15% of the TOC in raw water was mineralized [78,79,106]. The combination of O3 and UV at high O3 dosages can reduce the TOC by 31%. According to Lamsal et al. [96], the concentration of ozone relative to •OH was expected to be the main degradation pathway. The reduction of THMFP in the UV/O3 treatment process is 75% [96], and the combined UV/O3 process was more effective than the addition of O3 alone for the breakdown of THM precursors in surface water sources [107]. Page 18 of 55

4.4

Hydrolysis

Hydrolysis at elevated temperatures can lead to the reduction of THM in drinking water. Currently, boiling tap water for hot drinks is common across the world, and boiling can have a good impact on the DBP levels [108]. The effects of boiling significantly depend on the chemical stability and volatility of the DBPs, the heating duration when used as a part of a combined method, and the free chlorine residuals in the water [109]. Usually, when the heating time is 5 min, the THM levels decrease. Free chlorine residuals can react with THM precursors, and the net outcome of boiling is the total loss of THMs by hydrolysis and volatilization [7,110]. Tap water usually includes DBP precursors, DBPs and disinfectant residuals. If hot boiled water is kept in a closed container for an adequately long time, the concentration of THM will initially increase and then decrease. Meanwhile, the quantity of volatile THMs that escape into the atmosphere is significantly reduced [109]. All four THM components experienced significant hydrolysis in water with an initial pH range between 6.1 and 8.2 at temperatures of 65 °C to 95 °C [109].

4.5

Membrane Filtration

Several studies have been conducted using membrane filtration (microfiltration, ultrafiltration and nanofiltration) to eliminate THM precursors [111-119]. The membrane process provides a much better removal efficiency of THM precursors than other treatment processes and has been found to be the best available method to eliminate THM precursors to meet drinking water standards. However, the main challenge in using membrane filtration is the operational cost, which is mostly related to membrane replacement due to fouling, electricity consumption and operational scaling [59]. As reported by Yang et al. [59], the combination of ultrafiltration (UF) with nanofiltration (NF) membranes was determined to provide optimal quality in finished

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water, with a THMFP removal of up to 84.3%. The removal of NOM by the UF process can reach 30%, which may result in a decrease in THM formation by 50%, implying a greater removal of THMFP related to larger organic molecules by UF [120]. Sutherland et al. [118] investigated the elimination of THM precursors in Scottish surface water using a UF system. In the study, almost 82% of the TOC can be removed from the surface water. Moreover, when using a microfiltration (MF) system, the THM precursor removal was poor compared to that of UF, as Khan et al. [115] achieved only 5% TOC removal. Recently, another study reported using a combination of superhydrophilic and superhydrophobic nanofiltration membranes to remove NOM and DOC, and the removal of both THM precursors reached 83.6% and 73.3%, respectively [121]. During the membrane filtration process, membrane fouling may occur due to the existence of several bacteria and micropollutants. According to Bodzek et al. [122], for chloroform removal, the UF membrane process is far more efficient than that of NF and reverse osmosis membranes. This difference is because the removal of chloroform is dependent on various membrane characteristics, such as the membrane material and the membrane pore size. Membranes constructed using hydrophobic polymers are more sensitive to fouling, which is evidenced by the decrease in the yield and the lack of stability in the volumetric permeate flux during the filtration process [122]. On the membrane surface, the development of a filtration cake may facilitate the additional removal of chloroform throughout the ultrafiltration process [122]. Moreover, previous research on MF and UF also reached the same conclusions as Bodzek et al. [122]. Furthermore, based on Karnik et al. [20], the removal percentages of TTHMs and DOC by UF were 8–12.2% and 16.3–18.3%, respectively, which represent lower removal rates for THMs and its precursors compared with the combined methods (Table 4).

4.6

Biodegradation

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Biodegradation is the degradation of compounds by subsurface microorganisms. Biodegradation represents a potentially important removal process for a group of chlorination by-products in distribution and water treatment systems, but there is little knowledge about the organisms involved, such as the substrate range, identity and biodegradation kinetics [123]. The THM precursor NOM can be biodegraded, whereas other THMs have been reported to be nonbiodegradable [124]. The non-humic fraction of NOM is usually easily biodegradable and supports the development of bacteria in water distribution systems [22]. In addition to biodegradation, sorption has been proposed to be an essential process for the removal of acidic and hydrophobic fractions [125]. Reports over the last three and half decades indicate that CHCl2Br, CHClBr2 and CHBr3 are not reduced by microbial degradation [126]. However, starting in 2000, the biological removal of DBPs through biodegradation by microorganisms has been investigated and may perform well [102,123,124,127]. THMs are reportedly removed under aerobic conditions by ammonia-oxidizing bacteria (AOB) [102,128]. The effective microbes involved in the biodegradation of DBPs include Nitrosomonas europaea, Nitrosomonas oligotropha [102] and Xanthobacter autotrophicus [129]. The amount of effective microbes involved in the biodegradation of DBPs plays a major role in removal [129]. The removal of THMs by a mixed AOB culture was comparable to that metabolized by a pure AOB culture (Nitrosomonas europaea), thereby indicating that large-scale applications do not require a pure culture (which may be impossible regardless) because mixed AOB cultures exist naturally in water sources, according to Keener and Arp [130]. It is predicted that, in the presence of ammonia monooxygenase enzymes, THMs will bind to the hydrophobic sites of AOB where ammonia also binds. Thus, a higher concentration of ammonia will increase the removal of THM because the degradation rate of THMs depends on the concentrations of both ammonia and THM. As previously reported, brominated THMs are more easily removed by microbial degradation

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compared with chlorinated THMs. This difference is because brominated THMs are more nucleophilic (less electrophilic) than chlorine; thus, the hydrogen on THMs could reduce its partial charge, causing the compound to become relatively more hydrophobic as bromine substitution increases.

4.6.1 Biological Filtration Biological filtration is known to improve the water treatment process [115,116]. This process is used to support the amount of bacteria required to breakdown the pollutants [131]. The filter media hosting the bacteria must provide a large surface area and must have a rough surface on which the bacteria can easily grow. Such media include activated carbon [71,132] gravel, lytag, bio-balls, bio-glass, plastic media, flocor and aqua rock [131]. Biological filtration has been investigated for the removal of NOM [71,133,134] and DOC [132,135]. Using membrane bioreactor (MBR) filtration, a THMFP reduction of 75% generally resulted from the removal of 60% of the TOC [136]. As reported by Pramanik et al. [132], the use of a biological aerated filter (BAF) and biological activated carbon (BAC) filter achieved DOC removal rates of 51% and 56%, respectively. The combination of activated carbon and effective microbial function contributed to the BAC filter generating the greatest DOC removal. The flow configuration of the BAC filter plays a role in effective DOC removal. Upflow and downflow BAC filters removed approximately 37.9% and 31.5% of the DOC, respectively [71]. This difference was because the bacteria in the upflow BAC system were located more towards the middle of the filter (thus increasing the elimination of DOC from water), whereas the bacteria in the downflow BAC system were located on top of the filter [71]. However, biological filtration using expended clay operated in the downflow configuration resulted in 5–65% DOC removal for empty bed contact times (EBCTs) of 5.7–11.3 min [133]. These results indicate that

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biological filtration using adsorbent-based packing for bacteria growth and attachment is effective for THM precursor removal.

4.6.2 Soil Aquifer Treatment (SAT) SAT is part of a biodegradation process that uses biological substances in the process. In Arizona and California, pilot-scale SAT systems showed that the most significant removal of DOC occurred within the top 0–0.30 m soil layer [126,137,138]. Moreover, a significant structural transformation of DOM occurs during SAT operation [139,140]. This result indicates that the mechanism of biodegradation may include the transformation of biodegradable matter, mostly non-aromatic components, to less polar and larger compounds [55]. Hence, the dominance of biodegradation in DOM removal during SAT has been highlighted [141]. The reductions in DOC and THMFP using SAT were 65.1% and 45.2%, respectively [55].

4.6.3 Biosand Column Biosand columns are usually operated under gravity flow, and THM precursor removal using sand columns is generally carried out by microbial consortia placed in sand matrices, termed biofilms [142,143]. The employment of sand columns for DOM removal from groundwater resulted in quite low DOC removal efficiencies of only 5–15% owing to the hydrophobic acid fractions and aromatic hydrophilicity of DOC, which is usually recalcitrant and is difficult to preferentially remove [143]. Wei et al. [55] reported that DOC and THMFP removal efficiencies of 44.1% and 32.9%, respectively, were achieved by sand columns. Another study by Linlin et al. [116], which used a downflow sand column with a sand size of 0.4–0.8 mm, achieved a DOC removal of almost 60%.

4.6.4 Horizontal Subsurface Flow Wetlands (HSFW)

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Constructed horizontal subsurface flow wetlands (HSFW) [55] and vertical surface flow wetlands (VSFW) [144,145] were studied for the removal of DOC. The schematics of the HSFW and VSFW are shown in Figure 5. The emergent species planted in the HSFW were Canna indica and Acorus calamus, whereas those in the VSFW were Acorus calamus [145], Polygonum lapathifolium, Typha latifolia, Echinochloa crusgalli, and Schoenoplectus californicus [144]. The DOC and THMFP removal percentages by the HSFW with a 4-day retention time were up to 23.3% and 42.2%, respectively [55]. Moreover, the VSFW were able to remove up to 47.6% of the DOC [145] with a hydraulic retention time (HRT) of 1.07 m3/m·day.

4.7

Integrated Technology

4.7.1 Ozonation and Biological Granular Activated Carbon (O3/BGAC) Ozonation followed by biological treatment is a promising process for the elimination of THM precursors and THMFP from raw water [147-150]. Unsaturated bonds in aromatic moieties that are found in NOM can be split by ozone, which reduces the size of organic molecules and makes them more biodegradable and more readily oxidized; therefore, the NOM is transformed into a biodegradable form, such as biodegradable dissolved organic carbon (BDOC) [151]. Ozone can also be used in substrates to encourage microbial development in biofiltration systems [152,153]. During ozonation, the highest value of BDOC produced was approximately 30% relative to the total DOC in raw water, even with increased reaction time or ozone dose [154]. In BGAC, THM precursors present in the water are eliminated through two parallel processes: adsorption by the activated carbon and biodegradation by the microbial community. Activated carbon has been discovered to accelerate the decomposition of ozone into very oxidative forms, such as hydroxyl radicals (•OH) [155]. By using the O3/BGAC combination, the bed life of O3/BGAC is approximately 2–3.5 times higher than the bed life of GAC [150]. In addition, the

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O3/BGAC combination also contributes to biologically stable water [149,154]. The removal of DOC using O3/BGAC reached 33.3% [155], 62.9% [59] and 38% [150]. The combination of O3/BGAC also shows a synergistic interaction for removing NOM [150].

4.7.2 Ozonation and Ultrafiltration (O3/UF) The oxidation of THM precursors during ozonation makes them more polar and easier to degrade, whereas membrane filtration effectively eliminates organic matter from water and wastewater. However, because of membrane fouling, decreased permeate flux is one of the main obstacles related to the operation of membrane filtration plants [156]. The concept of UF is based on the physical separation of dissolved solids, turbidity and microorganisms by the small pore sizes of the membranes. However, few studies have been performed on the removal of THM precursors using an O3/UF system. From a previous study by Karnik et al. [20], the removal of DOC and TTHMs using this system was 26.4 to 45% and 41.4 to 48%, respectively. Using ozonation increased the fouling resistance and reduced the cake resistance of the UF membrane, but the system did not achieve a similarly high removal of TOC, as observed by Hyung et al. [157].

4.7.3 Ozonation/Ultraviolet-BAC (O3/UV-BAC) Ozonation-ultraviolet (O3/UV) is an AOP combination process that has been recommended for AOP pretreatment prior to biological filtration and can reduce bacteria development in distribution systems [158]. More biodegradable compounds tend to be easily utilized as primary food sources by microorganisms in BAC filtration [83]. Therefore, AOP treatment followed by BAC is particularly effective, and this method is already used in water treatment plants in a number of cities and countries to minimize the amount of chlorine used for disinfection, to

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control taste and odour and to prevent the formation of DBPs. A schematic diagram of a pilotscale O3/UV-BAC system is shown in Figure 6. The combination O3/UV-BAC treatment results in an extensive decline in TOC (19%), DOC (18%), TTHMs (71%) and THMFP (39.3%) compared with BAC or O3/UV treatment alone, which removed only 8.2 and 10% and 8.6 and 11.7% DOC and TOC, respectively [158]. For that reason, the O3/UV-BAC method continues to be recommended as a promising treatment for the reduction of hazardous DBPs along with its precursors, including DOC [158].

4.7.4 Granular Activated Carbon/Soil Aquifer Treatment (GAC-SAT) Soil exhibits better sorption of hydrophobic fractions if the compounds have relatively high molecular weights. The combination of adsorption and biodegradation in a soil aquifer results in higher DOC reduction for both aromatic and non-aromatic components [55]. Using GAC/SAT, Wei et al. [55] achieved DOC and THMFP removal values of 70.1% and 67.2%, respectively.

4.7.5 Granular Activated Carbon-Filter Adsorption (GAC-FA) Granular activated carbon-filter adsorption (GAC-FA) systems can be easily installed as a retrofit of sand filters whenever DBPs are difficult to manage using conventional water treatment. Sand filters retrofitted to a GAC-FA system for the removal of DOC and previously formed DBPs were first studied in September 2003 by the Buyeo wastewater treatment plant [159]. The average removal of DOC using GAC-FA was 30% higher than that of a sand filter alone based on 6 months of operation, after which the percentage removal decreased gradually over time due to saturated conditions [57]. Temperature influences the adsorption and desorption of THMs on GAC-FC. For example, during the warm season, the removal of THMs was 21.9%, whereas during the cold season, THM desorption occurred, creating new sites for

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further adsorption [159]. During the use of GAC-FA to remove THMs at concentrations of 4.5– 84.3 µg/L, GAC-FA breakthrough occurred after 3 months [159].

4.7.6 Activated Carbon/Nanoscale Zero-Valent Iron (AC/NZVI) Nanoscale zero-valent iron (NZVI) has been identified as a potential reductant, particularly for halogenated organic contaminants, because of its high reductive reaction capacity and high specific surface area [160,161]. The reduction in the THM components CHCl3, CHBrCl2, CHBr2Cl and CHBr3 during a HRT of 15 min were 62%, 77%, 82% and 98.5%, respectively [162]. The THM reduction is quite high considering this process required only 15 min compared to other treatments that require more time to achieve high removal percentages for THMs or TTHMs. Unfortunately, several challenges impede practical application. For example, NZVI is easily agglomerated and is easily oxidized in ambient conditions by the presence of oxidative species [163]. This could affect the activity of NZVI, leading to a reduction in the efficiency and reaction rate.

4.8 Advantages and Challenges of Treatment Systems in the Removal of THMs, THMFP and THM Precursors Every technology has its own advantages and challenges during operation of the treatment process. As discussed previously in this paper, various technologies can be used to remove THM precursors, THMFP and THMs through either chemical or biological methods or a combination thereof. Table 5 lists the detailed advantages and challenges of each technology. Advanced oxidation processes (AOPs) are especially able to reduce the aromaticity of NOM [163]. They can transform large NOM molecules to smaller ones and form readily

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biodegradable compounds. In addition, AOPs can also reduce the hydrophobicity of NOM [163]. However, there are still challenges to overcome in this process, including the cost of the treatment processes, which are beyond the range of economic acceptability for commercial drinking water, and the production of by-products, such as aldehydes and ketones. In addition, combined technological methods have many advantages in removing THM precursors, THMFP and TTHMs. By using technological combinations, the disadvantage of one method can be compensated by another. For example, in the combination of O3 and UF, the UF technology involves an easily fouled membrane, but the use of O3 in the initial stage can help remove and reduce the pollutant level in water and wastewater. Thus, at the membrane filtration stage, the use of O3 reduces membrane fouling and helps maintain permeate flux. Filtration technology using membranes can reject large organic molecules and hydrophobic organics (major THM precursors), thus preventing the formation of THMs during the chlorination process, and it is also effective in removing THMs. Membrane filtration has a high potential but is easily fouled, causing a decrease in permeate flux and requiring membrane replacement, which can be expensive. Meanwhile, treatment through biodegradation is accomplished by living organisms, mostly plants and microorganisms. This process is inexpensive to operate but requires a large plantation area. Moreover, the investment cost of biological technology is high since it requires a large area and the price of land is quite expensive, unless the treatment plant is located in a rural area, which is typically much cheaper. Though the investment cost is high, operation and maintenance is inexpensive, as other treatments, such as chemical technology, require the addition of chemicals to the treatment and AOP technology requires 1) the addition of chemicals and ozone, 2) the replacement of the UV lamp and 3) high energy consumption to run the system.

5.0

Green Technology for THM Precursor and THM Removal

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Green technology involves the use of one or more environmental and natural resources while performing environmental monitoring and protection to curb the negative impacts of human involvement. The term also describes the creation, design and use of chemicals and processes that reduce or eliminate the utilization and/or production of hazardous substances. In this case, biodegradation processes are environmentally friendly processes that do not involve harmful chemicals that may contribute to toxicity and environmental pollution. The biodegradation process is a green technology because it uses natural resources such as yeast, algae, bacteria and fungi to degrade chemicals [169]. This process is also inexpensive because the natural resources are readily available. The natural resources used in this process are bacteria, which act as an adsorbent and degrade the THMs and THM precursors present in water and wastewater. As described above, there are various types of water treatment processes using different chemical or biological technologies. For chemical technologies, the materials used are costly but are highly effective. In contrast, the materials used in biological technology are inexpensive and environmentally friendly, and these methods exhibit high efficacy in the water recovery process [169]. These results show that the use of green biological technology is safer and more effective for water treatment. Biodegradation has potential for further applications in the process of removing THMs and THM precursors. Table 6 shows the cost estimation for different types of technologies for water and wastewater treatment. AOP systems combined with ultrasonic (US) technology, such as US+UV+O3, US+UV+H2O2 and US+UV, are more effective treatment systems compared to one type of oxidation process alone [170]. However, the treatment system is expensive compared to other treatment systems. Though the capacity of an ultrafiltration system is 62.5 times higher than the capacity of an AOP system, the cost of the treatment system is $15,391,000/year for ultrafiltration and $3,585,087–12,465,887/year for AOP. The AOP

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treatment system is expensive because it requires a large amount of dissolved ozone, high energy consumption, and the replacement of expensive materials, such as UV lamps and ultrasonic devices. In coagulation process that require chemicals, such as alum, the cost of the treatment system is in the range of $1,112,000–1,315,000/year for a capacity of 3,785,412 L/day. According to Stenzel and Merz [171], a GAC system with a capacity of 1,641,600 L/day with a single stage that runs for an EBCT of 17 min has an annual cost of $42,800/year, while a system with a capacity of 1,088,640 L/day with a double stage including a transfer tank that runs for an EBCT of 26 min has an annual cost of $245,000/year. Hence, the cost of using a double stage is 5.72 times higher than using single stage. Stenzel and Merz [171] used a double stage to remove target contaminants present in groundwater. In comparing GAC and coagulation technology with the same capacity (3,785,412 L/day), the costs of the treatment systems are $152,205/year and $1,112,000–1,315,000/year, respectively. Thus, the GAC treatment system required the lowest annual cost, as GAC is inexpensive and can be reused with reactivation on-site or off-site. Table 6 includes the cost of GAC reactivated off-site, because to reactivate GAC on-site, extra costs, space and construction are necessary, which will increase the annual cost of the treatment system. For a membrane bioreactor (MBR), the combination of membrane filtration and biological technology is highly effective in the treatment process. However, the application of membrane filtration increased the cost of the treatment system since the capacity of the MBR is only 40,000 L/day with an annual cost of $6,384,000/year. On the other hand, biological treatment can be combined with other technologies, such as activated carbon, sand, soil, etc., since activated carbon is inexpensive and can remove THMs, THM precursors and THMFP effectively, according to previous studies.

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6.0 CONCLUSIONS Many removal technologies for THMFP, THM precursors and THMs have been studied. The technologies resulting in the greatest removal of THMs are UV, H2O2/UV and AC/NZVI, whereas the technologies resulting in the greatest removal of THMFP are biofiltration, UV/O3, H2O2/O3, H2O2/UV and UF+NF. In addition, the removal of THM precursors can be performed by SAT, GAC+SAT and H2O2/O3. Based on previous studies, considerable research has focused on eliminating THM precursors compared to eliminating the THMs themselves. However, some technologies are able to remove TTHMs effectively. Although the highest removal rates of these contaminants are typically achieved by chemical methods, such as AOP systems and membrane filtration, these processes require high capital, operation and maintenance costs. Alternatively, biological methods, such as BAC and biodegradation, and combined processes with a biological component have gained more attention and have the potential to reduce THMFP, THM precursor and THM contamination in water and wastewater. However, their metabolite mechanisms must be examined in future studies to further improve the performance of these technologies. Moreover, the application of biological processes is more environmentally friendly and features a low operational cost. Thus, biological processes are a promising future technology for the removal of THM precursors and THMs from water and wastewater.

7.0 ACKNOWLEDGEMENTS This research was financially supported by Universiti Kebangsaan Malaysia (UKM) through Dana Impak Perdana (DIP) with grant number DIP-2016-030.

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Page 42 of 55

DBPs - THMs, HAAs, HANs

PPCPs - Pharmaceuticals, Personal Care Products (PCPs)

EDCs - Pesticides, Additives, Drugs

Heavy Metal

Figure 1 Catogory of micropollutants

Page 43 of 55

Technology Biosorption GAC

Coagulation

AOPs

Hydrolysis O3

PAC

UV

CNTs

H2O2/UV

IER

H2O2/O3

Biodegradation

Membrane filtration

Biological Filtration SAT Biosand Column HSFW

UV/O3

Integrated Technology O3/BGAC O3/UF O3/UV-BAC GAC-SAT GAC-FA AC/NZVI

Figure 2 Technology for the removal of THMs, THM precursors and THMFP

Page 44 of 55

Figure 3 Technology for the removal of THMs, THM precursors and THMFP

Page 45 of 55

Figure 4 Mechanism for the ozonation of organic compounds [97].

(a)

(b)

Figure 5 Schematic diagrams of constructed wetlands [146].

Page 1 of 55

KI solution LINE I Feed wate

Ozone/UV reactor

Retention Tank

BAC Column

Effluent

BAC Column

Effluent

Ozone generator LINE II Figure 6 Schematic diagram of a pilot-scale O3/UV-BAC plant

Page 2 of 55

Table 1 Physico-chemical properties of THMs

Molecular formula Molecular weight (g mol-1) Melting point (°C) Boiling point (°C)

Chloroform

Bromodichloro -methane

Dibromochloro Bromoform -methane

CHCl3

CHBrCl2

CHBr2Cl

CHBr3

119.369

163.823

208.277

252.73

-63.5

-57

-22

-4 to 16

61.15

90

119

147 to 151

4.5 (20 °C)

2.5

3.2 (30 °C)

1.980 g cm-3

2.451 g mL-1

2.89 g mL-1

10.62 (0 °C) Solubility in 8.09 (20 °C) water (g L-1) 7.32 (60 °C) 1.564 g cm-3 (-20 °C) 1.489 g cm-3 (25 °C) Density 1.394 g cm-3 (60 °C)

Page 3 of 55

Table 2 Different national approaches to the regulation of THMs in drinking water Country

Organizat Date ion Set

2004 2007 1998

CHCl CHB rCl2 3 (µg/L (µg/L ) ) 16 -

CHB r2Cl (µg/L ) -

CHB r3 (µg/L ) -

TTH Ms (µg/L ) 250 100 100

Australia Canada European Union Germany Japan Netherlands New Zealand South Africa Sweden UK: England/Wales UK: Northern Ireland UK: Scotland USA World Malaysia

NHMRC HC EC FHM MHLW VROM MH DWAF NFA DWI

2001 2004 2000 2005 2005 2001 2000

60 200 -

30 60 -

100 150 -

90 100 -

50 100 25 a ˂200 100 100

DWI

2007

-

-

-

-

100

DWR EPA WHO MH

2001 2001 2006 2010

80 300 200

80 60 60

80 100 100

80 100 100

100 80 100 a

Refere nce

[39]

[40]

Table 3 Criteria for the surface water quality of Class I water [37] THMs CHCl3 CHBrCl2 CHBr2Cl CHBr3 TTHMs

Class I: Potable Water Supply (µg/L) ≤ 5.67 ≤ 0.27 ≤ 0.41 ≤ 4.30 ≤ 10.65

Table 4 Technology for the removal of THM precursors, THMFP and THMs Year

1990

Technolo gy

Type

Coagulatio n +

TTHM s (% Reducti on)

THMF P (% Reduct ion)

THM precursors DOC TOC (% (% Reduct Reduct ion) ion)

EBC Reference T/H RT (Ho ur)

-

35.1

-

-

-

[74]

Page 4 of 55

2000 2002 2003

Flocculati on H2O2/O3 Coagulatio Alum n Biofiltrati on CNTs

-

PAC

-

UF O3+UF

Tubular ceramic Tubular ceramic

O3

-

UV

-

H2O2 Coagulatio n Coagulatio n GAC

Ferric chloride

2005

2006

-

87

75

-

4

[85]

-

-

-

47

-

[75]

-

75±6

-

60

1

[136]

-

-

-

3

[61]

-

-

-

6

[61]

49.8±4. 2 69.09±6 .71 10.1±2. 1 44.7±3. 3 35.4±1. 2 CHCl3: 34 CHBrl2: 98 CHBr2C l: 98 CHBr3: 98 14±4

-

17.3±1 .0 37.1±7 .9 42.1±1 .2

[20] 2

-

-

-

1.17

[97]

-

-

-

1

[97]

-

27.7

45

40

1.5

[76]

Alum

-

25.2

30

30

1.5

[76]

Pseudomo nas fluorescen s + Spirillum sp. Strain Coconut Soil

-

-

-

23.1

-

[167]

-

-

33.3

-

-

[163]

-

55.9 45.2

45 65.1

-

-

[55] [55]

Coconut

-

67.2

70.1

-

-

[55]

HSFW

Acorus calamus + Canna indica

23.3

42.2

-

-

[55]

Sandcolumn

Soil

32.9

44.1

-

-

[55]

O3-BGAC

GAC SAT GAC+SA T 2009

-

Page 5 of 55

2010

-

-

31.8

-

-

Filtration

UF+NF

-

84.3

-

-

O3-BGAC

-

-

-

38

-

O3-BGAC

-

-

51.1

-

-

-

61.9±1. 3

-

-

-

4

[25]

CHBr2C l: 80 CHBr3: 99 -

55 69 15 77

-

13±10

0.5 0.5 0.5

[80] [96] [96] [96]

-

-

-

-

[101]

70 75

[96] [96]

-

8.0±2 31 7.0±1. 6 9.9±1. 8

0.5 0.5

-

6.5±1. 7 8.0±2. 0

0.42

[158]

0.42

[158]

18

19

0.42

[158]

-

-

0.25

[162]

-

-

-

[92]

-

-

-

[92]

IonExchange Resins H2O2/UV O3 UV H2O2/UV 2011

-

H2O2/UV

-

H2O2/O3 UV/O3

-

UV/O3 BAC

-

-

-

O3/UVBAC

-

71

39.3

-

CHCl3: 62 CHBrl2: 77 CHBr2C l: 82 CHBr3: 98.5 25 -

-

34

2014 GAC/NZ VI

2015

0.45 2 0.65 2 2.3 0.98 5

GAC

O3 O3/tBuOH

-

-

[59] [59] [150] [59]

Table 5 Advantages and challenges of various adsorption technologies Technology

Type

Advantages

Challenges

Page 6 of 55

Adsorption

Filtration

PAC

-

Flexible and minimal capital expenditure for feeding and contacting equipment

BAC

-

Effective for organic removal from water and wastewater

IonExchange Resins

-

Effective for NOM removal and THMFP reduction

MF, UF and NF

-

CNTs

-

Biological Filter

-

Coagulation/ Flocculation

Coagulation -

Biodegradation SAT, Wetland

-

Can be used to replace the chlorination process, thus minimizing disinfectant demand Effective for removing AOC, THMs, HAAs and EDCs Can be simply built-in with other media Effective for NOM removal Soluble CNTs are not toxic to cell viability [164] Effective for organic degradation Provide complete nitrification Remove large organic molecules and hydrophobic organics (major THM precursors) Identified to be effective for removing TOC, DOC and SUVA [76]

-

-

-

-

-

Ease of operation and maintenance NOM becomes more oxygenated, more aromatic and easier to remove -

Requires microbial monitoring after water treatment owing to the formation of bacterial population Regeneration of the ion-exchange resin requires a difficult procedure Membrane fouling causes a decrease in permeate flux High operation and maintenance costs

High operational cost [165] Membrane integrity and fouling potential [166] Biofouling membrane Loss of adsorbent media from the filter [47]

Addition of chemicals is required Produces hazardous solid waste that requires further treatment Causes significant structural modification of DOM Increases the effluent of SUVA Must be transplanted initially before use Requires a large plantation area DOM is recalcitrant and not easily removed Page 7 of 55

by [143]

Advanced Oxidation Process (AOP)

O3

-

-

The reaction does not produce toxic halogenated compounds Transforms higher molecular weight compounds to smaller ones

-

UV

-

H2O2

H2O2/UV

UV/O3

Integrated Technology

O3 + UF

Low maintenance Does not react with chlorine gas Does not produce dangerous fumes -

Strong oxidation agent for organic contaminants - No other chemical addition required - Mineralization of NOM during the treatment process - Significant reduction in DBP-FPs - Effective destruction of refractory and toxic organics in water - Generates higher levels of •OH and reduction of THMFP - Ozonation reduces -

biodegradation

Formation of aldehydes, ketones and alcohols More complex than chlorination and UV irradiation and requires relatively complicated equipment Highly reactive and corrosive (materials and equipment used must be resistant to corrosion) High capital and operational costs Not effective against changes in the taste, odour or colour of water Fewer effects on organic components in raw water

-

-

membrane fouling Effective for THM and DOC reduction High efficiency, space saving and easy to operate

Requires high energy

The process is more complex compared to others

Same challenges as ozonation Reduces permeate flux and membrane fouling

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O3/t-BuOH

-

O3/UVBAC

-

O3-BGAC

-

-

Decomposition of ozone was slowed [167] Enhanced the reaction of O3 towards THM precursors Biodegradation of BAC minimizes the chlorine dose Manages odorous and badtasting substances Prevents the formation of DBPs

Can possibly form acetone and butan-2one [168] Challenges in maintaining and regenerating the activated carbon either by chemical or biological regeneration methods

Accelerate the decomposition of ozone into highly oxidative species Leads to biologically stable water [149] Effective for removing NOM

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Table 6 Cost estimation for different technologies for water and wastewater treatment Technology

Type

US+UV+O3 US+UV+H2 Phenolic Wastewater O2 US+UV Ultrafiltrati Seawater on Groundwater

GAC Drinking Water

Coagulatio Wastewater n Wastewater MBR *NA=Not Available

Capacity Treatment

1,440,000 L/day 90,000,000 L/day 1,641,600 L/day 1,088,640 L/day 1,892,706 L/day 3,785,412 L/day 9,463,530 L/day 3,785,412 L/day 40,000 L/day

Retention Time/ Contact Time

Comparative Annual Cost

Refer ence

$12,465,887/year 13 min $9,048,806/year (EBCT) $3,585,087/year

[170]

97 L/m2 hr

[172]

17 (EBCT) 26 (EBCT) 15 (EBCT) 15 (EBCT) 15 (EBCT) NA NA

min min min min min

$15,391,000/year $42,800/year

[171] $245,000/year $103,112/year $152,205/year

[173]

$316,638/year $1,112,000– 1,315,000/year $6,384,000/year

[174] [175]

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