Chapter 1

General Introduction

Mika Sillanpää     Lappeenranta University of Technology, LUT Faculty of Technology, LUT Chemtech, Laboratory of Green Chemistry, Sammonkatu 12, 50130 Mikkeli, Finland

Abstract

Natural organic matter (NOM) is a complex matrix of organic materials and a key component in aquatic environments. As a result of the interactions between the hydrologic cycle and the biosphere and geosphere, the water sources of drinking water generally contain NOM. The amount, character, and properties of NOM vary considerably according to the origins of the waters and depend on the biogeochemical cycles of their surrounding environments. Also, the interrelation between NOM and climate change has attracted a great deal of attention in recent research. NOM has a significant impact on many aspects of water treatment, including the performance of unit processes, necessity for and application of water treatment chemicals, and the biological stability of the water. As a result, NOM affects potable water quality as a carrier of metals and hydrophobic organic chemicals and by contributing to undesirable color, taste, and odor problems. Moreover, NOM has been found to be the major contributor to disinfection by-product (DBP) formation. Changes in NOM quantity and quality have a significant influence on the selection, design, and operation of water treatment processes. These changes also cause operational difficulties in water utilities. High seasonal variability and the trend toward elevated levels of NOM concentration pose challenges to water treatment facilities in terms of operational optimization and proper process control. To improve and optimize these processes, it is vital to characterize and quantify NOM at various stages during the purification and treatment process. It is also essential to be able to understand and predict the reactivity of NOM or its fractions during different phases of the treatment. Once the composition and quantity of NOM in the water source has been examined, suitable methods for efficient NOM removal can be applied. No single process alone can be used to treat NOM due to its high variability. The most common and economically feasible process available is coagulation and flocculation followed by sedimentation/flotation and filtration. Other treatment options for NOM removal include magnetic ion exchange resin (MIEX®) techniques, activated carbon filtration, membrane filtration methods, and advanced oxidation processes.

Keywords

Characterization; Climate change; Coagulation; Drinking water; Natural organic matter (NOM); Water analysis; Water treatment
Abbreviations
AOP
   Advanced oxidation process
DBP
   Disinfection by-product
FA
   Fulvic acids
FTICR-MS
   Fourier transform ion cyclotron resonance mass spectrometry
GAC
   Granulated activated carbon
HA
   Humic acids
HAAs
   Haloacetic acids
HMM
   High molecular mass
HMW
   High molecular weight
LMM
   Low molecular mass
LMW
   Low molecular weight
MIEX®
   Magnetic ion exchange resin
NF
   Nanofiltration
NMR
   Nuclear magnetic resonance
NOM
   Natural organic matter
Py-GC-MS
   Pyrolysis gas chromatography-mass spectrometry
SEC
   Size exclusion chromatography
SUVA
   Specific UV absorbance
THM
   Trihalomethane
TOC
   Total organic carbon
UV–Vis
   Ultraviolet and visible
Natural organic matter (NOM) is ubiquitous in waters, sediments, and soils. Aquatic NOM is derived both from the breakdown of terrestrial plants and as the by-product of bacteria, algae, and aquatic plants. NOM is defined as a complex matrix of organic materials present in all natural waters.
A wide range of terminology is used to describe NOM in the environment. These terms are listed and comprehensively discussed in recent reviews by Filella (2009) and Uyguner-Demirel and Bekbolet (2011). NOM is a key component in aquatic environments and is present in all fresh waters, particularly surface waters. As a result of the interactions between the hydrologic cycle and the biosphere and geosphere, the water sources of drinking water generally contain NOM. It consists of a range of compounds with a wide variety of chemical charges, from highly charged to uncharged. These also vary widely according to chemical composition and molecular size, molecular weight (the molecular masses of humic substances range from several hundreds to tens of thousands), and structure. NOM compounds are complex mixtures possessing unique combinations of various functional groups, including esteric, phenolic, quinine, carboxylic, hydroxyl, amino, and nitroso, which are usually negatively charged at neutral pH (Gjessing, 1976). Humic substances, which are the major constituents of NOM in waters, are amorphous, dark colored, and acidic in nature. Structurally, they consist of substituted aromatic rings linked by aliphatic chains (Uyguner-Demirel and Bekbolet 2011). NOM also influences the acidity, mobility, and toxicity of metals and organic pollutants and weathering (Winterdahl, 2013).
Water systems often have multiple sources of NOM, thus different organic carbon fractions (Rigobello et al., 2011). The amount, character, and properties of NOM vary considerably according to the origins of the waters, and depend on the biogeochemical cycles of their surrounding environments (Fabris et al., 2008). The factors that determine the composition of NOM are location dependent and include the source of organic matter, the water chemistry, temperature, pH, and biological processes (Leenheer and Croué, 2003). Thus, the character of NOM varies with source and season (Sharp et al., 2006a,b; Fabris et al., 2008; Rigobello et al., 2011; Nkambule et al., 2012). Moreover, the range of organic components in NOM can vary seasonally at the same location (Sharp et al., 2006a,b; Smith and Kamal, 2009), for example, due to rainfall, snowmelt runoff, floods or droughts. Floods and droughts are the main indications of the impact of climate change on water availability and quality. It has been suggested that these changes may explain the increase in the total amount of NOM (Delpla et al., 2009; Evans et al., 2005), which has been observed in several parts of the world during the past 20 years (Eikebrokk et al., 2004; Evans et al., 2005; Korth et al., 2004; De Wit et al., 2007; Monteith et al., 2007; Worral and Burt, 2007, 2009; Couture et al., 2012; Jarvis et al., 2012; Winterdahl, 2013; Gough et al., 2014).
NOM is an integral part of the carbon cycle, which may have indirect climatic effects (Winterdahl, 2013). The interrelation between NOM and climate change has attracted a great deal of attention in recent research (Maurice et al., 2002; Hejzlar et al., 2003; Granskog et al., 2007; Park et al., 2007; Clements et al., 2008; Soh et al., 2008; Porcal et al., 2009; Sulzberger and Durisch-Kaiser, 2009; Bunting et al., 2010; Chen et al., 2012; Pautler et al., 2012; Brezonik and Arnold, 2012; Diem et al., 2013; Ritson et al., 2014). Not only the quantity of NOM, but also its quality, have been observed to change when other important characteristics of NOM, such as specific UV absorbance (SUVA), are taken into account (Eikebrokk et al., 2004). Winterdahl (2013) has emphasized that intraannual NOM variability is often larger than year-to-year changes by several orders of magnitude. Unravelling the controls on intraannual NOM dynamics is thus essential to understanding long-term changes, since climate change could alter NOM dynamics in ways not reflected in interannual trends. Several potential factors, including an increase in air and surface water temperature, rainfall intensity, and atmospheric CO2 and/or a decrease in acid deposition, have been proposed to explain the increased amount of NOM, but there is yet no scientific consensus on this issue (Delpla et al., 2009).
Tang et al. (2014) demonstrated that NOM has complex effects on the environmental behavior of and removal of heavy metal ions by nanomaterials. The impacts were reported to be controversial, depending both on the type of nanomaterials and metal ions and on geochemical conditions. They also demonstrated that the presence of NOM can modify the mechanisms controlling metal ion removal and transportation by nanomaterials in heterogeneous aquatic environments (Tang et al., 2014). Another emerging field of research is the effect of NOM on nanoparticle aggregation (Louie et al., 2013). Collin et al. (2014) recently concluded that NOM decreased the toxicity and bioaccumulation of nanoparticles. The impact of photooxidation on the optical, electrochemical, and photochemical properties of NOM has recently been investigated systematically (Sharpless et al., 2014). This sheds light on the relationships between NOM aromaticity, redox state, and photoreactivity.
NOM found in natural waters consists of both hydrophobic and hydrophilic components, of which the largest fraction is generally hydrophobic acids, making up approximately 50% of the total organic carbon (TOC) in water (Thurman, 1985). These hydrophobic acids can be described as humic substances comprising (1) humic acids (HA), which are soluble in alkali, but insoluble in acid, (2) fulvic acids (FA), which are soluble in both alkali and acid, and (3) humins, which are insoluble in both alkali and acid. FAs constitute a major fraction of these humic substances, which, while structurally comparable, vary in molecular size and functional group content (Table 1.1). Hydrophobic NOM is rich in aromatic carbon, phenolic structures, and conjugated double bonds, while hydrophilic NOM contains more aliphatic carbon and nitrogenous compounds, such as carbohydrates, sugars, and amino acids. The physical and chemical fractionation of aquatic NOM at specific pH can be used to classify organic solutes into these broadly defined hydrophobic and hydrophilic fractions (Chow et al., 2004; Leenher, 2004; Sharp et al., 2006a,c). While these fractions are more operationally than structurally defined, organic compounds can be assigned to a particular fraction according to their chain length and functional groups (Swietlik et al., 2004; Buchanan et al., 2005). The hypothetical molecular structure of HA is presented in Figure 1.1.

Table 1.1

Common properties of humic acid (HA) and fulvic acid (FA) (Snoeyink and Jenkins, 1980; Xing, 2010; Chamoli, 2013)

PropertyHAFA
Elemental composition (% by weight)
 Carbon50–6040–50
 Hydrogen4–64–6
 Oxygen30–3544–50
 Nitrogen2–4<1–3
 Sulfur1–20–2
Solubility in strong acidNot solubleSoluble
Apparent molecular weight range (atomic mass units)Few 100 to several million180–10,000
Functional group distribution (% of oxygen is indicated in functional groups)
 Carboxyl (–COOH)14–4558–65
 Phenol (–Ph)10–389–19
 Alcohol (–R–OH)13–1511–16
 Carbonyl (–CO)4–234–11
 Methoxyl (–O–CH3)1–51–2

image

Aquatic HAs are larger than aquatic FAs, but FAs have more carboxylic functional groups and oxygen, hence less carbon on a mass basis than HAs (Chamoli, 2013). FAs also are more soluble in water, because they have more polar groups per unit mass than HAs.

image
Figure 1.1 Hypothetical molecular structure of humic acid (HA). Adapted from Duan and Gregory (2003).
NOM has a significant impact on many aspects of water treatment, including the performance of unit processes, necessity for and application of water treatment chemicals, and the biological stability of the water. As a result, NOM affects potable water quality as a carrier of metals and hydrophobic organic chemicals and by contributing to undesirable color, taste, and odor problems. In addition, NOM necessitates the majority of the coagulant and disinfectant used in water treatment. It tends to interfere with the performance of unit operations, such as biofilm growth on media, causing rapid filter clogging and fast saturation of activated carbon beds. NOM is also responsible for the fouling of membranes. NOM contributes to corrosion, is a source of nutrients for heterotrophic bacteria, and acts as a substrate for bacterial growth in distribution systems (Jacangelo et al., 1995). Moreover, NOM has been found to be the major contributor to disinfection by-product (DBP) formation (Trang et al., 2012). NOM also forms stable complexes with metal ions.
Thus, the removal of NOM from water is an emerging issue, and a robust and efficient treatment technology is needed to address it. The number of publications related to NOM research in indexed journals has been escalating, indicating the scientific community’s burgeoning interest in NOM research (Figure 1.2).
Interest in DBPs of water treatment has grown significantly in the past few decades. More than 700 compounds of DBPs have been confirmed; among them, trihalomethanes (THMs) and haloacetic acids (HAAs) are the two groups found most frequently and in the highest concentrations in drinking waters worldwide (Krasner et al., 2006). Chlorine, ozone, chlorine dioxide, and chloramine are the most common disinfectants used today, and each produces its own suite of chemical DBPs in drinking water (Krasner et al., 2006). Although chlorinated DBPs have been the subject of the greatest concern, brominated DBPs have been considered even more hazardous than their chlorinated counterparts (Singer, 2006), while the formation of iodinated and nitrogen-containing DBPs has been studied with increasing intensity (Krasner et al., 2006; Hua and Reckhow, 2007; Zhao et al., 2008). DBPs have been associated with adverse health effects such as bladder cancer, spontaneous abortions, and birth defects (Singer, 2006; Gough et al., 2014). Thus, their occurrence in drinking water has been regulated in most countries. NOM has generally been considered the main precursor to DBPs, especially hydrophobic and high molecular mass (HMM) NOM, with its high aromatic carbon content (Hua and Reckhow, 2007; Bond et al., 2009; Liu et al., 2007). It has also been observed that hydrophilic and low molecular mass (LMM) NOM play a significant role in DBP formation (Hua and Reckhow, 2007; Bond et al., 2009). On the one hand, bromine and iodine appear more reactive with hydrophilic and LMM fractions of NOM in the formation of THMs and HAAs. On the other hand, chlorine has been shown to react more readily with HMM and hydrophobic NOM compounds (Hua and Reckhow, 2007).
image
Figure 1.2 Record of the number of publications in indexed journals containing the keyword “natural organic matter” in the abstract between 1990 and 2013. Scopus, January 27, 2014.
Thus, water treatment should be optimized to remove both hydrophobic and hydrophilic organic compounds, mitigating the formation of DBPs. More efficient, but economical, NOM removal methods are needed to meet stricter drinking water treatment regulations and overcome problems with water quality.
Changes in NOM quantity and quality have a significant influence on the selection, design, and operation of water treatment processes. These changes also cause operational difficulties in water utilities. High seasonal variability and the trend toward elevated levels of NOM concentration pose challenges to water treatment facilities in terms of operational optimization and proper process control. A sustained increase in NOM in raw water will lower the efficiency of water treatment processes and increase the demand for water purification. Changes in the properties of NOM also influence the treatment significantly (Eikebrokk et al., 2004; Sharp et al., 2006a,c). Water treatment facilities thus need to invest in additional NOM removal methods where existing water purification processes become insufficient.
To improve and optimize these processes, it is vital to characterize and quantify NOM at various stages during the purification and treatment process. It is also essential to be able to understand and predict the reactivity of NOM or its fractions during different phases of the treatment. Methods used in the characterization of NOM include resin adsorption, size exclusion chromatography, nuclear magnetic resonance (NMR) spectroscopy, and fluorescence spectroscopy. The amount of NOM in water has been predicted with parameters including ultraviolet and visible (UV–Vis), TOC, and SUVA. More precise methods for determining NOM structures have been developed recently: pyrolysis gas chromatography mass spectrometry, multidimensional NMR techniques, and Fourier transform ion cyclotron resonance mass spectrometry.
Once the composition and quantity of NOM in the water source has been examined, suitable methods for efficient NOM removal can be applied (Nkambule et al., 2012). No single process alone can be used to treat NOM due to its high variability. The development of rapid NOM characterization methods enables the selection of the proper treatment for the water concerned (Nkambule et al., 2012). Thus, NOM characterization and its removal during water treatment are closely connected. Effective selection criteria for precursor removal processes arise from increased knowledge of the nature of the precursor in individual water, facilitating the choice of appropriate technologies for precursor treatment (Bond et al., 2011). One reported variable is the nature of the reactive precursors present. Optimized coagulation treatment may be sufficient in hydrophobic waters, but if the postcoagulation residual remains reactive in DBP formation, technologies such as ion exchange for carboxylic acid precursors and/or granular activated carbon for hydrophobic precursors and/or nanofiltration for hydrophilic precursors are recommended (Bond et al., 2011).
The most common and economically feasible process available is coagulation and flocculation followed by sedimentation/flotation and filtration. Most of the NOM can be removed by coagulation, although the hydrophilic, low molecular weight (LMW) fractions of NOM are apparently removed less efficiently than the hydrophobic, high molecular weight (HMW) compounds (Jacangelo et al., 1995; Matilainen et al., 2010; Sharp et al., 2006a,c). This preference may be due to the more aromatic character, therefore more hydrophobic nature, of the latter (Sharp et al., 2006a,c). Moreover, the hydrophobic fraction generally has a higher specific colloidal charge; more highly charged fractions are more amenable to removal (Sharp et al., 2006a; Bose and Reckhow, 2007). Thus, LMW and hydrophilic NOM dominate the residual organic matter after coagulation (Zhao et al., 2009; Liu et al., 2007).
Other treatment options for NOM removal include magnetic ion exchange resin (MIEX®) techniques, activated carbon filtration, membrane filtration methods, and advanced oxidation processes (AOPs) (Jacangelo et al., 1995; Singer and Bilyk 2002; Matilainen et al., 2006a,b; Zularisam et al., 2006; Toor and Mohseni 2007; Matilainen and Sillanpää 2010; Parsons and Byrne, 2004; Pera-Titus et al., 2004; Suty et al., 2004; Agustina et al., 2005; Malato et al., 2007; Comninellis et al., 2008; Klavarioti et al., 2009; Malato et al., 2009). Although the MIEX® technique has been reported to remove even hydrophilic NOM, none of the above-named alternative treatment methods successfully remove all the NOM fractions present in raw water. Bioprocesses typically entail the development of a biofilm on a sand or activated carbon filter during water treatment (Bond et al., 2011). Biotreatment is reported to have a significant impact on precursor removal where reactive precursors are readily biodegradable, which is more likely in waters with high amounts of biologically derived NOM (Bond et al., 2011). The pros and cons of some treatment methods of NOM removal are presented in Table 1.2.

Table 1.2

Advantages and disadvantages of treatment methods for natural organic matter (NOM) removal (Jarvis et al., 2008; Bond et al., 2011; Shestakova and Sillanpää, 2013)

Treatment methodAdvantagesDisadvantages
AdsorptionHigh NOM removal efficiency is achievable.
Systems are available for various flow rates and concentrations of pollutants.
Easy to implement.
Removes hydrophobic NOM fraction.		Requires multiple regenerations and partial replacement of adsorbent.
In some cases, adsorbent is not regenerated and needs to be disposed of, causing secondary pollution of the environment.
Pretreatment of influent from suspended solids is required.
Effectiveness depends on the temperature and pH.
Table Continued

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Treatment methodAdvantagesDisadvantages
Advanced oxidation processes (AOP)Lower chemical consumption in some AOPs.
Complete mineralization of pollutant is achievable.
Reactions are often rapid.
Unselective oxidants.
Harmful to any microorganisms that may be present in the water.
Can often be installed in existing water treatment plants.
Oxidation of Disinfection by-products (DBPs).		Requires powdered photocatalyst separation from treated water.
Poorer performance for NOM removal than ferric salt coagulation.
Hydrogen peroxide residual is toxic.
Effectiveness depends on the pH value.
Requires additional reagents.
Dependent on the pH value.
Radical scavenging.
UV irradiation is a high-energy intensive process.
Recalcitrant NOM is poorly removed.
Ozone is a toxic gas, thus careful safety monitoring is required.
Water is corrosive due to high oxidation power.
Short duration of ozone exposure.
BiologicalRemoves some major NOM fractions that are biologically degradableRequires a relatively large area.
Activated sludge from aerobic treatment is a waste that requires disposal.
Requires additional nutrients.
Process is sensitive to conditions and concentration variations.
Table Continued

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Treatment methodAdvantagesDisadvantages
CoagulationCost efficient.
Conventional method with high levels of NOM removal.
Suitable for large molecules.		Sludge production.
Electrochemical methodsElectrocoagulation produces compact flocs.
Innovative, inexpensive, and effective.
Little or no chemical needed to facilitate NOM removal.
Less coagulant is needed in electrocoagulation; consequently, less sludge is formed.		Energy costs may limit practical use.
Electrode materials can be expensive for electrochemical oxidation.
Passivation of electrodes due to the presence of oxides and precipitation layers on the electrode surface, dissolving of the electrodes, and low conductivity of the surface waters.
Formation of DBPs.
Ion exchangeProven technology.
Potentially highly efficient.
Very low DBP formation.
Efficient in treating the transphilic fraction of NOM.		Additional treatment stage required.
Membrane technologyLarge, strong flocs formed.
Nanofiltration is efficient in the removal of low charged amino acids and carbohydrates.
Potential to remove low NOM fraction.
Can be integrated into other processes.		Energy-intensive process.
Additional treatment stage required.

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This book has two objectives. First, it investigates the techniques for NOM characterization, including conventional bulk parameters and the most advanced instrumental analytical tools. Second, it assesses the water treatment methods relevant to NOM removal, such as coagulation, electrochemical methods, membrane technology, AOPs, adsorption, and ion exchange.

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